External Dose Estimates for Future Bikini Atoll Inhabitants

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EXTERNAL DOSE ESTIMATES FOR FUTURE BIKINI ATOLL INHABITANTS Prepared for U.S. Energy Research & Development Ad m i ni s t rat ion u nder cont fact No. W-7405-Eng-48 University of Caliiornra /Liverinwe

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“ This report was prepared as an account of work sponsored by the United Statea Government. Neither the United States nor the United States Energy Research & Development Administration, nor any of their employees,nor any of their contractors,subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability o r responsibility for the accuracy,completeness or usefulness of any disclosed, or represents that its use would not Printed in the United States of America National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Price: Printed Copy $ Page Range Price Page Range Price *Add $2.50 for each additional 100 page increment from 601 to 1,000 pages: add $4.50 for each additional 100 page increment over 1,000 pages.

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LAWRENCE LIVERMORE LABORATORY EXTERNAL DOSE ESTIMATES FOR FUTURE BIKINI A TOLL INHABITANTS

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Techniques Used to Measure Gama-Ray Exposure Rates . External Dose Estimation ...............

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EXTERNAL DOSE ESTIMATES FOR FUTURE BIKINI ATOLL INHABITANTS To evaluate the potential radiation doses that may be received by the returning Bikini an s,wesur - veyed the residual radioactivity on Bikini and Eneu Islands in June of 1975. An integral part of the survey included meashrements of gama - ray exposure rates which are used t o estimate external gamma-ray doses. The survey showed that on Bikini Island the rates are highly variable : values near the shores are generally of the order of 10 t o 20 uR/h, while those within the interior average about 40 pR/h with a range of roughly ever,is characterized by more o r less uniformly distributed gama radiation levels of less than 10 pR/h over the entire island. These data,in conjunction with population statistics and expected lifestyles, allowed usto estimate the potential external gamma-ray doses associated with proposed housing locations along the lagoon road and within the interior portions of Bikini Island as well as along the lagoon side of Eneu Island.As expected,living on Eneu Island results in the lowest doses : 0.12 rem during the first year and 2.9 rem during 30 years. The highest values, 0.2% rem during the first year and 5.9remover 30 years, may potentially be received by inhabitants living within the interior of Bikini Island. Other options under consideration produce intermediate values. Bikini Atoll was one of the coral reef surrounding a lagoon U.S. nuclear weapons testing sites with major and minor axes having in the Pacific. It is situated dimensions of 35 and 2 7 km, re spec - in the northern part of Micronesia t iv ely(Fig. 1). The total land in the Central Pacific Ocean area is about6 km , and the land about 3600 km southwest of Honolulu. height generally averages3to5m The atoll consists of a number above mean sea level. The islands of small islands on an elliptical vary in size from small sandbars of

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Fig. 1. Map of Bikini Atoll. a few hundred square meters t o Eneu are the most likely islands to be re inhabited. A total of 23nuclear tests took place during the testing period. Most of t h e tests were conducted on barges anchored in the lagoon or on the reef.All islands were subjected to varying degrees of close - i n winds transported the radioactive debris clouds toward the southwest. One exception, however, occurred during the Bravo event when unexpected changes inthe wind directions caused the cloud to travel toward the east over Bikini Island. Most of the radioactive contamination on Bikini Island is due to this event. This recent survey was designed to evaluate the potential external gamma doses associated with proposed housing locations on Bikini and Eneu Islands, and to evaluate the potential doses received through the major terrestrial food crops on the atoll.In this report we only assess the external gamma doses.

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Techniques Used to Measure Gamma-Ray Exposure Rates Because the external dose is radio nuclides,with only minor contributions from alpha and beta emitters, w e had to obtain the best possible description of the geographical variability of the gamma-ray exposure rates on Bikini and Eneu Islands. Any technique for measuring gamma exposure rates has its own set of limitations portability of equipment, and extent of geographical coverage). We therefore used four different techniques to obtain the detailed geographical coverage and accuracy available pressurized ion chamber, and two types of thermoluminescent dosimeThe portable scintillation detectors consisted of a 2.5-cm-diam meter readout. The detectors were calibrated in micro roentgens per source on the primary calibration range of the National Environmental Research Center,Las Vegas, Nevada. Calibration was repeated on selected instruments following the survey. The detectors measured the expo sure rates at1m above the ground at about 2500 locations on a 30-m rectangular grid over the entire sur - face of Bikini Island, and at about 200 locations on a 120-m grid on Eneu Island.Since the response of the detectors was energy-dependent and they were calibrated with a point respond tothe gamma flux on the atoll because the flux is depth d is - tribute d and has a higher scatter energy - than the point source. detectors could be carried easily, which allowed usto make measurements a t many locations on a uniform grid of the islands. They are virtually insensitive to cosmic radiation. The response of the detector was compared with that of the pre s - sur ize dion chamber over the entire range of observed exposure rates. The ion chamber consists of a stainless steel sphere filled with high - pressure ultra - pure argon. The current produced by the radiation - induced ionization within the chamber is measured by a sensitive electrometer with digital readout. The detector was calibrated by the manufacturer and verified by several ERDA laboratories. It exhibits a relatively flat energy response over the gamma-ray energies of interest in at y pica ! environmental

