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 WNGELIAP 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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GAMMA DOSE RiW’ES AT RONGIZAP ATOLL, 1954-1963 Rongelap Atoll was contaminated with device on March 1, 1954. Gamma-dose 3.5 r/hz to 35 r/hr. Until 1959 the measured in the field followed tinetheoretical 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 long-livedgamma-emitter Cs137 in the soil. Redistribution of fallout had little effect on gamma dose rates except in the intertidal zone. It is concluded that predicted gamma dose rates from fallout based on the theoretical decay CUJWe of rad3.0nuclides from fission of U would tend to be equal to or greater than actual levels.

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GAMMA DOSE RATES AT RONGELAP ATOLL, 1954-1963 taminated 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 . The decline of gamma dose rates resulting from the fallout is discussed in this report. Gamma dose rates at Rongelap Atoll on D + 1 (time of detonationplus 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 extraWlations of measurements made two days after initial fallout . The subsequent decline of gamma meter readings taken three feet and Kabelle islets, is compared dose rates, based on sumey above the ground at Rongelap with the theoretical decay (3)

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2 and 3. The theoretical from slow neutron fiss~(-in no fractionation and thak qllrveis based on fission and the assumptions that the radionuclides are disover an infinite plane, while at Rongelap measur?ible amounts of activity remained on the trees. The actual measurefrtentsfit closely to the theoretical decay curve for Rongelap islet and at least for the first four years aftierfallout at Kabelle islet in spite of the assumptions made in determining the theoretical curve and the variability of the field measurements. The measurements on one small islet may vary by a factorof 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 gamma 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 measurable amount of contamination, as was seen by the shortIived 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 u~ie~pere-n: of the fallout deposited in 1954. Therefore the I?edwingand Hardtack fallout are insignificant when considering the long-range picture. Selectionof areas and reproducibility of repeated measurements.,— In March 1958, stakes were set out in variollsparts e: Rongelap islet in an attempt to provide a means of repeating measurements at identical locatiens. Stakes proved to be unsatisfactory since they only sertiad.to attract the curious, which resulted in trampling and disturbance of the areas and in SOIRGcases removal of the stakes. A practical solution to this problem was to select general areas within which measurements were to be taken. These to pathways, roads, buildings, areas were located with relation and measured distances frum hndmarka . in each general area measurements were taken mer different t~es 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 isletiand twice at both Eniaetok and Kabelle islets. At Rongelap islet the average for each set of readings ranged from 0.046 to 0.067 m/hr. 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, P=ticularly under Pandanus trees where litter had accumulated. The highest levels at Rongelap islet in 1955 were measured in the remaining palm-frond huts where fallout remained trapped in the roof and &he wall thatching. Readings the floor were Ynwer th~ those at thrtiefeet and readings close to the walls and roof were There were relatively high readings over some one inch abwe above the ground highest of all. open areas where soil algae were ahu~dtint. !Knesoil algae form a crust roughly one centimstcr thick and retain most of the radionuclides from Return to baekqround level The return to background level,~ 0.02 mr/%r, occurred first, as would be expected, in the intertidal zone, except for a few mall areas of beach rock covered with a film of algae. Levels ofAO.02 mr/hr were measured in July, 1957 in the inte&- tidal zones at Kabelle and Rongelap islets and in the newly constructedvillage 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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Local differences in dose rates at one inch Survey meter readings were also taken at one inch above the groundwith the beta shield both open and closed. Th@r@ was no apparentcorrelation between these readings and the gamma dose at three feet, except in a very general way, but were useful in selecting areas from which to sample 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 the 30il and there were markedly higher levels of activity over areas cwered with soil algae than over bare sand. Attempts also were made with a survey meter to determine local differences in activity in trec~. ~is was Unsuccessful since the general levels of activity masked local effects within the trees, even though laboratory analyses showed that the activity in lichens and mosses collected from the bark was several times higher than in the bare portions of the trees. The use of survey meters to determine the vertical distribution 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

