fif
if
EPA-520/1-76-001
POTENTIAL RADIOLOGICAL IMPACT OF AIRBORNE RELEASES
AND DIRECT GAMMA RADIATION TO INDIVIDUALS LIVING
NEAR INACTIVE URANIUM MILL TAILINGS PILES
U.S. ENVIRONMENTAL PROTECTION
>-- of Radiation Programs
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POTENTIAL RADIOLOGICAL IMPACT OF AIRBORNE RELEASES
AND DIRECT GAMMA RADIATION TO INDIVIDUALS LIVING
NEAR INACTIVE URANIUM MILL TAILINGS PILES
Ul
O
Jerry J. Swift
James M. Hardin
Harry W. Galley
January 1976
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Radiation Programs
Environmental Analysis Division
Washington, D.C. 20460
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FOREWORD
The Office of Radiation Programs carries out a National Program
designed to evaluate the exposure of man to ionizing and nonionizing
radiation and to promote development of controls necessary to protect
the public health and safety and assure environmental quality.
As man's appreciation of the risks of exposure to radiation has
developed, an awareness has developed that the tailings piles of
inactive uranium mills may be causing a risk to the public from radi-
ation of sufficient magnitude to warrant remedial action and develop-
ment of. necessary controls. As part of a cooperative program with the
Energy Research and Development Administration to assess the risk and
provide remedial action if needed, this report has been prepared to
provide a basis for an initial assessment of the potential radiation
doses from the tailings piles, and to point out the kinds of informa-
tion needed to prepare a thorough evaluation of the local radiation
risks from individual tailings piles.
Comments on this analysis would be appreciated. These should be
sent to the Director, Environmental Analysis Division, of the Office
of Radiation Programs.
W. D. Rowe, Ph.D.
Deputy MAiAtan
Radiation ?JWQ>iamt>
-------
Potential Radiological Impact of Airborne Releases
and Direct Gamma Radiation to Individuals Living
Near Inactive Uranium Mill Tailings Piles
Abstract
As part of a program to determine which measures to use to
control radioactivity from tailings piles at inactive uranium
mills, the U.S. Environmental Protection Agency has prepared
estimates of the potential for exposure to radioactivity pre-
sented by those piles. The gamma radiation field and the radon-
222 release rate are estimated on the basis of the radium-226
concentration in each pile. Potential exposures to airborne
particulate uranium, thorium-230 and radium-226 are estimated
from field measurements. Dispersion of airborne material was
calculated with adaptations of computer codes which use common
models for annual-average and single-plume dispersion.
The estimated potential annual dose from radioactive radon
decay products to individuals in dwellings in the vicinity of an
average inactive pile is approximately 8 rem to the tracheobronchial
region of the lungs at about 50 meters from the pile, 0.3 rem at 1
kilometer, and 0.1 rem at about 2.2 kilometers. The corresponding
doses to the pulmonary region of the lungs from airborne uranium
thorium-230, and radium-226 are estimated to be about one-third as
large, within 1 kilometer of the pile. Gamma exposure rates on the
tailings are up to 1 mR/hr. Estimated exposure rates are in reason-
able agreement with the limited data from field measurements.
Should an individual be exposed continuously to a dose equiva-
lent of 8 rem/yr to the tracheobronchial region of the lung, it
would require 100 years of exposure to double his risk of bronchial
cancer. Also, this level of exposure is considered equivalent to
0.5 Working Level Months per year, which is approximately 1/10 of
the radiation protection guidance for the protection of underground
uranium miners issued by the Environmental Protection Agency.
Average individuals exposed over a lifetime to a dose equivalent
of 0.3 rem/yr and 0.1 rem/yr would increase their risk of bronchial
cancer by about 3 percent and 1 percent respectively.
-------
Although there are uncertainties in these estimates, the
indicated conclusions are that the radon releases, which are
difficult to control, are the hazard of greatest significance,
that the radioactive particulate matter releases may sometimes
be significant also, and that thorough measurements are needed
for assessment of individual piles.
The potential national and worldwide radiological impact of
radon-222 was not evaluated.
vi
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Contents
Foreword [[[ iii
Abstract [[[ v
1.0 Summary [[[ 1
2.0 Introduction ............................. .................... 2
2 . 1 Background .............................................. 2
2.2 Present State of Knowledge .............................. 3
3.0 Radiological Assessment of Emissions from Uranium Mill
Tailings Piles ............................................... 6
3 . 1 Radioactivity Emitted from Tailings Piles ............... 7
3.1.1 Radon ............................................ 7
3.1.2 Radioactive Particulate Material ................. 13
3.1.3 External Gamma Ray Exposures ..................... 14
3.2 Airborne Dispersion of Radionuclides .................... 15
3.3 Radon-222 Dosimetry ..................................... 16
3.4 Airborne Thorium-230 and Radium-226 Dosimetry ........... 18
3.5 Gamma Ray Dosimetry ..................................... 22
4.0 Results [[[ 22
4. 1 Air Concentrations ...................................... 22
4.2 Radiation Doses ......................................... 25
5.0 Discussion [[[ 28
5.1 Results for the Average Inactive Tailings Pile .......... 28
5.2 Uncertainties ........................................... 30
5.2.1 Source Term ...................................... 31
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Tables
Page
Section 2
2.2-1 Summary of Onpile and Offpile Stations at the Four Study
Areas 5
Section 3
3.1-1 Inactive Uranium Mill Tailings Piles 11
3.3-1 Calculated Alpha Dose Rates (millirem/yr) from Inhalation
of Short-Lived 222Rn Daughter Products to the Basal Cell
Nuclei of Segmental Bronchi 19
3.3-2 Conversion Factors and Equivalents for Radon-222 and
Daughters 20
3.4-1 Dose Conversion Factors for Airborne Thorium-230 and
Radium-226 21
3.4-2 Percent Deposition of Particles in the Pulmonary Region of
the Lung as a Function of Activity Mean Aerodynamic
Diameter 23
Figures
Section 3
3.0-1 Uranium-238 Natural Radioactive Decay Series 8
Section 4
4.1-1 Ground-Level Radon-222 Concentrations and Dose Equivalent
Rates from an Area Source Compared to a Point Source (Axis
of a Single Plume) 24
4.1-2 The Distribution in the Crosswind Direction of Radon-222
Concentrations 300 m Downward from the Center of a 200-m-
Radius Area Source Compared to a Point Source (Single
Plume) 26
4.1-3 Annual Average Concentrations of Radon-222 Around a 0.4 km
Diameter Area Source Averaged Over All Directions 27
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1.0 Summary
This study is part of an evaluation of abandoned uranium
mill tailings to determine the need for remedial measures to
reduce their radiological health effects. The public health
significance of the mill tailings was estimated on the basis
of average conditions for 20 inactive mill sites. While this
assessment may not typify actual conditions at all uranium
mill tailings sites, several important observations were made.
First, both theoretical estimates and experimental evidence
indicate that individuals in the general population may be
receiving high radiation doses to the lung as a result of the
release of radon and its short-lived daughter products.
Respirable airborne tailings particles bearing radionuclides
provide another significant mode of potential radiation expo-
sure at small downwind distances, as does the gairnna radiation
emitted from the tailings piles. Secondly, in order to accu-
rately assess the potential public health significance of
uranium tailings sites for purposes of determining remedial
measures, a more comprehensive assessment of the problem using
actual data applicable to existing sites rather than assumptions
applicable to a model site is required.
i
This report has two main objectives: (1) to assess
realistically the potential radiological implications of air-
borne releases and direct gamma radiation from inactive uranium
mill tailings sites, and (2) to discuss the uncertainties inherent
in each aspect of the present assessment methodology.
The radiological assessment of the radon-222 emissions indi-
cates that the estimated annual average dose from radon-222 decay
products to individuals in dwellings in the vicinity of an average
uranium tailings site is approximately 8 rem to the lungs at about
50 meters from the pile, 0.3 rem at 1 kilometer, and 0.1 rem at
approximately 2.2 kilometers. The radiological effects of wind-
blown particulates containing thorium-230 and radium-226 were also
evaluated and were determined to be less of a problem. While there
is the possibility of doses to nearby individuals from these
radionuclides fully as large as those from decay products of radon-
222, the annual dose to the lungs of an individual within 1 kilo-
meter of the pile from uranium, thorium, and radium is estimated
conservatively to be about one-third as large. For a typical
uncovered tailings pile, the external gamma dose rate is estimated
to be 10 rem/yr at 3 feet from the surface of the pile, decreasing
to less than 1 mrem/yr at 1.0 kilometer.
It is estimated that the average individual exposed contin-
uously over a lifetime to a dose equivalent of 8 rem/yr to the
bronchial epithelium region of the lung will incur about a 70
percent increase in the risk of bronchial cancer. Average indi-
viduals similarly exposed to a dose equivalent of 0.3 rem/yr and
-------
0.1 rem/yr would increase their risk of bronchial cancer by
about 3 percent and 1 percent respectively. The relative
risk from particulate material blown from the pile is not as
critical, because the radiation dose from inhaled radioactive
particles is lower and, in addition, is delivered to the pul-
monary region of the lung. Irradiation of the pulmonary lung
is believed to be less likely to produce a health effect
compared to an identical dose equivalent delivered to the bron-
chial epithelium. The relative increase in risk caused by gamma
radiation from the average tailings pile is also estimated to be
less significant than the increase in risks caused by radon-222
from the pile.
2.0 Introduction
2.1 Background
In March 1974, the U.S. Environmental Protection Agency and
the U.S. Atomic Energy Commission testified at hearings before
the Joint Committee on Atomic Energy (JCAE) to present their
respective views on pending bills S-2566 and HR-11378. These
were identical bills which provided for a cooperative program
between the AEG and the State of Utah for the stabilization and
containment of a uranium tailings pile at the.site of the now
inactive Vitro Mill near Salt Lake City, Utah. Both Federal
agencies acknowledged that there are other uranium tailings
sites besides the Vitro Mill tailings pile that may present
public health problems, and that legislation that deals with
all inactive mill sites and not just a single site would be more
effective in controlling these potential problems. Consequently,
both the EPA and the AEG recommended and subsequently proceeded
to implement a study to determine the current condition of all
inactive uranium mill sites in the various states. This study is
a cooperative two-phase undertaking involving the states concerned
and appropriate Federal agencies.
