132 Health Physics Table 4. '’Cs/*?**°Pu activity ratios measured in 1978 soil samples (Robisonet al. 1981) and estimated fractionation (R/V) for Bravo fallout. The estimated uncertainty (1 SD) of the fractionation estimate is shown in parenthesis. Atoll or island BICSPPPy Northern Rongelap Atoll Rongelap Atoll: Rongelap Island Alinginae Atoll Rongerik Atoll: Rongerik Island Rongerik Atoll: Eniwetak Island Taka Atoll Utrik Atoll Ailuk Atoll Mejit Island Likiep Atoll Ujelang Atoll WothoAtoll 4 6 8 14 3 13 6 9 8 12 12 17 RIV (Est. SD) 2.0 1.4 1.3 1.0 1.5 1.0 1.0 1.0 0.9 0.7 0.5 0.5 (0.5) (0.2) (0.2) (0.2) (0.2) (0.2) (0.2) (0.1) (0.1) (0.2) (0.1) (0.1) August 2010, Volume 99, Number 2 the soil inventory measurements discussed below, suggest that our fractionation estimates are indeed reasonably based. Variations in fallout nuclide composition with time The atom ratios of various nuclides released in different nuclear weaponstests is knownto vary slightly due to differencesin fissile material and device construction (Hicks 1981). As discussed above, the radionuclide ratios in fallout at any particular location will also depend on the assumed degree of fractionation. However, for a given value of R/V, the differences from test to test are small compared to the uncertainty in the measured or estimated deposition, as illustrated in Table 5 for selected nuclide ratios. Because only fallout from Bravo was fractionated at Marshall Island sites, the variations from test to test for fractionated fallout are not relevant here. Note that the ''I to '"’Csratio is quite insensitive to approximate ratios expected based on the ratio of TOA to T.,. Based on the estimated TOAto T,, ratios for Bravo, we expect an R/V at Rongelap (where TOA/T.. ~ 0.12) on the order of 2, about 1.5 at Rongerik (where TOA/T,, ~ 0.16), and an R/V of about 1.0—1.3 for Utrik (TOA/T,, ~ 0.44). Smaller R/V would be expected for Likiep and Mejit Islands. Although webelieve our estimates of R/V for Bravo the particular test, even for non-TN compared to TN to be reasonable, the uncertainty can be large. Further- typical **’Pu-fueled weapon that was detonated at the more, the relative degree of fractionation assumed for 7Cs as a function of other volatile nuclides (Table 3) is a relatively crude estimate based on a modelfor fractionation for surface tests that is very sensitive to the particular test conditions and type of soil (Freiling 1961, 1962, 1963; Freiling et al. 1965). Because most of our estimates of '*’Cs deposition are from measurements of exposure rate, and because the value of R/V has a substantial influence on the temporal variation of the exposure rate, any error in the estimated R/V for Bravo fallout and the estimated activity ratios as a function of R/V (given in Tables | and 3) will amplify the error in the estimated '*’Cs deposited by the Bravo test. Thus, if the tests. However, the ratios for other nuclides contributing to either external or internal dose differ between TN tests and non-TN tests for some nuclides, reflecting the different fission yields for **’Pu fission as a function of 381) fast fission. Therefore, the radionuclide mix for the Bravo test was used for deposition-density estimates for all TN tests, while for non-TN tests, Tesla, which was a NTS(Hicks 1981), was taken to be representative of the non-TN tests conducted in the Marshall Islands. RESULTS Tests depositing fallout in the Marshall Islands The 20 tests we estimate deposited substantial fallout on any of the inhabited atolls of the Marshall Islands are presented in Simon et al. (2010a, Table 1). The list includes one test in 1948, two in 1951, two in 1952, six in 1954, three in 1956, and six in 1958. Though there were 45 tests conducted at Bikini and Enewetak (DNA 1979; Simon and Robison 1997) in addition to those actual Bravo R/V at Rongelap Islandis actually 1.0 or 1.5 as opposed to our current estimate of 1.3, the '°’Cs deposited could be in error by as much as 50%. Even if the R/V estimates are valid, the estimated ratio of '°’Cs to other volatile nuclides is based on a general fractionation model andis likely to vary from test to test and, thus, may notreflect the actual ratios for Bravo. Hence, even if our estimate of fractionation is correct, the error in the "Cs to E12 ratio couldstill be on the order of 10-15% just from the uncertainty in the estimated '*’Cs to ratio. Although the R/V estimates for Bravo fallout are very uncertain, the good agreement between the '*’Cs deposi- tion estimates (which incorporate our R/V estimates), and Table 5. Variation in selected nuclide activity ratios and in 'Cs/E12 quotients (Bq m~” per mR h“') for various thermonu- clear tests resulting in significant fallout in the Marshall Islands. Values for a typical NTS fission test (Tesla) are also given for comparison. All ratios are for R/V = 0.5 and H+12. Test BY/P7Cs S1/'7Cs S1Cs/E12 MB a/'Cs Mike Bravo Romeo Yankee Zuni Tewa Tesla 823 838 840 840 873 899 866 158 124 145 124 129 123 70 31 32 31 31 33 32 44 695 674 679 682 685 684 528