Gamma Dose Rates at Rongelap Atoll 1954-1963 (Bravo Event, Castle Operation Fallout on Rongelap Time Study)

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Biology and Medicine GAMMA DOSE RATES AT RCNGELAP ATOLL, 1954-1963 Laboratory of Radiation Biology University of Washington Work performed under Contract AT(45-1)1385 with the United States Atomic Energy Commission.

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Rongelap Atoll was contaminated with fallout from a device on March 1, 1954. Gamma-dose rates on D + 1 3.5 r/hr to 35 r/’hr. Until 1959 the decline of gamma-dose rates measured in the field followed the theoretical decay of mixed fission products from U235 calculated by Miller and Loeb. The reduction of gamma dose rates to approximately half the predicted levels in 19S9-63 probably reflects the downward movement of the t fallout had little effect on gamma dose rates except in the intertidal zone. St is concluded that predicted gamma dose rates from fallout based on the theoretical decay curve of radionuclides from fission of U would tend to be egual to or greater than actual levels.

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GAMMA DOSE RATES AT RONGELAP ATOLL, 1954-1963 Rongelap Atoll, Marshall Islands, was accidentally contaminated on March 1, 1954 with radioactive fallout from a thermonucleardevice detonated at Bikini Atoll some 80 miles to the west. Eighty-two natives residing on Rongelap Island were evacuated and repatriated in June 1957. The atoll, its inhabitantsand its economy have been briefly characterized(1)l The decline of gamma dose rates resulting from the fallout is Q discussed in this report. Gamma dose rates at Rongelap Atoll on D + 1 (time of detonation plUS one day) were estimated to be 3.5 r per hour at the inhabited islet of Rongelap in the south and 35 r per hour at uninhabited Lomuilal islet in the northern part of theatoll(2), Fig. 1. These estimates were based on extraPolations of measurements made two days after initial fallout . The subsequent decline of gamma meter readings taken three feet ~d Kabelle islets, is compared dose rates, based on survey above the ground at Rongelap with the theoretical decay(3)

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In Figs. 2 and 3. products from slow The theoretical rluve is based on fission neutron fisshn and the assumptions that there is no fractionation and that the radionuclide~ are dis. tributed over an infinite plane, while at Rongelap measurable amounts of activity remained on the trees. The actual measurements fit closely to the theoretical decay curve for Rongelap islet and at least for the first four years after fallout at I@belle islet in spite of the theoretical curve and made in determining of the field measurements. The measurements on one small islet may vary by a ? factor of more than three, even when the identical instrument is used by the same person. Decline followinq first storm About two weeks after initial contamination there was a storm with heavy rain, and a subsequent reduction in gamna dose rate somewhat greater than would have been expected on Fallout in 1956 and 1958 The rises in gamma dose rates in 1956 and 1958 were due to operations Redwing and Hardtack. Even though there was a m~asurab~e amo~t of contamination, as was seen by the short- ~ived radionuclides present and by following the beta decay rates in plants collected in 1958, the total contribution was

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a fraction of o~~e *Wr e-n: of the fallout deposited in 1954. ‘Xhereforethe Redwing and Hardtack fallout are insignificant when considering the long-range picture. Selection of areas and reproducibility of repeated measurements In March 1958, stakes were set out in variotlsparts of Rongelap islet in an attempt to provide a means of repeating measurements at identical Iocatinns. Stakes proved to be unsatisfactory since they only served.to attract the curious, Which resulted in tr~pling and disturbance of the areas and ~ in some cases remval of the stakes. A practical solution to this problem was to select general areas within which measurements were to be taken. These areas were located with relation tO pathways, roads, buildings, and measured distances from landmarksl Zn each general area measurements were taken over different types of vegetation, soil, the pathways themselves, over litter, and under Pandanus trees and other tall plants. In August 1958, the set of measurements was repeated three times at Rongelap islet and twice at both Eniaetok and Kabelle islets. At Range-lapislet the average for each set of readings ranged from 0.046 to 0.067 mr/ht. At Eniaetok the range of the average readings was 0.073 to 0.079 mr/hr and at Kabelle islet,

