External Exposure Measurements at Bikini Atoll

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BROOK HAVEN NATIONAL LABORATORY ASSOCIATED UNIVERSITIES, INC. UNDER CONTRACT NO. EY-76-C-02-0016 WITH THE UNITED STATES DEPARTMENT OF ENERGY

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This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy (DOE), nor any of their employees, nor any of their contractor, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal Iiabllity or responsibility for the accuracy, completenem or usefulness of any information, apparatus, product or process dkclosed, or represents that its use would not infringe privately owned rights. Printed in the United States of America National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Price: Printed Copy $4.50; Microfiche $3.00

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External exposure rate surveys from 1975 to 1977 on the islands Nam, Eneu and Bikini of Bikini Atoll gave average external exposure rates of 24, 5.7 and 32 ~R/hr respectively. The exposure rate on Eneu Island is uniform, whereas those on Bikini and Nam range from 7.0 to 80. pR/hr. Based on an assumed living pattern at Bikini Island, the adult male Bikinian is estimated to be in the presence of an external radiation field corresponding to 16 pR/hr due to debris and fallout from the 1954 BRAVO incident. This corresponds to a 30 year dose equivalent of 2.8 rem.

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In April 1975, Brookhaven National Laboratory initiated an external survey of Bikini Atoll in order to obtain information concerning the ambient external radiation levels resulting from the mid 1950’s weapons testing program and to make dose equivalent commitment determinations for the individuals living in the surveyed area. From 1975 to 1977, measurements were made to provide sufficient information on the external exposure received by the Marshallese people. Most of the information concerning Bikini and Eneu Islands was obtained in April 1975, when environmental ionization chamber measurements were made. In addition, thermoluminescent dosimeters (TLDs) were placed in the field and exposed for six months at Bikini Island to verify the uniformity of the exposure. Other groups assisted in these surveys. The team from Lawrence Livermore Laboratory (UCRL) made a detailed survey of Bikini and Eneu Islands in June 19754,9, and they refer to the information presented in this report as BNL unpublished data. In general, their results are substantiated by the exposure and dose equivalent commitments calculated here. The equipment used in 1975 consisted of a Reuter Stokes environmental radiation monitor model RSS-111 and a Baird-Atomic scintillation detector consisting of a sodium iodide detector (2.5 cm in diameter by 3.9 cm in length) connected to a ratemeter readout. Portable survey meters were used to help locate gross changes in the external exposure rate. Lithium fluoride thermoluminescent dosimeters were left on Bikini Island and retrieved in Environmental exposure levels were assessed via the RSS-111 and a NaI gamma spectrometer whose purpose was to determine the photon energy distribution and to compensate for the nonlinearity in the RSS-111 instrument response. This report presents all of the external exposure. data collected to date for Bikini Atoll by BNL. These data have been used to make external exposure estimates for the people living on Bikini Island, and the BNL data have been compared with UCRL data9 for Bikini Atoll. INSTRUMENTATION AND METHODS A) Ion Chamber Measurements All environmental exposure rate measurements were obtained with a Reuter Stokes environmental radiation monitor model RSS-111, which is designed to measure environmental radiation as low as 100 pR/yr. The RSS-111 consists of a spherical high pressure ion chamber filled with argon to a pressure of 25 atm. Incident radiation produces ion pairs within the active volume of the chamber which result in an ionization current. The current flow is mea;ured by an electrometer and is directly related to the free air ionization rate . active volume of the stainless steel ionization chamber is known to ionization current produced in the chamber is a function of incident from an external field, cosmic-ray response, and contamination present