Phase I involved a visit to each site to determine the physi-
cal conditions of the site, need for remedial action, ownership,
proximity to populated areas, prospects that the number of people
living around the site would increase, approximate areas of tailings,
etc. A preliminary report, which will serve as a basis for deter-
mining if additional assessment is necessary, was prepared for each
site. These reports were provided to the JCAE (October 1974) before
proceeding with Phase II.
Phase II of the study, which started in July 1975, includes a
detailed radiological and engineering assessment of the piles, an
examination of alternative remedial measures and the preparation of
cost estimates, detailed plans and specifications for various
remedial actions.
-------
This part of the study also includes extensive physical measure-
ments to determine the degree of potential radiation exposure of
the public. Additional research and development may be needed on
stabilization techniques to prevent wind and water erosion of the
tailings piles and to prevent radon emission from them. An assess-
ment will be made of the total environmental impact of proposed
remedial actions.
The EPA has recently attempted to quantify the potential
public health problems associated with uranium mill tailings at
both active and discontinued milling operations and place them
in perspective relative to other segments of the uranium fuel cycle.
The results of these analyses are contained in reference (1).
These analyses were done on the basis of a model tailings
pile which was assumed to be representative of all tailings sites.
Numerous assumptions were made where information was lacking or
where large uncertainties existed with the available information.
This generic assessment indicated that uranium mill tailings piles
present possible radiological health problems and that a more rig-
orous analysis of them is required.
Uranium mill tailings piles contain most of the decay products
of the radioactive decay of uranium and a little residual uranium.
The principal radionuclides of concern, in order of their appear-
ance in the decay chain, are thorium-230, radium->-22-6, radon-222,
polonium-218, lead-214, bismuth-214, polonium-214, and polonium-
210.
Uranium mill tailings piles release airborne radioactivity
as radon gas and also as airborne particulates when wind blowing
over the pile lifts material into the air. Radioactivity may be
leached from the pile into surface and ground water by precipita-
tion and by surface runoff waters. Also,sufficient radioactivity
is in the tailings to create a field of gamma radiation in the
..vicinity,of the tailings. Because of the presence of thorium-230
in the tailings, which by its decay maintains the radium inventory,
the .radioactivity in the pile can remain almost constant for
thousands of years. The scope of these problems and the remedial
actions required for their control vary with each site and; there-
fore, must be determined for each inactive tailings pile.
2.2 Present State of Knowledge
The Environmental Protection Agency and its predecessor
radiological health program in the U.S. Public Health Service have
been active for years in programs to determine the nature and mag-
nitude of the public health problems associated with uranium tailings.
More widespread appreciation of the magnitude of the problems involved
has brought more support for these programs.
3
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Under a joint agreement between the AEG and PHS, a project
was begun in June 1967, in cooperation with the Colorado Depart-
ment of Public Health and the Utah State Division of Health to
evaluate the public health aspects of atmospheric concentrations
of radon-222 in the vicinity of uranium tailings piles. The ob-
jectives of this project were to (1) develop techniques for
taking integrated air samples for radon near uranium tailings
piles, (2) evaluate atmospheric concentrations of radon in areas
near tailings piles as an index of radiation exposure to the popu-
lation, (3) determine the effect of stabilization and covering on
the emission of radon gas from tailings piles, and (4) develop
joint recommendations (or guidelines), if necessary, to control
exposures based upon measured radon concentrations. The results
of the study are reported in a 1969 DHEW publication titled
"Evaluation of Radon-222 Near Uranium Tailings Piles" (2).
Study areas were chosen to reflect conditions at both opera-
ting and inactive uranium mills. Locations were chosen at Grand
Junction, Colorado (then an operating mill with uncovered tailings),
Durango, Colorado (inactive mill, uncovered tailings), and
Monticello, Utah (inactive mill, covered tailings). The Vitro Mill
tailings pile near Salt Lake City, Utah, was added to the study in
October 1967. The Vitro Mill tailings were uncovered, and the mill
ceased operation in July 1968, while the study was being conducted.
At each of these study areas, a number of sampling sites were
selected that were representative of onpile, near-pile, and general
community conditions. Locations were chosen taking into considera-
tion the prevailing wind patterns, population densities, and geo-
graphical factors.
Table 2.2-1 summarizes the measured radon concentrations at
onpile and offpile sampling stations at the four study areas. For
perspective, these values may be compared with the following regu-
lations and guides.
Federal regulations 10 CFR Part 20 (3) presently permit AEC
licensed operations to release average concentrations of radon-222
to the air in unrestricted areas of 3 x 10"^ yCi/ml (3 pCi/1) above
natural background. [Such releases may be subject to reduction if
it appears that a suitable sample of an_exposed population.may have
an intake of the decay product radionuclides greater than they would
have if exposed to average concentrations of one-third of the
3 x 10-9 yCi/ml, i.e., 1 pCi/1.]
Recommendations of the International Commission on Radio-
logical Protection (ICRP) state that the annual radiation dose
limits for individual members of the general public should be
one-tenth of the corresponding annual occupational dose for con-
-------
Table 2.2-1. Summary of Onpile and Offpile Stations
at the Four Study Areas
(From "Evaluation of Radon-222 Near Uranium Tailings Piles" (2))
Location
Grand Junction, Colo.
Onpile
Near-pile
Other
Durango , Colo .
Onpile
Station 54 )
10
4.2
0.38(b)
Range
j.,1 -
.50 -
.13 -
.48 -
.44 -
.09 -
.12 -
.03 -
1.6 -
2.3 -
.06 -
28.0
4.5
4.4
34.0
2.3
1.3
12
1.3
22
6.6
1.4
Va'average of about 17 measurements at each location over a period of 12
.months
; ^assumed to be the local background concentration
bout 0.4 mi southeast of site
'about 0.4 mi west of site
-------
tinuous exposure as listed in the Committee II report (4). Appli-
cation of these recommendations to the 168-hour concentration
(10 pCi/1) for occupational exposure to radon-222 (with daughter
products present at equilibrium concentrations) leads to a value
of 1 pCi/1 for continuous exposure to individuals in the general
population. This concentration is in addition to the natural
background radon concentration which is on the order of 0.3 pCi/1.
More recently, EPA has conducted environmental surveys (5)
around the uranium tailings pile at the site of the now inactive
Vitro Mill near Salt Lake City, Utah. The surveys included measure-
ment of external gamma radiation and airborne radioactivity. The
results of the surveys indicated that the external radiation levels
on the tailings area exceeded recommended dose equivalent limits
for individuals in the general population (500 mrem/yr); that
ambient levels of radon over the pile and in some of the structures
built Immediately adjacent to the tailings pile were above the
currently recommended concentrations for individuals in the general
population (1 pCi/1); and that the working level (WL) exposures in
the adjacent buildings exceed 0.01 WL. The relation between radon
concentrations in pCi/1 and working level (WL) is discussed in
section 3.3.
Because the mill tailings piles have not been the subject of
as much interest as some other sources of radiation, there has been
little development of methods specifically designed for estimating
exposures to radiation originating with the pile. Schiager (6)
presents methods that require minimal data inputs for estimating
potential radiaton exposures from abandoned uranium tailings piles
based upon the average radium concentration in the tailings. The
reductions in exposure rates that can be achieved by covering the
tailings with earth or concrete are also discussed. Based upon a
case study of data on one tailings pile (Vitro pile), Schiager
concludes that contributions to the average annual outdoor radon
daughter concentrations exceeding 0.003 WL are very unlikely on or
near any tailings pile. However, estimates of indoor radon daughter
concentrations under the same conditions, which would lead to a more
accurate assessment of potential radiation exposures for most sites,
are not presented.
3.0 Radiological Assessment of Emissions from Uranium Mill
Tailings Piles
Mill tailings piles present a potential for exposure to radi-
ation by several pathways. The most important pathway is believed
to be that of the radon-222. Radium-226 in the pile decays by alpha
particle emission and becomes radon-222, a radioactive noble gas.
The radon-222 gas that is released into the spaces between the grains
-------
of tailings material diffuses toward the overlying tailings sur-
face; some reaches the surface and some undergoes radioactive
decay enroute. The radon-222 that reaches the surface escapes
into the air above, where it is mixed into the passing airstream
by normal local air turbulences. The wind carries the radon-222
with it and continually decreases the concentration by further
mixing and dilution. Persons downwind of the tailings pile will
be exposed to some concentration of radon-222 and its particulate
radioactive decay products in the air they breathe. The air that
enters a building downwind remains inside it for some period of
time, depending on the ventilation rate, i.e., on the rate at
which air enters and leaves the building. The radon-222 in the
air undergoes its normal rate of radioactive decay, forming a
series of decay products which may be inhaled by persons in the
building. Some of the radioactive daughters of the radon-222
are retained in the tracheobronchial region of the lungs, irra-
diating the fluids and tissues, and thus increasing the risk of
cancer formation there. In contrast, persons outdoors who are
exposed to the same radon-222 concentration from a nearby tailings
pile may receive an exposure to the lungs from the radon decay
products which is appreciably smaller (e.g., a factor of 10) than
that received indoors, due to the lack of a delay time which
permits daughter product ingrowth.
Another pathway for exposure to radiation is that in which
the wind lifts particles containing radionuclides from the sur-
face of the tailings pile and carries them downwind, with simul-
taneous mixing, dilution, and deposition, until they reach persons
inside or outside of buildings. Inhalation of the particles leads
to exposure in several ways, but it is believed that the principal
exposure is to the pulmonary region (the exchange space, non-cili-
ated) of the lungs in this case. It is likely that indoor exposures
by this pathway are smaller than those outdoors because filtration
and sedimentation processes reduce the particle concentrations.
The third principal pathway for exposure to radiation from
the tailings piles is that in which the radionuclides in the pile
emit gamma radiation which may penetrate the overlying material
and air to interact with the body tissues of persons on or near
the tailings piles.
The decay chain containing radon-222 and daughters is shown
in figure 3.0-1.