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response is often used as a reference to which other measurements may be compared. The chamber walls are sufficiently thick to render the detector insensitive to the beta radiation present in fallout fields. The instrument is, however, sensitive t o cosmic Further gamma exposure rate comparisons were made by means of dosimeter s (TLD's) placed at 80 an essentially flat energy response and excellent thermal stability. The response of L iF is within approximately 1%of being air equivalent for atypical en v iron - mental radiation field. The CaF2: Dy TLD ' s have an enhanced energy response at low energies, and were used to detect possible lowenergy radiation fields by comparison CaF chips were matched t o 5% and 4% respectively within each batch. The TLD's were annealed on the atoll immediately before being placed on the two islands. Two Lawrence Livermore Laboratory (LLL) plastic personnel badges containing three LiF and three CaF chips were placed a t each field location. The TLD packets were attached to trees by nylon straps or placed on wooden stakes at a height of 1 m above the ground. The locations were carefully chosen to obtain exposures over the full range of gamma exposure rates observed by the portable instrument period,the dosimeter s were retrieved and hand carried (by air)in a lead container to Livermore € or readout. We studied calibration and signal fading by exposing separate sets of and after the exposure period.A special low - scatter calibration fixture was constructed for field use which aided in obtaining uniform, reproducible exposures. The intensity of the Cs calibration source was Comparing the response of a set calibrated6o Co source to that obtained from the calibration The calibration is known within +- 3% a t one standard deviation. We stored a set of control TLD's in a lead pig on a " clean " island in the Marshall s during the *Reference toa company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Energy Research & Development Administration to the exclusion of others that may be suitable,

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exposure period for background determination. The background exposure was essentially all con tri - buted by cosmic radiation during the 3-month exposure period and during the aircraft flight to LLL. Additional TLD's were stored on the periphery of the lead pig to identify possible inadvertent exposures. The average background exposure for the two types of TLD's was subtracted from all field measurements so that the results represent only the terrestrial radiation exposure rates. We found that sunlight had a negligible effect on this packaging arrangement. The correspondence between t h e results obtained with the NaI scintilla tor and the pressurized ion chamber is presented in Fig.2. The ion chamber readings have been contribution at that latitude. The figure shows that the NaI scintilla - tor overresponded because of its nonlinear energy characteristics. The discontinuity at about 30 pR/h occurs at a range switching point on the scintilla tor. Three locations were measured on both low and high range, and those results are shown in solid circles. On the scintillation instrument ' s low range of 0to 30 pR/h, a correspondence near 1:l is observed. On the higher Fig.2. Comparison of responses of the Nal scintilla tor and the pressurized ion chamber. linear,deviates more markedly from The TLD results indicated that approximately 21% relative to the LiF. This is consistent with similar studies made a t Enewetak Atoll and with environmental monitoring perresponse varies with energy and this average gamma energy of about 500 keV. This is reasonable based oa and t h e predominance of 137Cs activities distributed in the soil. To assess the beta contribution

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thicknesses of aluminum absorbers were placed over an array of dosimeter s at three sites on Bikini Island.A feather analysis of the beta at tenu a - tion curves gave a maximum beta energy between 1.5 and 2.2 MeV. Given t h e known predominance of 90Sr-90Y beta activities in the soil, this energy range is consistent with the 2.27 MeV beta radiation. The analysis also revealed that the average beta contribution to the a rather significant contribution. Therefore,it was necessary to reduce the L iF results by this amount t o The comparison between the ion ray exposure rates is presented in Fig. 3. A linear regression of the two data sets gives agreement of about 13% between the two methods. One also finds that the correlation of points in Fig.3isnot as good as that in Fig. 2. This difference i s most likely due to the beta contri but ion to the LiF results, which may vary throughout the islands, causing spread in the data. Departure from the 1:l relationship correction of the TLD data for beta response or to insufficient consideration of the ion chamber data for energy dependence. Calibration of the pressurized ion chamber against a point226Ra source,the method used with the instrument in this study,leads to about a 3% overestimate in the tal fields in this country.Ifa similar correction were made t o these data,the agreement of the two independent exposure - rate TLD) would be within 10%. This is considered to be satisfactory agreement between the two reference techniques used in this work. Hence, on the basis of these results, the NaI scintillation readings were normalized to the output of the pressurized ion chamber. Fig.3. Comparison of responses of the pressurized ion chamber but ion due t o cosmic radiation has been subtracted.