United Stales
Environmental Protection
Agency
Office of
Radiation Programs
Washington, D.C. 20460
iPA 520/1-8WJ09
August 1986
Radiation
c/EPA
Final Rule for
Radon - 222 Emissions from
Licensed Uranium Mill
Tailings
Background Information
Document
-------
40 CFR Part 61 EPA 520/1-86-009
National Emission Standards
for Harzardous Air Pollutants
BACKGROUND INFORMATION DOCUMENT
STANDARD FOR RADON-222 EMISSIONS
FROM LICENSED URANIUM MILL TAILINGS
August 15, 1986
U.S. Environmental Protection Agency
Office of Radiation Programs
Washington, D.C. 20460
tt
-------
CONTENTS
Figures . vi
Tables viii
1. INTRODUCTION 1-1
1.1 History of Standard Development 1-1
1.2 Content 1-3
1.3 Other EPA Standards Affecting Uranium Mills 1-3
1.4 Other Regulations Affecting Uranium Mills 1-5
2. ESTIMATING THE RISK DUE TO EXPOSURE TO RADON-222 DECAY
PRODUCTS 2-1
2.1 Introduction 2-1
2,2 Radon-222 Exposure Pathways 2-2
2.2.1 Physical Considerations 2-2
2.2.2 Characterizing Exposures to the
General Population vis-a-vis
Underground Miners 2-5
2.3 Health Risk From Exposure to Radon-222 Decay
Products 2-7
2.3.1 Risk Models 2-7
2.3.2 The EPA Relative Risk Model 2-10
2.3.3 Comparison of Risk Estimates 2-12
2.3.4 Selection of'Risk Coefficients 2-17
2.4 Estimating the.Risks 2-18
2.4.1 Exposure 2-18
2.4.2 Risk Estimation 2-19
References 2-24
3. RADON-222 SOURCES, ENVIRONMENTAL TRANSPORT, AND RISK
ESTIMATES 3-1
3.1 Introduction 3-1
3.2 Origin and Properties of Radon-222 3-1
3.3 Sources of Radon-222 Emissions in the Milling
Process , 3-4
3.4 Characterization of Emissions 3-14
3.4.1 Ore Handling and Preparation 3-18
3.4.2 Mill Emissions 3-19
3.4.3 Emissions From Tailings Disposal • 3-20
3.5 Transport and Risk Assessment 3-21
3.5.1 Air Dispersion Estimates 3-22
3.5.2 Risk Estimates 3-22
111
-------
CONTENTS (continued)
3.6 Measurement of Radon-222 3-22
3.6.1 Ambient Air Samplers , 3-23
3.6.2 Concentrating Samplers That Measure
- , 'Radon-:2'22 Emanation From Surfaces 3-23
References • ' \ • . • •' • ••';'.• •' 3-25
4. INDUSTRY DESCRIPTION : . •'.'''. 4-1
4.1 Overview,. ; . • • . 4-1
4.2 Site-Specific Characteristics 4-1
4.2-.-1, Colorado 4-5
• • • • 4.. 2.2 , New Mexico 4-12
• , • 4.2.3 Texas • 4-19
4.2.4 Utah • ' 4-22
. 4.2.5 -Washington 4-25
• ,4.2.6 Wyoming , 4-28
4.3 Population Within 5 km (3.1 mi) of Existing
. Tailings Impoundments • • 4-35
References " •••-•:. • , 4-38
5. INDUSTRY RADON-222 EMISSION 'ESTIMATES . 5-1
5., 1 Introduction " • , . • 5-1
5.2 Estimating Emissions , 5-1
References , • , • 5-7
6. BASELINE "INDUSTRY RISK ASSESSMENT , 6-1
6.1 Introduction 6-1
6.2 Risk Estimates 6-1
6.2.1 Nearby Individuals ' 6-1
6.2.2 Regional Population 6-2
6.2.3 National 6-6
6.2.4 Risks from New Tailings Impoundments 6-8
References '6-10
7. RADON-222 CONTROL TECHNIQUES 7-1
7.1 Description of Control Practices 7-1
7.1.1 Earth Covers ' 7-2
7.1.2 Water Cover - 7-7
• 7.1.3 Water Spraying • 7-9
',7.1.4 Other .Control Techniques 7-n
IV
-------
CONTENTS (continued)
Page
7.2 Control Practices Applicable to-".Exist ing
Tailings Impoundments .;- 7-14
7.2,1 Interim-Controls -. - 7-14
7,2.2 Final Reclamation 7-16
7.2.3 Comparison of Interim.and Final
Controls , ' 7-16
7.3 Control Practices Applicable to New Tailings
Impoundments -'•' 7-18
7.3.1 Single Cell Tailings Impoundment 7-18
7,3,2 Phased Disposal Tailings Impoundment 7-24
7.3.3 Continuous Disposal-' .. - . 7-30
7.4 Summary of Radon-222 Control Practices ' 7-38
References •• ". • • ' • 7-41
8. SUMMARY AND COMPARISON OF WORK PRACTICES • 8-1
8.1 Single-Cell' Impoundments v, . . 8-1
8.2 Phased Disposal •••"• 8-3
8.3 Continuous Disposal '• 8-7
8.4 Comparison of Work Practices 8-7
Appendices
A - Diagrams of Uranium Mill Sites and Tailings Impound-
ments • • :•" • A-l
B - Cost Estimates for Existing and Model New Uranium
Mill Tailings Impoundments B-l
C - Evaluation of Interim Cover as a Control Option C-l
v
-------
FIGURES
Number " Page
2-1 Radon-222 decay series • , 2-3
2-2 U.S. lung cancer mortality by age—1970 •. 2-9
3-1 Uranium-238 decay chain and half-lives of principal
radionuclides . . -3-2
3-2 Schematic illustration of the radon sources at a
uranium mill . - 3-5
3-3 Simplified flow diagram of the acid leach process 3-10
3-4 Simplified flow diagram of the alkaline leach-caus-
tic precipitation process 3-11
3-5 Qualitative illustration of radon-222 emissions from
licensed uranium milling process 3-15
3-6 Effect of ore pile depth on hyperbolic tangent term
in radon-222 flux equation 3-17
4-1 Approximate locations of licensed conventional
uranium mills 4-4
4-2 Location of mills in Colorado 4-10
4-3 Location of mills in New Mexico 4-14
4-4 Location of mills in Texas 4-21
4-5 Location of mills in Utah 4-23
4-6 Location of mills in Washington 4-26
4-7 Location of mills in Wyoming 4-29
7-1 Changes in radon-222 penetration with earth cover
thickness 7-4
7-2 Radon emanation coefficients for tailings samples 7-10
7-3 Size and layout of the model single cell tailings
impoundment 7-20
7-4 Estimated radon-222 emissions from a model single-
cell tailings impoundment 7-21
VI
-------
FIGURES (continued)
Number Page
7-5 Size and layout of model phased—disposal impoundment 7-26
7-6 Estimated radon-222 emissions from a model phased- •
disposal impoundment 7-27
7-7 Size and layout of the model continuous-disposal
impoundment 7-32
7-8 Estimated radon-222 emissions from a model continuous-
disposal impoundment 7-35
7-9 Estimated radon-222 emissions from a model continuous
single-cell disposal impoundment 7-36
Vll
-------
TABLES
Number Page
2-1 Annual exposure equivalent (WLM) as a function of age
for members of the general public continuously
exposed to radon progency 2-7
2-2 Age-dependent risk coefficients and minimum induction
period for lung cancer due to inhaling radon-222
progeny ••-.- • 2-12
2-3 Estimated risk from exposure to radon-222 progeny 2-15
2-4 Radon-222 decay product equilibrium fraction at
selected distances from the center of a tailings
impoundment- • 2-20
2-5 Lifetime risk for lifetime exposure to a given level
of radon-222 progeny 2-22
3-1 Properties of .radon-222 3-3
4-1 Operating status and capacity of licensed conventional
uranium mills as of August 4, 1986 4-2
4-2 Summary of current uranium mill tailings impoundment
areas and radium-226 content . 4-6
4-3 Estimate of the population living within 0 to 5 km
from the eentroid of tailings impoundments of
active and standby mills in 1983 4-36
4-4 Estimate of the population living within 0 to 5 km
from the centroid of tailings impoundments of
active and standby mills in 1985 . 4-37
5-1 Summary of radon-222 emissions from uranium mill
tailings impoundments 5-3
6-1 Estimated risk of fatal lung cancer from maximum ex-
posure for an individual living near tailings
impoundment; 6-3
6-2 AIRDQS-EPA code inputs and estimated risks 6-4
6-3 Summary of regional health-effects from existing
tailings impoundments 6-5
Vlll
-------
TABLES (continued)
Number Page
6-4 Summary of nationwide health effects for tailings
impoundments 6-7
6-5 Summary of fatal cancers from current tailings
impoundments ' 6-9
7-1 Percent reduction in radon-222 emissions attained by
applying various types of earth cover 7-5
7-2 Summary of unit costs for estimating earth cover costs 7-6
7-3 Earth moving and placement costs (thousands of dollars
per hectare) of attenuating radon-222 as a function
of thickness (meters of different soils) and type of
earth ' 7-8
7-4 Benefits and Cost of alternatives that apply
earth cover to existing tailings impoundments 7-17
7-5 Average radon-222 emission rate from model single-
cell tailings impoundments ••' 7-22
7-6 Estimated costs for a model single-cell tailings
impoundment •''" 7-23
7-7 Average radon-222 emission rate for model single-
cell and phased-disposal tailings impoundments 7-28
7-8 Estimated costs for a model phased-disposal im-
poundment ' 7-33
7-9 Estimated radon-222 emission rates for- model
single-cell, phased-disposal, and continuous-
disposal tailings impoundments 7-34
7-10 Estimated costs for a model continuous-disposal
impoundment • . 7-37
7-11 Summary of estimated radon-222 emissions from new
model tailings impoundments 7-39
7-12 Summary of estimated costs for new model tailings
impoundment • • 7-40
ix
-------
TABLES (continued)
Number - Page
8-1 Emission and cost comparison for single cell impound-
ment with'-final cover applied at 0, 20, and
40 years after reaching capacity 8-2
8-2 Comparison of estimated deaths and benefits for a
single cell model impoundment with final cover ap-
plied at 0, 20, and 40 years after reaching capacity 8-4
8-3 Emissions and costs for model phased-disposal im-
poundments 8-5
8-4 Comparison of estimated death for model phased-
disposal impoundments . 8-6
8-5 Emissions and cost of model below-grade trench type
continuous -disposal impoundment ' 8-8
8-6 Comparison of work practices for new model tailings
impoundments 8-9
8-7 Comparison of cost and benefits between model Base
Case I and new work practices - 8-11
8-8 Comparison of cost and benefits between model Base
Case II and new work practices 8-12
-------
Chapter 1: INTRODUCTION
This Background Information Document supports the Agency's
final rule on radon-222 emissions from licensed uranium milling
activities. It is an integrated risk assessment that provides
the scientific basis for this action. Although the U.S.
Environmental Protection Agency (EPA) has considered radon-222 in
several regulatory actions, no specific emis'slon standard for
this radionuclide has yet been promulgated for operating licensed
uranium mills.
1.1 H1 story_of _S tanday d^Deye.lopment -:=
On January 13, 1977 (42 FR 2858), EPA issued Environmental
Radiation Protection Standards for Nuclear Power Operations.
These standards, promulgated in Title 40, Code of Federal
Regulations Part 190 (40 CFR 190), limit the total individual
radiation dose due 'to emissions from uranium fuel-cycle
facilities, including licensed uranium mills,-' At the time 40 CFR
190 was promulgated, considerable uncertainty existed regarding
the public health impact of levels of radon-222 in the air and
the best method for managing new man-made sources of this
radionuclide. Therefore, the Agency exempted radon-222 from
control under 40 CFR 190. '--.
On September 30, 1983, the Agency issued standards under the
Uranium Mill Tailings Radiation Control Act :(UMTRCA) (40 CFR 192,
Subparts D and E) for the management of tailings at locations
licensed by the Nuclear Regulatory Commission (NRC) or the States
under. Title II of the UMTRCA. These standards do not
specifically limit radon-222 emissions until after closure of a
facility? however, they require as low as reasonably 'achievable
(ALARA) procedures for radon-222 control, and the NRC does
consider ALARA procedures in licensing a mill. When the UMTRCA
standards were promulgated, the Agency stated that it would issue
an Advance Notice of Proposed Rulemaking with respect to control
o"f radon-222 emissions from uranium tailings piles during the
operational period of a uranium mill. •
On April 6, 1983, standards for NRC licensees were proposed
under the Clean Air Act (48 FR 15076, April 6, 1983); however,
uranium fuel-cycle facilities, which included operating uranium
mills, were excluded because these sources are subject to EPA's
40 CFR Part 190 standard.
1-1
-------
During the comment period for the Clean Air Act standards, it -was
noted that radon-222 emitted from operating uranium mills and
their actively used tailings piles were not subject to any '
current or proposed 'EPA standards, and that such emissions could
pose significant risks. .. : •
On October 31, 1984, EPA published an Advance Notice of
Proposed Rulemaking 'in the Federal Register, 49 FR 43916, for
radon-222 emissions .'from licensed uranium mills. The 'notice
stated that -the' Agency is considering emissions standards for
licensed uranium mills and solicited information in the' following
areas; ' ' ••
o Radon-222 -emission rates from uranium mills and
assoclated'i'tailings piles
o Local;and ^regional impacts due to emissions of
radon-222 from-uranium mills and associated tailings
piles prior. to'.permanent disposal'
o. Applicable--:radon-222. control options and'strategies,
including work practices
o Feasibility and cost .of'radon-222 control options and
strategies • _ '• • .
' o • 'Methods off-determining compliance with a work practice
type of standard to control radon-222 emissions
o Impact of.:.radon-222 controls on the uranium industry
Pursuant to the^citizens1 suit provision of the Act, the .
U.S. District Court for the Northern District of California
directed EPA to promulgate.standards for other sources of
radionuclide'emissions, which could include radon-222 emissions
, fr'om licensed uranium-mills. Thus, discussions between EPA and
the Sierra Club regarding a"schedule for developing a standard .
led'to an agreement .'to submit a schedule for the promulgation of
a standard in one year rather than having-the'Court establish a
schedule. This motion was submitted to the Court on August 5,
1985, and the court Bordered the"EPA to issue final standards for
radon-222 emissions from licensed uranium mills and mill tailings
impoundments by May,l, 1986. This date was later moved to August
15, 1986 to allow additional time for public comment.
• The EPA then issued the.proposed rulemaking for "National-
Emission'Standards for Hazardous Air Pollutants; -Standards for
Radon-222 Emissions:-,from Licensed Uranium Mill. Tailings," on
February 21, 1986 (51 FR 6382-6387). Subsequent, to the
announcement,of the proposed rule, a public hearing was held on
March 25, 1986 in Denver, Colorado (51 FR 8205) and a second
comment-period.was held open until April 28, 1986.
1-2
-------
1 ,• 2 Content ''.'•.. ' . ..•••?,; ' • ' •
The health effects of radon-222 and the;- risk assessment
procedure are summarized in .Chapter .2. The .'incidence of lung
cancer and resulting deaths among miners exposed to radon-222 are
described, and the range of risk' factors-is presented.
The sources of radon-222 in uranium milling and the factors
affecting the rate of radon-222 emissions ar.e described in
Chapter 3. 'This-chapter also includes a general description of
EPA's risk-estimating procedure, along -with ,,the methods of
measuring radon-222. -
A description of each licensed mill., its associated tailings
impoundments, and its estimated milling .production rates are
contained in Chapter 4. Estimates of radon-222 emissions from
the existing tailings impoundments are presented in Chapter 5.
The' baseline industry risk assessment' for individuals and
regional and national populations and the control techniques and
work practices that can be used to reduce radon-^222 emissions are
described in Chapters 6 and 7'respectively. The resulting
emissions after application of these;control 'methods are
estimated. A comparison of work practices,-costs, and
effectiveness is presented in Chapter 8. :• •
Information for this study was compiled"- from the technical
literature, previous studies by EPA and the:-Nuclear Regulatory
Commission, comments resulting from rulemaking notices, and
discussions with industry representatives. ;;Comments "received
during the public comment period were incorporated into this
final document as appropriate. No significant change in the
technical information was made except for the Agency's revision -
of the risk factors associated with radon-22'2 exposure. . These
risk factors were increased from a range of;i,250-lQQO. deaths, per
million person working level months to a range.of'380-1520 deaths
per million person working level month. In-.:addition, mill , -.
site-specific information was corrected andrthe''- discussion of- -
interim cover was revised. .<-,; "; ' •• -. , - '
1.3 Other EPA Standards Affecting Uranium Mills•.
On December 3,. 1982, EPA issued guidelines-, under the'Clean
Water Act.for effluent>limitations for New Source Performance
Standards for wastewater,discharges from the mining and dressing
of uranium, 'radium, • and vanadium .ores (40 CFR-Part • 440, 47"-FR
54598). . These effluent guidelines cover discharges of both
radioactive and nonradioactive', materials tor surf ace waters from
uranium byproduct materials.. ,'. - ' '• • , " rx ..
1-3 • • " *;.''•
-------
The EPA promulgated 40 CFR Part 261, Subpart F
—Groundwater Protection—on July 26, 1982 (47 FR 32274)
under the1 Solid Waste Disposal Act (SWDA) as amended by the
Resource Recovery and Conservation Act. This Act requires
that standards for nonradioactive hazards from uranium
byproduct materials be consistent with standards promulgated
under SWDA for such hazards. The Act also requires that the
NRC establish general requirements that are, insofar as
possible, at least comparable to requirements applying to
the possession, transfer, and disposal of similar hazardous
material regulated by EPA under the SWDA.
The EPA issued standards for cleanup of contaminated
open lands and buildings and for disposal of tailings at
inactive uranium processing sites on January 5, 1983 (48 FR
590) under UMTRCA. For inactive mills, the standard
specified in 40 CFR 192.02 requires that controls:
"(a) Be effective for up to one thousand' years, to the
extent reasonably achievable, and, in any case,
for at least 200 years, and,
(b) Provide reasonable assurance that releases of
radon-222 from residual radioactive material to
the atmosphere will not:
(1) Exceed1 an average release rate of 20
picocuries per square meter per second, or
(2) Increase the annual average concentration of
radon-222 in air at or above any location
outside the disposal site by more than
one-half picocurie per liter."
This standard was later amended -under Section 84 of the
Atomic Energy Act of-1954 to include standards for
radionuclides during and after processing of uranium ore
sites (48 FR 45946, October 7, 1983). These regulations in
40 CFR 192.30 specify concentration limits and construction
standards for surface impoundments to ensure ground-water
protection. In addition, Part 192.32 addresses radon-222 at
active mills in a generic manner by requiring the mill owner
to "make every effort to maintain radiation doses from
radon-222- emissions from surface impoundments of uranium
byproduct materials as far below the Federal Radiation
Protection Guides as'.;is practicable at each licensed site."
1-4
-------
This standard also specifies that radqn-222 emissions
are limited to 20'picocuries per square meter per second,
(pCi/m s) after mill closure. This limitation does not
apply to sites that contain a radium-226 concentration from
mill tailings that does not exceed the background level by
more than 5 pci per gram over the top 15 cm of soil and
15 pCi per gram over each successive 15-cm layer of soil
below the top 15 cm.
'1-4 Other Regulations Affecting Uranium Mills
All uranium mills are licensed by the NRC or by States
that enforce the NRC regulations, and are subject to the
regulations contained in 10 CFR 20. Specific standards
pertaining to radon-222 limit atmospheric.radon-222
concentrations to 3 x 10 pCi/ml (30 pCi/liter) in
restricted areas (i.e., areas within the mill property) and
3 x 1Q~ pCi/ml (3- pCi/liter) in unrestricted areas,
These concentrations are approximately equivalent to
one-third and one-thirtieth of a working level,
^ ' respectively. The NRC has also recently issued
amendments to its regulations governing uranium mill
tailings disposal (100 CFR Part 40) as published on October
16, 1985 '(50 FR 41852). These amendments''Conform to the EPA
regulations for tailings disposal.
The NRC has entered into agreement with a number of
States to provide enforcement of the NRC regulations. These
States are referred to as "Agreement States." The Agreement
States that have uranium mills are Colorado, New Mexico,
Texas, and Washington' .
State regulations pertain to the construction of
tailings impoundments to minimize ground-water
contamination. In addition, States inspect tailing
impoundment dams to ensure that they are built and
maintained to minimize safety problems.
• A .working level is defined in Chapter 2. The
relationship between radon-222 and-working levels-
depends on the degree of equilibrium'between radon-222
and its decay products.
^ ' Utah also is an Agreement State in nuclear licensing
areas other than uranium milling. New Mexico returned
licensing authority to the NRC on May 1, 1986.
1-5
-------
Chapter 2: ESTIMATING THE RISK DUE TO EXPOSURE
TO RADON-222 DECAY PRODUCTS
2.1 Introduction
The methodology- the EPA uses to estimate the exposure and
the health detriment (i.e., lung cancer) due to radon-222 in the
general environment is described in this chapter. Radon-222
exposure pathways are explained, the EPA risk model is described,
estimates of risks due to radon-222 progeny (radon-222 decay
products) made by various scientific groups are compared, and the
risk coefficients to be used in this risk assessment are
selected. Earlier studies have shown that a degree of
uncertainty exists in all risk estimates (EPA84); therefore, EPA
uses more than a single coefficient to indicate the range of this
uncertainty.
The occurrence of radiation-induced cancer is infrequent
compared with the current incidence of all cancers. Even among
heavily irradiated populations (e.g., some of the uranium mine
workers in epidemiologic studies), the precision and accuracy of
the estimate of the number of lung cancers resulting from
radiation is uncertain because of the small sampling segment and
because the data vary greatly. Also, the small sampling of
exposed populations has not been followed for their full
lifetime? therefore, information on the ultimate effects of their
exposure is limited.
Only human epidemiological data are used to derive risk
estimates for effects of exposure to radon-222 progeny, but
animal studies support the risk estimates. In a series of
studies performed with rats, French investigators have shown a
dose-effect relationship similar to that obtained in surveys of
uranium miners (Ch84, 85). In these studies, the risk per
working level month at. 20 cumulative working level months (CWLM)
is about four times greater than at 3000 or more CWLM (Ch84,.
85). The lowest exposure studied to date, 20 CWLM, which is
about 10 times the background exposure, doubled the incidence of
lung cancer in the rats (Ch84, 85).
"When considered in light of experiments with animals and
various theories of carcinogenesis and mutagenesis, the
observational data on cancers related to human exposure to
radiation are subject to a number of interpretations. These
various interpretations lead to differing estimates of radiation
risks by both individual radiation scientists and expert advisory
groups. Readers should bear in mind that estimating radiation
risks is not a mature science and that the evaluation of the risk
due to radon-222 decay products (progeny) will change as
additional information becomes available.
2-1
-------
Nevertheless, a substantial data base is available for use in
developing risk estimates, and the Agency believes these
estimates can be used in the development of regulatory-
requirements. ' '
2.2 Radon-2.2.2 Exposure Pathways
2.2.1 Physical Considerations
Radon-222 from uranium milling operations enters the general
environment from stockpiled ore and mill exhaust systems and
through waste materials from milling operations. The half-life
of radon-222 is 3.8 days; therefore, when it is released into the
atmosphere, some atoms of gaseous radon-222 can travel thousands
of miles through the atmosphere before they decay. As shown in
Figure 2-1, the radon-222 decay process involves seven principal
decay products before the radon-222 becomes nonradioactive lead.
The first four short-half-life radioactive decay products of
radon-222 are the most importa-nt sources of cancer risk. Members
of the decay chain with relatively long half-lives (beginning
with lead-210, which has a 22-year'half-life) are more likely to
be ingested than inhaled and generally present much smaller
risks. .''•'•
The principal short-half-life products of radon-222 are
polonium- 218, lead-214, bismuth-214, and polonium-214.
Polonium-218, the first decay, product, has a half-life of just
over 3 minutes. This is long-' enough for most of the electrically
charged polonium atoms to- attach themselves to microscopic
airborne dust particles that are typically less than a millionth
of a meter in diameter. ' When inhaled, these small particles have
a' good chance of sticking- to the- moist epithelial lining of the
bronchi. Most inhaled particles are eventually cleared (removed)
from the bronchi by mucus, but not quickly enough to keep the
bronchial epithelium from being exposed to alpha particles from
the decay of polonium-218 and polonium-214. This highly ionizing
radiation passes through and delivers radiation doses to several
types'of "lung cells.
. Adequate characterization cannot be made of the exact doses
delivered to cells that eventually become cancerous. Knowledge
of the deposition pattern of the radioactive particles in the
lung is based on theoretical models, and the'distances from the
radioactive particles to cells that are susceptible can only be
assumed. Further, some disagreement exists about the types of
bronchial cells in which cancer originates.- Therefore, EPA
estimates of lung cancer -risk-'are based on the amount of inhaled
radon-222 decay products to which people are exposed rather than
on the dose absorbed by the lung.
2-2
-------
3.05 min.
21
4
Pb
26 £.
mm.
214
83Bi
»-> » i
19.8
mm.
214
84Po
OUT ,
1.64
Figure 2-1. Radon-222 .decay series,
2-3
-------
Ingrowth of Radon-222 Decay Products
At - the point where radon-222 diffuses out of the tailings
pile surface, the concentration of associated radon-222 decay
products is zero because those decay products generated prior to
diffusion from .the surface have been captured in the tailings or
cover. As soon as radon-222 is airborne, ingrowth of decay
products commences and secular equilibrium between the radon-222
and the short half-life1decay, products'is eventually obtained.
At secular equilibrium, the activities of radon-222 and of all
its short-half-life decay products are equal, and the alpha
activity per unit of radon-222 concentration is at its maximum.,
As a means of accounting' for the incomplete equilibrium before
this 'state is reached,, the "equilibrium fraction" is defined as
the ratio of the potential alpha energy' from those decay products
actually present to the potential alpha energy that would be '
present at complete equilibrium. As radon-222' and its decay
products are transported by the-wind, the equilibrium fraction
increases ;with distance from the tailings pile/ and at great
distances, approaches the theoretical maximum value of oner
however, depletion processes, such as dry deposition 'and
precipitation scavenging, selectively remove decay products (but
not radon), so complete equilibrium of the short-lived decay
products with the radon-222 is seldom, if ever, reached.
When radon^222 and its decay products enter a structure, the
building ventilation rate is the principal factor affecting the
indoor equilibrium fraction. The equilibrium fraction can also
be affected by other considerations, however,'such as the indoor
surface-to-volume ratio and the dust loading in indoor air
(Po78).
.In estimating the exposures of nearby individuals to
radon-222 decay products (in Chapter 6), the model uses the
calculated effective equilibrium'fraction at selected distances-
from a tailings pile (see Table.' 2-4 presented later in this
chapter). For estimating population exposures, a '
population-distance weighted effective equilibrium fraction would
be appropriate, 'but, it is impractical to calculate this
fraction. Indoor exposure is the dominant form of exposure due
to radon-222 [Americans spend about 75 percent of their time
indoors (Mo76, oa72)], and the indoor effective equilibrium
fraction does not depend.greatly, on the distance from the
tailings pile. In this assessment, an effective equilibrium
fraction of 70 percent is assumed ;for calculating the exposure of
populations because most of the affected individuals are at some
distance from the tailings pile (see Section. 2.4.1).
2-4
-------
2.2.2 Characterizing Exposures- to__the General Population
Vis-a-vis Underground Miners
Although considerable progress has been made in modeling the
deposition of particulate material in the lung (Ha82, JaSQ,
Ja81), adequate characterization of the bronchial dose delivered
by alpha particles from inhaled radon-222 progeny attached to
dust particles is not yet possible. Knowledge is still lacking
concerning the kinds of cells'in which bronchial cancer is
initiated (Mc78, Mc83) and the depth of these cells in the
bronchial epithelium. Current estimates of the exposure dose of
inhaled radon-222 progeny actually causing radiogenic cancer are
based on average doses/ which may.or may not be-relevant (E185).
Until more reliable estimates of .the bronchial dose become
available, following the precedents set in the 1972 and 1980
National Academy of Sciences reports appears to be a prudent
approach (NAS72, NAS80). Therefore, the EPA estimates the risk
due to radon-222 progeny on the basis of exposure rather than
dose per se. This is called the epidemiological approach; i.e.,
risk is estimated on the basis, of' observed cancers after
occupational exposure to radon-222 progeny.
Exposures to radon-222 decay products under working
conditions are commonly reported in a special unit called the
working level (WL). One working level is any concentration of
short half-life radon-222 progeny having 1.3 x 105 MeV per liter
of potential alpha energy (FRC67) ., (A WL is also equivalent to
approximately 100 pCi/liter of radon-222 in secular equilibrium
with its short-lived decay products.) This unit was developed
because the concentration of specific radon-222 progeny depends
on ventilation rates and other factors. A working level month
(WLM) is the unit used to characterize a mine worker's exposure
to one working level of radon-222.progeny for a working month of
170 hours. Inasmuch as the results of epidemiological studies
are -expressed in units of WL and WLM, comparable estimates of'"
'exposure were developed for members of the general population •
exposed to radon-222 progeny, as explained in the following
paragraphs. : •
1 For a given concentration of -radon-222 progeny, the amount
of potential alpha energy a member of the general population .
inhales in armonth is more than the amount a mine worker receives
in a working•month. Although members of the general population
are exposed longer (up to 24 hours- per day, 7 days a week),, the
average amount of air inhaled per minute (minute .volume) is less
in this group than that for a mine worker when periods of•
2-5
-------
'sleeping and resting are taken' into account (EPA79, Th82). The
radon-222 progeny exposure-of a mine worker can be compared with
that of a member of the general population by considering the
amount of potential alpha energy each inhales per year (Ev69),
'That radon daughter deposition (and dose) 'in the conducting
airways of the lung is proportional 'to ventilation rate (quantity
inhaled)- has also been recommended by other investigators (Ra85,
Ho82).
The IPA assumes that a mine worker inhales 30 liters per
minute (averaged over a work day). This average corresponds to
about 4 hours of light activity and 4 hours of moderately heavy
work per day (ICRP75). The new ICRP radon-222 model, however,
assumes an inhalation rate of 20 liters per minute for mine
workers, which corresponds to 8 hours of light activity per day
(ICRP81) . ' -This may be appropriate for nuclear workers »• however,
studies of the metabolic rate of mine workers clearly show that
they are not engaged in light activity only (Sp56, ICRP75,
NASA?3). Therefore, 30 liters appears to be a more realistic
estimate of the average minute volume for this group. Based on
this minute volume, a mine worker inhales 3.6 x 10 cubic
meters in a working year of 2000 hours £ICRP79). One working
level of radon-222 progeny is 2.08 x 10 joules per cubic
meter (1.3 x 10 MeV per liter); therefore, in a working year,
the potential alpha energy inhaled by a mine worker exposed to
one working level is 7.5 x 10 joules.
According to the ICRP Task Group report on reference man
(ICRP75), an inhaled air volume of 2.3 x 10 liters per day is.
assumed for adult males in the general population and 2.1 x 10
liters per day for adult females, or an average of 2.2 x 10
liters per day for members of the adult population. This average
volume results in an intake of 8.04 x 10 cubic meters of air
and 1.67 x 10 joules per year of inhaled potential alpha
energy from a continuous exposure of an adult member of the
population to one working level of radon-222 progeny for
365,25 days.
Although it may be technically inappropriate to quantify the
amount of potential alpha particle energy inhaled by a member of
the general population in working level months, continuous
exposure to 1' WL corresponds to about the same inhaled -potential
alpha energy as'27,WLM would'to a miner. Hence, for an adult
member o'f the general population, a one working level
concentration of radon progeny results in a 27 WLM annual
exposure equivalent (see Table 2-1). As stated earlier, an
occupancy factor of 0.75 is assumed for indoor exposure; thus, an
indoor exposure to one WL results in an annual exposure
equivalent of "20 WLM (EPA79) in' terms of the amount of potential
alpha energy actually inhaled.
2-6
-------
The- smaller- bronchial area of children as compared with that
of adults more. than of fsets . their lower per-minute volume;
therefore, tor a, given concentration of radon-222 progeny, the
dose to children-' s bronchi is greater. This problem has been •
addressed in -a paper by Hofmann and Steinhausler (Ho77) , in which
they estimate- that "doses received, during childhood are about 50
percent greater than adult doses. ' This information was. used to
prepare Table,, 2-1, which lists the age-dependent potential
exposure equivalent used in the risk assessment described in the
next subsection.; ' The larger effective exposure to children
relative to that : to' adults increases the estimated mortality due
to lifetime exposure from birth by about 20 percent.
"•• Table '2-1'. 'Annual exposure equivalent (WLM) as a
•. function of, 'age for members of the general public
continuously exposed to radon progeny at one working
•level (2.08 x 10 joules per cubic meter)
of • . Exposure
general population Equivalent
(years) . (WLM)
0-2 35
3-5 43
6-11 49
12-15 43
16-19 • " 38
20-22 . . -32
23 'or 'more 27
Lifetime Average 31
2-3 Health Risk. From Exposure to Radon-222 Decay .Products.
2.3.1 ,' Risk Models
A wealth of- data indicates that radon-222 exposure of the
bronchial epithelium of underground mine workers causes an
increase in bronchial•lung cancer among both smokers and
nonsmokers. Among recent reviews (ICRP81,.NA580, NCRP84,
N105HS5, Th82), two are of particular interest.
^a' The assumptions on minute volume, etc., for mine workers
and the" general population just described are the same as those
used in the preparation of the EPA report .entitled "Indoor
Radiation Exposure Due to Radium-226 in Florida Phosphate Lands"
(EPA79J and Final Environmental Impact Statements (EPA82, 83a).
2-7
-------
The 1980 NAS BEIR-3 Report (NAS80) contains a review of
epidemiological studies on mine workers and develops an age
specific absolute risk model. A lengthy report entitled "Risk
Estimates for the Health Effects of Alpha Radiation," which.was
prepared by D. C. Thomas and K. C. McNeil for the Atomic Energy-
Control Board (AECB) of Canada, reanalyzes many of these
epidemiological studies in a consistent fashion so that the
modeling assumption's are the same for all of the data sets and
develops a relative risk coefficient which fits most studies
(Th82) .
The manner in which radiogenic lung cancers are distributed
in time, after a minimum induction period, is a crucial factor in
numerical risk estimates. For radiation-induced leukemia and
bone cancer, the period of risk expression is relatively brief?
.most occur within 25 years of exposure. For other
radiation-induced cancers (including lung cancer), however, it
appears that people are at risk for the remainder of their lives
(NAS80). None of the epidemiological studies of underground mine
workers provides information on lifetime expression; indeed, most
of the study populations are still alive and still at risk.
Lifetime risks cannot be estimated only on the basis of
observations to date,' therefore, a model is needed to project the
risk beyond the period of direct observation. As discussed in
the 1980 NAS BEIR report, there are two basic models of risk
projection: (1) the absolute risk projection model, in which it
is assumed that the observed annual numerical excess cancer risk
per unit exposure (or dose) continues throughout life? and (2)
the relative risk projection model, in which it is assumed that
the observed percent age increase of the baseline cancer risk per
unit exposure (or dose) is constant with time (NAS80).
In the case of lung cancer and most'other solid cancers, a •
relative risk model leads to larger estimated risks than1 the
absolute risk model because of the generally increasing incidence
of such cancers with increasing age. The number of lung cancer
deaths that occurred in the U.S. population as a function of age
in 1970 and in 1980 is shown in Figure 2-2. The decrease in the
number of deaths for ages greater than 65 years is due in part to
depletion of the population by competing risks, and in part to a
decrease in the age-specific incidence of lung cancer mortality,
which peaks in males at about age 75 but is relatively constant
in females until age 95 (NCHS73, NCHS8.3) (see Figure 2-3). The
age-specific mortality of underground mine workers dying of
radiogenic lung cancer shows the same pattern of death as a
function of•age as the general male population (Ra84, E185). In
a recent review (1185), it was shown that a relative risk model
can adequately account for the temporal pattern of cancer deaths
observed in underground mine workers, whereas absolute risk
projection models fail to do so.
2-8
-------
X
H
U
Q
CC
UJ
o
z
o
0
z
3
15,000-
14,000-
13,000-
12,000-
11,000-
10,000-
9,000-
8,000-
7,000-
6,000-
5,000-
4,000-
3,000-
2,000-
1,000-
•null
MALES 1980
60
70
80
90
100
AGE in YEARS
Figure 2-2. U.S. lung cancer mortality by age—1970 and 1980.
2-9
-------
i
i—
o
W
n:
M
C
o
UJ Q.
O »
O g.
o -
z
5,000-
4,500-
4,000-
MALES 1980
40
50
90
100
60 70 80
AGE in YEARS
Figure 2-3. Age-specific U.S. lung cancer mortality rates—1970 and 1980
110
-------
2.3.2 The EPA_R_elative Risk Model
Since 1978, the Agency has based risk estimates due to
inhaled radon-222 progeny on a linear dose-response function, a
relative risk projection model, and a minimum induction period
of 10 years. Lifetime risks are projected on the assumption
that exposure to 1 WLM increases the age-specific risk of lung
cancer by 3 percent over the age-specific rate in the U.S.
population as a whole (EPA79). The life table analysis
described in BuSl and EPA84 is used to project this risk over a -
full life span.
The EPA model has been described in detail (EPA79, E179).
A review of this model in light of the more recent information
described herein revealed that the major assumptions, linear
response, and relative risk projection have been affirmed. The
A-bomb survivor data clearly indicate that the absolute risk of
radiogenic lung cancer has continued to increase among these
survivors, whereas their relative risk has remained reasonably
constant (Ka82). The UNSCEAR, the ICRP, and the 1980 NAS
Committee have continued to use a linear dose response to
estimate the risk of lung cancer due to inhaled radon-222
progeny. Thomas and McNeill!s analysis (Th82) indicates that
the use of linearity is not unduly conservative -and actually may
underestimate the risk at low doses. The 1980 NAS BEIR
Committee reached a similar conclusion (NAS8Q).
A major limitation of earlier EPA risk estimates is the
uncertainty in the relative risk coefficient used, 3 percent
increase in the age-specific lung cancer mortality rate per
WLM. This value is based on the excess mortality caused by lung
cancer among exposed mine workers of various ages, many of whom
smoked. Therefore, it represents an average value for a mixed
population of smokers, former smokers, and.nonsmokers. This
assumption may tend to inflate the risk estimate (as discussed
herein).because smoking was more prevalent among some groups of
mine workers studied than it is among the U.S. general
population today.
