Long Term Activity Estimates for the Northern Marshall Islands

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Long ?erm Activity Estimates This paper provides preliminary upper-bound estimates of the residual garma activity on the northern Marshall Islands due to U.S. atnnspheric testing at Bikini. These estimates are intended to be indicative of the activity to be detemined by up-cor,in~ det~iled surveys. Estimates are also provided for islands in the Enewe:?k atoll and compared with the 1972 survey. Finally, an an?lysis of wind profiles and fallout patterns is presented which serves to dglineate those northern Marshall islands which were uncontaminated by fallout fro: the Bikini tests. After 20 ywrs or SO, the princiPal fission Pr~~Q~ts of interest . are S-go and CS13? whose characteristics are sunnarized below. The fractional contribution of Cs137 to the one-hour dose rate is not the same as the fraction of total Curies at one hour since the ~s137 y energy is lower than that average energy for all fissio~ PrDducts (.66 PteVVS. 2 HeV). This results in a mentgen response for ~s13? that 1s 0.41 times that for the inventory taken as a WhOle. At some time after burst, when Cs’37 is the only remaining fission product q-em!tter, the dose rate $s given by DOEARCHW where T is $n years. Note that beta activity is not being considered ~3here on the presuqtion that the survey techniques distinguish between

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beta and ~amma. The above equation permits estimating the long term The first step in the anelysis WaS to co~?are the dose-l-~te estimates developed as prescribed ab~ve with recent surveys perform~d for the Enewetak atoll. This comparison would inc!icatethe m?gn:tude of the difference due to neglecting the mig~ation of the isotopes into the soil and plant uptake. Figure 1 is a map of the Enewetak atoll showing the location of 3 islands chosen for the comparison--~~ice, Janet, and Yvonne. Table 1 lists the measured dose rete fro~the 1951-5~ operations for these th=e islands as,w$ll as the 1972 estimates for The 1972 survey (reported in tW03-149) provides average expssura fi~~ion pr~d~ct but results from weepon debris activation). In ad~ition, soil depth for Alice and Janet. It is important to note that there lviolentlyhave been no cleanup activities (which would invalidate the comparisons discussed here) on Alice and Janet. Yvonne is a different. situation because of construction and earth moving activities during the testing period. Large var~ations in exposure rates occur on Yvonn?; \ thus, mean levels are misleading. For this reeson, Yvonne will be dropped Table 2 provide; the CS137 suwey data for Alice and Jan’et. The dose rates can be compared directly with the estimates of Table 1. As lxpected, the estimates are high since anmng other reasons it was lssumed that the activity was all on the surface. The soil profiles of activity concentration versus depth can be used to develop a pseudo dose rate by relocating the activity back to the surface. Aconparison of this value’with the estimate is useful in that the difference is~

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then attributable not to soil migration but rather to plant uptake and - other losses. 10 develop this pseudo dose rate, the fOlloklingequation where o is the activity density in pCi/g, z is the depth in CK,,p is the soil density (1.89/CIT3)and the factor of 10-B provides the cont version froxpCi to Ci and from cm to m . The dose rate for Cs ~ is given by Table 3 surmrizes the comparison between the estimated and measured ; CS137 dose rate and the.pseudo dose tate as well. AS can be seen, the ! estimate is a factor of about 20 higher than the measured value and i that roughly half of this difference can b~ accounted for by mech?nis~,s 4: other th?n soil migration. This comparison indicates that simpl~ estifm:es can be used to provide bounding upper limits and that it I might be possible to refine these estimates to within an order of megnitude by correcting for soil migration. The conditions ”for this refinemnt would be: a.) that for the location of interest, there had . . been no cleanup or major earth movin~ prior ti the survey and b.) that the soil profiles would be similar to that found on undisturbed Enewetak islands receiving Having compared dose rate lstimates with survey results for “ Enewetak, we can now turn to those islands in the northern Marshalls that were contaminated by fallout from shots at Bikini. Because the lstimating scheme being used requires the one-hour dose rate as fnput, $t $s important to first establish that off-site i measun~nts were made in all cases where there was fallout on the . fslands of interest. If these data are incomplete, estimations cannot

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Comparison of Estimated and Measured Cs lCalculated by nlocating activity to surface.