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Local differences in dose rates at three feet The highest levels measured were generally under trees, The highest levels Pandanus trees where litter had accumulated. at Rongelap islet in 1955 were measured in the remaining palm-frond huts where fallout remained trapped in the roof and tihewall thatching. Readings one inch abme the floor were Imer than those at three feet above the ground and readings close to There were relatively the walZs and roof were highest of all.. high readings over some open areas where soil algae %;ereabundant. Tile&il algae form a crust roug~ly t one centimeter thick and retain .mmt of the radionuclides from Return to background level first, as would for a few small be expected, in the intertidal zone, except areas of beach rock covered with a film of algae. Levels af LO.02 mr/hr were measured in July, 1957 in the intertidal zones at Kabelle and Rongelap islets and in the newly constructed village on Rongelap islet. Construction of the village entailed the removal of the thatched huts and bulldozing of a considerable part of the area.

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Survey meter readings were also taken at one inch above the shield both open and closed. There was no between these readings and the gamma dose groundwith the beta rate readings at three feet, except in a very general way, such readings were useful in selecting areas from which to and tidicated local distribution of the activity. For example, when measurements were made one inch over the ground with the shield open in 1959, the levels were higher after the litter was removed from of activity over the soil and there were areas covered with soil algae than over sand. Attempts also were made with a survey meter to determine local differences in activity in tree%, This was since the general levels of activity masked local the trees, even though laboratory analyses showed activity in lichens and mosses collected from the several times higher than in the bare ‘Theuse of survey metess to determine of activity in soil pits was impracticable due to the high background levels from surrounding contamination and the fact that the bulk of the radioactivity was in the surface inch or less the vertical distribution

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In September 1959, film badges sealed against moisture were exposed at Rongelap and Kabelle islets to measure gamma doses in different areas. The badges were provided and set out by Radiation Safety personnel at the Pacific Proving Ground. The limit of detection was an accumulated dose of 10 mr. Three badges were placed at each location, one suspended by strings three feet above the ground and away from tree tr~s, one three feet above the ground attached to a tree, and one on the ground. Gamma dose rates measured with a survey meter at each location indicated ! that the accumulated dose in 69 out of 116 film badges would be in excess of 10 mr, but less than 20 mr. The results were, however, negative for all badges. The discrepancy between the doses calculated from the survey meter measurements and those obtainedwith film badges may be explained by differences in sensitivityof the two methods to the gamma energies present in the field. Calibration was based on a than on actual fallout material. This invalidatethe decline curves in Fi9s. retical curve (solid line) Is based on radium standard rather discrepancy does not 2 and 3 since the meter simflar to those used for the various measurements However, the discrepancy does point out that while levels of activity can be determined accurately by any one type measurement, absolute values depend on calibration with

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radioactive sources having the same range and proportion of——— energies as the fallout material. Decline and fallout composition It has turned out in practice at Rongelap Atoll that when a large ntier ef survey meter readings are taken and these are averaged, a pattern of decline ofgamma-dose rates consistent witi the theoretical decay for mixed*fission products emerges. is true even though the theoretical curve is based on the an infinite plane and disregards differences in both the composition and distribution of fallout radfonuclides in the actual The fallout at Rongelap consisted of mixed The induced activities contributing to the gamma activit-yare of shorter half life than the long-lived fission prothe early decline in gamma dose rates at more rapid than the theoretical decay of have been expected that Rongelap would have been mixed fission products alone. As the art of producing thermonuclear devices progresses the fission yield per kiloton will decrease. There will be a higher pro~rtion of induced radionuclides to fission products