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in the stainless steel. The instrument response is energy dependent, and data from the manufacturer indicate an error of as much as 6 to 10% could result if energy corrections are not made to the gross readings8. The RSS-lllS used in this study were calibrated at the factory against radium sources whose calibration is traceable to the National Bureau of Standards. The calibration of the instruments was also checked at the Environmental Monitoring Laboratory (formerly Health and Safety Laboratory) before and after In the report on external exposure for all other atolls surveyed by BNL3, energy dependence corrections were calculated for data from Rongelap and Rongerik Atolls. The factors needed to compensate the RSS-111 response for energy dependence ranged from 1.01 to 1.05. The mean correction was approxiLithium fluoride (LiF) thermoluminescent dosimeter chips l/4-inch square were used5, for several reasons. LiF is approximately a tissue equivalent material, and its response is essentially energy independent for photon energies greater than 20 keV up to several MeV. The system is precise to *2% and has a long term retention of 5% loss at room temperature for one year. These qualities made the LiF ideal for use in the Marshall Islands. All TLDs were cleaned with analytical grade methanol before departure for the Marshall Islands and prior to analysis. Prior to irradiation, the TLDs were annealed at 400*C for one hour and them at 100°C for 2 hr. After field exposure and before reading, the TLDs were annealed at 100*C for 10 min. In addition to the TLDs exposed in the field at Bikini and Eneu, several sets of TLDs were assembled for use in correcting field measurements for background, fading and air transportation contributions. Several TLDs were annealed and then immediately stored in a lead pig in the BNL analytical counting area. An equal number of TLDs were irradiated to 100 mR and stored with the background TLDs to determine fading losses. Four other TLDs were sent to Kwajalein and stored there in a lead pig to determine in-transit contributions to the response. All TLD results have been corrected for these parameters. sults are directly related to the external exposure and beta absorbed dose that would be received by individuals living on Bikini and Eneu Islands. Because the total response must be differentiated into beta and gamma components, a TLD holder was developed that would eliminate nearly 100% of the 90y beta of 2.27 MelJ (Figure 1). Four TLDs are used per holder. TWO are covered by 1100 mg/cm2 of aluminum and Mylar which is of sufficient mass density thickness to eliminate beta response; these provide the gamma response. The two 2 Mylar to respond to the total gamma-betaother TLDs are shielded by J’ 15 mg/cm contribution at one meter above the earth’s surface. The difference between the

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responses of the two TLD sets gives field were positioned with the open Because shielding part of the made to predict the resulting error the beta response. TLDs placed in the windows facing the soil. dosimeter may bias the data, an attempt was by randomly placing four of the dosimeters 90Sr-90Y as a source, placed 30 cm from the TLDs. The open and closed windows were varied to cover all combinations of field positioning. The error using a field situation represents a distributed plane source, and the source-todosimeter distance was between 50 and 100 cm, the field situation should have a minimal positioning error associated with the results (Figure 2). A total of 203 RSS-111 measurements were made on Bikini Atoll. Each data point is the average of at least 20 individual readings. This assures the precision of the value, and the initial calibration guarantees accuracy. The mean exposure rate is reported with one standard deviation calculated by assuming that the data obtained from a specific site follow a Gaussian distribution. Tables 1 through 5 represent all data taken on Bikini Atoll. Table 2 lists the data from Nam Island, located at the northwest corner of the atoll, closest to ground zero of the BRAVO device. The average external exposure rate over the land areas monitored isfl 24 pR/hr. This is six times higher than the background levels at Wotje, Ailuk or Utirik Atolls3. This average value should not be interpreted as a true value for the Nam island average, since dense vegetation prevented a representative sample of readings over the whole island. Nam is uninhabited at present and is not used for food production. The exposure rate is non-uniform and varies significantly as a function of location. Table 3 presents the data from Eneu Island, located south and west of Bikini Island. Eneu received the least fallout contamination as evinced from the average external radiation exposure rate of 5.7 pR/hr. This value is 1.5 times the natural background and is the lowest external exposure rate on any of the islands surveyed. Figure 3 shows the sample sites and the exposure rate measured at each site. These data demonstrate the uniformity of exposure rate on this island. The external exposure rate on Bikini Island is a strong function of location (Figure 4A-E). It is the lowest in the areas closest to the lagoon and current housing*, highest in the center of the island and intermediate in other areas. The average exposure rate for the island, based on an average of all the data is 32.1 pR/hr. Table 4 lists exposure rate measurements made in the living areas of the available housing. Table 5 lists all other exposure rate measurements made at Bikini Island. *In 1978, the Department of Interior made the decision to relocate the inhabitants of Bikini Atoll to either Ejit Island, Majuro Atoll, or Kili Island. The relocation took place in August 1978.

