Technical Note
ORP/LV-75-7(A)
RADON EXHALATION FROM
URANIUM MILL TAILINGS PILES
Description and Verification
of the Measurement Method
D. E. Bernhardt
F. B. Johns
R. F. Kaufmann
NOVEMBER 1975
OFFICE OF RADIATION PROGRAMS - LAS VEGAS FACILITY
U.S. ENVIRONMENTAL PROTECTION AGENC
LAS VEGAS, NEVADA 89114
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Technical Note
ORP/LV-75-7(A)
RADON EXHALATION FROM URANIUM
MILL TAILINGS PILES
Description and Verification
of the Measurement Method
r». E. Bernhardt
F. B. Johns*
R. F. Kaufmann
November 1975
OFFICE OF RADIATION PROGRAMS-LAS VEGAS FACILITY
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89114
*Associated with Environmental Monitoring and Support Laboratory
Office of Research and Development, Las Vegas, Nevada 89114
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This report has been reviewed hy the Office of
Radiation Proprams-Las Vepas Facility, Hnvironmental
Protection Agency, and approved for publication.
Mention ol" trade names or commercial products does
not constitute endorsement or recommendation for
use.
11
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PREFACE
Uranium mills are a part of the nuclear fuel cycle designed
to extract uranium from ore which contains radioactive isotopes
of the naturally-occurring uranium series decay chain. These
isotopes, some of which are extremely long lived, are discarded
as mill wastes into large ponds and tailings piles. Wind
and water erosion have scattered the mill wastes over large
parts of the mill sites' local environs, resulting in land
contamination and increased population radiation exposure.
Releases of radon-222, the noble gas progeny of radium-226,
from the tailings piles can result in an inhalation dose
to people in adjoining areas.
This study was made in cooperation with the Energy Research
and Development Administration to evaluate existing conditions
at inactive uranium mill sites in order that appropriate
remedial actions can be taken to decontaminate the site environs,
minimize erosion, and reduce population exposures.
Donald W. Hendricks
Director, Office of
Radiations Programs, LVF
111
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RADON EXHALATION FROM URANIUM MILL TAILINGS PILES
Description and Verification of the Measurement Method
Abstract
Uranium mill tailings piles result in several sources
of radiation exposure. These exposures are primarily from
concentrations of the uranium progeny, thorium-230, radium-
226, and radon-222, in the tailings. Radon-222 and its progeny
are a source of external gamma and lung exposure.
Using the accumulation technique, field measurements
of the radon flux from uranium mill tailings were made at
three mills and at one experimental plot. The sample collection
technique, method of calculating results, and reproducibility
of the technique are described.
2
The exhalation data (fCi/cm -sec) reveal that reproducibility
is within about 10 percent and that the variation is less
than the uncertainty associated with the linear regression
analysis of the accumulated radon concentration versus time.
Long term measurements (greater than about 8 hours) result
in accumulated concentrations that approach the radon concentrations
in the surface soil gas, and invalidate the assumptions inherent
in the accumulation technique.
IV
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CONTENTS
Page
Abstract iv
List of Figures vi
List of Tables vi
Introduction 1
Scope of Study 2
Experimental Procedure 4
Technique 4
Sample Analysis 7
Calculations 8
Results 13
Discussion and Conclusions 22
References 25
Appendices
A-Calculations 27
B-Radon Flux Data 30
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LIST OF FIGURES
Number
1
2
3
4
Radon Accumulation Vessel
Radon Concentration Build-up With Time
Radon Flux in Uranium Mill Tailings
Radon Concentration Build-up With Time,
Long-Term Sampling
Page
4
9
14
19
LIST OF TABLES
Number
1
2
3
Radon Flux Calculation
Replicate Radon Samples
Replicate Flux Samples
12
15
17
VI
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ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of
Allen B. Tanner of the U. S. Geological Survey, Reston, Virginia, and
Professor M. H. Wilkening of the New Mexico Institute of Mining and
Technology, Socorro, New Mexico who assisted in providing technical
literature and valuable consultation. Thanks are extended to the
Bureau of Mines staff in Boulder City, Nevada, for their help and
use of test plots at their facility. Recognition is also given to
Mr. Charles Russell of EPA, ORP-LVF who performed many of the
mathematical and statistical calculations and to Mr. Michael Lowry,
EPA, Las Vegas who assisted in the field operations.
Funding for this study was provided in part by the Office of
Energy, Minerals, and Industry, USEPA Office of Research and
Development.
VII
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INTRODUCTION
Uranium mill tails are natural materials which have
been manipulated by man. The uranium ore is removed from
its original geological setting by standard mining techniques.
At the mill site the ore is crushed and treated with acid
or basic solutions to remove uranium and other metals. Organic
solvents are also used in the processing. The fine particulate
(micrometers to millimeters in diameter) waste material and
the bulk of the ore, along with spent solvents and process
water is transferred to tailings ponds and piles which are
in direct contact with the biosphere.
The tailings contain about 10 percent or less of the
original uranium isotopes, plus elevated concentrations of
the progeny of uranium, notably thorium-230 and radium-226.
