External Radiation Survey and Dose Predictions for Rongelap, Utirik, Rongerik, Ailuk, and Wotje Atolls

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Table of Content Instrumentation and Methods ......................................... 3 Energy Dependence Corrections ...................................... 4 Discussion of Results .............................................. 7

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External radiation measurements were made at several atolls in the northern Marshall Islands, which are known or suspected to have been the recipients of tropospheric fallout during the Pacific Testing Programs. Sufficient data were available to ascertain realistic dose predictions for the inhabitants of Rongelap and Utirik Atolls where the 30 year integral doses from external sources exclusive of background radiation were 0.65 and 0.06 rem respectively. These estimates are based on realistic lifestyle models based on observations of each atoll community. Ailuk and Wotje Atolls were found to be representatives of regional background radia-

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In 1976, BrookhavenJational Laboratory initiated a program of external radiation survey for -the Rongelap, Rongerik, Ailuk, Wotje and Utirik Atolls. The purpose of these surveys was to provide sufficient information concerning the ambient radiation levels resulting from the mid 1950's weapons testing program to make external dose calculations for the individuals living in the surveyed areas. During the last two years, sufficient measurements were made to provide external dose information for most of the populations in the region. The data from Rongerik, Ailuk, Wotje, Rongelap and Utirik Atolls were acquired during trips in September 1976, May 1977 and October 1977. All the exposure rate information gathered from these atolls was obtained with a pressurized ion chamber. The equipment used in these studies consisted of a Reuter Stokes Environmental Radiation Monitor, Model RSS-111 and a gamma spectroscopy system consisting of a sodium iodide detector coupled to a portable multichannel analyzer. Environmental exposure levels were assessed via the RSS-111, and the NaI gamma spectrometer was used to determine the energy dependence correction factors for the The field trips were staffed by BNL personnel and guest scientists from other institutions. Participants are listed later in the report. This report represents all of the external exposure data collected.to date by BNL from these atolls. From these data, we have made external exposure estimates for the people living on Rongelap, Ailuk, Wotje and Utirik Atolls.

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Instrumentation and Methods A) Ion Chamber Measurements All environmental exposure rate measurements were obtained using a Reuter Stokes environmental radiation monitor model RSS-111. The instrument is designed to measure environmental radiation as low as 100 @Rad/year. The REX-111 consists of a spherical high pressure ion chamber filled to 25 atmospheres of argon. Incident radiation produces ion pairs within the active volume of the chamber which result in a current flow. The current flow is measured by an electrometer and is directly related to the free air exposure The active volume of the stainless steel ionization chamber is known to +01X. The current produced in the chamber is a function of incident radiation from an external field,cosmic ray-response and contamination found in the stainless steel. The equation relating instrument response to energy of the incident radiation is: current produced in the chamber by the incident proportionality constant stating the variability instrument responseto the energy of the incident intensity of the gamma field in PR/hr current produced by activity in the stainless steel proportionality constant for cosmic rays intensity of cosmic rays ~oEARCH'~~

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For a given area, the values of Kc and Ic will be constant along with I&Y. Since we measure RT, the only unknown are Kj and Ij. The value of Kj can be determined once the ambient gmmua spectrum is known. Data from the manufacturer indicates anerror of as much as 6 to 10% could result if energy corrections are not made to the gross readings. The RSS-111s used in this study were calibrated at the factory using radium sources whose calibration is traceable to the National Bureau bration of the instruments were also checked by EXL (formerly HASL) prior to field Energy Dependence Corrections In the 1977 surveys, RNL used a sodium iodide detector, whose output was coupled tcr a multichannel analyzer. The purpose'was to enable the BNL team to acquire spectra of the terrestrial background radiation at one meter above the surface. This was done at the same height and in the same areas where the RSS-111 measurements were taken. Consequently, energy dependence factors could be calculated by examining the environmental gamma scan for the energies of those nuclides most predominant in the terrestrial environment. The equipment used to accomplish this part of the work was a computing Gamma Spectrometer, Model LEA 74-008 Pll built by Lawrence Livermore Laboratory (2). The system uses a Harshaw 5.08 cm diameter x 5.08 cm thick NaI(T1) scintillation detector. The spectrometer can be operated from AC power or on internal batteries. Spectra are visually displayed on a CRT, and transferred to magnetic tape for storage. Using the math package with the system, each spectrum was examined in 100 KeV increments, and folded into the RSS-111 energy response cume to determine the energy dependence factors. The range of factors needed to compensate the RSS-111 response due to energy