3.1 Radioactivity Emitted from Tailings Piles
3.1.1 Radon
Most of the uranium ores being milled in this country are
sandstones consisting of silica grains poorly cemented together
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Production of Radon-222
23& 4-2 MeV a } 234Th 0.19 MeV B
4.51 x 109 yr 24.1 days
234p
4.7 MeV a
1.17 min 2.47 x 105 yr
9qn 4.6 MeV a 09fi 4.8 MeV a 00<>
230Th - 7 - » 226Ra - " 222Rn
8.0 x 104 yr 1,600 yr
Short Half-Life Radioactive Decay Products of Radon-222
m5.5 MeV a 91B . .. 6.0 MeV a _ .
^ - j. 218po (RaA) - "
3.82 days 3.05 min
9iA_ 0.7 MeV 3 O-IA 3.3 MeV B
214Pb (RaB) 7 - - " 214B± (RaC)
26.8 m 19.7 min
7.7 MeV a
164 u sec
910
210Pb
Lone Half-Life Radioactive Decay Products of Radon-222
210pb (RaD) 0.02 MeV f , 210B1 (RaE)
1.1 MeV f
21 yr 5,01 days
21°P° ^ 138 days" " *****
Figure 3.0-1. Uranitim-238 Natural Radioactive Decay Series
8
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with materials such as calcium carbonate (7} . In the milling
process, the sandstone is broken down and after the uranium (and
sometimes vanadium) is extracted by chemical leaching, essen-
tially the whole original mass remains as tailings. The tailings
consist of the original sand grains plus slimes made up of
materials from between the sandstone grains and waste chemical
products of the mill process (7_).
Sampling of the material flow streams at numerous points in
several mills has shown that the tailings contain essentially all
of the radioactivity that was originally in the ore except for
the uranium; usually less than 10 percent of the original uranium
content remains in the tailings (7_,8) However, analyses of
samples from inactive tailings piles show lesser radioactivity
concentrations than expected from uranium concentrations in the
ore (9,_10,_11) . The principal decay chain in the tailings radio-
activity is that beginning with uranium-238 and ending with lead-
206; among its many members are the long-lived radionuclides
thorium-230, radium-226, and lead-210. The thorium-230 is the
effective beginning of this decay chain in the tailings. The
other radionuclide decay chain present, beginning with uranium-
235 and ending with lead-207, contributes about 5 percent of the
total radioactivity. There are no reports of radionuclides found
in the tailings in appreciable concentrations which are not members
of either of these two decay chains. For the majority of the
uranium ores mined in the United States, the ratios of the con-
centrations of the radionuclides in the decay chains are approx-
imately the ratios expected under the condition of secular equi-
librium (7); if the uranium-238 decay chain was in undisturbed
secular equilibrium, for each 0.1 percent by weight of uranium
oxide (UgOs) that had been in the ore, the tailings should contain
about 280 pCi/g of thorium-230, of radium-226, and of each of the
succeeding members of the decay chain. Measurements of the radio-
nuclides in the tailings show that commonly the activity of the
thorium-230 in the tailings is about the same as the activity of
the radium-226 (11,12).
The radioactivity that was pumped to the tailings pond was
disproportionately divided between the sands, the slimes, and
the dissolved material (T) The dissolved material generally
contained less than one percent of the mass and less than one
percent of the radioactivity of the tailings (7^8). Although
there is considerable variation from mill to mill, the slimes
solids are usually about one-third of the total weight of
tailings solids and have most of the radioactivity, about three-
quarters of the total. It is inferred from a statement of
Tsivoglou et al. (7) that the slimes particles may be as fine
as minus-400-mesh (less than 25 micrometers in diameter) like
particles of silt or clay, and that the sands are generally ulus-
200-mesh (greater than 74 micrometers in diameter), i.e., in the
normal size range for sand particles.
9
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Culot, Olson, and Schiager (13) have measured radon emana-
tion from the sand fraction of tailings from the Grand Junction
pile and determined that about 23% of the total radium inventory
in the sand releases radon to the atmosphere. This emanating
fraction will be assumed to apply to the slimes also, although
Pearson (14) indicates that the smaller particles may release a
greater fraction. The overall release from tailings particles
is assumed to be about 20% of the total radon produced by the
decay of radium-226.
The 20 percent emanation fraction does not completely deter-
mine the radon release rate from tailings piles. Radon released
at a depth in the pile must migrate to the surface, and may decay
during the process. This migration of the radon, a diffusion
process, produces a rate of radon emission from the upper surface
of the pile which, for a given uniform radon production rate
within the pile, is .a function of the depth of the pile. This
rate approaches an upper limit as the pile depth is increased.
Schiager indicates the depth at which the limit is approached by
stating that, for dry tailings, "6 to 10 feet in depth is required
to produce the maximum radon flux" at the surface of the pile (6).
This may be considered an "infinite thickness;" i.e., a thickness
such that the rate at which radon released from all greater depths
(even if they were infinite) reaches the surface is insignificant
in comparison, increasing the total release rate negligibly.
Schiager also provides a relation between the radon emission rate
and the radium content. He estimates on theoretical grounds that
at the maximum rate of radon emission from the tailings, 1.6 pCi/m^-
radon-222 are released for each pCi/g radium-226 in the tailings (6).
A variety of factors influence the rate of radon release from
a tailings pile. The radon release rate at any one location is
known to vary over a factor of 10 due to effects of weather, i.e.,
wind speed, barometric pressure, atmospheric stability, rainfall,
and snow cover (14). Moisture in the tailings is particularly
effective in reducing the emission rate; calculations (15) indi-
cate that completely saturating a dry tailings pile with water will
reduce its radon emissions by a factor of about 25. Other data
(13,^.4) indicate that addition of moisture occupying 15 to 85
percent of the interstitial space will reduce the emission rate by
a factor of 2 to 3. It has been estimated from theory that a 2-foot
layer of soil over the tailings will also reduce the radon emission
rate by about 25 percent (13)-
A list of inactive uranium mill tailings piles is given in
table 3.1-1. The piles average 35 acres in area, with a range
from 2 to 107 acres. Their average radium-226 concentration is
10
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Table 3.1-1 Inactive Uranium Mill Tailings Piles (1974)
Ariz.
Colo.
N.M.
Ore.
Tex.
,
t
Utah
Wyo.
Idaho
Location W
Tuba City
Monument Valley(d>
Grand Junction
Gunnison
Maybell
Rifle (new)
Rifle (old)
Slick Rock (2 piles)
Durango
Naturita
Grants'6'
Grants (f)
Shiprock
Lakeview
Falls City
Ray Point
Mexican Hat
Green River
Salt Lake City
Monti cello
Converse County
Riverton
Lowman(d>
Company
El Paso Natural
Gas Co. ;
Vanadium Corp. :.
Amax Uranium Corp;"
Colorado Venture
Union Carbide
Union Carbide
Union Carbide
Union Carbide
Foote Mineral
Foote Mineral
United Nuclear
United Nuclear
Foote Mineral
Oregon Pacific Ind.
Susquehana Western
Atlantic Richfield
A Z Minerals
U.S. Air Force
Vitro Corp.
AEC
Western Nuclear
Susquehana Western''
Porter Brothers Corp.
Sur
Acres <'
25
20
55
33
51
40
15
12
4
25
20
55
40
37
28
55
45
50
7
107
40
2
40
race Area
M2 (xlO5)
1.01
0.81
2.23
i 1.34
'' 2.06
1.64
0.62
0.49
0.16
1.01
0.81
2.23
1.62
1.50
1.13
2.23
1.82
2.02
0.28
4.33
1.62
0.08
1.62 -
Quantity*"' »"'
Tons (x106)
0.80
1.10
T 1-9
0.55
2.56
2.6
0.35
0.40
0.04
1.62
0.72
2.68
1.22
1.55
0.13
2.78
0.47
2.20
0.12
1.67
0.90
0.18
0.91
Ra-226 Activity""
pCi/gm
924
50
784
420
252
504
980
171
784
840
756
760
670
700
420
448
518
784
140
896
910
356
660
Ra-226 Quantit
Ci
690
45
1,350
209
585
1,188
311
62
28
1,234
493
1,847
741
984
49
1,129
220
1,564
15
1,357
744
58
544
Y
Stabilized
No
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Partial
Yes
No
Partial
Partial
No
Partial
(in prog.)
Yes
No
No
No
Yes
No
(in pit)
Partial
(in prog.)
(a) Statement of Dr. James L. Liverman, USAEC, before the JCAE Subcommittee on Raw Materials, Hearings on Uranium Mill Tailings located in the
State of Utah (March 12,1974)
(b) Data updated, June 1974, personal communication from Frank McGinley, USAEC
(c) AEC correspondence from Martin B. Biles, Director of Operational Safety to Vernon S. McKenzie, Dept. Director, Bureau of Disease Prevention
and Occupational Health, PHS, HEW, March 13,1967
(d) Tailings pile from ore upgrading process
(e) 18 miles north (Ambrosia Lake) of Grants
(f) 10 miles northwest of Grants
-------
about 620 pCi/g with reported concentrations (estimated from
uranium production records) ranging from 90 pCi/g to 980 pCi/g.
Assuming a uniform density of 1.6 g/cm^ (13), the average depth
of all piles, calculated from their reported areas in table 3.1-1,
is about 4.8 m, ranging from 1.5 to 12.6 m for individual piles.
The piles are frequently roughly rectangular, but in general
irregular in shape and, if unstabilized, are made more irregular
by wind erosion. Table 3.1-1 indicates that about two-thirds of
the piles, both by number and by area, have been stabilized to
some degree. Photographs of more than a dozen tailings piles
are in references (11) and (16) . The Phase I study referred to
in the introduction has produced recent photographs of these piles.
The overall radon emission rate from tailings piles may be
estimated by applying the theoretical factor of 1.6 pCi/m2-s
radon-222 released per pCi/g radium-226 in the tailings to the
areas and concentrations in table 3.1-1, with adjustments for
conditions which influence the emission rate. Because the
analyses of samples from tailings piles (9_,JLO,_11) indicate lower
radium-226 concentrations than those of table 3.1-1, a corre-
sponding value of about 4/5 the average will be used, i.e., 500
pCi radium-226 per gram will be used in all calculations that
project the potential exposures from an "average" inactive
uranium mill tailings pile.