The milling process is very selective for uranium, thus the
progeny radionuclides are in concentrations equivalent to
the original ore content of uranium (0.1 to 0.5% U308).
The progeny concentrations range up to about 3000 pCi/g with
nominal values around 1000 pCi/g, or roughly 1000 times normal
average background concentrations.
Radionuclides in the tailings are in contact with the
biosphere and present several sources of radiation exposure
for man. This report is concerned with radon-222, the noble
gas progeny of radium-226.* The inhalation of airborne progeny
*References in this report to radium and radon refer to
radium-226 and radon-222, respectively, unless otherwise indicated.
-------
of rndon, resulting from radon diffusion or exhalation from
uranium mill tailings, result in a lung dose to man. There
is an additional dose to other organs as a result of transfer
of material from the lungs to the blood and lymph systems.
Several studies of ambient radon concentrations around mill
tailings piles (Snelling, 1971; Shearer et al., 1969; Duncan
and Eadie, 1974; and more notably Swift et al., in press)
characterize the inhalation dose from radon progeny as being
among the highest doses to man from uranium mill tailings
piles. This is especially true for piles where stabilization
efforts to reduce erosion by wind and water have been made.
Scope of Study
There appears to be no previously reported data on the
radon exhalation rate or (fri/cm -sec) from uranium mill
tailings piles.* This report describes the equipment and
technique used in a recent field study to measure the flux
from three inactive uranium mill tailings piles. Measurements
were also made on a laboratory test plot. The calculation
techniques and data which verify the sampling technique
are presented. Correlation of the radon flux with the various
uranium mill tailings uile parameters (e.g., radium concentration,
moisture content, and porosity) will be discussed in future
papers.
*A private communication in October, 1975, with Mr. Andreas
George of the Energy Research and Development Agency, Health
and Safety Laboratory, indicates they made flux measurements
on the Edgemont, South Dakota pile in the summer of 1975.
2
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Several techniques for measuring the radon flux
are reported in the literature. Pearson (1967) reports
sampling the radon from a stream of air flowing across the
ground-air interface. Megumi and Mamuro (1972) present a
technique where radon is directly accumulated in a bed of
granular activated charcoal placed on the soil surface.
The technique used in this study is similar to that used
by Wilkening et al. (1972). Radon is accumulated in an open
faced vessel inverted and placed on the. surface and periodic
aliquots of the accumulated mixture are taken for analysis
of radon.
The validity of the accumulation method is based on several
assumptions:
1. The accumulation time must be short compared
to the half-life of radon or else decay
corrections must be made.
2. The concentration of radon in the accumulator
must remain significantly below the soil gas
concentration. Otherwise, back diffusion will
occur and the vertical soil profile of the radon
concentration will change. Wilkening et al.
(1972) suggest that the accumulated concentration
be kept below 10 percent of the soil gas
concentration at a depth of 13 cm, as noted by
Kraner et al. (1969).
3. The accumulator must be emplaced so as not to
disturb the soil.
3
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EXPERIMENTAL PROCEDURES
Technique
The sampling technique was hased on measuring the increasing
radon concentration in nn open-ended barrel, placed open-
end down on the tailings. Figure 1 is a schematic of the
sampling system.
The sampling or accumulation vessel was a barrel, with
one end removed. The bead or barrel rim was left on to
provide support for the barrel on the tailings surface.
Both 30- and 55-gallon barrels were used in the field study.
Two one-quarter inch diameter connections were welded to
the side of the barrels, and one was located on top. These
DRUM WITH
OPEN FACE
ON SURFACE
WETTED
BENTONITE
SEAL
SAMPLING PORT
WITH VALVE
THERMOMETER
FAN
FAN WIRES
Figure 1
Radon Accumulation Vessel
-------
were used for sampling ports, to measure the temperature
inside the barrel, and to route wires for a fan which insured
mixing of the gases.
The use of the sample ports varied. In several instances
concurrent samples were taken from the top and bottom ports
to determine if gases in the barrel were mixed. The temperature
of gases in the barrel was monitored by inserting a glass
thermometer into the barrel through one of the ports. For
several experiments a 12-volt fan was mounted inside the
barrel to insure mixing of the air and exhaled radon. Caulking
material was used to seal the thermometer and wiring ports.
Temperature fluctuations of the gases in the barrel result
in associated pressure or gas volume fluctuations. Although
the barrels were originally painted with aluminum paint
to reflect solar radiation, measurements during the initial
experiments indicated temperature increases of up to 8°C.
Additional temperature control was accomplished by utilizing
evaporative cooling from wet towels placed on the barrel.
This limited the difference between the ambient and the
barrel temperature to several degrees Centigrade. The final
technique involved wrapping the barrel (top and sides) with
2.5-cm foam rubber and covering it with a "space blanket"
(combination of aluminum foil and mylar layers). This technique
was independent of relative humidity and provided adequate
temperature control (about 1°C).
