The Distribution of Cesium-137 and Pu-239+240 from Nevada Test Site Detonations

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THE DISTRIBUTION THROUGHOUT UTAH OF 137Cs AND 3 39+3 40 Pu FROM NEVADA TEST SITE DETONATIONS f'-y DEPARTMENT OF ENERGY

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THE DISTRIBUTION THROUGHOUT UTAH OF 1 37 Cs AND 230+240 Pu FROM NEVADA TEST SITE DETONATIONS Philip W. Krey and Harold L. Beck Environmental Measurements Laboratory New York, New York 10014 This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any,warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, complete ness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific com mercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency Printed in the United States of America National Technical Information Service 5285 Port Royal Road NTIS price codes

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From the analyses of the 137Cs and 23‘'+24clpu content and from the plutonium mass isotopic composition of soil samples recovered in Utah, the fraction of these radionuclides that were deposited in Utah by global and by Nevada Test Site followed similar patterns with elevated values existing in the southwestern corner of Utah around St. George, and then decreasing rapidly with distance along a northeasterly direction from NTS. Then midway along this diagonal through Utah, NTS fallout of these radionuclides increased again and remained elevated throughout the remainder of the state. In the populated Provo and Salt Lake City 23B+24oPu deposits were even greater than at St. George. These data will serve as the basis for estimating the population exposures from NTS debris which will be addressed in a future report. Based on a preliminary but conservative estimate of 100 mrad per mCi km" of Cs from NTS debris, the cumulative exposure to the population of southwestern Utah from external radiation is in reasonable agreement with the exposure based on postshot monitoring data. Furthermore, the population weighted exposure in man rads may prove to be significantly higher in other parts of Utah than in the south western section of the state.

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Considerable interest has been shown by the Department of Energy and by the scientific community in reassessing the radiological dose from nuclear tests at the Nevada Test Site (NTS) to the general population in areas surrounding the NTS Reflecting that interest the Environmental Measurements Laboratory (EML) has conducted several experiments to estimate the cumulative free air dose and population exposure from NTS debris which was deposited throughout Nevada and Utah (Beck, 1979; Krey ejt £l., 1980; Beck and Krey, 1980; Miller et^ aK, 1980). These estimates will be based on current measurements and will be independent of the historic postshot radiological surveys. In this report we present the results of the radiochemical and the plutonium mass isotopic analyses of soil samples collected in Utah and Colorado in 1979. From these measurements we calculate the global and NTS contributions to the deposition values will be converted into estimates of cumulative free air exposures and population exposures based on the relative concentrations of other SOIL SAMPLING AND ANALYSIS In situ gamma-ray spectrometric and high pressure ionization chamber measurements were obtained at over 150 sites and in 56 cities and towns through out Utah in 1979 (Beck and Krey, 1980). The sites at which soil samples were taken are identified in Table 1 and all the population centers in which measure ments were made are located in Figure 1. Whenever possible, each site was a flat well-kept lawn without any trees or structures within a radius of 10 m, and reportedly undisturbed since 1950. Most sites were lawns of tabernacles, schools city offices, parks, or private homes which were heavily watered because of the arid climate. We have also identified in Table 1 some urban sites from earlier The standard EML soil sampling method was used (Harley, 1972) in which eight to ten, 8.9 cm diameter cores spaced 0.5m apart were taken to a depth of 30 cm. The sampling for each was done sequentially by first removing a 5 cm deep surface core. Usually this was then sliced into two separate 2.5 cm fractions. Then a 10 cm corer was inserted into the hole to obtain a 5 to 10 cm cut. Finally the remainder of the core was removed with an auger down to 30 cm or to a shallower depth when large rocks or stones prevented a deeper sample from being taken. Some profiles omitted the 5 to 10 cm cut and went directly from 5 to 30 cm. At other sites only surface soil samples (0 to 2.5 cm, 2.5 to