Based on meteorological data for the region (17_, _18_ ,19^t2Q) ,
it is assumed that during the year, as a result of precipitation
and freezing weather, there are about 80 days during which the
release rate is reduced to 1/3 of the value for dry tailings.
In effect, this reduces the yearly value by 15 percent. This
may be conservative, because some piles are moist years after
cessation of mill operations (9) .
An adjustment may be considered for areas of tailings piles
which have less than an "infinite thickness" of tailings under
them. Although each pile is less than the "infinite thickness"
at its edges, and although about one-quarter of the piles
average less than 3 meters in depth, the overall effect of areas
having less than the "infinite thickness" is small, and is esti-
mated from the information given by Schiager (6) to be about 5
percent .
The overall radon-222 emission rate per unit area for the
average unstabilized tailings pile is estimated from these considera-
tions to be 500 pCi/g 226Ra x 1.6 pCi/m2-s 222Rn per pCi/g 226Ra
x 0.85 x .95 = 640 pCi/m2-s radon-222. For the average stabilized
pile, this rate is estimated to be reduced by one-quarter to 480
pCi/m2-s radon-222 (6).
For the average inactive unstabilized pile of 35 acres, the
total radon-222 release rate for the pile is about 90 uCi/s (2,900
Ci/yr); for a similar stabilized pile, 68 uCi/s (2,200 Ci/yr) . For
12
-------
the more than 800 acres of piles in table 3.1-1, the total is
about 2 mCi/s (6 x 104 Ci/yr).
3.1.2 Radioactive Particulate Material
The radium-226, thorium-230, and more than a dozen other
radionuclides in tailings piles are associated with particles
which are picked up by the wind and transported through the
environment. This discussion will concentrate on those radio-
nuclides whose presence in the air is clearly due to wind action
and on the thorium-230 in particular, because it is among the
more hazardous once inhaled.
The ability of the wind to pick up and transport particles
depends on the size and density of the particles and on the local
wind speed. It is likely that the wind picks up only small
numbers of particles bearing thorium until the wind speed is
sufficient to allow the wind to move the larger sand grains.
Under such conditions, the wind will pick up sand particles and
with them the finer particles that were once the tailings slimes.
The smaller particles have an unknown size distribution but indi-
cations are that an appreciable fraction are smaller than 25 micro-
meters in diameter (7.) The larger grains are deposited quickly,
after a short travel distance. At one site, it has been noted
that wind-carried tailings on the ground are readily visible
beyond the original pile for some 500 feet in the predominant
wind direction (21).
Meteorological data (17_,1.9,Z2) indicate that, allowing for
rain and snow cover, roughly 15 percent of the time the wind may
have the opportunity and sufficient speed to blow the tailing
material around, and about 0.1 percent of the time heavy dust
blowing occurs. It is likely that moisture is of lesser conse-
quence in this case, as the surface layers dry quickly. The data
are insufficient to permit determination of a quantitative depen-
dence upon the wind speed. Stabilized piles are sometimes con-
sidered as if they release no tailings particles to the air. How-
ever, it appears that, due to the spread of contamination during
the period preceding stabilization, some particulate radionuclides
are available for resuspension from the surroundings in every case.
This is supported by measurements showing higher than background
concentrations (0.004 and 0.008 pCi/m^ thorium-230) in the air
upwind of piles (23).
Reported data (£,JLO,21,.23_) indicate that average exposure
rates on tailings piles may be about 10 rem/year to the pulmonary
region of the lung from airborne radium, thorium, and uranium,
and in the general range of 0.01 to 4 rem/year within 1,000 m of
13
-------
the pile, with most data in the range of 0.1 to 1 rem/year.
Measurements made under dusty conditions show exposure rates
sometimes exceeding the equivalent of 60 rem/year at locations
on wind-deposited tailings near the pile (21).
Thorium-230 and other uranium-related radionuclides are
normally present in the air due to wind erosion of natural soil
which contains concentrations on the order of 1 to 5 pCi/g of
these radionuclides (13_ ^24), with additions to the air from the
burning of coal and other human activities. Their normal back-
ground concentrations in air in the general environment are
estimated from reported, uranium concentrations (25) to be in
the range of 0.00001 to 0.0002 pCi/m3.
A peculiarity of reported particulate concentrations in air
in the vicinity of inactive mill tailings piles is that many show
uranium concentrations comparable to the radium and thorium con-
centrations, although milling operations commonly remove more
than 90 percent of the uranium. Some of the airborne uranium may
originate with ore fragments contaminating the mill site. However,
it is difficult to believe this contamination is the principal
source of uranium in particulates because of the implication that
it is also the source of the thorium and radium, thus exonerating
the tailings piles.
3.1.3 External Gamma Ray Exposures
Tailings piles containing radium-226 are large enough so
that for purposes of calculating the gamma ray exposures at the
surface of the pile near its center, they may be treated as
radiation-emitting slabs of infinite area. Every disintegration
of an atom of radium-226 eventually results in the production,
through decay of its daughter products, of an average of 2.18
photons. These photons have a mean energy of 0.824 MeV. More
than 95 percent of this gamma energy is from radon daughters
(24). The total gamma ray flux at the surface of the pile is
made up of unscattered primary gamma rays and secondary gamma
rays that have been scattered by material inside the pile but
still reach the surface. The primary gamma ray flux is a func-
tion of the gamma ray emission rate and energies, the linear
attenuation coefficient of the tailings, and the thickness of
the tailings. The secondary or scattered gamma ray flux may be
described by a buildup factor which is a function of the linear
attenuation coefficient, the energy of the primary gamma rays,
and the thickness of the slab.
Schiager (6) has performed some theoretical calculations for
uranium mill tailings piles and has reduced them to the following:
x (yR/hr) = 2.5 CRfl (pCi/g)
14 Ra
-------
This simple formula allows the estimation of the exposure rate
(x) above a tailings pile if the concentration of the radium in
the tailings is known. Schiager assumed that the density of the
tailings pile was 1.6 g/cm3 and that the linear attenuation co-
efficient was 0.11 cm~l. The buildup factor increases only
slowly after 1 to 2 feet of tailings. Thus, the formula, which
includes buildup, does not have to be corrected unless the depth
of the tailings is less than about 1 foot, in which case the
exposure at the surface of the pile will be smaller than pre-
dicted by this equation.
Moisture in the pile increases the gamma radiation field
because of greater retention of radon which decays to gamma-
emitting daughters. Conversely, moisture reduces the gamma
radiation by increasing the mass of material with which the
gamma rays interact before escaping from the pile.
For the average pile containing 500 pCi/g of radium, the
gamma exposure rate at the surface is estimated to be approxi-
mately 1,200 uR/hr or approximately 10 R per year. In theory,
each foot of packed earth (1.6 g/cm3) covering the pile (6)
will reduce this exposure rate by a factor of approximately 10;
it would require only about 4 feet'of earth to reduce the expo-
sure rate at the surface from the pile to 1 mR per year. Diffu-
sion and decay of radon-222 in the soil will, however, produce
gamma emissions from the upper layers, thus counteracting in
part the decrease in the gamma field due to attenuation. For
a typical uncovered tailings pile, it is estimated that the
exposure rate will decrease to less than 1 mR/yr at 1.0 kilo-
meter.
Experimental measurements of the gamma radiation over the
Vitro uranium mill tailings pile by Salt Lake City have been
made using TLD dosimeters (_5). This pile has been reported
(table 3.1-1) to contain about 900 picocuries radium-226 per
gram. Schiagerfs equation using this concentration would
predict an exposure rate on the surface of this pile of 2.2
raR/hr. TLD readings, 3 feet above the surface of the pile,
ranged from 0.2 to 1 mR/hr at the edge of this pile and from
approximately 0.4 to 2.2 mR/hr at the center of the top surface
of this pile. These predicted and measured dose rates are in
reasonable agreement.
3.2 Airborne Dispersion of Radionuclides
The radionuclides released from the^tailings pile surface
are mixed into the passing airstream by local turbulences and
carried off by the wind, which continually decreases the concen-
tration by mixing and dilution. This process of airborne dis-
persion eventually delivers a dilute concentration of radio-
nuclides to the downwind locations of people who may inhale them.
15
-------
Tailings.piles are in general large in area and not very
high compared to the distance across them. For computational
convenience, an example tailings pile has been chosen as a flat
circular area emitting radon-?22 uniformly over its area. The
circumstances considered were the single plume case in which the
wind blows continuously from one direction, and the annual average
case in which the set of different wind directions and velocities
occurring over a year are treated.
There are several methods for treating single plume disper-
sion from area sources. It may be modeled by a virtual point
source or by a line source (26). It may be treated by integra-
ting over the source, analytically or numerically (27). For
exposures near or on the source, it may be treated by considering
a volume of air moving over the area source (6). The evaluations
made in this study were accomplished using a numerical integration
over the area source for the single plume cases, and using a line
source representation for the calculation of annual average
exposures. The area source considered is a circular area with a
400 m (about 1/4 mile) diameter; it has an area of about 31 acres,
a little smaller than the 35 acre average area of inactive piles.
The single plume results were obtained from an adaptation of the
RSAC code (28), and the annual average results from an adaptation
of the AIREM code (29).
Meteorological data from the Ft. St. Vrain reactor site
were adapted for the calculation of annual average exposures
because the detailed meteorological data required for these cal-
culations were not readily available for any of the specific
tailings pile sites (17). Although the Ft. St. Vrain reactor
is not in the neighborhood of a tailings pile, it has meteor-
ological conditions with many points of similarity to those at
many tailings pile locations, including a highly anisotropic
wind rose (30), in which the wind frequency ranges from 2.9 to
17 percent per 22.5 degree compass sector.
3.3 Radon-222 Dosimetry
When a person breathes the air downwind of a tailings pile,
he inhales the dilute concentration of radionuclides in the air.
The effect of the radionuclides depends on the nature of the
radiation they emit, on the rate at which they emit radiation,
on the parts of the body they are retained in and the period of
time retained there, and on the manner in which the radiation
interacts with the body tissues on a microscopic scale. The
significance of radon-222 is believed to be primarily due to
its radioactive decay products; it is the inhalation of these
decay products which is considered to be a cause of cancer in
the tracheobronchial region of the lung.