After selecting a location on the pile with a flat,
continuous (non-cracked) surface, the barrel was carefully
5
-------
placed on the pile surface. As emphasized by Pearson (1967),
efforts were made not to disturb the surface. Areas with
large rocks which would reduce the effective diffusion area
were avoided. The vessel was sealed to the surface by sprinkling
dry bentonite around the edge of the barrel. The bentonite
was then moistened to provide a wet clay seal. Analysis
of the bentonite indicated a radium-226 content of 2.5 pCi/g.
After the barrel was emplaced and sealed on the surface,
a minimum of four samples of the air/radon mixture were
taken at approximately one-half hour increments. Sampling
periods of up to 12 or even 48 hours were used in several
instances. Samples were taken from the top port through
a filter holder into an evacuated scintillation cell. The
filter holder contained a valve for closing the sampling
train, and a 0.45 micrometer filter for removal of the radon
progeny and any dust. Prior to taking samples, about 100
ml of air were withdrawn to remove stagnant air from the
sampling train and to insure that the sample would be representative
of the air/radon mixture in the barrel. Connections between
the scintillation cell and the sample train consisted of
flat glass joints, a rubber 0-ring, and a spring clamp.
Radon progeny were allowed to accumulate in the cells for
about four hours prior to counting. The ingrowth half-life
is 30 minutes; thus, 4 hours allows for 99.6 percent of
equilibrium.
The scintillation cells were similar to those described
by Lucas (1957). Scintillations from alpha particle interactions
6
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with the zinc sulfide coating inside the cell were counted
through the end window of the cell, on a photomultiplier
(PM) tube. The cells, which have a volume of 125 ml, were
evacuated and purged with nitrogen or aged air at the laboratory
prior to use in the field. In the field the cells were
alternately evacuated and purged with ambient air for a period
of an hour prior to reuse. These techniques removed the alpha
emitting progeny, thus minimizing the background counting rate.
Sample Analysis
The counting instrument consisted of a 5-cm (2-inch)
diameter PM tube and preamplifier mounted in a counting
chamber. A Ludlum Model 20 sealer provided high voltage,
low pulse-height noise discriminator, time, and sealer functions.
The counting chamber was sealed to prevent the influx of
light during counting. Safety switches insured that the
high voltage was turned off when the chamber was opened.
The counting equipment was operated in a darkened room
to minimize the impact of potential light leaks. Fluorescent
room lighting was not used, thus minimizing the photon activation
of the scintillation cells.
Scintillation cells were placed directly on the PM
tube. After turning on the high voltage there was a delay
of several minutes prior to counting the sample, to insure
decay of any photon (light) activation of the scintillator
and to allow stabilization of the high voltage.
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Calculations
The data were transformed from observed counts per
2
minute to exhalation-rate in fCi/cm -sec. These calculations
are described in the following three sequences:
-Conversion of the counts per minute in the
scintillation cell to pCi/1.
-Regression analysis of pCi/1 as a function of time
-Conversion of pCi/1-min in barrel to radon flux
2
in fCi/cm -sec.
Samples of the air/radon mixture were taken in evacuated
scintillation cells and counted on a PM tube after allowing for
ingrowth of the progeny. The results, in counts per minute,
were converted to pCi/1 at standard temperature and pressure
by use of an empirical cell factor (1.7 pCi/1 per count per
minute) and corrections for temperature and pressure differences.
More details are given in Appendix A.
Lucas and Woodward (1964) note that the disintergrations
from the radon progeny do not meet the assumptions necessary
for the validity of the normal distribution (variance equal
to the number of counts). Their tabulation of correction
factors indicates that for counting intervals of about one
hour and a counting efficiency of 70 percent, the variance
is 1.624 times the number of radon and progeny counts.
The 95 percent confidence interval counting error is about
2.5 (1.3 x 1.96) times the square root of the number of counts.
8
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10,000
9,000
8.000
7,000
Z 6.000
O
H
Z 5.000
LU
O
O
O
4.000
3.000
2.000
1.000
30
Figure 2
RADON CONCENTRATION
BUILDUP WITH TIME
Shiprock, NM
Slope
(pCi/l - mm.)
65 ± 3
I
Site 3
Correlation
Coefficient
Squared
0.999
60 90 120
TIME (min.)
150
180
-------
Figure 2 is a representative plot of data, with a linear
regression line, from one of the sampling sites. The data
is plotted as pCi/1 versus time after starting the accumulation
in the barrel. There are several possible techniques for
calculating the rate of build-up of the concentration.
These include:
-A regression analysis of the data; i.e. the slope
of the regression line (pCi/1-min).
-The change in concentration divided by the accumulation
time; e.g. concentration at 30 minutes minus the initial
concentration, divided by 30 minutes.
The "difference method" provides a build-up rate for each
sampling interval. But, the calculated rate is dependent
on the selection of the accumulation time period: e.g.j
starting time to 30 minutes, or 30 to 60 minutes.
If the accumulation or sampling time is short compared
to the half-life of radon C3.83 days), the build-up is linear
and can be expressed by a linear regression of the data
(pCi/1 versus time). This results in a single average value
for a set of measurements. For each case, the validity
of the linear regression approach is limited to the assumption
of steady-state conditions; i.e., the parameters affecting
the flux did not change during the sampling time. These
parameters include the moisture content and temperature
of the medium Csoil or tailings), and the atmospheric pressure.
