United States Office of EPA 520/1 -86-001
Environmental Protection Radiation Programs January 1986
Agency Washington, D.C. 20460
Radiation
4>EPA Proposed Standard for
Radon - 222 Emissions from
Licensed Uranium Mill
Tailings
Draft Background Information
Document
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40 CFR Part 61 EPA 520/1-86-001
National Emission Standards
for Hazardous Air Pollutants
DRAFT BACKGROUND INFORMATION DOCUMENT
PROPOSED STANDARD FOR RADON-222 EMISSIONS
FROM LICENSED URANIUM MILL TAILINGS
January 23, 1986
U.S. Environmental Protection Agency
Office of Radiation Programs
Washington, D.C. 20460
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CONTENTS
Figures vi
Tables viii
1. INTRODUCTION 1-1
1.1 History of Standard Development 1-1
1.2 Purpose 1-2
1.3 Content 1-2
1.4 Other EPA Standards Affecting Uranium Mills 1-3
1.5 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 2-4
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 2-7
2.3.3 Comparison of Risk Estimates 2-10
2.3.4 Selection of Risk Coefficients 2-13
2.4 Estimating the Risks 2-15
2.4.1 Exposure 2-15
2.4.2 Risk Estimation 2-17
References 2-19
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 3-12
3.4.1 Ore Handling and Preparation 3-14
3.4.2 Mill Emissions 3-16
3.4.3 Emissions From Tailings Disposal 3-17
3.5 Transport and Risk Assessment 3-18
3.5.1 Overview of EPA Analysis 3-18
3.5.2 Air Dispersion Estimates 3-19
3.5.3 Risk Estimates 3-20
iii
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CONTENTS (continued)
3.6 Measurement of Radon-222 3-20
3.6.1 Ambient Air Samplers 3-20
3.6.2 Concentrating Samplers That Measure
Radon-222 Emanation From Surfaces 3-20
References 3-22
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 4-18
4.2.4 Utah 4-20
4.2.5 Washington 4-23
4.2.6 Wyoming 4-26
4.3 Population Within 5 km (3.1 mi) of Existing
Tailings Impoundments 4-32
References 4735
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 Individual 6-1
6.2.2 Regional Population 6-2
6.2.3 National 6-5
6.2.4 Risks from New Tailings Impoundments 6-6
References 6-9
7. CONTROL TECHNIQUES 7-1
7.1 Description of Control Practices 7-1
7.1.1 Earth Covers 7-2
7.1.2 Water Cover 7-5
7.1.3 Water Spraying 7-8
7.1.4 Other Control Techniques 7-8
IV
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CONTENTS (continued)
Page
7.2 Control Practices Applicable to Existing
Tailings Impoundments 7-12
7.2.1 Interim Controls 7-12
7.2.2 Final Reclamation 7-14
7.2.3 Comparison of Interim and Final
Controls 7-15
7.3 Control Practices Applicable to New Tailings
Impoundments 7-15
7.3.1 Single Cell Tailings Impoundment 7-15
7.3.2 Phased Disposal Tailings Impoundment 7-21
7.3.3 Continuous Disposal 7-25
7.4 Summary of Radon-222 Control Practices 7-30
References 7-37
Appendices
A - Diagrams of Uranium Mill Sites and Tailings Impound-
ment s A-1
B - Cost Estimates for Existing and Model New Uranium
Mill Tailings Impoundments B-l
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FIGURES
Number Page
2-1 Radon-222 decay series 2-3
2-2 U.S. lung cancer mortality by age—1970 2-8
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 3-9
3-4 Simplified flow diagram of the alkaline leach-caus-
tic precipitation process 3-10
3-5 Qualitative illustration of radon-222 emissions from
licensed uranium milling process 3-13
3-6 Effect of ore pile depth on hyperbolic tangent term
in radon-222 flux equation 3-15
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 4-13
4-4 Location of mills in Texas 4-19
4-5 Location of mills in Utah 4-21
4-6 Location of mills in Washington 4-24
4-7 Location of mills in Wyoming 4-27
7-1 Changes in radon-222 penetration with earth cover
thickness 7-3
7-2 Radon emanation coefficients for tailings samples 7-9
7-3 Size and layout of the model single cell tailings
impoundment 7-18
7-4 Estimated radon-222 emissions from a model single
cell tailings impoundment 7-19
7-5 Size and layout of model phased disposal impoundment 7-23
VI
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FIGURES (continued)
Number Page
7-6 Estimated radon-222 emissions from a model phased-
disposal impoundment 7-24
7-7 Size and layout of the model continuous-disposal im-
poundment 7-28
7-8 Estimated radon-222 emissions from a model continuous-
disposal impoundment 7-31
7-9 Estimated radon-222 emissions from a model continuous/
single-cell disposal impoundment 7-32
vii
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TABLES
Number Page
2-1 Exposure equivalent for one year of exposure to one
working level (2.08 x 10~5 joules per cubic meter)
as a function of age 2-6
2-2 Age-dependent risk coefficients and minimum induction
period for lung cancer due to inhaling radon-222
progeny 2-10
2-3 Estimated risk from exposures to radon-222 progeny 2-12
2-4 Radon-222 decay product equilibrium fraction at
selected distances from the center of a tailings
impoundment 2-16
2-5 Lifetime risk for lifetime exposure to a given level
of radon-222 progeny 2-17
3-1 Properties of radon-222 3-3
4-1 Operating status and capacity of licensed conventional
uranium mills as of November 1985 4-2
4-2 Summary of current uranium mill tailings impoundment
area and radium-226 content 4-6
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 4-33
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 4-34
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 AIRDOS-EPA code inputs and estimated exposure based
on emissions of 1 kCi/y 6-4
6-3 Summary of health effects from existing tailings
impoundments 6-5
6-4 Summary of nationwide health effects for tailings
impoundments 6-6
viii
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TABLES (continued)
Number Page
6-5 Summary of fatal cancers from current tailings
impoundments 6-6
7-1 Percent reduction in radon-222 emissions attained by
applying various types of earth cover 7-4
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 flux as a
function of thickness (meters of different soils)
and type of earth 7-7
7-4 Cost and Effectiveness of applying earth cover to
existing tailings impoundments 7-16
7-5 Average radon-222 emission rate from model single-
cell tailings impoundments 7-17
7-6 Estimated costs for a model single-cell tailings
impoundment 7-20
7-7 Average radon-222 emission rate for model single-cell
and phased-disposal tailings impoundments 7-22
7-8 Estimated costs for a model phased-disposal impound-
ment 7-26
7-9 Estimated radon-222 emission rates for model single-
cell, phased disposal, and continuous-disposal tail-
ings impoundments 7-29
7-10 Estimated costs for a model continuous disposal im-
poundment 7-33
7-11 Summary of estimated radon-222 emissions from new
model tailings impoundments 7-34
7-12 Summary of estimated costs for new model tailings
impoundment 7-36
IX
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Chapter 1: INTRODUCTION
1.1 History of Standard Development
Although the U.S. Environmental Protection Agency (EPA) has consid-
ered radon-222 in several regulatory actions, no specific emission stan-
dard for this radionuclide has yet been proposed or promulgated for
operating licensed uranium mills.
On January 13, 1977 (42 FR 2858), EPA issued Environmental Radiation
Protection Standards for Nuclear Power Operations. These standards,
promulgated in Title AO, 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 regard-
ing 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 achiev-
able (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 of 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, a
standard that, except for radon-222 emissions, provided protection equiva-
lent to that of the Clean Air Act. During the comment period for the
Clean Air Act standards, it was noted that radon-222 emitted from opera-
ting uranium mills and their actively used tailings piles were not sub-
ject to any current or proposed EPA standards, and that such emissions
could pose significant risks.
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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 con-
sidering emissions standards for licensed uranium mills and solicited
information in the following areas:
0 Radon-222 emission rates from uranium mills and associated
tailings piles
0 Local and regional impacts- due to emissions of radon-222 from
uranium mills and associated tailings piles prior to permanent
disposal
Applicable radon-222 control options and strategies, including
work practices
0 Feasibility and cost of radon-222 control options and strat-
egies
0 Methods of determining compliance with a work practice type of
standard to control radon-222 emissions
0 Impact of radon-222 controls on the uranium industry
Pursuant to the citizens' suit provision of the Act, the U.S. Dis-
trict Court for the Northern District of California directed EPA to
promulgate standards for other sources of radionuclide emissions, which
could include radon-222 emissions from licensed commitments. 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 has ordered the EPA to issue final standards for
radon-222 emissions from licensed uranium mills and mill tailings impound-
ments by May 1, 1986.
1.2 Purpose
This Background Information Document supports the Agency's proposed
rules on radon-222 emissions from licensed uranium milling activities.
The integrated risk assessment that it includes provides the scientific
basis for these actions.
1.3 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.
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The sources of radon-222 in uranium milling and the factors affect-
ing the rate of radon-222 emissions are 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.
Chapter 4 contains a description of each licensed mill, its associ-
ated tailings impoundments, and its estimated milling production rates.
Chapter 5 presents estimates of radon-222 emissions from the existing
tailings impoundments.
Chapter 6 presents the maximum baseline industry risk assessment for
individuals and regional and national populations. The control tech-
niques and work practices that can be used to reduce radon-222 emissions
are described in Chapter 7, and the resulting emissions after application
of these control methods are estimated.
Information for this study was compiled from the technical litera-
ture, previous studies by EPA and the Nuclear Regulatory Commission,
comments resulting from rulemaking notices, and discussions with industry
representatives.
1.4 Other EPA Standards Affecting Uranium Mills
Several EPA environmental standards already apply to uranium mill
tailings. The previously mentioned Title 40, Code of Federal Regulations
(CFR) Part 190, Environmental Radiation Protection Standards for Uranium
Fuel Cycle Operations was promulgated by EPA on January 13, 1977 (42 FR
2858). These standards specify the maximum allowable radiation levels
during normal operations of the uranium fuel cycle facility. These
standards cover radiation exposures due to environmental release of
uranium byproduct material, with the exception of emissions of radon-222
and its decay products. 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 nonradioac-
tive materials to surface waters from uranium byproduct materials.
The EPA promulgated 40 CFR Part 261, Subpart F—Groundwater Protec-
tion—on July 26, 1982 (47 FR 32274) under the 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 re-
quirements applying to the possession, transfer, and disposal of similar
hazardous material regulated by EPA under the SWDA.
Under the Uranium Mill Tailings Radiation Control Act of 1978 (UMTRCA),
Public Law 95-604, 42 USC7901, the EPA has promulgated standards for the
control of radionuclides from both inactive and active uranium mills.
Under this Act, Congress directed EPA to "promulgate standards of general
1-3
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application for the protection of the public health, safety, and the en-
vironment from radiological and nonradiological hazards associated with
the processing and with the possession, transfer, and disposal of by-
product material...at sites at which ores are processed primarily for
their source material content or which are used for the disposal of such
byproduct material." For these sites, the Act defines the term "byprod-
uct material" as "...the tailings or wastes produced by the extraction or
concentration of uranium or thorium from any ore processed primarily for
its source material content." The Act assigns the responsibility for
implementation and enforcement of these standards to the NRC and its
Agreement States through their licensing activities.
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). 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 rea-
sonably 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) Exceed 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 impound-
ments of uranium byproduct materials as far below the Federal Radiation
Protection Guides as is practicable at each licensed site."
This standard also specifies that radon-222 emissions are limited to
20 picocuries per square meter per second (pCi/m2s) after mill closure.
This limitation does not apply to sites that contain a radium-226 concen-
tration 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
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1.5 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. Specific standards pertaining to radon-222, which are found in
Part 20, limit atmospheric radon-222 concentrations to 3 x 10~8 yCi/ml
(30 pCi/liter) in restricted areas (i.e., areas within the mill property)
and 3 x 10~9 yCi/ml (3 pCi/liter) in unrestricted areas. These concen-
trations are approximately equivalent to one-third and one-thirtieth of a
working level,* respectively. The NRC has also recently issued amend-
ments to its regulations governing uranium mill tailings disposal (100 FR
Part 40) as published on October 16, 1985 (50 FR 41852). These amend-
ments conform to the EPA regulations for tailings disposal.
The NRC has entered into agreement with a number of States to pro-
vide 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 impound-
ments to minimize ground-water contamination. In addition, States in-
spect tailing impoundment dams to ensure that they are built and main-
tained 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.
1-5
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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 number of lung cancers re-
sulting 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 esti-
mates of radiation risks by both individual radiation scientists and
expert advisory groups. Readers should bear in mind that estimating
2-1
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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. 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 require-
ments.
2.2 Radon-222 Exposure Pathways
2.2.1 Physical Considerations
Radon-222 from uranium milling operations enters the general en-
vironment 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 important 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 par-
ticles 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 polo-
nium-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 de-
livered 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 disagree-
ment exists about the types of bronchial cells in which cancer origi-
nates. 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.
Ingrowth of Radon-222 Decay Products
At the point where radon-222 diffuses out of the tailings pile sur-
face, the concentration of associated radon-222 decay products is zero
2-2
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10
-4
21
82
0
Pb
22.3
yrs.
210
83Bl
P
5.01
days
21
84
0
Po
Figure 2-1. Radon-222 decay series.
2-3
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because those decay products generated prior to diffusion from the sur-
face 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-life decay products is even-
tually obtained. At secular equilibrium, the activity of radon-222 and
all its short-half-life decay products is equal and the alpha activity
per unit of radon-222 concentration is at its maximum. As a means of ac-
counting 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 one; however, depletion
processes, such as dry deposition and precipitation scavenging, selec-
tively 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 build-
ing ventilation rate is the principal factor affecting the indoor equili-
brium 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 equi-
librium 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 in-
door effective equilibrium fraction does not depend greatly on the dis-
tance from the tailings pile. In this assessment, an effective equilib-
rium 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).
2.2.2 Characterizing Exposures to the General Population Vis-a-vis
Underground Miners
Although considerable progress has been made in modeling the deposi-
tion of particulate material in the lung (Ha82, Ja80, 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 bron-
chial 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
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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 occupa-
tional 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 devel-
oped 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 gen-
eral population exposed to radon-222 progeny, as explained in the fol-
lowing paragraphs.
For a given concentration of radon-222 progeny, the amount of poten-
tial alpha energy a member of the general population inhales in a month
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 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).
The EPA 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, NASA73).