16
-------
When the radon-bearing airstream is flowing around a building,
the radon enters the building through the ventilation system
and through the cracks and openings in it. Some of the radon
daughters entrained in the air enter also, and additional
amounts of radon daughters are formed in the air inside the
building during the period the air remains there before the
ventilation replaces it with more air from outside. Thus,
the air inside a building downwind from a tailings pile will
probably have greater concentrations of radon daughters than
the air outside. The time period available for formation of
greater indoor concentrations of radon daughters depends upon
the ventilation rate; for normal ventilation (31) an amount of
air equal to that contained within the structure leaves it and
is replaced at least every hour. Ventilation rates of 2 to 6
changes per hour are common. The levels of radon daughter
products, greater thaja 0.01 WL, measured in buildings adjacent
to the Salt Lake City pile may be partly attributed to the
retention of radon in the structures (_5) .
The assumption is made that an average individual chron-
ically exposed to a concentration in air of 1 pCi/m^ of radon-
222 receives a radiation dose of 4 ,mrem per year to the lung
from radon daughters (1). This dose conversion factor is in
agreement with recommendations in a recent UNSCEAR report (32)
and assumes the person is inside a building with adequate ven-
tilation, a quality factor (Q) for alpha particles of 10 and a
penetration depth to cells at risk (bronchial epithelium of the
lung) of 60 ym. It does not represent the worst conceivable
case; but, because of uncertainties, it is considered an accept-
able conversion factor for estimating the average dose to large
segments of the general population.
There are many uncertainties when the dose to the lung is
calculated from exposure to a known concentration of radon-222
(31). The estimated dose is a function of:
a. The degree of equilibrium between radon and its radio-
active daughters.
b. The fraction of daughter ions Which remain free, or
unattached to aerosols at the moment of inhalation.
c. Assumptions with regard to the critical mode of expo-
sure to certain tissues within the lung. Cancers in
uranium miners considered to have been caused by
excessive exposure to radon daughters, predominantly
appear in the area of the large bronchi and are
believed to occur as a result of deposition of alpha
particle energy to the basal layer cells of the upper
bronchial epithelium.
17
-------
Table 3.3-1 (33) gives calculated alpha dose rates to the
basal cell nuclei of segmental bronchi resulting from the inha-
lation of short-lived radon-222 daughter products in air con-
taining 1 pCi/m3 radon-222. The dose rate resulting from a
given concentration of radon-222 is seen to have a wide range
of values depending on the conditions of the exposure and the
assumptions regarding the critical mode of exposure to tissues
of the respiratory system.
Working level months (WLM) is a term commonly used to
express a miner's calculated exposure to radon daughter products
found in mine air. This unit is derived from the working level
(WL), a unit which is defined as any combination of short-lived
radon daughters in one liter of air that will result in the
ultimate emission of 1.3 x 105 MeV of alpha energy (34). A
working level month (WLM) is the exposure to 1 WL concentration
of radon daughters for 170 hours or an equivalent product of
radon-daughter concentrations and time.
If the concentrations of radon daughters are known (or
assumed) relative to the concentration of radon, the WL concen-
tration can be expressed in units of pCi/m^ radon-222. For a
daughter ratio of 1.0/0.9/0.5/0.35, which is the ratio assumed
for dose calculations in this report, 1 pCi/m3 of radon-222 is
equivalent (table 3.3-2) to 5 x 10~6 WL. This same ratio was
used to calculate the radiation dose of 4 mrem/yr resulting
from chronic exposure to 1 pCi/m3 radon-222. These relation-
ships are summarized in table 3.3-2.
3.4 Airborne Thorium-230 and Radium-226 Dosimetry
Table 3.4-1 gives the dose conversion Jractors for thorium-
230 and radium-226 used to estimate the average dose to large
segments of the general population (1).
Particulate material from a tailings pile that is lifted
into the air by the wind will contain thorium-230, radium-226,
and radon-222 and its daughter products. If these particles
are inhaled, the resultant radiation doses will depend pri-
marily on:
a. The activity levels of the radionuclides.
b. The particle size distribution of the aerosol particles.
c. The solubility of the particles in fluids of the lung.
d. The energies of the alpha particles emitted in the lung.
18
-------
Table 3.3-1. Calculated Alpha Dose Rates (millirem/yr) from Inhalation
of Short-Lived 222Rn Daughter Products to the Basal Cell
Nuclei of Segmental Bronchi (33)
Penetration Depth to Cells at Risk
Living Accommodation:
adequate ventilation'3'
Living Accommodation:
inadequate ventilation^)
Industrial Premises (c)
Air Conditioned Sites (d>
(a)222Rnj 218Po> 214pb,
pCi/mr respectively.
15
49
59
115
72
and 214BJ
30
25
31
61
36
I concentrations:
45
8.9
13
26
13
1.0,
60
3.6
4.7
10
3.9
0.90, 0.51,
(um)
70
0.13
0.18
0.41
0.15
0.35,
(b,)222Rn,^218po, 214Pb, and 214Bi concentrations: 1.0, 0.95, 0.70, 0.57,
respectively.
c-)222Rll) 218p0> 214Pb> and 214Bi concentrations: 1.0, 0.97, 0.84, 0.78,
m respectively.
W)222Rnj 218PO) 214pb, and 214Bi
respectively.
Bi concentrations: 1.0, 0.88, 0.49, 0.35,
19
-------
Table 3.3-2. Conversion Factors and Equivalents
for Radon-222 and Daughters
Concentration Working Dose
of Radon-222 Level Equivalent
Continuous'3' Continuous 4 mrem/year
exposure to exposure to to the lung
1 pCi/m3 5 x 10~6 WL
Continuous Continuous 8 x 10^ mrem/year
exposure to exposure to to the lung
2 x 105 pCi/m3 1 WI/C)
Continuous Continuous 1 mrem/year
exposure to exposure to to the lung
0.25 pCi/m3 1.3 x 10~6 WL
(a) Assumed radon-222 and daughter ratios 1.0/0.9/0.5/0.35 (for
living accommodation with normal ventilation) .
delivered to basal cell nuclei of segmental bronchi;
Q, 10; penetration depth to cells at risk, 60 urn.
^C'A combination of short-lived radon daughters in 1 liter of air
that will result in the ultimate emission of 1.3 x 105 MeV of alpha
energy .
20
-------
Table 3.4-1. Dose Conversion Factors for Airborne
Thorium-230 and Radium-226 (1)
Radionuclide
Thorium-230
Radium-226
Solubility
(Class)
Soluble (D)
Insoluble (Y)
Soluble (D)
Insoluble (Y)
Critical
Organ
Bone
Lung«>)
Bone
Lung(a)(b)(c)
Dose Conversion Factor
mrem/yr per
pCi/m3
38,000
11,000
300
11,000
(a)
00,
Pulmonary region of the lung.
'Assumes particle has an AMAD of between 0.5 and 1.0 y, the pulmonary
region of the lung weighs 450 gms, and the mean retention time of an insoluble
class Y particle is 1,000 days in the pulmonary region of the lung.
(c)
its daughters.
Assumes only radium-226 contributes to the dose and not radon-222 nor
21
-------
The probability of inhaled particles being retained in the
lung is determined by the particle size distribution of the
aerosol (33). Some of the particles are exhaled, and a fraction
of the others that is initially retained in the lung is subse-
quently removed to the gastrointestinal tract. Of the remaining
retained particles, those that are insoluble will remain a long
time and deliver their radiation primarily to their region of the
lung; those that are soluble are dissolved and their radionuclides
will be translocated by the blood to other organs where they may
be retained selectively. Soluble thorium and radium are bone
seekers; thus, the critical organ doses will be delivered to the
bone.
Dose conversion factors given in table 3.4-1 are suitable
for aerosol particles with a particle size distribution charac-
terized by an activity mean aerodynamic diameter (AMAD) of 0.5
to 1.0 micrometer. As an illustration of the effect of particle
size, table 3.4-2 gives the deposition of particles in the pul-
monary region of the lung as a fraction of the AMAD and indicates
how the dose conversion factor could change as a function of par-
ticle size distribution. For insoluble particles, the dose con-
version factors are also directly related to the mass of the pul-
monary region of the lung (450 g) and the mean residence time of
particles in the lung (tb = 1,000 days). For insoluble particles
with a t|j of 250 days and an AMAD of 5 micrometers, the dose con-
version factors could be as much as a factor of 10 smaller. The
dose conversion factors in table 3.4-1 represent upper limit or
conservative values, with the exception of the assumption that
only the radium contributes to the dose, and not its decay product
radon-222 nor radon daughters.
3.5 Gamma Ray Dosimetry
The gamma ray exposure from the surface of a uranium mill
tailings pile is assumed to be whole body radiation potentially
delivering an absorbed dose with a quality factor of 1.0. The
exposure rate in mR/hr is therefore assumed equal to a Dose
Equivalent Rate in units of mrem/hr to the whole body.
4.0 Results
4.1 Air Concentrations
Figure 4.1-1 shows the radon-222 concentrations in air as
a function of distance for a single plume from an ar^a source
compared with those from a point source. The downwind distance
is measured from the center of the area source to the receptor.
22
-------
Table 3.4-2. Percent Deposition of Particles in the Pulmonary
Region of the Lung as a Function of Activity
Mean Aerodynamic Diameter (33)
Deposition of Particles
Particle in the Pulmonary Region
AMAD of the Lung
Micrometers %
10
5
2
1
0.5
0.2
0.1
9
11
20
25
31
40
50
23
-------
100,000
10,000
o
Q.
<
cc
in
u
I
o
Q
1,000
EDGE OF AREA SOURCE
FROM POINT SOURCE
FROM 400 M
DIAMETER AREA
SOURCE
10.0
5.0
2.0
1.0
0.1 1.0
DOWNWIND DISTANCE FROM CENTER OF AREA SOURCE, km
10
FIGURE 4.1-1 GROUND-LEVEL RADON-222 CONCENTRATIONS AND DOSE
EQUIVALENT RATES FROM AN AREA SOURCE COMPARED
TO A POINT SOURCE (AXIS OF A SINGLE PLUME)
cc
D
O
I
uj
CC
5
m"
UJ
g
UJ
UJ
8
O
UJ
0.5 E
0.2
o
o
cc
O
24
-------
Figure 4.1-2 shows a similar comparison of the radon-222
concentrations in air as a function of distance in the cross-
wind direction at a downwind distance of 300 m.