In a recent paper, Radford and Renard (Ra84) reported on
the results of a long-term study of Swedish iron miners who were
exposed to radon-222 progeny. This study is unique in that most
of the miners were exposed to less than 100 WLM and the risks to
smokers and nonsmokers were considered separately. The absolute
risks of the two groups were similar, 20 fatalities per 10
person-year WLM for smokers compared with 16 fatalities for
nonsmokers. The total number of lung cancer fatalities for
nonsmokers is small; therefore, the estimate of 16 fatalities' is
not too reliable. • Although absolute risks were comparable for
2-11
-------
the smoking and nonsmoking miners, relative risks were not.
Nonsmokers have a much lower baseline incidence of lung cancer
mortality than smokers. . This resulted in a relative risk
coefficient for nonsmoking exposed miners relative to unexposed
nonsmokers that was about four times larger than the relative
risk coefficient" for exposed smokers. This larger relative risk
does not, however, fully compensate for the lower baseline
incidence of lung cancer mortality among nonsmokers. Therefore,
this study indicates that a relative risk coefficient derived
from data on miners maybe biased high when applied to the
population as a whole. Further'follow-up of this and other •
groups of mine workers may provide more reliable data* on the
risk to nonsmokers, and EPA expects to incorporate separate
consideration of smokers and-'nonsmokers into its analyses as
more data become available.
Although occupational -exposures 'to pollutants other than
radon-222 progeny are probably not important factors in the- ••
observed lung cancer risk for underground mine workers (E179,
Th82, Mu83, Ra84), the use of occupational risk data to estimate
the risk of a general population is far from optimal, as it
provides no information on the effect of radon-222 .progeny
exposures to children and women. Although the assumption has
continued that the risk per unit exposure during childhood is no
more effective than that occurring to adults, this assumption
may not be correct. The -A-bomb survivor data indicate that, in
general, the risk resulting from childhood exposure to low
linear energy transfer (LET) radiation is greater than that
resulting from adult exposure,-and this greater risk continues
for at least 33 years (Ka82). As yet, however, no specific data
pertaining to the effect of age at irradiation on lung cancer
have been published (Ka82). Another limitation of the data for
underground mine workers is the absence of women in the studied
populations. The A-bomb survivor data indicate that"women are
about as sensitive as men to radiogenic lung cancer, even though
they tend to smoke less as a group (Pr83). These data are not
conclusive, however.
2-12
-------
2,3,3 Comparison of Risk Estimates
National Academy of Sciences BEIR-3
Several estimates of the risk due to radon-222 progeny have
been published since the EPA model was developed. One of
particular interest was developed by the National Academy of
Sciences BEIR Committee (NAS80). The BEIR-3 Committee formulated
an age-dependent absolute risk model with 'increasing risk for
older age groups. Estimates of the risk per WLM for various ages
and the estimated minimum induction period for lung cancer after
exposure (NAS80, pp. 325 and 327, .respectively) are summarized in
Table 2-2. These have been used to calculate the lifetime risk
of lung cancer .mortality due to lifetime exposure of persons in
the general population.
Table 2-2. Age-dependent risk coefficients and minimum induction
period for lung cancer due' to inhaling radon-222 progeny (NAS80)
Age at Excess lung cancers * Minimum
D
diagnosis '. (cases per 10 ' induction period
(years) person-year WLM) ' • (years)
0-15 0 25
16-36 ' 0 25-15
36-50 10 • 10
51-64 - 20 10
65 or more 50 . 10
This was done by means of the same life table analysis that was
used to calculate other EPA risk estimates (BuSl).
2-13
-------
The zero risk shown in Table 2-2 for those under 35 years of
age at diagnosis does not mean that no harm occurs; rather, it
means that the risk is not expressed until the person is more
than 35 years old, i.e., only after the minimum induction
period. The sequence of increasing risk with age shown in this
table is not unlike the increase in lung cancer with age observed
in unexposed populations; therefore, the pattern of excess risk
over time is similar to that found by the use of a relative risk
projection model.
Atomic Energy Control Board of Canada
In their recently conducted thorough analysis of the
incidence of lung cancer among uranium mine workers for the
Atomic Energy Control Board (AECB) of Canada, Thomas and McNeill
tested a number of risk'models on all of the epidemiological
studies that contained enough data to define a dose-response
function (Th82). They concluded that lung cancer per WLM among
males increased 2.3 percent and "that a relative risk projection
model was more consistent with the incidence of excess lung
cancer observed in groups of underground mine workers than any of
the other models they tested. This is the only analysis that
treated each data set in consistent fashion and used, to the
extent possible, modern epidemiological techniques such as
controlling for age at exposure and duration of followup.
The estimate for lifetime exposure to Canadian males is 830
fatalities per million person WLM (Th82). For presentation in
Table 2-3, this estimate has been adjusted to 600 fatalities per•
million person WLM (which would be the appropriate estimate for
the U.S. 1970 general population) by determining the "best
estimate" risk (see p. 114 in ThS2). This estimate was then
multiplied by the ratio of lung cancers caused by radon-222 in
the U.S. 1970 general population to lung cancers in' the U.S. 1970
male population as calculated in the EPA model.. The 1978
reference life tables for Canadian males and U.S. males are quite
similar? therefore, the simple,proportional relationship of ,
general population deaths to male deaths should give a reasonable
estimate. . '
2-14
-------
International Commission onRadiologicalProtection'
The International Commission, on Radiological Protection
(ICRP) has made risk 'estimates for occupational exposure of
working adults- (ICRP81) . The larger ICRP estimate (shown in . .
Table 2-3) is based on an epid.emiologic.al approach; i.e., the
exposure to mine workers in • WLM and, the risk per VJLM observed in •
epidemiological studies of underground mine workers. The ICRP
epidemiological approach assumes an average expression period of
30 years for lung cancer. Children, who have a much longer
average expression period, are excluded from this estimate. The
ICRP has not explicitly projected the-risk to mine workers beyond
the years of observation, even though most of the mine workers on
whom these estimates are based -are still alive and continue to
die of .lung'cancer. . •• '
The smaller of. the two ICRP estimates listed-in Table 2-3 is
based on their dosimetric approach.- These.estimates are in the
lower part of the range shown for.the.ICRP estimate in Table
2-3. In the dosimetric approach, the ICRP assumes that the risk
per rad for lung tissue is, 0.12 of the risk of cancer and genetic
damage after whole-body exposure (ICRP77). For exposure to
radon-222 progeny, the ICRP divides-this factor of 0.12 into two
equal parts. A weighting factor of 0.06,is used to assess the
risk from a, high dose to bronchial tissue,, where radiogenic lung.
cancer is observed in exposed underground mine workers. The
other half of the lung cancer weighting factor, another 0.06 of
the total body risk, is used to assess the risk to the pulmonary
region, which receives a comparatively small dose-from radon-22.2
progeny-and. where-human- lung cancer is seldom, if ever, observed..
UNSGEAR - - • ' - • -
The United Nations Scientific Committee on the Effects of
Atomic Radiation- (UNSCEAR) estimate shown,in Table 2-3 is for a
general population and assumes an expression time _.of.. 40 years :
(UNSCEAR77) >. •-. Like the ICRP, UNSCEAR did not. make- use'of an '
explicit projection of risk of fatal lung' cancer over a full
lifetime.
2-15
-------
Table 2-3. ' Estimated risk from exposures to radon-222 progeny
Organization
EPA(a)
W
NAS BEIR-3^
AECB
ICRP
UNSCEAR
NCRPTQ)
Fatalities per
10 person WLM
760 (460)
730 (440)
600 (300)
150-450
200-450
130
(b)
(b)
Exposure
period
Lifetime
Lifetime
Lifetime
Working
lifetime
Lifetime
Lifetime
Expression Reference
'period
Lifetime
Lifetime
Lifetime
30 years
40 years
Lifetime
EPA84
NAS 80
The 2
ICRP81
UNSCEAR77
NCRP84
(a) The number of fatalities per million-person WLM listed for
EPA and NAS BEIR-3 differs from those previously published
bygEPA [860 fatalities per 10 PWLM and 850 fatalities per
10 .PWLM, respectively (EPA83a) because the increased
exposure equivalent applicable, to childhood has now been
included. Risk'estimates for various sources of radon-222
in the environment have not changed because all were
calculated in a life table analysis yielding deaths per
100,000 exposed rather than deaths per 10 PWLM.
(b) The EPA and AECB estimates of risk for the general
population are based on an exposure equivalent, corrected
for breathing rate (and other factors). For comparison
purposes, the values in parentheses express the risk in more
customary form, in which a continuous exposure to 1 WL for a
year corresponds to 51.6 WLM.
(c) Adjusted for the 1970 U.S. general population,- see text.
(d) Assumes risk diminishes exponentially with a 20-year
halftime.
2-16
-------
National Council on RadiationProtection and Measurements
The National Council on Radiation Protection and
Measurements (NCRP) risk estimate in Table 2-3 is based on an
analysis by Harley and Pasternack (Ha82). This estimate is of
particular interest because, like the EPA and AECB estimates, it
is based on a life table analysis of the lifetime risk from
lifetime exposure (NCRP84). This estimate uses an absolute risk
projection model with a relatively low risk coefficient, 10 cases
per 10 person WLM per year at risk, which is the smallest of
those listed by the NAS BEIR-3 Committee (cf. Table 2-2).
Moreover, they have assumed that the risk of lung cancer after
irradiation decreases exponentially with a 20-year half-life and,
therefore, exposures occurring early in life present very little
risk.
The NCRP assumption of a 20-year half-life for radiation
injury reduces the estimated lifetime risk by about a factor of
2.5. Without this assumption, the NCRP risk estimate would be
the same as the midpoint of the UNSCEAR estimate (325 fatalities
per million person WLM). The assumed decrease in risk used by
NCRP is questionable. If lung cancer risk decreased over time
with a 20-year half-life, the excess lung cancer observed in
Japanese A-bomb survivors (following the minimum latent period)
would have decreased during the period this group has been
followed (1950-1982); but to the contrary, their absolute lung
cancer risk has increased markedly (Ka82).
Comparison of .Estimates
Good agreement exists among the EPA, NAS (BEIR-3),• and the
AECB estimates listed in Table 2-3. Each of these estimates is
based on lifetime exposure and lifetime expression of the
incurred risk-. Conversely, the three lower risk estimates shown
in Table 2r3 either do not explicitly include these, conditions or
they include other modifying factors. Nevertheless, Table 2-3
indicates a divergence, by a factor of about 6, in risk estimates
for exposure to radon-222 progeny. Thus, the use of a single
risk coefficient may. not be appropriate, as it could give the
impression, that the risk is well known when obviously it is not.
The EPA, BEIR-3, and--AECB estimates may be slightly high because
they represent relative risks based on adult males, many of whom
smoked. The actual .risk may be smaller for a population that
includes adult females, children, and nonsmokers. The UNSCEAR
and ICRP estimates are probably low because they represent
absolute risk estimates that do not completely take into account
the duration of the .exposure and/or the duration of the risk
during a lifetime. The NCRP estimate is likely to be very low,
.as a low risk coefficient was used in an absolute risk model, and
it was assumed that the risk decreases exponentially after the
exposure.
2-17
-------
2.3.4 Selection of Risk Coefficients
To estimate the range of reasonable risks from exposure to
radon-222 progeny for use in the Background Information Document
for Underground Uranium Mines (EPA85), SPA averaged the estimates
of BEIR-3, the EPA model, and the AECB to establish an upper
bound of the range. The lower bound of the range was established
by averaging the UNSCEAR and ICRP estimates. The Agency chose
not to include the NCRP estimate in its determination of the
lower bound because this estimate used an absolute risk
projection model with a relatively low-risk coefficient.
Therefore, the EPA chose relative risk coefficients of 1.2
percent per WLM and 2.8 percent per WLM (300 to 700 fatalities
per million-person WLM) as reasonable estimates for the possible
range of effects from inhaling radon-222 progeny for a full life
time. Although these two risk estimates do not encompass the
full range of uncertainty, they appeared to illustrate the
breadth of much of current scientific opinion.
The lower limit of the range of relative risk coefficients,
1.2 percent per WLM, is similar to that derived by the Ad Hoc
Working Group to Develop Radioepidemiological Tables, which also
used 1.2 percent per WLM (NIH85). Some other estimates based
only on U.S. and Czech miner data average 1 percent per WLM
(Ja85) or 1.1 percent per WLM (St85).
A possible 0.5 percent per WLM lower bound of risk mentioned
by the Environmental Protection Agency Radiation Advisory
Committee (SAB85) appears too low. Estimates of this magnitude
of risk are usually based on data from the entire cohort of U.S.
white uranium miners (Th82, Wh83, Ja85, St85). The risk of
exposure of 600 cumulative WLM or less, however, is usually 2,4
times or more higher than the risk for the entire cohort (Lu71,
Ar79, Th82). For.this reason, the 0.5 percent per WLM relative
risk coefficient was not used.
The upper limit is lower than what might be justified by
some current reports. Although the Swedish iron miners study
(Ra84) suggested a rather high relative risk coefficient, this is
a comparatively small study. In 1985, the National Institute of
Occupational Safety and Health estimated the relative risk
coefficient in these Swedish miners was 3.6 percent per WLM
(NIDSH85). In the same year, a report on 8500 Saskatchewan
^uranium miners (Ho85) estimated a relative risk of 3.3 percent
per WLM. In addition, a small study was made of persons exposed
to different levels of radon-222 daughters and smoking in
2-18
-------
dwellings on the Swedish island of Oeland (EdS3, 84). Data from
this study could justify a relative risk coefficient of about 3.6
percent per WLM.
These three studies indicate a relative risk coefficient
greater than 3 percent per WLM; therefore, the EPA is increasing
the upper limit of its estimated range of relative risk
coefficients. To estimate the risk due to exposure to radon-222
progeny, the EPA will use the range of relative risk coefficients
of 1 to 4 percent per WLM. These risk coefficients were obtained
by rounding off the coefficients listed above to the nearest
whole number,'
These changes are in agreement with the recommendations of
the Radiation Advisory Committee of the Science Advisory Board of
EPA ( SABS 5) which recommended that EPA use a risk coefficient
range of 1 to 4 percent per WLM, as they believed that both
overestimations of exposure and the effect of random error could
have biased the risk coefficients downward, and a risk
coefficient of 4% was recommended ' as an upper bound. The
Committee also recommended use of single-digit risk coefficients
to avoid the suggestion of a precision that does not exist. In
response to these recommendations, EPA used risk coefficients of
1 to 4 percent per WLM. These risk coefficients were obtained by
rounding off the coefficients discussed above. The basis for
these relative risk coefficients was reviewed for this final
report, but no changes were made and the risk estimates are based
on 1 and 4 percent per WLM.
It may be noted here that using a 1% to 4% relative risk per
WLM with the WLM Exposure Equivalent defined earlier is
numerically the same as using a 0,6% to 2.4% relative risk per
WLM with the conventional WLM, (see table 2-3).
2 .4 Estimating the Risks
2.4.1 Exposure
The exposure to radon-222 progeny at a site of interest is
based on the calculated radon-222 concentration and the
calculated radon-222 progeny equilibrium fraction:
Radon progeny Radon Radon progeny 9.84 x 10~
concentration = cone, x equil. fraction x (WL per pci/liter)
(WL) pci/1) (f err) _ -
For individuals and regional populations, emission data and
meteorological data are used with the AIRDOS-EPA model (Mo79) to
calculate air concentrations of radon-222," for national
populations, emission data and meteorological data are used with
the NOAA Trajectory Dispersion Model (NRC79) .
2-19
-------
Calculations of radon-222 progeny equilibrium fractions are
based on distance from a source and the time required to reach • ,
the exposure site. By using the ingrowth model of Evans (Ev69) •
and the potential alpha energy data of UNSCEAR (UNSCEAR77) , the
outdoor equilibrium fraction can be calculated by the expression:
f °Ut=1.0 - 0-t4'39 -38-6 -t/28 ' 4
where t is the travel time in minutes (distance/transport
velocity) .
The indoor equilibrium fraction, presumes that those decay
products associated with the radon-222 release also enter the
building and that a ventilation rate of 1 h (one air change
per hour) in combination with indoor removal processes (e.g.,
deposition- onto room surfaces) produces an indoor equilibrium
fraction of 0.35 when there are no decay products , in the
ventilation air and 0.70 when .the decay products are in
equilibrium with the radon-222 in the ventilation air (EPA83b)
A simple linear interpolation is used to obtain the indoor
equilibrium . fraction ;
. ;fe in- 0.35 (1+ fe
If one further assumes that a person spends 75" percent of
his or her time indoors and. the remaining 25 percent outdoors at
the same location, the effective equilibrium fraction is given
by: . . • . . .
fe 6ff = 0.75 fe ^n + 0.25 fe °Ut = 0.2625 + 0.5125 ftf °Ut
An example of the case for a 3.5 in/s windspeed and various'
distances .from the source . is given in Table 2-4. , Removal
processes outdoors were assumed to limit the equilibrium
fraction -to 0.85, which corresponds to an- indoor equilibrium
fraction of 0.65 and an effective fraction of 0.70... Table
2-4 shows that this limit is reached at a distance of
19-, 550. meters.
2.4.2 Risk Estimation
After the exposure equivalent has been calculated, the risk
can be estimated for an individual or a population.
Individual
Individual risks are calculated by using the life table
methodology described by Hunger et al. (Bu81) . Relative risk
2-20
-------
Table 2-4. ' Radon-222 decay product equilibrium fraction at
selected distances from the center of a 80 ha, tailings
impoundment( '
Distance
(m)
f
out
in
eff
0
100
150
200
250
300
400
500
600
800
1,000
1,500
2,000
2,500
3,000
4,000
5,000
6,000
8,000
10,000
15,000
19,550
(a)
0.008-
0.009
0.013
0.020
0.026
0.031
0.041
0.051
0.060
0.078
0.094
0.133
0.168
0.201
0,234
0.295
0.353
0.407
0.507
0.593
0.755
0.850
(b)
0.353
0.353
0.355
0'.357
0.359
0.361
0.364
0.368
0.371
0.377
0.383
0.397
0.409
0.421
0.432
0.453
0.473
0.493
0.527
0.558
0.614
0.648
0.267
0.267
0.269
0.273
0.276
0.278
0.284
0.289
0'. 293
0. 302
,311
,331
0.349
0.366
0.382
O.:414
0.443
0.471
0.522
0.566
0.650
0.698
0
0,
Calculations (tabulated to 3 decimal places to facilitate
comparisons) presume: a 3.5 m/s windspeed for the outdoor
equilibrium fraction," an indoor equilibrium fraction of 0.35
for no radon-222 decay products in the ventilation air and
0.70 for ventilation air with'100 percent equilibrium between'
radon-222 and.its decay products? and an effective
equilibrium fraction based on 75 percent of time indoors and
25 percent of time outdoors.
2-21
-------
projections for lifetime exposure based on coefficients of 1.0
percent and 4.0 percent per WLM for the radiation-induced
increase in lung cancer yield rounded-off estimates of 380
deaths/10 person WLM and 1520 deaths/10 person WLM,
respectively when using updated age specific mortality and the
1980 life table data. These risk projections compare to the
estimate of 250 and 1000 deaths/10 person WLM used in the
Draft Background information Document which were based on the , •
1970 life tables. The updated estimates used in this final
document are based on the same risk coefficients but yield higher
death rates since there are more people in each age category and
there is a higher total incidence of lung cancer.,
These risk coefficients can be used in the CAIRD Code (Co78)
to calculate the risk from any exposure to radon-222 progeny
across any time period. Usually, the lifetime risk from lifetime
exposure at a constant level is calculated. The age-specific
differences in exposure equivalent listed in Table 2-1 are
included in calculations of the lifetime risk.
One of the characteristics of the life table based
calculations is that the same risk coefficients will yield
different estimates of life time risk when different life tables
are used. This is particularly true of relative risk projections
when both the life table and the age-specific mortality data in
the calculation may be changed. Prior ORP relative risk
estimates were based on the 1970 life table (NCHS75) and the 1970
mortality data (NCHS73). For this document the basis for
calculation has been changed to the recently available 1980 life
table (NCHS85) and 1980 mortality data'(NCHS83).
Although-this change provides risk estimates more
appropriate for the 1980s, the increase in the life span
reflected in the life table and, more significantly, the increase
in lung cancer mortality (Figures 2-2 and 2-3) have caused an
appreciable upward change in the risk estimate. Lifetime risk
estimates made using the relative risk projection with 1980 vital
statistics are about 50% greater than those made earlier using
the 1970 vital statistics. Thus, the updated estimates used in
this final document are based on the same risk coefficients (1%
and 4% increase per WLM), but yield highter numerical risks since
there are more people in each age catagory and there is a higher
rate of lund cancer mortality for each age.
Results of representative calculations of lifetime risk
using 1980 data are given in Table 2-5.
2-22
-------
Table 2-5.- Lifetime risk for 'lifetime exposure to a given
level of radon-222 progeny
(1980 Life Table, 1980 Mortality Data)
Lifetime risk of lung cancer
Radon-222 progeny
concentration•(WL)
0.0001
0.001
0.01
0.1
0.2
4 percent increase
per WLM
3
3
3
2
5 X 10
5 X 10
4 X 10
8 X 10
.5 X 10"
-4
-3
-2
-1
1 percent increase
per WLM
8.8 X 10
8.8 X 10
8.8 X 10
8.3 X 10
1.6 x 10
-5
-4
-3
-2
-1
The lifetime risk estimates shown in Table 2-5 are for lifetime
exposure at a constant level of radon-222 progeny. These risk
estimates were used with WL-'exposures that_were calculated by
using radon-222 concentrations and an f determined as
shown in Table 2-4 to estimate the risks of fatal lung cancer
due to maximum exposure of individuals living nearest the
tailings impoundments (Table 6-1).
Lifetime risk factors for selected concentrations of
radon-222 in air with relative risk coefficients of l percent
and 4 percent per WLM are shown in Table 2-6 in a manner similar
to Table 2-5.
Table 2-6.
Lifetime risk for lifetime exposure to a given
, level of radon-222 in air
Lifetime jrisk__gf_lunq cancer
(a)
Radon-222 .
concentration
(pCi/l)
10
3
1
0. 3
0.1
6.9 x.-lO
2.1 X- 10
6.9 X 10
2.1 x 10
6.9 X 10
-2
-2
-3
-3
-4
4 .percent'increase
per WLM
,2.1
7.1
X 10
X 10
2.4 x'10
•7,4 X 10
2.4 X 10
-1
-2
-2
-3
* O
At equilibrium fraction of 0,7.
1 percent increase
per WLM
5.9 x 10
1.8 X 10
6.1 X 10
1.8 X 10
6.1 X 10
-2
-2
-3
-4
2-23
-------
Regional
Collective (population) risks foe the region ace calculated
from the annual collective exposure (person WLM) for the
population in the assessment area by a computerized methodology
known as AIRDOS-IPA (Mo79). An effective equilibrium fraction
of 0.7 is presumed because little collective exposure takes
place near the source.
Formally, the annual collective exposure, Sg. can be
defined as:
S_ = JE n(E) dE
t o
where Sg is the collective exposure (person WLM), E is the
exposure level (WLM), and n(E) is the population density at
exposure level E (person/ WLM) .
Practically, however. the collective exposure is
calculated by dividing the assessment area into cells and then
calculating the population, N^ (persons), and the annual
exposure, Ej (WLM), for each one. The collective exposure is
then calculated by the following expression:
S = IE N
E i i
where the summation is carried out over all the cells.
Customarily, the regional population exposure is limited to
persons within 80 km of the source.
The same risk factors used for the individual risk
calculations (4 percent increase per WLM or 1 percent increase
per WLM) are also used to calculate the population risk.
National
Radon-222 released from a source can be transported beyond
the 80-km regional cutoff. A trajectory dispersion model
developed by NOAA (NRC79) has been used to estimate the
national impact of radon-222 releases from a source. This
model calculates the average radon-222 exposure to the U.S.
population from unit releases at four typical uranium mining
and milling sites. The model yields radon-222 concentrations
(in picocuries per liter) in air, which are then converted to
decay product exposures by assuming an effective equilibrium
fraction of 0.7. National annual collective exposures
(person-WLM) are calculated for distances beyond the 80-km
regional limit. The exposures and risks are calculated for a
total population of 200 million persons.
2-24
-------
REFERENCES
Ar79 Archer V. E., Radford E. P., and Axelson 0, "Factors in
Exposure-Response Relationships of Radon Daughter
Injury", in: Conference/Workshop on Lung Cancer
Epidemiology and industrial Applications of Sputum
Cytology, Colorado School of Mines, Golden, Colorado,
pp. 324-367, 1979.
Bu81 Hunger B., Cook J. R. , and Barrick M. K. , "Life Table
Methodology for Evaluating Radiation Risk: An
Application Based on Occupational Exposure", Health
Physics 40, 439-455, 1981.
Ch84 Chameaud J., Masse R., and Lafuma J., "Influence of
Radon Daughter Exposure at Low Doses on Occurrence of
Lung Cancer in Rats", Radiat. Prot. Dos. 2, 385-388,
1984 .
Ch85 Chameaud J,, Kasse R., Morin M., and Lafuma J., "iung
Cancer Induction by Radon Daughters in Rats", in:
Occupational Radiation Safety in Mining, Vol. 1, H.
stocker, editor, Canadian Nuclear Assoc., Toronto,
Canada, pp. 350-353, 1985.
Co78 Cook J. R., Bunger B. M., and Barrick M. K., "A
Computer Code for Cohort Analysis of Increased Risk of
Death (CAIRO)", Technical Report 520/4-78-12, Office of
Radiation Programs, USEPA, Washington, D.C., 1978.
Ed83 Idling C., Kling H., and Axelson O., "Radon in Homes -
A Possible Cause of Lung Cancer", in: Lung Cancer and
Radon Daughter Exposure in Mines and Dwellings.
• Linkoping University Medical Dissertations No. 157, by
Christer Edling, Department of Occupational Medicine,
Linkoping University, Linkoping, Sweden, pp. 123-149,
1983.
Ed84 Edling C., Wingren G., and Axelson 0., "Radon Daughter
Exposure in Dwellings and Lung Cancer", in: Indoor
Air, Volume 2: Radon, Passive Smoking, Particulates
and Housing Epidemiology, B. Berglund, T. Lindvall and
J. Sundell, editors, Swedish Council for Building
Research, Stockholm, Sweden, pp. 29-34, 1984.
2-25
-------
E179
E185
EPA79
EPA82
EPA83a
EPA83b
EPA84
EPA85
Ev69
Ellett W. H. and Nelson N, S., "Environmental Hazards
From Radon Daughter Radiation", in: Conference/
Workshop on Lung Cancer Epidemiology and Industrial
Applications of Sputum Cytology, Colorado School of
Mines Press, Golden, Colorado, pp. 114-148, 1979.
Ellett W. H. and Nelson N. S., "Epidemiology and Risk
Assessment: Tests of Models for Lung Cancer induction,
in: Indoor Air and Human Health", R. B. Gammage and S.
V. Kaye, editors, Lewis Publishers, Inc., Chelsea,
Michigan, pp. 79-107, 1985.
Environmental Protection Agency, "Indoor Radiation
Exposure Due to Radium-226 in Florida Phosphate Lands",
EPA 520-4-78-013, Office of Radiation Programs, USEPA,
Washington, D. C., revised printing, July 1979.
Environmental Protection Agency, "Final Environmental
Impact Statement for Remedial Action Standards for
Inactive Uranium Processing Sites (40 CFR 192), Volume
I", EPA 520/4-82-013-1, Office of Radiation Programs,
USEPA, Washington, D.C., 1982.
Environmental Protection Agency, "Final Environmental
Impact Statement for Standards for the Control of
Byproduct Materials from Uranium Ore Processing (40 CFR
192), Volume I", EPA 520/1- 83-008-1, Office of
Radiation Programs, USEPA,'Washington, 'D.c., 1983.
Environmental•Protection Agency, "Final'Environmental
Impact Statement for Standards for the Control of
Byproduct Materials From Uranium Ore Processing (40 CFR
192), Volume II", p. A.2-33, EPA 520/1-83-008-2, Office
of Radiation Programs, USEPA, Washington, D.C., 1983.
Environmental Protection Agency, "Radionuclides
Background Information Document for Final Rules, Volume
I", EPA 520/1-84- 022-1,'Office of Radiation Programs,
USEPA, Washington, D.C. 1984. ••
Environmental Protection;Agency, "Background
Information Document-—Standard for Radon-222 Emissions
from Underground Uranium Mines", EPA 502/1-85-010,
Office of Radiation Programs, USEPA, Washington, D.C.
1985.
Evans R., "Engineers Guide to the Elementary Behavior
of Radon Daughters", Health Physics, 17, 229-252, 1969.
2-26
-------
FRC67 Federal Radiation Council, "Radiation Guidance for'
Federal Agencies, Memorandum for the President",
July 21, 1967, 32 FR 11183, 84, August 1, 1967.
Ha82 Harley N. H. and Pasternack B. S., "Environmental Radon
Daughter Alpha Dose Factors in a Five-Lobed Human
Lung", Health Physics, 42, 789-799, 1982,
Ho77 Hofmann W. and Steinhausler F., "Dose Calculations for
Infants and Youths Due to the Inhalation of Radon and
Its Decay Products in the Normal Environment", in:
• Proceedings of the 4th International Congress of the
International Radiation Protection Association, Paris,
2, 497-500, 1977.
Ho82 Hofmann, W. "Cellular Lung Dosimetry for Inhaled Radon
Decay Products as a Base for Radiation-Induced Lung
Cancer Risk Assessment", Radiat. Environ, Biophys.,
2^:95-112 (1982).
Ho85 ' Howe G. R. , "Presentation at the Society for
Epidemiological Research", 19th Annual Meeting, Chapel
Hill, North Carolina, June 18-21, 1985.
ICRP75' International Commission on Radiological Protection,
"Report of the Task Group on Reference Man", ICRP
Publ. 23, Pergamon Press, New York, 1975.
ICRP77 International Commission on Radiological Protection,
"Recommendations of the International Commission on
Radiological Protection", ICRP Publ. 26, Ann. ICRP, I
(1), Pergamon Press, 1977.
ICRP79 International Commission on Radiological Protection,
"Limits for Intakes of Radionuclides by Workers", ICRP
Publication 30, Part 1, Ann. ICRP, 2. (3/4) , Pergamon
Press, New York, 1979.
ICRP81 International Commission on Radiological Protection,
"Limits for Inhalation of Radon Daughters by Workers",
ICRP Publication 32, Ann. ICRP, 6, Pergamon Press,
1981'.
Ja80 Jacobi W. and Eisfeld K., "Dose to Tissue and Effective
Dose Equivalent by Inhalation of Radon-222 and
Radon-220 and Their Short-Lived Daughters", GFS Report
S-626, Gesellschaftfur Strahlen und Unweltforschung
mbH, Munich, 1980.
2-27
-------
Ja81 James A. C., Jacobi W. and Steinhausler F.,
"Respiratory Tract Dosimetry of Radon and Thoron
• Daughters; The State-of-the-Art and Implications for
Epidemiology and Radiology", in: Proceedings of the
'International Conference on Hazards in Mining;
.•'Control, Measurement, and Medical Aspects', October 4-9,
i'981, Golden, Colorado, editor'Manual Gomez, Society of
' Mining Engineers, New York, pp. 42-54, 1981.
Ja85 'Jacobi W., Paretzke H. G. and Schindel F., "Lung Cancer
Risk Assessment of Radon-Exposed Miners on the Basis of
a Proportional Hazard Model", in: Occupational
Radiation Safety in Mining, Volume 1, H. Stocker, •
editor, Canadian.Nuclear Association, Toronto, Ontario,
Canada, pp. 17-24, 1985.
Ka82 Kato H. and- Schull W. J., "Studies of the Mortality of
A-bomb Survivors, 7. Mortality, 1950-1978: Part I",
Cancer Mortality, -Rad. Research, 90., 395-432, 1982.
(Also published by the Radiation Effect Research
Foundation-as: RERF TR 12-80, Life Span Study
Report 9. Part 1.)
Lu71 Lundin F. E. Jr., Wagoner J. K, and Archer V. 1.,
"Radon Daughter Exposure and Respiratory Cancer,
Quantitative and Temporal Aspects", Joint Monograph Mo.
1, U.S. Public Health Service, U.S. Department of
Health, Education and Welfare, Washington, D.C., 1971.
Me78 McDowell E. M., McLaughlin J. S., Merenyi D. K.,
Kieffer R. F., Harris C. C., and Trump B. F., "The
Respiratory Epithelium V. Histogenesis of Lung
Carcinomas in Humans", J. Natl. Cancer Inst., 6JL,
587-606, 1978.
Mc83 . McDowell E. M. and Trump B. F., "Histogenesis of
Preneoplastic and Neoplastic Lesions in '
Tracheobronchial Epithelium", Surv. Synth. Path. Res.,
1,235-279, 1983.
Mo76 Moeller D. W. and Underhill D. W., ":Final Report on
Study of the Effects of Building Materials on
-Population Dose Equivalent", School of Public Health,
, Harvard University, Boston, Massachusetts, December
1976.
2-28
-------
Mo79
Mu83
NAS72
MAS 80
NASA?3
NCHS73
NCHS75
NCHS83
Moore R, E., Baes C. F. Ill, McDowell-Boyer L. M.,
Watson A. P., Hoffman F.-O., Pleasant J. C., and Miller
C. W. , "AIRDOS-EPA: A Computerized Methodology for
Estimating Environmental Concentrations and Doses to
Man from Airborne Releases of Radionuclides",
ORNL-5532, Oak Ridge National Laboratory, Oak Ridge,
Tennessee, 1979. • . .
Muller J., Wheeler W. C., Gentleman'J.'F., Suranyi G,,
and Kusiak R. A. , "Study of Mortality of Ontario
Miners, 1955-1977, Part I", Ontario Ministry of Labor,
Ontario, Canada, May 1983,
National Academy of Sciences - National Research
Council, "The Effects of Populations of. Exposures to
Low Levels of Ionizing Radiation", Report of the
Committee on the Biological.Effects of'Ionizing
Radiations (BEIR Report) , NAS', Washington, D.C., 1972.
National Academy of Sciences - National Research
Council, "The Effects of Populations of Exposures to
Low Levels of Ionizing Radiation: 1980", Committee on
the Biological Effects of Ionizing Radiation (BEIR-3
Report), NAS, Washington, D.C., 1980.
National Aeronautics and Space Administration,
"Bioastronautics Data Book", NASA SP-3QQ6, 2nd Edition,
J. R. Parker and V. R. West - editors, NASA,
Washington, D.C., 1973.
National Center for Health'Statistics, "Public Use •
Tape, Vital Statistics - Mortality, Cause'of Death
Summary - 1970", PB80- 133333, NTIS, Washington, B.C.,
1973.
National Center for Health Statistics, "U.S. Decennial
Life Tables for 1969-1971, Volume 1, No. 1", DHEW - -
Publication No. (HRA) 75-1150. U.S.' Department of
Health, Education and Welfare, USPHS, NCHS, Rockville,
Maryland, 1985. - '
National Center for Health Statistics, "Public Use
•Tape, Vital Statistics - Mortality, Cause of Death
•Summary -'1980-", PB83-261578, "NTIS,. Washington, D.C.,
•1983. ' '
2-29
-------
NCHS85
NCRP84
NIH85
NIOSH85
NRC79
Oa72
P078
Pr83
Ra84
Ra85
National Center for Health Statistics, "U.S. Decennial
Life Tables for 1979-1981, Volume 1, No. 1, United
States Life Tables", PHS Publication No. 85-1150-1,
Department of Health and Human Services, USPHS, NCHS,
Hyattsville, Maryland, 1985.
National Council on Radiation Protection and
Measurements, "Evaluation of Occupational and
Environmental Exposures to Radon and Recommendations",
NCRP Report No. 78, NCRPM, Washington, D.C., 1984.
National Institutes of Health, "Report of the National
Institutes of Health Ad Hoc Working Group to Develop
Radioepidemiological Tables", NIH Publication
No. 85-2748, U.S. Department of Health and Human
Services, NIH, Bethesda, Maryland, 1985.
National Institute for Occupational Safety and Health,
"Evaluation of Epidemiologic Studies Examining the Lung
Cancer Mortality of Underground Miners", National
Institute for Occupational Safety and Health, CDC,
Cincinnati, Ohio, May 9, 1985.
Nuclear Regulatory Commission, Draft Generic
Environmental Impact Statement on Uranium Milling,
Volume II, NUREG-0511, USNRC, Washington, D.C., 1979. •
Oakley D. T., "Natural Radiation Exposure in the United
States", ORP/SID 72-1, USEPA, Washington, D.C., 1972.
Porstendorfer J., Wicke A., and Schraub A., "The
Influence of Exhalation, Ventilation, and Deposition
Processes Upon the Concentration of Radon, Thoron and
Their Decay Products in Room Air", Health Physics, 34,
465-473, 1978.
Prentice R. L., Yoshimoto ¥., and Mason M. W.,
"Relationship of Cigarette Smoking and Radiation
Exposure to Cancer Mortality in Hiroshima and
Nagasaki", J. Nat. Cancer Inst., 70. 611-622, 1983.
Radford E. P., and Renard K. G. St. Cl., "Lung Cancer
in Swedish Iron Miners Exposed to Low Doses of Radon
Daughters", N. Engl. J. Med., 310, 1485-1494, 1984.
Radford, E.P. "Radon Daughters and Lung Cancer".
Engl. J. Med., 313:1610-1611 (1985).
N.
2-30
-------
SAB85 Science Advisory Board, "Letter from the Radiation
Advisory Committee of the Science Advisory Board of EPA
to Lee M, Thomas, Administrator of EPA, dated November
5, 1985", on the subject of EPA radon risk estimates.
Sp56 Spector W. S,, editor, "Handbook of Biological Data,
Table 314, Energy Cost, Work: Man", W. B. Sanders Co.,
Philadelphia, 1956.
St85 Steinhausler F,, and Hofmann W., "Inherent Dosimetric
and Epidemiological Uncertainties Associated with Lung
Cancer Risk Assessment in Mining Populations", in:
Occupational Radiation Safety in Mining, Volume 1, H.
Stocker, editor, Canadian Nuclear Association, Toronto,
Ontario, Canada, pp. 327-334, 1985,
Th82 Thomas D. C„, and McNeill K. G., "Risk Estimates for
the Health Effects of Alpha Radiation", Report
INFO-0081, Atomic Energy Control Board, Ottawa, Canada,
1982.
UNSCEAR77 United Nations Scientific Committee on the Effects of
Atomic Radiation, "Sources and Effects of Ionizing
Radiation", Report to the General Assembly, with
Annexes, UN publication E. 77 IX.1.,.United Nations,
New York, 1977.
Wh83
Whitemore A. S. and McMillar A., "Lung Cancer Mortality
Among U.S. Uranium Miners: A Reappraisal", Technical
Report No. 68, Department of Statistics, Stanford
University, California, 1983.
