Fallout deposition in the Marshall Islands @ H. L. Beck er AL. northern group of atolls and islands that included Ailinginae Atoll (normally uninhabited), Rongelap Island (home to the Rongelap community), Rongerik Atoll (usually uninhabited), and Bikar Atoll (never inhabited) were considerably higher, ranging from 50 to 200 kBq m~. Thoseatolls clearly received fallout deposition from the Bravotest. The results of our study indicate that most of the total fallout deposition in the Marshall Islands occurred in 1954 from the Castle series tests. However, contrary to almost every previous report, the Castle-Bravo test was not the single test that contributed the largest fraction of fallout at every atoll. As a means to emphasize this point, we graphically compare deposition density of '*’Cs in Fig. 3. In the southern atolls of the Marshall Islands, epitomized by Majuro, the Castle-Romeotest contributed the largest fraction ofthe total '*’Cs deposited. Conversely, in the northern Marshall Islands including Utrik and Rongelap, the Bravo test was easily the largest contributor. At atolls at midlatitudes (Kwajalein and others), Castle-Yankee contributed most of the fallout. Estimates of deposition of radionuclides other than '*’Cs Out of the several hundreds of fission products and activation products produced in a nuclear weaponstest, the @ CASTLE-BRAVO 139 63 most important radionuclides, including '°’Cs, represent- ing over 99% of the potential ingestion dose, have been selected. Deposition densities of those 63 radionuclides have been estimated for each of the 20tests that resulted in any fallout deposition in the Marshall Islands (other than at the test site atolls) and for each of the 32 atolls and separate islands that are considered. As discussed earlier, the dependence of activity ratios on fractionation was calculated by appropriately adjusting Hicks’ (1981) activity ratios for Bravo unfractionated fallout. Because of the widespread interest in the ground contamination by plutonium, deposi- tion densities of *°'“°Pu also have been estimated, but only for the total activity deposited from all tests. A representative sample of the estimated deposition densities is shown in Table 10. In general, the uncertainty in these deposition density estimates is driven by the uncertainty in the '"’Cs deposition density estimates. The uncertainties in the Hicks (1981) activity composition as a function of R/V are small compared to the uncertainties in the estimates of R/V used to estimate '’Cs deposition density from exposure rate measurements. However, as discussed earlier, the good agreement between the sum of the various '’Cs deposition density estimates with retrospective soil sample data, even for atolls with fractionated fallout, suggests that the deposition density estimates for all radionuclides of interest are sufficiently accurate to allow reasonable estimates to be made of radiation doses from external and internal exposures at all of the populated atolls of the Marshall Islands. Exponential fit to BRAVO --G- CASTLE-ROMEO SUMMARY AND CONCLUSION --A-- Entire CASTLE Series (1954) 100 = 2 a 80 - 4 60 F 4 40 F - 2+ | ¢ g = 8 oO a n Oo 3 o FE 6 =. S 8 2 a 0 Latitude (°N) Fig. 3. Percentage of total ground deposition density from Bravo, Romeo,and all Castle series (1954) tests at 25 inhabited atolls and islands. The Castle series data points include the contributions from Bravo and Romeoas wellasall othertests in that series. The abscissa represents the latitude (°N) of separate reef islands or the centroid location of atolls (except location for Kwajalein which represents the southern half of atoll at about 9.1°N). While there have been numerous measurements made over the decades of radioactivity in soil collected from many of the atolls (particularly the northern atolls and primarily of '*’Cs), no assessmentofthe deposition ofall of the major radionuclides contributing to radiation exposure from each test has previously been made forall of the atolls of the Marshall Islands. In this paper, for the first time, the deposition densities of all the major radionuclidesin fallout from individual nuclear weapons tests at Bikini and Enewetak Atolls have been estimated at each atoll of the Marshall Islands. These new deposition estimates are based on a thorough analysisof all the available data. All together, depositions at 32 atolls have been estimated for 63 radionuclides originating from the 20 nuclear tests which were likely to have deposited substantial amounts of fallout anywhere in the Marshall Islands. The agreement of the total estimated fallout at each atoll with measurements of soil radioactivity inventory from two different studies conducted years apart provides strong confirmation of the validity of the deposition estimates andthatall incidences of substantial fallout are accounted for.