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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 m. Three badges placed at each location, one suspended by strings three feet above the ground and away from tree trunks, one three feet abme 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. ‘I’hadiscrepancy 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 radium standard rather than on actual fallout material. T%is discrepancy does not invalidatethe decline curves in Figs* 2 and 3 since the theoretical curve (solid line) is based on measurements with a survey meter similar to those used for the various measurements made. Hwever, the discrepancy does point out that while relative levels of activity can be determined accurately by any one type of measurement, absolute values depend on calibration with

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sources having the same range and proportion of the fallout material. Decline and fallout composition It has turned out in practice at Rongelap Atoll that when a large number of survey meter readings are taken and these are averaged, a pattern of decline ofgamma-dose rates consistent wifi the theoretical decay for mixed fission products emerges. This is true even though the theoretical curve is based on the decay of mixed fission products from U distributed uniformly over an infinite plane and disregards differences in both the Composition and distribution of fallout radionuclides in the actual field situation. The fallout at Rongelap consisted of Axed activities contributing to the gamma fession products and neutron-induced radionuclides from a thermonuclear device. The induced activity are of shorter half the early decline in gamma dose rates at Rongelap would have been more rapid than the theoretical decay of mixed fission products alone. As the art of the fission yield per higher proportion of induced r.adionuclidesto fission pr~ucts producing thermonuclear devices progresses kiloton will decrease. There will be a

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and the decline of gamma dose rates will be more rapid. fore, in the future, long-range predictions of residual dose rates based on the Rongelap experience would be likely to yield higher values than would actually occur. In addition to the differences in composition of fallout from different devices and variations in measurements there is fractionationof the radj.onuclides,a change in species compositim with time or distance from origin. The various factors involved in fractionation are discussed in detail in the Congressional Hearings, 1959(5) and with specific reference to the March 1, 1554 explosion in “The Effects of Nuclear Weapons, ” 1962(4)l Reduction of gamma-dose rate levels due to redistribution of falloutpossibly occurred during the first storm after fallout (Dunning,1957), but thereafter redistribution had very little effak 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 be 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 Bhieldingeffect by additional moisture in the soil. Xt 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 f3kW. Por example, cores taken in immature soils in tained 90 per cent of the activity in the top However, the reduction of gamma-dose rates to half the predicted levels in 1959-63 probably downward movement Of the long-lived gamma-emitter Cs 137 in the Cesium-137 and Sb125 are very slowly leached in the atoll and Mn’4 tend to remain at the surface(1,6,7)0 Although the gamma-dose rate values at Rongelap islet in 1959-63 fall on the theoretical curve, it appears that the levels due to the 19s4 fallout have fallen below values 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 gamma-dose rates due to the fallout and due to background would be approximately twice background. The Rongelap experience has shown that the decline of gamma dose rates can be ~Pproxima&.ed from the decay cme fOr

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~235 fission products in a local or intermediate fallout situation. As instrumentation, techniques of calibration and the predictability of the radionuclide spectrum from nuclear devices continue to improve, so will the usefulness of gamma-dose rate rneasurethe experience and judgement measurements, the variety of such measurements will depend upon of the individuals making the environmental situations encountered and the the available for making such measurements. Their reliability is further substantiated by the fact that the levels of specific!radionuclides in the various land organisms at different islands were roughly correlated with the ganmuldose 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 known mixtures of :adionuclides simulating the fallout radionuclides for a particular device, OE bettez, a sample of the raw .:alloutmaterial coZlected at each site to be studied. This mixture could then be used to calibrate instruments, film badges and chemical dosimekers 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 mixture, with decline of gamma-dose rates in the field, would give a more accurate indication of overall effect of the redistribution of radionuclides on gamma dose rates.

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‘k Estimated dose rate Gamma Dose Rates \ operation Operation on Rongelap Island 0 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 Gaxmna Dose Rates at Rongelap Island.

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Gamma Dose Rates on \ products as measured by the \ feet above an infinite plane and March 1 of Fig. 3. Decline of Gamma each year indicated Dose Rates at K?@elle Islet.

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E.E. Held, Qualitative Distribution of Radionuclides at Rongelap Atoll, In Radioecoloqy, 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 heas in the Pacific Ocean from Nuclear Tests. U.S. Attic 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 Committee Session, U.S. Government Printing Office (1959). (See Studies of Ion and Coral Atoll Soils. Moisture Movement in Glacial Till and

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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). (Xn Press)