Although there were undoubtedly minor changes in some of these
10
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parameters, it is concluded that the linear regression line,
with the radon concentration as the dependent variable,
is an adequate representation of the radon flux. The equation
for the regression line is:
yCpCi/1) = a (pCi/1) + b (pCi/1-min) . x (min)
Where:
y = the dependent variable, radon concentration in
pCi/1.
a = the intercept of the regression line in pCi/1.
b = the slope of the regression line in pCi/1-min.
X = the time of accumulation in min.
In addition to the regression coefficients, error terms
for the slope and intercept, and the correlation coefficient
of the regression line were calculated. The error term for the
slope indicates the uncertainty of the slope based on the
scatter of the data and the number of data points. The
correlation coefficient squared, r f is the proportion of the
variance of the values of y explained by the linear regression
on x.
The error term for the intercept indicates its significance.
Given the basic assumptions that the initial radon
concentration in the drum was zero, that the site was not
disturbed when emplacing the barrel, and that the linear
regression is valid, the intercept should be zero.
The calculations were done using standard equations,
such as those indicated by Natrella (1963), Riggs (1968),
11
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and Remington and Schork (1970). Additional information on
the calculation of error terms is given in Appendix A.
2
The radon exhalation flux ((fCi/cm -sec) is calculated
as the product of the concentration build-up rate (pCi/1-min)
and the ratio of the volume and cross-sectional area of the
sampling container. Table 1 summarizes these values for the
two containers (55- and 30-gallon barrels) used in this study.
Table 1
Radon Flux Calculation
2
Container Volume Cross Sectional Area fCi/cm -sec
2 pCi/1-min
(liters) (cm ) .
55-gallon 209 2420 1.44
30-gallon 120 1690 1.18
The following equation was used to calculate the
values in the right hand column of Table 1.
pCi x Barrel volume in 1 x min x 10 fCi
1-min Barrel area cm2 60 sec pCi
fCi/cm2 -sec
pCi/1-min
12
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RESULTS
Figure 3 is a bar graph summarizing the radon flux values
from field studies at three uranium mill tailings sites
and from test plots at the Boulder City, Nevada, IJ. S. Bureau
of Mines facility. This report focuses on the validity and
reproducibility of the measurement method. Interpretation
of the results, based on variations of pertinent parameters,
will be the subject of subsequent reports. All of the radon
"^s.
exhalation flux values and intercepts, duration of sampling
periods, and error terms are included in Appendix B.
The counting error for the radon analyses was generally
less than 5 percent for the first sample (lowest quantity
of radon) and less than 2 percent for subsequent samples.
Thus, in most instances the variation due to the counting
statistics has a minor impact on the total variation of
the results.
An additional analytical error stems from the use of
a common cell factor (pCi/1 per count per minute). The
significance of this error can be evaluated from the data
in Table 2. The percent difference of the results averages
about 5 percent and is somewhat higher than the counting
errors (generally 1 percent). Aldrich et al. (1975) report
a standard error of about 6 percent for the calibration
or cell factor of six cells. Although this error is greater
than the counting error, the overall significance is limited.
13
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-------
TABLE 2
Replicate Radon Samples
(Errors at 95 Percent Confidence Level)
Series
Salt Lake City
6B-1
6BA-1
6BA-2
6BA-3
6BA-4
6BA-5
Lakeview
10-1
Boulder City
7
8
Radon
Cone.
CpCi/1)
32,000
32,000
3,800
3,400
3,600
5,000
5,100
5,400
7,300
6,900
7,500
7,300
10,000
11,000
300
260
230
200
13
16
Counting
Error
CpCi/n
280
280
59
57
58
68
73
71
82
81
83
83
98
100
11
11
2.3
6.8
1.7
1.9
Percent
Difference
Average From Average
32,000
3,600 6
5,170 4
7,100 3
7,400 0.1
10,000 5
280 7
215 7
14.5 10
Several tests were conducted to insure that the gases in
the barrel were thoroughly mixed. The difference between samples
taken from the top and bottom sampling ports was less than
5 percent and less than the 2-sigma counting error. Samples
taken before and after operation of the fan during daytime hours
also indicated insignificant differences. One set of samples
taken at night, before and after mixing with the fan, indicated
the possibility of incomplete mixing.
15
-------
It appears that solar heating of the barrel insures self-
mixing. Mechanical mixing appears to be prudent to insure
mixing of the gas volume prior to taking samples at night.
Replicate sample results are presented in Table 3.
These samples were taken over the exact same area, or within
about 2 meters of each other, in areas where the tailings
had uniform physical and chemical characteristics. Agreement
within the 95 percent CL error of the regression line attests
to the reproducibility of the method within the stated regression
error. These values further indicate reproducibility not
only with sequential measurements using one barrel, but
also with barrels of different sizes.
Three measurements were made at Salt Lake City Site 1.