Therefore, 30 liters appears to be a more realistic estimate of the aver-
age minute volume for this group. Based on this minute volume, a mine
worker inhales 3.6 x 103 cubic meters in a working year of 2000 hours
(ICRP79). One working level of radon-222 progeny is 2.08 x 10~5 joules
per cubic meter (1.3 x 105 MeV per liter); therefore, in a working year,
the potential alpha energy inhaled by a mine worker exposed to one work-
ing level is 7.5 x 10 2 joules.
According to the ICRP Task Group report on reference man (ICRP75),
an inhaled air volume of 2.3 x 101* liters per day is assumed for adult
males in the general population and 2.1 x lO*4 liters per day for adult
females, or an average of 2.2 x 104 liters per day for members of the
2-5
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adult population. This average volume results in 1.67 x 10 l joules per
year of inhaled potential alpha energy from an 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, this amounts to an annual exposure
equivalent of 27 WLM to an adult member of the general population exposed
24 hours a day. 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.
The smaller bronchial area of children as compared with that of
adults more than offsets their lower per-minute volume; therefore, for 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 informa-
tion was used to prepare Table 2-1, which lists the age-dependent poten-
tial exposure equivalent used in the risk assessments 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. Exposure equivalent for one year of exposure
to one working level (2.08 x 10 5 joules per cubic meter)
as a function of age
Age of
general population
(years)
0-2
3-5
6-11
12-15
16-19
20-22
23 or more
Lifetime Average
Exposure
equivalent
(WLM) W
35
43
49
43
38
32
27
31.4
Assuming a WLM corresponds to about 6.2 x 10~3 joules of potential
alpha particle energy inhaled (see text).
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" (EPA79) and Final Environmental Impact
Statements (EPA82, 83a).
2-6
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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. Two recent reviews are of
particular interest. The 1980 NAS BEIR-3 Report (NAS80) contains a
review of epidemiological studies on mine workers. 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 assumptions are the
same for all of the data sets (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 expo-
sure. 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 obser-
vation. 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 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 than the absolute risk model
because of the high prevalence of such cancers at old age. The number of
lung cancer deaths that occurred in the U.S. population as a function of
age in 1970 is shown in Figure 2-2. The decrease in the number of deaths
for ages greater than 65 years is due to depletion of the population by
competing risks, not a decrease in the age-specific incidence of lung
cancer mortality, which is relatively constant until age 95 (NCHS73).
The age-specific mortality of underground mine workers dying of radio-
genic lung cancer shows the same pattern of death as a function of age as
the general male population (Ra8A, E185). In a recent review (E185), 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.
2.3.2 The EPA Relative 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
2-7
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10.00
8.75
/ MALES \
7.50
6.25
LU
Q
(j
<
CD
5.00
3.75
2.50
1.25
I O-O-
••on
15
30
45
60
75
90
105
AGE AT DEATH (years)
Figure 2-2. U.S. lung cancer mortality by age—1970.
2-8
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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 Bu81 and EPA84 is used to project this risk over a full
lifespan.
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 in-
crease among these survivors, whereas their relative risk has remained
reasonably constant (Ka82). The UNSCEAR, the ICRP, and the 1980 NAS Com-
mittee 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 con-
servative and actually may underestimate the risk at low doses. The 1980
NAS BEIR Committee reached a similar conclusion (NAS80).
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 fatal-
ities per 106 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. Al-
though absolute risks were comparable for 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 of 3 percent per WLM may be too high when
applied to the population as a whole. Further followup 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.
2-9
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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, no specific
data for lung cancer have been collected (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 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.
2.3.3 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. This was done by means of the same life table analysis that
was used to calculate other EPA risk estimates (Bu81).
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
diagnosis (cases per 106 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
2-10
-------
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 relativfe
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 pro-
jection 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 AECB 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 popu-
lation) by determining the "best estimate" risk (see p. 114 in Th82).
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; there-
fore, the simple proportional relationship of general population deaths
to male deaths should give a reasonable estimate.
International Commission on Radiological Protection
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 epide-
miological approach; i.e., the exposure to mine workers in WLM and the
risk per WLM 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.
2-11
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Table 2-3. Estimated risk from exposures to radon-222 progeny
Organization
EPA(a)
NAS BEIR-3
AECB
ICRP
UNSCEAR
NCRPW
Fatalities per
106 person WLM
760 (460)^
730 (440) J?;
600 (300) W
150-450
200-450
130
Exposure
period
Lifetime
Lifetime
Lifetime
Working
lifetime
Lifetime
Lifetime
Expression
period
Lifetime
Lifetime
Lifetime
30 years
40 years
Lifetime
Reference
EPA84
NAS 80
Th82
ICRP81
UNSCEAR? 7
NCRP84
(a)
The number of fatalities per million-person WLM listed for EPA and
NAS BEIR-3 differs from those previously published by EPA [860 fatal-
ities per 106 PWLM and 850 fatalities per 106 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 106 PWLM.
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 ex-
posure to 1 WL for a year corresponds to 51.6 WLM.
(c)
Adjusted for the 1970 U.S. general population; see text.
Assumes risk diminishes exponentially with a 20-year half time.
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 compar-
atively small dose from radon-222 progeny and where human lung cancer is
seldom, if ever, observed.
UNSCEAR
* ;• T™co Na^ions Scientific Committee on the Effects of Atomic
Radiation (UNSCEAR) estimate shown in Table 2-3 is for a general popula-
tion and assumes an expression time of 40 years (UNSCEAR77) . Like the
2-12
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ICRP, UNSCEAR did not make use of an explicit projection of risk of fatal
lung cancer over a full lifetime.
National Council on Radiation Protection 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 Paster-
nack (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 106 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 ex-
ponentially 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 as-
sumed 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 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. Con-
versely, the three lower risk estimates shown in Table 2-3 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 repre-
sent 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 as-
sumed that the risk decreases exponentially after the exposure.
2.3.A 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), EPA averaged the estimates of BEIR-3, the EPA
2-13
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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 is believed to be
outside the lower bound. Therefore, the EPA chose relative risk co-
efficients 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 pos-
sible 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 appear 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 averages 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 CWLM 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 cur-
rent 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 (NIOSH85). In the same year, a report on 8500 Saskatche-
wan 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 in dwellings on the Swedish island of
Oeland (Ed83, 84). Data from this study could justify a relative risk
coefficient of about 3.6 percent per WLM.
Three studies now 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 co-
efficients were obtained by rounding off the coefficients listed above to
the nearest whole number.
The Radiation Advisory Committee of the Science Advisory Board of
EPA (SAB85) recommended that EPA use a range of 1 to 4 percent per WLM,
as they believed that both underestimations of exposure and the effect of
2-14
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random error could have biased the risk coefficient downward. The Com-
mittee also recommended use of single-digit risk coefficients to avoid
the suggestion of a precision that does not exist.
2.4 Estimating the Risks
2. A. 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 x Radon progeny x 9.84 x 10 3
concentration concentration equilibrium fraction
(WL) (pCi/liter) ^e^^ (WL per Pci/liter)
For individuals and regional populations, emission data and meteorologi-
cal 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) .
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 out = 1.0 - 0.0479e-t/4'39 -2. 1963e-t/38'6 + 1.2442e-t/28'4
e
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 1 (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 equilib-
rium with the radou-222 in the ventilation air (EPA83b) . A simple linear
interpolation is used to obtain the indoor equilibrium fraction:
f in = 0.35 (1 + f °Ut).
e e
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:
2-15
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feeff = 0.75
0.25
= 0.2625 + 0.5125 f
out
An example of the case for a 3.5 m/s windspeed and various distances
from the source is given in Table 2-4.
Table 2-4. Radon-222 decay product equilibrium fraction , .
at selected distances from the center of a tailings impoundment
Distance (m)
(a)
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
0.008
0.008
0.008
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
0.353
0.353
0.353
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
0.267
0.273
0.276
0.278
0.284
0.289
0.293
0.302
0.311
0.331
0.349
0.366
0.382
0.414
0.443
0.471
0.522
0.566
0.650
0.698
Calculations (tabulated to 3 decimal places to facilitate compari-
sons) presume a 3.5 m/s windspeed for the outdoor equilibrium frac-
tion; 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 effectiv£ equilibrium fraction based on 75 percent of time
indoors and 25 percent of time outdoors.
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.
2-16
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2.4.2 Risk Estimation
After the exposure 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 Bunger et al. (Bu81). Relative risk projections for life-
time 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 250 deaths/106 person WLM and 1000 deaths/106 person WLM
respectively.
These risk coefficients in the CAIRD Code (Co78) can be used 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.
Results of representative calculations of lifetime risk are given in
Table 2-5.
Table 2-5. Lifetime risk for lifetime exposure to a given
level of radon-222 progeny
Lifetime risk of lung cancer
Radon-222 progeny 4 percent increase 1 percent increase
concentration (WL) per WLM per WLM
0.
0.
0.
0.
0.
0001
001
01
1
2
2.
2.
2.
2.
3.
3
3
3
0
6
X
X
X
X
X
10~
10
10
10
10
1+
3
2
1
1
5.
5.
5.
5.
1.
6
6
6
4
0
X
X
X
X
X
10
10
10
10
10
~5
-it
-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 factors were
used with WL exposures that were calculated by using radon-222 concentra-
tions and an f determined as shown in Table 2-4 to estimate the risks
£
listed in Table 6-1 for the maximum individual risk.
Regional
Collective (population) risks for the region are calculated from the
annual collective exposure (person WLM) for the population in the assess-
ment area by a computerized methodology known as AIRDOS-EPA (Mo79). An
effective equilibrium fraction of 0.7 is presumed because little collec-
tive exposure takes place near the source.
2-17
-------
where
(WLM)
WLM).
Formally, the annual collective exposure, SE, can be defined as:
SE = /"En(E)dE
S is the collective exposure (person WLM), E is the exposure level
, and n(E) is the population density at exposure level E (person/
Practically, however, the collective exposure is calculated by
dividing the assessment area into cells and then calculating the popula-
tion, N. (persons), and the annual exposure, E (WLM), for each one. The
collective exposure is then calculated by the following expression:
S = ZEiNi
SE 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.
2-18
-------
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2-19
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Ja80 Jacobi W. and Eisfeld K., Dose to Tissue and Effective Dose
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Ja81 James A. C., Jacobi W. and Steinhausler F., Respiratory Tract
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Colorado, edited by Manual Gomez, Society of Mining Engineers,
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Assessment of Radon-Exposed Miners on the Basis of a Propor-
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Ka82 Kato H. and Schull W. J., Studies of the Mortality of A-bomb
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Rad. Research, 9£, 395-432, 1982. (Also published by the
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Span Study Report 9. Part 1.)
2-21
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Lu71
Mc78
Mc83
Mo76
Mo79
Mu83
NAS72
NAS80
NASA73
NCHS73
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Lundin F. E. Jr., Wagoner J. K. and Archer V. E., Radon
Daughter Exposure and Respiratory Cancer, Quantitative and
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Service, U.S. Department of Health, Education and Welfare,
Washington, D.C., 1971.
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
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McDowell E. M. and Trump B. F., Histogenesis of Preneoplastic
and Neoplastic Lesions in Tracheobronchial Epithelium, Surv.
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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, Massachu-
setts, December 1976.
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 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.
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Effects of Populations of Exposures to Low Levels of Ionizing
Radiation: 1980, Committee on the Biological Effects of Ioniz-
ing Radiation (BEIR-3 Report), NAS, Washington, D.C., 1980.
National Aeronautics and Space Administration, Bioastronautics
Data Book, NASA SP-3006, 2nd Edition, edited by J. R. Parker
and V. R. West, NASA, Washington, D.C., 1973.
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Statistics - Mortality, Cause of Death Summary - 1970, PB80-
133333, NTIS, Washington, D.C., 1973.
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.
2-22
-------
NIH85 National Institutes of Health, Report of the National Insti-
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NRC79 Nuclear Regulatory Commission, Draft Generic Environmental
Impact Statement on Uranium Milling, Volume II, NUREG-0511,
USNRC, Washington, D.C., 1979.
Oa72 Oakley D. T., Natural Radiation Exposure in the United States,
ORP/SID 72-1, USEPA, Washington, D.C., 1972.
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Exhalation, Ventilation, and Deposition Processes Upon the
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Air, Health Physics, J34_, 465-473, 1978.
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1983.
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Energy Cost, Work: Man, W. B. Sanders Co., Philadelphia, 1956.
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2-23
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UNSCEAR77 United Nations Scientific Committee on the Effects of Atomic
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Department of Statistics, Stanford University, California,
1983.
2-24
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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 Origin and Properties 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 uranium-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 envi-
ronmental impact due to the short half-lives of the decay products.
Important properties of radon-222 are presented in Table 3-1 for infor-
mation 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 hun-
dreds 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
3-1
-------
1.17 m
a,y 1.6 x 10J y
5.01 d
138.4 d
y - years
d - days
m - minutes
s - seconds
Figure 3-1.
Uranium-238 decay chain and half-lives of
principal radionuclides.
3-2
-------
Table 3-1. Properties of radon-222
(a)
Property
Value
Atomic number
Atomic weight
Boiling point
Melting point
Density
Solubility in water
Half-life
Decay modes and energy
86
222
-62°C
-71°C
9.73 grams/liter
51 cm3 in 100 grams at 0°C
8.5 cm3 in 100 grams at 60°C
3.824 days
5.4897 MeV
0.512 MeV
(a)
Source: Chemical Engineer's Handbook, Perry, J. H. (editor),
McGraw-Hill Book Co., New York, New York, 1983, and Chart of the
Nuclides, Knolls Atomic Power Laboratory, Operated by General
Electric Co. for U.S. Dept. of Energy, 12th Edition, April 1977-
3-3
-------
become attached to dust particles in the air and can be inhaled and
deposited in the lungs (NRC81).
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 indis-
tinguishable 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 tail-
ings disposal at licensed mills currently contributes about 140,000 Ci/y
(PEI85).
3.3 Sources of Radon-222 Emissions in the 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, U02, or UFg. Conventional uranium milling involves a series of
unit operations, including ore handling and preparation, extraction,
concentration and precipitation, product preparation, and tailings dis-
posal.
Ore stockpiles, crushing and grinding operations, the extraction
circuit, and tailings piles are sources of radon-222 at operational
uranium mills, 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 pico-
curies radium-226 present. This release can then be expressed in terms
of the amount of U309 produced by the mill.