2-31
-------
Chapter 3: RADON-222 SOURCES, ENVIRONMENTAL TRANSPORT, AND
RISK ESTIMATES
3.1 Introduction
This chapter presents the physical and chemical properties
of radon-222, where and how it is emitted from the uranium
milling process, and how it is transported through the
environment. Also presented are the methods used to model the
dispersion of the radon-222 and a description of how the health
risks associated with these emissions are estimated.
3.2 OriginandProperties of Radon-222
Uranium ore contains both uranium and its decay products,
including significant concentrations of radium-226. Radon-222 is
a naturally occurring radioactive gaseous element that is formed
by the radioactive decay of radium-226. Radium-226 is a
long-lived (1620-year half-life) decay product of the uraniuiti-238
series. In nature, uranium is about 99.3 percent uranium-238?
thus, it is the decay products of uranium-238 (shown in Figure
3-1) that govern the radioactive content of the ore (NRC81).
Other isotopes of radon (radon-219 and radon-220) occur from the
decay of uranium-235 and thorium-232, but these isotopes have
short half-lives of 3.96 and 55.6- seconds, respectively, and have
little environmental impact due to the short half-lives of the
decay products. Important properties of radon-222 are presented
in Table 3-1 for information purposes only.
Mined uranium ore is milled to extract the uranium-238.
Milling removes about 90 percent of the uranium-238 from the
ore. The remaining uranium-238 and essentially all other
radioactive elements (including thorium-230) present in the ore
are left behind and disposed of with the mill waste (tailings).
These tailings will remain radioactive for hundreds of thousands
of years.
Radon-222 is the only member of the decay chain that is a
gas. It is a noble gas and therefore does not usually combine
with other elements to form nongaseous compounds. As a gas,
radon-222 is released to the atmosphere if it escapes (emanates
from) the mineral matrix that contains its parent, radium-226.
The subsequent radioactive decay of radon-222 produces a series
of solid radioactive products called "radon progeny." If
radon-222 is airborne at the time of its decay, these radon
progeny become attached to dust particles in the air and can be
inhaled and deposited in the lungs (NRC81).
3-1
-------
4.5 x 1(T y
24.1 d
1.17 m
5.01 d
138.4 d
y
d
m
s
years
days
minutes
seconds
Figure 3-1. Uranium-238 decay chain and half-lives of
principal radionuclides.
3-2
-------
Table 3-1. Properties of radon-222
Property . Value
Atomic number 86
Atomic weight 222
Boiling point -62°C
Melting point ' -71*C
Density 9.73 grams/liter
Solubility in water 51 cm3 in 100 grams at 0°C
8.5 cm3 in 100 grams at 60°C
Half-life ' 3.824 days
Decay modes and energy
a •. . . 5.4897 MeV
Y 0.512 MeV
Source: Chemical Engineer's Handbook, Perry, J. H. (editor),
McGraw-Hill Book Co., New York, New York, 1983, and Chart of the
Nuclides, Rnolls Atomic Power Laboratory, Operated by General
Electric Co. for U.S. Dept. of Energy, 12th Edition, April 1977.
3-3
-------
Radon-222 that enters the•atmosphere can be transported over
great distances. At distances beyond about a mile, however, the
contribution of radon-222 concentrations from the mills and
tailings piles is. indistinguishable from natural background
(NRC81). Some uranium-238, 1-2 ppm, is present in most soils?
therefore, radon-222 is emitted constantly from the Earth's
surface (NRC81). It is estimated that -120 million Ci/y of
radon-222 is emitted from undisturbed soil and an additional 3
million Ci/y is emitted from tilled soil (NRC81). In comparison,
uranium tailings disposal at licensed mills currently contributes
about 140,000 Ci/y (PEI85).
3.3 Sources•of Radon-222Emissionsinthe'Milling Process
Uranium ore is processed in mills to recover and concentrate
uranium to an intermediate, semirefined product often called
"yellowcake." This yellowcake is sent to separate refining
facilities that produce uranium metal, UO , or UF-.
Conventional uranium milling involves a series of unit
operations, including ore handling and preparation, extraction,
concentration, and precipitation, product preparation, and
tailings disposal.
Ore stockpiles, crushing and grinding operations, the
extraction circuit, and tailings piles are sources of radon-222
at operational uranium mill's1'," "as "illustrated in Figure 3-2.
Other sources, such as contaminated former ore storage areas,
also release 'radon-222. • These sources, however, are
comparatively small in comparison with-tailings and of such
uncertainty in size, source strength, and frequency of occurrence
that they are omitted from the present analyses.
Radon-222 releases can be characterized as total-release
events or continual, diffusion-limited releases. ~ Thick or deep
sources, such as ore-storage piles 'and mill tailings
impoundments, that remain undisturbed for extended periods of
time, release radon-222 by diffusive and advective mechanisms.
Accordingly, the radon-222 emission is often characterized by a
mathematical diffusion expression of the radon-222 flux.
Conversely,- sources'that rapidly release radon-222 during a
mechanical' disturbance, such as the crushing and grinding
operation,' are. best characterized by a radon-222 release per unit
mass? e.g., picocuries.of radon-222 per picocuries radium-226
present. ' This release can then be expressed in terms of the
amount- of U_OQ produced by the mill.
j O
The domestic uranium ores currently mined contain an average
of about 0.1 percent uranium. When uranium ore lies underground,
only a very small fraction (if any) 'of'the radon-222 it produces
3-4
-------
LO
I
DIFFUSION-LIMITED
RADON RELEASE
ORB STOCKPILES
TOTAL RADON
RELEASE
DUMPING. CRUSHING. PROCESSING
DIFFUSION LIMITED
RADON RELEASE
UNSATURATED
BEACH
SATURATED
BEACH
DIKE
1
MIXED
Figure 3-2. Schematic illustration of the radon sources at a uranium mill (PEI85).
-------
escapes.to the atmosphere, Radon-222 has a half-life of only
3.8 days;'therefore, most of the radon-222 that is generated more
than-a few meters below the surface decays into nongaseous
radionuclides before it can migrate through the soil pore space
(the air space between soil particles) and escape into the
atmosphere. When uranium ore is mined and milled, however, the
handling and grinding* operations, liberate radon-222 contained in
the pores in the ore. Milling of the ore to sand-sized particles
also' allows a greater portion of the radon-222 that forms in the
tailings to be released into the'atmosphere by diffusive and
advective mechanisms. Both-the' increased surface area of the
particles and increased porosity resulting from the milling
process cause an increase in the portion of radon-222 that
escapes to the atmosphere,
Ore Handling and Preparation
Ore handling and 'preparation include ore blending, storage,
crushing, fine ore storage, and grinding. Ore blending ensures
that the mill feed is of uniform grade, which is necessary to
achieve maximum-efficiency in the mill circuit. Blending may be
performed' at either the mine or the mill. Ore is stored in
stockpiles on ore pads at the mill site. The stockpiles provide
sufficient feed for a continuous supply to the mill. Ore
received from the mine often has a high moisture content;
however, the dry climate typical of the major uranium districts
causes rapid drying. For this reason, some ore storage piles are
sprayed with water to maintain their moisture content and to
reduce dusting. *•
Storage pads typically cover several acres and provide
enough ore storage to feed the mill for one or two months of
operation. Ore usually is not kept on the storage pad when the
mills are on standby status. Similarly, when operations are
reduced because of a depressed economy, as they currently are, a
lesser quantity of ore is stockpiled at the mill site than would
be i'f the mill were operating at full capacity. The ore
residence time in storage pile's'varied from 4 to 180 days, with a
mean and standard deviation -of 8-7 + 72 days,' at seven mills
surveyed in Wyoming (Th82).
•• ' The- number of piles can be estimated by the product of the
mill feed rate (weight/day) and the stockpile residence time
(days) divided by the mass of .a'pile. The piles vary in shape
among different mills, but they are frequently conical, oblong,
or--wedge-shaped. " A maximum height-of 10 m (30 ft) and 45-degree
sloping sides are common. The volume and surface area of a
typical pile have been estimated to be 8000 m and 2500 m ,
respectively (Th82). Emissions of radon-222 from stockpiles are
3-6
-------
considered to emanate from an infinitely deep or thick source •
from all surfaces, even though some parts may be shallow or
thin. The resulting high radon-222 emission estimate for some.of
the pile areas;is justified by the variable sizes, shapes, and
other characteristics of ore .stockpiles.
Stockpiles initially emit no radon-222 because all of the
emanated radon-222 stored"in the pores of the ore was released as
the ore was mined and transported to the stockpiles. As new
radon-222 emanates "into the pore-..space of the ore, the
interstitial radon-222 levels and the escaping radon-222 flux
increase. After several weeks, a nearly constant radon-222 flux
(emission rate) is attained.
Crushing is 'the first stage of size reduction and involves
the use of impact and/or gyratory crushers. Crushing typically
reduces mine run ore'to between minus 3/4 inch and minus 1-1/2
inch size (Me71) . .• Fine'ores (undersized material) bypass the
crushing circuit and,are conveyed directly to fine-ore storage
bins. Air exhaust hoods with dust collectors are located on
crushers and screens and at transfer -points to minimize
particulate emissions, and air is exhausted to the atmosphere via
vents. The dust collectors do not capture .radon-222 emanating
from the ore during these processes, and it is vented to the
atmosphere. Crushing plant capacities range from 70 to 320 tons
per hour (NEC80) . . " '• ' • -
Crushed and undersized ore is stored in cylindrical fine-ore
bins about 7 to 10 m (25 to_ 35 feet) in-.diameter. These bins
provide a fine-ore storage capacity up to double the rated daily
milling capacity (NRC80). Radon-222 that emanates from the fine
ore in storage -is vented to the atmosphere.
Belt-type feeders convey the ore from the crushing circuit
and fine-ore bins to the grinding circuit, where rod and ball
mills or semiautogenous mills are used to reduce the ore size -
further. Occasionally, the ore is roasted before it is sent to
the grinding, circuit to reduce moisture before grinding, to
increase the solubility of other valuable constituents (e.g.,
vanadium), or to improve the physical characteristics of the
ore, The, ores are ground dry and then slurried with water or
wet-ground to- yield a pulp density of 50 to 65 percent solids •
(NRC80)i Classifiers, thickeners, cyclones, or screens are used
to size the ore, and coarser particles are returned for further
grinding. One mill uses an alkaline leaching process, which
requires the ore- be ground much finer (200-mesh) than for acid
leaching.(28-mesh). -
3-7
-------
Wet, semiautogenous grinding is being used increas-
ingly in place of dry crushing or ball and rod mill grinding
operations, which may be run wet or dry. The semiautogenous
grinder performs the ore sizing function of these operations and
reduces or eliminates dry ore handling.
The total release of radon-222 from the dumping, crushing,
and extraction processes occurs mostly during the process of
transferring and. dumping the ore ihto the mill 'feed area. The
ore is typically reduced to sizes of less than 40 cm, which is
the relaxation diameter for radon-222?diffusion from ore pieces
with diffusion coefficients of 10 cm /s; therefore,
radon-222 escapes readily from the pores of the ore when it is
handled and results in the total release of accumulated
radon-222. During the remainder of the short milling process,
little additional radon-222 escapes from the'ore for release.
Hard-rock uranium ores are an exception, in that they have very
Iow5diffusion coefficients for radon-222 (10~ to
10~ cm /s). The 4 to 14cm particles of these ores can
significantly reduce radon-222 releases," hence, the sharp
one-time release is less and is delayed until the ore is ground
to smaller particle sizes during milling.
Extraction
Hydrometallurgical leaching technigues are used to recover
uranium from the ground ore slurry. Little radon-222 is released
from the extraction process.because the radon-222 contained in
the ore is released during initial ore handling and size
reduction steps and the relatively short milling time (less than
24 hours) does not permit significant formation of new
radon-222. The extraction process uses sulfuric acid or' an
alkaline carbonate solution for lixivation. Acid leaching is
preferred for ores with low lime content (12 percent or less)
(NRC80) and is the predominant leach process,in the United
States. A flow diagram of the acid leach/solvent extraction
process is shown in Figure 3-3.
The leaching circuit consists of a series of mechanically.
agitated tanks having a total ore residence time of approximately
7 hours. The pH in the tanks is maintained between 0.5 and 2.0
by adding- sulfuric acid. The free acid concentration is from 1
to 90 grams of acid per liter during the contact period (NRC80).
Acid leaching is carried out at atmospheric pressure and slightly
above room temperature.
After leaching, the pregnant leach solution is separated
from the tailing solids in a countercurrent decantation,(CCD)
circuit.' The sands and slimes are pumped to a tailings pond for
disposal.
3-8
-------
Alkaline leaching, which is best suited to ores with high
lime content, nay be used in combination with ion exchange or
caustic precipitation to concentrate and purify uranium.' A flow
diagram of the alkaline -leach/caustic precipitation process is
shown in Figure 3-4.
The ore slurry is.leached,in a two-stage system (pressure
leaching followed by atmospheric leaching). The leach solution
contains sodium carbonate (40 to .50 grams per liter) and sodium
bicarbonate (10 to 20 grams per liter). Circular tanks are used
and air is added to oxidize the'uranium to the hexavalent state.
Residence time varies from 21 to 33 hours. The pregnant leach
solution is separated from the tailings in a series of CCD
filtrations.
Concentration, a_nd_ _Precip_itation
Three techniques are used to concentrate uranium from the
pregnant leach solution: ion exchange, solvent extraction, and
the Eluex process, which is a combination of ion exchange and
solvent extraction. Uranium that has been concentrated by one of
these methods is precipitated from the solution by the addition .
of gaseous ammonia (NH3), sodium hydroxide (NaOH), hydrogen
peroxide (H202), or magnesia (MgO) in several stages under
controlled pH. Most mills use gaseous ammonia. The precipitated
uranium is dewatered in thickeners and then filtered and washed
in drum, plate, or frame filters. At this point, the resulting
filter cake still contains considerable moisture.
'! 1
Product Preparation
The uranium filter cake (yellowcake) is dried in a
continuous steam-heated dryer or in a multiple-hearth dryer. The
dried yellowcake is crushed.and screened to the required size and
packaged in 55-gallon drums for shipment. Some" radon-222
emanates from this operation and is vented to the atmosphere.
Tailings Disposal
With the exception of the uranium extracted during milling,
the dry weight of the tailings represents the total dry weight of
the processed ore. Ore contains only about 0.1 percent uranium;
therefore, the tailings consist of 99.9 percent of the ore,
including all the 'radioactive decay products. The tailings
discharge is composed of three fractions: (1) the sands, which'
consist of solids greater than 200 mesh (74 mm); (2) the slimes,
which consist of solids less than 200-mesh; and (3) the liquid
solution containing milling reagents and dissolved ore solids.
3-9
-------
ORE
CRUSHING AND
GRINDING
WATER
WET
GRINDING
SULFURIC
ACID AND
SODIUM CHLORATE
I
LEACHING
I
FLOCCULANT
WATER
COUNTER CURRENT
DECANTATION
(CCD)
TAILINGS POND
(TAILINGS SAND AND SLIMES,
LIQUID WASTES)
BARREN RAFFINATE
PREGNANT LIQUOR'
SOLVENT
EXTRACTION
FURTHER
>ROCESSING
Figure 3-3, Simplified flow diagram of the acid leach process.
3-10
-------
CRUSHING AND
GRINDING
WATER
WET
GRINDING
NaHCO
I
LEACHING
FLOCCULANT
WATER
COUNTER CURRENT
FILTRATION
CAUSTIC SODA
PRECIPITATION/
PURIFICATION
I
TAILINGS PILE
TAILINGS
CO-
RECARBONATJON
-TO GRINDING
AND LEACHING
DRYING AND
PACKAGING
YELLOWCAKE
Figure 3-4. Simplified flow diagram of the alkaline
leach-caustic precipitation process.
3-11
-------
Dry tailings from an acid leach mill are typically composed of 20
to 37 percent slimes by weight (NRC80). Tailings are discharged
from the mill as a slurry at an average ratio, by weight, of
about 1:1 (solids to liquids') and are sent to an impoundment,
where the tailings settle. . - .
About 10 percent of the;uranium-238 and virtually all of the
other radionuclides in the ..ore .are contained in the tailings.
Tailings represent the largest -'and longest lasting source of
radon-222 emissions from-licensed conventional uranium mills
because of the large exposed area and the significant
concentrations of radium-226 present. The fine slimes fraction
contains the majority of radium-226'in the tailings (up to 80
percent) (NRCSQ). The sands fraction contains radium-226 in
concentrations ranging from 26 to 100 pCi/gram (NRC80), and the
tailings liquid (raffinate) contains 1.7 to 35,000 pCi/liter of
radium-226 and 50 to 250,000 pCi/liter of thorium-230 (EPA83).
The methods used to construct and fill tailings impoundments
causes segregation of the slimes and sands. During spigoting,
the sands are deposited on the' perimeter of the impoundment and
the slimes are carried to the central portions of the impoundment
with the raffinate. The more porous sands are deposited away
from the center of the pile and are therefore typically drier
than the slimes, which are usually saturated with moisture of
actually covered with standing process fluids.
Except for a small percentage used for backfill in
underground mines, virtually" all tailings are disposed of in
impoundments. Disposal is- below grade in mined-out or excavated
pits and above grade behind dams. The majority of the tailing
impoundments at licensed mills are above'grade. Currently, new
dams are constructed of earthen material, whereas in the past
they were constructed of tailings sands. Impoundment sizes vary
from 10 to about 121 ha (25 to 300 acres) (EPA85).
Site topography dictates the general shape of above-grade
surface impoundments. One-sided, two-sided, and three-sided dams
are constructed across valleys -and along hillsides. Dams
constructed on relatively flat terrain, where the tailings cannot
be contained by the natural topography, are four-sided.
Embankments are generally constructed of earthen material, but
some (at six mills) are constructed of the snad fraction of the
tailings.
The water level in a tailings impoundment is controlled
through the use of decant towers, pumps, or siphons to recycle -
the water or to transfer it-.to evaporation ponds for proper
maintenance of freeboard. Most mills operate with zero liquid
discharge (40 CFR Part 440) and',rely on evaporation.
3-12
-------
Constructing impoundments with earthen embankments or below
grade is the preferred method at new milling•operations or. for
new impoundments at existing mills because they inherently have
greater short-term and long-term stability. In addition,
tailings disposed of below grade are typically covered with
raffinate, which effectively controls dusting and reduces
radon-222 emissions during the mill's active life.
Radon-222 is emitted from all exposed tailings in
impoundments. Emission rates vary in different areas and over
time. A qualitative illustration of the variation in radon-222
emissions over the life'of a milling operation is shown in Figure
3-5. These emissions occur during the licensed phase of mill
operations and continue for hundreds of thousands of years after
closure of the mill. Radon-222 and radium^226 both have much
shorter half-lives than their precursor thorium-230; therefore,
their radioactivity remains the same as that for, thorium-230
(EPA83). The radon-222 emissions decrease only as the
thorium-230, which has a half life of 77,000 years^ decreases
(EPA83). It would require about 265,000 years for the radon-222
emissions to be reduced to 10 percent of its initial value
(EPA83). If control techniques are not imposed, the radon-222
emissions will remain relatively constant,.on a year-to-year
basis for many tens of thousands of years.
3.4 Characterization of Emissions
The amount of radon-222 emitted _from ore storage piles,
milling circuits, evaporation ponds, and tailings impoundments
depends on a number of highly variable factors, such as ore
grade, emanation fraction, porosity, moisture, temperature, and
barometric pressure. These factors, in turn, vary between
milling sites, between locations on the same site, and with time
(PEI85). These variations make it difficult to assess the
radon-222 emission rate. For these reasons, mathematical model's
typically have been used to estimate average radon-222 emissions
on a theoretical basis. A few systematic measurements have been
made of radon-222 emissions from licensed uranium mills and
tailings piles,-and studies have demonstrated good agreement
between actual measurements and estimates based on,mathematical'
models (EPA83) .;
Considerable research has been conducted to develop and
refine ways of calculating average radon-222 flux from infinitely
thicM or deep sources (i.e., at least 1 meter deep). This work
has largely been carried out in support of the Uranium Mill
Tailings Remedial Action Program (UMTRAP). Although these
calculations were developed for inactive mill tailings piles,
they are directly applicable to ore storage piles and tailings
impoundments at licensed mills. .
3-13
-------
1/5
O
i-^
uo
C/}
*—t
UJ
eg
CM
O
Q
•ACTIVE LIFE
DRYING INACTIVE RECLAMATION
L PERIOD
TAILINGS IMPOUNDMENT .EMISSIONS
MILLING EMISSIONS
_SL
APPROXIMATELY 30
TIME, years
Figure 3-5. Qualitative illustration of radon-222 emissions
frciii licensed uranium milling process.
3-14
-------
A one-dimensional. steady-state, radon-222 diffusion
equation has been developed foe soucces (e.g., ore piles and
tailings) that ace more than several meters thick (Ni82, Fc84)
The equation is:
4 1/2
Jt = 10 RpE ("K.D) (3-1)
where J is the cadon-222 flux at the surface of the source
2
(pCi/ra s); R is the specific activity of radium-226 in ore
or tailings equal to 2812 x uranium ore grade in percent
(pCi/g); ft is the bulk dry density of source (g/cra ); 1 is
the radon-222 emanating fraction of source, dimensionless;
-6
\ is the radon-222 decay constant (2.1 x 10 /s); D is the
effective diffusion coefficient foe cadon-222, equal to
2
bulk radon diffusion coefficient/porosity De/p (cm /s); and
p is the porosity, equal to l-(bulk density/specific
gravity).
For piles that are less than a few meters thick,
Equation 3-1 should be multiplied by a hyperbolic tangent
function that varies with depth or thickness (T), as shown
in Figure 3-6, With the exception of, the radon^222 decay
constant, these parameters can vary significantly from
location to location within the source, both horizontally
and with depth, in a given ore pile or tailings
impoundment. Except for the decay constant and bulk
density, these parameters ace difficult to measure. They
are based on the physical characteristics of the soucce
materials, which vary over time (e.g., radium-226 content
may decrease over the life of the mill as oce grade
declines), seasonally, or with changing mill operation
(e.g., moisture content changes seasonally and with changes
in mill operations and directly affects the emanation and
diffusion coefficients).
A radon-222 release rate of 1 pCi Rn-222/m s per pel
of Sa-226 per gram of tailings is used in this background
report because of emission rate variations and the lack of
specific information required to use the more detailed
mathematical equations (NRC80) (Ha85). Using an average,
specific flux does not take into account site-specific
conditions such as moisture, porosity, and emanation
coefficients. It is useful for estimating industry-wide
emissions, however, and is consistent with previous EPA
studies (EPA83). In the following sections, a model mill
handling 1800 t/day of ore with 0.1 percent U_O_ will be
J a
used to illustrate radon-222 emission calculations.
Assumptions are made for the parameters required to
calculate emissions with the diffusion equations, and for
comparison a specific flux of 1 pCi Rn-222/m s per pCi of
Ra-226/g is also used to estimate emissions.
3-15
-------
1.0
0.9
r
0.8
°7
0.6
i 0,5
0.4
0.3
0.2
0.1
0.0
'De=0.001
Oe=0.005
De=O.Q1
De=0.05
Oe=01
= BULK DIFFUSION COEFFICIENT
0 40 120 200 280 360
DEPTH, cm
440'
520
600
Figure 3-6. Effect of ore pile depth on hyperbolic tangent
term in radon-222 flux equation (Ha85).
3-16
-------
3.4.1 Ore Handling and Preparation
Stockpiles are blended to the average or optimum feed grade
upon entry to the mill. Emissions can be based on the average
radium-226 content, as both the initial total radon~222 release
and the longer-term, diffusion-controlled radon-222 releases vary
linearly with radiura-226 content. The radium-226 content is
typically estimated from ore grades, assuming secular equilibrium
between the uranium-238 and the radium-226.
Ore storage piles are typically more than 3 meters deep.
Thus. Equation 3-1 can be used to estimate radon-222 emissions if
the various values are known, or a specific flux of L pci
Rn-222/m s per pci Ra-226 per gram of ore can be used.
As an example, consider the ore pad at a hypothetical mill
with the following parameters:
4 2
A = area of ore pile = 6 acres or 2.4 x 10 m
T = depth of ore pile = 3m minimum
R - Ra-226 concentration = 2812 x 0.1 U_0
3 8
281 pCi/g
E ~ emanating power of ore =0.2
p = density = 1.6 g/cm
2
D = diffusion coefficient a 0.05 cm /s
4 1/2
J = 10 RpE (XD)
= 281 X 0.2 X 1.6 (2.1 X 10~6 X $ 0.05)1/2 X
,4 2, 2
10 cm /ra
2
= 291 pCi Rn-222/m s
The ore pad would have the following calculated radon-222
emissions:
291 pCi Rn-222/m2 x 2.4 x 104m x 3.156 x
7 -12
10 s/y x 10 Ci/pCi = 221 Ci/y
3-17
-------
Or if a specific flux of 1 pCi Rn-222/m s per pCi Ra-226 is
assumed, the estimated emissions ace:
1 pCi Rn-222/m s/pCi Ra-226/g x 281 pCi Ra-226/g x
2.4 x lQ4m2 x 3.156 x io?s/y x I0~12ci/pci =
213 ei/y
3.4.2 Mill Emissions
The throughput is relatively large (several thousand tons
pec day); therefore, the residence time of ore in the mill is
less than one day. This short residence time means that little
new radon-222 is formed in the milling operation. Hence, the ore
does not release large quantities of radon-222 in the mill
circuit unless the cadon-222 that previously emanated from the
ore was not released completely during storage, handling, and
crushing and grinding.
Most milling emissions of cadon-222 occur during the
transferring and dumping of the ore into the mill feed area
because the ore has usually been reduced to sizes of less than 40
cm, which allow trapped cadon-222 to escape. Emissions from
dumping, crushing, and grinding can be estimated by assuming 10
percent of the radon is released, as shown here:
1800 t/day x 310 days/y x 281 pCi/g x 106g/t x
10~12Ci/pCi x 0.1 = 16 Ci/y
7
Alternatively, an average emission factor of 3.8 K 10 pCi/lb
U 0 nay be used to estimate Rn-222 emissions from milling
j B
(PEI85).
1800 t/day x 310 days/y x 2200 Ib/t x 0.001 Ib
U_0Q/lb ore x 3.8 x 10?pCi/lb U_O0 x
•j Q , .3 a
10~12 Ci/pCi = 47 Ci/y
Radon-222 emissions from the leaching and extraction
processes of the mill circuit are" very low because these are wet
processes and most of the radon-222 in the ore was already
released during storage and handling prior to milling. Emissions
from packaging the yeilowcake product are also low, as very
little (less than 0.1 percent) of the radium-226 that produces
the radon-222 remains in the yellowcake.
3-18
-------
3.4.3 EmissionsFcom Tail ings Disposal
The large area occupied by tailings impoundments and the
extent of the exposed surface area make these impoundments the
major potential source of radon-222. Tailings include the barren
crushed ore material plus process solutions. These tailings
consist of mixtures of sand and slimes (coarse and fine
tailings). Evaporation ponds used to contain excess liquid from
tailings impoundments also contain suspended and dissolved
tailings and are included in this analysis. The size of these
ponds was documented in a recent report (EPA.85). Tailings solids
ace assumed to be carried with the process liquids and deposited
on the bottoms of these ponds. If exposed, these solids are
assumed to emit cadon-222 at the same specific flux as tailings
impoundments.
The procedure for estimating radon-222 emissions will depend
on the amount of site-specific information available. If
site-specific information on the radium-226 concentration,
moisture content, porosity, density, and emanating power are
known, the diffusion equation to estimate radon-222 flux may be
used. Where specific information is not available, a simplified
relationship of 1 pCi Rn-222/ra s per pCi Ra-226/g of tailings may
be used to estimate emissions from dcy areas of tailings
impoundments (wet and ponded areas are not assumed to emit
radon-222). An example of the calculation used to estimate
radon-222 emissions from tailings by both calculation procedures
is presented here for a 50-ha (120-aere) impoundment. Of the
total area, 50 percent consists of saturated or liquid-eovered
tailings and 50 percent is dry. The tailings solids in. the
impoundment are 10 m {30 ft) deep.
Emission estimates made by using diffusion Equation3-1
4 1 I 7
Hadon-222 flux J = 10 EpE (\D)
R = 281 pCi Ra-226/g of tailings
E = 0.2 (based on measurement; varies from -0.1 to
-0.4)
p = density = 1.6 gm/cm
\ = 2.1 x I0~6/s
D = diffusion coefficient for tailings
2 5
O.07 exp (4mp - 4ra - 4m )
where m is the moisture saturation fraction (-0.35), p is the
porosity (1-p/g), and g is the specific gravity (-2.7 g/cm ).
3-19
-------
Thus:
p = 1- 1.6/2". 7, = 0.407
D = 0.07 exp [4 K 0.35.X (0.407) - 4 X
i 0.35 - 4 x (0.35 }5]
2
= 0.0213 Cm /S ' ,
J = 281 X 0.2 X 1.6 (2-1 X 10~ X
x 104cm2/m2
• •'•'" 2 '
1= '• 190 pCi/m S
Total annual emissions are determined by multiplying J by the dry
area and seconds pec year.
Rn-222 = 190 pCi/m2s x 25 x 104m2 x 3.156 x
107s/y x 10~12 Ci/pCi
= 1505 Ci/y = -1.5 kCi/y
Emissions estimate based on. specific flux o£
2 r
1 pci Rn-222/m s pec pel Ra-226/q
Hn-222 = 1 pCi Rn-222/mZs/pCi Ra-226/g x 281
4 2
pCl Ra-226/g x 25 X 10 m X 3.156 X
• . 7 -12 .
10 s/y x 10 Cl/pCl
= 2223 Ci/y = -2.2 kCi/y
The simplified calculation based on a specific flux of 1 pCi
Rn-222/m s pec pCi Ra-226/g yields a similar but higher emission
estimate in this example case.
In almost all cases, the tailings impoundments are by far
the largest source of radon-222 emissions. For mills on standby,
the tailings, impoundments account for practically all the
radon-222 emissions. The tailings impoundment, which is the most
significant source of radon-222 emissions from the mill site,
accounts for about 80 percent of the total radon-222 emissions at
an active licensed mill and practically 100 percent at an
inactive or standby licensed mill.
3.5 Transport and Risk assessment
Two separate, steps are required to estimate the health
impact of a specific source of radon-222: (1) determining its
dispersion and estimating, at various locations, its
concentration and t.he corresponding exposure to its decay
products in units -of WLM and (2) calculating the risk.
3-20
-------
3.5.1 Air Dispersion Estimates '.
EPA uses the AIRDOS-EPA code (Mo79, Ba81) to analyze the
transport of radionuclide emissions into air from a specific
source. This analysis'estimates radionuclide concentrations in
air at various distances from-the source,
The AIRDOS-EPA code uses a modified Gaussian plume equation
to estimate airborne dispersion. Calculations are site-specific
and require the joint frequency distribution of wind direction,
windspeed, and atmospheric stability. The accuracy of these
projections decreases with distance; -therefore, calculations with
this method are limited to regional areas (e.g., less than 80 km
from the source), The values calculated represent annual
averages because diurnal or seasonal variations are included in
the joint frequency distribution. Calculations of working-level
exposures for the inhalation of radon-222 progeny are then made
based on estimates of radon-222 concentrations in air.
Radon-222 emitted from tailings impoundments can be
transported beyond the 80-km regional area. Results from a
trajectory dispersion model developed by the National Oceanic and
Atmospheric Administration (Tr79) were used to estimate the
national impact of radon-222 emissions. The model yields
radon-222. concentrations in the air (in picocuries/ liter), which
are converted to decay product concentrations and expressed in
terms of working levels.
3.5.2 Risk... Estimates
After the exposure to radon-222 decay products has been
estimated in terms of working level months for a specific source
by means of the environmental transport code,:the,risk of fatal
lung cancer is calculated using the risk factors discussed in.
Chapter 2. The risk is scaled up to the total population risk by
multiplying-by the population exposed to that working level'over
a lifetime. • .
3.6 Measurement of Radon-222 ' '
Although all radon-222 emission levels in this report
represent calculated estimates, it.is possible to make direct
measurements on specific sources. . Radon-222 measurement
methodologies are discussed in the following subsections.
Ambient samplers are generally used to measure radon-222
emissions? however, some concentrating samplers are also used.
The latter operate in a grab or-continuous mode and sample
radon-222 "as it emanates from a source. Ambient gas samplers
measure the accumulation of radon-222 present in the ambient air,
and typically have short sample collection periods (i.e.,
minutes). Concentrating samplers use a- medium such as activated
charcoal to adsorb radon-222. Sample collection periods for
concentrating samplers are typically 24 to 72 hours.
3-21 '
-------
3.6.1 Ambient Air Samplers
The most common type of ambient air sampler for the
collection of radon-222 grab samples is the accumulator can.
Accumulator can design and construction vary widely; however, all
accumulator cans are constructed with an open-ended container
fitted with a sampling port for periodic withdrawal of radon-222 •
air samples. During collection of a radon-222 sample, the open
end of the container is sealed to the sample medium (e.g., • '
tailings) by simple insertion, caulking, or the use of permanent
fixtures. After an adequate length of time (on the order of
minutes) has been allowed for the radon-222 to accumulate in the
container, a fixed air volume is withdrawn from the container
through the sampling port and the alpha activity is counted.
Another type of ambient sampler, which operates continuously
rather than collecting grab samples, uses the same sampling
procedure as the accumulator can except air is pumped through the
can at a rate equivalent to one air volume per sampling period.
The air is pumped through a filtered inlet to a calibrated
scintillation cell and alpha activity is counted continuously.
3.6.2 Concentrating Samplers That Measure Radon-222 Emanation
From Surfaces
There are two types of concentrating samplers equipped with
activated charcoal to adsorb radon-222. These include the
passive charcoal canister samplers and the active,
circulating-air test sampler. The charcoal canisters, which are
available in a variety of sizes, are placed directly on the soil
or tailings surface, exposed'for 24 to 72 hours, and use
activated charcoal as the concentrating medium. Their physical
dimensions and the quantity of charcoal used to collect a
radon-222 sample vary widely (Ni84).
Selection of a specific charcoal sampler depends on the
particular application. Large-area samplers (e.g., greater than
1000 cm ) improve the representativeness of the sample by
sampling a larger area, but small samplers are more economical
and logistically simpler.
The circulating-pir test sampler covers a much larger area
than the canisters (i.e., 9290 cm (Ni84). It is a continuous,
active sampler in which air is circulated across the soil or
tailings surface enclosed by the sampler, and -continues through a
section of corrugated tubing containing the activated charcoal.
The tubing is sectioned into two halves, which allows for the
detection of any carryover. The sampler is typically operated
for 24 hours at a flow rate of about 2 liters per minute. The
circulating-air test sampler is a cumbersome technique and is
less effective than charcoal canisters considering cost and labor
(Yo83).
3-22
-------
Activated charcoal used, for the collection of radon-222 is
sealed in an air-tight container and set aside"for a few hours to
allow the short-lived redon daughters to come to equilibrium
(Yo83). The amount of radon adsorbed by the activated charcoal
(no matter which concentrating sampler is used) is quantified by
gamma-ray spectroscopy of the charcoal using a Nal(Tl) -crystal or
germanium diode and multichannel analyzer. Typically the
Bismuth-214 609-keV peak is used to determine radon-222 activity,
but other Bismuth-214 or Lead-214 peaks could by used.
3-23
-------
REFERENCES
Ba81
EPA79
1PA83
EPA85
Fr84
Ha85
Me71
Mo79
Baes C. F. Ill and Sharp R. D., "A Directory of
Parameters Used in a Series of Assessment Applications
of the AIRDOS-EPA and. DARTAB Computer Codes",
QRNL-572Q, Oak Ridge National Laboratory, Oak Ridge,
Tennessee, March 1981.
U.S. Environmental Protection Agency, "Indoor Radiation
Exposure Due to Radium-226 in Florida Phosphate Lands",
EPA 520/4-78-013, Office of Radiation Programs, U.S.
EPA, Washington, B.C., July 1979.
U.S. Environmental Protection Agency, "Final
Environmental Impact Statement for Standards for the
Control of Byproduct Material From Uranium Ore
Processing", EPA 520/1-83-008-1, Office of Radiation
Programs, U.S. EPA, Washington, D.C., September 1983.
U.S. Environmental Protection Agency, "Draft
Document-Estimates of Population Distributions and
Tailings Areas Around Licensed Uranium Mill Sites",
Office of Radiation Programs, U.S. EPA, Washington,
D.C., November 1985.
Freeman H. D., and Hartley J. N. , "Predicting Radon
Flux From Uranium Mill Tailings", in: Sixth Symposium
on Uranium Mill Tailings Management, Fort Collins,
Colorado, February 1-3, 1984.
Hartley J. N., Glissmeyer J. A., and Hill 0. F.,
"Methods for Estimating Radioactive and Toxic Airborne
Source Terms for Uranium Milling Operations", PNL for
U.S. Nuclear Regulatory Commission, Washington, D.C.,
NUREG/CR-4088 June 1985.
Merritt, R. C., "The Extractive Metallurgy of Uranium",
prepared under contract with the United states Atomic
Energy Commission, 1971.
Moore R. E., Baes C. F. Ill, McDowell-Boyer L. M.,
Watson A. P., Hoffman F. 0., Pleasant J. C., and Miller
C. W., "AIRDOS-EPA: A Computerized Methodology for
Estimating Environmental Concentrations and Dose to Man
From Airborne Releases of Radionuclides", EPA
520/1-79-009, Office of Radiation Programs, U.S. EPA,
Washington, D.C., December 1979.
3-24
-------
Ni82
N184
NRC80
NRC81
PEI85
Th82
Tr79
Y083
Nielsen K. K., et al., "Radon Emanation Characteristics
of Uranium Mill Tailings", in: Symposium on Uranium
Mill Tailings Management, Fort Collins, Colorado,
December 9-10, 1982.
Neilson K. K.' and Rogers V. C., "Radon Flux Measurement
Methodologies, Management of Uranium Mill Tailings,
Low-Level Waste and Hazardous Waste", in: Proceedings
of the Sixth Symposium, Colorado State University, Fort
Collins, Colorado, February 1-3, 1984.
Nuclear Regulatory Commission, "Final Generic
Environmental Impact Statement on Uranium Milling",
NUREG-0706, September 1980.
Nuclear Regulatory Commission, "MILDOS - A Computer
Program for Calculating Environmental Radiation Doses
From Uranium Recovery Operations", NUREG/CR-2011,
PNL-3767, prepared by Pacific Northwest Laboratory,
Battelle Memorial Institute for the U.S. Nuclear
Regulatory Commission, April 1981.
PEI Associates, Inc., "Radon-222 Emissions and Control
Practices for Licensed Uranium Mills and Their
Associated Tailings Piles", prepared for U.S.