Simultaneous measurements were made on two plots one meter
apart (1A and IB). The 30-gal. barrel was then used inside
of the area where the 55-gal. barrel had been (Site 1AA).
Tests 1A and IB were conducted between 0900 and 1130 hours.
Test 1AA was conducted between 1135 and 1330 hours.
SLC Tests 6A and 6B were conducted about 2 meters apart.
Test 6A was conducted between 1110 and 1410 hours, 6B between
1420 and 1720 hours of the same day.
Tests 3 and 3A at Lakeview were conducted simultaneously.
The 5-gallon accumulator belonged to the Oregon State Health
Department. The large regression error primarily relates to
only taking two samples. Thus, the regression line is based
on the (0,0) intercept and two sampling points, with the result
that there is only 1 degree of freedom (n-2).
16
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The Boulder City samples were taken on two different test
plots covered with two inches of tailings from Shiprock, New
Mexico, uranium mill tailings pile. Although the tests were
taken two weeks apart, they were taken on days with similar
climatological conditions.
Table 3
Replicate Flux Samples
Location
Salt Lake City
Salt Lake City
Lakeview
Boulder City
Site
Sample
No. *
1A
1AA
IB
6A
6B
3
3
1
2
Radon
Flux 2
fCi/cm -sec
15.1
13.0
15.8
65
54
0.37
0.38
1.10
1.09
Percent
Error
20
21
54
21
29
55
130
10
3.7
Value/Mean
1.03
0.89
1.08
1.09
0.91
0.99
1.01
1.00
1.00
Sampling
Vessel (Gallon)
55
30
30
55
55
30
5
30
30
*Salt Lake City Site 1AA was actually located inside of the
area covered by Site 1A. The other duplicate measurements were
based on adjoining areas which had similar physical characteristics;
e.g. soil moisture, radium content, etc.
17
-------
The y-axis (pCi/1) intercepts and 95 percent Confidence
Level (CL) error terms are given in Appendix B. With several
minor exceptions, the error terms are such that the intercepts
are not significantly different from zero at the 95 percent
CL. In all cases the intercepts are less than the initial
radon concentration (not given in Appendix B) taken roughly
30 minutes after barrel emplacement. Thus, it is reasoned
that the sites were not significantly disturbed and that
the linear regression line is valid. It is concluded that
the y-axis intercepts, excluding several notable cases,
are not significantly different from zero.
The site locations designated by a prime (e.g., Salt
Lake City, 6BA1) are locations where the sampling barrel
was left in place over night, and measurements were taken
the next day. Most of these data sets indicate y-axis intercepts
significantly different from zero and a decreased exhalation
flux with increased measuring time. The intercept of the
line also increases, indicating that the later points not
only decrease the slope (the exhalation flux), but cause
the line to deviate from the (0,0) intercept.
Figure 4 presents the data for Salt Lake City Site 6BA.
Several lines are shown. One for the daytime measurements
(line a), one including the daytime and nighttime measurements
(line b), and a line also including a measurement from the
next morning (line c). It is apparent that the data sets
which include the readings taken at night should not be
18
-------
36,000
30.000
Figure 4
RADON CONCENTRATION
BUILDUP WITH TIME
LONG-TERM SAMPLING
Salt Lake City
Correlation
Coefficient
Squared
Slope
(pCi/l - mm.)
200
400
600 800 1000
TIME (min.)
1200
1400
1600
19
-------
fitted with a single straight line. Rather, they should
be fitted with a sequence of lines: one for the initial
daytime values and two for the nighttime segments. This
implies a change in the diurnal radon flux. Wilkening et
al. (1972) indicate there is no significant change in the
diurnal flux. Thus, it appears that these long term measure-
ments involve conditions that violated the assumptions inherent
in the concepts of linear accumulation of radon.
Although soil gas concentrations were not measured,
they can be estimated from calculations and measurements
of other investigations. Data from the present study indicated
average parameters for Salt Lake City, Site 6BA of 1000
pCi/g of radium, porosity 0.38, and dry bulk density of
3
about 1.4 g/cm . Culot et al. (1973) indicate an emanating
power (EP) of 20 percent for tailings. An estimate of the
maximum radon concentration in soil gas can be made by using
the above parameters and assuming all the radon remain? in the
soil volume where it is produced. This is largely representative
of the situation at a depth of several meters, where the radon
diffusing out of a volume is largely replaced by radon diffusing
in from the surrounding media. The maximized estimate is
700,000 pCi/1.* Vohra et al. (1964) indicate normal soil
6
*1000 pCi Ra x 2.1 x 10' pCi/sec of Rn x 1.4 g x 0.2 EP
g pCi of Ra cm
3
x Decay Const, x Soil Volume x 1000 cm = 737,000 pCi/1
2.1 x 10' sec'1 0.38 void volume liter
20
-------
gas concentrations up to 1000 pCi/1, which relates to 1,000,000
pCi/1 for tailings.* Kraner et al. (1964) report values
of over 300 pCi/1 at a depth of 1 meter or roughly 300,000
pCi/1 for tailings. Information from Kraner et al. (1964),
and Aleksexev et al. (1957) indicate soil gas concentrations
at 10 cm are roughly 10 percent of those at depth, or about
50 to 100,000 pCi/1 for uranium mill tailings. Wilkening
et al. (1972) report a value of 17 pCi/1 of radon at 13-cm
depth in soil with a radium concentration of 1.25 pCi/g.