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 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
3-4
-------
l
Ul
DIFFUSION-LIMITED
RADON RELEASE
ORE STOCKPILES
TOTAL RADON
RELEASE
DUMPING. CRUSHING. PROCESSING
DIFFUSION LIMITED
RADON RELEASE
UNSATURATED
BEACH
SATURATED
BEACH
DIKE
MIXED
Figure 3-2. Schematic illustration of the radon sources at a uranium mill (PEI85).
-------
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 re-
sulting 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, crush-
ing, fine ore storage, and grinding. Ore blending ensures that the mill
feed is of uniform grade, which is necessary to achieve maximum effi-
ciency 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'if the mill were operating at full capacity. The ore
residence time in storage piles varied from 4 to 180 days, with a mean
and standard deviation of 87 ± 72 days, at seven mills surveyed in Wyom-
ing (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 m3 and 2500 m2,
respectively (Th82). Emissions of radon-222 from stockpiles are consid-
ered to emanate from an infinitely deep or thick source from all sur-
faces, even though some parts may be shallow or thin. The resulting
conservatively-high radon-222 emission estimate for some of the pile
areas is justified by the variable sizes, shapes, and other character-
istics 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.
3-6
-------
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 collec-
tors are located on crushers and screens and at transfer points to mini-
mize 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 (NRC80).
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 semi-
autogenous 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).
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).
Wet, semiautogenous grinding is being used increasingly 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 into 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 3 cm2/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 low diffusion coefficients for
radon-222 (10~H to 10~5 cm2/s). The 4- to 14-cm 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 par-
ticle sizes during milling.
Extraction
Hydrometallurgical leaching techniques are used to recover uranium
from the ground ore slurry. Little radon-222 is released from the ex-
traction process because the radon-222 contained in the ore is released
3-7
-------
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 pre-
ferred 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 decantatipn (CCD) circuit. The sands
and slimes are pumped to a tailings pond for disposal.
Alkaline leaching, which is best suited to ores with high lime
content, may be used in combination with ion exchange or caustic precip-
itation to concentrate and purify uranium. A flow diagram of the alka-
line 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 and Precipitation
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.
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.
3-8
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ORE
WATER
SULFURIC
ACID AND
SODIUM CHLORATE'
FLOCCULANT
WATER
1
CRUSHING AND
GRINDING
WET
GRINDING
1
LEACHING
COUNTER CURRENT
DECANTATION
(CCD)
TAILINGS POND
(TAILINGS SAND AND SLIMES,
LIQUID WASTES)
BARREN RAFFINATE
PREGNANT LIQUOR
SOLVENT
EXTRACTION
FURTHER
PROCESSING
Figure 3-3. Simplified flow diagram of the acid leach process.
3-9
-------
\ '
CRUSHING AND
GRINDING
WATER
WET
GRINDING
Na2C03
NaHCO
3
I
LEACHING
FLOCCULANT
WATER
I
COUNTER CURRENT
FILTRATION
CAUSTIC SODA
PRECIPITATION/
PURIFICATION
I
DRYING AND
PACKAGING
TAILINGS PILE
TAILINGS
CO,
RECARBONATTON
-TO GRINDING
AND LEACHING
YELLOWCAKE
Figure 3-4. Simplified flow diagram of the alkaline
leach-caustic precipitation process.
3-10
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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 tail-
ings 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 ym);
(2) the slimes, which consist of solids less than 200-mesh; and (3) the
liquid solution containing milling reagents and dissolved ore solids.
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 repre-
sent 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) (NRC80). The sands fraction contains radium-226 in concentra-
tions 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 or 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-, two-, and three-sided dams are constructed across
valleys and along hillsides. Dams constructed on relatively flat ter-
rain, where the tailings cannot be contained by the natural topography,
are four-sided. Embankments are generally constructed of earthen mate-
rial, but some (at six mills) are constructed of the sand fraction of the
tailings.
3-11
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The water level in a tailings impoundment is controlled through the
use of decant towers, pumps, or siphons to recycle the water or to trans-
fer it to evaporation ponds for proper maintenance of freeboard. Most
mills operate with zero liquid discharge (40 CFR Part 440) and rely on
evaporation.
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 typi-
cally 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 thou-
sands 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 models typi-
cally have been used to estimate average radon-222 emissions on a theo-
retical 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 esti-
mates based on mathematical models (EPA83).
Considerable research has been conducted to develop and refine ways
of calculating average radon-222 flux from infinitely thick or deep
sources (i.e., at least 1 meter deep). This work has largely been car-
ried 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.
3-12
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•ACTIVE LIFE
DRYING INACTIVE RECLAMATION
PERIOD
CM
CM
CM
I
O
O
TAILINGS IMPOUNDMENT EMISSIONS
MILLING EMISSIONS
APPROXIMATELY 30
TIME, years
Figure 3-5. Qualitative illustration of radon-222 emissions
from licensed uranium milling process.
3-13
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A one-dimensional, steady-state, radon-222 diffusion equation has
been developed for sources (e.g., ore piles and tailings) that are more
than several meters thick (Ni82, Fr84). The equation is:
Jt = 10" RpE (AD)*5 (3-1)
where J is the radon-222 flux at the surface of the source (pCi/m2s); R
is the specific activity of radium-226 in ore or tailings equal to 2812 x
(uranium ore grade in percent), pCi/g; p is the bulk dry density of
source (g/cm3); E is the radon-222 emanating fraction_of source, dimen-
sionless; A is the radon-222 decay constant (2.1 x 10~6/s); D is the
effective diffusion coefficient for radon-222, equal to bulk radon dif-
fusion coefficient/porosity De/p (cm2/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 are difficult to measure. They are
based on the physical characteristics of the source materials, which vary
over time (e.g., radium-226 content may decrease over the life of the
mill as ore 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/m2s per pCi of Ra-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 esti-
mating 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 l^Gg will be 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/m2s per pCi of Ra-226/g is
also used to estimate emissions.
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 radium-226
content. The radium-226 content is typically estimated from ore grades,
assuming secular equilibrium between the uranium-238 and the radium-226.
3-14
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Dg= BULK DIFFUSION COEFFICIENT
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).
3-15
-------
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 1 pCi Rn-222/m2s 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:
A = area of ore pile = 6 acres or 2.4 x 1(T m2
T = depth of ore pile = 3 p minimum
R = Ra-226 concentration = 2812 x 0.1 U308 = 281 pCi/g
E = emanating power of ore =0.2
p = density = 1.6 g/cm3
D = diffusion coefficient = 0.05 cm2/s
J = 101* RpE (X D)*5
= 281 x 0.2 x 1.6 (2.1 x 10~6 • 0.05)^ x lO*4 cm2/m2
= 291 pCi Rn-222/m2s
The ore pad would have the following calculated radon-222 emissions:
291 pCi Rn-222/m2s x 2.4 x 10H m2 x 3.156 x 107 s/y x 10~12 Ci/pCi
= 221 Ci/y
Or if a specific flux of 1 pCi Rn-222/m2s per pCi Ra-226 is assumed, the
estimated emissions are:
1 pCi Rn-222/m2s/pCi_Ra-226/g x 281 pCi Ra-226/g x 2.4 x 10V2 x
3.156 x 107 s/y x 10 12 Ci/pCi = 213 Ci/y
3.4.2 Mill Emissions
The throughput is relatively large (several thousand tons per 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 radon-222 that previously
emanated from the ore was not released completely during storage, han-
dling, and crushing and grinding.
Most milling emissions of radon-222 occur during the transferring
and dumping of the ore into the mill feed area because the ore has usu-
ally been reduced to sizes of less than 40 cm, which allow trapped radon-
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 106 g/t x 10~12 Ci/pCi x
0.1 = 16 Ci/y
3-16
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Alternatively, an average emission factor of 3.8 x 107 pCi/lb U308 may be
used to estimate Rn-222 emissions from milling (PEI85).
1800 t/day x 310 days/y x (2200) Ib/t x 0.001 Ib U308/lb ore x
3.8 x 107 pCi/lb U308 x 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 yellowcake 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.
3.4.3 Emissions From Tailings 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 (EPA85). Tailings solids are
assumed to be carried with the process liquids and deposited on the
bottoms of these ponds. If exposed, these solids are assumed to emit
radon-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 informa-
tion on the radium-226 concentration, moisture content, porosity, den-
sity, 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/m2s per pCi Ra-226/g of tail-
ings may be used to estimate emissions from dry areas of tailings im-
poundments (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-acre) impoundment. Of the total area, 50 percent consists of sat-
urated or liquid-covered tailings and 50 percent is dry. The tailings
solids in the impoundment are 10 m (30 ft) deep.
Emission estimates made by using diffusion Equation 3-1
Radon-222 flux J = 10^ RpE (AD)*5
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/cm3
A = 2.1 x 10"6/s
D = diffusion coefficient for tailings
= 0.07 exp (4mp2 - 4m - 4m5)
3-17
-------
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/cm3).
Thus:
p = 1- 1.6/2.7 = 0.407
D = 0.07 exp [4 x 0.35 x (0.407)2 - 4 x 0.35 - 4 x (0.35)5]
= 0.0213 cm2/s •,
J = 281 x 0.2 x 1.6 (2.1 x 10"6 x 0.0213)"* x 101* cm2/m2 =
190 pCi/m2s
Total annual emissions are determined by multiplying J by the dry area
and seconds per year.
Rn-222 = 190 pCi/m2s x 25 x 101* m2 x 3.156 x 107s/y x 10~12
Ci/pCi
= 1505 Ci/y -vl.5 kCi/y
Emissions estimate based on specific flux of 1 pCi Rn-222/m2s per pCi
Ra-226/g
Rn-222 = 1 pCi Rn-222/m2s/pCi Ra-226/g x 281 pCi Ra-226/g
x 25 x lOSn2 x 3.156 x 107 s/y x 10~l2 Ci/pCi
= 2223 Ci/y ^2.2 kCi/y
The simplified calculation based on a specific flux of 1 pCi Rn-222/m2s
per 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 larg-
est 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 per-
cent at an inactive or standby licensed mill.
3.5 Transport and Risk Assessment
3.5.1 Overview of EPA Analysis
Three separate steps are required to estimate the health impact of a
specific source of radioactivity: (1) determining the transport and dis-
persion of a radionuclide and estimating, at various locations, its con-
centration and annual intake resulting from specific sources of radio-
activity in the environment; (2) calculating the estimated exposure and
risk resulting from the estimated concentration of radioactivity in the
environment; and (3) using a means of relating the exposure estimates to
match the specific source strength.
3-18
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3.5.2 Air Dispersion Estimates
Various computer codes are available for predicting how radionu-
clides are transported through environmental pathways. The EPA uses the
AIRDOS-EPA code (Mo79, Ba81) to analyze the transport of radionuclide
emissions into air from a specific source. This analysis produces esti-
mates of air and ground surface radionuclide concentrations at various
distances from the source.
The AIRDOS-EPA code uses a modified Gaussian plume equation to esti-
mate airborne dispersion. Calculations are site-specific and require the
joint frequency distribution of wind direction, windspeed, and atmo-
spheric stability. The accuracy of these projections decreases with
distance; therefore, calculations with this method are limited to reg-
ional areas (e.g., less than 80 km from the source). The values calcu-
lated represent annual averages because diurnal or seasonal variations
are included in the joint frequency distribution. Calculations of work-
ing-level exposures for the inhalation of radon-222 progency are also
based on the energy of these decay products.
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.
Another computer program, MILDOS, designed by the NRC, calculates
environmental radiation doses from uranium recovery operations (milling
facilities) (NRC81). The program calculates radon-222 exposure to the
regional population within an 80-km radius of a facility. It can be used
to calculate maximum individual exposure or maximum concentrations in the
air around a facility. The program applies a dispersion model to radon-
222 emissions from point or area sources. A transport model in the
program accounts for particle deposition, resuspension, radioactive de-
cay, and the ingrowth of daughters. The program considers several expo-
sure pathways:
1) Inhalation
2) External exposure from ground concentrations
3) External exposure from cloud immersion
A) Ingestion of vegetables
5) Ingestion of meat
6) Ingestion of milk
3-19
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3.5.3 Risk Estimates
After the radionuclide intakes and concentrations have been calcu-
lated in terms of working level months for a specific source by means of
the environmental transport code, they are related to the risk of getting
fatal lung cancer. Epidemiological data are used to determine this risk,
as described earlier in Sections 2.3 and 2.4. 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 mea-
sure 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.
3.6.1 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 con-
structed 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 con-
tainer, 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 acti-
vated charcoal to adsorb radon-222. These include the passive charcoal
canister samplers and the active, circulating-air test sampler. The
3-20
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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 cm2) improve
the representativeness of the sample by sampling a larger area, but small
samplers are more economical and logistically simpler.
The circulating-air test sampler covers a much larger area than the
canisters (i.e., 9290 cm2 (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 contain-
ing the activated charcoal. The tubing is sectioned into two halves,
which allows for the detection of any carryover. The sampler is typi-
cally 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).
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 radon 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 be used.
3-21
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REFERENCES
Ba81 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, ORNL-5720, Oak Ridge National Labora-
tory, Oak Ridge, Tennessee, March 1981.
EPA79 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, D.C., July
1979.
EPA83 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.
EPA85 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.
Fr84 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.
Ha85 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.
Me71 Merritt, R. C., The Extractive Metallurgy of Uranium, prepared
under contract with the United States Atomic Energy Commission,
1971.
Mo79 Moore R. E., Baes C. F. Ill, McDowell-Beyer 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 Radio-
nuclides, EPA 520/1-79-009, Office of Radiation Programs, U.S.
EPA, Washington, D.C., December 1979.
Ni82 Nielson 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.
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Ni84 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.
NRC80 Nuclear Regulatory Commission, Final Generic Environmental Im-
pact Statement on Uranium Milling, NUREG-0706, September 1980.
NRC81 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.
PEI85 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).
Th82 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.
Tr79 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.
Yo83 Young J. A., et al. Recommended Procedures for Measuring Radon
Fluxes From Disposal Sites of Residual Radioactive Materials,
NUREG/CR-3166, PNL-4597, March 1983.
3-23
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Chapter 4: INDUSTRY DESCRIPTION
4.1 Overview
Currently (January 1986), the conventional uranium milling industry
in the United States consists of 26 licensed facilities. Three addition-
al mills have been licensed, but have never been constructed. Only 4 of
the 26 licensed facilities are operating; 16 are on standby status, and 6
are 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 Chap-
ter 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 tail-
ings impoundments were taken from a 1984 survey that Battelle Memorial
4-1
-------
Table 4-1. Operating status and capacity of licensed conventional
uranium mills as of November 1985
State
Colorado
New Mexico
South Dakota
Texas
Utah
Washington
Mill
Canon City
Uravan
L-Bar
Churchrock
Bluewater
Quivira
Grants
Edgemont
Panna Maria
Conquista
Ray Point
White Mesa
La Sal
Moab
Shootaring Canyon
Ford
Sherwood
Owner
Cotter Corp.