Environmental Protection Agency, Office of Radiation
Programs, June 1985 (revised November 1985).
Thomas V. W., Nielson K. K., and Mauch M. L., "Radon
and Aerosol Release From Open Pit Uranium Mining",
NUREG/CR-2407, Nuclear Regulatory Commission, 1982.
Travis C. C., Watson A. P., McDowell-Boyer L. M.,
Cotter S. J., Randolph M. L., and Fiedls D. E., A
"Radiological Assessment of Radon-222 Released From
Uranium Mills and Other Natural and Technologically
Enhanced Sources", ORNL/NUREG-55, Oak Ridge National
Laboratory, Oak Ridge, Tennessee, 1979.
¥oung J. A., et al. "Recommended Procedures for
Measuring Radon Fluxes From Disposal Sites of Residual
Radioactive Materials", NUREG/CR-3166, PNL-4597, March
1983.
3-25
-------
Chapter 4; INDUSTRY DESCRIPTION
4,1 Overview
In January 198-6, the conventional uranium milling industry
in the United States consisted of 26 licensed facilities. Three
additional mills have been licensed, but either have never been
constructed or have never operated. Only 4 of the 26 licensed
facilities were operating," 16 were on standby status, and 6 were
being or have been decommissioned. The mills on standby status
are being maintained, but they are not processing uranium ore.
When the demand for uranium increases, these standby mills could
resume milling. The decommissioned mills have been dismantled
and have been removed off site or disposed of on site? therefore,
these mills will never resume operations. Their associated
tailings impoundments are either being reclaimed or there are
plans to reclaim them. The current operational status and
capacity of each licensed conventional mill are shown in Table
4-1.
The Secretary of Energy has determined that the domestic
uranium mining and milling industries were not viable in 1984
(ELP85). In 1984, the annual domestic uranium production was the
lowest since the mid-1950's, and employment was down 75 percent
from 1981 to 1984 (ELP85) .
4.2 Site-Specific Characteristics
The licensed conventional uranium mills are in Colorado, New
Mexico, South Dakota, Texas, Utah, Washington, and Wyoming.
Their approximate locations are shown in Figure 4-1. Brief,
site-specific summaries of all the active or standby conventional
uranium mills were prepared as part of this document and are
presented in this section. As described in Chapter 3, the
tailings disposal operations represent the largest source of
radon-222 emissions; therefore, the summaries focus largely on
these operations.
The site summaries were compiled'from data contained in
other EPA, NRC, and DOE documents. A recent EPA report (EPA85)
entitled "Estimates of Population Distributions and Tailings
Areas Around Licensed Uranium Mill Sites" was the source of the
measurements of the surface areas of impoundments. The
populations in the 0- to 5-km range around the tailings
impoundments were taken from a 1984 survey that Battelle
4-1
-------
Table 4-1. Operating status and capacity of licensed conventional
uranium mills as of August 4, 1986l '
State
Mill
Owner
status
Operating
capacity
(tons/day)
(c)
Colorado
New Mexico
.-
South Dakota
Texas
Utah
Washington
Canon City
Uravan
L-Bar
Churchrock
Bluewater
Quivira
Grants
Edgemont
Panna Maria
Conquista
Ray Point
White Mesa
La Sal
Moab
Shootaring Canyon
Ford
Sherwood
Cotter Corp.
Umetco Minerals
Sohio/Kennecott
United Nuclear
Anaconda
Kerr-McGee
Homestake
TVA
Chevron
Conoco/Pioneer
Exxon
Umetco Minerals
Rio Algom
Atlas
Plateau Resources
Dawn Mining
Western Nuclear
Standby
Standby
Decommissioning , , (
I Cl I
Decommissioning* '
. Decommissioning^ '
Standby
Active^6'
Decommissioned
Active
Decommissioned
Decommissioned
Active ; •
Active (g)
Standby
Standby
Standby
. Standby
1200
1300
1650
4000 - ,
6000
7000
3400
. •
2600
—
—
2000
750
. 1400
800
600
2000
-------
Table 4-1. Operating status and capacity o
uranium mills as of August 4,. 1986 ^
licensed conventional
(continued)
State
Mill
Owner
Operating
c+-a1-n=VD7
status
Operating
capacity ,
(tons/day)* '
i
LO
Wyoming
Highland
Gas Hills
Shirley Basin
Gas Hills
Split Rock
Gas Hills
Bear Creek
Shirley Basin
Sweetwater
Exxon
American Nuclear
Corp.
Petrotomics
Pathfinder
Western Nuclear
Umetco Minerals
Rocky Mt. Energy
Pathfinder
Minerals Exploration
Decommissioned
Decommissioned
Decommissioned
Standby
Standby
Standby . ,.
Decommissioning^ '
Active
Standby
—
—
—
2500
1700
1400
2000
1800
3000
Total
5 Active
11 Standby
10 Decommissioned or
intend to decommission
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Data obtained from conversations with Agreement States, NRG representatives, and mill operators.
Does not include mills licensed but not constructed.
Active mills are currently processing ore and producing yellowcake. Standby mills are not currently
processing ore, but are capable of restarting. The mill structure has been dismantled at
decommissioned mills and tailings-piles are currently undergoing reclamation or will be.
Tons indicates short tons equal to 2000 Ibs.
Submitted letter of intent to decommission.
Operating only a few days each month.
Current contract will allow operation for 12-18 months.
Likely to go to standby status.
-------
-p-
I
-P-
MILL STATUS IN NOVEMBER 1985
• ACTIVE
• STANDBY
A DECOMMISSIONED
Figure 4-1. Approximate locations of licensed conventional uranium mills,
-------
Memorial Institute conducted for the EPA (PNL84) . In addition-,
color aerial photographs of each active and standby mill site
were provided by the Office of Radiation Programs to augment the
available data base (EPA85).
A summary of current conditions and the extent of tailings
impoundments and evaporation ponds at these sites is presented in
Table 4-2. Diagrams of each mill site are included in
Appendix A. Additional details regarding these mills and the
impoundments are provided in the following text under the
appropriate state.
4.2.1 Colorado
The two licensed uranium mills located in Colorado are
operated by Cotter Corporation and Umetco Minerals (Union
Carbide) in Canon City and Uravan (see Figure 4-2). A third
mill, Pioneer Nuclear's proposed San Miguel mill in San Miguel
County, was licensed but never constructed. The license for this
mill is under litigation '(NJRC84) .
CanonCity Mill
The Cotter Corporation, a 'subsidiary of Commonwealth Edison,
operates a two-stage acid leach mill at Canon City, Colorado,
which recovers' uranium and vanadium. A small alkaline leach mill
also was operated on this site from 1968 until its
decommissioning in 1979, The existing mill, which began
operations in September 1979, has a capacity of 1200 tons of ore
per day. The ore grade ranges between 0.23 and 0.35 percent
U O (NRC84). The mill has-been on standby status since
February 1985.
Tailings generated since September 1979 have been placed in
an above-grade clay- and membrane-lined impoundment that covers
34 ha (84 acres) and has earthen embankments (EPA85). Plans call
for the dam to be raised to its ultimate height of 35 m
(115 feet) in one additional stage. The tailings solution
currently covers 31 ha (77 acres) and varies in depth from less
than,0.3 to more than 6 m (<1 to >20 feet)(EPA85, Mc85).
Currently, the area of exposed tailings beach covers 3 ha
(7 acres), of which 1.8 ha (4.5 acres) is dry (EPA85). The
tailings discharge into the pond is moved along the perimeter
during operations to keep the tailings wet and evenly
distributed. This impoundment now contains 0.9 x 10 tons of
tailings and has a capacity of 14 x 10 tons (NRC84). The
tailings are reported to contain 780 pCi/g of radium-226
(EPA83a).
4-5
-------
Table 4-2. Summary of. current uranium mill tailings impoundment areas
and radium-226 content
Owner / Impoundment
Colorado
Cotter Corp.
Primary
Secondary
Umetco
Uravan 1 & 2
Uravan 3
Sludge pile
Evap. pond
New Mexico
Sohio
L-Bar
Type of
impoundment ' a'
2/SL
2/SL
1
1
1
1
1
(c
)
Average
Surface area(acres) Ra-226^d^
•Status 00
S
C
c
c
c
c
S
Total
84
31
66
32
20
17
128
Ponded
77
1
0
0
0
0
28
Wet
3
1
4
3
1
2
55
Dry
4
30
62
29
19
15
45
pCi/g)
780
780
480
480
480
480
500
United Nuclear
Churchrock
Anaconda
Bluewater 1
Bluewater 2
Bluewater 3
Evap. ponds
Kerr-McGee
Quivira 1
Quivira 2a
Quivira 2b
Quivira 2c
Evap. ponds
1
1
1
1
2
148
76
65
290
S
c
c
S
S
S
S
S
S
239
47
24
162
269
105
28
30
372
0
0
0
97
14
10
0
0
268
0
0
0
17
64
35
3
4
10
239
47
24
48
191
60
25
26
95
620
620
620
620
620
620
620
620
620
-------
Table 4-2. Summary of current uranium mill tailings impoundment areas
and radium-226 content (continued)
Owner / Impoundment
Home stake
Home stake 1
Home stake 2
Texas
Chevron
Panna Maria
Utah
•P- Ume t c o
-J White Mesa I
White Mesa 2
White Mesa 3
Rio Algom
Rio Algom 1
Rio Algom 2
Atlas
Moab
Plateau Resources
Shootaring
Washington
Dawn Mining
Ford 1,2,3
Ford 4
Type of
impoundment' a)
1
2
2
3/SL
3/SL
3/SL
2
2
1
2
2
3/SL
(c)
Surface area(acres)
Status(b)
S
C
S
A
A
A
A
A
S
S
C
S
Total
205
44
124
48
61
53
44
32
147
7
95
28
Ponded
63
4
68
7
10
39
4
12
54
2
0
17
Wet
33
0
20
7
6
0
2
5
4
1
0
0
Dry
109
36
36
34
45
14
38
15
90
4
95
11
Average
pCi/g)
385
385
196
350
350
350
560
560
540
280
240^e)
240 (e^
-------
Table 4-2. Summary of current uranium mill tailings impoundment areas
and radium-226 content (continued)
00
Owner /Impoundment
Western Nuclear
Sherwood
Evap. pond
Wyoming
Pathfinder
: ' 'Gas Hills 1
Gas Hills 2
Gas Hills 3
',Ga:s 'Hills 4 "L
Western ; Nuclear
. Split Rock
Umetco
Gas Hills
A-9 Pit
Leach pile
Evap. ponds
Type of
impoundment ^a'
2/SL
2/SL
2
2
, .. -2 -.
2
2
3/CL
2
2
Status^)
S
S
S
C
S
S
C
S
S
S
(c)
Surface area(acres)
Total
94
16
124
54
22
89 :
156
\
151
25
22
20
Ponded
18
16
2
2
19
73
94
~
0
2
0
20
Wet
7
0
3
12
2
4
19
0
9
0
0
Dry
70
0
119
40
2
11 '
43
151
14
22
0
Average
pCi/g)
200
200
420'
420
420
. 420
430
310
310
310
310
Rocky Mountain Energy
Bear Creek
121
45
23
53
420
Pathfinder
Shirley Basin
261
179
22
60
540
-------
Table 4-2. Summary of current uranium mill tailings impoundment areas
and radium-226 content (continued)
Owne r / Impoundme n t
Minerals Exploration
Sweetwater
Totals
Type of
' / \
impoundment \a '
2/SL
Surface area(
Status (-D>' Total Ponded
S 37 30
3882 1282
(c)
acres)
Wet , Dry
0 7
457 2140
Average
pCi/g)
280
, -
(a) Type of impoundment; 1 = dam constructed of coarse tailings; 2 = earthen dam; 3 = below grade;
SL = synthetic liner; CL = clay liner. ,
•*" = .(") Status of impoundment; A- = active; S =? standby (will be used when operations resume) ; C = filled to
"^ capacity (will not be used again).
Source: EPA85
(d) Source: EPA83
(g) Source: EPA86
-------
flj Cotter Corp.
Canon City Mill
IMetco Minerals
Uravan Mill
Figure 4-2, Location of mills in Colorado,
4-10
-------
A 12-ha (31-acre) secondary impoundment containing
1.5 x 10 tons commingled tailings (defense-related tailings
generated under Atomic Energy Commission contracts commingled
with tailings generated under commercial contracts) generated in
pre-1979 operations has been constructed adjacent to the main
impoundment. . Approximately 0.4'ha (1 acre) is covered with
ponded solution, 0.4 ha (1 acre) consists of exposed saturated
tailings, and about 12 ha (30 acres) are dry (EPA85). These
impoundments are actually two cells of one large impoundment.
The secondary impoundment also is'used for disposal of
nontailings solid waste generated on site .and will be used for
disposal of decommissioning waste during closure operations
(DOE82). The old tailings have not been covered, but they have
been furrowed to control dusting. The costs for constructing the
main and secondary impoundments were $15,800,000 and $7,200,000,
respectively (DOE82).
Canon city is located about 3.2 km .(2 mi} north of the mill
site. The area Immediately surrounding the mill site is
unpopulated, and the land -is used primarily for livestock grazing
(DOE82). The nearest residents are 184 people who live between
2 and 3 km (1.2 and 1.9 mi) from the impoundment (PNL84), A 1983
survey indicated 5933 people lived within 5 km (3.1 mi) of the
tailings impoundment (PNL84).
The climate in the area is semiarid and temperate; average
annual precipitation is 30 cm (12 in.) (DOE82). Windspeeds are
variable, with a mean of 13 km/h (8 mi/h) (DOE82).
•Uravan Mill ....
Umet'co Mineral's uranium mill in Uravan, Colorado, an area
of rugged canyons and mesas, is 80 km (50 mi) south of Grand
Junction. Uranium, vanadium, and radium-226 recovery operations
were begun at this site in 1915. The mill''has been on standby
status since' November 1984 and will likely be on standby for at
least 2 years and possibly permanently (Kr85). The existing
tailings disposal facilities have reached their maximum capacity,
and a new disposal area must be planned and approved' be-fore mill
operations are restarted (Kr85). The capacity of this mill is
1300 tons of ore per day.
The mill uses a hot, highly oxidizing, two-stage acid leach .
to recover uranium and vanadium. During milling operations, ore
has been received from more than 200 mines in the Uravan mineral
belt. Tailings have been generated under AEC, Army, and
commercial contracts and have been commingled and disposed of on
site. The impoundments contain'an estimated 10 x 10 tons of
tailings. These tailings impoundments are situated on mesas
4-11
-------
above Uravan. Impoundments 1 and 2 are adjacent and overlapping
and actually constitute just one impoundment. The impoundments
are constructed behind dikes of coarse tailings on the outward
face and contained by the native terrain on the inward side,
Tailings were discharged to the impoundments from spigots
situated around the berm. Gravity settling deposited the sands
near the dike, and slimes were carried to the interior with the
tailings solution.
Impoundments 1 and 2 cover a combined area1 of 27 ha
(66 acres) and have a maximum dam height of 46 m (155 ft) (EPA85,
DOES2). Impoundment 3 covers 13 ha (32 acres), and the dike is
about 33 m (110 ft) high. Eight other impoundments, which either
contain tailings or have been constructed of tailings, were
mainly used for evaporation. These eight impoundments cover
15 ha (37 acres). The radium-226 content of the Uravan tailings
has been reported to be 480 pCi/gram (EPA83b).
The Uravan operation uses several other ponds in its water
'management .system. Six solvent, extraction (SX) raf f inate
evaporation/seepage ponds receive barren solution from the
vanadium SX section. Residue in these ponds will be placed in
the tailings ponds at closure. The SX ponds cover 15 ha
(36 acres) (NRC84). '
The general- area is sparsely populated. A recent survey
indicates 349 people living from 2 to 5 km (1.2 to 3.0 mi) away
from the main tailings,impoundments. The survey showed nobody
living within 0.5 km (0.3 mi) of these impoundments, but
147 people lived 0.5 to 1.0 km (0.3 to 0.6 mi) distant (PNL84).
The climate at Uravan.is semiarid, with only about 25 cm
(10 inches) -of precipitation a year. Evaporation is about 142 cm
(56 inches) per year (EPA83b).
4.2.2 New Mexico
The five licensed mills located in New Mexico are operated
by Sohio/Kennecott -Minerals, United Nuclear Corporation, Anaconda
(Atlantic Richfield), Kerr-McGee Corp. (Quivira Mining), and
Homes.take Mining Co. (see Figure 4-3) . Two additional mills,
Bokum Resources Corporation and Gulf Minerals, were licensed but
have never operated.
4-12.
-------
L-Bar-Mill
• The Sohio/Kennecott • L-Bar Uranium Mill is located near
Seboyeta in 'Gibola County, in an area of hilly terrain about
71 km (44 mi) west of Albuquerque and 16 km (10.mi) north of"
Laguna, New Mexico. Ore is obtained from an underground mine in
the Jackpile sandstone formation. The acid-leach mill began
operations -in 1976, but has been on standby status since May 1981
(NRC84).•'The ore processing capacity of the mill is 1650 tons
per day. Ore'reserves are' adequate to provide for 10 to 15 years
of operation. The ore grade varies from 0.05 to 0.30 percent
U-O8 and averages 0.225 percent (NRC84). Size'reduction is
accomplished by semiautogenous grinding.
Mill tailings are contained in a single tailings
impoundment. 'The L-Bar'tailings dam was one of the last dams
permitted in the industry -in which the upstream construction
method was used (JoSO) . The tailings impoundment is built above
grade with an'earthen starter dam to the west that keys into
natural topography'on the' north" a-nd south. A smaller saddle dam
is constructed 'to the east. Tailings have been discharged to the
impoundment from-a single pipe that was moved along the dam'.
Coarse sands settled near the dike, 'whereas'slimes deposited in
the interior area. Water was decanted and'pumped back to the
mill. During operations, the edge'of the1tailings solution was
maintained about-60 m. (200-'ft) from the dam crest. A light-track
pressure dozer, was - vised to. construct raises with the sand'
tailings. The total impoundment area'covers 72 ha (180 acres),
about 51.2 'ha ,(-128 acres)''of which are'covered with tailings
(NRC84). Approximately 11.2 ha (28 acres) of the tailings are
covered with tailings solution (EPA85). The impoundment consists
of about 1.6'"x'10. tons'of tailings. The maximum height of the
dam is 15 m (50,'ft)' (NRC80) . The'facility was designed to ;
provide an ultimate storage'capacity of 7.-5 x 10 tons of
tailings (JoSO). The tailings, are reported to contain 500 pCi/g
of radium-226 (EPA83b)..'-..-
During operations,- ore is stockpiled at the mill on an ore
pad and apron feeder." Since the plant.went on standby status in
1981, no ore -has been stored 'on these areas, but a short supply
has been stored north of the tailings area (NM85). , '
The surrounding area is sparsely populated. A i983 -survey
indicated ho -population residing within a 3-km (1.9-mi) radius of
the tailings'impoundment (PNL84). Reportedly 42 people'live
between 3 and^4 km (1.8 and 2.5 mi) away and 129 live between
4 and 5 km .(2.5 and 3.1 mi) (PNL84).
4-13
-------
Sohio
L-Bar Mill
©
fT) United Nuclear Corp.
Churchrock Mill
©
Anaconda Minerals Co,
Bluewater Mill
Kerr-McGee Nuclear Corp,
Quivara Mill
Homestake Mining Co.
Homestake Mill
Figure 4-3. Location of mills in New Mexico
4-14
-------
Churchrock Mill
United Nuclear Corporation's Churchrock Mill is located
32 km (20 mi) northeast of Gallup, New Mexico, on an
alluvial plain situated near an arroyo. The mill, which
opened in 1977, is designed to use acid-leach extraction to
process about 4000 tons of ore per day from the
company-owned underground mines. The ore contains 0.035 to
0.381 percent U30g (average is 0.12 percent) in a
sandstone matrix. Fresh water for mill operations is
obtained from underground mines. The mill has been on
standby status since 1982.
The tailings impoundment is formed by a dam built from
native clays and compacted coarse tailings. It has three
compartments separated by earthen embankments. The total
surface area of tailings is 59 ha (148 acres) (EPA85). The
surface area of liquid on the tailings impoundment is 3 ha
(7 acres). The maximum depth of tailings is about 15 m
(50 ft) A The storage capacity of -the pond is about
10 x 10 m (365 x 10 ft ) (NRC84). The tailings are
reported to contain 290 pCi/g of radium-226 (EPA83b).
The area around the mill is sparsely populated. The
1983 population survey indicated 25 people residing within
2 km (1.25 mi) and 77 living within 3 km (1.9 mi) (PNL84).
The survey also indicated a total of 213 people living
within 5 km (3.1 mi) of the mill, but none within 1 km
(0.6 mi) (PNL84).
In July 1979, a break in the tailings dam caused about
350 x 106 liters (93 x 106 gal) of effluent and 1100 tons of
tailings to spill on or into nearby soil and streams
(NRC84). This spill resulted in the release of almost all
of the impounded liquid, but less than 1 percent of the
solids. The streams carried the spilled tailings into the
Rio Puerco River, which flows, through Navajo grazing lands,
and finally into Arizona. The mill was closed from. July
1979 until the fall of 1979 while measures were taken to
clean up the streams contaminated by the spill. The cleanup
of the streams has been completed. The mill has been
inactive since 1982, and corrective action to clean up the
contaminated groundwater is continuing (NRC84).
4-15
-------
Bluewater Mill ' -' • ; •..',.-••-.;.
Anaconda's Bluewater .Uranium Mill is located - i"n- the
Grants Mineral Belt about 16 'km (10 -mi) northwest'of Grants,
New Mexico. The site is in a small valley characterized by
an undulating, relatively level surface with gentle-swales
and small rounded hills (DOE82). The mill 'was originally
constructed in 1953 and operated until 1982, .when.it went to
standby status. Since 1953, the milling operations have
gone through several major modifications. Capacity has been
expanded to 6000 tons of ore (0,2 percent U_0g)
(NRC84). Production has been under both AEC'\I956 to 1970)
and commercial contracts. Through 1981, the Bluewater mill
had processed more-than 23.5 x 10 tons of ore ranging. -
from 0.06 to 0.60 percent U-,0. (DOE82). .Some
decommissioning .activities nave been initiated at this ' .
mill. • . -. .
The mill site has three tailings impoundments,
Carbonate tailings from eartly operations- were deposited in
an area immediately northwest of the mill in a flat-lying
impoundment (No. 2) covering about 19 ha (47 acres),
(DOE82). This inactive impoundment has been covered with
native soil to an average depth of 0.8 m (2.5 ft)._(DOE82) .
Other tailings from the early carbonate processing were
emplaced in what is now the main tailings impoundment for
acid tailings (No. 1). A third tailings impoundment, the
north area acid pile, is situated immediately'northwest of
the.main pond. It covers 10 ha (24 acres), and .in 1977 was
covered with about 0.8 m (2.5 ft) of native soil (DOE82).
The main•impoundment (No. 1), which was put: in
operation in 19,56, covers 96 ha (239 acres) (EPA85) . It is
currently dry. /The dam surrounding the pond is .constructed
of compacted natural.soils.and alluvium and is about 18 m
(60 ft) high at the 'south end-and 6 m'(20 ft) .high at the
north end (DOE82). Tailings are discharged along the
southern part of the dam. This impoundment contains
25 x 10 tons of tailings (NRC84). '.'',-': . ' • .. • . - _ •
There are also 162 acres of evaporation''ponds- in the
mill,water management circuit. ' Currently, 97 acres are
covered with solution, 17 acres -are exposed and "wet, and
48 acres are exposed and dry (EPA85), -Some tailings solids
are carried with the water to these•evaporation ponds where
they remain' after the solution .evaporates. .-.•,,
.4-16
-------
The specific activity of radium-226 in the old tailings has
been reported to be 520 pCi/gram and 280 pCi/gram in the tailings
in the main pond (NM85); however, it has also been estimated to
average 620 pCi/g (EPA83a).
The area around the Bluewater Mill is sparsely populated. A
1983 survey indicated 907 people living within 5 km '(3.1 mi) of
the mill (PNL84). Of this total, 142 lived within 3 km
(1,9 mi.-). No one lives within 2 km (1.2 mi.) of the mill
(PNL84).
Annual precipitation averages 22 cm (8.8 inches)—most as
rain, but some as snow. Wind is channeled through the valley in
a westerly direction. The site is in the "southwest mountains"
climatological subdivision of New Mexico.
Quivira Mill
Kerr McGee's Quivira mill has been on standby status since
February 1985. The largest acid leach mill in the United States,
its current capacity is 6350 t (7000 tons) of ore per day
(NCR84). The Quivira mill is in a flat area of the Grants
Mineral Belt about 40 km (25 mi) north of Grants, New Mexico.
The mill began operation in 1958 with"-a capacity of 3270 t of
(3600 tons) sandstone ore per day.
All of the tailings from the mill are contained in two main
impoundments, (Tailings impoundments Nos. 1 and 2a) and two
ancillary impoundments (2b and 2c). Impoundment No. 1 was the
most recently active area for tailings deposition. It extends
southeasterly from the mill for about 1370 m (4500 ft); its
greatest width is about 820 m (2700 ft), and the outside berm
ranges from 8 to 27 m (25 to 90 ft) above ground level (DOE82).
An earthen starter dike was used along with the upstream method
of tailings disposal. .Tailings were discharged to the pond from
multiple spigots located along the crest at 9-m (30-ft)
intervals. The bulk of the sands is deposited on a beach inside
the berm, and the slimes and liquid flow into- the central-
depression to form a lake (DOE82). The operator maintains a
150-m (500-ft) wide beach and a 1.5 m (5 ft) freeboard during
operation. Impoundment No. 1 covers 108 ha (269 acres) and
contains a liquid covered area of about 6 ha (14 a'cres) (EPA85) .
Approximately 76 ha (191 acres) are dry and the remaining 26 ha
(64 acres) remain saturated (EPA85).
Tailings Impoundment No. 2a covers about 42 ha (105 acres)
and is west of and contiguous with Pond No. 1 (EPA85).
Impoundments Nos. 1 and 2a have been in use sigce 1958. These
two impoundments contain approximately 26 x 10 tons of
tailings. Some tailings are used as backfill in a nearby
underground mine. Tailings set aside for use as backfill
4-17
-------
are contained in Impoundment No. 2ta.' Heap leached tailings are
contained in Impoundment No. 2c. Impoundments 2b and 2c cover
11 and 12 ha (28 and 30 acres), respectively. Although no water
is currently ponded in either of these impoundments, 1 to 1.5 ha
(3 or 4 acres) of each are saturated (EPA85). The tailings are
reported to contain 620 pCi/g of radium-226 (EPA83b).
The Quivira mill uses 15 evaporation ponds in its water
management system. These ponds currently cover a total of 149 ha
(372 acres) (EPA85). Of this total surface area, 107 ha
(268 acres) are-covered with solution, 4 ha-(10 acres) are wet,
and 38 ha (95 acres) are dry (EPA85). Some tailings solids are
carried with the liquid solution and are deposited in these
evaporation ponds. • -
The area surrounding the-mill is sparsely populated. The
1983 population survey indicated only one person living within
5 km (3.1 mi) of the mill (PNL84), and that person lived between
2 and 3 km (1.2 and 1.9 mi.) from the impoundment (PNL84).
Precipitation averages 22 cm (8.8 in.) per year (DOE82).
Local winds are channeled" by the valley, and gusts can exceed
80 km (50 mi) per hour.
Horoestake Mill ;
Homestake Mining Company's mill is 16 km (10 mi) northwest
of Grants, New Mexico. The mill began production in 1958, Since
its beginning, its capacity has been increased from 675 t
(742 tons) to its present 3200 t (3400 tons) of ore per day
(DOE82)-. The Homestake Mill "uses the alkaline leach process. •
The mill has been on standby status since mid-1985. The ore
grade milled -at•Homestake has ranged from 0.05 to'0.30 percent
U,00 (NRC84).
.2 O
The mill site is relatively flat and covers about 600 ha
(1500 acres). Two tailings impoundments, one on standby and the
other inactive,.are located on site. The inactive impoundment
contains tailings generated between 1958 and 1962 under AEC
contracts. The 1.1 x 10 t (1.2 x 10 tons) of AEC tailings
cover -about 18 ha (44 acres) "and :are contained within an 8-m
(25-ft) high earthen embankment (DOE82). There currently is
1.6 ha (4 acres) of ponded water-on the impoundment (EPA85),
Approximately 20 percent, 3.2 ha (8 acres), of this tailings
impoundment has been covered with a meter of contaminated soil
excavated from an area affected by a past spill from the active
impoundment (DOE82), Efforts have been made to revegetate the
impoundment to reduce dusting.
4-18
-------
The active impoundment contains about 20 x 10 tons of
commingled tailings (DOE82). The impoundment is shaped like a
large rectangular-base prism that rises above the flat ground
surface (DOE82) , It has a surface area of ,82 ha (205 acres)
(including the sides) and is about 26 m (85 ft) high. The slopes
of the four sides are about 2:1 (h:v). The top of the
impoundment is divided into two cells which are used alternately
for tailings discharge. Most of the interior, of both cells is
covered with tailings solution. The total surface area of the
ponded fluid in these two cells is about 25 ha (63 acres)
(EPA85). Homestake maintains a 15-m (50-ft) beach -and 1.5-m
(5-ft) freeboard.8 The embankments are constructed of coarse
tailings (sands) built up by the centerline method of
construction. A mobile cyclone is used to separate the sands and
slimes. Decanted pond liquid is recycled back to the mill.
Surface water sprays and chemical .treatments are applied to the
embankment faces to inhibit dusting. The tailings are reported
to contain 385 pCi/g of radium-226 (EPA83b).
Residential areas are located within 1.6 km (1 mi) of the
mill. Homestake's 1982 license renewal application and the 1983
survey both indicated no population within 1 km (0.6 mi). The
1983 survey indicated that 190 people live between 1 and 2 km
(0.6 to 1.2 mi.) from the impoundment (PNL84). The survey
counted a population of 396 people within 5 km (3,1 mi.) of the
mill. (PNL84). Homestake has purchased additional land adjacent
to the mill-site to provide a 0.8-km (0.5-mi.) buffer zone
(DOE82)....
The site's climate is characterized by low precipitation .
[22 cm (8.8 iri.)/y average), sunny days (75 to 80 percent), low
humidity, wind gusts to 80 kilometers per hour (50 mph), and
moderate temperatures with large diurnal and annual fluctuations
(DOE82). .
4.2,3 Texas ^ .
The three licensed mills in Texas are owned by Chevron
Resources, Conoco-Pioneer, and-Exxon Minerals. Their locations
are indicated in Figure 4-4. One additional mill, Anaconda
•Minerals Rhode Branch Mill, was licensed in 1982, but was never
constructed. Only the Panna Maria Mill is described herein, as
the others are being decommissioned.
Panna Maria Mill
The Panna Maria Uranium Project•of -Chevron Resources Company
is located in South Texas about 160 km (100 mi) northwest of
Corpus christi and 10 km (6 mi) north of Karnes City. The mill
processes about 2600 tons per day of a mixture of sandy clay ore
4-19
-------
averaging 0.05 percent U3Qa (Ma85). this facility^ which
uses semi-autogeneous grinding followed by acid leaching, began
operation in January 1979 and has been on standby status since
June 1985 (Ma85).
Tailings are contained dn a ...single above-ground impoundment
contained by earthen dikes. -Material for the dikes was excavated
from the area beneath the impoundment. The tailings area covers
50 ha (124 acres); 14 ha (36 acres) consist of dry, exposed
beach, and about 27 ha (68 acres):. are covered with tailings
solution (EPA85), The impoundment contains approximately
3,3 x 10 tons of tailings (NRC84). It was designed to contain
all the tailings projected .to be.generated over•the .life of the
mill. The maximum height of the earthen;dam" surrounding the pile
is 19 m (62 ft), the crest width/is 6'» (20 ft), and the
downstream slope is 3:1 (h:v) (Ki80). ' Designed maximum storage
of tailings in this impoundment is 10 .x 10_ tons) (Ki80)* The
average density of the tailings is 1.2 t/m (0.04 ton/ft,),
and the specific gravity is- 2.55 : (Ki80). ••.".'.•
During operations, the tailings discharge to the impoundment
is periodically moved around - the!perimeter of the impoundment.
An exposed beach of coarsevtailings forms along the dike and the
tailings solution gathers-'in the'center portion of .the pond. The
depth of the solution varies from an average of 1.5 m .(5 ft) on
the east side to 5- to 6m (15 to 20 ft) on the west (Ma85) .
The radon-222 flux from the'tailings has not. been measured.
The radium-226 content of the tailings is estimated to- be
196 pCi/g. '•
The ore pad at this facility covers'approximately 12 ha
(30 acres). During normal operations, a:1-month supply of ore
[69,000 t (76,000 tons) at capacity] is stockpiled on-'the pad.
A 1983 survey of population in the area: indicated 453 people
living within 5 km (3.1 mi) of the tailings impoundment,
12 people within 1 km (0.6 mi), 42 people within i"and 2 km
(0.6 and 1.25 mi), and 33 people within 2 and 3 km (1.25 and 1.9
mi) (PNL84) . , .' ' ' . :- '
The average annual -rainfall ;at "the location of the :
impoundment is 76 cm'(30 in.), and the net-annual evaporation is
89 cm (35 in.) . , - . ' • ,- ' ';.
4-20
-------
©
1) Conoco/Pioneer^ Nuclear .•-.
Conquista Project , - •
o
£y Chevron Resources Co.
Panna Maria Mill
(^) .Exxon Minerals
Ray Point
Figure 4-4. Location of mills in Texas,
4-21
-------
4.2.4 Utah
The four licensed mills located in Utah (see Figure 4-5) are
owned by Atlas Minerals, Plateau Resources, Ltd.,, Umetco
Minerals, and Rio Algom Corporation.
Umetco White -Mesa Mill . !
The Umetco Minerals White Mesa mill,-which is about 8 km
(5 mi.) south of Blanding, Utah, began operating in July 1980.
This mill is currently active. Semi-autogenous grinding,
acid-leaching, and solvent-extraction are used to process ores
containing about 0.13 percent U,0_ (NRC84). The capacity of
the mill is 1800 t (2000 tons) of ore per day (NRC84).
Approximately 500,000 t (550,000 tons) of tailings are
contained in three cells of a proposed six-cell disposal system.
The cells contain 19, 24, and 21 ha (48, 61, and 53 acres) of
tailings for a total of 64 ha (162 acres) (EPA85). A total of
22 ha (56 acres) is covered by solution, 5 ha (13 acres) are
saturated, and 42 ha (106 acres) are dry (EPA85). • The proposed
system was planned to feature simultaneous construction, [
operation, closure, and reclamation. The tailings impoundments
are lined with synthetic liners. The tailings are1 reported to
contain 350 pCi/g of radium-226 (EPA83b).
A 1983 population survey indicated no people living within a
4-km (25-mi) radius of the tailings impoundment (PNL84). The
same survey indicated eight people living between 4 and 5 km
(2.5 and 3.1 mi) of the tailings disposal area (PNL84).
Rio Alcfom Mill
The Rio Algom Kill is near La Sal, Utah, about 48 km (30 mi)
southeast of Moab. This mill is currently active and has been in
operation since 1971. Ore obtained from adjacent underground
mining operations is processed by alkaline leaching and ion
exchange. The'mill's designed' throughput is 700 t (750 tons) of
ore per day.
Over 1.6 x 10 t (1.8 x 106 tons) of tailings have been
generated at this mill (NRC84). The tailings are contained in
two unlined'tailings impoundments retained by natural soil
embankments-placed across' a drainage course, one immediately
upstream of'"the other (NRC84) . The lower impoundment has been in
use since 19'72, the upper since 1976. The total area of tailings
is 30 ha (75 acres) (EPA85). Approximately 6 ha (16 acres) are
covered with solution, 3 ha (7 acres) are saturated, and 21 ha
(53 acres) are dry (EPA85). The tailings are reported to contain
560 pCi/g of radium-226 (EPA83b) . ''
4-22
-------
f 1 j Umetco Minerals
^""^ White Mesa Mill
2.) Rio Algom ,Corp .
La Sal Mill
©
Atlas Minerals
Moab Mill
4 ) Plateau Resources, Ltd.
Shootaring Canyon Mill
Figure 4-5, Location of mills in Utah,
4-23
-------
A 1983 survey of the population in the area indicated no
inhabitants living within 0.5 km (0,3 mi.) of the tailings
impoundment (PNL84). Eight inhabitants were reported to live
between 0.5 and 1.0 km (0.3 and 0.6 mi) from the impoundment, and
105 people between 1 and 2 km (0.6 and 1.2 mi) from the
impoundment (PNL84)„
Moab Mill
The Atlas Corporation Mill is located on the Colorado River
in a long, narrow valley of a mountainous area about 5 km (3 mi.)
northwest of Moab, Utah. The mill, which began operations in
October 1956, is on standby status. This mill has combined acid
and alkaline circuits, which give it greater flexibility in
handling a variety of ores (DOE82). Uranium has been produced
for sale to both government and commercial buyers. Capacity of
the mill is 1980 tons of ore per day (NRC84).
Prior to 1977, mill tailings were discharged to the Colorado
River (NRC84). Since that time, all tailings have been placed in
a single tailings impoundment. The dam has been constructed
mainly of coarse tailings. Tailings are discharged from multiple
spigots around the perimeter of the dam. The coarse sand is
deposited on and near the dam, whereas the fines are carried to
the interior of the impoundment with the tailings solution. The
impoundment's total surface area is 60 ha (147 acres (EPA85). Of
the total area, 22 ha (54 acres) are covered by ponded solution,
2 ha (4 acres) are saturated, and 36 ha (90 acres) are exposed
dry tailings {including the dams) (EPA85). Because the
impoundment is on a sloping surface, its height varies from 6 to
about 36 m (20 to about 120 ft) above ground (DOE82). Between
7 and 9 x 106 t (8 and 10 x 106 tons) of tailings are contained
in this impoundment (DOE82, NRC84)-.
The radium-226 content of the tailings has been reported to
be 540 pCi/gram (EPA83)„ Ore grade ranges from 0.20 to
0.25 percent U OD (NRC84).
o
Moab is the only nearby incorporated community. A
1983 survey indicated a total population of 2361 within a 5-km
(3.1-mi) radius of the tailings pile (PNL84). The same survey
indicated no people living within 1.0 km (0.6 mi) and 9,people
within 1 and 2 km (0.6 and 1.2 mi) from the impoundment. The
survey also indicated that 2319 people were living between 3 and
5 km (1.8 and 3.1 mi) of the mill (PNL84).
