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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- 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. ------- 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 ------- 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 ------- 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 ------- 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 ------- 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 ------- !,- I - -se u ' w ft ALATION FLUX ; i S - UJ ~ ? -.-. X; SI SI is rri IS IS X' IS :SI IS n SI IS. ::: 1 >v ;i;i *»" IS 'ill :|l| :|l| I-: ;X |. __ ;.;. IS 11342756 SHIMOU. NN RADON FLUX IN URANIUM MILL TAILINGS BaraTebe? Bare Tailings Crusted Area '//////////////////////////////\ '//////////////////////////////A Y//////////////////////////////A 1 i i V////////////////////////////S c« ! ? Y///////////////////////////////////// r 1 1 1 //////////////////////////////////////A c 1 //////////////////////////////////\ rl * « ret th i?e I i T =5 Y///////////////////////////////. re ft Js UU U 2 354 7 « M(U I 11 U ULT UKE CITY. UT Covered Tailings -X |l|l S: lll- IS IS S :: I IS SI IS ~ SI 1 S 3 31 31 4 1 2 N UIEVIEW. 01 Test Plot 1 1 ^ ////////// ^ 1721 muaciY. i r i ₯ ^ ad 1M II A 1fl LOCATION OF TALMGS Pl£ AND SfTE NUMBER Figure 3 ------- 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 ------- 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) ------- |