Umetco Minerals
Sohio /Kennecott
United Nuclear
Anaconda
Kerr-McGee
Home stake
TVA
Chevron
Conoco /Pioneer
Exxon
Umetco Minerals
Rio Algom
Atlas
Plateau Resources
Dawn Mining
Western Nuclear
Operating
status '
Standby
Standby
Standby
Standby
Standby
Standby
Standby
Decommissioned
Standby
Decommissioned
Decommissioned
Active^
Active
Standby
Standby
Standby
Standby
Operating
capacity , -.
(tons /day)
1200
1300
1650
4000
6000
7000
3400
—
2600
—
—
2000
750
1400
800
600
2000
(continued)
-------
Table 4-1 (continued)
State
Wyoming
-p-
i
u>
Facility
Highland
Gas Hills
Shirley Basin
Gas Hills
Split Rock
Gas Hills
Bear Creek
Shirley Basin
Sweetwater
Owner
Exxon
American Nuclear
Corp.
Petrotomics
Pathfinder
Western Nuclear
Umetco Minerals
Rocky Mt . Energy
Pathfinder
Minerals Exploration
Operating
status
Decommissioned
Decommissioned
Decommissioned
Standby
Standby
Standbyf ^
Active) (
v C )
Active
Standby
Operating
capacity , ..
(tons/day)
.._
—
—
2500
1700
1400
2000
1800
3000
Total
4 Active
16 Standby
6 Decommissioned
(a)
(b)
(c)
(d)
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.
Current contract will allow operation for 12-18 months.
(e)
Likely to go to standby status soon, in the second quarter of 1986.
Operating at 20 percent of capacity, likely to go to standby in the second quarter of 1986.
-------
MILL STATUS IN NOVEMBER 1985
• ACTIVE
• STANDBY
* DECOMMISSIONED
Figure 4-1. Approximate locations of licensed conventional uranium mills.
-------
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.
A summary of current conditions and the extent of tailings impound-
ments and evaporation ponds at these sites is presented in Table 4-2.
Diagrams of each mill site are included in Appendix A. Additional de-
tails 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 (NRC84).
Canon City Mill
The Cotter Corporation, a subsidiary of Commonwealth Edison, oper-
ates 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 1100 t
(1200 tons) of ore per day. The ore grade ranges between 0.23 and 0.35
percent U30g (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 0.8 x 106 t
(0.9 x 106 tons) of tailings and has a capacity of 13 x 106 t (14 x 106
tons) (NRC84). The tailings are reported to contain 780 pCi/g of radium-
226 (EPA83a).
A 12-ha (31--acre) secondary impoundment containing 1.4 x 106 t
(1.5 x 106 tons) commingled tailings (defense-related tailings generated
under Atomic Energy Commission contracts commingled with tailings gener-
ated under commercial contracts) generated in pre-1979 operations has
Metric tons (tonnes) are equal to 2200 Ibs and are abbreviated as t in
this document. Short tons are equal to 2000 Ibs and are abbreviated
tons.
4-5
-------
Table 4-2. Summary of current uranium mill tailings impoundment areas
and radium-226 content
Company / Impoundment
Colorado
Cotter Corp.
Primary
Secondary
Umetco
Uravan 1 & 2
Uravan 3
Sludge pile
Evap . pond
New Mexico
Sohio
L-Bar
Type of }
impoundment
2/SL
2/SL
1
1
1
1
1
Surface area
Status (b)
S
C
c
C
c
c
S
Total
84
31
66
32
20
17
128
Ponded
77
1
0
0
0
0
28
(acres) (c)
Wet
3
1
4
3
1
2
55
Dry
4
30
62
29
19
15
45
Average,,
Ra-226^ ;
(pCi/g)
780
780
480
480
480
480
500
United Nuclear
Churchrock
148
76
65
290
Anaconda
Bluewater 1
Bluewater 2
Bluewater 3
Evap. ponds
Kerr-McGee
Quivira 1
Quivira 2a
Quivira 2b
Quivira 2c
Evap. ponds
(continued)
2
2
2
2
S
C
C
S
S
S
S
S
S
239
47
24
162
0
0
0
97
0
0
0
17
239
47
24
48
620
620
620
620
269
105
28
30
372
14
10
0
0
268
64
35
3
4
10
191
60
25
26
95
620
620
620
620
620
-------
Table 4-2 (continued)
I
-J
Company /Impoundment
Home stake
Home stake 1
Home stake 2
Texas
Chevron
Panna Maria
Utah
Umetco
White Mesa 1
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 (a)
impoundment
1
2
2
3/CL
3/CL
3/CL
2
2
1
2
2
3/SL
Surface area
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
(acres) (c)
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,,
Ra-226V '
(pCi/g)
385
385
196
350
350
350
560
560
540
280
850
850
(continued)
-------
Table 4-2 (continued)
i
00
Company / Impoundment
Western Nuclear
Sherwood
Evap . pond
Wyoming
Pathfinder
Gas Hills 1
Gas Hills 2
Gas Hills 3
Gas Hills 4
Western Nuclear
Split Rock
Umetco
Gas Hills
A-9 Pit
Leach pile
Evap . ponds
Type of ,
impoundment
2/SL
2/SL
2
2
2
2
2
2
3/CL
2
2
Surface area
Status (b)
S
S
S
c
S
S
S
c
S
S
S
Total
94
16
124
54
22
89
156
151
25
22
20
Ponded
18
16
2
2
19
73
94
0
2
0
20
(acres)
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
Ra-226^ '
(pCi/g)
200
200
420
420
420
420
430
310
310
310
310
Rocky Mountain Energy
Bear Creek
Pathfinder
Shirley Basin
121
261
45
179
23
22
53
60
420
540
(continued)
-------
Table 4-2 (continued)
(c)
, Surface area (acres)
Type of , , , . -
Company/Impoundment impoundment Status Total Ponded Wet Dry (pCi/g)
Average }
Ra-226l '
Minerals Exploration
Sweetwater
2/SL
37
30
280
Totals
3882
1282
457
2140
(a)
(b)
(c)
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
-------
(V) Cotter Corp.
Canon City Mill
2 ) Umetco Minerals
Uravan Mill
Figure 4-2. Location of mills in Colorado,
4-10
-------
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 ex-
posed 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 resi-
dents 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
Umetco 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 before
mill operations are restarted (Kr85). The capacity of this mill is 1200
t (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 9 x 106 t (10 x 106 tons) of tailings. These tailings impound-
ments are situated on mesas above Uravan. Impoundments 1 and 2 are ad-
jacent 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 area of 27 ha (66 acres) and
have a maximum dam height of 46 m (155 ft) (EPA85, DOE82). Impoundment 3
covers 13 ha (32 acres), and the dike is about 33 m (110 ft) high. Eight
4-11
-------
other impoundments, which either contain tailings or have been construc-
ted of tailings, were mainly used for evaporation. These eight impound-
ments cover 15 ha (37 acres). The radium-226 content of the Uravan tail-
ings has been reported to be 480 pCi/gram (EPA83b).
The Uravan operation uses several other ponds in its water manage-
ment system. Six solvent extraction (SX) raffinate 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 Rich-
field), Kerr-McGee Corp. (Quivira Mining), and Homestake Mining Co. (see
Figure 4-3). Two additional mills, Bokum Resources Corporation and Gulf
Minerals, were licensed but have never operated.
L-Bar Mill
The Sohio/Kennecott L-Bar Uranium Mill is located near Seboyeta in
Cibola 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 ob-
tained 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
1500 t (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
U308 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 (Jo80). The tailings
impoundment is built above grade with an earthen starter dam to the west
that keys into natural topography on the north and 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 opera-
tions, the edge of the tailings solution was maintained about 60 m (200
4-12
-------
(T) Sohio
L-Bar Mill
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,
4-13
-------
ft) from the dam crest. A light-track pressure dozer was used to con-
struct 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 1.5 x 106 t (1.6 x 106 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 6.8 x 106 t (7.5 x 106 tons) of tailings
(Jo80). 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 1983 survey indicated
no 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).
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 3600 t (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)
The storage capacity of the pond is about 10 x 106 m3 (365 x 106 ft3)
(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 1000 t (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 per-
cent of the solids. The streams carried the spilled tailings into the
Rio Puerco River, which flows through Navajo grazing lands, and finally
4-14
-------
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).
Bluewater Mill
Anaconda's Bluewater Uranium Mill is located in 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 origin-
ally 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 5400 t (6000 tons) of
ore (0.2 percent U308) (NRC84). Production has been under both AEC (1956
to 1970) and commercial contracts. Through 1981, the Bluewater mill had
processed more than 21.4 x 106 t (23.5 x 106 tons) of ore ranging from
0.06 to 0.60 percent U308 (DOE82). Some decommissioning activities have
been initiated at this mill.
The mill site has three tailings impoundments. Carbonate tailings
from early 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 im-
poundment, 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 1956,
covers 96 ha (239 acres) (EPA85). It is currently dry. The dam surround-
ing 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 23 x 106 (25 x 106 tons) t of tail-
ings (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.
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 mai
pond (NM85); however, it has also been estimated to average 620 pCi/g
(EPA83a).
4-15
-------
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 direc-
tion. The site is in the "southwest mountains" climatological subdivi-
sion 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 impound-
ments, (Tailings impoundments Nos. 1 and 2a) and two ancillary impound-
ments (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) free-
board during operation. Impoundment No. 1 covers 108 ha (269 acres) and
contains a liquid covered area of about 6 ha (14 acres) (EPA85). Approxi-
mately 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 since 1958. These two impoundments contain approx-
imately 24 x 106 t (26 x 106 tons) of tailings. Some tailings are used
as backfill in a nearby underground mine. Tailings set aside for use as
backfill are contained in Impoundment No. 2b. 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
4-16
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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.
Homestake 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 U30e
(NRC84).
The mill site is relatively flat and covers about 600 ha (1500
acres). Two tailings impoundments, one on standby and the other inac-
tive, are located on site. The inactive impoundment contains tailings
generated between 1958 and 1962 under AEC contracts. The 1.1 x 106 t
(1.2 x 106 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.
The active impoundment contains about 18 x 106 t (20 x 106 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. 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 embank-
ment faces to inhibit dusting. The tailings are reported to contain 385
pCi/g of radium-226 (EPA83b).
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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 impound-
ment (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
(DOES2).
The site's climate is characterized by low precipitation [22 cm (8.8
in.)/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).
A.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 2400 t
(2600 tons) per day of a mixture of sandy clay ore averaging 0.05 percent
U308 (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 in a single above-ground impoundment con-
tained 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 3.0 x 106 t (3.3 x 106 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 surround-
ing the pile is 19 m (62 ft), the crest width is 6 m (20 ft), and the
downstream slope is 3:1 (h:v) (Ki80). Designed maximum storage of tail-
ings in this impoundment is 9 x 106 t (10 x 106 tons) (Ki80). The
average density of the tailings is 1.2 t/m3 (0.04 ton/ft3), and the spe-
cific 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 coarse -tailings forms along the dike and the tailings solution
gathers in the center portion of the pond. The depth of the solution
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©
Conoco/Pioneer Nuclear
Conquista Project
Chevron Resources Co.
Panna Maria Mill
Exxon Minerals
Ray Point
Figure 4-4. Location of mills in Texas,
4-19
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varies from an average of 1.5 m (5 ft) on the east side to 5 to 6 m (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 1 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.).
4.2.4 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 U30g
(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 solu-
tion, 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 either synthetic liners or compacted clay. The tailings are 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 Algom Mill
The Rio Algom Mill is near La Sal, Utah, about 48 km (30 mi) south-
east of Moab. This mill is currently active and has been in operation
since 1971. Ore obtained from adjacent underground mining operations is
4-20
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©
Umetco Minerals
White Mesa Mill
2 ) Rio Algom Corp.
La Sal Mill
Atlas Minerals
Moab Mill
Plateau Resources, Ltd.
Shootaring Canyon Mill
Figure 4-5. Location of mills in Utah.
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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 106 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 drain-
age course, one immediately upstream of the other (NRC84). The lower
impoundment has been in use since 1971, 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).
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 the AEC and commercial buyers.
Capacity of the mill is 1800 t (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 U308 (NRC84)
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).
4-22
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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 (DOE82).
Plateau Resources Mill
The Plateau Resources Shootaring Canyon Mill is located near Hanks-
ville, 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 U308,
ranging from 0.07 to 0.24 percent (NRC84). An average of approximately
88,000 t (97,000 tons) of surface mined ore is stockpiled on site when
the mill is running at capacity (Ge85). The primary mill circuit in-
volves 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 dam, 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 con-
tain 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 evapo-
ration 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 stabiliza-
tion 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 impound-
ment (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 li-
censed, 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.
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 Corpo-
ration and Midnight Mines, Inc. It began operations in 1957 and operated
through 1964 under the AEC concentrate purchase program. The mill was
4-23
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(T) Dawn Mining Co.
Ford Mill
(T) Western Nuclear, Inc.
Sherwood Mill
Figure 4-6. Location of mills in Washington.
4-24
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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 ammo-
nia. The Midnight mining open-pit mine produces ore between 0.10 and
0.25 percent U308 (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 sepa-
rate impoundments, three of which are above grade, unlined, and construc-
ted behind earthen dams. These three impoundments have been filled to
capacity and are inactive. Impoundment Nos. 1 and 2 contain an estimated
1.06 x 106 t (1.2 x 106 tons) of tailings from AEG 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). Impound-
ment No. 3, which contains about 1.5 x 106 t (1.6 x 106 tons) of tail-
ings, 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.con-
tent of the Dawn Mill tailings is reported to be 850 pCi/g (EPA83b).
The community of Ford is located within 3.2 km (2 mi) of the tail-
ings 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 aver-
age annual precipitation is 30 to 46 cm (12 to 18 in); annual evaporation
is about 127 cm (50 in) (EPA83b).