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dependence was 1.01 to 1.05. The mean correction was approximately quently, we felt no need to correct the remaining 1976 or 1977 data minor energy dependence encountered. A total of II2 RSS-111 measurements were taken on five atolls. Each data point is the average of at least 20 individual readings. This assures the precision of the value while the initial calibration guarantees accuracy. The one sigma error is on the mean exposure rate. All exposure rate values include natural background except where othewlse noted. Figure 1 graphically presents the data obtained at Enlwetak Island, Rongerlk Atoll. On this island, random measurements were taken along a central northsouth transect. Table 1 presents the raw data collected with one sigma error. The average exposure rate for this island is 6.3 PR/hr. This is about 1.5 times hlgher than the cosmic/terrestrial date rate found on uncontaminated coral islands. Enlwetak was the island surveyed in the Rongerik Atoll due to presence of U. S. servicemen at the weather station there at the time of the BRAVO fallout incident. Tables 2, 3. 4, 5 and 6 present the raw data from Rongelap Atoll. The islands surveyed were Kabelle, Naen, Enlaetok and Rongelap. Naen is located at the northwest corner of the atoll, and Kabelle at the northeast corner. Kabelle is a significant copra resource; and both of these islands may be.used for brief visits, but neither of them is permanently inhabited. These islands received a significant amount of fallout debris and consequently, are still substantially more contaminated than the islands of Rongelap and Enlaetok, located in the south- _ east and eastern parts of the atoll. The current values for external exposure rates on these islands are listed below and in Table 14. The entire population presently Island Average Exposure Rate in PR/hr

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lives on Rongelap Island. The people obtain most of their food from Rongelap with occasional supplemental trips to Eniaetok and to other southern islands in the atoll. Little or no activities currentlytakesplace on Naen or Kabelle, or other islands in the north. Figure 2 is a graphic presentation of the measurement points and exposure rates along the main road of Rongelap Island. The exposure rate is fairly uniform averaging 7.3 kR/hr over the island. This is about twice the background radiation level of uncontaminated atolls in the Marshall Islands. Tables 7, 8 and 9 present the data for the islands surveyed in the Utirik Atoll. These islands, Aon, Eorukku and Utirik, represent the major islands within the atoll. Aon, located in the southwest corner and Utirik located in the southeast corner of the atoll, are the major areas for living and food production. The external exposure rate for all these islands is about 4 pR/hr, i.e., very near the regional background level. Tables 10, 11, 12 and 13 present the RSS-111 survey results for Wormej and Wotje Islands of Wotje Atoll and for Bigen and Ailuk Islands of Ailuk Atoll. These islands were surveyed to determine whether they were representative of baseline external exposure rates for the Marshall Islands. The individual island averages are found in Table 14, but range from 3.7 PR/hr to 3.9 pR/hr. These exposure rates are about the same as that for Kwajalein and other areas not exposed to gross contamination from fallout; we assumed them to be representative of ambient background radiation levels for the region. Discussion of Results The average exposure rate as measured for each island is listed in Table 14. In all areas, except for Rongelap Atoll and Rongerik Atoll where only Eniwetak Island was visited, there is essentially an uniform exposure rate within the islands of a given atoll. For hypothetical inhabitants of Eniwetak Island at Rongerik Atoll,

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and for the people living at Utirik Atoll, external dose estimates were made, and the results are presented in Table 15. These dose estimates were made based upon the following assumptions or observations: 1) The exposure rate was relatively uniform throughout the atoll. 2) The average exposure rate represents the average for all islands within 3) Wotje and Ailuk Atolls are representative oftlmnatural background in the Northern Marshall Islands. It is difficult to estimate an external dose for the inhabitants of Rongelap Atoll apart from typical residents who spend most of their time on Rongelap Island. The reason lies in the nonuniform distribution of radioactive material from island to island within the atoll. While the southern islands of Rongelap were determined to have uniform exposure rates on a per island basis, there were significant differences in the exposure rates between islands and substantial heterogeneity in exposure rates on any given island in the northern sector. In UCRL 51879 Rev. 1 (3,4), this problem was approached by estimating the fraction of the time that an individual spends on various activities. This estimate is reprinted here as Table 16. Using this as a basic assmstion, we have constructed external exposure rate estimates for the various living activities based upon our measurements reported in Tables 1-13. The value for the lagoon exposure rate was assumed to be the same as that for uncontaminated atolls in the _the Marshallese would spend an equal amount of time on each of the other islands which we surveyed. All other estimates are made by taking the average of all measurements made within the area of interest.