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to 5.0 cm, or 0 to 5 cm) were taken. At a few sites where core samples were difficult to obtain due to terrain features or soil texture, samples were recovered from one or more areas of 400 to 900 cm each as outlined by square metal templates. This technique has been used in earlier soil studies under similar conditions (Hardy, 1976; Krey and Hardy, 1970). The soil samples are air dried, crushed, pulverized and blended according to EML standard procedures (Harley, 1972). About 100 g of the pulverized fractions were analyzed for Cs by gamma-ray spectrometry at EML as reported earlier (Beck and Krey, 1980), Individual 100 g aliquots of selected samples were also analyzed for Sr. These Sr measurements were performed as a point selected 1 Cs as our reference fission product for this study rather than 00 Sr because of its ease of measurement and because of its lesser mobility in soil. We were concerned that some of the Sr might have percolated down below the 30 cm depth of our normal sampling methods. One kg aliquots were removed from the pulverized fractions for Pu and Pu analyses. At a number of sites, the samples taken at each depth interval were analyzed independently for 339+240pu to indicate its distribution within the soil. At most sites we were interested in the total integrated deposit of plutonium. Therefore, we prepared a 1 kg aliquot of the entire core to a depth of 30 cm by compositing aliquots from each depth interval which reflected the fractional mass of each depth sample to the total mass in the core. Although these composite samples were homogenized, the efficiency of this homogenization is not relevant because the entire 1 kg aliquot was analyzed radiochemically. On a selected number of these 1 kg samples, Am analyses were also performed sequentially with the plutonium analyses. Americium-241 was selected for a limited investigation because, like Pu, it should be deficient relative to Pu in NTS debris as compared to global fallout. This follows because the high neutron flux in the large nuclear explosions,which were responsible for most of the global fallout, elevated the 241 Pu content (the 241 Am precursor) along with the 340 Pu content. Consequently, the 241 Am analysis should provide an independent though less precise check of our plutonium results. After the plutonium radioassays were completed, the separated plutonium fractions were analyzed for their isotopic composition by mass spectrometry. The radiochemical analyses were performed by the Environmental Analysis Laboratories, Inc. (EAL), Richmond, CA. The samples were leached according to a version of the EML soil leaching procedure (Harley, 1972). An earlier study of soil samples recovered from Nevada, Utah and Idaho showed that acid extraction was as effective as complete dissolution in recovering the plutonium present (Hardy, 1976) . The mass spectrometry was performed by the Analytical and Nuclear Research Section of the Pacific Northwest Laboratories (PNL), Battelle,

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The analyses of 90 Sr, 33BPu, 239+240 Pu, 241 Am and Che plutonium mass isotopic composition decay corrected to 1980 at each site are presented in Table 2. In Table 2 and other tables of this report 239+240 pu will be represented as Pu as a matter of convenience. The Pu/ Pu atom ratios are not reported because a Pu contaminant was Introduced during the chemical analyses of the sample. The total inventory of Cs in the soil samples (mCi km ) as of 1979 previously reported by Beck and Krey (1980) are included in Table 2 for ease of reference. Recent data from several sites in the eastern United States are also included as characteristic of global fallout. Table 3 gives the activity ratios of the nuclides measured in this study. Table A summarizes the precision of duplicate soil sampling and analysis from the same city or town. We have limited Table 4 to the 239+240 pu deposition and the plutonium atom ratio because these are the critical parameters in estimating the NTS contribution at each site. There are several sites in which duplicate collections were made that are reported upon in Table 2 but not in Table 4 because those sets of samples are not considered representative of the sampling precision achievable by the EML coring method. Jordan Park in Salt Lake City is not considered a duplicate of the two other sites in that city because it is located in the western region of the city where the rainfall is probably less than in the eastern and more elevated region where the other two sites are located. In addition a bottle cap and some bits of glass were found in the soil sample suggesting that the site may have been disturbed. The Enterprise Reservoir samples and the samples from the Wilkens residence in Duchesne are omitted from Table 4 because they were retrieved by a variably sized template method. The core sample from the Heber Tabernacle lawn was not representative because a 5 cm asphalt layer was encountered at a depth of 15 cm. Subsequent to collection, local officials recalled the prior existence of a tennis court at the site which was removed in the mid sixties. Finally the two samples from the Scoville residence in Green River are omitted as representative duplicates in Table 4 because each consisted of only 3 cores instead of the usual 10 (an area of 186 cm2 instead of 620 cm2). Table 5 gives the results of the coded blank and reference soil samples which were submitted blind to the contractor. The quality of the estimate of the representivity of the soil sampling and NTS Cs at any site is related to the aliquoting, and to the quality of the