Western Nuclear Sherwood Mill
Western Nuclear's Sherwood uranium mill is located in eastern Wash-
ington about 64 km (40 mi) northwest of Spokane. Ore taken from a nearby
surface mine has averaged 0.05 to 0.09 percent U308 (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
4-25
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from the mill was neutralized with lime before being pumped to the Hypa-
lon-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 manage-
ment is estimated to be 1.5 x 106 t (1.6 x 106 tons) (NRC8A). The tail-
ings 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 impound-
ment (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 Fremont 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 ca-
pacity 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.
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 U308 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 for discharge.
The tailings retention system consists of four tailings impoundments
having surface areas of 50, 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 forma-
tion. 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
4-26
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(V) Pathfinder Mines Corp.
Gas Hills Mill
2 ) Western Nuclear, Inc.
Split Rock Mill
Umetco
Gas Hills Mill
4 ) Rocky Mountain Energy
Bear Creek Mill
5) Pathfinder Mines Corp.
Shirley Basin Mill
(6j Minerals Exploration Co.
Sweetwater Mill
Petrotomics
Shirley Basin Mill
8) American Nuclear Corp.
Gas Hills Mill
Exxon Corp.
Highland Mill
Figure 4-7. Location of mills in Wyoming.
4-27
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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 10.5 x 106 t
(11.5 x 106 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-km (3-mi) radius of the tailings ~piles (PNL84).
In 1963 a flood at the mill site resulted in the release of 8.7 x
107 liters (2.3 x 107 gal) of impounded tailings solution to the environ-
ment. As a result of this incident, the tailings impoundment was en-
larged to its current capacity. The existing system, with a minimum of 1
m (3 ft) of freeboard, is estimated to provide 12.6 x 108 liters (3.3 x
108 gal) of emergency storage.
Western Nuclear 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 850 t (935 tons) of yellowcake per year (Bo85). Maximum through-
put was about 1500 t (1700 tons) of ore per day (NRC84). The ore grade
has ranged from 0.15 to 0.30 percent UsOg in the past and is expected to
range from 0.05 to 0.15 percent in the future (NRC84). Milling opera-
tions involve semi-autogenous grinding, an acid leach, and solvent extrac-
tion. The mill usually stockpiles 1800 to 4500 t (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 11 x 106 t (12 x 106 tons) of commingled
tailings are under management (NRC84).
The average radium-226 concentration of the tailings is approxi-
mately 100 pCi/g (99.5 ± 42 pCi/g) (Bo85). Radium-226 values in the
4-28
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sands and slimes were determined to be 63 pCi/g and 87 pCi/g, respec-
tively (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/m2-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).
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 rela-
tively 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 capac-
ity of about 1000 t (1100 tons) per day; in January 1980, the capacity
was increased to 1300 t (1400 tons) per day. In June 1983, milling of
mined ore was temporarily curtailed, and only the heap leach facility was
kept in operation. During milling operations, a 2-month stockpile of ore
is maintained at the mill (Wo85). This amounts to 78,000 t (85,800 tons)
when the mill is operating at capacity.
During the anticipated total active life of the project (1960 to
1986), about 12 x 106 t (13 x 106 tons) of mill tailings will have been
produced. The retention capacity [7.6 x 106 t (8.4 x 106 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 2.3 x 106 t (2.5 x 106 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 expan-
sions) 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 106 t (6.4 x 106
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 evapora-
tion area consists of three ponds with a combined surface area of 8 ha
(20 acres).
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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).
The area is sparsely populated. A 1983 survey indicated no people
living within a 5-km (3-mi) radius of the 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 2.6 m (8.5 ft) of overburden
over the entire tailings area. This will require about 210,000 m3 (7.5 x
106 ft3) of clay, at a cost of $1,129,000, and 1.8 x 106 m3 (65 x 106
ft3) 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 U308 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 Mill
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
1800 t (2000 tons) of ore per day (NRC84). The U308 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 60,000 t (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 quar-
ter of 1986.
Mill tailings are contained in a single tailings impoundment en-
closed 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).
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).
4-30
-------
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 semi-
autogenous 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). Opera-
tions 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 acres) are dry beaches. The
impoundment contains 5.8 x 106 t (6.4 x 106 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 Exploration 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 U308 (Hi85).
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 106 t (1 x 106 tons) of tailings have been generated
and are contained in this impoundment. Plans 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-dis-
posal facility that has gone through several iterations during devel-
opment. 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/mz-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).
4-31
-------
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 tail-
ings impoundments. This more recent .survey, which was based on interpre-
tation 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.
4-32
-------
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/Mill 0
Colorado
Cotter
Uravan
New Mexico
Sohio
United
Nuclear
Anaconda
Kerr-McGee
Home stake
Texas
Chevron
Utah
Umetco
Rio Algom
Atlas
.0-0.5
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
1.0-2.0
0
193
0
25
6
0
190
42
0
105
9
2.0-3.0
184
6
0
52
136
1
104
33
0
154
33
3.0-4.0
2767
3
42
85
666
0
45
81
0
32
1094
4.0-5.0
2982
0
124
150
99
0
57
285
8
44
1225
Total
5933
349
166
312
907
1
396
453
8
343
2361
Plateau
Resources
171
171
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
3
0
0
3
0
0
0
0
173
93
0
0
0
0
0
0
0
663
157
0
58
30
0
0
0
0
948
96
32
0
697
0
0
0
0
5640
62
17
0
176
0
0
0
0
5400
411
49
58
906
0
0
0
0
12,824
PNL84.
4-33
-------
Table 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
State/Mill 0.0-0.5
Colorado
Cotter
Uravan
New Mexico
Sohio
United
Nuclear
Anaconda
Kerr-McGee
Home stake
Texas
Chevron
Utah
Umetco
Rio Algom
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
(a)
EPA85.
4-34
-------
REFERENCES
An84 Andrew R. E., Uranium Mills Program, Department of Social and
Health Services, State of Washington, Correspondence with PEI
Associates, Inc., December 1984.
Bo85 Bogden G., Western Nuclear, Inc., Correspondence with PEI
Associates, Inc., January 1985.
DOE82 Department of Energy. Office of Defense Waste and Byproducts
Management, Commingled Uranium Tailings Study (DOE/DP-0011),
Washington, D.C., June 30, 1982.
DOE84 Department of Energy, United States Mining and Milling
Industry, DOE/2-0028, May 1984.
ELP85 Uranium Industry Not Viable in 1984, Determines DOE's Harrington.
Electric Light and Power. November 1985.
EPA83a Environmental Protection Agency, Regulatory Impact Analysis of
Environmental Standards for Uranium Mill Tailings at Active
Sites, EPA 520/1-82-023, March 1983.
EPA83b Environmental Protection Agency, Office of Radiation Programs,
Final Environmental Statement for Standards for the Control of
Byproduct Materials from Uranium Ore Processing (40 CFR 192) ,
(EPA 520/1-83-008-1), Washington, D.C., September 1983.
EPA85 U.S. Environmental Protection Agency, Office of Radiation
Programs, Draft Document-Estimates of Population Distributions
and Tailings Areas Around Licensed Uranium Mill Sites, November
1985.
Ge85 Gerdemann F. W., Plateau Resources Limited, Correspondence with
PEI Associates, Inc., January 1985.
Ha85 Hardgrove T., Pathfinder Mines Corporation, Correspondence with
PEI Associates, Inc., January 1985.
Hi85 Hill C., Minerals Exploration Company, Correspondence with PEI
Associates, Inc., January 1985.
4-35
-------
Jo80 Johnson T. D., Sohio Western Mining Company Tailings Dam, in:
First International Conference on Uranium Mine Disposal, C. 0.
Brawer, editor, Society of Mining Engineers of AIME, New York,
1980.
Ki80 King K. and Lavander R., Design and Construction of Uranium
Disposal Facilities for the Panna Maria Project, Texas, Pre-
sented at First International Conference on Uranium Mine Waste
Disposal, Vancouver British Columbia, May 19, 1980.
Kr85 Kray E., State of Colorado, Correspondence with PEI Associates,
Inc., January 1985.
Ma85 Manka M., Cheveron Resources Company, Panna Mana Project,
Correspondence with PEI Associates, Inc., February 1985.
Mc85 McClusky J., Cotter Corporation, Cprrespondence with PEI Asso-
ciates, Inc., February 1985.
Me85 Medlock R., Bear Creek Uranium, Correspondence with PEI Asso-
ciates, Inc., February 1985.
NM85 State of New Mexico, Radiation Protection Bureau, Correspond-
ence with PEI Associates, Inc., January 1985.
NRC80 Nuclear Regulatory Commission, Final Generic Environmental
Impact Statement on Uranuim Milling, NUREG-0706, September
1980.
NRC84 Nuclear Regulatory Commission, Office of State Programs,
Directory and Profile of Licensed Uranium-Recovery Facilities,
(NREG/CR-2869), Washington, D.C., March 1984.
PNL84 Pacific Northwest Laboratory. Estimated Population Near
Uranium Tailings. (PNL-4959). January 1984.
Si85 Simchuk G. J., Pathfinder Mines Corporation, Correspondence
with PEI Associates, Inc., February 1985.
WA86 State of Washington, Department of Social and Health Services,
Uranium Mills Program, Correspondence with PEI Associates, Inc.
January 1986.
Wo85 Wong T., Umetco (Union Carbide), Correspondence with PEI Asso-
ciates, Inc., February 1985.
4-36
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Chapter 5: INDUSTRY RADON-222 EMISSION ESTIMATES
5.1 Introduction
This chapter presents a discussion of the methodology used to esti-
mate 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 con-
tent have a major influence in controlling the amount of radon-222 that
is released; therefore, dry conditions must be considered in the determi-
nation 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 Estimating Emissions
As discussed in Chapter 3, estimates of radon-222 emissions are
based on an assumed emission rate that equals the specific flux of 1 pCi
radon-222/m2s per pCi radium-226/g tailings for dry tailings times the
dry area. It has also been assumed that areas of 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 esti-
mate emissions.
For the specific flux of 1 pCi radon-222/m2s 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 compila-
tion of these earlier data bases, as demonstrated by the drop in uranium
production (and thus tailings generation), the initiation of decommis-
sioning 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
5-1
-------
the results 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 solu-
tion, 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 im-
poundments 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/m2s per pCi of
Ra-226 per gram 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, EPA83). 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 calcula-
tion was used to estimate emissions from dry areas:
kCi Rn-222/y = dry area, m2 x 1 pCi Rn-222/m2s per pCi Ra-226/g
x pCi Ra-226/g x 3.15 x 107 s/y x 10~15 kCi/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 the preceding calcula-
tion. 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 138 kCi/y under current conditions and to
rise to about 247 kCi/y after all the areas have dried.
5-2
-------
Table 5-1. Summary of radon-222 emissions from uranium
mill tailings impoundments
Site/Impoundment
Emissions (kCi/y)
Current
conditions
(flux = 1)
, .
U'
Current
conditions
(factored)
After
drying
Colorado
Cotter Corp.
Primary
Secondary
Umetco
Impoundments 1 & 2
Impoundment 3
Sludge pile
Evaporation pond
New Mexico
Sohio
L-Bar
0.4
3.0
3.8
1.8
1.2
0.9
2.9
0.5
3.0
3.9
1.8
1.2
1.0
3.9
8.4
3.1
4.0
2.0
1.2
1.0
8.2
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
Panna Maria
2.4
19
3.7
1.9
3.8
15
4.7
2.0
2.1
7.5
5.4
1.8
0.9
3.2
19
3.7
1.9
4.2
17
5.6
2.0
2.2
7.7
5.8
1.8
1.0
5.5
19
3.7
1.9
13
21
8.3
2.2
2.4
29
10
2.2
3.1
(continued)
5-3
-------
Table 5-1 (continued)
Emissions (kCi/y)
Site /Impoundment
Utah
Umetco
White Mesa 1
White Mesa 2
White Mesa 3
Rio Algom
1
2
Current
conditions.. .
(flux = ira;
1.5
2.0
0.6
2.7
1.1
Current
conditions x,v
(factored)*1 ;
1.6
2.1
0.6
2.8
1.2
After
drying
2.1
2.7
2.4
3.1
2.3
Atlas
Moab
6.2
6.3
10
Plateau Resources
Shootaring Canyon
Washington
Dawn Mining
Ford 1, 2, 3
Ford 4
Western Nuclear
Sherwood
Evaporation pond
Wyoming
Pathfinder
Gas Hills 1
Gas Hills 2
Gas Hills 3
Gas Hills 4
Western Nuclear
Split Rock
Umetco
Gas Hills
A-9 Pit
Leach pile
Evaporation ponds
(continued)
0.1
10
1.2
1.8
0
6.4
2.1
0.1
0.6
2.4
6.0
0.6
0.9
0
0.1
10
1.2
1.8
0
6.4
2.3
0.1
0.7
2.7
6.0
0.7
0.9
0
0.2
10
3.0
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 (continued)
Emissions (kCi/y)
Current Current
conditions.. . conditions,, , After
Site/Impoundment (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 138 146 247
^a' Based on a specific flux of 1 pCi Rn-222/m2s per pCi Ra-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/m2s per pCi Ra-226 per gram of tailings
for wet tailings area, 1 pCi Rn-222/m2s per pCi Ra-226 per gram of
tailings for dry area, and zero for ponded areas.
5-5
-------
Although a specific flux of 1 pCi radon-222/m2s 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/m2 per pCi radium-226/g; zero from ponded areas,
as previously discussed; and 0.3 pCi radon-222/m2s 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 146 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/m2s 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 informa-
tion. The site-specific information was based on a number of assump-
tions, 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/m2s per pCi radium-226/g) for dry areas and zero for ponded and
saturated areas were used in this report. The emission estimates pre-
sented 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 Radio-
active 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 tb 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 inter-
preted 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 im-
poundments .
6.2 Risk Estimates
6.2.1 Nearby Individuals
Individual risks are calculated by using the life table methodology
described by Hunger 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 AIKDOS-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 1 percent (1E-2) occurs at
Anaconda, New Mexico at a distance of 2 km from the center of the im-
poundment .
6.2.2 Regional Population
Collective (population) risks fo'r the region are calculated from the
annual collective exposure (person WLM) for the population in the assess-
ment area by a computerized methodology known as AIRDOS-EPA (Mo79). An
effective equilibrium fraction of 0.7 is presumed because little collec-
tive exposure takes place near the-mill.