Figure 4.1-3 shows the calculated annual average radon-222
concentrations as a function of distance and averaged over all
directions, compared to the measured data from which table 2.2-1
was compiled (2). The majority of the measured concentrations
did not differ appreciably from the concentration assumed to be
the local background. Only measured results which did differ
appreciably from background are shown; their statistical error
ranges well over 100 percent.
4.2 Radiation Doses
o
If the dose conversion factor of 4 mrem/yr per pCi/mJ
radon-222 is applied to the radon-222 concentrations in figure
4.1-3, the dose to the bronchial epithelium of the lungs of
persons in buildings at the respective distance from the center
of the tailings pile is obtained. The estimated average radia-
tion dose due to radon emission is about 8 rem to the lung per
year at 50 m from the edge of the pile, about 0.3 rem per year
at 1 km and about 0.1 rem per year at 2.2 km.
For individuals outdoors near tailings piles, the exposure
to the airborne decay products of radon-222 released from the
pile is probably insignificant, because of dilution by the wind
before much radioactive decay occurs. However, most people spend
the majority of their time indoors.
Another mode of exposure is that from particulate radio-
active material airborne due to wind action. The average expo-
sure rate from airborne uranium, thorium, and radium to the lungs
of individuals within 1.0 km of a tailings pile is estimated to
be generally one-third or less of that from decay products of
radon released from the pile. The principal exposure from the
particulate uranium, thorium, and radium is believed to be to the
non-ciliated, pulmonary region of the lung, in contrast to the
principal exposure by daughters of airborne radon being to the
bronchial epithelium. Another contrast between these modes of
exposure to the lung is that the exposure from particles is
likely to be less indoors than out, because of removal of par-
ticles by filtration and sedimentation from the airstream
entering a building.
The average exposure rates from gamma radiation to the whole
body are similar to the exposure rates to the lung from airborne
uranium, thorium, and radium at locations close to a tailings
pile, but decrease more rapidly with distance, to less than 1
mR/year at 1.0'km.
25
-------
oc
HI
o
o
o
o
§
cc
- 30,000
- 20,000
PLUME AXIS
POINT SOURCE
DISTRIBUTION
AREA SOURCE
DISTRIBUTION
- 10,000
300
200
100 0 100
CROSSWIND DISTANCE, m
200
300
FIGURE 4.1-2
THE DISTRIBUTION IN THE CROSSWIND DIRECTION OF RADON-222
CONCENTRATIONS 300 m DOWNWIND FROM THE CENTER OF A 200-m-
RADIUS AREA SOURCE COMPARED TO A POINT SOURCE (SINGLE
PLUME). '
26
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10,000
1,000
1=
<
cc
LU
U
I
CM
CM
§
100
10.0
1.0
©G
D =DURANGO
G = GRAND JUNCTION
S = SALT LAKE CITY
O MEASURED NET CONCENTRATIONS
NORMALIZED ON THE BASIS OF
RADIUM CONCENTRATION IN PILE
- CALCULATED CONCENTRATIONS
456789
DISTANCE FROM PILE CENTER, Km
10
11
12
FIGURE 4.1-3 ANNUAL AVERAGE CONCENTRATIONS OF RADON-222 AROUND A 0.4 Km
DIAMEJER AREA SOURCE AVERAGED OVER ALL DIRECTIONS
27
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The presence in the neighborhood of contamination such as
scattered ore fragments and windblown tailings may cause expo-
sures to remain higher with distance from the pile than these
estimates indicate.
5.0 Discussion
Radiological dose assessment calculations were performed
for the average, uncovered, dry tailings pile 1.3 x 10^ m2
(31 acres) containing 500 pCi/g each of thorium-230 and radium-
226. For piles of different radionuclide concentrations, the
results given in figure 4.1-3 can be scaled linearly to estimate
approximately the corresponding radiation dose rates. For example,
the Vitro pile is approximately 3 times larger and contains
almost twice the radioactivity as the average pile. Therefore,
allowing for its size, radiation doses from radon and thorium
are expected to be 2 times greater in adjacent areas and, at a
distance, 5 or 6 times greater than those predicted for the
average pile. In addition, the dose rate in the prevailing wind
direction may be twice that of the average dose rate. The "worst
case" is downwind from the Vitro pile where dose rates in buildings
adjacent to the pile are estimated to be about 3 times that pre-
dicted for the average dose rate in the vicinity of the average
inactive tailings pile.
The credibility, accuracy, and precision of estimates of
dose equivalent from emissions from tailings piles can be greatly
improved with site-specific information on:
a. The emission rate of radon from inactive piles, experi-
mentally measured.
b. Meteorological data, assembled for the inactive piles
for comparison with assumptions used in this report.
c. Population data around each pile.
d. Measurements of airborne radionuclide concentrations,
correlated with meteorological conditions.
e. Activity vs. particle size distributions of airborne
particles containing thorium-230 and radium-226.
5.1 Results for the Average Inactive Tailings Pile
For persons in dwellings in the neighborhood of a 31 acre
inactive tailings pile assumed to contain 500 pCi/gm radium-226,
the estimated average radiation dose rates are given in
28
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section 4.2. These dose rates, high in comparison to those
around nuclear facilities, indicate a need to evaluate measures
to reduce exposures near inactive tailings piles.
The comparison of the area source with a point source indi-
cates that the size of the source causes average concentrations
of radon in the neighborhood of a pile to be more evenly dis-
tributed around the compass than average concentrations from a
point source. Thus, a location upwind of the pile in the most
frequent wind direction will not be as favored as might be
expected from the wind frequency data.
The results indicate that the suspension of fine particles
by the wind is not as large a problem as that of the radon
emissions. Stabilization of piles by addition of a foot or more
of soil cover is believed to effectively stop further spread of
wind suspended contamination as long as the cover is not dis-
placed; previously distributed contamination not cleaned up at
the time of stabilization can continue to contribute small dose
rates.
Similarly, the potential gamma radiation dose rates, ranging
up to about 10 rem/yr on a tailings pile, may be largely avoided
by stabilization of the pile with one or more feet of soil cover
and cleanup of distributed contamination. The permanence of the
benefits of such stabilization may be questioned. Measurements
at the Mpnticello pile indicate that the cover soil has reduced
the gamma exposure rate by a factor of about 5 (35).
Data in the report "Evaluation of Radon-222 Concentrations
Near Uranium Tailings Piles" (2) may be compared to the estimated
average values of the example calculation, shown in figure 4.1-3.
The data representing measurements on the piles (see table 2.2-1)
may be compared to the estimated value in figure 4.1-3 of roughly
2,000 pCi/m^ at 50 m from a pile; all average concentrations on
piles are higher, some by a factor of 8. Annual average concen-
trations measured in Durango, Grand Junction, and Salt Lake City
(2) are plotted in figure 4.1-3 for comparison, after subtracting
background. Predicted values are not in major disagreement with
the measured values, considering their wide scatter.
It appears from the results of this assessment that the
principal long term radiological health problem associated with
tailings piles is caused by radon releases. The critical pathway
is radon emission from the pile surface followed by diffusion,
holdup in dwellings long enough for radon daughters to grow in,
and finally radiation exposure to the lung due to the inhalation
of the radioactive daughters of radon. Radon is not appreciably
contained by covering the pile with a few feet of earth cover.
29
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Therefore, the evaluation of remedial actions for individual
tailings piles to prevent excessive exposure to members of the
general population must include a consideration of the radia-
tion dose delivered through the radon exposure pathway.
Particulate material of respirable size carried off a pile
by wind delivers a radiation dose to the lung which is less
than the radon dose. Radiation exposure through this pathway
appears to be relatively easily controlled by stabilizing the
pile with a few feet of earth cover. Direct gamma radiation
from the pile is also reduced although not eliminated by a few
feet of packed earth.
Radioactive material may be carried from the pile by winds
or by human actions, creating offsite exposures from radon and
from gamma radiation. Wind action brings tailings and conse-
quent radiation exposures to locations whose distance once pro-
vided appreciable protection from the radioactivity in the pile.
Radium can be leached from the pile if the pile is exposed to
water. These are special cases of contamination, not considered
in this report, that must be considered for each individual
tailings pile site.
When radon-222 is released from a tailings pile there is,
in addition to the exposure of persons living near the pile, a
potential for national and world wide exposure. Radon being a
noble gas does not deposit quickly on the ground like particu-
late matter; thus, the 3.8 day half life allows sufficient time
for wind to carry most of the radon very long distances before
radioactive decay occurs. At long distances from the pile, the
radon-222 exposure pathway is identical to the local exposure
pathway. But in addition to the radiation dose to the bronchial
epithelium caused by short half-life daughters, radon-222 even-
tually decays to long half-life radionuclides of lead, bismuth,
and polonium. These deposit on the ground and have a potential
for radiation exposure through food pathways. These potential
national and world wide radiological impacts are not evaluated
in this report. They are very small when compared to the
natural radiation background as caused by these same radio-
nuclides .
5.2 Uncertainties
The radiological assessment of uranium mill tailings piles
performed in this report makes use of some parameters for which
the values are not well known.
30
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5.2.1 Source Term
One of the primary factors affecting the emission of radon-
222 is the quantity of its precursor, radium-226, that is in the
tailings. Under the assumption that the radium-226 content of
the ore is in secular equilibrium with the uranium-238 content,
the quantity of radium-226 that went to the tailings at each
mill site has been estimated from the mill's production records;
the values in table 3.1-1 were derived in this manner. Analyses
of samples from tailings piles (£,.10,1^) show smaller concentra-
tions, down to half the expected value. Whether these discrepan-
cies are due to errors in the predicted concentration or as a
result of sampling which is not representative is not clear; indi-
vidual samples from the Mexican Hat pile differed by a factor of
3 or more (9) . The ratio of sand to slimes particles in the
sample may influence the analysis, as the slimes solids contain
higher radium-226 concentrations. It is probable that in some
instances the assumption of secular equilibrium is incorrect; for
example, ground water may have leached some elements from the ore.