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and the decline of gamma dose rates will be more rapid. Therefore, in the future, long-range predictions of residual gama dose rates based on yield higher values the Rongelap experience would be likely to than would actually occur. In addition to the differences in composition of fallout from different devices and variations in measurements there is fractionationof the radionuclides, a change in species compositim with time or distance from origi~. The various factors involved in fractionation are discussed in detail in the Congressional Hearings, 1959(5) and with specific reference to the March 1, 1954 explosion in “The Effects of Nuclear Weapons, ” 1962(4)l Reduction ofgamma-dose rate levels due to redistribution of falloutpossibly occurred during the first storm after fallout (Dunning,1957), but thereafter redistribution had very little eff~ on the gamma dose rates during the first four years after fallout. The exception, of course, is the relatively rapid decline of radioactivity in the intertidal zone. Such rapid decline would also b@ anticipated in areas in which there is heavy erosion. An example is the man-made erosion by bulldozers in the village area at Rongelap. The reduction of gamma dose rates following the storm could have been due to the washing of fallout material

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from the leaves of the vegetation and perhaps also to some shieldingeffect by additional moisture in the soil. It was certainlynot due to rapid vertical movement of material in the soil. Analysis of soil leachates and soil cores shows that verticalmovement of radionuclides in atoll soils is very SIUW. hr example, cores taken in immature soils in 1963 still contained 90 per cent of the activity in the top centimeter. However, the reduction of gamma-dose rates to approximately half the predicted levels in 1959-63 probably reflects the soils while other and Sb12’ are very slowly leached in the atoll the gamma-dose rate values at Rongelap islet in 1959-63 fall on the theoretical curve, it the 1954 fallout have fallen the levels due to predicted by the theoretical curve here also. Since the theoretical curve had reached background levels by 1959 it would be expected that the sum of due to the fallout and due to background would The Rongelap experience has shown that the decline of gamma dose rates can be approximated from the decay c-e for

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~235 fission products in a local or intermediate fallout sltuatfon. AS instrumentation, tedxniques of calibration and the predictability of the radionuclide spectrum from nuclear devices continue ments fox predicting the decline of gamma-dose rates. Practic~ the experience and judgement measurements, the variety of ~d the the available reliability is further such measurements will depend upon of the individuals making the for making such measurements. substantiated by the fact that Of specific radionuclides in the various land organisms at different islands were roughly correlated with the ganunadose rates. Errors in predicted levels will tend to be conservative, i.e., higher than actual levels. It would be useful in any future operations to have available kn&n mixtures of xadionuclides simulating the fallout radionuclides for a particular device, or better, a sample of the raw lallout material collected at each site to be studied. This mixture could then be used to calibrate instruments, film badges and chemical doslmeters as time went on and as the spectrum of gamma-energies changed. Comparison of the decay of

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gamma-dose rates from the rates in the field, would mixture, with decline of gamma-dose give a more accurate indication redistribution of radionuclides

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I‘= Estimated dose rate Gamma Dose Rates on Rongelap Island \ Redwing Hardtack \ Ho University of Washington radiac at 3 feet above an infinite plane (Miller and Days after March 1,!954 and Marchl of each year indicated Fig. 2. Decline of Gaxma Dose Rates at Rongelap Island.

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products as measured by the \, feet above an infinite plane and March 1 of Fig. 3. Decline of Gaeach yeor indicated Dose Rates at Kal@~e, Islet.

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E.E. Held, Qualitative Distribution of Radionuclides at RongeXap Atoll. In Radioecolonv, Schultz and Klement (Editors), Reinhold Pub. Corp. and the American Institute of Biological Sciences, New York (1963). G.M. Dunning (Editor), Radioactive Contamination of Certain Areas in the Pacific Ocean from Nuclear Tests. U.S. Atomic Rate of Fission Products Ionization Rate and Photon Pulse from Slow Neutron Fission of U . Atomic Energy Commission (1962) (See Fallout from Nuclear Weapons Tests, Hearings Before the Special Subcommittee on Radiation of the Joint Cdttee on Atomic Energy, Congress of the U.S. 86th Congress, Ist Studies of Ion and Moisture Movement in Glacial Till and Coral Atoll Soils. soil Sci. Sot. of Amer. Proc. 25, 4(1961).—. .— ——

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Autoradiography of Sectioned Soil Cores. Proceedings of the Symposium on Radioisotope Sample Measurement Techniques in Medicine and Biology, I.A.E.A. (1965). (In Press)