In this study, population data in the 0- .to 5-km and 5- to 80-km re-
gions around each mill were obtained from an earlier detailed study by
EPA and are summarized in Chapter 4 (EPA83). Collective exposure calcu-
latiojls expressed in person WLM were performed for each mill by multiply-
ing 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 normalized emission rate of 1 kCi/y 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 2.8
percent (700 deaths per 106 person WLM). These estimates were then
multiplied by 1000/700 or 250/700 to adjust to the risk coefficients of 4
and 1 percent, respectively (1000 and 250 deaths per 106 person WLM).
This result was then multiplied by each mill's estimated annual emission
rate (in kilocuries/year as shown in Table 5-1) to obtain the total
number of fatal cancers. 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 en-
tirely 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 2.1 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
(a)
Distance
(km)
(b)
Colorado
New Mexico
Texas
Utah
Washington
Wyoming
Cotter
Umetco
Kerr-McGee
Anaconda
United Nuclear
Homestake
Sohio
Chevron
Umetco
RioAlgom
Atlas
Plateau Res.
Dawn
Western Nuclear
Minerals Exploration
Umetco
Pathfinder
Shirley Basin
Gas Hills
Rocky Mt.
Western Nuclear
3E-4 (7E-5)
3E-3 (8E-4)
6E-4 (1E-4)
1E-2 (3E-3)
1E-3 (4E-4)
6E-3 (2E-3)
6E-4 (1E-4)
2E-3 (4E-4)
1E-3 (3E-4)
8E-3 (2E-3)
2E-3 (5E-4)
3E-5 (8E-6)
8E-3 (2E-3)
2E-4 (4E-5)
4E-6 (9E-7)
1E-4 (3E-5)
8E-5 (2E-5)
8E-5 (2E-5)
9E-5 (2E-5)
6E-4 (2E-4)
2
1
10
2
1
1
3
0.6
2
1
1
2
1
3
30
10
10
20
10
1
(a)
(b)
The value in the first column is based on a risk factor of 1000 deaths/
106 person WLM, and the values in parentheses are based on 250 deaths/
106 person WLM.
Distance from center of a homogenous circular equivalent impoundment.
6-3
-------
Table 6-2. AIKDOS-EPA code inputs and estimated exposure based on emissions of 1 kCi/y
AIRDOS code inputs
State
Colorado
New Mexico
Texas
Utah
Washington
Wyoming
Company
Cotter
Umetco
Kerr-McGee
Anaconda
United Nuclear
Homes take
Sohio
Chevron
Umetco
RioAlgom
Atlas
Plateau Res.
Dawn
Western Nuclear
Minerals Explora-
tion
Umetco
Pathfinder
Shirley Basin
Gas Hills
Rocky Mt.
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
33.9
35.4
28.0
Approximate
Ambient impoundment
temperature area
(°C) (ha x 10=)
10
10
11
11
11
11
11
21
13
13
13
13
9
9
6
6
6
6
6
6
0.8
0.3
1.0
2.0
0.4
0.8
0.7
0.6
0.3
0.1
0.5
0.04
0.5
0.5
0.2
0.6
1.0
0.8
0.6
0.5
Exposure
(person-WL)
0-5 km
1.59E-1
1.27E-1
2.53E-4
6.14E-2
3.23E-2
9.26E-2
1.18E-2
3.14E-2
2.00E-4
2.45E-2
1.30E-1
1.48E-2
1.73E-2
l.OOE-3
_
_(a)
-
9.19E-3
-
2.58E-2
5-80 km
6.89E-1
1.22E-1
2.66E-1
4.24E-1
3.33E-1
5.33E-1
6.66E-1
1 . 30E-0
6.69E-2
5.25E-2
l.OE-1
2.88E-2
4.51E-1
2.99E-1
1.86E-2
1.3E-2
5.0E-2
1 . 30E-2
5.0E-2
l.OE-2
(a)
Zero population in the 0-5 km region.
-------
Table 6-3. Summary of health effects from existing
tailings impoundments
, » Committed fatal cancers per year
Condition of Emissions
tailings (kCi/y) 0-5 km 5-80 km 0-80 km
Current
All dry
138
247
0.2 (0.035)
0.3 (0.071)
1.2 (0.3)
2.2 (0.6)
1.4 (0.3)
2.5 (0.6)
(a)
Based on radon-222 flux of 1 pCi/m2s per pCi of Ra-226 per gram
of tailings.
Values in first column are based on 1000 deaths due to lung cancer
per 10& person WLM. The values in parentheses are based on 250
deaths per 106 person WLM.
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 effec-
tive 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 calcula-
tions are shown below and summarized in Table 6-4.*
2.47 deaths 1QQ . _, , 1000
138
202.7 kCi/y 60
2.47 deaths -„_ . _ . , 250
138
202.7 kCi/y 860 '
For the dry tailings condition with emissions of 247 kCi/y, the
corresponding values are 3.4 and 0.9 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 860 deaths per 106 person WLM.
6-5
-------
Table 6-4. Summary of nationwide health effects
for tailings impoundments
Condition of
tailings
Emissions
(kCi/y)
Committed fatal
cancers per year
(a)
Current
All dry
138
247
1.9 (0.5)
3.4 (0.9)
(a)
Values in first column are based on 1000 deaths due to lung cancer
per 106 person WLM. The values in parentheses are based on 250
deaths per 106 person WLM.
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 emis-
sions increased.
Table 6-5. Summary of fatal cancers from
current tailings impoundments
Fatal cancers per year
(a)
Condition
of tailings
Current
Dry
0-5 km
0.2 (0.03)
0.3 (0.1)
Region
5-80 km
1.2 (0.3)
2.2 (0.6)
National
1.9 (0.5)
3.4 (0.9)
Total
3.3 (0.8)
6.0 (1.5)
(a)
Values in first column are based on 1000 deaths due to lung cancer
per 105 person WLM. The values in parentheses are based on 250
deaths per 106 person WLM.
6.2.4 Risks from New Tailings Impoundments
Radon-222 emissions will not increase greatly until the current
impoundments reach capacity and new impoundments are built. This need
for new impoundments is directly related to industry growth. The health
effects caused by new impoundments are in direct proportion to their
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. In order to estimate
the risks from these new impoundments, the risk per kCi/y (calculated
from Table 6-4) can be multiplied by annual emissions from a new
impoundment.
6-6
-------
REFERENCES
Bu81 Bunger B., Cook J. R. , and Barrick M. K., Life Table Method-
ology for Evaluating Radiation Risk: An Application Based on
Occupational Exposure, Health Physics 40, 439-455, 1981.
EPA83 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,
B.C., 1983.
Mo79 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 Doses to Man from Airborne Releases of
Radionuclides, ORNL-5532, Oak Ridge National Laboratory, Oak
Ridge, Tennessee, 1979.
NRC79 Nuclear Regulatory Commission, Draft Generic Environmental
Impact Statement on Uranium Milling, Volume II, NUREG-0511,
USNRC, Washington, D.C., 1979.
6-7
-------
Chapter 7: CONTROL TECHNIQUES
The reduction of radon-222 emissions at licensed uranium mills 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 emis-
sions. 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 dis-
posal 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 classi-
fied 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 impound-
ments. The depth of earth 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 processes such as molecular diffu-
sion, described mathematically by Pick's law. These complex diffusion
parameters have been evaluated by Rogers and Nielson (Ro81). They deter-
mined that diffusion depends greatly on the porosity and moisture content
of the medium through which it occurs. Ideally, the diffusion coeffi-
cient 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).
Cover thickness may be calculated by using the same diffusion equa-
tions that apply to emissions from uncovered tailings as shown in the
following equations (Ro84):
J = J exp (-b x )
c t ^ v c c
where J is the flux through cov^r (pCi/m2s); J is the flux through
tailings (pCi/m2s); b is (A/D )'5; A is the radon-222 decay constant (2.1
x 10 6/s); D is the SiffusionCcoefficient of cover, 0.07 exp [-4(m-mp2 +
m5)]; m is tfie moisture saturation fraction [0.01 M(l/p - 1/g)"1]; M is
the moisture content of cover material (percent dry weight); p is the
bulk density (g/cm3); g is the specific gravity (g/cm3); p is the poros-
ity (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 cover depth and the uncovered tailings flux.
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"
7-2
-------
100
I
T
c
OJ
u
QJ
CL
o
o
o
o
o
-------
(HVL). The HVL is that thickness of cover material (earth) that reduces
the radon-222 flux to one-half its uncovered value. High-moisture con-
tent 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 0.5-, 1-, 2-, and 3-meter depths is shown in Table 7-1.
Table 7-1. Percentage reduction in radon-222 emissions attained by
applying various types of earth cover
Depth of earth cover (m)
Earth type
A
B
C
D
E
HVL(m)
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
(a)
See Figure 7-1.
In practice, earthen cover designs must take into account uncertain-
ties in the measurements of the properties of the specific cover mate-
rials used, the tailings to be covered, and especially the predicted
long-term values of equilibrium moisture content for the specific loca-
tion. 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 predict-
ing 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 excava-
tion, 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
7-4
-------
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 con-
tingency. Indirect costs would add approximately 30 percent to the
direct charges.
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/m2s. 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/m2s) is $5.2 x 106 with a fairly dry type A earth* and $1.4 x 106
for a more moist type D earth.
Earth cover is applied to dry tailings with conventional earth-mov-
ing 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 recircula-
tion 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 impound-
ments 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 impound-
ments (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 installa-
tions.
50 ha x $105,000/ha = $5,250,000
7-5
-------
Table 7-2. Summary of unit costs for estimating earth cover costs
(a)
Task
Unit cost ($)
Grading, self-propelled scraper, 1000-ft haul
Excavation, elevating scraper, 5000-ft haul
Compaction, vibrating
Excavation, front-end loader, truck-loaded
Hauling, 12-yd3 dump truck, 2-mile round trip
Fencing, 6-ft, aluminized steel
Riprap, machine-placed slope protection
Borrow, bank-run gravel
1.16/yd3
2.46/yd3
1.00/yd3
0.84/yd3
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
-------
(a)
Table 7-3. Earth moving and placement costs (thousands of dollars per hectare) of attenuating radon-222
flux as a function of thickness (meters of different soils) and type of earth
Earth Type
Final flux(b)
(pCi/m2s)
20
50
100
200
Cost
267
174
104
34
A
Thickness
3
2
1
0
.81
.49
.49
.49
:B
Cost Thickness
200
130
78
25
2.
1.
1.
0.
86
86
11
36
C_
Cost Thickness
133
87
52
17
1.
1.
0.
0.
90
24
74
24
D
Cost Thickness
80
52
31
10
1.14
0.75
0.45
0.15
Cost
32
21
12
4
E_
Thickness
0.46
0.30
0.18
0.06
(a)
(b)
Cost basis: $7.00/m3 ($5.35/yd3) of soil cover material; includes excavating ($0.84/yd3), hauling
($2.35/yd3), spreading ($1.16/yd3), and compacting ($1.00/yd3), in 1985 dollars.
Based on initial radon-222 emission rate of 280 pCi/m2s.
-------
Effectiveness and Cost
The diffusion coefficient of water is very low (1.1 x 10 5 cm2/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 effective-
ness 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 pre-
sented in Section 3. Water-covered tailings have a radon-222 flux of
about 0.02 pCi/m2s per pCi of radium-226 per gram of tailings compared
with a dry tailings flux of about 1 pCi/m2s 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 through
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 coeffi-
cient 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 ini-
tially increase because of a larger emanation coefficient. As the mois-
ture 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
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 Docu-
ment .
7-8
-------
0.6
,_ 0.4
HI
o
tZ
LL
LU
O 0.3
O
•z.
g
i-
LU
•z.
O
Q
oU-
0
TAILINGS MOISTURE, M (dry wt. %)
10
20
30
MONTICELLO ALKALINE
GRAND JUNCTION SLIME
GRAND JUNCTION SAND
MEXICAN HAT
DURANGO
AMBROSIA LAKE
MONTICELLO ACID
VITRO SLIME
RAY POINT
VITRO SAND
MONUMENT VALLEY
RIVERTON
ASSUMING m- 2.7M
0.2 0.4 0.6 0.8
MOISTURE SATURATION, m
1.0
Figure 7-2. Radon emanation coefficients for tailings samples
(Ro84)
7-9
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Synthetic Covers
Synthetic material, such as polyethylene sheet, can reduce radon-222
emissions if carefully placed on dry beach areas and sealed. Diffusion
coefficients of less than 10~6 cm2/s have been measured for synthetic
materials (Ro81). Such covering could be used on portions of the tail-
ings on a temporary basis and then removed or covered with fresh tail-
ings. Such a barrier also would aid, at least temporarily, in the con-
trol 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/ft2 per mil of thickness or $0.50/ft2 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 suppress-
ing 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).
Costs of applying a full-scale asphalt cover were estimated to be
$24.20/m2 ($20.23/yd2) in 1981 dollars or $100,000/acre (Ba84). These
cost estimates are probably applicable to relatively flat sites. Exis-
ting 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 sys-
tem. 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.
7-10
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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 showed 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 prac-
tical disposal method, information is- needed on the following:
(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 stabi-
lized 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, protec-
tion 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 poten-
tial for ground-water contamination may require the use of liners in a
disposal area.
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 (Wm81). 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).
7-11
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Soil Cement Covers
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 rela-
tively 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 con-
trol has not been evaluated.
Deep-Mine Disposal
Disposal of tailings in worked-out deep mines offers several advan-
tages 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 disposal. Radon-222 releases to the
atmosphere would be reduced, as would erosion and external radiation.
This method, however, has potential for ground-water contamination prob-
lems. Also, it could be costly, depending on the mine location and the
controls required to guard against potential ground-water contamination.
7.2 Control Practices Applicable to Existing Tailings Impoundments
Control practices that are applicable to existing tailings impound-
ments 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 require-
ments of final reclamation. Standards for final reclamation include
requirements for reducing average radon-222 emissions to 20 pCi/m2-s and
for long-term (1000 y) stability and protection against misuse.
7.2.1 Interim Controls
Interim controls would significantly reduce radon-222 emissions and
particulate emissions over the period of licensed operation and prior to
final reclamation. For example, earth covers of 0.5 m (1.6 ft) or 1 m
(3.3 ft) that have a 7.5 percent moisture content (type B soil) would
result in reductions of radon-222 emissions of 37 and 60 percent, respec-
tively (Table 7-1). Similarly; increased water cover, when feasible,
could be used to reduce emissions at lined impoundments. Interim
7-12
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controls would not provide the long-term control required for final
reclamation. The objective of applying an interim cover would be to
reduce radon-222 emissions, and therefore health effects, over the period
during licensed operations but prior to final reclamation.