This relates to about 15,000 pCi/1 for tailings which contain
1000 pCi/g of radium.
From the foregoing it is evident that accumulated con-
centrations associated with long-term flux measurements (up to
30,000 pCi/1) were a significant fraction of the soil gas
concentrations and that diffusion was impeded.
The radon in the surface soil gas is the immediate source
of the radon in the accumulation vessel. Thus, the radon
concentration in the surface soil exceeds that in the barrel.
During sampling periods when the accumulated radon concentration
reaches a significant fraction of the radon concentration
in the surface soil gas, there is an increase in the radon
concentration of the surface soil gas. Radon not only diffuses
*Assume a linear relationship; i.e., 1000 pCi/1 per 1 pCi/g
of Ra in soil times 1000 pCi/g of Ra in tailings, equals
1,000,000 pCi/1.
21
-------
out of the soil with the soil gas, but back into the ground.
The end result, shown in Figure 4, is a decrease in the
radon flux, as evidenced by the decrease in the slope of
the curves for increased sampling times.
DISCUSSION AND CONCLUSIONS
Radon exhalation rate (flux) measurements were made at
four uranium mill tailings sites using the accumulation
technique, similar to that used by Wilkening et al. (1972).
This report describes the equipment and technique used, and the
verification of the technique.
Four replicate sets of samples indicated variations
from the mean of 10 percent or less. The maximum spread
of the results was 20 percent. This variation is similar
to and generally less than the 95 percent confidence level
(CL) errors calculated for the regression line analyses
for the individual flux measurements (see Appendix B).
The calculated 95 percent CL uncertainties of the y-axis
intercepts of the regression lines were greater than the
intercepts, indicating that the intercepts were not significantly
different from zero. This signifies that the site conditions
affecting the flux were not markedly altered by the measurement
techniques and sampling periods of several hours duration.
The results for measurements made over long periods
of time (over 8 hours or overnight) indicated a decrease in
the flux, i.e., a shift in the slope of the regression line.
22
-------
Furthermore, the intercepts of the regression lines were also
statistically different from zero, indicating a change in
the site characteristics during the measurements. Calculations
indicated that the accumulated radon concentrations were
approaching the soil gas concentrations. Thus, the depth
profile concentration of the soil gas was affected since the
surface boundary layer was no longer an effective infinite sink.
Several limited experiments to assess the adequacy of
mixing of the accumulated radon/air mixture indicated that
there were no problems, especially during the day, with
solar heating of the accumulation barrel. It appears to
be prudent to insure mixing during night time sampling conditions
Solar heating of the accumulation barrel causes significant
temperature increases (8°C) of the accumulated gases. The
temperature increase and fluctuation cause an associated
pressure or volume fluctuation which may affect the radon
flux. The temperature was initially controlled by placing
wet towels on the accumulation vessel to provide evaporative
cooling. In several instances water dripping from the barrel
wetted the surface of the ground under the barrel. Since
moist soil reduces the radon diffusion coefficient and thus
the flux (Tanner, 1964), results had to be corrected or
discarded. In later tests, temperature control was more
successfully accomplished by insulating the barrel with
foam rubber or polyurethane. Although specific measurements
were not made to assess interferences in the radon flux
23
-------
measurements as a result of temperature increases in the
sampling container, it appears to be prudent to control
the accumulation vessel temperatures so as to minimize pressure
or volume changes.
24
-------
References
Aldrich, L. K., M. K. Sasser, and D. A. Conners (1975), Evaluation
of Radon Concentrations in North Carolina Ground Water Supplies,
Environmental Radiation Surveillance Program, Department
of Human Resources, State of North Carolina, P. 0. Box 12200,
Raleigh, North Carolina, 27605, January 1975.
Alexsexev, V. V., A. G. Grammakov, A. I. Nikonov, G. P. Tafeev
(1957), Radiometric Methods in the Prospecting and Exploration
of Uranium Ores, AEC-tr-5758 (Books 1 and ZJ translated from
a publication Of the State Scientific Technical Publishers
of Literature on Geology and Mineral Resources Conservation,
Moscow (1957).
Culot, M. V. J., H. G. Olson, K. J. Schiager (1973) Radon
Progeny Control in Buildings, Colorado State University (1973).
Duncan, D. L. and G. G. Eadie (1974), Environmental Surveys
of the Uranium Mill Tailings Pile and Surrounding Areas, Salt
Lake City, Utah, EPA-520/6-74-006 (1974).
Kraner, H. W., G. L. Schroeder, and R. D. Evans (1964), Measurements
of the effects of atmospheric variables on radon-222 flux
and soil-gas concentrations, The Natural Radiation Environment.
University of Chicago Press (T9~6~4j"!