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The TLD data for Bikini Island (Table 1) agree with the RSS-111 measurements, but no constant relationship is seen between beta dose and gamma exposure. Non-uniform deposition of fallout material in the areas surveyed and translocation of material are major factors governing this result. DISCUSSION OF RESULTS The average exposure rate as measured for each island is listed in Table 6. Estimation of the dose equivalent for the inhabitants of Bikini Atoll is debatable due to the nonuniform distribution of radioactive material within given areas of the atoll. The exposure rates measured on Eneu Island are fairly uniform, but those on Bikini Island showed significant differences between areas (Table 5 along with Figure 4A -4E). In the UCRL work4, this problem was approached and a solution derived by estimating the fractions of an individual’s time spent in various areas. These estimates4 are used here (Table 7) to construct external exposure rate estimates for the various activities based on the measurements reported in Tables 2 through 5. The exposure rate for the lagoon was obtained by assuming that it would be less than or equal to that in the areas of continual habitation. The values for other islands were obtained by assuming that the Marshallese would spend an equal amount of time on each of the other islands surveyed. All other estimates were made by taking the average of all measurements made within the area of interest. Table 8 shows the estimated exposure rate for each pattern of activity in Table 7 based on continuous occupancy of Bikini Atoll. Table 9 shows the estimated exposure rate for each age group as weighted by the percent of time spent in each area, for inhabitants of Bikini Atoll. Summation of the exposure rates in all the areas provides the average total-body exposure rate for each age Using the average hourly exposure rate, the long term external dose equivalent was calculated (Table 10). The data were corrected for background (terrestrial and cosmic radiation) by using the average exposure rate on Wotj$ and Ailuk as representative samples of the normal Marshall Island environment . These data for Bikini residents are lower than UCRL data9 for living patterns 2 and 3, which give the estimated integral external gamma dose equivalent for 30 years as 4 rem, because the present estimates include the measured exposure rate for habitation of the newly constructed housing. These indoor values ar: 39% lower than those previously reported and their use reduces the total estimated reduction in the 30 year dose equivalent commitment by 32%. The ICRP suggests6 that population groups should not receive a 30-year dose equivalent of more than 5.0 rem to the whole body from sources other than medical equipment or natural background. For the external radiation component at Bikini Atoll, this requirement is met; the problem is that external radiation is not the sole source of radiation exposure to the Marshallese. The dietary pathway, based on UCRL data9, could increase the 30-year total body dose equivalent commitment by a factor of 4. Whole-body counting data taken in 19741, 19772 and 19787 indicate that the dietary pathway became the prime source of radiation exposure after January

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1977. Current in vi~o data indicate that the equilibrium body burdens for 137cs will range from 3 PCI to 30 VCi in the Bikini population. This corresponds to a 30-year internal dose equivalent that falls in the range of 11 to 110 rem. Bioassay data obtained from Bikinians during 1978 indicate that bone marrow dose e uivalents for 30 years of habitation would be between 0.4 and 1.0 rem from Reviewing the Bikini dose commitment in this light, one immediately realizes that the inhabitants would receive a total body dose equivalent exceeding the ICRP criteria6. Thus, for Bikini Atoll, we concur with the UCRL recommendation that more must be done to lower the total body and bone marrow radiation exposures so that the Marshallese can live within the population dose

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The field portion of the accomplished by a very intense ent organizations. The number radiological survey of the Marshall Islands was and thorough effort by people representing differof samples collected and the amount of information obtained during each survey was a direct result of the cooperation and diligent effort of the following individuals: Owen H. Davis Pacific Gas and Electric Company Kim S. Kastens Marine Science Center, SUNY at Stony Brook Alan V. Kuehner Brookhaven National Laboratory Janakiram R. Naidu Brookhaven National Laboratory Victor A. Nelson University of Washington, LRE Margaret A. Reilly State of Pennsylvania, Bureau of Radiological Health Michael S. Terpilak DHEW, Bureau of Radiological Health We are also deeply indebted to the following BNL personnel who complemented the field work by performing TLD analyses on the numerous samples that were collected and by pretesting all equipment prior to use in the field: The survey crew extends its thanks to the Nevada Operations Office and Pacific Area Support Office for support services that resulted in a smooth and efficient survey. Support from the Kwajalein Missile Range and the site contractor, Global Associates, and from the crew of the R. V. Liktanur is The outstanding cooperation of personnel from the Trust Territory of the Pacific Islands and from the Office of the District Administrator of the Marshall Islands, as well as that of the Bikini people, played an important part in the successful …