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) would influence the type of interim
controls or the extent to which they could be applied. Interim controls
(e.g., earth cover) could be applied immediately to all dry areas of
existing impoundments. Currently, 55 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.
The most feasible method of reducing radon-222 emissions from active
tailings impoundments depends on the specific characteristics of the
milling process and the impoundment. These characteristics include
layout and dike construction, dike height, stability, phreatic level and
permeability, plant water balance, pond evaporation rates, and
availability of suitable earth cover material. Operating factors such as
expected uranium production rate, length and number of standby periods,
pond capacity, and expected mill life also affect the controls that could
be realistically selected.
At active impoundments, only those portions that are not to be used
further would be covered. Which portion and how much of the tailings
area to cover are a function of expected mill life and quantity of tail-
ings, the size of tailings impoundment, and the level of tailings gen-
erated (percentage of capacity). 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 dike may affect the
size of the equipment that can be used to transport and spread the cover
material. More soil may have to be added to the dam or embankments to
decrease their slope and increase stability. 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 tail-
ings 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
7-13
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process will eventually cover the tailings areas and reduce emissions to
20 pCi/m2s as required by Federal regulations. Mills that wish to retain
their operating licenses do not have to begin their final decommissioning
process, but they could take some interim actions to minimize radon-222
emissions. Methods for reducing radon-222 emissions to air from opera-
tional tailings areas are difficult to apply as new tailings beach areas
are continuously being formed. 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-spe-
cific 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-222 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 option.
Based on the information on tailings impoundments and areas for
licensed mills presented in Chapter 4, covering the currently dry beach
areas with 1 meter of earth and maintaining the current water cover on
the ponded and wet beach areas would reduce radon-222 emissions from 138
kCi/y to about 53 kCi/y, a reduction of 60 percent,* at a cost of about
$80 x 106 (1985 dollars). Fugitive dust emissions would also be reduced.
At tailings impoundments with sand tailings dams, these would also be
covered with earth. Placing 1 meter of earth on the areas of existing
impoundments that are now dry provides an interim control measure. No
steps would necessarily be taken during this interim period that would
help bring the impoundment to final reclamation. Depending on how
interim control is applied, a portion of its cost may be recoverable at
the time of final reclamation.
7.2.2 Final Reclamation
If all existing impoundments were allowed to dry, and were covered
with enough earth to achieve a flux of 20 pCi/m2s, the total radon-222
emissions would be reduced to 8 kCi/y. The cost would be about $660 x
106.
*
Based on soil with 7.5 percent moisture content and current emissions
of 138 kCi/y.
7-14
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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 substan-
tially 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 cost
estimates presented in this section. The cost of enough earth (8%
moisture) to attenuate the radon-222 flux to 20 pCi/m2s 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 alterna-
tives are summarized in Table 7-4. Covering the currently dry areas with
a meter of earth achieves an estimated reduction in emissions of 61
percent at a cost of $80 x 106 (1985 dollars) and prevents 1.2 cancers
per year nationally. An estimated emission reduction of 94 percent can
be achieved by applying sufficient cover to achieve 20 pCi/m2s; this
would cost $660 x 106 (1985 dollars) and prevent 1.8 cancers each year
nationally. (These cost estimates are for the control practice only and
do not include the cost of establishing new impoundments. In addition,
these estimates have not been discounted.)
7.3 Control Practices Applicable to New Tailings Impoundments
New tailings-disposal impoundments at uranium mills can be designed
to incorporate radon-222 control measures. Three different kinds of new
model impoundments are considered: single-cell, phased disposal, and
continuous disposal of dewatered tailings. Descriptions of radon-222
emissions and estimated costs of the three types of new model tailings
impoundments are presented in the following sections.
Below-grade impoundments are the NRC's preference, as this method
minimizes potential for windblown emissions and water erosion and elim-
inates the potential for dam failure (NRC80). Although below-grade
disposal is preferable, well-designed and operated above-grade tailings
impoundments can also provide adequate safety and be licensed by the NRC.
7.3.1 Single-Cell Tailings Impoundment
New tailings disposal areas must conform with Federal regulations
(40 CFR 190 and 192 and 10 CFR 40) for prevention of ground-water contam-
ination and airborne particulate emissions. New impoundments will also
be designed to facilitate final closure as required by current Federal
Standards. New tailings areas will have synthetic liners, will probably
be built below or partially below grade, and will have earthen dams or
7-15
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Table 7-4. Cost and effectiveness of applying earth cover
to existing tailings impoundments
Alternative
Radon-222
emissions
(kCi/y)
Avoided fata
cancers/y
($ x 106)
Cease use of current
impoundments, allow
to dry and apply final
cover.
Cover current dry areas
with 1 m of earth.
Before
After 0-80 km
National
138
138
8 1.3 (0.3) 1.8 (0.5)
53 1.1 (0.2) 1.2 (0.3)
660
80
Values are based on 1000 deaths due to lung cancer per 106 person
WLM. The values in parentheses are based on 250 deaths per 106
person WLM.
Total cost, including indirect charges. Final cover includes earth
required to achieve 20 pCi/m2s, regrading sand tailings dams to 5:1
(H:V) slope, riprap on sides, and gravel on top of impoundments (1985
dollars).
7-16
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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 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 1800 t
(^2000 tons)/day of tailings over a 15-year active period. During opera-
tion, 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.
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
0.30 with 3 meters of earth
__
For 47-ha new model impoundment with 15-
year life and 5-year drying-out period.
Emissions based on 280 pCi Ra-226/g and a
specific flux of 1 pCi Rn-222/m2s per pCi
Ra-226/g of tailings when dry.
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 $41.3 x 10 ,
7-17
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3 m FINAL COVER,
GRADE LEVEL FOR
TAILINGS
BELOW GRADE IMPOUNDMENT
LINER
637 m
SECTION A-A
6 m
24 m
637 m
t
24 m
I
637 m
TAILINGS CAPACITY = 1800 t/d x 310 d/y x 15 y = 8.4 x 10° t
TAILINGS VOLUME = 8.4 x 106 t * 1.6 t/m3 = 5.25 x 106 m3
FINAL TAILINGS SURFACE AREA = 47 ha (116 acres)
DIAGRAMS ARE NOT TO SCALE.
Figure 7-3. Size and layout of the model single-cell tailings impoundment.
7-18
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>^
•^
O
c\j
OJ
CSJ
§
4 -
3 -
1 1 1
RADIUM CONCENTRATION - 280 pCi Ra-226/g
SPECIFIC FLUX - 1 pCi Rn-222/ra2s per
pCi Ra-226/g FOR DRY AREAS NO
~ DURING ACTIVE LIFE, 20% OF /
AREA IS DRY BEACH /
DRYING 3'
PHASE
I 1 1* *
0 5 10 15 2
YEAR
»
COVER, 4.15 kCi/y{
(280 pCi/m2s)
-m COVER, 0.30 kCi/y
? '
/ OO r>ri /rr c \
\c.(j pLi/m S; /
0 >20
? _
1 -
Figure 7-4. Estimated radon-222 emissions from a model
single-cell tailings impoundment.
7-19
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Table 7-6. Estimated costs for a model single-cell tailings
impoundment
Item
Excavation
Synthetic liner (30-mil)
Grading
Drainage system
Dam construction
Cover (3-m)
Gravel cap (0.5-m)
Riprap on slopes
Subtotal direct cost
(c)
Indirect cost
Total cost
Costs
Below grade
21.51
3.03
0.40
0.40
-
4.05
1.92
-
31.31
10.02
41.33
($ x 106)
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)
(b)
(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.
7-20
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including a final cover cost of about $6.0 x 106 ($4.15 x 106 for earth
cover and $1.9 x 106 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 x 106.
Final closure costs are slightly higher at $7.8 x 106, as riprap is
required on the sides of the dam.
7.3.2 Phased-Disposal 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 dewa-
tered and allowed to dry. After the first cell has dried, it would be
covered with earth obtained from the cells excavation. This process con-
tinues 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
periods, the cell could be dewatered and an earth or synthetic cover
7-21
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applied. To estimate radon-222 emissions, a model phased-disposal im-
poundment comparable to the model single-cell impoundment was used. This
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 m2 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 dewa-
tered 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 impound-
ment are 13.5 kCi. Average radon-222 emission rates are shown in Table
7-7.
Table 7-7. Average radon-222 emission rate for model single-
cell and phased-disposal tailings impoundments
(a)
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
Phased-disposal 0.7 0.33 covered with 3 m of earth
For new model impoundment with 15-y life and 5-y drying
period for each cell. Emissions based on 280 pCi Ra-226/g and
specific flux of 1 pCi Rn-222/m2s per pCi Ra-226/g of tailings when
dry.
Assumes a 5-y drying-out period for each cell and immediate cover of
3 m of earth.
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
7-22
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3 m FINAL COVER
24 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 1O6 t/CELL
TAILINGS VOLUME PER CELL - 1.4 x 1O6 t/CELL - 1.6 t/m3 = 8.75 x 105 m3/CELL
FINAL TAILINGS SURFACE AREA = 8.6 ha/CELL (21.3 acre/CELL)
DIAGRAM IS NOT TO SCALE
Figure 7-5. Size and layout of model phased disposal impoundment.
7-23
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>,
o
RADIUM CONCENTRATION - 280 pCi Ra-226/g
SPECIFIC FLUX - 1 pCi Rn-222/m2s per
pCi Ra-226/g FOR DRY AREAS
I/O
o
I—I
oo
C\J
CM
CM
START COVER-
OF CELL 1
/
0
• —^ ^ __j
/ LU * LU
i» O O
vo
I
10 15
YEAR
0.33 kCi/y
<>
.(20 pCi/m^s)
20
•5
Figure 7-6. Estimated radon-222 emissions from a model
phased-disposal impoundment.
7-24
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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
106, 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 impound-
ment). An evaporation pond is included as part of the phased-disposal
system. The cost for a partially above-grade phased-disposal system is
about $6.9 x 106 per cell, or a total of $41.5 x 106. The decreased cost
of excavation is partially offset by the dam construction cost and the
riprap on the sides.
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 placed
and 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 nonfer-
rous 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
blinding. Corrosion-resistant materials would be required in any tail-
ings dewatering system because of the highly-corrosive solutions that
must be handled. Although it is used in some foreign countries, contin-
uous 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
dwatered 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.
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,
7-25
-------
(a)
Table 7-8. Estimated costs for a model,phased-disposal impoundment
($ x 10°)
Item
Excavation
Synthetic liner
(30-mil)
Grading
Drainage system
Dam contruction
Cover (3-m)
Riprap on slopes
(0.5 m)
Gravel cap (0.5-m)
Evaporation pond
Subtotal direct
cost
Indirect cost
Total cost
Below
One cell
3.68
0.57
0.07
0.07
-
0.76
-
0.37
0.52
6.04
1.93
7.97
grade
All cells
22.08
3.40
0.45
0.40
-
4.57
-
2.21
3.09
36.20
11.58
47.78
Partially
One cell
1.28
0.57
0.07
0.07
1.27
0.76
0.32
0.39
0.52
5.25
1.68
6.93
above grade
All cells
7.70
3.40
0.45
0.40
7.61
4.57
1.91
2.34
3.09
31.47
10.07
41.54
(a)
Below-grade impoundments are constructed so that the top of the final
(b)
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,
7-26
-------
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 m (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 to (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 tail-
ings. Excavation, filling, and covering would be carried out simultan-
eously. Umetco Minerals proposed a continuous disposal system that would
be located on a mesa adjacent to the Uravan, Colorado, mill. The exis-
ting 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 tail-
ings. A 6-m berm separates the trenches to allow for tailings placement.
A diagram of the model continuous-disposal impoundment is shown in Figure
7-7. The total tailings surface area at capacity is 572,000 m2, or
57,200 m2 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 re-
quired for the single-cell impoundment. A partially below-grade contin-
uous/single-cell disposal impoundment is also considered because it
minimizes the excavation cost as well as the cost of dam construction.
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 contin-
uous-disposal impoundments. This is evident in Table 7-9, which shows
the average emission rates for continuous-disposal and the single-cell
model impoundments.
7-27
-------
GRADE LEVEL FOR
-137 m-
BELOH-GRADE IMP0UNDMENT'
3 m
12 m
77 m
COVER
GRADE LEVEL FOR
BELOH-GRADE IMPOUNDMENT
NOTES:
TAILINGS
SECTION A-A
-469.5 m-
3 m
I V
12 m
-409.5 m-
SECTION B-B
COVER
TAILINGS
TAILINGS CAPACITY PER TRENCH = 1800 t/d x 310 d/y x 15y t 10 TRENCHES = 8.4 x 10b t
TAILINGS VOLUME PER TRENCH = 8.4 x 105 t t 1.6 't7m3 = 5.25 x 1Q5 m3
FINAL TAILINGS SURFACE AREA = 5.74 ha/TRENCH (13.7 acre/TRENCH)
DIAGRAM IS NOT TO SCALE
Figure 7-7. Size and layout of the model continuous-disposal impoundment.
7-28
-------
Table 7-9. Estimated radon-222 emission rates for model single-cell,
phased disposal, and continuous-disposal tailings impoundments
(a)
Average emission rate (kCi/y)
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
a
For new model impoundments with 15-y operational life emissions based
on 280 pCi Ra-226/g and specific flux of 1 pCi Rn-222/m2s per pCi
Ra-226/g of tailings when dry.
Includes 5-y drying-out period.
7-29
-------
Figures 7-8 and 7-9 show the radon-222 emission rates for the model
continuous-disposal impoundments of single-cell and trench designs, re-
spectively. It has been assumed that 4 ha (10 acres) of dewatered tail-
ings 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 contin-
uous 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 106, and the
cost of a partially above-grade trench design system, at about $61.0 x
106. 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 .
7.4 Summary of Radon-222 Control Practices
A summary of the radon-222 emissions from new model impoundments
serving an 1800 t/day mill is presented in Table 7-11. Three types of
emissions are presented: operational, post-operational, and total emis-
sions. The emissions from a model single-cell impoundment represent
those with and without final cover to provide a perspective on the emis-
sion reductions.
Operational emissions are those that occur during the operating 15-y
life of the mill plus those due to the impoundment's 5-y 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-y lifetime. Emission
rates for the active and drying-out periods of phased- and continuous-dis-
posal 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-y 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/m2s. 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/m2s; therefore, variations
in emissions from the various covered impoundments are due to different
operational emissions and small differences in the tailings surface
areas.