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Table 17 represent6 the exposure rate at each pattern of activity as listed in Table 16 calculated eseuming 100% occup8ncy for ROngel8p Atoll. Table 18 presents an estimate of the exposure rate for each age group, weighted by the percent of time Spent in each area for inhabitant6 of Rongelap Atoll based on the Lawrence Livermore lifestyle Model (3,4). Sunanation of the exposure .rates in each are8 provides the average exposure rates to the Rongelapese. Using the average hourly exposure rates, the long term external dose was culated. These data, presented in Table 9 for Rongelap Atoll, have been corrected for background (terrestrial and cosmic) radiation by using the average exposure rate of Wotje and Ailuk Atolls as a representative sample of the normal (unexposed) Marshall Island environment. We feel that this is 8 very conservative estimate for Rongelsp Atoll Since the people rately visit the more heavily contaminated islands in the north, and tend to restrict their “other islands” visits to the southern sector where exposure r8te6 are Similar to that on Rongelap Island itself. This observation wa6 supported by an independent living pattern assessment from which data became available in the fall of 1977 (5). Specific living pattern information for Rongelap was obtained on a field trip in October 1977 (5). This information is presented in Tabie 20. It should be noted that 86 previously mentioned, the Rongelap “lifestyle” involves very little time away from Rongelap Island where a constant exposure rates of 7.3 yR/hr is assumed. Revised external dose predictions based on the observed Rongelap living pattern are given in Table6 21, 22 and 23. These doses include correction6 for physic81 decay for 137 Cs and 6oCo which are responsible for >999. of the total external exposure r8te above background. The cesium and cobalt ratios were obtained using the averages of soil sample activities from analyses by BNL (6) and the University of Washington (LRR) (7). It was assumed for this assessment that no radionuclide loss mechanisms are operative other than physical decay.

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IcXP #g suggests that in 30 years, the general public should receive a dose of less than 5.0 rem from total body sources other than medical or natural background (8). In all cases examined here, this requirement is Inet. The problem arise8 that the external gm radiation is only one source of exposure to the Marshallese. The dietary pathway could contribute a substantial increment as an internal dose cmtment. Reviewing all atoll dose commitment8 in thie light, we feel that inhabitant8 of Rongelap Atoll may have difficulty meeting the ICRP #9 criterion of 5 rem in 30 years, but Should be within the 0.5 rem/year standard for individuals. The interal dose assessment for the people of Rongelap will be the subject of a separate report. At this time, we do not recommend any remedial action until a complete dose commitment can be determined by means of examining the external, dietary and whole body counting data available to date. : The other islands and atolls surveyed are well within the ICRP recouunended levels. As such, little more than minimal followup should be done on these atolls. The main task of the environmental program8 should be one of detecting significant changes in the environment or lifestyle whichmight warrant a reassessment of these

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The field portion of the radiological survey of the Marshall Islands was accomplished by a very intense and thorough effort by people representing different organizations. The number of samples collected and the amount of information obtained during the survey was a direct result of the cooperation and diligent effort of the following individuals: We are also deeply indebted to the following BNL the field work by performing radionuclide analyses on of Washington, LRE personnel who complemented numerous samples that were collected and by pretesting all equipment prior to use in the field: The survey crew extends its thanks for the Nevada Operations Office and Pacific Area Support Office for support services which resulted in a smooth and efficient survey. Support from the Kwajalein Missile Range and the site contractor, Global Associates, as well as 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 on important part in the successful completion of the survey.

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1. No Author Cited, Environmental Radiation Monitor Model RSS-111 Operational Manual, (Reuter Stokes Instruments, Inc., Cleveland, Ohio). Future Bikini Atoll Inhabitants, Lawrence Livermore Laboratory, Rept. UCRLAtoll, Lawrence Livermore Laboratory, Rept. UCRL-51879 Part 5, (June 1977). 5. G. Knight, unpublished data. 6. N. A. Greenhouse, et al, Radiological Analyses of Marshall Islands Environmental Samples from 1974 through 1976. Brookhaven National Laboratory Report 7. V. A. …