A review of analyses of more than 50 soil samples (j.2) having
radioactivity concentrations characteristic of uranium ore (i.e.,
concentrations of radionuclides that, in equilibrium, would be
associated with uranium concentrations greater than 0.05%),
showed that about one-third had concentrations of radium-226
and thorium-230 within 10% of equilibrium and one-third had con-
centrations differing from equilibrium by more than 50%, the
remaining third falling in between. At Mexican Hat, the sample
analyses show thorium-230 concentrations five times the radium-
226 concentrations (9_). In general, either case is possible;
that the ore had uranium-238 radioactivity greater than its
radium-226 radioactivity, or smaller.
The radon emitted from the surface of the pile may be
assumed to have originated at a distance below the surface which
is less than the "infinite thickness;" however, the radium may
not be uniformly distributed in the pile. At various times,
ores having different radium concentrations may have been proc-
essed by a mill, with their associated tailings ending in different
parts of the pile. Another factor is the greater radioactivity
of the slimes solids (T) because any process that tended to
separate slimes solids from sands in the tailings would make the
pile non-uniform in its distribution of radium-226, and thus non-
uniform in its radon-222 emission rate over the surface. At least
one mill had separate tailings ponds for the sands and for the
slimes (8).
Another source of uncertainty in the radon emission estimate
is the 0.2 emanation fraction. The determination of a 0.23 frac-
31
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tion by Schiager et al. (13), although carefully performed, is
properly applicable only to tailings sands from the Grand Junction
pile. The emanation fraction may be different for the slimes
solids, and may differ from pile to pile. It is unlikely that
it is much less than 0.2 and also that it is greater than 0.8.
Although the tailings piles have been discussed in terms of
an average pile, they are not uniform in character. Though fre-
quently roughly rectangular, the piles are generally irregular
in size and shape. Several of the piles listed in table 3.1-1
are actually two or more piles in the same vicinity, with the
possibility that each has individual characteristics as a radio-
activity source.
The piles may vary considerably in the soil characteristics
of the material comprising the pile. Some piles, from opera-
tions called upgrading operations or concentrators, may be
relatively pure sand with little of the fines from the slimes
and little radioactivity. This is because the slimes have been
shipped to another mill for further processing (7) . Piles of
this type might be expected to be easily windblown, having no
fines to help hold the material together. Other piles may have
a large fraction of the fine particles from slimes in their make-
up, due to the nature of the ore they came from and possibly to
slimes from upgrading operations having been added to them.
Reference to piles being muddy (_5) and clay-like (9) is reason-
able confirmation of characteristics unlike sand. Such piles
will retain moisture longer and have reduced radon emission rates
as a result. They may also be. somewhat less subject to being
wind drifted.
These differences in the soil characteristics of a tailings
pile affect its relaxation length for radon (a measure of the
depth from which radon can migrate to the surface) when wet or
dry and the rate at which the pile will dry out when moistened
by precipitation. Some piles appear to dry slowly (£); their
radon emission rates may be higher in the future when they have
dried out. The dry depth available for radon emission may also
be restricted by the underlying soil characteristics and topog-
raphy, which may prevent rapid drainage of moisture from the pile.
Moisture, especially together with a high slimes content, will
reduce the radon emission rate, but it is unlikely that it would
be reduced by more than two-thirds. Moisture may also serve to
leach radium and other radionuclides into the underlying soil and
possibly into the cover soil if it is applied for stabilization.
Stabilizing a pile by the addition of soil and by growing vegeta-
tion may further reduce the radon emission rate by the enhance-
ment of moisture retention.
32
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Although we have used c. callings density of 1.6 g/cm^, it
is known that the dry density of tailings varies from one pile
to another (13). One effect of uncertainty in the density is
to introduce uncertainty in our calculation of the average depth
of tailings in a pile from its area and tonnage. In regard to
other pile characteristics, it should be noted that the slimes
solids have been reported as being more dense than the sands (8).
The uncertainty in density may also affect the relation (6)
between the radium concentration in pCi/g and the rate of radon
emission in pCi/m^-s from the surface of a pile of "infinite
thickness." There seems to be some discrepancy among the pa-
rameters relating density, emanation fraction, and infinite
thickness; if the density (1.6 g/cm-*) and emanation fraction
(0.2) are reasonably accurate, the infinite thickness (6 to 10
feet) appears to be underestimated. It is unlikely that the
density would be less^-than 1.4 or greater than 2.4 g/cm^.
The thorium-230 content of the tailings piles is subject
to greater uncertainties than the radium content, because it has
not been so widely measured. It has the same types of uncer-
tainties in quantities, concentrations, and distribution as the
radium. Since its chemical behavior is different from that of
radium, its distribution may be different from the radium, both
by natural causes in the ore and by the physical and chemical
processes in the mill and tailings.
The soil characteristics of the tailings may also have a
large influence on the amount of material that becomes airborne
in that a relatively clean sand of coarse grains may not release
many particles to the wind whereas tailings noticeable for their
dusty appearance may release large quantities (23). Tailings
which are subject to wind action may be so blown around that the
source is enlarged, and the reported exposures may reflect wind-
blown tailings much nearer than the tailings pile. This may
account in part for the slow decrease in exposure rate with down-
wind distance (23).
Some of the radioactive radon-222 decay products in the air
may have been lifted into the air in particulate form (in the
same manner as the uranium, thorium, and radium) rather than
being formed directly in the air by radon-222 decay. The avail-
able data does little to distinguish airborne radon daughters by
their history, especially if they are associated with particles.
The degree and nature of their association with particles plays
an important role in their potential for exposure of different
regions of the lung or of other organs. Our estimates of doses
from radon daughters include only those daughters formed in the
air.
33
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Some of the factors that affect the radon emission estimates
also affect the estimates of the gamma radiation field around the
piles; differences in density, in composition, and in moisture
content can affect the linear attenuation coefficient and the
buildup factor. The linear attenuation coefficient will vary
almost directly with the bulk density of the tailings and thus
varies as the moisture content affects the density. Comparison
of estimated radium concentrations with measured exposure rates
indicates that the theoretical factor employed in the gamma radi-
ation estimates may estimate exposure rates which are high in some
cases, perhaps up to a factor of 3. Since most of the external
gamma dose is due to radon and its decay products in the pile,
the rate at which radon escapes influences the gamma dose rate.
Sealing the tailings surface with a thin coating preventing
radon escape could be expected to result in an increase in the
local gamma field, but not more than 60 percent (13). The
external gamma dose rate from the airborne material is small,
less than one mrem per year.
5.2.2 Meteorology
The meteorological data employed, from the Fort St. Vrain
reactor site in Colorado (17), is different from that at specific
tailings piles. The uncertainties due to differences at each
tailings location include the uncertainties due to the effects
of terrain on the meteorology at low elevations. These effects
are quite local so that a good estimate of the annual average
dispersion to locations within a half mile of a pile would
require data obtained from that neighborhood.
The dispersion of radon from the piles was estimated with
adaptations of the common model for Gaussian diffusion from a
continuous point source emitting at a uniform rate. The only
changes made were to consider an area source rather thamsa point
source. It has been observed that the radon emission rate is
affected by meteorological conditions and, thus, is not uniform.
One effect is reported to be an emission rate proportional to wind
speed (14). Such an effect could deplete the radon in the upper
layers of a pile so that following a high wind, the emission rate
would be depressed, under circumstances in which the common -Gaussian
diffusion model would, due to its inverse dependence era windspeed,
predict an increased concentration exposure rate. It is-.possible
that there are other meteorological factors, such as variations
in barometric pressure and stability of the lower atmosphere,
which may cause average exposures to differ from those predicted
by the model for a uniform continuous source. Pearson {14)
suggests that atmospheric instability may produce a depletion
analogous to that suggested for high wind speeds. Either or
34
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both of these depleting effects would lower the annual average
exposure to radon from the piles.
It should be noted that not all the airborne radioactivity
in the vicinity of an inactive tailings pile originates in the
pile. The poor correlation between the direction of the wind
across the tailings pile and the measured concentrations (2)
lends support to this. Some airborne activity is due to natural
background which may be anomalously high because of higher than
average radionuclide concentrations in the soils and rocks in the
vicinity, and some may be due to contamination of the surroundings
by ore fragments and windblown tailings from the mill site (_5,j^,
J.O^,J.l) . Urban areas may have higher concentrations of some par-
ticulate radionuclides in the air due to increased dust levels
resulting from vehicular traffic and construction activities (19).
Thus, the radioactivity levels reported in the vicinity of several
piles (_2,J>) may arise from several sources. These different sources
complicate field measurements and the interpretation of the data
obtained. The assumption that the tailings pile is the only source
may lead to overestimating its effects.
Several conditions can lead to high dust concentrations
and related higher thorium and radium concentrations. The lack
of protective vegetation on the tailings piles permits higher
wind velocities at the surface so that dust-blowing conditions
may result more frequently than in vegetated areas. Measurements
made in dust-blowing conditions may show exposure rates ten or a
hundred times the average (21).
Dust devils provide another mechanism for providing high
dust concentrations and thus higher potential exposure rates
(19,23). Dust devils occur commonly on bare dry ground which
has been heated by the sun, warming the adjacent air so that it
is appreciably lighter than the overlying air (36). Most dust
devils last only a few minutes but in some locations they may
occur frequently (36). It has been reported that one which
lasted several hours removed about a cubic yard of sand per
hour (36). Measurements made with dust devils on a tailings
pile show exposure rates as much as ten times the average (23).
They are unlikely to be present more than a few percent of the
time, however.
Since construction activities and vehicular traffic add sig-
nificantly to dust concentrations (19), allowance should be made
in planning actions to stabilize or dispose of tailings piles to
avoid unnecessary exposure of workers and persons living in the
vicinity.
35
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5.2.3 Dose Conversion Factors
The dose conversion factor for radon-222 is a function of
the degree of equilibrium between radon and its daughters, the
fraction of daughters which remain unattached to aerosols at
the moment of inhalation, and assumptions with regard to the
critical mode of exposures to certain tissues within the lung.