7-30
-------
RADIUM CONCENTRATION - 280 pCi Ra-226/g
SPECIFIC FLUX - 1 pCi Rn-222/m2s per
pCi Ra-226/g FOR DRY AREAS
>,
•^-
O
O
t—t
C/)
CM
CM
CM
I
O
O
-------
T
RADIUM CONCENTRATION - 280 pCi Ra-226/g
SPECIFIC FLUX - 1 pCi Rn-222/m2s per
pCi Ra-226/g FOR DRY AREAS
oo
O
CVJ
CM
CM
I
z:
o
o
0.30 kCi/y (20 pCi/nTs)
10
15
20
YEAR
Figure 7-9. Estimated radon-222 emissions from a model
continuous/single-cell disposal impoundment.
7-32
-------
Table 7-10. Estimated costs for a model, continuous disposal impoundment
($ x 106)U;
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
Total cost
Below-grade
trench design
22.75
3.82
0.51
-
5.15
-
2.54
4.80
1.46
41.03
13.13
54.16
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
37.44
Trench
design
7.24
3.82
0.51
8.06
5.15
2.15
2.99
4.80
1.46
46.18
14.78
60.96
(a)
(b)
(c)
In 1985 dollars.
Below-grade impoundments are constructed so that the top of the final
cover is at grade. Partially above-grade design is 6 m deep and 6 m
above grade.
Indirect costs are estimated to be 32 percent of direct costs.
7-33
-------
Table 7-11. Summary of estimated radon-222 emissions from new model tailings impoundments
(a)
I
U)
Alternative
Operational emissions
(kCi/y)
Active Dry-out
Cumulative
Post-operational emissions emissions total
(kCi/y) (kCi)
With
(b)
(15 y) (5 y) Average Uncovered final cover 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.
0.
0.
0.
2(d)
7(d)
$
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
1.2
(d)
4.2
NA
25
108
191
NA - Not applicable.
(a)
(b)
(d)
(e)
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.
Assumes 20% of the impoundment area is dry beach during the 15-y active life; remainder of
area is water-covered.
Based on 20-y life: 15 y active, and 5 y drying out.
Based on 15-y life.
-------
Cost estimates for constructing new model tailings impoundments are
summarized in Table 7-12. The partially above-grade single-cell impound-
ment cost, $29.7 x 106, is the lowest cost alternative, but most of the
costs are incurred during initial construction. Its completely below-
grade counterpart costs are estimated to be $41.3 x 106. The difference
is largely due to increased excavation costs. Phased- and continuous-dis-
posal impoundments are more costly, but the costs are spread out over the
life of the impoundment.
7-35
-------
Table 7-12. Summary of estimated costs for new model tailings impoundment
(1985 $ x 10 )
^J
I
OJ
Single-cell
Below grade
Direct cost 31.3
Indirect cost 10.0
Partially
above grade
22.5
7.2
Phased-disposal
Partially
Below grade above grade
36.2 31.5
11.6 10.0
Continuous-disposal
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 Baker E. G., Hartley J. N., Freeman H. D., Gates T. E., Nel-
son D. A, and Dunning R. L., Asphalt Emulsion Radon Barrier
Systems for Uranium Mill Tailings - An Overview of the Tech-
nology, DOE/UMT-0214, PNL-4840, March 1984.
Dr81 Dreesen D. R. , Williams J. M. , and Cokal E. J., Thermal Sta-
bilization of Uranium Mill Tailings, in: Proceedings of the
Fourth Symposium on Uranium Mill Tailings Management, Fort
Collins, Colorado, October 1981.
EPA83 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.
Ge84 Gee G. W., Nielsen 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.
Ha83 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.
Ha84 Hartley J. N., and Gee G. W., Uranium Mill Tailings Remedial
Action Technology, in: Proceedings of the Second Annual Haz-
ardous Materials Management Conference, Philadelphia, Pennsyl-
vania, June 1984.
Ha85 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.
Ma83 Marline Uranium Corp. and Union Carbide Corp. An Evaluation of
Uranium Development in Pittsylvania County, Virginia, October 15,
1983, Section E.3.
NRC80 Nuclear Regulatory Commission, Final Generic Environmental
Impact Statement on Uranium Milling, NUREG-0706, September
1980.
NRC81 Nuclear Regulatory Commission, Environmental Assessment Related
to the Operation of San Miguel Uranium Project, NUREG-0723,
January 1981.
7-37
-------
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 Determina-
tion 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 Attenua-
tion 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,
^2_, 27-32, January 1982.
Th81 Thode E. F. and Dreesen D. R., Technico-Economic Analysis of
Uranium Mill Tailings Conditioning Alternatives, in: Proceed-
ings of the Fourth Symposium on Uranium Mill Tailings Manage-
ment, 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 Tail-
ings Management, Fort Collins, Colorado, October 1981.
7-38
-------
APPENDIX A
DIAGRAMS OF URANIUM MILL SITES AND
TAILINGS IMPOUNDMENTS
A-l
-------
Diagrams of each of the 20 licensed uranium mill sites that were in-
cluded in this evaluation are presented in this appendix. These diagrams
were adapted from aerial photographs taken by the Office of Radiation Pro-
grams. 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
-------
>
SECONDARY
IMPOUNDMENT
PROCESS
WATER
PONDS
PRIMARY
IMPOUNDMENT
WATER COVERED
TAILINGS
N
I
1000 ft
COTTER CORP. MILL
CANON CITY, CO
DATE: 8/12/85
-------
TAILINGS
IMPOUNDMENT 3
EVAPORATION
POND SOLIDS
TAILINGS
^IMPOUNDMENTS
1 AND 2
0
I
1000 ft
I
UMETCO MINERALS MILL
URAVAN, CO.
DATE: 8/7/85
EXPOSED TAILINGS
MILL BUILDINGS
o
EVAPORATION PONDS
FORMER RESIDENTIAL AREA
EVAPORATION PONDS
PONDED WATER
-------
ACCESS ROAD
Ul
L-BAR TAILINGS
IMPOUNDMENT
WATER COVERED
TAILINGS
SOHIO MILL
CEBOLLETA, NM
DATE: 10/5/85
0
mm EXPOSED TAIL INGS mm
1000 ft
MINEWATER
POND
MINE
BUILDINGS
o
-------
MINE SHAFT
MINE
SHAFT
MINE SHAFT
MINE
BUILDINGS
MINE WATER
PONDS
0
1000
I
UNITED NUCLEAR MILL
GALLUP, NM
DATE: 10/5/85
2000 ft
i
CHURCHROCK TAILINGS
IMPOUNDMENT
A-6
-------
EVAPORATION
PONDS
BLUEWATER 1
TAILINGS IMPOUNDMENT
0
L_
1000
I
2000 ft
i
ANACONDA MILL
BLUEWATER, NM
DATE: 8/24/85
EXPOSED
TAILINGS
BLUEWATER 2
TAILINGS IMPOUNDMENT
BLUEWATER 3
TAILINGS IMPOUNDMENT
MILL BUILDINGS
-------
EVAPORATION
PONDS
KERR-McGEE CORP. MILL
GRANTS, NM
DATE: 8/24/85
r t
OO
?i QUIV IRA 2A .QUIVI RAJ
WATER
COVERED
TAILINGS
1000
i
QUIVIRA 2C
-------
ROAD
>
HOMESTAKE MILL
GRANTS, NM
DATE: 8/24/85
HOMESTAKE 2
TAILINGS
IMPOUNDMENT
0
u
1000 ft
I
EXPOSED TAILINGS
HOMESTAKE 1
TAILINGS
IMPOUNDMENT
EXPOSED TAILINGS
HZ)
CD
WATER COVERED
TAILINGS
MILL BUILDINGS
HIGHWAY 53
-------
N
>
i
0
1000 ft
I
CHEVRON RESOURCES MILL
PANNA MARIA, TX
DATE: 11/4/85
WATER COVERED
TAILINGS
PANNA MARIA
TAILINGS
IMPOUNDMENT
-------
>
I
WHITE MESA TAILINGS
IMPOUNDMENT NO.l
EXPOSED TAILINGS U.
WATER COVERED SS
TAILINGS
ORE PILES
c-p MILL
" BUILDINGS
WHITE MESA TAILINGS
IMPOUNDMENT NO.2
UMETCO MILL
BLAND ING, UT
DATE: 8/7/85
WHITE MESA TAILINGS
IMPOUNDMENT NO.3
0
1000 ft
I
-------
I
M
NJ
RIO ALGOM MILL
LA SAL, UT
DATE: 8/7/85
WATER COVERED
TAILINGS
MILL
BUILDINGS <£
^EXPOSED TAILINGS
SURFACE
WATER
IMPOUNDMENT NO. 2
IMPOUNDMENT NO.l
EXPOSED
TAILINGS
-------
ATLAS CORP. MILL
MOAB, UT
DATE: 8/4/85
N
MOAB TAILINGS
IMPOUNDMENT
-------
WATER
COVERED
TAILINGS
EXPOSED
TAILINGS
SHOOTARING TAILINGS
IMPOUNDMENT
SURFACE
WATER
MILL
BUILDINGS
0
L
PLATEAU RESOURCES MILL
SHOOTARING CANYON, UT
DATE: 8/4/85
1000 ft
J
-------
FORD TAILINGS
IMPOUNDMENT
NO. 4
WATER COVERED
TAILINGS -
M
Ln
FORD TAILINGS
mm IMPOUNDMENTS NOS
mm 1, 2, AND 3
mm EXPOSED TAILINGS
STREAM
DAWN MINING MILL
FORD, WA
DATE: 8/14/85
1000 ft
MILL
BUILDINGS
-------
WATER
COVERED
TAILINGS
WESTERN NUCLEAR MILL
WELLPINIT, WA
DATE: 8/14/85
SHERWOOD
IMPOUNDMENT
1000
2000 ft
i
EVAPORATION
POND
A-16
-------
GAS HILLS
TAILINGS
IMPOUNDMENT NO. 4
•WATER COVERED
TAILINGS
GAS HILLS
TAILINGS
^IMPOUNDMENT NO. 3
PATHFINDER GAS HILLS MILL
SHIRLEY BASIN, WY
DATE: 8/8/85
1000 ft
GAS HILLS
TAILINGS
IMPOUNDMENT NO. 2
EXPOSED >ipS£
TAILINGS YX:::-':
DJ1MILL BUILDINGS
A-17
-------
ACCESS ROADS
PROCESS WATER
POND
I
I—'
CD
WATER COVERED
TAILINGS
EXPOSED
$ TAILINGS
WESTERN NUCLEAR MILL
JEFFREY CITY, WY
DATE: 8/8/85
-------
SURFACE
MINE
I
M
VO
1000
UMETCO MILL
GAS HILLS, WY
DATE: 8/8/85
GAS HILLS
TAILINGS
IMPOUNDMENT
MILL BUILDINGS-*^
-------
SURFACE
WATER
>
o
SURFACE MINE
MINE
WATER
PONDS
ACCESS ROAD
MILL
BUILDINGS
WATER
COVERED
TAILINGS
EXPOSED
^TAILINGS
BEAR CREEK TAILINGS
IMPOUNDMENT
ROCKY MT. ENERGY MILL
BEAR CRE£K, WY
DATE: 9/13/85
-------
SHIRLEY BASIN
TAI LINGS IMPOUNDMENT
^^^ WATER COVERED I
TAILINGS
^^^^ —^^ ——^—«••»» • • • •' *-•_' -^^^-——-^ —— »^^ ^i^j - _ —i —
EXPOSED
TAILINGS!
ACCESS ROAD
PATHFINDER SHIRLEY BASIN MILL
SHIRLEY BASIN, WY
DATE: 9/13/85
A-21
-------
M
N)
0
u
:-MINE WATER POND
1000
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
most 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.
Table B-l. Indirect cost factors used in the cost estimation of
uranium mill tailings impoundments
Indirect cost item
Percentage
Range
Value used
Engineering and design
Insurance
Performance bond
Permits
Overhead and profit
Contingency at conceptual stage
Total
2.5 - 6.0
0.1 - 0.82
0.39 - 1.2
0.5 - 2.0
10 - 15
15 - 20
32.0
Source: "Means Site Work Cost Data 1985," 4th Annual Edition, R.S. Means
Co., Inc.
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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/yd3 ($7.00/m3) was used to estimate the cost of placing the interim
cover. This includes the direct costs of excavation, hauling, spreading,
and compacting the cover.
Finaj. Reclamation
Measures for effecting final reclamation of existing uranium mill
tailings impoundments are those required to reduce the radon-222 flux to
20 pCi/m2s and to place the impoundment in a state of permanent, long-
term stability. 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 pos-
sible immediately after an impoundment had dried. No cost for attaining
dry-out was assumed. The measures taken and the costs of final reclama-
tion 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/m2s 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 tailings 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
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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 slop-
ing 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 compar-
ability 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
106 t of tailings with a volume of 5.25 x 106 m3.
Single-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 of
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
30-mil synthetic liner and a drainage system to facilitate dewatering
when the impoundment has reached capacity. 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 esti-
mated costs for the below-grade and the partially below grade single-cell
impoundments are $41.3 x 106 and $29.7 x 106 (1985 dollars), respectively.
The difference is largely due to the additional excavation required for a
below-grade impoundment.
Phased Disposal Impoundments
The phased-disposal impoundment consists of a series of small im-
poundments 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.
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Excavation to a depth of 6 meters for the partially below-grade
phased-disposal impoundment does not provide sufficient earth to con-
struct the dam and to place a 3-meter earth cover over the tailings.
Thus, the costs of obtaining additional earth and reclaiming a borrow pit
are included in the cost of the dam construction. The total estimated
costs for the below-grade and the partially below-grade phased disposal
impoundments are $47.8 x 106 and $41.5 x 106 (1985 dollars), respec-
tively.
Continuous Disposal Impoundments
A series of 10 rectangular trenches are included in the model con-
tinuous-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 evapo-
ration 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 106 and $61.0 x 106
(1985 dollars), respectively.
Continuous/Single Cell Disposal Impoundment
The design of the continuous/single-cell disposal impoundment in-
cludes 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 106 (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.
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REFERENCES
DOE82 Department of Energy, Commingled Uranium Tailings Study, DOE/
DP-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, Office of State Programs, Direc-
tory and Profile of Licensed Uranium-Recovery Facilities,
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).
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''U.S. GOVERNMENT PRINTING OFFICE: 1986-151-097 :
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