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analysis of 137CsJ 239+24opu antj plutonium mass isotopic composition. We will address each of these factors separately. If the criteria for selecting soil sampling site are satisfied, the EML sampling method has been shown to provide reasonable estimates of the local, regional, and global fallout (Hardy 1975; Krey, 1976; Hardy et al., 1973). Beck and Krey (1980) have shown that the prepared soil samples from this Utah study are sufficiently homogeneous that the137Cs measurements of duplicate aliquots agree to within two standard deviations of the counting rates. Table 2 corroborates that finding in that the measurements of duplicate aliquots for Pu, Pu and Am also agree to within two standard deviations of the radioassay value. However, one of the two sets of Sr analyses of duplicate aliquots does disagree by slightly more than two standard deviations of the The measurements of the 340 Pu/^39Pu atom ratios are much more precise, usually within a few tenths of a percent for the former and two to three times as much for the latter. While the mass isotopic compositions of the duplicate aliquots usually differ by more than twice the counting error in Table 2, the standard deviations about the mean are generally <±1^. This suggests that the homogeneity of the soil sample relative to the plutonium isotopic composition is >99$), but not quite 100$.. A notable exception is the 13$ standard deviation of the mean Pu/ Pu atom ratio at Tremonton, for which we have no ready We have assumed that, except for Salt Lake City, there is little gradient in the cumulative fallout within each city. From a series of duplicate soil collections and Cs measurements at a number of Utah cities and towns, Beck and Krey (1980) inferred a precision of soil sampling and analysis for Cs of ±8^i. By taking the root mean square of the percent deviations between duplicate °Pu measurements given in Table 4, we arrive at a similar precision estimate of soil sampling and analyses for233+24°Pu of ±9$. Similar calcula tions with the 240 Pu/239 Pu and 241Pu/239 Pu atom ratios in Table 4 yield precision estimates of ±3.0 and ±4.1$, respectively, for the determination of these values at any given site. The results of the coded blank samples in Table 5 indicate that any contamination of Sr, Pu and Pu introduced during the preparation or analyses of the samples was insignificant compared to the activities present in the samples. The four blind analyses of the Chester reference sample for 2 39 + 2 40Pu agreed with EML's assigned value within one standard deviation of the combined errors. Because of a Pu contamination that existed at the Laboratory at the time that the Chester reference sample was analyzed, EML does not report any Pu data for comparison. Similarly the analysis of the blank samples spiked with PNL s pure Pu agree to within two standard deviations of the

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counting errors with PNL's expected value. In addition the mass isotopic analyses of these pure Pu samples indicate that normal sampling and analysis does not introduce sufficient Pu or Pu to significantly affect the plutonium atom ratios in the samples. Finally, the 90 Sr analyses of the reference sample • agreed with the EML value within two standard deviations of the combined errors. The Chester reference soil was blended with a small amount of Rocky Flats, CO soil which markedly decreased the 340 Pu/239 Pu and 241 Pu/339 Pu atom ratios. The individual aliquots reflected some scatter in the plutonium mass isotopic composition which might be expected for a blended composite of this kind. The standards used to calibrate the gamma spectrometers for137Cs at EML, the beta counters for 90 Sr and alpha spectrometers for the Pu isotopes and 241 Am at EAL, and the mass spectrometers for the plutonium atom ratio measurements at for 90 Sr, 2.4$ for alpha spectrometry and about 0.3$ for the mass isotopic Depth Profile of Plutonium Table 2 indicates that most of the plutonium at sites where depth profiles were analyzed resides in the lowest depth interval recovered. This was also found to be true for Cs (Beck and Krey, 1980), and we believe it is due to the excessive watering necessary for lawn maintenance in this arid region. However, if we adjust for the varying intervals of depth sampled, it is apparent that we recovered essentially all the plutonium deposited in the soil. As a …