4-24
-------
The climate at the site is semiarid. Annual precipitation
is 20 cm (8 inches), and the annual evaporation rate is 163 cm
(64 inches) (EPA83). As a means of minimizing dusting, the dried
tailings are sometimes wetted with sprinklers and/or a chemical
dust suppressant, such as Coherex (DOE82). Windspeeds usually
are quite low (DOES2).
Plateau Resources Mill
The Plateau Resources Shootaring Canyon Mill is located near
Hanksville, Utah. This mill was operational only from April to
October 1982 and is currently on standby status. The capacity of
the mill is 725 t (800 tons) per day (NRC84). The average ore
grade is 0.15 percent U»0_, ranging from 0.07 to 0.24 percent
(NRC84). An average of approximately 97,000 tons of surface
mined ore is stockpiled on site when the mill is running at
capacity (Ge85). The primary mill circuit involves
semi-autogenous grinding of the sandstone ores, followed by a
sulfuric acid leach. Tailings are disposed of in a planned,
phased disposal system. An earthen dam has been constructed
across the valley. Behind the earthen dara, berms have been
constructed to form six cells for tailings disposal. Because of
the short period of operation, only one cell contains a
significant quantity of tailings. Two other cells contain only
minor quantities, and the other three cells contain none. The '
area of the tailings is only 3 ha (7 acres), and about 0.8 ha
(2 acres) of these are covered with water (EPA85). Plateau
Resources has taken steps to stabilize this impoundment
temporarily by inducing water evaporation and placing a 0.3-m
(1-ft) cover of local soil over 1.2 ha (3 acres) of the tailings
to limit windblown dust. This interim stabilization process will
be completed in approximately 3 years. Radon-222 flux from the
tailings has not been measured.
The area around the mill is sparsely populated; no
inhabitants live within a 4-km (2.4-mi.) radius (PNL84). The
1983 survey indicated 171 people living within 4 and 5 km
(2.4 and 3.1 mi) of the tailings impoundment (PNL84).
4.2.5 Washington
Washington has two licensed conventional mills, owned by
Dawn Mining (Newmont Mining/Midnight Mines) and Western Nuclear,
Inc. (Phelps Dodge) (see Figure 4-6). Another mill, owned by Joy
Mining Company, was licensed, but was never fully operational, ;
This latter mill is not typical as it processed a bog material on
a leach pad. Only 820 t (900 tons) of tailings (heap leached bog
material) was generated. It is reported that this residue has a
low radium-226 content (WA86). The license for this mill was
suspended in June 1985.
4-25
-------
QT) Dawn Mining Co.
Ford Mill
(T) Western Nuclear, 'Inc.
Sherwood Mill
Figure 4-6. Location of mills in Washington,
• 4-26
-------
Dawn Mining_Mill
The Dawn Mining Mill, which is near Ford, Washington, about
72 km (45 mi) northwest of Spokane, is jointly owned by Newmont
Mining Corporation and Midnight Mines, Inc. It began operations
in 1957 and operated through 1964 under the AEC concentrate
purchase program. The'mill was shut down and rehabilitated'
between 1965 and 1969. It operated between 1969 and 1982, but
has been inactive and on standby status since 1982.
The production capacity of the mill is 550 t .(600 -tons) of
ore per day. The mill circuit incorporates a two-stage agitation
acid leach process followed by ion exchange and precipitation of
uranium with ammonia. The Midnight mining open-pit mine produces
ore between 0.10 and 0.25 percent U.Og (NRC84). During
operations, a 1-year supply of ore [193,000 t (212,300 tons)] was
maintained on a 6-ha (14-acre) stockpile at the mill site
(DOE82).
The tailings generated by the Dawn Mill are contained in
four separate impoundments, three of which are above grade,
unlined, and constructed behind earthen dams. These three
impoundments have been filled to capacity'and are inactive.
Impoundment Nos. 1 and 2 contain an estimated 1.2 x 10 tons of
tailings from government contract production. They have been
covered with about 0.61 m (2 ft) of sandy soil and wood chips for
dust control and interim stabilization (DOE82, An84).
Impoundment No. 3, which contains about 1.6 x 10 tons of
tailings, has also been covered with sandy soil and wood chips.
These three impoundments have a surface area of 38 ha (95 acres),
all of which is dry (EPA85). Impoundment No. 4 is an excavated,
below-grade, lined (Hypalon) pond covering 11 ha (28 acres).
Seven hectares (17 acres) are covered by solution and 4 ha
(11 acres) are dry (EPA85). The tailings are covered with water
to a depth of 1.2 to 1.5 m (4 to 5 ft). The radium-226 content
of the Dawn Mill tailings .is reported to be 240 pCi/g (EPA86).
The community of Ford is located within 3.2 km (2 mi) of the
tailings impoundments. In 1983 approximately 411 people were
living within 5 km (3.1 mi.) of the tailings impoundments
(PNL84). No one lived within 0.5 km (0.3 mi) and 3 people'lived
within 0.5 and 1.0 km (0.3 and 0.6 mi). Ninety-three people
lived within 1 and 2 km (0,6 and 1.2 mi) .and 157 lived'within
2 and 3 km (1.2 and 1.9 mi) of the impoundments (PNL84),
The area's topography is characterized by rolling hills.
The average annual precipitation is 30 to 46 cm. (12 to 18 in);
annual evaporation is about1127 cm (50 in) (EPA83b).
4-27
-------
Western Nuclear Sherwood Mill
Western Nuclear's Sherwood uranium mill is located in
eastern Washington about 64 km (40 mi) northwest of Spokane. Ore
taken from a nearby surface mine has averaged 0.05 to 0.09
percent U_0_ (EPA83). This mining and milling operation,
which began in 1978, has been inactive and on standby status
since July 1984.
The tailings generated by acid leaching at the Western
Nuclear Mill have been placed in a single above-grade impoundment
behind an earthen dam. The area covered by tailings is 38 ha
(94 acres) (EPA85). Of this total, 7 ha (18 acres) are covered
with tailings solution, 28 ha (70 acres) are dry, and the
remainder is saturated (EPA85). Tailings slurry from the mill
was neutralized with lime'before being-pumped to the
Hypalon-lined impoundment. Tailings solution decanted from the
impoundment was pumped to a 16-acre evaporation pond situated
immediately upstream of the tailings impoundment. The current
amount of tailings under management is-estimated to be (1.6 x
10 tons) (NRC84). The tailings are reported to contain
200 pCi/g of radium-226 (EPA83).
The area is sparsely populated. A 1983 survey indicated
49 people living between 3 and 5 km (1.9 and 3.1 mi) away from
the tailings impoundment (PNL84). This survey also indicated
that no one was living within 3 km (1.9 mi) of the impoundment.
Annual precipitation is 25 to 38 cm (10 to 15 in.), and annual
evaporation is about 127 cm (50 in.) (EPA83b).
4.2.6 Wyoming
As shown in Figure 4-7, nine mills are located in Wyoming.
Three of these have been decommissioned, two are active, and four
are on standby status. Descriptions of the active and standby
mills are presented in the following 'subsections.
Pathfinder Gas Hills Mill
The Pathfinder Mines Corp. (formerly Lucky Me.Corp.) Gas
Hills Mill is located in the Gas Hills region of Fremond County,
Wyoming, about 40 km (25 mi.) northeast of Jeffrey ..City.
This mill first began producing yellowcake in 1958' with a
nominal ore-processing capacity of 850 t (935 tons) per day.
Since then, the capacity has been.expanded to about 2273 t (2500
tons) of ore per day. The mill uses an acid-leach process and
was the first in the United States to incorporate the moving-bed,
ion-exchange technique originally developed in South Africa. It
is also the only domestic uranium mill that uses anion exchange
for concentration of uranium from the feed solution.
4-28
-------
(l) Pathfinder Mines Corp,
Gas Hills Mill
(2) Western Nuclear, Inc.
Split Rock Mill
©
(4) Rocky Mountain Energy
Spj
Umetco
Gas Hills Mill
f6J Minerals Exploration Co,
©
Sweetwater Mill
Bear Creek Mill
Petrotoznics
Shirley Basin Mill
8j American Nuclear Corp,
Gas Hills Mill
Exxon Corp,
Highland Mill
(5) Pathfinder Mines Corp,
Shirley Basin Mill
Figure 4-7. Location of mills in Wyoming.
4-29
-------
Company-owned open-pit mining operations, located 1.5 to
3 km (1 to 2 mi) from .the mill, supply 90 percent of the ore; the
remaining 10 percent is produced at Pathfinder's Big Eagle Mine
near Jeffrey City. The ore grade has averaged 0.21 percent U.OQ
3 o
in past operations and is expected to average 0.11 percent in the
future (Ha85). Although mines adjacent to the mill also could
provide fresh water for ore processing, the availability of hot
[57°C (135°F)] well water at the site makes it advantageous, from
a process standpoint, to use well water in the mill and to treat
mine water Cor discharge.
The tailings retention system consists of four tailings
impoundments having surface areas of 508 22, 9, and 36 ha (124,
54, 22 and 89 acres) (EPA85) The impoundments are situated
sequentially in the head of a draw north-northeast of the mill
and are dug into an underlying shale formation. The clay core
dams are keyed into the shale. The average tailings depth is now
12 m (40 ft) and is expected to increase to 18 m (60 ft) by the
end of the projected milling operation in 1996 (Ha85). Water is
sprayed over 8 ha (19 acres) of the dry tailings during warm
weather to control dust (Ha85). Dry beaches account for 69 ha
(172 acres) of the total, whereas 38 ha (96 acres) are covered
with tailings solution. The remaining 8 ha (21 acres) of exposed
tailings are saturated with solution (EPA85). The current amount
of tailings under management is 11.5 x 10 tons) (Ha85).
The radium-226 activity for the solid tailings, combined
sands, and slimes is about 160 pCi/g (Ha85). An earlier EPA
report estimated the radium-226 content at 420 pCi/g (EPA83b) .
The radium-226 activity of the tailings liquid is approximately
200 pci/liter (Ha85).
The Pathfinder Gas Hills Mill is in a remote location away
from permanent habitation. The nearest residence is
approximately 19 km (12 mi) away (Ha85). A 1983 survey also
indicates no population within a 5-kit (3-mi) radius of the
tailings piles (PNL84).
In 1963 a flood at the mill site resulted in the release of
7 7
8.7 x 10 liters (2.3 x 10 gal) of impounded tailings solution
to the environment. As a result of this incident, the tailings
impoundment was enlarged to its current capacity. The existing
system, with a minimum of 1 m (3 ft) of freeboard, is estimated
8 8
to provide 12.6 x 10 liters (3.3 x 10 gal) of emergency
storage.
4-30
-------
Western Jfuclear Split Rock Mill
Western Nuclear's Split Rock Mill is located 3.2 km
(2 miles) north of Jeffrey City, Wyoming. This mill began
operation in 1957 and has been on standby status since June
1981. When running at capacity, the mill produced 935 tons of
yellowcake per year (Bo85). Maximum throughput was about
1700 tons of ore per day (NRC84). The ore grade has ranged from
0.15 to 0.30 percent U_0R in the past and is expected to
range from 0.05 to 0.15 percent in the future (NRC84). Milling
operations involve semi-autogenous grinding, an acid leach, and
solvent extraction. The mill usually stockpiles 2000 to
5000 tons of ore when it is operating. Two 8-m (25-ft) diameter
bins are used to store fine ore.
The tailings generated by the Split Rock Mill are contained
in a single tailings impoundment that is enclosed by an earthen
dam. The tailings impoundment has a surface area of 62 ha
(156 acres), and the maximum depth is about 29 m (95 ft)(EPA85,
Bo85). Currently, 38 ha (94 acres) of the impoundment are
covered by tailings solution (EPA85). There are 17 ha (43 acres)
of dry tailings in the impoundment (EPA85). Tailings are
discharged from the crest of the dam? the point of discharge is
periodically moved along with the crest. Western Nuclear uses a
sprinkler system to control dusting from the pond during
nonfreezing months. Wind fences, chemical sprays, and vegetation
seeding are also used to control dusting. About 12 x 10 tons
of commingled tailings are under management (NRC84).
The average radium-226 concentration of the tailings is
approximately 100 pCi/g (99.5 ± 42 pCi/g) (Bo85). Radium-226
values in the sands and slimes were determined to be 63 pCi/g and
87 pCi/g, respectively (Bo85). Western Nuclear has used charcoal
canisters to measure radon-222 flux from the tailings. The
average flux measurements, made in 1977-1978, were 2+1.1
pCi/m s (Bo85). An earlier EPA report indicated that 430 pCi/g
of radium-226 was present in the tailings (EPA83b).
A 1983 population survey indicated that three people lived
between 0.5 and 1.0 km (0.3 and 0.6 mi) from the tailings
impoundment (PNL84). This survey further indicated that
30 people resided within 2 and 3 km (1.2 and 1.9 mi) of the
tailings impoundment, 697 people within 3 and 4 km (1.9 and
2.5 mi), and 176 people within 4 and 5 km (2.5 and 3.1 mi)
(PNL84).
4-31
-------
Umetco Gas Hills Mill
The Umetco Minerals Gas Hills- Mill is located in the
southeastern portion of the Wind River Basin of Wyoming. The
mill is about 95 km (60 mi) west of Casper in an area of rolling
hills interspersed with relatively flat areas. The mill is
currently on standby status.
An acid-leach system (RIP-Eluex system) is used to recover
uranium. Recycled solution from the impoundment system is used
to wash sands -after sand-slime separation. Additional pond
decant solution is used for tailings dilution. The mill began
operation in early 1960 with a capacity of about 1100 tons per
day? in January 1980, the capacity was increased to 1400 tons per
day. In June 1983, milling of mined ore was temporarily
curtailed, and only the heap leach facility waskept in
operation. During milling operations, a 2-month stockpile of ore
is maintained at the mill (Wo85). This amounts to 85,800 tons
when the mill is operating at capacity.
During the anticipated total active life of the project
(1960 to 1986), about 13 x 10 tons of mill tailings will have
been produced. The retention capacity [7.6 x 10 t (8.4 x
10 tons)] of the mill's original above-grade tailings
impoundment has been reached, and since January 1980, tailings
have been discharged to a depleted open-pit mine (A-9 Pit), which
has a capacity of 2.5 x 10 tons. This has an area of 10 ha
(25 acres), is clay-lined on the bottom, and has an in-pit
dewatering system. The A-9 Pit has an exposed dry tailings beach
area of about 6 ha (14 acres) (EPA85). The maximum height of the
embankment of the original above-grade tailings impoundment (and
expansions) is about 14 m (45 ft), This impoundment has a
surface area of 60 ha (151 acres), all of which is dry, and
contains 5.8 x 10 t (6.4 x 10 tons) of commingled tailings
(EPA85, Wo85). The inactive tailings area, which has not been
used since January 1980, is currently in a preliminary phase of
reclamation. The inactive impoundment has been covered with an
average thickness of 1.2 m (4 feet) of overburden (Wo85). The
tailings are reported to contain 310 pCi/g of radium-226
(EPA83b). The evaporation area consists of three ponds with a
combined surface area of 8 ha (20 acres).
An EPA report estimates the radium-226 content of the
tailings to be 310 pCi/g (EPA83b). No measurements of radon-222
flux from the tailings impoundment have been made at this site
(Wo85).
4-32
-------
The area is sparsely populated. A 1983 survey indicated no
people living within a 5-km (3-mi) radius of th'e tailings
impoundment (PNL84). Average annual precipitation is 25 cm
(10 in.) ,-and evaporation is 17 cm (42 inches) (-EPA83b) .
Under the current reclamation plan, Umetco is committed to
provide a uniform cover of 0.3 m (1 ft) of clay and 2.6m
(8,5 ft) of overburden over the entire tailings area. This will
require about 210,000 m £7.5 x 10 ft ) of clay, at a cost of
$1,129,000, and 1.8 x 10 m (65 x 10 ft ) of overburden,
at a cost of $1,840,000 (NRC84). When the cost of revegetation
is added, the basic materials-needed for the reclamation program
will cost about $3,800,000.
Umetco also operates a heap leach facility in the mill area
at its Gas Hill site. The water used in the process
[1.7 liters/s (27 gal/min)] is taken from a nearby tailings area,
and U 0 is recovered from high-grade leach liquor by a
solvent-extraction process. The organic phase is pumped to the
mill circuit. Heap leach pads cover about 9 ha (22 acres) at
this site (EPA85).
Rocky Mountain Energy Mi11
Rocky Mountain Energy's Bear Creek Mill is part of a uranium
project that includes open-pit mining operations in the Powder
River area of Converse County, Wyoming, about 72 Km (45 miles)
northeast of Casper. The operation, which was dedicated in
September 1977, has a capacity of 2000 tons of ore per day
(NRC84). The U30g content of the ore ranges from less than
0.1 to 1.0 percent (NRC84). Ore is stockpiled' at the mill on an
8-ha (20-acre) pad; approximately 66,000 tons are currently on
hand (Me85). The mill is currently operating at about 20 percent
of its capacity and is milling stockpiled ore. It is likely that
the mill will go to standby status sometime during the second
quarter of 1986.
Mill tailings are contained in a single tailings impoundment
enclosed by an earthen dam. The surface area of tailings is '
48 ha (121 acres), of which 18 ha (45 acres) are covered with
tailings solution and 21 ha (53 acres) are dry tailings beaches
.(Me85). A portion, 13 ha (32 acres), of the pile has been
covered with 30 cm (1 foot) of soil to control fugitive dust
(Me85).
4-33
-------
No measurements of radon-222 flux from tailings have been
made .at this site. The,radium-226 content of the Bear Creek
tailings is reported to be 420 pCi/g (EPA83b) . - • '•
A 1983 survey indicated no one living within a 5-km (3.1-mi)
radius of the tailings pile (PNL84) . The 'annual precipitation in
the area is about 30 cm (12 in.)/ and annual evaporation is 102
cm (40 in,) (EPA83),
Pathfinder Shirley Basin
The Pathfinder Mines Corporation Shirley Basin Uranium Mill
is located in an area of plains and rolling -hills about 72 km
(45 mi) south of Casper, Wyoming. The mill, which began
operation in 1971, uses semiautogenous grinding, leaching, and
ion exchange. Current mill capacity is 1600 t (1800 tons) of ore
per day (NRC84), The mill is currently active and has a
throughput of 900 t (990 tons) per day (Si85). Operations are
projected to continue through 1994.
Tailings.are contained' in a single onsite tailings
impoundment that is contained above grade by a single-sided
earthen- retention dam 18 m (60 ft) high. The surface area of the
tailings impoundment is 10 ha (261 acres), of which 72 ha
(179 acres) are covered with ponded tailings solution (EPA85),
Twenty-four hectares (60 acres1 are dry beaches. The impoundment
contains 5.8 x 106 t (6.4 x 10 tons) of tailings (NRC85). The
tailings are reported to contain 540 pCi/g of radium-226
(EPA83b).
A 1983 survey of the population in the vicinity of the
Pathfinder Shirley Basin Mill indicated no inhabitants living
within 3 km (1.9 mi.) of the tailings impoundment (PNL84). Six
people, who lived between 3 - and- 4 km (1.9 and 2.5 mi) from the
impoundment, were the only inhabitants within 5 km (3.1 mi)
(PNL84).
/Minerals Explo_ration_Mill '
The Minerals Exploration Company's Sweetwater Mill is
located within the Red Desert portion of Wyoming's Great Divide
Basin, about 64 km (40 mi) northwest of Rawlins. The mill, which
began operations in early 1981, has been inactive since November
1981 and is currently on standby status. The capacity of the
mill is 2700 t (3000 tons) per day. The average ore grade
processed to date has been 0.03 percent U,0_ (Hi85). ;
j a
4-34
-------
All tailings have been placed in a single tailings
impoundment. It is a lined (synthetic) impoundment that is
partially below grade and has earthen embankments. The total
surface area of the tailings is 15 ha (37 acres)* (EPA85) , With
the exception of a 3-ha (7-acre) delta at the tailings discharge
point, the tailings are covered by. tailings solution.
Approximately 0.9 x 10 t (1 x 10 tons) of tailings have
been generated and are contained in this impoundment. Flans call
for a second cell to be constructed to the north,of the existing
cell if-additional, capacity is required. The/Sweetwater tailings
disposal system is a phased-disposal facility that has gone
through several iterations during development. The impoundment
was originally designed to be square, below-grade, and divided
into four cells. The Minerals Exploration Company reports that
measurements of radon-222 flux made on the tailings solids ranged
from 90 to 100 pCi/m s (Hi85).
A 1983 survey indicated no population living within 5 km
(3.1 mi.) of the tailings impoundment (PNL84). The annual
precipitation in the area is 15 to 20 cm (6 to 8 in,), and annual
evaporation is 102 to 178 cm (40 to 70 in.) (EPA83).
4.3 Population Within 5 km (3.1 mi) of Existing Tailings
Impoundments
A 1983 estimate indicated that 12,824 persons lived within
5 km (3.1 mi) from the centroid of the tailings impoundments at
the active and standby sites (PNL84). No one lived within 0.5 km
(0.3 mi), whereas 173- people lived between 0.5 and 1 km (0.6 and
1.2 mi). Nobody lived within 5 km (3.1 mi) of four of these
mills, all of which were in' Wyoming. A summary of this
information by state and by mill is presented in Table 4-3. '-By
comparison, a population survey conducted by EPA in 1985 showed
that there were 11,483 people living within 5 km (3.1 mi) of
these tailings impoundments. This more recent survey, which was
based on interpretation of aerial photographs, indicated that no
one lived within 5 km (3.1 mi) of six of these tailings
impoundments. The results of this later survey are presented in
Table 4-4.
4-35
-------
Table 4-3. Estimate of the population living within 0 to 5 km from the
centroid of tailings impoundments of active and. standby mills in 1983
State/Owner 0.0-0.5
Colorado
Cotter
Umetco
New Mexico
Sohio
United
Nuclear
Anaconda
Kerr-MoGee
Homestake
Texas
Chevron
Utah
Umetco
Rio Algora
Atlas
Plateau
Resources
Washington
Dawn
Western
Nuclear
Wyoming
Pathfinder
(Gas Hills)
Western
Nuclear
Umetco
Rocky Mt.
Energy
Pathfinder
(Shirley
Basin)
Minerals
Exp.
Total
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.5-1.0
0
147
0
0
0
0
0
12
0
8
0
0
3
0
0
3
0
0
0
0
173
1.0-2.0
0
193
0
25
6
0
190
42
0
105
9
0
93
0
0
0
0
0
0
0 .
663
2.0-3.0
184
6
0
52
136
1
104
33
0
154
33
0
157
0
58
30
0
0
0
0
948
3.0-4.0
2767
3
42
85
666
0
45
81
0
32
1094
0
96
32
0
697
0
0
0
0
5640
4.0-5.0
2982
0
124
150
99
0
57
285
8
44
1225
171
62
17
0
176
0
•o
0
0
5400
Tota!
5933
349
166
312
907
1
396
453
8
343
2361
171
411
49
58
906
0
0
0
0
12,82^
PNL84.
4-36
-------
Table 4-4. Estimate of the population living within 0 to 5 km from ths
centroid of tailings iitpoundments of active and standby mills in 1985 ^
State/Owner 0.0-0.5
Colorado
Cotter
Umetco
New Mexico
Sohio
United
Nuclear
Anaconda
Kerr-MoGee
Homestake
Texas
Chevron
Utah
Umetco
Rio Algoro
Atlas
Plateau
Resources
Washington
Dawn
Western
Nuclear .
Wyoming
Pathfinder
(Gas Hills)
Western
Nuclear
Umetco
Rocky Mt.
Energy
Pathfinder
(Shirley
Basin)
Minerals
Exp.
Total
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.5-1.0
0
0
0
0
0
0
0
12
0
0
0
0
0
0
0
0
0
0
0
0
12
1.0-2.0
0
14
0
34
0
0
267
108
0
12
9
0
119
0
0
6
0
0
0
0
569
2.0-3.0
90
0
10
90
67
0
118
104
4
16
24
9
253
0
0
48
0
0
0
0
833
3.0-4.0
1693
0
60
105
574
0
41
253
8
186
923
115
75
56
0
737
0
0
0
0
4826
4.0-5.0
3029
0
161
213
146
0
80
313
4
88
632
100
71
48
0
358
0
0
0
0
5243
Total
4812
14
231
442
787
0
506
790
16
302
1588
224
518
104
0
1149
0
0
0
0
11,483
EPA85.
4-37
-------
REFERENCES
An84
Bo85
DOES 2
DOES 4
ELP85
EPA83a
EPA83b
EPA85
EPA86
Ge85
Andrew R. E., "Uranium Mills Program, Department, of
Social and Health Services, State of Washington",
Correspondence with PEI Associates, Inc., December
1984.
Bogden 'G., Western Nuclear, Inc., Correspondence with
PEI Associates, Inc., January 1985.
Department of Energy, Office of Defense Waste and
Byproducts Management, "Commingled Uranium Tailings
Study," DOE/DP-0011, Washington, D.C., June 30, 1982.
Department of Energy, "United States Mining and Milling
Industry", DOE/2-0028, May 1984.
Electric Light and Power, "Uranium Industry Not Viable
in 1984, Determines DOE's Harrington", November 1985,
Environmental Protection Agency, "Regulatory Impact
Analysis of Environmental Standards for Uranium Mill
Tailings at Active Sites", EPA 520/1-82-023, March
1983.
Environmental Protection Agency, "Final Environmental
Statement for standards for the Control of Byproduct
Materials from Uranium Ore Processing (40 CFR 192}",
(EPA 520/1-83-008-1), Office of Radiation Programs,
Washington, D.C., September 1983.
U.S. Environmental Protection Agency, "Draft
Document-Estimates of Population Distributions and
Tailings Areas'Around Licensed Uranium Mill Sites",
Office of Radiation Programs, November 1985.
Environmental Protection Agency, "Technical information
submitted by letter dated January 27, 1986 from T. R
Strong, Head, Radiation Control Section, State of
Washington, Department of Social and Health Services,
Olympia, Washington to Sheldon Meyers, Acting
Director", Office of Radiation Programs, U.S. EPA.
Gerdemann F. w., "Correspondence with PEI Associates,
Inc.", Plateau Resources Limited, 'January 1985.
4-38
-------
Ha85
H185
Jo80
Ki80
Kr85
Ma85
Mc85
Me85
NM85
NRC80
NRC84
PNL84
Si85
Hardgrove T., Pathfinder Mines Corporation,
Correspondence with PEI Associates, Inc., January
1985,
Hill C., Minerals Exploration Company, Correspondence
with PEI Associates, Inc., January 1985.
Johnson T. D., Sohio Western Mining Company Tailings
Dam, in: First International Conference on Uranium
Mine Disposal, C. 0. Braver, editor, Society of Mining
Engineers of AIME, New York, 1980.
King K. and Lavander R., Design and Construction of
Uranium Disposal Facilities for the Panna Maria
Project, Texas, Presented at First International
Conference on Uranium Mine Waste Disposal, Vancouver
British Columbia, May 19, 1980.
Kray E., State of Colorado, Correspondence with PEI
Associates, Inc., January 1985.
Manka M., Cheveron Resources Company, Panna Mana
Project, Correspondence with PEI Associates, Inc.,
February 1985.
McClusky J., Cotter Corporation, Correspondence with
PEI Associates, Inc., February 1985,
Medlock R., Bear Creek Uranium, Correspondence with PEI
Associates, Inc., February 1985.
State of New Mexico, Radiation Protection Bureau,
Correspondence with PEI Associates, Inc., January
1985.
Nuclear Regulatory Commission, Final Generic
Environmental Impact Statement on Uranuim Milling,
NUREG-0706, September 1980.
Nuclear Regulatory Commission, Office of State
Programs, Directory and Profile of Licensed
Uranium-Recovery Facilities, (NREG/CR-2869),
Washington, D.C., March 1984.
Pacific Northwest Laboratory. Estimated Population
Near Uranium Tailings. (PNL-4959). January 1984.
Simchuk G, J., Pathfinder Mines Corporation,
Correspondence with P1I Associates, Inc., February
1985.
4-39
-------
WA86 ' '• ~State of Washington,' Department of Social and Health
Services, Uranium Mills Program, Correspondence with
PEI Associates, Ina. January 1986.
Wo85 Wong T., Umetco (Union Carbide), Correspondence with
PEI Associates, Inc.-, February 1985.
4-40
-------
. Chapter 5: INDUSTRY.RADON-222 EMISSION ESTIMATES
5.1 Introduction
This chapter presents a -discussion of ,the methodology used
to estimate the quantity of radon-222 emitted from tailings
impoundments and evaporation ponds at licensed uranium mills. As
mentioned in Chapter 3, ore storage and' milling operations emit
relatively low amounts.of radon-222 compared with the amounts
emitted by tailings impoundments. Mills that are on standby
generate almost no radon-222 other than that from their tailings
impoundments. The quantity of radon-222 emitted annually from
each site is estimated both for current conditions (i.e.,
fraction of tailings area with current water cover) and for
anticipated future conditions (i.e., dry tailings). Water cover
and tailings moisture content have a major influence in
controlling the amount of radon-222 that is released; therefore,
dry conditions must be considered in the determination of the
potential maximum amount of radon-222 that could be emitted .
(i.e., future conditions). Emissions are estimated for each
tailings impoundment and evaporation pond at each licensed
uranium mill except the six mills that have already initiated
decommissioning activities and are subject to other Federal
standards,
5.2 Estimat ing Emis si on s
Estimates' of radon-222 'emissions are based on an assumed
emission rate that equals the specific flux of 1 pci
radon-222/m s per pCi radium-226/g tailings for dry tailings
times the dry area. ,It has been assumed that tailings that are
either saturated with or covered by. tailings solution do not emit
radon-222. These assumptions were applied to the site-specific
data to estimate emissions.
2
For the specific flux of 1 pCi radon-222/in s per pCi
radium-226/g to be used, both the dry surface area and the
radium-226 concentration of the tailings impoundment must be
known. The surface area of existing tailings impoundments has
been documented previously (EPA83, NRC80). The uranium industry,
however, has changed significantly since the compilation of these
earlier data bases, as demonstrated by the drop in uranium
production (and thus tailings generation),- the initiation of
decommissioning activities at six mills, and the drying of
tailings impoundments at others because they are not in use. To
obtain an updated data base, EPA's Office of Radiation Programs
completed a study entitled "Estimates of Population Distribution
and Tailings Areas Around Licensed Uranium Mill Sites" (EPA85).
As discussed in Chapter 4, this document summarizes the results
5-1
-------
of a survey the EPA conducted of 22 uranium mill sites in 1985.
This survey produced estimates of the total surface area of the
tailings impoundments, which includes the area covered by
tailings solution, the saturated "area, and the dry surface area
of tailings. The same information was also compiled for
evaporation ponds. These estimates of tailings areas were used
as the basis for estimating radon-222 emissions in this report
(See Table 4-2 in Chapter 4). This tabulation includes a
listing, by state, of each known tailings impoundment and
evaporation pond at the licensed mills. The type of impoundment
is also identified, i.e., earthen dam, sand tailings dam, or
below-grade impoundment. The status of each impoundment (active,
standby, or at capacity) is shown, and estimates of the average
radium-226 content in the tailings are listed for each mill. The
total impoundment and evaporation pond area is 1570 ha (3882
acres), over 50 percent of which is dry. Only four mills with
seven tailings impoundments are currently active? 32 tailings
impoundments are on a, standby basis or have been filled to
capacity,
Concentrations of radium-226 present in tailings vary from
site to site. The EPA's Final Environmental Impact Statement for
Standards for the Control of Byproduct Materials from Uranium Ore
Processing listed radium-226 concentrations in tailings for each
licensed mill (EPA83). These values were used in this report to
estimate emissions of radon-222.
Emissions were estimated for two conditions: current
water-cover conditions (as of late summer of "1985) and after
drying. Under current conditions, it was assumed that radon-222
was emitted only from dry areas of the tailings impoundments or
evaporation ponds. In the estimates of radon-222 emissions, a
specific flux of 1 pCi Rn-222/m s per pCi of Ra-226 per grain of
tailings was used for dry tailings and a specific flux of zero,
for -ponded and saturated tailings. As discussed in Chapter 3,
this assumed specific flux calculation has been previously
documented and used (NRC80,- SPAS3) . This average conservative
flux, which provides an approximate estimate of emissions, is
useful when the many other factors affecting the flux, such as
tailings moisture content, •.diffusion factors, and emanation
coefficients', are not well known. The following calculation was
used to estimate emissions from dry areas:
2 2
kCi Rn-222/y = dry area, m x 1 pCi.Rn-222/m s per pCi
Ra-226/g x pCi Ra-226/g x 3.15 x 107s/y x 10~15kCi/pCi
The radium-226 concentration"in picocuries/gram of tailings is
shown in Table 5-1. For estimates of emissions after drying, the
total tailings area was substituted for the dry tailings area in
5-2
-------
Table 5-1, Summary of radon-222 emissions from uranium
mill tailings iitpoundraents
Owner/lnqpounctaent
Radon-222 emissions CkCi/y]
Current
conditions
(flux = 1}
Current
conditions,.
(factored)( '
After
drying
Colorado
Cotter Corp.
Primary
Secondary
. Urnetoo
Inpoundraents 1 & 2
Impoundment 3
Sludge pile
Evaporation pond
New Mexico
Sohio
Ir-Bar
United Nuclear
Churchrock
Anaconda
Bluewater 1
Bluewater 2
Bluewater 3
Evaporation ponds
Kerr-McGee
Quivira 1
Quivira 2a
Quivira .2b
Quivira". 2c
Evaporation ponds
Homestake
" Homestake 1
Homestake 2
Texas
Chevron
Parma Maria
0,4
3.0
3.8
1.8
1.2
0.9
2.9
2.4
19
3.7
1.9
3.8
15
' '4.7
2.0
2.1
7. -5
5.4
1.8
0.9
0.5
3.0
3.9
1.8
1.2
1.0
3.9
3.2
19
3.7
1.9
4.2
17
5.6
2.0
2.2
7.7
5.8
1.8
1.0
8.4
3.1
4,
2,
.0
.0
1.2
1.0
8,2
5.5
19
3.7
1.9
13
21
8.3
2.2
2.4
29
10
2.2
3.1
5-3
-------
Table'5-1. Summary of radon-222 emissions from uranium
• mill, tailings iirpoundments (continued)
Owner/InpourKJment '
Utah
Umetco
White Mesa 1
White Mesa 2
White Mesa 3
Rio Algom
1
2
Atlas
Moab
Radon-222
- . Current
, • conditions
• '(flux - 1) l }
1.5
2.0
0.6
2.7
• 1.1
6.2
emissions fkCi/v)
Current
conditions,.
(factored) ( }
1.6
• 2,1
0.6.
2.8
1.2
6.3
After
drying
2.1
•2.7
2.4
3.1
2.3
10
Plateau. Resources. • •.
Shootaring Canyon
Washington
Dawn Mining
'Ford l, 2, 3
Ford 4
Western Nuclear
Sherwood
Evaporation pond
Wyoming
Pathfinder
Gas Hills l
Gas Hills 2
Gas Hills 3
Gas Hills 4
Western Nuclear
Split Rock
Umetco
Gas Hills
A-9 Pit
Leach pile
Evaporation ponds
0.1
2.9
0.3
1.8
6.4
2.1
0.1
0.6
2.4
6.0
0.6
0.9
0.1
2,9
0.3
1.8
6.4
2.3
0.1
0.7
2.7
6.0
0.7
0.9
0.2
2.9
0.9
2.4
0.4
6.6
2.9
1.2
4.8
8.6
6.0
1.0
0.9
0.8
5-4
-------
Table 5-1. Summary of radon-222 emissions from uranium
mill tailings iitpoundments (continued)
Radon-222emissions fkCi/y)
- Current Current
conditions . conditions. . After
Owner/Iitpoundment (flux = 1) * ' (factored) ( ' drying
Rocky Mountain Energy
Bear Creek 2.8 3.2 6.5
Pathfinder
Shirley Basin 4.1 4.6 18
Minerals Exploration
Sweetwater 0.2 0.2 , 1.3
Totals 129 137 238
fal 2
1 ' Based on a specific flux of l pel Rn-222/m s per pCi Ba-226 per
gram of tailings for dry areas and a flux of zero for ponded and
wet areas.
* ' Specific flux of 0.3 pCi Rn-222/ra s per pCi Ra5226 per gram of
tailings for wet tailings area, 1 pCi Rn-222/m s per pCi Ba-226
per gram of tailings for dry area, and zero for ponded areas.
5-5
-------
the preceding calculation. The results of the calculations for'
impoundments at each mill considered in this report are presented
in Table 5-1. Total radon-222 emissions are estimated to be 129
kci/y under current conditions and to rise to about 238 kCi/y
after all the areas have dried.
2
Although a specific flux of 1 pCi radon-222/m s per pCi
radium-226/g tailings is commonly used and recommended by NRC
(NRC85) when specific data are lacking, alternative methods of
flux estimation are available. One alternative method is to
assume that the radon-222 flux from dry areas is 1 pCi
radon-222/m per pCi radium-226/g; zero from ponded areas, as
previously discussed; and 0.3 pCi radon-222/m per pCi
radium-226/g for saturated areas instead of zero (NRC80).
Estimates of radon-222 emissions made by using this method of
calculation indicate 137 kCi/y, as shown in Table 5-1. '
Other alternative methods of estimating radon-222 emissions
require site-specific data. As discussed in Chapter 3,
information on radium-226 and on the moisture content/ porosity,
density, and emanating power of tailings can be substituted into
the diffusion equation to estimate a site-specific flux for
each area of a tailings impoundment. An attempt was made to
complete such an estimate for each mill in a recent study
(PEI85). That study indicated that using a specific flux of
1 pCi radon-222/m s per pCi radium-226/g tailings for dry areas
and zero for ponded and saturated areas resulted in a
conservative (high) estimate of radon-222 emissions. Total
emissions estimated by using the assumed specific flux were about
twice as high as those made using site-specific information.
The site-specific information was based on a number of
assumptions, however, as not all of the necessary tailings data
are currently available at licensed mill sites. Also, estimating
radon-222 emissions from tailings after drying would require
additional assumptions regarding their physical characteristics.
The current data base is_ not sufficient to allow more accurate
calculation of emissions based 'on site-specific tailings
characteristics; therefore, the specific flux (1 pCi
radon-222/m s per pCi radium-226/g) for dry areas and zero for
ponded and saturated areas were used in this report. The
emission estimates presented herein may be conservative compared
with estimates made by other means, but insufficient' specific
data are available to draw any definite conclusions.
5-6
-------
REFERENCES
EPA83 Environmental Protection Agency, "Final Environmental
Impact Statement for Standards for the Control by
Byproduct Material from Uranium Ore Processing", EPA
520/1-83-008-1, Office of Radiation Programs, U.S. EPA,
Washington, D.C., September 1983.