Lucas, H. F. (1957), Improved low-level alpha-scintillation
counter for radon, The Review of Scientific Instruments. 28/9:
680-683 (1957).
Lucas, H. F. and D. A. Woodward (1964), Effect of long decay
chains on the counting statistics in the analysis of radium-
224 and radon-222, Journal of Applied Physics, 35/2: 452-
456 (1964).
Megumi, K. and T. Mamuro (1972), A method for measuring radon
and thoron exhalation from the ground, J. Geophys. Res., 77:
3052-3056 (1972).
Natrella, M. G., (1963) Experimental Statistics, National
Bureau of Standards Handbook 91 (1963).
Pearson, J. E. (1967), Natural Environmental Radioactivity
from Radon-222, U. S. Department of Health, Education and
Welfare, Environmental Health Series PH-26 (1967).
Remington, R. D. and M. A. Schork (1970), Statistics with
Applications to tl
Hall Inc. (1970").
Applications to the Biological and Health Sciences, Prentice-
ill "
Riggs, H. C. (1968), Some statistical tools in hydrology,
Chapter Al; Hydrologic Analysis and Interpretation, book 4,
Techniques of Water-Resources Investigations of the United
States Geological Survey (1968).
25
-------
Shearer, S. D. et al. (1969), Evaluation of Radon-222 Near
Uranium Tailings Piles, PER 69-1, Department of Health, Education
and Welfare (1969) .
Snelling, R. N. (1971") , Environmental survey of uranium mill
tailings pile, Mexican Hat, Utah, Radiol. Health Data and
Reports . 12: 17-27, January 1971.
Swift, J. J., J. M. Hardin, and H. W. Galley (in press),
Assessment of Potential Radiological Impact of Airborne Releases
and Direct Gamma Radiation From Inactive Uranium Mill Tailings
Piles, Environmental Protection Agency, Office of Radiation
Programs (in press) .
Tanner, A. B, (1964), Radon migration in the ground; a review,
The Natural Radiation Environment t University of Chicago Press
~
Wilkening, M. H. , W. E. Clements, and D. Stanley (1972) Radon
222 flux measurements in widely separated regions, Second
International Symposium on the Natural Radiation Environment
II, August 1972.
Vohra, K. G., M. C. Subbaramu, and A. M. Mohan Rao, (1964),
Measurement of radon in soil gas, Nature, 201: 3739 (1964).
26
-------
Appendix A
Calculations
Conversion of Counts per Minute
in a Cell to pCi/1
The radon concentration in pCi/1 is calculated from the
following equation:
pCi/1 = net counts per minute
C. F. x V
Where:
C. F. = the cell factor (counts/minute per pCi
of radon standard).
V = the cell volume, 0.125 liter.
Thus, with a cell factor of 4.6 counts/min per pCi, one count
per minute corresponds to 1.7 pCi Rn/1.
The cell factor can also be directly related to the cell
counting efficiency and volume.
Counts/min x Radon alpha x Cell Efficiency
Cell 3 radon and 5T7
progeny alphas
x Cell Volume x pCi = 1.7 pCi/1
0.125 liter 2.22 dis/min count/min
The radon concentrations were converted to standard
temperature and pressure, and corrected for any decay between
collection and counting of the sample.
27
-------
Regression or Least Squares Calculations
The regression analyses or least squares analyses were
performed using the standard Hewlett Packard statistical
packages. The calculation techniques are the same as those
given by Natrella (1963) and Remington and Schork (1970).
The slope b is equal to:
b = Zxy-Zx £y/n
The intercept a is equal to:
a = "y - bx (pCi/1)
Where:
x = the time of radon accumulation in minutes
y = the accumulated radon concentration in pCi/1
x and y = the average x and y values.
The correlation coefficient, r, is:
r = b Sx Sy
Where:
2 2
Sx and Sy are the x and y variances, respectively.
For example:
Sx = £x2 - (EX) 2/n
rPT
Where n is the number of values.
The variance for the slope is:
Sl = Syx/Sx Cn-1)
28
-------
Where Syx is the standard error of estimate, or
Syx = Cn-1) (S* - b2 S*)
Cn-2)
The variance for the intercept is:
(1/n x2/S* (n-1)
The appropriate t value for various confidence levels
is for n-2 degrees of freedom.