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be made. Table 4 summarizes the fallout pattern characteristics fror, the Bikini tests. The last column inmost cases indicates that the wind directions precluded fallout on the islands. The detinite exceptions are Bravo and Yankee. For Bravo and Yankee, t)ff-$itemeasurener,ts were in fact made. fJDne cf the Enewetak shots resulted in fallout on 3ikini or other islands to the e~st, SD the test operations in Table 1 can be ignored. lFigure 2 shows the Marshall Islands relative to the test lo:ations. “The Bravo fallout pattern has been reconstructed independently $s based on extensive surveys. The one-hour dose rates for ’affected islands are given in Table 5. All of the listed islands are outside ‘ barely); the levels ~re stated only to”the nearest decade since extrapolation had to be used. The range of values for Rongelap ant ; Ronge’rikis due tc the variation of the Bravo pattern across the ~ respective island. BY and large, Brzvo is the predominant contributor. Table 6 provides 1977 estimates of the (s137 dose rate for. ! these islands. On the basis of the limited comparison performed for ~ the Enewetak caje, these values could be reduced by a factor of about j 6 to~ccount for soil rnigration,provided the geology is similar to thet The final part of this paper ts devoted to identifying with “ high confidence which islands did not receive fallout from the Bikini tests. Table4, as discussed above, Indicates that only Bravo and . Yankee definitely resulted in fallout on the islands; this is based on the me of off-site measurements to reconstruct their respective fallout patterns. The other shots in the Castle operation, for which there wepe no off-site measurements, apparently were not a problem. however, a detailed investigation is warranted and is reported on in \ AISO contained there is an extrapolation of the Bravo. and Yankee patterns,to a level consistent with background.

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On the basis of extnmely unlikely to h tests at levels higher this investigation, the following islands are ave received fallout from the B~k~n\ or Enew~t than the background exposure of 200 wWem/ye?r: and any other islands circumscribed by the above. The following islands may have received some It is unlikely that the intensit~es would have in an exposure ofmn th~n 2 rem the first yezr; subsequent an~~zl lxposures would have been less than background: The following islands did receive fallout with intensities r~nsing fronl 1 to 2990 R/hr at 1 hr. They are listed in estim?tei order of decreasing residual activity: Taongi (based on cloud drift only - no survey data available) The above estiiates, lven when corrected for soil migration, can only be considered preliminary; they are very likely to be upper bounds. Note that only Cs The addition of Wat{on) are necessary tn completing the estimates of the total a~tiv

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The distribution of the activity in the soil, plants and orgariIsms will not be detemined by a s’ - The estimates in this pawr, a10n9 in deteminin!l such a distribution water table height anl rnple survey of surface Contamjndtion. with such a survey, uoul~ be useful fromth fO~~Ohfin~ kinds Of additional physical characteristics of the soil strata plant categories and root depth.

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The Bravo and Yankee shots, as previously dis~ussed, both deposited fallout on the Islands last-of Bikini. In both ca$es, the lowest reported contour level was not low enough to circumscribe th~ tot?l fallout deposition. Extrapolation was used to define the 0.1 1(/tlR(H+]) contour; this level was chosen because it results in an exposure the first year of about 200 mrem,which is about the annuel background dose. Shown in Figure 2 is the southern periphery of the Bravo and Ydnkee patterns yelative to the location of the Islands. 6 The other Cestle shots are Romea, Koon and Union; off-site fallout measurements are not available so that their respective wind profiles have to be examined. l above 67,000 ft. Below this altitude the dominant direction of the profile is to the north; while not measured for the test, ‘the higher altitude winds are uniformly to the west. Thus it is safe to state that the Romeo fallout dio not reach any of the off-site Marshall Islands. Shot Koon winds were documented for all levels of interest. Except for near-surface, no winds had a northerly component that would have carried any fallout to the south and east. It can be high confidence that Koon fallout carried to the north and did not reach any of the Marshall islands. Shot Union presented M rather unique wind problem. the Ioweralt$tude winds were from the east, strong northerly a;d westerly components existed from 12,000 to 50,000 feet. The influence ‘ of the winds is not readily apparent without further examination. Therefore a crude reconstruction of the fallout pattern was performed bydetemining the displacement of 50, 100 and 200J particles whfch are Initially assumed to be at cloud top and at cloud bottom. This ( permits the construction of an envelope of all such particles in the.

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cloud. The H+6wind profile was used and constent fall rates of .15, .57 and 2.lrn/sec,respectively,were used for the three particle sizes. (Including the altitude dependence of fall rate is probably ~n overspecification,considering the uncertainty in the spatial variatior, of the wind). “ Shown in Figure 3 is this envelope. Taongi is definitely affe’cted by the Union fallou~ but the other islands are outside the