EPA85 Environmental Protection Agency, "Estimates of
Population Distribution and Tailings Areas Around
Licensed Uranium Mill Sites (Draft)" Office of
Radiation Programs, Las Vegas Facility, Las Vegas,
Nevada, 1985.
NRC80 Nuclear Regulatory Commission, "Final Generic
Environmental Impact Statement on Uranium Milling",
NUREG-0706, September 1980.
NRC85 Nuclear Regulatory Commission, "Methods for Estimating
Radioactive and Toxic Airborne Source Terms for Uranium
. Milling Operations", NUREG/CR-4088, June 1985.
PEI85 PEI Associates, Inc., "Radon-222 Emissions and Control
Practices for Licensed Uranium Mills and Their
Associated Tailings Piles", EPA Contract 68-02-3878,
June 1985.
5-7
-------
Chapter 6: BASELINE INDUSTRY RISK ASSESSMENT
6.1 Introduction
This chapter contains an assessment of the risks of fatal
lung cancer caused by radon-222 emissions from uranium tailings
impoundments. Two measures of risk are presented: risks to
nearby individuals and risks to the total population. The first
measure refers to the estimated increased lifetime risk imposed
upon individuals who spend their entire lifetime at a location
near a tailings impoundment, where the predicted radon-222
concentrations are highest. Risks to nearby individuals are
expressed as a probability, i.e., 0.001 (1/1000) or 1E-3. This
means that the increased chance of lung cancer in an exposed
person's lifetime is 1 in 1000. Estimates of risks to nearby
individuals must be interpreted cautiously, as few people
generally spend their whole lives at such locations. The second
measure, risks to the total population, refers to all people
exposed to radon-222 emissions from all of the licensed uranium
mill tailings impoundments'. Expressed in terms of the number of
fatal cancer cases caused by the amount of radon-222 emitted
annually, this provides a measure of the overall public health
impact.
An epidemiological approach is used to estimate risks which
are based on relative risk from exposures to radon-222 expressed
in working level months (WLM). • The WLM is in turn related to a
concentration of radon-222 decay products, expressed in
picocuries/liter. Risks are directly proportional to emissions;
therefore, one can estimate the deaths due to radon-222 in the •
future by assuming that new tailings impoundments will be
located in the same general area of existing impoundments.
6.2 Risk Estimates
6.2.1 Nearby Individuals
Individual risks are calculated by using the life table
methodology described by Bunger et al. (Bu81). The relative
risk projections used for lifetime exposure were based on
relative risk coefficients of 1 and 4 percent per WLM for the
radiation-induced increase in lung cancer. See discussion in
Section 2.3,
6-1
-------
The AIRDOS-EPA and DARTAB codes and an assumed radon-222
decay product equilibrium fraction determined as shown in Table
2-4 were used to estimate the increased chance of lung cancer
for individuals living near a tailings impoundment and'receiving
the maximum exposure. Results are shown in Table 6-1. The
maximum risk of 2 percent (21-2) occurs at Anaconda, New Mexico
at a distance of 1.5 km from the center of the impoundment.
6,2.2 Regional Population ' '
Collective (population) risks for the region are calculated
from the annual collective exposure (person WLMJ for the
population in the assessment area by a computerized methodology
known as AIRDOS-EPA (Mo79). An effective equilibrium fraction
of 0,7 is presumed because little collective exposure takes
place near the mill.
In this study, population data in the 0- to 5-km and 5- to
80-km regions around each mill were obtained from an earlier
detailed study by EPA and, are summarized in Chapter 4 (EPA83).
Collective exposure calculations expressed in person WLM were
performed for each mill by multiplying the estimated
concentration in each annular sector by the population in that '
sector. The parameters used in the AIRDOS-EPA code are shown in
Table 6-2. An approximate emission height of 1 meter was
assumed in all cases. Meteorological parameters from selected
weather stations were used for each mill. Included in this
table are the resulting exposure for that mill based on the
emission rate and the population near the mill. Estimates of
the number of fatal cancers corresponding to this exposure were
made by using a risk factor of 3 percent (760 deaths per
10 person WLM). These estimates were then multiplied by
1520/760 or 380/760 to adjust to the risk coefficients of 4 and
1 percent, respectively (1520 and 380 deaths per
10 person WLM). A summary of the estimated fatal cancers due
to radon-222 from existing tailings impoundments is shown in
Table 6-3 under the current .(partially wet and partially dry)
conditions and under entirely dry conditions.
These estimated health effects for the 20 mills considered
compare favorably with the previous EPA study (EPA83) for
uranium byproduct materials. In the earlier study, a model
plant approach was used at 26 sites, and 0.38- and 2<.l deaths
were estimated for the 0-5 km and 5-80 km regions, respectively,
for post-operational (dry) conditions (Page 6-14 in EPA83).
6-2
-------
Table 6-1. Estimated risk of fatal lung cancer from maximum exposure
for an individual living near tailings impoundment
State
Mill -owner
Maximum lifetime .,,
risk to individual^ '
Distance^3'
meters
Colorado Cotter 3E-4 (8E-5)
Umetco 8E-3 (2E-3)
New Mexico Kerr-MoGee . •1E-2 (2E-3)
Anaconda 2E-2 (5E-3)
United Nuclear . 2E-3 (4E-4)
Homestake 6E-3 (1E-3)
Sohio 7E-4 (2E-4)
Texas Chevron 2E-3 (4E-4)
Utah • Umetco
RioAlgom
Atlas
Plateau Res.
Washington Dawn
Western Nuclear
Wyoming Minerals Exploration 4E-6 (9E-7)
Pathfinder
Gas Hills . • 2E-3 (6E-4)
Shirley Basin 9E-5 (2E-5)
' Rocky Mt. 9E-5 (2E-5)
Umetco 1E-4 (3E-5)
Western Nuclear 2E-3 (5E-4)
2E-4
3E-3
4E-3
2E-5
3E-3
2E-4
(6E-5)
(7E-4)
(1E-3)
(4E-6)
(6E-4)
(5E-5)
2500
750
2500
1500
1500
1500
3500
750
4500
750
1500
4500
750
4500
30000
2500
15000
15000
15000
750
(a)
(b)
Distance from center of a homogenous circular equivalent
impoundment.
The value in the.first column is based on a risk factor of 1520
deaths/10 person WLM, and the values in parentheses are based on
380 deaths/10 person WIM.
6-3
-------
Table 6-2. AIRDOS-EPA code inputs and estimated risks
AIRDOS code inputs
State
Colorado
New Mexico
Texas
Utah
Washington
Wyoming
Company
Cotter
Umetco
Kerr-McGee
Anaconda
United Nuclear
Homestake
Sohio
Chevron
Umetco
RioAlgom
Atlas
Plateau Res.
Dawn
Western Nuclear
Minerals Explora-
tion
Pathfinder
Gas Hills
Shirley Basin
Rocky Mt.
Umetco
Western Nuclear
Atmospheric
mixing
depth (m)
700
700
800
800
800
800
800
1000
700
700
700
700
600
600
700
700
700
700
700
700
Precipitation
(cm/y)
38.8
40.2
29.1
• 27.0
29.1
27.0
27.0
76.6
22.2
22.2
22.1
25.2
54.2
54.2
27.3
28.0
29.6
35.4
33.9
28.0
Ambient
temperature
( C)
10
10
11
11
.11
11
11
21
13
13
13
13
9
9
6
6
6
6
6
6
. Approximate
impoundment
area Deaths/year3
(ha)
10
50
200
100
30
60
20
10
40
20
40
2
40
30
3
70
20
20
80
20
0-5 km
. 1.2E-2
2 . 3E-2
2 . OE-4
5.0E-2
1.7E-3
1.5E-2
7.6E-4
6.7E-4
1.8E-5
2 . 2E-3
1.8E-2
3 . 3E-5
3.5E-3
8.7E-5
-
1.9E-3
-
• 1.4E-3
5-80 km
5.2E-2
2.1E-2
1.9E-1
2.7E-1
1.8E-2
8.6E-2
4.3E-2
2.6E-2
6.1E-3
4.6E-3
1.4E-2
6.4E-6
3.0E-2
1.2E-2
8.3E-5
2.9E-3
4.6E-3
3.7E-3
2.2E-3
5.2E-4
(b)
Based on 760 deaths per 10 person WLM.
Zero population in the 0-5 km region.
-------
Table 6-3. Summary of regional health effects from existing
tailings iatpoundments
Condition of Emissions* ^ Committed fatal cancers per year ';
tailings (kCi/y)
0-5 ton 5-80 km 0-80 km
Current 129 0.3 (0.1) 1.6 (0.4) 1.8 (0.5)
Ml dry 238 0.4 (0.1) 2.9 (0.7) 3.3 (0.8)
(a.) 2
k ; Based on radon-222 flux of 1 pCi/m per pCi of Ra-226 per gram
of tailings.
^ ' Values-in first column are based on 1520 deaths due to lung cancer
per 10 person WLM. The values in parentheses are based on
380 deaths per 10 person WIM.
6-5
-------
6.2.3 National
Radon-222 released from mills can be transported beyond the
80-km regional cutoff, A trajectory dispersion model developed
by NOAA (NRC79) has been used to estimate the national impact of
radon-222 releases. The model yields radon-222 concentrations
(in picocuries per liter) in air, which are then converted to
decay product exposures by assuming an effective equilibrium
fraction of 0.7. National annual collective exposures (person
WLM) are calculated for distances beyond the 80-km regional
limit for a total population of 200 million persons. This model
was used in a previous EPA study on byproduct material from
uranium ore processing (EPA83), Inasmuch as all- mills are still
in the same location, the results of this earlier study were
used to estimate current national health effects by ratioing the
estimated deaths to the current emission estimates and adjusting
for the revised risk factor ranges. .The calculations are shown
below and summarized in Table 6-4 ^ ' .
2.47 deaths x 129 kCi/y x 1520 =3.1 deaths/y
202.7 kCi/y 760
2.47 deaths x 129 kCi/y x 380 = 0.8- death/y
202.7 kCi/y 760
For the dry tailings condition with emissions of 238 kCi/y,
the corresponding values are 5.8 and 1.4 .deaths per year.
The 2.47 deaths from emissions of 202.7 kCi/y are.from
EPA's 1983 report and were based on a risk of 760 deaths
per 10 person WLM.
6-6
-------
Table 6-4. Summary of health effects beyond the 80-km region
from tailings impoundments
Condition of
tailings
Current
All dry
Emissions
fkCi/v)
129
238
Committed fatal
cancers_ per year
3.1 (0,8)
5,7 (1,4)
(a)
(a)
Values in first column are based on 1520 deaths due to lung
cancer per 10 person WLM.g The values in parentheses are
based on 380 deaths per 10 person WLM.
6-7
-------
The estimated health effects from existing impoundments is
shown in Table 6-5, This summary shows that about 3 fatal
cancers per year can be attributed to tailings impoundments in
their current conditions, and this could increase to 6 deaths
per year if the impoundments dried and emissions increased,
6.2.4 Risks from Mew Tailings Impoundments
Radon-222 emissions will not increase greatly until the
current impoundments reach capacity and new impoundments are
built. The need for new impoundments is directly related to
industry growth. The health effects caused by new impoundments
may be estimated by assuming a direct proportion of effects to
emissions. This procedure assumes that new impoundments will be
located in the same geographical area as the existing
impoundments and will have the same impact on surrounding
populations. Emissions from model new tailings impoundments are
estimated in Chapter 7 and will vary with the design and work
practice 'used.
6-8
-------
Table 6-5. Summary of fatal cancers from
current tailings impoundments
Condition
of tailinqs
Current
Dry
0-5
0
0
.3
.4
ton
(0.
(0.
1)
1)
Fatal
5-80
1.6
2.9
cancers per vear^ '
tan National
(0.
(0.
4)
7)
3.1
5.7
(0
(1
.8)
.4)
Total
4.9 (1.2)
9.0 (2.3)
Values in first column are based on 1520 deaths due to lung cancer
per 10 persongWIM. The values in parentheses are based on 380
deaths per 10 person WLM.
6-9
-------
REFERENCES
BU81
EPA83
Mo79
NRC79
Hunger B./. Cook J. R., and Barrick'.M.. K., "Life Table.
Methodology for Evaluating Radiation Risk: An
Application Based on Occupational Exposure", Health
Physics-40,.439-455, 1981. . -
Environmental Protection Agency, "Final Environmental
Impact Statement for Standards for the Control of
Byproduct Materials from Uranium Ore Processing (40,
CFR 192), Volume I", EPA 520/1-83-008-1, Office of
Radiation Programs, USEPA, Washington, D.C., 1983.
Moore R. E., Baes C. F. Ill, McDowell-Boyer L. M.,
Watson A. P., Hoffman F. O., Pleasant J. C., and
.Miller C. w., "AIRDOS-EPA: A Computerized Methodology
for Estimating Environmental Concentrations and Doses
to Man from Airborne Releases of Radionuclides",
ORNL-5532, Oak Ridge National Laboratory, Oak Ridge,
Tennessee, 1979,
Nuclear Regulatory Commission, "Draft Generic
Environmental Impact Statement on Uranium Milling,
Volume II", NUREG-0511, USNRC, Washington, D.C., 1979.
6-10
-------
Chapter 7: RADON-222 CONTROL TECHNIQUES
The reduction of radon-222 emissions at licensed uranium
mill sites is accomplished most effectively by reducing the
emissions from the tailings disposal area, Radon-222 emissions
from the balance of the milling circuit are relatively small and
are not easily controlled. At mills that are not operating and
are on a standby basis, almost all of the radon-222 emissions
come from the tailings disposal area.
This chapter is concerned with control techniques that can
be applied to licensed uranium mill tailings impoundments to
reduce radon-222 emissions. A general discussion of radon-222
control techniques is followed by more detailed discussion of
controls for existing and new impoundments.
Radon-222 emissions from uranium mill tailings can be
controlled most easily by keeping the tailings covered with
water or by covering them with earthen material. At new
tailings impoundments, phased disposal of the tailings or
continuous disposal- by dewatering and immediate covering
represent systematic ways of" controlling radon-222 emissions
using water or earth covers. Extraction of radium-226 from the
tailings, chemical fixation, and sintering of tailings have been
explored as means of reducing radon-222 emissions, but they have
not been applied on a large scale and they appear to be too
costly for general application (NRC80).
The applicability and effectiveness of .control techniques
depend primarily on the design of the mill tailings disposal
area and the mill's operating schedule. Thus, the control
techniques can be broadly classified as applicable to
(1) existing tailings disposal areas at existing uranium mills,
and (2) new tailings disposal areas at either new or existing
uranium mills.
7.1 Description of Control Practices , '
The most effective way of controlling radon-222 emissions
is to cover the radium-bearing tailings with an impervious';
material. Earth and water are the cover materials most commonly
used and are effective in reducing radon-222 emissions. :;These
cover materials retard the movement of radon-222 long enough for
it to decay in the cover material; thus, the decay products
remain in the cover.
7-1
-------
7.1.1 Earth Covers
Covering the dried beach area with earthen materials has
been used to control dust and radon-222 emissions at inactive
tailings impoundments. The depth of eacth required for a given
amount of control varies with the type of earth and the rate at
which radon-222 emanates from the bare tailings.
Earth cover restricts the diffusion of radon-222 long
enough so that it will decay in the cover material. Radon-222
diffusion through earth is a complex phenomenon affected by
such as molecular diffusion, described mathematically
law. These complex diffusion parameters have been
by Rogers and Nielson (Ro81). They determined that
depends greatly on the porosity and moisture content
of the medium through which it occurs. Ideally, the diffusion
coefficient should be measured experimentally for a given earth
cover at its ambient moisture content and expected compaction
level. This coefficient can, however, be estimated based on
the moisture content and porosity of the material. Clay soils
have superior moisture retention (9 to 12 percent moisture) and
are best for covering tailings; clay soils are found in the
uranium milling regions of Colorado, New Mexico, Utah, and
Wyoming (Ro81) .
processes
by Pick's
evaluated
diffusion
Cover thickness may be calculated by using the same
diffusion eguations that apply to emissions from uncovered
tailings as shown in the following equations (Ro84):
Jc - Jt exp (-bc xc)
where J, is the flux through cover (pCi/m s); Jf is the flux
; g is the radon-222
is the diffusion coefficient
is (\/D }
C
m )];
m is the moisture
through tailings (pCi/m s); b
u
-6
decay constant (2.1 x 10 /s); D
of cover8 0.07 exp [~4(ra-rap +
_ j_
saturation fraction [0.01 M(l/p - 1/g) ]; M is the moisture
content of cover material (percent dry weight); p is the bulk
density (g/cro ); g is the specific gravity (g/cm ) ,* p is the
porosity (1 - p/g),* and x is the depth of cover material (cm).
This simplified equation assumes that the physical
parameters of the cover material, such as its density, specific
gravity, moisture content, and porosity, are similar to those
of the tailings, and that the tailings are sufficiently thick
so that other terms approach a value of one. The flux through
the cover material may be estimated by substituting values for
the covet depth and the uncovered tailings flux.
7-2
-------
Effectiveness and Cost
The approximate effectiveness of various types of earth
cover in reducing radon-222 emissions is shown in Figure
7-1. The application of almost any type of earth will
initially achieve a rapid decrease in radon-222 emissions,
One meter's depth of high-moisture-content earth such as
clay will reduce radon-222 emissions by about 90 percent.
In Figure 7-1 the earth types are categorized by their
"half-value layer" (HVL). The HVL is that thickness of
cover material (earth) that reduces the radon-222 flux to
one-half its uncovered value. High-moisture content earth
provides greater radon-222 emission reduction because of its
smaller diffusion coefficients and its lower HVL values.
The approximate reduction in radon-222 emissions achieved by
applying selected types of earth at 0.5m, 1m, 2m, and
3 meter depths is shown in Table 7-1.
In practice, earthen cover designs must take into
account uncertainties in the measurements of the properties
of the specific cover materials used, the tailings to be
covered, and especially the predicted long-term values of
equilibrium moisture content for the specific location.
Predicting long-term moisture content requires specific
knowledge of the earthen cover to be used and the climatic
conditions (Ha84, Ge84). Proper consideration of these
factors at the design stage help ensure that radon-222
emissions remain constant over the long term. In predicting
reductions in radon-222 flux, uncertainty increases when the
required radon-222 emission limit is very low,
The cost of applying earth covers varies widely with
location of the tailings impoundment, its layout, and
availability of earth. Costs also depend on the size and
topography of the disposal site, its surroundings, the
amount of earth required, and the hauling distance. Another
factor affecting the costs of cover material is ease of
excavation and the type of excavating equipment used. In
general, the more difficult the excavation, the more
elaborate and expensive the equipment is and the higher the
cost. The availability of such materials as clay will also
affect costs'. Large deposits of bentonite and similar clays
are found in Wyoming and Utah, and smaller deposits are
found in all the Western States. If the necessary materials
are readily available locally, no incremental costs would be
incurred; if they must be purchased or hauled, costs could
increase significantly. Cost factors for earth cover
application are given in Table 7-2, and more detailed cost
factors are presented in Appendix B. These are direct
costs, and they do not include indirect costs such as
engineering design and permit costs, insurance, or a
contingency. Indirect costs would add approximately
30 percent to the direct charges.
7-3
-------
•M
C
OJ
u
v-
o
CL
L_J
o
o
o
•=£
ct
o.
z
o
100
90
80
70
60
50
40
30
20
10
1 I
EARTH TYPE
1 I I
HVL(m) I MOISTURE
A SANDY SOIL 1.0
B SOIL 0.75
C SOIL 0.5
D COMPACTED MOIST SOIL 0,3
0.12
3.4
7.5
12.6
17
21.5
1 2 ' 3 4 5 6
EARTH COVER THICKNESS (meters)
Figure 7-1. Changes in radon-222 penetration with earth
cover thickness. (adapted from FPA83)
7-4
-------
Table 7-1, Percentage reduction in rad.on-222 emissions attained by
applying various types of earth cover
Depth of earth cover (ml
Earth type^
A
B
C
D
E
HVL(TO)
1.0
0.75
0.5
0.3
0.12
0.5
29
37
50
68
94
1.0
50
60
75
90
>99
2.0
75
84
94
99
>99
3,0
88
94
98
>99
>99
Figore 7-1.
7-5
-------
Table 7-2. Summary of unit costs for estimating earth caver costs
(a)
Task
Unit cost ($)
Grading, self-propelled scraper, 1000-ft haul
Excavation, elevating scraper, 5000-ft haul
Compaction, vibrating
Excavation, front-end loader, truck-loaded
Hauling, 12-yd dump truck, 2-mile round trip
Fencing, 6-ft, aluminized steel
Riprap, machine-placed slope protection ,
Borrow, bank-run gravel
1.16/yd
2.46/yd3
1.00/yd2
0.84/yd
2.35/yd3
11.30/linear ft
21.00/yd3
6.60/yd3
(a)
Building Construction Cost Data 1985, R. S. Means Co., Inc.,
43rd Annual Edition, 1984.
7-6
-------
Based on the cost factors and the required earth thickness
shown in Figure 7-1, the resulting total costs per hectare for
earth cover can be estimated (as shown in Table 7-3) for
selected emission or flux levels and a bare tailings radon-222
emission rate of 280 pCi/m s. These costs only take into
account the earth moving and placement costs; they do not
include any indirect charges or final closure costs, such as
riprap or reclaiming borrow pits. They are presented to show
the variation in costs among the different types of soil.
For a model 50-ha (124-acre),tailings impoundment, the
approximate direct earth moving cost to achieve a 64 percent
reduction (from 280 to 100 pCi/m s)' is $5.2 x 10 (50 ha x
$105,000/ha = $5,250,000) with a fairly dry type A earth and
$1.4 x 10 for a more moist type D earth.
Earth cover is applied to dry tailings with conventional
earth-moving equipment and engineering practices. However, some
areas, especially the sloped sides of dams constructed of coarse
tailings, may be difficult to cover without recontouring the
pile. Dams constructed of coarse tailings are located at six
mill sites, mainly in New Mexico. The slope of the sides of
these dams is 2:1 or steeper. Some of these dams have heights
of 100 ft or more. These sloped areas represent about 8 percent
of the total tailings area. At least one site, Uravan in
Colorado, has applied a partial earth cover to the sloped sides
of dams constructed of tailings, which would indicate that this
is a feasible practice.
7.1,2 Water Cover'
Maintaining a water cover over tailings reduces radon-222
emissions. The degree of radon-222 control increases slightly
with the depth of the water. Factors affecting this practice
include the mill water recirculation rate (if any), evaporation
and precipitation rates, impoundment construction and slope,
phreatic levels, ground-water contamination potential, and dike
or dam stability. Some above-ground tailings impoundments
minimize the depth of water to reduce seepage and possible
ground-water contamination by draining the water through an
overflow pipe to a separate evaporation pond. All uranium mill
surface impoundments are subject to ground-water concentration
standards as specified in 40 CFR Subpart D 192.32 and
incorporated in NRC criteria, for tailings impoundments (10 CFR
40, Appendix A). These strict ground-water contamination
standards will frequently determine the type of impoundment
design and degree of water cover maintained in an active area.
An impoundment liner and ground-water monitoring programs will
be required for new 'installations.
7-7
-------
-J
I
CO
Table 7-3. Earth moving and -placement costs (thousands of dollars per hectare) ^a' of attenuating
radon-222 as a function of thickness (meters of different soils) and type of earth
Earth
A B C D E
Final flux(c)
(pCi/m s) Cost Thickness Cost Thickness Cost Thickness Cost Thickness Cost Thickness
20
50
100
200
267
174
104
34
3.81
2.49
1.49
0.49
200
130
78
25
2.86
1.86
1.11
0.36
133
87
52
17
1.90
1.24
0.74
0.24
80
52
31
10
1.14
0.75
0.45
0.15
32
21
12
4
0.46
0.30
0.18
0.06
'a' Cost basis: $7.00/m ($5.35/yd ) of soil cover material; includes excavating ($0.84/yd ), hauling
3 3 3
($2.35/yd ), spreading ($1.16/yd ), and compacting ($1.00/yd ), in 1985 dollars.
^ See Figure 7-1.
(c\ 2
*• ' Based on initial radon-222 emission rate of 280 pCi/m s.
-------
Effectiveness and Cost
The diffusion coefficient of water is very low (1.1 x
10 cm /s), about one-thousandth of that of soil with a
9 percent moisture content. Thus, water is an effective barrier
for radon-222. In shallow areas, the release of radon-222
dissolved in water is increased by thermal gradients and wave
motion, and emissions approach those of saturated tailings.
Increased radium-226 content in the water reduces its overall
effectiveness in controlling radon-222 because the solution also
releases radon-222. For a water depth less than 1 meter, the
flux rate is similar to that of saturated tailings' and may be
estimated by Equation 3-1 as presented in Section 3.
Water-covered tailings have a radon-222 flux of about
0.02 pCi/m s per pCi of radium-226 per gram of tailings
compared'with a dry tailings flux of about 1 pCi/m s per pCi
of radium-226 per gram, or a radon-222 reduction efficiency of
about 98 percent (PEI85). Emission estimates of zero are
frequently used for ponded and saturated areas,, and that
assumption is used throughout this report (Ha85) (EPA83).
If a pond is initially designed and built to maintain a
water cover, there is no added cost for this form of radon-222
control. Continued -monitoring is required to determine if any
seepage is occurring through1 the dam. or sides, and ground-water
samples may be required periodically as a check for
contamination.
7.1.3 Water Spraying
Water (or tailings liquid) sprays can be used to maintain a
higher level of moisture in the tailings beach areas. This
reduces fugitive dust emissions and may reduce the diffusion of
radon-222 through the tailings; however, ground-water
.contamination may be increased at some sites. The effectiveness
of this method varies with- the moisture content of the
tailings. As shown in Figure 7-2, the radon-222 emanation
coefficient initially increases with increasing moisture content
up to about 5 to 10 weight, percent moisture and then"remains
fairly constant. Thus, if water is applied to a very dry beach
area, radon-222 emissions may initially increase because of a
larger emanation coefficient. As the moisture increases,
however, the diffusion coefficient will decrease. These
mechanisms (both affecting radon-222 emissions) "compete" at low
moisture levels. Whereas some reports (NRC80) estimate that
wetting can achieve an overall radon-222 reduction of 20
percent, others (ST82) have stated that'by wetting tailings at
low moisture levels, a larger emanation coefficient may outweigh"
the effects of a lower diffusion coefficient and result in
increased emissions at low moisture contents. The overall
7-9
-------
0.6
|_ 0.4
z
yj
g
a.
u.
UJ
O 0,3
o
z -
o
t-
0.2
UJ
z
o
Q
TAILINGS MOISTURE. M (dry wt.
10 20 90
MONTICELLO ALKALINE
GRAND JUNCTION SLIME
GRAND JUNCTION SAND
MEXICAN HAT
DURANQO
AMBROSIA LAKE
MONTICELLO ACID
VITRO SLIME
RAY POINT
VITRO SAND
MONUMENT VALLiY
RIVERTON
ASSUMING m= 2.7M
0.2 O,4 0.6 0.8
MOISTURE SATURATION, m
1.0
Figure 7-2.. Radon emanation coefficients for tailings samples
(Ro84).
7-10
-------
feasibility of wetting to achieve significant radon-222
reductions is questionable, especially in arid regions, because
large quantities of liquid are required to maintain high moisture
levels.
7.1.4 Other Control Techniques
Several other radon-222 control techniques have been
evaluated. Although none of these methods has been applied on a
large scale, they are described briefly here as part of this
Background Information Document,
Synthetic Covers
Synthetic material, such as polyethylene sheet, can reduce
radon-222 emissions if carefully placed on dry b|ach areas and
sealed. Diffusion coefficients of less than 10 cm /s have
been measured for synthetic materials (Ro81). Such covering
could be used on portions of the tailings on a temporary basis
and then removed or covered with fresh tailings. Such a barrier
also would aid, at least temporarily, in the control of radon-222
if a soil cover material were subsequently applied. The overall
effectiveness of synthetic covers is not known because leaks
occur around the edges and at seams and breaks. Synthetic covers
have a limited life, especially in dry, sunny, windy areas, and
will not provide a long-term barrier to radon-222._ The cost of
installing polyethylene material is about $0.01/ft per mil of
thickness or $0.50/ft for 50 mil material, which is equivalent
to about $53,800/ha ($21,750/acre).
Chemical stabilization sprays that form coatings on the dry
tailings are effective for controlling dust, but they are not
useful for suppressing radon-222 because they do not provide an
impermeable cover.
Asphalt Covers
Asphalt cover systems have been proposed as a radon-222
control technique because such systems exhibit very low radon-222
diffusion coefficients. The Pacific Northwest Laboratory (PNL)
has investigated controlling the release of radon-222 through use
of asphalt emulsion covers for several years for DOE's Uranium
Mill Tailings Remedial Action Project (UMTRAP), Results have
shown asphalt emulsion cover systems to be effective at
substantially reducing radon-222 emissions, and field tests
indicate that such systems have the properties necessary for
long-term'effectiveness and stability. Of the various types of
asphalt cover systems that were researched, an asphalt emulsion
admix seal was found to be the most effective (Ha84, Ba84),
7-11
-------
Costs of applying a full-scale asphalt cover were estimated
to be $24.20/m ($2Q.23/yd) in 1981 dollars or $100,000/acre
(Ba84), These cost estimates are probably applicable to
relatively flat sites. Existing uranium mill tailings
impoundments may have to be regraded before these techniques
could be applied. Cover protection, in the form of gravel or
revegetation, above an earthen cover applied over the asphalt
radon-222 barrier to protect it may also have to be considered.
Asphalt cover systems could prove to be economically competitive
with earthen covers at some existing sites. Site-specific
evaluations would have to be performed that analyzed the amount
of earth required as well as its availability and cost versus the
cost of applying an asphalt cover system. An ample supply of
earthen material should be available as a final cover of new
uranium mill tailings impoundments that are constructed below or
partially below grade,' such a supply would probably make an
asphalt cover system economically unattractive.
Thermal Stabilization
Thermal stabilization is a process in which tailings are
sintered at high temperatures. The Los Alamos National
Laboratory has conducted a series of tests on tailings from four
different inactive mill sites where tailings were sintered at
temperatures ranging from 500 to 1200 C (Dr81).
The results show that thermal stabilization effectively
prevented the release (emanation) of radon-222 from tailings.
The authors note, however, that before thermal stabilization can
be considered as a practical disposal method, information is
needed on the follox^ing:
(1) The long-term stability of the sintered material
exposed to physical degradation and chemical attack
(e.g., solubility of new minerals and amorphous
material found in thermally stabilized tailings).
(2) The interactions of the tailings with the refractory
materials lining a kiln.
(3) The gaseous and particulate emissions produced during
sintering of tailings.
(4) Revised engineering and economic analysis as more
information is developed.
Gamma radiation is still released after sintering;
therefore, protection against the misuse of sintered tailings
would be required. Although the potential health risk from
external gamma radiation is not as great as that from the
radon-222 decay products, it can produce unacceptably high
exposure levels in and around occupied buildings. Also, the
potential for ground-water contamination may require the use of
liners in a disposal area.
7-12
-------
Chemical Processing
The Los Alamos National Laboratory has also studied various
chemical processes for the extraction of thorium-230 and
radium-226 (precursors of radon-222) from the tailings along with
other minerals (WraSl). 'After their removal from the tailings,
the thorium-230 and radium-226 can be concentrated and fixed in a
matrix such as asphalt or concrete. This greatly reduces the
volume of these radioactive materials and permits disposal with a
higher degree of isolation than economically achievable with
tailings.
The major question regarding chemical extraction is whether
it reduces the thorium-230 and radium-226 values in the stripped
tailings to safe levels. If processing efficiencies of 80
percent to 90 percent were attained, radium-226 concentrations in
tailings would still be in the range of 30 to 60 pCi/g. Thus,
careful disposal of the stripped tailings would still be required
to prevent misuse. Another disadvantage of chemical processing
is the high cost, although some of the costs might be recovered
from the sale of other minerals recovered in the process (Th81).
Soil CementCovers
A mixture of soil and portland cement, called soil cement,
is widely used for stabilizing and conditioning soils (PC79).
The aggregate sizes of tailings appear suitable for producing
soil cement, which is relatively tough, withstands freeze/thaw
cycles, and has a compressive strength of 300 to 800 psi. when
combined in a disposal system with a 1-meter earth cover over it,
soil (tailings) cement would likely provide reasonable resistance
to erosion and intrusion, could be expected to reduce radon-222
releases, and would shield against penetrating radiation. The
costs are expected to be comparable to those of thick earth
covers.
The long-term performance of soil cement is unknown, •
especially as tailings impoundments shift or subside with age.
Also, soil cement cracks at intervals when placed over large
surface areas, The importance of this cracking on the
effectiveness of soil cement for radon-222 control has not been
evaluated.
Deep-Mine Disposal
Disposal of tailings in worked-out deep mines offers several
advantages and disadvantages compared with surface disposal
options. The probability of intrusion into and misuse of
tailings in a deep mine is much less than that of surface
7-13
-------
disposal. Radon-222 releases ''tcx the atmosphere would be reduced,
as would erosion and external radiation. This method, however,
has potential for ground-water -contamination problems. ' Also, it
could be costly, depending on- the'nine location-and the controls
required to -guard against potential ground-water contamination.
7,.2 Control'Practices..Applicable to Existing Tailings
Imp oundments •
Control practices that are applicable to existing tailings
impoundments are limited to application of earthen covers, or
possibly asphalt mixtures, to dry areas, and maintaining or
expanding the area of tailings covered by water (if it were
determined that ground-water impacts would not result). Either
interim (i.e., short-term) or final (i.e., long-term) controls
could be applied. Interim control is the application of a cover
that reduces radon-222 emissions but that does not meet the
requirements of final reclamation. Standards for final
reclamation include requirements 'for reducing average radon-222
•emissions to 20 pCi/m s and for long-term (1000 y) stability
and protection against misuse'. •• ' .
> _ 'i ,.-, j.. .
7.2.1 Interim Controls -
Application of an interim earthen cover on the dry portions
of tailings impoundments could reduce radon-222 emissions over
the period of licensed operation and prior to final reclamation.
For example, '-a 0.3 m (1 ft) or 1 m (3.3 ft) thick earth cover
having 8 percent moisture content would -theoretically reduce
radon-222 emissions by about 25 and 62 percent, respectively
(Table 7-1). "There are many unknowns regarding the
effectiveness, applicability,' timing,' and operational aspects of
.interim "cover. These items' a're discussed below and more fully in
Appendix C. • .'"•••. - "•'• ';
The operational status (at capacity, standby, or active) and
the type of .construction (dams"constructed of coarse tailings,
earth dams', or below-grade" lined impoundments) control the extent
to which interim jcover could be'applied. Interim cover could be
applied immediate!y'''to most dry areas of existing impoundments
(excluding '-dams)".- -Currently, 'about'50 percent of the total area
of existing impoundments is dry (Table 4-2). Ten existing
impoundments have been filled to capacity. These impoundments
represent about 14 percent of the total area and "'about' 25 percent
of'the total area, that -is currently dry (the dry'"areas are the
'-major'sources of radon-222 emissions as discussed in Chapter 3).
Impoundments' that are - at capacity could be covered immediately
because 'they have already dried and'because they•will never'be
used again for tailings"disposal.
7-14
-------
Site characteristics that control or prohibit the
applicability of interim cover include impoundment design and
construction; dam height; stability; phreatic level;
permeability; site/water balance?, evaporation rates;, presence and
location of movement monitors,"monitor wells or piezometers; and
availability of suitable earth cover material. Operating factors
such as expected uranium production 'rate, length and number of
standby periods, impoundment capacity, 'and expected'mill life
also affect the- applicability of interim cover.
At. active impoundments, only. those 'areas that are not to be
used further would be covered'. Which areas could be covered are
a function of expected mill life and quantity of- tailings, the
size of tailings impoundment, the level of tailings generated
(percentage of capacity), and the operational practices used to
construct the .impoundment. In addition, a source of cover
material must be obtained.and a technique must be developed for
hauling, dumping, spreading 'and compacting the earth cover onto
the beach area. Limited access to -the tailings area and the
stability of the dam would affect the.size of the equipment that
can be used to transport and spread .the cover material. Metal
gratings or timbers may be required to distribute vehicle wheel
loads on the dike or dried beach area to facilitate the use of
earthmoving equipment. These site-specific factors would
increase earthmoving costs.
- '? '
Of the existing tailings impoundments, 11 have sand tailings
dams and are above ground, 22 have earthen dams and are above
ground (4 of these are lined), and.5 are below grade and lined.
Currently, all tailings impoundments at licensed mills must limit
radon-222 to as low as reasonably achievable (ALARA) levels, as"
specified in 40 CFR.192. Work practices or emission limits are
not specified, however. Mills that are on standby and have begun
or are about to begin the decommissioning process will eventually
cover the tailings areas and reduce emissions to 20 pCi/m s as
required by Federal regulations. Mills that wish .to .retain their
operating licenses do not have to feegin their final
decommissioning process, but they'could take some-interim actions
to minimize,radon-222 emissions. .-Interim cover as a means of.
reducing,radonr222 emissions to air from operational tailings
impoundments is. difficult t'o apply as .new tailings beach areas
are continuously being formed..' , - , . '- , ' .
Covering the currently dry beach-areas, excluding dams, -with
1 meter of earth and maintaining .the current water cover on the
ponded and wet- beach areas would reduce radon-222 emissions-from
129 kci/y to about 69 kci/y, a reduction of 46 percent, / at a
cost of about $63 x 10 (1985 dollars). Additional details
regarding the applicability, timing, and operational aspects of
interim cover are discussed in Appendix C.
(a\
% ' Based on soil with 7.5 percent moisture content.
7-15
-------
The feasibility of maintaining water cover is limited
because of.potential site-specific factors such as seepage,
ground-water contamination, and dam stability problems. For an
existing above-ground tailings impoundment, many site-specific
factors cannot be readily changed, and the feasibility of water
cover is limited, mainly because of dike stability and seepage.
Also, during extended standby periods, maintaining the water
cover would be difficult, especially in arid areas. Ideally, the
impoundment would be lined and constructed to allow approximately
a 1-meter depth of water cover and have an overflow pipe leading
to an adjacent evaporation pond and/or for recycling to the
mill. The use of water cover would require maintaining
sufficient freeboard to prevent .overflow and the monitoring of
ground water. Eight impoundments are lined, representing
11 percent of the total tailings area and 9 percent of the dry
exposed tailings areas. Five of these impoundments are below
grade. The water cover on these lined impoundments could be
increased to reduce radon-2'22 emissions from the 200 acres of dry
tailings that they currently contain. The potential for
increased ground-water contamination, however, would limit the
use of this radon-222 control option.