Depending on the conditions of the exposure and assumptions
regarding the way tissues of the lung are exposed, the dose
conversion factor ranges widely in value as seen in table
3.3-1. The value of the factor used in this report of 4
mrem/yr per pCi/m-* radon-222 does not represent the worst
case, but because of uncertainties it is considered acceptable
for estimating the average dose to large segments of the general
population.
The dose conversion factors for airborne thorium-230 and
radium-226 depend greatly on knowledge of the solubility and
size of the radioactive aerosol particles. For thorium-230,
the dose conversion factor varies by a factor of 3.5 between
insoluble and soluble particles; for radium-226 by a factor of
330. The dose conversion factor for both may vary by a factor
of 2, larger or smaller, depending on whether the particle size
distribution may be described by an activity mean aerodynamic
diameter (AMAD) as small as 0.1 ym or as great as 10 ym. The
AMAD value of 0.5 to 1.0 ym assumed in this report is believed
to be reasonably typical of airborne particles carried more than
short distances by average wind velocities. Measurements reported
by Breslin and Glauberman (23) support this assumption; they indi-
cate a respirable fraction (to the pulmonary region) of about
one-third.
The solubility of the particles in lung fluids and in passing
through the gastrointestinal tract is undetermined. However,
experience with uranium miners indicates that most of the ore dust
they are exposed to is insoluble (34) and investigations of mill
flow streams show that very small percentages of the radium and
thorium are dissolved as the material passes through the mill (8)
The assumption made is that the airborne participates in the vicin-
ity of tailings piles are insoluble.
The dose conversion^factors used in this assessment for air-
borne, insoluble particles containing thorium-230 and radium-226
assume that the mass of the lung is 450 g and that the particles
have a half-value residence time (tfe) in the lung of 1,000 days.
These assumptions, which represent the best judgment of the Envi-
ronmental Protection Agency, have been selected with a tendency to
make the dose conversion factors conservative. For example, if t^
were 250 days, the factors would be 4 times smaller; if the lung
mass were assumed to be 1,000 grams, they would be 2 times smaller.
36
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The dose conversion factors used in this assessment for air-
borne, soluble particles containing thorium-230 and radium-226
and for gamma radiation are derived directly from recommendations
of the International Commission on Radiation Protection (ICRP)
and as such represent generally acceptable values.
5.3 Health Risk Estimate
There is believed to be a potential health risk to persons
living near uranium mill tailings as a result of the pile's
radon-222 emissions. The potential health risk associated with
this exposure to radon-222 and its daughters is a form of lung
(bronchial) cancer considered to be 100 percent fatal. It has
a latent period of 10 to 15 years after the onset of radiation
exposure that is followed by a plateau period of elevated risk
which lasts from 15 years to possibly a lifetime.
The potential health risk from windblown particulate material
from the pile is probably not as significant because the radiation
dose is smaller than the dose from radon-222, plus the particulate
material dose is delivered to the pulmonary region of the lung.
The radon-222 dose is delivered to the bronchial epithelium region
of the lung, and data from the BEIR (37) report indicates that the
bronchial epithelium is much more sensitive to radiation compared
to the pulmonary region of the lung.
The potential health risk from the pile's gamma radiation is
also of lesser significance because the whole body gamma radiation
dose is considerably smaller than the dose to the bronchial epithe-
lium from radon-222 daughter products. While more health effects
are produced per unit dose equivalent of whole body radiation than
per unit dose equivalent delivered to the bronchial epithelium only,
the much smaller relative dose from gamma rays causes the overall
potential health risk from gamma radiation to be considerably less
than the potential health risk from radon-222 emissions.
Because bronchial cancer is believed to have many possible
causes, the bronchial cancer risk from radon daughters is best
expressed in terms of the percentage increase in the expected
cancer risk, i.e., the relative risk, rather than the absolute
number of cancers expected per person-rem of exposure. The rela-
tive risk is not considered to be a function of the level of
absolute risk and is therefore the same for everyone while the
absolute risk of bronchial cancer to an individual depends
strongly on his personal history. For example, habitual cig-
arette smoking may increase the risk of bronchial cancer by a
factor of 2 to 10. Because the relative risk coefficient for
bronchial cancer from radon exposure is thought to be relatively
independent of smoking history, exposure to radon daughters
37
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sufficient to double the risk of bronchial cancer for
nonsmokers will also double the risk for smokers.
The health risk associated with exposure to radon-222
and its daughter products cannot be accurately assessed at
this time. Although epidemiological studies have indicated
increased incidence of bronchial cancer among uranium miners
and certain metal, hematite, and felspar miners who have been
exposed to high airborne concentrations of radon daughters,
the use of health risks derived from these results to provide
estimates of risk to members of the general population involves
considerable uncertainty. There are many differences between
the uranium miner populations and the general population living
near tailings piles, and there are critical differences between
radiation exposure conditions found in mines compared to houses
(38,39).
It is current best judgment that exposure to 110 working
level months (WLM) doubles the risk of bronchial cancer in
uranium mining populations (42) where "working level months"
is a measure of integrated exposure to radon daughter products
(38). Because uranium miners usually have chronic bronchitis
which reduces the dose owing to the protection provided to the
epithelium by the increased thickness of the mucus layer, an
equivalent radon daughter exposure level delivers a greater dose
to members of the general population than to mining populations.
The BEIR report (37) uses a factor of 2 to account for existing
chronic bronchitis when estimating the dose to the basal cell
layer of the epithelium of the larger bronchi on a probabilistic
basis. Therefore, despite uncertainties, 55 working level months
exposure will be assumed to double the risk of bronchial cancer
for members of the general population.
The general population doubling exposure of 55 WLM lies within
the limits of 250 WLM (40) in United States uranium miners and 29
WLM (41) in non-uranium miners which can be obtained in other
studies although the uncertainties in risk estimation below 100
WLM is very high (41). This general population risk estimate is
still likely to be low by a factor of 2 to 4 because of other
significant differences in exposure conditions between members of
the general population and the uranium mining population (42) which
have not been adequately assessed. The potential risk is assumed
to vary in direct proportion to exposure to radon daughter products
in units of WLM. Additional discussion is included in Reference (42)
The individual living in the immediate vicinity of the average
inactive tailings pile (50 meters) is estimated to receive a poten-
tial dose equivalent of 8 rem/yr to the bronchial epithelium. This
38
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dose equivalent is assumed to be equal to an exposure to 0.5
WLM/yr (38) of radon daughters. It would, therefore, require
approximately 100 years of exposure to radon daughters at the
rate of 0.5 WLM/yr to double the individual's risk of bronchial
cancer. This is the same as a 1 percent increase in potential
risk for each year of exposure or a 70 percent increase in
potential risk over a lifetime of 70 years. Also, 0.5 WLM/yr
is approximately 1/10 of the radiation guidance for the protec-
tion of underground uranium miners issued by the Environmental
Protection Agency (43). Individuals similarly exposed to 0.3
rem/yr at 1 km and 0.1 rem/yr at 2.2 km incur an increased
bronchial cancer risk of about 3 percent and 1 percent respec-
tively for a lifetime of exposure to an average inactive uranium
mill tailings pile.
6.0 Conclusions
Atmospheric concentrations of radon-222, thorium-230, and
radium-226, and direct gamma radiation levels from inactive
uranium tailings piles have been estimated, and the potential
radiation doses to individuals in the general population were
presented in section 4. These calculations indicate that indi-
viduals may be receiving significant doses to the lung out to
distances of approximately 1 kilometer from the tailings sites
from radon-222 and its daughter products, and that this is the
most significant long-term exposure pathway. Since some of
these sites are located in or near urban areas, large popula-
tion groups may be receiving these lung doses, thus increasing
the health risks. Stabilization of piles with compacted earth
may not be especially effective in reducing the radon emission
problem since, in theory, 2 feet of earth cover reduces the
emission rate by only about 25 percent and about 20 feet are
required to reduce it by more than 90 percent (6).
Windborne particulates contining uranium, thorium-230,
and radium-226 which are fine enough to be transported distances
of a kilometer or more result in other lung doses which are
smaller than those from radon daughters and which are delivered
to different lung tissues. Several feet of earth cover are
expected to be adequate to stabilize the pile so that wind ero-
sion of radioactive particulates is significantly reduced or
eliminated.
The direct gamma radiation exposure rate at about 3 feet
from the surface of an average tailings pile containing 500 pCi/g
of radium-226 is estimated to be about 1,200 yR/hr (^ 10,000
mrem/yr). In theory, each foot of compacted earth cover will
reduce this exposure rate by a factor of 10 (6). Therefore, 4
39
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feet of earth cover should eliminate any problems associated
with direct gamma radiation doses. The actual reduction in
exposure rate provided by earth cover appears to be less and
should be confirmed by experiment.
There is considerable need for field studies to refine the
various uncertainties discussed in section 5.2. If the poten-^
tial radiological impact of an inactive uranium mill tailings
pile is to be more accurately estimated, the following param-
eters should be measured:
a. Radium-226 and thorium-230 content of the pile.
b. Radon emission rate from the surface of the pile.
c. Average radon-222 and decay product concentration,
the concentration of ambient aerosol particles, particle size
distribution, and the fraction of the total potential alpha
energy that is uncombined with particles in the environment
around the pile out to a distance of at least 1 kilometer.
d. Radon-222 and daughter product concentrations and
the fraction of the total potential alpha energy that is
uncombined with particles inside a selected number of
dwellings around the tailing pile.
e. Direct gamma radiation levels as a function of
distance from the tailings pile.
f. Airborne levels of thorium-230 and radium-226 around
the pile as a function of distance, both indoors and outdoors,
together with the determination of the particle size distribu-
tion.
g. Solubility class of the radionuclide bearing particles
and their specific activity.
Finally, there is a need to refine dose conversion factors
for radon-222 and its decay products, and for thorium-230 and
radium-226, under those conditions typical of uranium mill
tailings pile sites.
In order to completely assess the environmental impact of
a tailings pile, the total health effects to the population at
risk would have to be considered on a national and worldwide
basis. This will be the subject of future investigations by the
Environmental Protection Agency.
40
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