29
-------
Appendix B
Radon Flux nata
(Error terms a*- 955 Confidence Level)
(r is the correlation coefficient)
LINEAR REGRESSION OF DATA
Sampling
Location
(Site Number)
Shinroc1:, NM
1 ''ncovered
2 "
3
4 Stabilized
5
ft
7
8
Salt Lake City
1A Crusted Area
1AA
IF
2
2'
2 A Trust
Broken
3 Dirt rover
A "
5
5'
Radon
fCi
cm 2-se
59
132
93
128
168
220
141
44
15.1
13.0
15.8
15.1
11.3
37
10.8
60
24.7
8.3
Exhalation
Error
c (Percent)
25
21
5.3
12
]0
24
21
99
20
21
54
13
12
27
58
15
25
17
Rate
2
r
0 . O7
o!«9
1.00
] .00
1 .n^
0.99
1.00
0.90
0.99
0.99
0.92
1 .00
0.98
O.Q8
0.99
0.99
0.98
0.97
y-Axis Intercept
Intercent Error
(pCi/1) (pCi/1)
*60
1900
2"0
-130
-250
440
240
360
-40
-19
-14
12
192
68
-29
130
-HO
810
1100
1500
310
620
1300
7. \nn
1200
1700
160
520
160
12C
710
500
150
750
420
Radon Counting
Error
(Percent)
< 5
< 5
< 5
< 5
< 5
< 5
< 5
< 5
< 4
< 7
< 4
< 4
< 4
< 4
< 6
< 3
< 4
720 , < 4
Number of
Samples
6
4
6
3
3
3
3
3
4
4
4
5
8
4
5
4
4
7
Sampling
Time
(Minutes)
ISO
120
150
90
90
90
90
90
120
120
115
145
1350
120
150
150
150
1275
-------
Appendix B (Continued)
Radon Flux Data
(Error terms at 951 Confidence Level)
(r is the correlation coefficient)
LINEAR REGRESSION OF DATA
Sampling
Location
(Site Number)
6A Bare Tailings
6B
6B'
6BA
6BA'
6BA" "
7
8 Sewage Sludge
9
10 Bare Tailings
11
Lakeview, Oregon
1
2
3
3A
TB
&
5 Old Pond Area
8
9
1Q
Boulder City, NV
1 Uncovered
2 Uncovered
BC 7 Sulfur Cover
8 Sulfur Cover
Radon Exhalation
fCi Error
cn2-sec (Percent)
65
54
35
78
51
43
49
32
IP
24
15.5
1.53
1.59
0.37
0.38
1.10
1.37
0.30
0.076
3.1
1.88
1.10
1.09
0.77
0.077
21
29
6.1
13
5.1
13
4.5
3.7
35
9. A
23
32
8.8
55
130
6.1
38
'8.4
39
37
26
10
3.7
31
30
Rate
2
n.pg
0.97
1 .00
O.Q7
0.99
0.93
1.00
1.00
0.97
1.00
0.98
0.07
] .00
0.92
0.99
1.00
0.96
1.00
0.96
0.96
0.98
0.99
1.00
0.93
0.95
Intercept
Intercent Error
(pCi/1) (nCi/1)
3nn
540
1700
220
2100
3600
131
-3
-25
22
120
-7.4
-2.9
1.2
2.4
0.17
13
1.12
1.03
1.53
3.14
-10
0.11
25
0.71
11.64
1200
890
780
850
2000
320
110
500
130
280
51
12
19
68
4.8
70
2.6
3.2
120
53
18
1.6
40
2.8
Radon Counting
Error
(Percent)
<2
<2
<2
<2
<2
<2
<2
<4
<5
<5
<4
<8
<6
<13
<10
<7
<5
<10
<20
<5
<7
<10
<18
<5
12 - 40
Number of
Samples
4
6
8
12
21
22
6
5
4
4
4
4
4
4
2
3
4
4
4
4
4
6
4
6
6
Sampling
Time
(Minutes)
180
180
1114
180
776
1432
310
270
191
132
159
226
220
180
123
150
313
230
215
295
177
330
96
300
268
Av. Background
0.05
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
I. REPORT NO.
ORP/LV-75-7(b)
2.
3. RECIPIENT'S ACCESSION*NO.
4. TITLE AND SUBTITLE
6. REPORT DATE
Radon Exhalation From Uranium Mill
. Tailings Piles
Description and Aerification of the
Measurement Method
6. PERFORMING ORGANIZATION CODE
7. AUTHORI8)
D. R. Bernhnrdt, F. B. Johns (Office of
Research and Development), R. F. Kaufmann
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Office of Radiation Programs-Las Vegas
Facility,
U.S. Environmental Protection Agency,
Las Vegas. NV 89114
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
Same as above
IB. SUPPLEMENTARY NOTES
16. ABSTRACT
ranium mill tailings piles result in several sources of radiatioi
exposure. These exposures are primarily from concentrations of the
uranium progeny thorium-230, radium-226, and radon-222 in the
tailings. Radon-222 and its progeny are a source of external gamma
and lung exposure.
Using the accumulation technique, field measurements of the
radon flux from uranium mill tailings were made at three mills and at
one experimental plot. The sample collection technique, method of
calculating results, and reproducibility of the technique are
described. 2
The exhalation data (fCi/cm -sec) reveal that reproducibility is
within about 10 percent and that the variation is less than the
uncertainty associated with the linear regression analysis of the
accumulated radon concentration versus time. Long term measurements
(greater than about 8 hours) result in accumulated concentrations that
approach the radon concentrations in the surface soil gas, and invali-
date the assumptions inherent in the accumulation technique.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
Radon, Radium,
Uranium,
Natural radioactivity
Mill tailings
07D
1ST)
18G
18H
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (This Report!
N/A
21. NO. OF PAGES
39
20. SECURITY CLASS (Thispage)
N/A
22. PRICE
EPA Form 2220-1 (9-73)
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