7.2.2 ' Final, Reclam_ation " '*
If all existing impoundments were allowed to dry, and were
covered with,enough earth to*achieve'aj flux of 20 pCi/m s, the
total radon-222 emissions would be reduced to 8 kCi/y. The cost
would be about $660 x 10 . 'For ongoing milling operations, new
tailings impoundments would be built and work practices would be
instituted to reduce emissions.
Bringing existing impoundments to final reclamation entails
substantially more effort than effecting interim control
measures. After the sand tailings dams have dried, they are
recontoured to 5:1 (H:V) slopes for long-term stability. Earth
dams were not recontoured in the cast estimates presented in this
section. The cost of enough earth (8% moisture) to attenuate
the radon-222 flux to 20 pCi/m s is placed over the tailings.
The earthen cap is covered with gravel to protect the top
surface, and the riprap is used to protect earth-covered side
slopes from erosion* The cost estimate also includes reclaiming
the on-site borrow pits that are assumed to be the source of
earthen cover material.
7.2.3 Comparison of Interim and Final Controls
Estimates of the reduction in emissions, the avoided fatal
cancers, and the costs of applying earth cover to achieve various
control alternatives are summarized in Table 7-4. Covering the
7-16
-------
Table 7-4. Benefits and costs of alternatives that apply earth cover
to existing tailings inpoundments
Alternative
Cease use of current im-
poundments, allow to dry
and apply final cover'.
Cover current dry areas
with 1 m of earth.
Radon-222
emissions
(kCi/y) .
Avoided fatal Cost
cancers/y^ '
(b)
($ x 1CT)
Current 'After 0-80 km National
129 8 1.7(0.5) 2.9(0.7) 660
129 69 0.8(0.2) 1.5(0.4) 63
(a)
(b)
Values are based on 1520 deaths due to lung cancer per 10 person WIM.
The values in parentheses are based on 380 deaths per 10 person WIM.
Total cost, including indirect charges., Final cover includes earth
required to achieve 20 pCi/m s, regrading sand tailings dams to
5:1 (H:V) slope, riprap on sides, and gravel on top of impoundments
(1985 dollars).
7_17
-------
currently dry areas, excluding dams and evaporation ponds, with a
meter of earth achieves a theoretical estimated reduction in
emissions of 46 percent'at a cost of $63 x' 10 (1985 dollars)
and prevents from 0,6 to 2.3-cancers per year (based on a range
of 380 to 1520 deaths per 10 person WLM). Total avoided
cancers are the sum of avoided cancers in the 0-80 km region and
the national (i.e.,' outside the 0-80 km region). An estimated
emission reduction of 94 percent can be achieved by applying
sufficient cover to achieve 20 .pci/ro s; this would cost .$660 x
10' (1985 dollars)1 and prevent from 1.2 to 4.6 cancers each
year. (These'cost estimates are for the control practice only
and do not include the cost'of establishing new impoundments. .In
addition, these' estimates--have not been discounted.) This
comparison shows one point in time only. It does not reflect -
reapplications of interim cover -required after restarting of
operations at- specific site's' or changes in emissions due to
interim cover deterioration. Annual maintenance costs that would
occur over time1 are also not included.
7.3 Control Practices Applicable to NewTailings Impoundments
New tailings-disposal impoundments at uranium mills can be
designed to incorporate radon-222 control measures. Three
different kinds of new model impoundments are considered;
single-cell, phased disposal, and continuous disposal 'of
dewatered tailings. Descriptions' of radon-222 emission's and
estimated costs of the three types of new model tailings
impoundments are presented in the following sections.
Below-grade impoundments are the NRC's preference, as this
method minimizes potential'for windblown emissions''and water
erosion and eliminates the potential for dam failure (NRC80).
Although below-girade disposal is preferable', well-designed and
operated above-grade tailings' 'impoundments can also provide
adequate safety'and be licensed by 'the NRC. "
7 • 3 .1 • Single-Cell Tailincrs Impoundment '
•'. ' New tailings- disposal-areas-must conform with Federal
regulations (40' CFR 190 and'-192 and 10 CFR 40) 'for" prevention of
•ground-water contamination and 'airborne p'articulate emissions.
New impoundments will" also be designed to facilitate "'final
closure as required by current Federal Standards. New tailings
areas w'i'1'1 have synthetic liners-, will probably be built below or
partially below grade, and will have earthen dams or
embankments. A means for dewatering the tailings at closure also
should be incorporated. This basic layout is amenable to
maintaining a water cover over nearly the.entire tailings area
during the operational phase and standby periods; therefore, it
7-18"
-------
will maintain a very low level of radon-222 emissions. The
drainage system can be used to accelerate dewatering of the •
tailings when the impoundment is full. . ' :•••.'
Effectiveness and Cost ; .
A model single-cell impoundment was used to estimate
radon-222 emissions and the effectiveness of single-cell tailings
impoundments. The basic design and layout of this impoundment
are consistent with previous uranium mill tailings studies.. The
impoundment is a square sloping pit containing a 12-meter depth
of tailings and having a final tailings surface area of 47 ha
(116 acres), as shown in Figure 7-3. A synthetic liner is placed
along the sides and bottom. It handles about 2000 tons/day of
tailings over a 15-year active period. During operation, 20
percent of the surface area is assumed to be dry beach and the
remainder is assumed to be water-covered. ,Cover.material is
applied after the impoundment has reached capacity or is not
going to be used further and the tailings have dried. Emissions
average 0.8 kCi/y during the operational 15-year life and
increase after drying begins, as shown in Figure 7-4 and
Table 7-5. '
• Emissions are .constant at approximately 4.2 kCi/y after the
tailings are dry. If an earth cover is applied after drying,
emissions can be reduced (as shown.in Figure 7-4 and Table 7-5)
to about 0.30 kCi/y with-3 meters of earth (Type B soil, 8
percent moisture as shown in Figure 7-1). Total emissions during
the 5-year drying period amount to 12.5 kCi.
The approximate costs for constructing a new • single-cell
impoundment are shown in Table 7-6 for a below-grade design and a
partially above-grade design. The cost of a new impoundment
would vary widely, depending mainly on the site-specific
topography and the ease of excavation. The total cost for a
below grade impoundment is approximately $41.3 x 10 , including
a final cover cost of about $6.0 x 10 ($4.15 x 10 for earth
cover and $1.9 x 10 for gravel cap). The partially
above-grade design is identical to,the below-grade design except
that 6 m (19.6 ft) of tailings are below grade and 6 m (19.6 ft)
are above grade and surrounded by.an earthen dam.. This design is
less costly because of the savings resulting -from decreased
excavation. The cost is about $29.7 xg10 . Final closure
costs are slightly higher at -$7.8 x 10 , as riprap is required
on the sides of the dam. .'
7-19
-------
3 m FINAL COVER,
TAILINGS
|;i2 in
637 m
SECTION A-A
GRADE LEVEL FOR
BELOW GRADE IMPOUNDMENT
LINER
6 m
24 m
637 m
f
24 m
637 m
TAILINGS CAPACITY = 1800 t/d x 310 d/y x 15 y = 8.4 x 10° t
TAILINGS VOLUME - 8.4 x 106 t 4 1.6 t/m3 = 5.25 x 106 m3
FINAL TAILINGS SURFACE AREA = 47 ha (116 acres)
DIAGRAMS ARE NOT TO SCALE.
Figure 7-3. Size and layout of the model single-cell tailings impoundment.
7-20
-------
CM
e\j
CJ
o
o
oc.
1 I ]
RADIUM CONCENTRATION - 280 pCi Ra-226/g
SPECIFIC FLUX - 1 pCi Rn-222/m2s per
pCi Ra-226/g FOR DRY AREAS N0
~ DURING ACTIVE LIFE, 20% OF /
AREA IS DRY BEACH /
DRYING 3-
PHASE
1 I 1* *
D 5 10 15 *' 2
YEAR
>
COVER, 4.15 kC1/y,
(280 pC1/m2s)
•m COVER, 0.30 kCi/y
? '
1 On rtfi Itf f \
(10 pLl/Ti SJ
0 >20
Figure 7-4. Estimated radon-222 emissions from a model
single-cell tailings impoundment.
7-21
-------
Table 7-5. Average radon-222 emission rate ,
from model single-cell tailings impoundments^
Time period Emissions (kCi/y)
Year 0-15 0.8
Year 15-20 2.5
Year >20 4.2 uncovered
0.3 with 3 meters
of earth.
^ ' For 47-ha new model impoundment with 15-year life
and 5-year drying-out period. Emissions are based
on 280 pei Ra-226/g and a specific flux of 1 pel
Rn-222/m s per pCi Ra-226/g of tailings when dry.
7-22
-------
Table 7-6. Estimated costs for a model single-cell tailings
irtpoundment ^ '
' Item
Excavation
Synthetic liner (30-mil)
Grading
Drainage system
Dam -construction
Cover (3-m)
Gravel cap (0.5-m)
Riprap on slopes
Subtotal direct, cost
Indirect cost^ '
Total cost . '
Costs ($ x 106)
Below grade
'21.51
3.03
: 0.40
0.40
4.05
1.92
31.31
10.02 - ~ '- :.r .:
41.33 , -
Partially ...
above grade ^ '
"8,14
3,03
0.40 ' .
0.40
. 2.75
• 4.05
1.99
1.74
22.50
7.21
29.71"
(a)
(c)
Below-grade impoundments are constructed so that the top of the
final cover is at grade.
Fifty percent below grade and 50 percent above grade.
Indirect costs are estimated to be 32 percent of direct costs.
7-23
-------
7.3,2 • Phased~P.isp.gsal Tailings Impoundment
In phased-disposal systems, a tailings area is partitioned
into sections or cells that are used independently of other
sections. After a cell has been filled, it can be dewatered, • -
dried, and covered while another section is in use. In practice,
one or two lined cells would be constructed initially. Tailings
are pumped to the first cell until it is filled and then pumped
to the second cell while the first cell is 'dewatered and allowed
to dry. After the first cell has dried, it would be covered with
earth obtained from the cells excavation. This process continues
sequentially. This system reduces emissions at any given time,
as a cell can be covered after use without interfering with the
operation of subsequent cells. Standby periods do not present as
great a problem and construction of new cells can easily be
postponed. Less total tailings surface area is thus uncovered at
any one time compared with operation of the model single-cell
impoundment, which is uncovered until mill closure and the .
impoundment dries.
Several existing mills have either proposed or implemented
phased-disposal systems. At the Plateau Resources Shootaring
Canyon Mill in Utah, an earthen dam'has been constructed across a
valley. Behind this dam, earthen beams have been constructed to
form six cells for tailings disposal. Currently, only one cell
contains a significant quantity of tailings, Umetco's White Mesa
Mill, also in Utah, uses a phased tailings disposal system
designed to feature simultaneous construction, operation, and
reclamation. Three cells of a proposed six-cell system have been
constructed. These impoundments are lined with either clay or
•synthetic liners. Minerals Exploration's Sweetwater Mill also
has a planned phased-disposal system. One cell of,a proposed
multicell impoundment system-has been constructed.- 'This system
has gone through several iterations during development.
Originally, it was designed to consist of four square,
below-grade cells.
Effectiveness and Cost -
Phased disposal is effective in reducing radon-222 emissions
because tailings are assumed to be completely covered with water
during cell operation and, finally, with soil. Only-'-during the
drying-out period- (about 5 years for each cell) do any radon-222
emissions occur, and these are from a relatively small area.
During mill standby periods, a water cover could be maintained on
the operational cell. For extended standby period's, •• the cell
could be dewatered and an earth or synthetic cover applied. To
estimate radon-222 emissions, a model phased-disposal impoundment
comparable to the model single-cell impoundment was used. This
7-24
-------
impoundment consists of six cells, and each cell holds one-sixth
of the mill tailings generated during a 15-year operational
period (i.e., 2 "years worth of tailings). Each cell is square
with a tailings depth of 12 meters and a trapezoidal cross
section, as shown in Figure 7-5. The total tailings surface area
at capacity is 86,260 m per cell.
Emissions from a cell during operation are zero because the
cell is covered with water. After the first cell reaches
capacity, it is dewatered and begins a 5-year drying period.
Over this period, radon-222 emissions gradually increase up to a
rate of about 0.8 kCi/y, at which time the cell is dry and soil
cover is applied. Meanwhile, the second cell has begun drying
and also contributing emissions.- Emissions thus increase at
2.5-year intervals as the cells reach capacity and begin their
drying out periods. The-emission rates occurring after 3 meters
of earth cover have been applied to dry cells are shown in Figure
7-6. Earth cover of the first cell is not started until after
7.5 years have elapsed. After the final 5-year drying period for
the last cell is complete (at the 20th year), this cell is also
covered and emissions are then constant at 0.33 kCi/y.
Total emissions during the 20-year operating life of this
impoundment are 13.5 kCi. Average radon-222 emission rates are
shown in Table 7-7. During the operational phase,.the average
emission rate of 0.7 kCi/y is lower than that for a-single cell
impoundment (.1.2 kCi/y) . In the post-operational period,
emissions from'a phased-disposal impoundment are much lower than
those from-uncovered single-cell impoundments and equivalent to
those from single^cell impoundments with the same respective
earth cover.
Estimated costs of building phased-disposal impoundments are
shown in Table 7-8.' The total cost-.of below-grade phased
disposal, at $47.88 x TO 6, is greater than the cost of a
single-cell impoundment with similar earth cover, but the costs
are incurred over a 20-y period. This cost is based on a 12-m
tailings depth (similar to the model single-cell impoundment).
An evaporation pond is included as part of the phased-disposal
system. The-, cost for a partially above-grade phased-disposal
sygtem is aboutj$6.9 x 10 per cell, or a total of $41.5 x
10 . The decreased cost of excavation is. partially offset by
the dam construction cost and the riprap on the sides.
Numerous variations, in the model phased-disposal impoundment
are conceivable. An impoundment could be designed to include any
number of cells, each capable of containing an equal amount of
the mill tailings.generated during a 15-year operational period.
As an example, a, below-grade, phased-disposal impoundment
utilizing three cells was investigated.
7-25
-------
f 3 m FINAL COVER
24 m
t
12 m
Z45.7 m
GRADE LEVEL FOR
BELOW GRADE DESIGN
24 m
SECTION A-A
NOTES:
TAILINGS CAPACITY PER CELL = 1800 t/d x 310d/y x 15y * 6 CELLS = 1.4 x 106 I/CELL
TAILINGS VOLUME PER CELL = 1.4 x 106 t/CELL f 1.6 t/m3. = 8.75 x TO5 m3/CELL
FINAL TAILINGS SURFACE AREA = 8.6 ha/CELL (21.,3 aere/CELL)
DIAGRAM IS NOT TO SCALE
Figure 7-5. Size and layout of model phased-*disposal impoundment.
7-26
-------
5 3
C_3
1/1
to
CM
CM
C\i
O
o
-------
Table 7-7. Average radon-222 emission rate for model single-
cell and phased-disposal tailings impoundments
Average emission rate (kCi/y)* ;
Operational phase' ' Post-operational phase
Single-cell 1.2 4.2 Uncovered
0.30 covered with 3 m of earth
Biased-disposal 0.7 0.33 covered with 3 ra of earth
^ For new model impoundment with 15 yr. life and 5 yr. drying period for
each cell. Emissions based- on 280 pCi Pa-226/g and specific flux of
1 pCi Hn-222/m s per pCi Ra-226/g of tailings when dry.
^ ^ Assumes a 5-y drying-out period for each cell and immediate cover of
3m of earth.
7-28
-------
Table 7-8. Estimated costs for a model phased disposal impoundment^'
Item
Excavation
Synthetic liner
(30-mil)
Grading
Drainage system
Dam contraction
Cover (3 ra)
Riprap on slopes
(0.5 m)
Gravel cap (0.5-m)
Evaporation pond
Below
One cell
3.68
• 0.5?
0.07
0.07
-
0.76
-
0.37
0.52
($ X 106)
crrade
All cells
22.08
3.40
0.45
0.40
-
4.57
-
2.21
3.09
Partially
One cell
1.28
0.57
0.07
0.07
1.27
0.76
0.32
0.39
0.52
above grade
All cells
7.70
3.40
0.45
0.40
7.61
4.57
1.91
2.34
3.09
Subtotal direct 6.04 36.20 5.25' 31.47
cost
Indirect cost^ ' 1.93 11.58 1.68; 10.07
Total cost 7.97 47.78 6.93 41.54
^ Below-grade impoundments are constructed so that :the top of the
final cover is at grade. Partially above-grade impoundment is
6 m below grade and 6 m above grade.
' ' Indirect costs are estimated to be 32 percent of direct costs.
7-29
-------
Compared, with the design of the previously-discussed
phased-disposal impoundment with six cells, the three-cell
impoundment is conceptually identical except that each cell's
capacity is now doubled. Because the total surface area of a
three-cell impoundment is somewhat less than that of a six-cell
impoundment, some reductions in cost and emissions are effected.
The estimated cost of a below-grade, phased-disposal impoundment
with three cells is $46.58 x 10 , compared with $47.88 x 10
for six cells. -The average radon-222 emission rate during the
operational phase of a three-cell impoundment is 0,62 kCi/y,
compared with 0.67 kCi/y for six cells, and during the
post-operational phase, the emissions are 0.31 and 0.33 kCi/y,
respectively.
7.3.3 Continuous Disposal
Water- can be removed from the tailings slurry prior to
disposal. The relatively dry, dewatered (25 to 30% moisture)
tailings can be placedand covered with soil almost immediately.
No extended drying phase is necessary. Ground-water problems
would also be reduced. Implementation of a dewatering system
would require added planning, design, and modification of current
designs. Acid-based leaching processes do not generally recycle
water, and larger evaporation ponds with ancillary piping and
pumping systems would be required to handle the liquid removed
from the tailings.
Tailings dewatering systems have been used successfully at
nonferrous ore beneficiation mills in the United States and
Canada (Ro78). Various filtering systems, such as rotary,
vacuum, and belt filters, are available and could be adapted to a
uranium tailings dewatering system. Experimental studies would
be required for a specific ore to determine the filter media and
dewatering properties of the sand and slime fractions. The
typical mill ore grinding circuit may have to be modified to
permit efficient dewatering and to prevent filter plugging or
binding. Corrosion-resistant materials would be required in any
tailings dewatering system because of the highly-corrosive
solutions that must be handled. Although it is used in some
foreign countries, continuous tailings dewatering is not"
practiced at any uranium mills in the United States; however, it
has been proposed for several sites. In a planned installation
in the Eastern United States, tailings were to be dewatered by a
belt filter system and trucked to a tailings disposal area, where
a 0.3-m (1-ft) clay cap would be :applied (Ma83). An active
working edge of 100 m (300 ft) was allowed for spreading, but no
more than 4.0 ha (10 acres) of tailings were to be exposed at any
one time. The clay cap was to be covered with 0.2 m (8 inches)
of gravel and about 2 ,_7 m (8 ft) of random fill. Additional
random fill and overburden from a surface mining operation were
to complete the tailings cover.
7-30
-------
At least three uranium mills have proposed the use of
continuous disposal systems. Anaconda submitted conceptual plans
of such a tailings disposal system prior to the downturn of the
uranium market. However, the plans were never implemented. The
system was to be a trench and fill type operation,. Tailings were
to be thickened to 60 percent solids prior to pumping to 91-m
(300-ft) by 2300-m (7500-ft) trenches excavated to a depth of
15 to 21 in (50 to 70 ft) . The tailings were then to be covered
with 5 m (16 ft) of earthen material. Pioneer Uravan, Inc.,
submitted plans to build the San Miguel Mill using continuous
tailings disposal at Slick Rock, Colorado (NRC81). The mill has
not been constructed. The planned tailings disposal operation
consisted of below-grade burial of belt-filtered tailings in a
series of 10 trenches. Excess water was to be transferred to two
evaporation ponds. Each trench would measure 76 by 760 m (250 by
2500 ft) and be 9 to 11 m (30 to 35 ft) below grade. Tailings
would be transferred from the mill to the trench via conveyor.
Six to 6.4 m.(20 to 21 ft) of earth cover would be placed over
the tailings. Excavation, filling, and covering would be carried
out simultaneously. Umetco Minerals proposed a continuous
disposal system that would be located on a mesa adjacent to the
Uravan, Colorado, mill. The existing impoundments at this site
have been filled to capacity.
Effectiveness and Cost
Continuous disposal is an effective means of reducing
radon-222 emissions, especially during the operational life of a
uranium mill. Dewatered tailings are placed in trenches and
covered with soil shortly after placement, which eliminates the
drying period associated with other tailings disposal
techniques. The model continuous-disposal impoundment consists
of a series of 10 trenches, each having the capacity for
one-tenth of the volume of tailings generated over the 15-y life
of the model mill. Each trench has sloping sides and contains a
12-m depth of tailings. A 6-m berm separates the trenches to
.allow for tailings placement. A diagram of the model '
continuous-disposal impoundment is shown in Figure 7-5. The
total tailings surface area at capacity is 572,000 m, or 57,200 m
per trench.
Another alternative method of continuous disposal of uranium
mill tailings entails a combination of two previously discussed
methods. Continuous/single-cell disposal involves placement of
dewatered tailings in a single large impoundment as opposed to
placement in a series of trenches. The size of the impoundment
would be comparable to that required for the single-cell
impoundment. A partially taelow-grade continuous/single-cell
disposal impoundment is also considered because it minimizes the
excavation cost as well as the cost of dam construction.
7-3,
-------
GWBE LEVEL FOR • •
BElW-GRAOE IMPOUNDMENT'S
\
3m / \ COVER / \ /
/ \ / \ /
12 m
TAILINGS
77
SECTION A-A
S««OE LEVEL FOR
BELW-SRADE
r«5"ffl*j
12 m
, COVER
TAILINGS
NOTES:
•409.5 m-
SECTION B-B
TAILINGS CAPACITY PER TRENCH = 1800 't/d x 310 d/y x 15y t- 10 TRENCHES = 8,4 x 10
TAILINGS VOLUME PER TRENCH - 8.4 x 105 t t- 1,6 -f/m3 = 5.25 x 1Q5 m3
FINAL TAILINGS SURFACE AREA = 5.72 ha/TRENCH (14,1 acre/TRENCH)
DIAGRAM IS NOT TO SCALE
Figure 7_7_ Size and layout of the model continuous -disposal impoundment.
7-32
-------
Emissions from continuous-disposal impoundments during
operation are low. Elimination of the drying-out period, which
is responsible for the majority of the operational radon-222
emissions associated with the other model disposal impoundments,
substantially reduces emissions from continuous-disposal
impoundments. This is evident in Table 7-9, .which shows the
average emission rates for continuous-disposal and the
single-cell model impoundments.
Figures 7-8 and 7-9 show the radon-222 emission rates for
the model continuous-disposal impoundments of single-cell and
trench designs, respectively. It has been assumed that 4 ha
(10 acres) of dewatered tailings are uncovered at any point in
time over the 15-y life because of the normal short interval
between placement and covering of tailings. At year 15, when the
impoundment is at capacity, the final, 4 ha of tailings are
covered. The final emission rates, 0.36 kCi/y or 0.30 kCi/y, are
similar to the other model impoundments. The estimated costs for
continuous disposal, shown in Table 7-10, include an evaporation
pond for the liquid removed from the tailings and a vacuum filter
system. The cost of a below-grade impoundment is estimated to be
about $54.2 x 10 , and the cost of a partially above-grade
trench design system, at about $61.0 x 10 . A design
consisting of a single large impoundment partially above grade
could reduce the large dam construction cost inherent in building
10 trenches. This alternative would cost about $37.4 x 10 .
7-33
-------
Table 7-9. Estimated radon-222 emission rates for model single-cell,
phased disposal, and continuous-disposal tailings ijnpoundments
Average emission rate (hCi/yV *
Operational phase Post-operational phase
Single cell 1.2' ' 4.2 uncovered
0.30 covered with 3m
of earth
Phased disposal 0.7* ' 0.33 covered with 3m
of earth
Continuous disposal 0.5 0.30 covered with 3m
(single-cell) of earth
Continuous disposal 0.5 0.36 covered with 3m
(trenched)' ' of earth
* ' For new model impoundments with 15-y operational life emissions based
on 280 pCi Ra-226/g and specific flux of 1 pCi Hn-222/m s per pCi
Ra-226/g of tailings when dry.
* ' Includes 5-y drying-out period.
7-34
-------
to
o
t—*
to
co
CM
CM
CM
i
RADIUM CONCENTRATION - 280 pCi Ra-226/g
SPECIFIC FLUX - 1 pCi Rn-222/m2s per
pCi Ra-226/g FOR DRY AREAS
0.36 kCi/y
(20 pCI/nrs)
10
15
20
YEAR
Figure 7-8. Estimated radon-222 emissions from a
model continuous-disposal impoundment.
7-35
-------
£ 3
*F""
O
O
•—*
t/1
l/l
X
Csl
I
o
o
RADIUM CONCENTRATION - 280 pCi Ra-226/g
SPECIFIC FLUX - 1 pCi Rn-222/in2s per
pCi Ra-226/g FOR DRY AREAS
0.30 kCi/y (20
j_
10 15
YEAR
20
Figure 7-JL Estimated radon-222 emissions from a model
cpntinuous/single-cell disposal impoundment.
7-36
-------
Table 7-10.
Estimated costs far
(S x
l continuous disposal impoundment
Partially above-grade
Item
Excavation^ '
Synthetic liner
(30-mil)
Grading
Dam construction
Cover (3-m)
Riprap on slopes
Gravel cap (0.5-m)
Evaporation pond
Vacuum filter
Subtotal direct cost
(c)
Indirect cost^ '
Below-grade
trench design
22.75
3.82
0.51
5,15
2.54
4.80
1.46
41,03
13 . 13
Single-cell
design
8.14
3.03
0,40
2.75
4,05
1.74
1.99
4.80
1.46
28.36
9.08
Trench
design
7.24
3.82
0.51
18.06
5,15
2.15
2.99
4.80
1.46
36.18
11.57
Total cost
54.16
37.44
47.75
(a)
(b)
(c)
In 1985 dollars.
Below-grade inpoundments are constructed so that the top of the final
cover is at grade. Partially above-grade design is 6 HI deep and 6 m
above grade.
Indirect costs are estimated to be 32 percent of direct costs.
7-37
-------
7,4 Summary of Radon-222 Control Practices
A summary of the radon-222 emissions from new model
impoundments serving an 1800 t/day mill is1 presented in Table
7-9, Three types of emissions are presented: operational,
post-operational, and total emissions. The emissions front a
model single-cell impoundment represent those with and without
final cover to provide a perspective on the emission reductions.
Operational emissions are those that occur during the
operating 15 yr. life of the mill plus those due to the
impoundment's 5 yr. drying-out period, if applicable.. For
determination of the average operational emission rates-
presented, the total amount of emitted radon-222 was calculated
and divided by the appropriate 20 or 15 yr. lifetime. Emission
rates for the active and drying-out periods of phased- and
continuous-disposal impoundments are not presented because these
values vary with time. Tailings are being dried at various
points in time in a phased-disposal system, and no
5 yr.drying-out period is required for continuous disposal.
Post-operational emissions occur at the end of an
impoundment's drying-out period. After the 15-y operational
period and the 5-y drying-out period of a single-cell
impoundment, radon-222 emissions increase to 4.2 kCi/y with no
cover. After compliance with Federal requirements, the emission
rate reduces to 0.3 kCi/y. The post-operational emission rates
for the model impoundments with final cover meet the Federal
emission limit of 20 pCi/m s. The emission rate for
continuous disposal (trench design) with' final cover is slightly
higher than the others because the tailings surface area is
slightly larger.
The final column of Table 7-11 presents cumulative
emissions over various time periods. Emissions over these
different time periods are the sum of those from the operational
phase of an impoundment as well as those occurring after final
cover (if applicable). All impoundments with final cover meet
an emission limit of 20 pCi/m s; therefore, variations in
emissions from' the various covered impoundments are due to
different operational emissions and small differences in the
tailings surface 'areas.
Cost estimates for constructing new model tailings
impoundments are summarized in. Table 7-12. The partially
above-grade single-cell impoundment cost, $29.7 x 10 , is the
lowest cost alternative, but most of the costs are incurred
during initial construction. Its completely belpw-grade
counterpart costs are estimated to be ,$41.3 x 10 . The
difference is largely due to increased excavation costs. Phased
and continuous disposal impoundments are more costly, but the
costs are spread out over the life of the impoundment.
7-38
-------
Table 7-11. Summary of estimated radon-222 emissions from new model tailings impoundments
(a)
I
u>
Cumulative
Post-operational emissions emissions total
(kCi/y) (kCi)
Alternative
Operational emissions
. (kCi/y)
Active Dry-out With
(15 y) (5 y) Average Uncovered final covervu' 20 y 40 y 60 y
1.
2.
3.
Single cell(c)
Phased disposal
Continuous
disposal
(trench)
(single-cell)
0.8
NA
NA
NA
2.5
NA
NA
NA
1.2
0.7(d)
0 5(e)
o!5(e)
NA
NA
NA
NA
0.
0.
0.
0.
30
33
36
30
25
13
10
9
31
20
17
15
37
27
24
21
4. No action
(single cell
without cover)
0.8
2.5
4.2
NA
25
108
191
NA - Not applicable.
(a)
Emission estimates based on a specific flux of 1 pCi/m2s radon-222 per pCi radium-226 per g
tailings and a radium-226 concentration of 280 pCi/g.
Final cover to meet 20 pCi/m2s standard.
(c\
Assumes 20% of the impoundment area is dry beach during the 15-y active life; remainder of
area is water-covered.
(d)
(e)
Based on 20-y life: 15 y active, and 5 y drying out.
Based on 15-y life.
-------
.Table 7-12. Summary of estimated costs for new model tailings impoundment
(1985 $ x 10 )
• _ Continuous-disposal
i
o
Single-cell
Partially
Below grade above grade
Direct cost 31.3 . 22.5
Indirect cost 10.0 7.2
Phased-disposal
Partially
Below grade above grade
36.2 31.5
11.6 10.0
Partially
above grade
Single-
Below grade cell
41.0 28.3
13.1 9.1
Trench
46.2
14.8
Total cost 41.3 29.7 47.8 41.5 54.1 37.4 61.0
-------
REFERENCES
Ba84
Dr81
EPA83
Ge84
Ha83
Ha84
Ha8 5
Ma83
NRC80
Baker E. G., Hartley J, N., Freeman H. D., Gates T.
E., Nelson D. A, and Dunning R. L., "Asphalt Emulsion
Radon Barrier Systems for 'Uranium Mill Tailings - An
Overview of the Technology", DOE/UMT-0214, PNL-4840,
March 1984,
Dreesen D. R., Williams J. M., and Cokal E. J.,
"Thermal Stabilization of Uranium Mill Tailings", in;
Proceedings of the Fourth Symposium on Uranium Mill
Tailings Management, Fort Collins, Colorado, October
1981.
Environmental Protection Agency, "Final Environmental
Impact Statement for Standards for the Control of
Byproduct Material from Uranium Ore Processing", EPA
520/1-83-008-1, Office of Radiation Programs, U.S.
EPA, Washington, D.C., September 1983.
Gee G. W., Nielson K. K, and Rogers V. c,, "Predicting
Long-Term Moisture Contents of Earthen Covers at
Uranium Mill Tailings Sites", DOE/UMT-0220, PNL-5047,
September 1984.
Hartley J. N., Gee G. w., Baker E. G., and Freeman H.
D., "1981 Radon-222 Barrier Field Test at Grand
Junction Uranium Mill Tailings Pile", DOE/UMT-0213,
PNL-4539, April 1983. '. •
Hartley J. N. , and Gee G. W., "Uranium Mill Tailings
Remedial Action Technology'^, in: Proceedings of the
Second Annual Hazardous Materials Management
Conference, Philadelphia, Pennsylvania, June 1984.
Hartley J. N. ,. Glissmeyer J. A., and. Hill 0. F.,
"Methods for Estimating Radioactive and'Toxic Airborne
Source Terms for Uranium Milling Operations",
NUREG/CR-4088, Nuclear Regulatory Commission,
Washington, D.C., June 1985.
Marline Uranium Corp. and Union Carbide.Corp, "An
Evaluation of Uranium 'Development in Pittsylvania
County, Virginia", October 15, 1983, Section E.3.
Nuclear Regulatory Commission, "Final Generic
Environmental Impact Statement on Uranium Milling",
NUREG-0706, September 1980. ' •
7-41
-------
NRC81 Nuclear Regulatory Commission, "Environmental
Assessment Related to the Operation of San Miguel
Uranium Project", NUREC— 0723, January 1981.
PC79 Portland Cement Assn. , "Soil-Cement Construction
Handbook", EB003.095, Skokie, 111. 1979.
PEI85 PEI Associates, Inc., "Radon-222 Emissions and Control
Practices for Licensed Uranium Mills and Their
Associated Tailings Piles", EPA Contract No.
68-02-3878, June 1985.
Ro78 Robinsky E. I., "Tailing Disposal by the Thickened
Discharge Method for Improved Economy and
Environmental Control", in; Volume 2, Proceedings of
the Second International Tailing Symposium, Denver,-
Colorado, May 1978.
Ro81 Rogers V. C. , and Nielson K. K. , "A Handbook for the
Determination of Radon-222 Attenuation Through Cover
Materials", NUREG/CR-2340, Nuclear Regulatory
Commission, Washington, D'.C., December 1981.
Ro84 Rogers V. C., Nielson K. K., and Kalkwarf D. R., Radon
"Attenuation Handbook for Uranium Mill Tailings Cover
Design", NUREG/CR-3533, 1984.
St82 Strong K. P. and Levins D. M., "Effect of Moisture
Content on Radon Emanation from Uranium Ore and.
Tailings", Health Physics, 42, 27-32, January 1982.
Th81 Thode E. F. and Dreesen D. R., "Technico-Economic
Analysis of Uranium Mill Tailings Conditioning
Alternatives", in: Proceedings of the Fourth
Symposium on Uranium Mill Tailings Management, Fort
Collins, Colorado, October 1981.
Wm81 Williams J. M., Cokal E. J., and Dreesen D. R.,
"Removal of Radioactivity and Mineral Values from
Uranium Mill Tailings", in: Proceedings of the Fourth
Symposium on Uranium Mill Tailings Management, Fort
Collins, Colorado, October 1981.
7-42
-------
Chapter 8; SUMMARY AND COMPARISON OF WORK PRACTICES
A number of alternatives are available to reduce radon-222
emissions and subsequent risks from tailings disposal. Both
timing and the disposal method effect emissions. The control
alternatives', their emissions, costs, and potential benefits are
presented in this chapter on a comparable basis by using the
model tailings impoundment described in Chapter 7. •
8.1 Single-Cell.Impoundments
The base case assumes disposal of tailings in a single cell
impoundment similar to current practice at many mills. This
nominal 50 ha (125 ac) impoundment (actually 47 ha or 116 acres)
has a 15 year active life. The surface area is 80 percent wet
or ponded during this active period and average radon-222
emissions are 0.8 K. Ci/y. Emissions then increase during a
5-year drying period to 4.2 kCi/y. Emissions after this time
depend on when the impoundment is covered to comply with Federal
and/or state regulations. For illustrative purposes time
periods of 0, 20, and 40 years are used before final cover is
applied. The total cost for constructing and eventually
covering a single cell impoundment is the same, but since the
final cover is applied at different times in,this example, the
net present value of this cost is different.* ' The longer
the cover cost is postponed, the smaller the net present value.
A summary of radon-222 emissions and costs for single cell
impoundments are presented in Table 8-1.
In Base Case I the impoundment is dry and uncovered for
40 years. This example case yields the highest emissions and
least cost since nothing is done for 40 years after the
impoundment is full and dry (60 years from start). In Base
Case II no cover is applied for 20 ^years after the impoundment
is full and dry (40 years from start). Initially emissions are
the same as the first example, but greatly reduced during the
40-'to 60-year period since final earth cover is applied. Costs
are increased by $700,000 since the cover cost is incurred
20 years sooner. Covering the impoundment as soon as possible
after it is full reduces radon-222 emissions still further and
increases the net present value cost by about $2,500,000 when
compared with Base Case I.
^ Net present value = current cost x [1/(1 + 0.05)n] at a
5 percent discount rate and where n = years in which cost
is incurred.
8-1
-------
Table 8-1. Emission and cost comparison for single cell impoundment
with final cover applied at 0, 20, and 40 years after reaching capacity
Cumulative
radon-222 emissions (kCi)
Work practice
Cover 40 years after
full - Base Case I
Cover 20 years after
full - Base Case II
Cover "when full
0-20 y
25
25
25
20-40 y
83
83
6
40-60 y
83
6
6
0-60 y
191
114
37
NF7,
costs ^
($10b)
33.9
34.6
36.4
*• ' At 5 percent discount rate.
8-2
-------
The risks incurred by leaving a model impoundment uncovered
•can be estimated from the radon-222 emission rate and assuming
the model impoundment has an impact in proportion to that of the
current licensed mills as shown belowr
Risks from model = nationwide risks emissions from
impoundment total emissions model impoundment
Based on the' current estimated emission rate of 138 kCi/y
from licensed mill impoundments and a nationwide fatal cancer
rate of 2.34 committed fatal cancers per year (based on
760 deaths per million person WLM), deaths at other emission
rates can be estimated.
For the single cell model impoundments deaths and benefits
(deaths avoided) were estimated for a 60-year period as shown in
Table 8-2. Benefits are determined by comparing with the Base
Case I, i.e., not covering for 40 years. When compared with the
cover in 20 years case, the benefits of covering immediately
when full are reduced to 1.3 deaths avoided over a 60-year
period.
8.2 Phased Disposal
Phased disposal provides a means of reducing emissions
since the smaller areas involved in each cell at any given time
are easier to keep flooded during operation and standby
periods. Also, during the drying phase less tailings are
exposed. Two model phased disposal impoundments with the same
capacity as the large single cell impoundment were characterized
to estimate emissions, cost, and potential benefits. A 6-cell,
20-acre-per-cell, and a 3-cell, 40-acre-per-cell impoundment
were used as models. Average emissions during a 20-year
operational period are.0.7 and 0.6 kCi/y for the 20 acre and
40-acre cell size, respectively. Average radon-222 emissions
after being-completely covered with earth are similar at 0.33
and 0.31 kCi/y for the 20-acre and 40-acre cells, respectively.
The total costs of a 6-cell, 20-acre-per-cell design and a
3-cell, 40 acre cell design are similar but the net present
value- for the 40-acre-per-cell design is less since some costs
are postponed compared with the 20-acre-per-cell design.
The emissions and cost data for below grade phased disposal
model impoundments are summarized in Table 8-3. Radon-222
emissions are very similar and the NFV for the 40-acre/cell
impoundment is about $1,500,000 less than the 20 acre/cell
design,
Committed fatal cancers for the model phased disposal
impoundments were also estimated as shown in Table 8-4. Only a
very slight difference in estimated deaths is seen, and this
would be expected since emissions are very similar.
8-3
-------
Table 8-2. • Comparison of estimated deaths and benefits for a single
cell model - inpDundment with final caver" applied at 0, 20, and 40 years
, after reaching capacity
Nationwide deaths,^a'
Benefits,
Work practice 0-60 y . 0-60 y
Base Case I - Cover 40 years
after full ' 3.5
Base Case II - Cover 20 years
after full ' 2.1 1.4
Cover when full 0.7 2.8
Based on 760 deaths per million person WIM.
8-4
-------
Table 8-3. Emissions and costs for model phased disposal impoundments
Work practice
6-cell, 20-acre/cell
3-cell, 40-acre/cell
NW
Kadon-222 emissions (kCi) costs ^a'
0-20 y 20-40 y 40-60 y 0-60 y ($ x 106)
13 7 . 7 27 36.1
12 6 6 24 34.6
At 5 percent discount rate.
8-5
-------
Table 8-4. Comparison of estimated death for model
phased-disposal impoundments
Nationwide deaths,^a'
Work practices ' ' 0-60 y
20 acre - 6 cell design ., 0.5
40 acre - 3 cell design ,. 0.4
^ Based on 760 deaths per million person WIM.
8-6
-------
8.3 Continuous Disposal
Dewatering and continuously covering tailings is an
attractive but untried method for tailings disposal in this
country. By exposing only a relatively small beach area,
radon-222 emissions are reduced during operation and a long
drying period is not required prior to final cover. A model
continuous disposal below-grade, trench type impoundment with
the same capacity as the single cell conventional impoundment
was used to estimate emissions and cost. Average emissions
during the operational period are 0.5 kCi/y and drop to
0.36 kCi/y after the final beach area is covered at the 15-year
point. As shown in Table 8-5, cumulative emissions over a
60-year period are 24 kCi. Based on this emission rate,
committed fatal cancers from this work practice at a model
impoundment amount to 0.4 over a 60-year period. Assuming that
costs are incurred at the beginning of each of three 5-year
periods, the net present value cost for a below-grade trench
impoundment is about $43 x 10 .'
8.4 Comparison ofWork Practices
Work practices for new model tailings impoundments are
summarized in Table 8-6 in order to compare their radon-222
emissions, net present value cost, and the resulting health
effects attributed to each model impoundment. The single-cell
impoundment with cover applied when dry has the highest
emissions during its operating life and thus, the highest
cumulative emissions. This higher emission rate results in a
higher health risk. Phased disposal yields lower emissions
during the operating period and thus lower cumulative
emissions. Costs are similar to the single-cell impoundment and
cumulative health effects are lower. Continuous-disposal
emissions are very similar to phased disposal and health effects
are thus also similar. Net present value costs for this trench
type of disposal are $43 x 10 ; higher than single-cell or
phased-disposal alternatives.
8-7
-------
Table 8-5. Emissions and cost of nodal below-grade trench type
continuous disposal impoundment
Cumulative radon-222 emissions (kCi) - • •
0-20 y 20-40 y 40-60 y 0-60 y NFV cost^
($ X 106)
10 7 7 24 43.3
^ ' At 5 percent discount rate.
8-8
-------
Table 8-6. Comparison of work practices for new model tailings impoundments
CO
1.
2.
3.
4.
Work practice
Single cell covered
when full (20 y
from start)
Phased disposal
20-acre cells
Phased disposal
40-acre cells
Continuous dis-
posal (trench-
type)
Cumulative
radon-222 emissions (kCi)
0-20 y 20-40 y 40-60 y 0-60 y
25 66 37
13 7 7 27
12 6 6 24
10 7 7 24
NPV of work
practice @ 5% - g
discount ($ x 10 )
36.4
36.1
34.6
43.3
Committed
fatal cancers^
0-60 y
0.6
0.5
0.4
0.4
f-
Nationwide, based on 760 deaths/10 person WIM. Assumes model plant is at average location of
existing mills.
-------
When compared with an uncovered single-cell impoundment,
all the-work practices yield similar benefits in the form of
avoided deaths. Costs of these-alternative'work practices are
also similar except for continuous disposal which is about
$9 x 10 higher. Tables 8-7 and 8-8 present a comparison
between the alternative work practices and a base case single
cell impoundment uncovered for 40 years and also 20 years
respectively. Depending on the base case selected, benefits of
about 1.4 to 2.8 deaths avoided can be realized for a model
impoundment over a 60-year period when alternative work
practices are used.
8-10
-------
Table 8-7, Comparison of cost and benefits between model
Base Case I and new work practices
• 'Difference in NFV Deaths.
from base-case avoided^
Work practice ($ x 10 j 0-60 y
Base Case I . —
Single cell covered 40 y
after full (60 y from start)
1. Single cell covered when 2.5 2.8
full (20 y from start)
2. fihased disposal 20-acre cells
3. Hiased disposal 40-acre cells
4. Continuous disposal (trench-type)
2.2
0.7
9.4
3.0
3.1
3.1
' Nationwide basis.
8-11
-------
Table 8-8. Comparison of cost and benefits between model
Base case II and new work practices
Difference in NW Deaths. .
front base-case avoided'^ '
Work practice (S x 10 ) 0-60 y
Base Case II
Single cell covered 20 y . -
after full (40 y from start)
1. Single cell covered when 1.8' • 1.4
full (20 y from start)
2, Ehased disposal 20-acre cells 1.5 1.6
3. Ehased disposal 40-acre cells - 1.7
4. Continuous disposal (trench-type) 8.7
fa,}
^ ' Nationwide basis.
8-12
-------
APPENDIX A
DIAGRAMS OF URANIUM MILL SITES AND
TAILINGS IMPOUNDMENTS
A-l
-------
Diagrams of each of the 20 licensed uranium mill sites that
were included in this evaluation are presented in this
appendix. These diagrams were adapted from aerial photographs
taken by the Office of Radiation Programs. The diagrams are
presented to show the relative location of the tailings
impoundments, mill structures, and other important site
features. Approximate scales,and the dates of the aerial
photograph are indicated on each diagram.
A-2
-------
PROCESS
WATER
PONDS
^S WATER COVERED
TAILINGS
m EXPOSED
mTAILINGS
SECONDARY
IMPOUNDMENT
1000 ft
COTTER CORP. MILL
CANON CITY, CO
DATE: 8/12/85
-------
TAILINGS
IMPOUNDMENT 3
EVAPORATION
POND SOLIDS
TAILINGS
^IMPOUNDMENTS
1 AND 2
0
1000 ft
UMETCO MINERALS MILL
URAVAN, CO.
DATE: 8/7/85
EXPOSED TAILINGS
MILL BUILDINGS
o
SAN MIGUEL RIVER
DRY SOLIDS
EVAPORATION PONDS
FORMER RESIDENTIAL AREA
EVAPORATION PONDS
PONDED WATER
-------
ACCESS ROAD
L-BAR TAILINGS
IMPOUNDMENT
WATER COVERED
TAILINGS
SOHIO MILL
CEBOLLETA, NM
DATE: 10/5/85
mm EXPOSED TAIL INGS Mm
1000 ft
MINEWATER
POND
MINE
BUILDINGS
o
-------
MINE SHAFT
MINE
SHAFT
MINE SHAFT
MINE
BUILDINGS
MINE WATER
PONDS
EXPOSED £
•• TAILINGS t
0
1000
I
UNITED NUCLEAR MILL
GALLUP, NM
DATE: 10/5/85
2000 ft
i
CHURCHROCK TAILINGS
IMPOUNDMENT
A-6
-------
EVAPORATION
PONDS
>
0
L_
1000
- I
2000 ft
i
ANACONDA MILL
BLUEWATER, NM
DATE: 8/24/85
BLUEWATER 1
TAILINGS IMPOUNDMENT
mm EXPOSED :
TAIL INGS
BLUEWATER 2
TAILINGS IMPOUNDMENT--'
BLUEWATER 3
TAILINGS IMPOUNDMENT
MILL BUILDINGS
-------
EVAPORATION
PONDS
KERR-McGEE CORP. MILL
GRANTS, NM
DATE: 8/24/85
>
WATER
COVERED
TAILINGS
1000
I
QUIVIRA 2C
-------
ROAD
CD
a
HOMESTAKE 1 ,
TAIUNGS- •-..;
IMPOUNDMENT: -
WATER COVERED
TAILINGS
MILL BUILDINGS
HOMESTAKE MILL
GRANTS, NM
DATE: 8/24/85
HOMESTAKE 2
TAILINGS
IMPOUNDMENT
1000 ft
EXPOSED TAILINGS
HIGHWAY 53
-------
>
i
0
CHEVRON RESOURCES MILL
PANNA MARIA, TX
DATE: 11/4/85
WATER COVERED
TAILINGS
PANNA MARIA
TAILINGS
.IMPOUNDMENT
-------
WHITE MESA TAILINGS
IMPOUNDMENT NO.l
n F
Lb $&«
TAILINGS Jffg
EXPOSED TAILINGS ^
WATER COVERED ^^^=^*~S
ORE PILES
WHITE MESA TAILINGS
IMPOUNDMENT NO.2
UMETCO MILL
BLANDING, UT
DATE: 8/7/85
WHITE MESA TAILINGS
IMPOUNDMENT NO.3
0
L
1000 ft
I
-------
RIO ALGOM MILL
LA SAL, UT
DATE: 8/7/85
WATER COVERED
TAILINGS
^EXPOSED TAILINGS££
SURFACE
WATER
EXPOSED
TAILINGS
IMPOUNDMENT NO. 2
-------
I
H1
OJ
ATLAS CORP. MILL
MOAB, UT
DATE: 8/4/85
N
MOAB TAILINGS
IMPOUNDMENT
^ MILL
° NBUILD INGS
WATER COVERED
TAILINGS
-------
>
.p-
WATER
COVERED
TAILINGS
EXPOSED
TAILINGS
SHOOTARING TAILINGS
IMPOUNDMENT
SURFACE
WATER
MILL
BUILDINGS
0
I
PLATEAU RESOURCES MILL
SHOOTARING CANYON, UT
DATE: 8/4/85
1000 ft
I
-------
WATER COVERED
TAILINGS -
o
Of
O
FORD TAILINGS
IMPOUNDMENT
NO. 4
FORD TAILINGS
IMPOUNDMENTS NOS.
1, 2,.AND 3
M;: EXPOSED TAILINGS
STREAM
DAWN MINING MILL
FORD, WA
DATE: 8/14/85
1000 ft
MILL
BUILDINGS
-------
WATER
COVERED
TAILINGS
SHERWOOD
IMPOUNDMENT
WESTERN NUCLEAR MILL
WELLPINIT, WA
DATE: 8/14/85
1000
i
2000 ft
EVAPORATION
POND
A-16
-------
GAS HILLS
TAILINGS
IMPOUNDMENT MO 4
•WATER COVERED
TAILINGS
GAS HILLS
TAILINGS
IMPOUNDMENT NO 2
GAS HILLS
TAILINGS
IMPOUNDMENT NO. 3
'
PATHFINDER- GAS HILLS MILL
SHIRLEY BASIN, NY L
DATE; 8/8/85
1000 ft
GAS HILLS "M
TAILINGS &?
IMPOUNDMENT NO. } ®$
EXPOSED }
TAILINGS
DO
MILL BUILDINGS
A-17
-------
ACCESS ROADS
CO
PROCESS WATER
POND
WATER COVERED
TAILINGS
# EXPOSED
STAILINGS
WESTERN NUCLEAR MILL
JEFFREY CITY, WY
DATE: 8/8/85
-------
MILL BUILDINGS
SURFACE
MINE
'GAS HILLS w®
TAILINGS «
IMPOUNDMENT m£
Wim EXPOSED
mzm: TAILINGS
EVAPORATION^
PONDS
/A-9 TAILINGS
IMPOUNDMENT
SURFACE
WATER
UMETCO MILL
GAS HILLS, WY
DATE: 8/8/85
-------
SURFACE
WATER
N>
O
WATER
COVERED
TAILINGS
BEAR CREEK TAILINGS
IMPOUNDMENT
ROCKY MT. ENERGY MILL
BEAR CREEK, WY
DATE: 9/13/85
-------
SHIRLEY BASIN
TAILINGS IMPOUNDMENT
WATER COVERED !
TAILINGS
^^^mm EXPOSED ;
^Ji^s^TA KINGS;
MILL BUILDINGS
-------
tv>
NJ
0
L_
::MINE WATER POND
MINERALS EXPLORATION MILL
SWEETWATER COUNTY, WY
DATE: 8/8/85
SWEETWATER TAILINGS
IMPOUNDMENT
-------
APPENDIX B
COST ESTIMATES FOR EXISTING AND MODEL
NEW URANIUM MILL TAILINGS IMPOUNDMENTS
B-l
-------
Appendix B: COST ESTIMATES FOR EXISTING AND MODEL NEW URANIUM
MILL TAILINGS IMPOUNDMENTS
This Appendix presents the approach, assumptions, and bases
used to generate the cost estimates of Chapter 7. For existing
impoundments, the roost recent available site-specific
information was used to estimate the cost of interim control and
final reclamation measures. For new tailings impoundments,
model impoundments were designed, which formed the basis of the
cost estimate,
All costs are presented in 1985 dollars, which have not
been discounted. Both direct and indirect costs are included.
In general, direct costs represent labor, equipment, and
material costs. A total of 32 percent was added ,to this_figure
to cover indirect cost items such as engineering, 'insurance,
contingency, etc. Table B-l presents information, on the
indirect- cost factors used in preparation of the cost estimates,
B.1 Existing Impoundments
Detailed data on each existing site were obtained from
various sources (DOE82, EPA85,'"NRC84, PEI85) . Two types of work
practice control measures were considered for control of
radon-222 from existing uranium mill tailings impoundments:
interim control and final reclamation.
Interim Control
Interim control involved placing 1 meter of earth on the
surface of all dry tailings areas of an impoundment. For sand
tailings dams, the amount of soil required to cover the
embankment slopes was also included. Interim control is
considered a temporary measure; therefore, neither the costs of
reclamation of the source of cover soil (borrow pits) nor the
costs of impoundment erosion control were included. A unit cost
of $4.35/yd ($7,00/m ) was used to estimate the cost of
placing the interim cover. This includes the direct costs of
excavation, hauling, spreading, and compacting the cover.
Final Reclamation
Measures for effecting final reclamation of existing
uranium mill tailings impoundments are those required to reduce
the radon-222 flux to 20 pci/m s and to place the impoundment
in a state of permanent, long-term stability.
B-2
-------
Table B-l. Indirect cost factors used in the cost estimation of
uranium mill tailings impoundments
Percentage
Indirect cost item
Engineering and design
Insurance
Performance bond
Permits
Overhead and profit
Contingency at conceptual stage
Total
Range
2.5 - 6.0
0.1 - 0.82
0.39 - 1.2
0.5 - 2.0
10-15
15-20
Value used
5.0
0.5
0.5
1.0
10.0
15.0
32.0
Source.: "Means Site Work Cost Data 1985," 4th Annual Edition, R.S. Means
Co., Inc.
B-3
-------
No credits for earth covers that may have previously been
placed for interim control measures were considered to be of
help in achieving final .reclamation. Final reclamation"was
assumed to be possible immediately after an impoundment had
dried. No cost for attaining 'dry-out was assumed.' The
measures taken and the costs of final reclamation depend on
the type of impoundment and its size.
An estimate of the cost of '-covering each impoundment
with sufficient earth to reduce 'the radon-222 flux to 20
pCi/m s was based on the radium-226 concentration of the
tailings. Costs of reclaiming a borrow pit (source of the
earth for cover) and placing an 18-inch thick gravel cap on
top also were included for each impoundment. For
impoundments that are constructed of sand tailing's dams, the
costs' for', regrading slopes to 5:1(H:V) and protection of the
slopes'-'earthen cover with 18 inches of riprap were also
included. ,For these cost estimates, it was assumed that the
slopes of each'dam constructed of tailings originally had
1:1 (H:V) -slopes. These slopes would be reshaped to 5:1
(H:V) before placement of the cover and riprap. As
discussed earlier, indirect costs were then added to the
direct costs to obtain the total cost of final reclamation
of existing'impoundments.
B.2 New Tailings Impoundments
Four types of model impoundments were defined for
estimation of the costs of constructing new uranium mill
tailings impoundments: -.single-cell, phased-disposal,
continuous-disposal, .and. continuous/single-cell disposal
impoundments. Costs of .the first three types of
impoundments were estimated•for below-grade placement of
tailings and for partially below-grade-placement. Only
partially (50 percent) below-grade"placement of tailings was
considered for the model continuous/single-cell disposal
impoundment. . : '
Each model impoundment was assumed to have 2:1 (H:V)
interior sloping sides, to contain a 12-meter;depth of
tailings, and to have 6 meters of' tailings below grade and 6
meters above-grade (in the case of the partially :below-grade
impoundment), This arrangement ensures the comparability of
the cost estimates for the various impoundments. Each model
impoundment is designed or sized.to handle the production
output of the model mill over its 15-year life (NRC80),
which is estimated to be'8;.4 x 10 t of tailings with a
volume of 5.25 x 10 m . . -
B-4 "
-------
Sinrgle-Cell_Impoundments • • '. ,
The single-cell impoundments are large, :square
impoundments. For the below-grade impoundment*,. .15 meters of
earth is excavated so that the final level of. the '-
impoundment, which will contain a 12 meter depth.:o,f tailings
and be .covered with 3 meters of earth, is at-grade;. For the
partially below-grade single-cell impoundment,, a depth of 6
meters of tailings, is below-grade? therefore, "'the top of the
impoundment after final cover' is 9 meters above grade. Each
type of impoundment'has a, 3.0-mil , synthetic 'liner' and a
drainage system to facilitate •••dewatering. when- the,
impoundment has reached capacity L .For the partially
below-grade impoundments, 'embankments-are constructed from
the excavated material, which is also .used'for the final
cover. The embankments are 9'meters high/ .have:a'6-meter
berm, and have interior and'exterior slopes'of- 2:1 and 5:1,
respectively. The exterior of the embankment.is covered
with riprap for erosion protection. An; 18-inch-gravel cap
is placed atop the final cover of .each'type'of impoundment
for protection. The total estimated costs for'the .•
below-grade and the partially below grade single-cell.
impoundments are $41.3 x 10 and $29.7 x '10 ; (.1985'
dollars), respectively. The difference is,largely due to
the additional excavation required for a below-grade
impoundment. ' -•,';';,
Pha s edDisposal Impoundments • '
The phased-disposal impoundment consists of a series of
small impoundments or cells that-are constructed .
sequentially, filled, and brought to final reclamation over
the life of the model mill. The six cells are similar.in
design to the single-cell impoundment, but the capacity of .
each is just one-sixth of the total tailings quantity.
Unlike the model single-cell impoundment, an
evaporation pond is included in the cost estimate of
phased-disposal impoundments. The impoundment surface area
available for evaporation is much smaller; therefore, an
evaporation pond is required. The estimate includes both
the' cost of construction and the cost of closure of the •
evaporation pond at the end of the mill's life.
Excavation to a depth of 6 meters for the1partially
below-grade phased-disposal impoundment does riot" -provide .
sufficient' earth to construct the dam and to pl;ace/a 3-meter
earth cover over the tailings. Thus, the costs--'of obtaining
". ', B-5;
-------
additional earth and reclaiming a borrow pit are included in
the cost of the dain construction. The total estimated costs
for the below-grade and the partially below-grade phased
disposal impoundments are $47.8 x 10 and $41.5 x 10
(1985 dollars), respectively.
Continuous,; Disposal' .Impoundments
A series of 10 rectangular trenches are included in the
model continuous-disposal impoundments. As in phased
disposal, the trenches would be constructed sequentially,
filled, and covered over the life of the model mill. Unlike
phased disposal, however, the tailings are dewatered to
allow for almost immediate placement of the cover. The
estimate includes the cost of a vacuum filter to dewater the
tailings. An evaporation pond (larger than that required
for the phased-disposal model) is also needed. The tailings
are dewatered prior to disposal; therefore, no drainage
system is necessary.
The volume excavated is insufficient to meet the earth
requirements for the partially below-grade
continuous-disposal impoundment dam. The shortfall is made
up by hauling earth from a borrow pit, which is later
reclaimed. These costs are included in that of the dam
construction. The total estimated costs for the below-grade
and the partially below-grade continuous-disposal
impoundments are $54.2 x 10 and $61.0 x 10 (1985
dollars), respectively.
Continuous/Sing,le__.Ce_ll_ Disposal Impoundment -
The design of the continuous/single-cell disposal
impoundment includes a single, partially below-grade
impoundment for placement of dewatered tailings, as opposed
to a series of trenches. Such a design substantially lowers
the estimated cost of the dam construction, as it eliminates
individual embankments between trenches and the need to haul
in additional earth. The total cost of $37.4 x 10 (1985
dollars) is essentially the same as that estimated for the
partially below-grade single-cell impoundment except that an
evaporation pond and vacuum filter are still required
because the tailings must be dewatered.
B-6
-------
R1FERENCES
DOE82 Department of Energy, "Commingled Uranium Tailings
Study", DOE/ DF-0011, Office of Defense Waste and
Byproducts Management, Washington, D.C., June 30,
1982 =
EPA85 U.S. Environmental Protection Agency, "Draft
Document-Estimates of Population Distributions and
Tailings Areas Around Licensed Uranium Mill
Sites", Office of Radiation Programs, November
1985.
NRC80 Nuclear Regulatory Commission, "Final Generic
Environmental Impact Statement on Uranium
Milling", NUREG-0706, September 1980.
NRC84 Nuclear Regulatory Commission, "Directory and
Profile of Licensed Uranium-Recovery Facilities",
Office of State Programs, NUREG/CR-2869,
Washington, D.C., March 1984.
PEI85 PEI Associates, Inc., "Radon-222 Emissions and
Control Practices for Licensed Uranium Mills and
Their Associated Tailings Piles, Final Report"/ -
prepared for the U.S. Environmental Protection
Agency, Office of Radiation Programs, Document No.
PEI 3642-6, June 1985 (revised November 1985).
B-7
-------
APPENDIX C
EVALUATION OF INTERIM COVER
AS A CONTROL OPTION
C-l
-------
CONTENTS
1.' ' Introduction ;"••"'• •-" j ' 1-1
2. ' 'Technical" Issues "••''.''"' ' . 2-1
2.1 Introduction . 2-1
2.2' Effectiveness of Interim Cover 2-2
2.3 Applicability of Interim Cover 2-4
2.4Timing of Interim Cover • ' ' ' ' 2-7
'•2.5 : Operational Aspects ' ' ' 2-8
C-2
-------
Chapter 1;, INTRODUCTION .
The,use of an earthen cover on the dry portion of inactive
tailings impoundments can potentially reduce radon-222 emissions
by restricting the diffusion of this gas long enoughL to allow •
decay. In developing the background information for the proposed
standard, the option of using a temporary or interim earthen
cover evolved as a possible work-practice.standard. This cover
would be placed on dry portions of impoundments that are not in
use. The cover would be about 1 foot or 1 meter in depth
(depending on the selected option). If and when the impoundment
returned to active use, tailings would be dumped on top of the
earth cover. Other methods of reducing radon-222 emissions
include water cover, and synthetic or asphalt covers.
Maintaining a water cover causes potential ground water and, at
some sites,-dam stability problems. If a mill is on standby,
water cover will be difficult to maintain due to evaporation.
Synthetic or asphalt covers have not been evaluated over longer
time periods on a large-scale and their true effectiveness is not
known. Thus, only a limited number of viable options are
available for reducing radon-222 emissions from existing tailings
impoundments, namely;
Apply a relatively shallow earthen (interim) cover
over the dry areas when the impoundments are not in use
(i.e., standby).
Discontinue tailings disposal' in current impoundments
and apply final cover per existing standards.
Various schedules can' be used with either of these options as
described in the Federal Register Notice of February 21, 1986.
An analysis of these alternatives for reducing radon-222
indicated that the application of an interim earthen cover
appeared to be a cost effective option if an-impoundment was not
used again. This option is therefore being reevaluated to better
assess its practicability, effectiveness and cost.
C-3
-------
Chapter 2: TECHNICAL ISSUES
2.1 Introduction'
Interim earthen covers of 0.3 or 1 m having 8 percent
moisture content theoretically reduce radon-222 emissions by
about 37 and 62 percent, respectively. The actual effectiveness
of such interim covers has never been demonstrated on licensed
tailings impoundments. Additionally, while use of earthen covers
is a demonstrated control technology at inactive uranium mill
tailings sites, it has never been used on a short term basis to
limit radon-222' emissions from licensed tailings impoundments on
active or standby status. Therefore the evaluation of interim
cover is based on best engineering judgment and not practical
experience. However,, the use of thick (3 m) earth covers to
control radon-222 and provide long-term'stabilization of inactive
tailings piles is demonstrated technology. The evaluation of
interim cover, particularly estimation of its effectiveness in
controlling radon-222, is based on research conducted under the
UMTRCA program. • ' •. • '
Several characteristics of,.the impoundments impact the
potential use of interim cover. Site-specific characteristics
such as evaporation rates, dam construction, phreatic level,
availability of cover material, presence of liners, expected
length of standby periods, remaining capacity and expected mill
life must be considered on a site by site basis.
Uranium mill tailings are deposited as a slurry in tailings
impoundments. Three major types of impoundments currently
exist: those where coarse tailings are used as dam construction
material (11 impoundments representing 32 percent of total
tailings area)? those using earthen dams (22 impoundments
representing 65 percent of the total area) »* and below-grade
impoundments (5 impoundments representing about 3 percent of the
total). As discussed in later sections, impoundment construction
affects the applicability of interim cover. Additionally,
climate plays an important role in determining how much time is
required to allow an impoundment to dry sufficiently before
interim cover can be applied. For example, some tailings
impoundments, are located in arid areas (i.e., New Mexico)
relatively wet, areas (i.e., Texas, Washington) and areas that
experience severe winter weather (i.e., Wyoming). The geology
beneath an impoundment also impacts the time required for
drying. The geologic settings vary from porous underlayments
(sandy soils of New Mexico) to relatively impermeable bases (clay
foundations in Texas). For example, impoundments in New Mexico
would dry * relatively quickly because of seepage through the
bottom coupled with high evaporation rates while the Panna Maria
C-4
-------
impoundment in Texas would require a longer drying period because
of the impermeable base that would inhibit dewatering by seepage
and the relatively high rainfall rate. There are also several
operational aspects that must be evaluated when considering
interim cover. For example, annual maintenance, periodic
inspections, enforcement, and loss of capacity must be included
in the evaluation. Each of these items are discussed in the
following sections. ' -
2.2 Effectiveness of Interim Cover
The effectiveness of any earthen cover depends mainly on its
moisture content and depth, and the homogeneity and integrity of
the cover layer. The effectiveness of an earth cover was
estimated in the Draft BID by using diffusion equations which
take into account the cover material and tailings density,
porosity, specific gravity and moisture content, and by assuming
these properties do not vary throughout the cover or tailings, or
with time. These idealized conditions would not typically be
achieved in practice and the actual effectiveness would probably
be less than the calculated effectiveness. The applicability of
the basic diffusion equation to relatively shallow earth covers,
such as 0.3 m, is also questionable.' •
The key variable effecting'the effectiveness of an earthen
cover of given depth in controlling radon-222 is its moisture
content. An example of "this variation is shown in Figure 2-1.
For a 1-meter depth of'cover, with 12 percent moisture, about
20 percent of the radon-222 released from the tailings surface
would still emanate from the cover. If the cover material dries
out to 6 percent, about 47 percent of the radon-222 from the
tailings would emanate from the :cover. Thus, the emissions
increased by a factor of 2.35 (47/20) or the effectiveness
decreased by about 33 percent.• Similar5losses in effectiveness-
are evident for all depths of earth cover. However, a thicker
cover will not dry out as completely or quickly as a thin cover,
and soils with' a higher silt and- clay content will retain more
moisture much longer than a sandy soil.
In addition to the cover material's moisture content, the
overall integrity of the cover must be maintained in order to
reduce radon-222 emanations. Wind and rain erode an earth cover,
thus reducing its depth and subsequent effectiveness. In
addition, cracks from freeze-thaw cycles, subsidence", or
burrowing animals decrease a shallow cover'.s effectiveness. When
final reclamation is implemented", "gravel, rip-rap, or vegetation
cover, and additional grading and runoff control'are included-to
decrease erosion'and ensure the long-term integrity of the
cover. These items are not included in interim earth covers
since, by definition, they are not designed as long-term control
techniques.
C-5'
-------
CJ
«z
cc
0.90
0.80 -
0.70 -
0.60 -
0.50 -
0.40
0.30 -
0.20 -
0.10 -
1 I I I I T i I 1 I l I I i I I I
MOISTURE CONTENT OF TAILING IS 8%
1 FOOT DEPTH
4 5 6. 7 8 9 10 11 12 13 14 15 16 17 18 19 20
EARTH COVER MOISTURE CONTENT, % by weight
Figure 2-1. Variation in.earth cover effectiveness with moisture content,
C-6
-------
The combination of surface drying, erosion, and loss of
integrity reduces the^ interim cover's effectiveness. This loss
in effectiveness would be especially evident in a shallow cover
of only 0,3 m. The exact decrease in effectiveness is not known
and cannot be calculated readily since these factors are highly
variable and site-specific. The loss in effectiveness can be
offset by frequent maintenance of the earth cover, as discussed
under Operational Aspects.
2.3 Applicability of Interim Cover
Limitations regarding the placement of interim cover are
associated with physical conditions of the tailings. The water
content of the tailing and the slope of the surface are
controlling factors. Tailings must be dry in order to support
earthmoving equipment and the cover itself. Tailings are
dewatered and dried by seepage from the impoundment and .by
evaporation. -: (The time required to achieve sufficient dryness is
discussed in Section 2.4.) The tops of tailings impoundments are
essentially flat, and if thoroughly dry, would pose no
difficulty to placement of interim cover. However,
11 impoundments at 6 mill sites are constructed with dams made of
coarse tailings (Type 1 impoundments). The outer faces of these
dams are steep (approximately 2.5:1, H:V). Placement of interim
cover on these dams would be difficult because of the steep
slope. In addition, seepage through the dams could cause
instability and slumping of the interim cover. Conversely, the
interim cover could cause the phreatic surface to rise in the dam
by inhibiting seepage through the dam. This occurrence could
cause a decrease in the stability of the dam itself.
In evaluating the applicability of interim cover, three
distinct situations currently exist on tailings impoundments.
These conditions and how they affect the applicability of interim
cover are addressed below.
Interim Cover' on Dams Constructed of 'Tailincfs '
The faces of these dams are at a~ slope of about 2.5:1 (H:V),
it probably would not be possible to apply and maintain
interim cover to these steep areas without recontouring the
impoundment. Recontouring'would result in a significant
loss of storage capacity. Additionally, it would be very
difficult to compact cover material placed on such a slope.
Uncompacted material would be subject to more rapid wind and
water erosion.
These slopes (300 acres, total) represent 8 percent of total
area and 15 percent of currently dry areas.
C-7
-------
20 percent of this sloped area is at impoundments that have
been filled to capacity (Uravan). These impoundments would
more logically apply final cover.
12 percent of this sloped'area is at impoundments at sites
that have indicated decommissioning will begin soon (L-Bar
and Churchrock). These impoundments would, more logically
apply final cover.
31 percent of the total sloped area is at one major
impoundment (Homestake), that is a 4 sided structure, with
steep slopes (2 to 2,5:1) that would be most difficult to
place interim cover on without recontouring.
Coarse tailings are reported to have lower Ra-226 content
than the slimes. Therefore, these areas have a lower source
term than the tops of the impoundments.
Piezometers and movement benchmarks used to monitor the
stability of these dams would have to be extended and their
use uninterrupted during application of interim cover.
It would be necessary to provide drainage between the
"tailings and the earthen cover to allow any seepage through
the dam to escape without building up a hydrostatic head
that could cause dam failure. Seepage through these dams is
inherent to their design and must be maintained. A drainage
system, such as a blanket drain, would also provide a
- permeable path for radon-222 migration, making at least the
lower portion of the cover less effective.
Interim Cover on Tops ofUnlined Type 1 and Type 2 Impoundments
Tops of Type 1 and 2 unlined impoundments account for 75
percent of the total tailings area.
Current dry areas on top of"these piles equal
44 percent of the total and 73 percent of the currently dry
areas.
These areas are flat and if thoroughly dry, interim cover
could be placed-easily.
The length of time required for drying prior to placement of
cover is•site-specific and will vary depending on
impoundment design, climate, and hydrogeology. Some
impoundments, particularly those in climates characterized
by high net evaporation and permeable soils (e.g., New
Mexico) would dry sufficiently in a relatively short time,
C-8
-------
1 year for example. Heavy equipment could access most .of
the area at that time. Other sites having lower
evaporation, more rainfall/snowfall and/or less permeable
soils that limit seepage could require considerably longer
to dewater and dry (5 to 10 years for example).
Placement of 0.3-meter cover on these dry areas is
demonstrated technology and is an NRC recommendation during
standby to control windblown tailings.
Interim Cover on LinedType 2 and Type 3 Impoundments
Tops of lined impoundments represent 14 percent of the total
area.
Dry areas on lined impoundments make up 11 percent of
currently dry areas.
Issue of lost capacity is more important on these
impoundments because their construction cost is greater.
The dry out period will be longer than in unlined piles
because seepage is limited.
Evaporation, Ponds
In addition to tailings impoundments, several mills use
evaporation ponds for water management. Decant water from the
tailings impoundments and often mine pump-out water and seepage
from the tailings impoundment is pumped to these evaporation
ponds. Some tailings slimes and dissolved radium-226 are .carried
along with the water and are deposited in these ponds. Upon
drying, these solids emit radon-222. Interim cover was applied
to dry areas of evaporation ponds in the Draft BID. In the
current evaluation,.interim cover is not applied to evaporation
ponds because: 1) these ponds receive water from sources other
than tailings impoundments and would need to remain in service
during standby periods; 2) the quantity of tailings present and
their contribution to the site's source term are not accurately
known; 3) these ponds will eventually be excavated and the
material placed on the tailings impoundments prior to.
reclamation; and 4) these ponds are lined to prevent seepage.
Movement of heavy equipment on these ponds could destroy the
integrity of the liners.
Summary of Applicability
Because of the significant uncertainties and perceived
difficulties and complications associated with the application of
C-9
-------
interim cover to the outward faces of dams-, constructed of coarse
tailings, in addition to the relatively lower source term of
these areas, the current evaluation of interim cover assumes that
these slopes remain uncovered. All other tailings surfaces can
be 'covered 'as soon' as they are dry enough to support earthmoving
equipment and the cover itself. In this evaluation .-of interim
cover, it is assumed that dry areas of evaporation ponds are not,
covered for the reasons stated above.
2 . 4 Timing of 'Interim Cover
The. evaluation of interim cover includes several
assumptions that are based on best engineering judgment,
regarding the timing. of interim cover applications (i.e., when
can interim cover be applied) . The assumptions are specified
below:
Dry areas (as specified in Table 4-2 of the BID) o'f tailings
impoundments that are on standby status or that have been
filled to capacity can be covered immediately.
' and, ponded areas of tailings impoundments that are on
.standby status or that have been filled to capacity will
dewater and dry over a 5-year period,' at which time it is
assumed interim cover could be applied.
Interim cover is 'not applied to operating impoundments. The
method of placing tailings1 in impoundments is to discharge
from several' points around- the perimeter or to- move the
discharge point around the perimeter? -in 'either case interim
cover would not be compatible with these operations. These
impoundments receive interim cover when they go to standby
status (i.e., dry areas covered immediately, wet and ponded
areas covered in 5 years) .
The useful life of an interim cover is limited by return to
active status at which time the earthen cover is covered
with new tailings. In the current evaluation, impoundments
would become active sometime between 1990 and 1995 and a
second application of interim cover would be made in the
year 2000.
It was initially assumed for the base case that an inactive
impoundment would remain uncovered for 40 years. This
appears unrealistic and a shorter time period of no more
than 20 years is more representative.
C-10
-------
2.5 Operational' Aspects
The effectiveness of an unmaintained interim cover in
limiting the escape of radon-222 can be expected to deteriorate
with time. The rate of deterioration is highly site-specific.
It depends upon many variables such as frequency and intensity of
precipitation and wind, characteristics of the interim cover
(e.g., moisture content, type of soil, compaction, grade, etc.)/
and drainage basin considerations (e.g., run-on and run-off). To
prevent or minimize deterioration of interim covers, maintenance
practices would be employed, .Annual maintenance would include
periodic regrading or placement of additional cover material.
Additionally,'periodic inspections of the interim cover system
would be required to ensure its integrity. Such expenses are
estimated to be 5 percent of the capital cost of the interim
cover per year.
One important aspect regarding interim cover is the issue of
lost capacity. An interim cover of 1 meter applied over a
tailings-impoundment that is on standby status results in a loss
of tailings capacity equal to the cover volume. .If interim cover
is applied more than once (i.e., a covered impoundment goes from
standby to operational status and- back to standby), the effect of
lost capacity is multiplied. Information received from the NRC
on the capacity of existing piles and the capacity loss
associated with an application of interim cover is presented in
Table 2-1. Some impoundments would have no remaining .capacity if
interim cover were applied, while others would lose as little as
9 percent of their remaining capacity. Information on the
remaining capacity at other sites- is not currently available.
C-ll
-------
Table 2-1. lost capacity associated with a single application of interim
cover (0.9 m) over the entire impoundment in nonagreement states^ .
(1000 tons)
Mill
Current
tailings
Licensed
quantity
of tailings
Quantity of interim
cover - 0.91 meter thick
(% of remaining capacity)
White Mesa 1,500
la Sal 2,954
Moab 10,600
Shootaring Canyon . 174
Gas Hills (Pathfinder) 11,762
Split Rock 7,700
Gas Hills (UMETCQ) 9,600
Bear Creek 4,100
Shirley Basin 6,800
Sweetwater 3,900
5,137
5,041
15,600
5,000
20,468
8,000
9,900
5,700
8,800
9,100
1,958
205
1,176
411
793
500
1,023
882
1,364
1,764
(54)
(10)
(24)
(9)
(9)
(100)
(100)
(55)
(68)
(34)
(a)
NIC Uranium Field Office, Denver, Colorado, £pril 1986. Information on
remaining capacity of inpoundroents in agreement states «as hot available.
C-12
-------
REFERENCES FOR CHAPTER 2
ORNL 83 Oak Ridge National Laboratory, "Guidance for Disposal
of Uranium Mill Tailings; Long-Term Stabilization of
Earthen Cover Materials," Prepared for U.S. Nuclear
Regulatory Commission, NUREG/CR-3199, ORNL/TM-8685.
October 1983.
C-13
*U. S, GOVERSHENT PRIHTMG OFFICE 1986; 621-735/60533
------- |