Gamma Dose Rates at Rongelap Atoll, 1954-1963

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GAMMA DOSE RATES AT RONGELAP ATOLL, 1954-1963 Laboratory of Radiation Biology University of Washington

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This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or reprenentetton, expreesed or implied, with respect to the accuracy, completene.m, or usefulness of the information contained in this report, or that the uae of any information, apparatua, method, or process diaciosed in this report may not infringe prlvatel y owned rights; or B. Aseumes any liabilltlen with respect to the uoe of, or for damagea resulting from the use of any information, apparatua, method, or process disclosed in this report. Aa used in the above, 1‘person acting on behalf of the Commission” includes any employee or contractor of tbe Commhmion, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, dieeeminatee, or provides accerm to, any information pursuant to his employment or contract with the Commimion, or his employment with such contractor. This reporthas been reproduceddirectlyfrom thebest

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BIOLOGY AND MEDICINE (TID-4500) GAMMA DOSE RATES AT RONGELAP 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 RATES AT RONGEIJU? ATOLL, 1954-1963 Rongelap Atoll was contaminated with . device on March 1, 1954. Ganuna-dose . 3.5 rfir to 35 r/%r. Until 1959 the fallout from a thermonuclear rates on D + 1 ranged from decline of gamma-dose rates measured i.nthe field followed the theoretical decay of mixed fission products from U 235 calculated by Miller and Loeb. The reduction of gamma dose rates to approximately half the predicted levels i.n 1959-63 probably reflects the downward movement of the 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 curve 235nuclides from fission of U would tend to be equal to 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 to the west. device detonated at Bikini Atoll some 80 miles Eighty-two natives residing on Rongelap Island were evacuated and repatriated in June 1957. The atoll, its inhabitants and 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 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 Lomullalislet 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 dose rates, based on survey meter readings taken three feet above the ground at Rongelap and Kabelle islets, is compared with the theoretical decay

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... ..>. in Figs. 2 and 3. The theoretical products from slow neutron fission there is no fractionation and that curve is based on fission and the assumptions that the radionuclides are distributed 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 Kabelle islet in spite of the assumptions made in determining the theoretj.cal curve and the variability 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 h 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 measurable amount of contand.nation, as was seen by the lived radionuclldes present and by following rates in plants collected in 1958, the total the beta decay

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a fraction of one per Therefore the Redwing cent of the fallout deposited in 1954. 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 various parts of . Rongelap islet in an attempt to provide a means of repeating measurements at identical locations. Stakes proved to be unsatisfactory since they only served to attract the curious, which resulted in trampling and disturbance of the areas and in some cases removal of the stakes. A practical solution to this problem was to select general areas wi.thi.nwhich measurements were to be taken. These areas were located with relation to pathways, roads, buildings, and measured distances from landmarks . In 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. August 1958, the set of measurements was repeated three times Rongelap islet and twice at both Eniaetok and Kabelle islets. Rongelap islet the average for each set of readings ranged from 0.046 to 0.067 mr/hr. At Eni.aetok 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, particularly 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 the wall thatching. Readings the floor were lower than those at three feet and readings close to the walls and roof were There were relatively high readings over some one inch above above the ground highest of all. open areas where soil algae were abundant. The soil algae form a crust roughly one centimeter thick and retain most of the radionuclides from Return to background level The return to background level,~ 0.02 mr/’hr, occurred first, as would be expected, in the intertidal zone, except for a few small areas of beach rock covered with a film of algae. Levels of Z 0.02 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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Local differences in dose rates at one inch Survey meter readings were also taken at one inch above the ground with the beta shield both open and closed. There was no apparent correlation 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 soil and there were markedly higher levels of activity over areas covered with soil algae than over bare sand. Attempts also were made with a survey meter to determine local differences in activity in trees. 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 portions of 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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,1 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 trunks, one three feet above the ground attached rates measured with to a tree, and one on the ground. Gamma dose 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 obtained with film badges may be sensitivity of the two methods to the field. Calibration was based than on actual fallout material. neter measurements and those =xplained by differences in the gamma energies present in invalidate the decline cumes in Figs. retical cume (solid line) is based on radium standard rather discrepancy does not 2 and 3 since the theomeasurements with a survey meter similar to those used for the various measurements made. However, 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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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 number of survey meter readings are taken and these are averaged, a pattern of decline ofg--dose rates consistent with the theoretical decay for mixed fission products emerges. This is true even though the theoretical curve is based on the decay 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 mixed fission products and neutron-induced radionuclides from a thermonuclear device. The induced activities contributing to the gamma activity are of shorter half life than the long-lived fission product, 30-year Cs . It therefore might have been expected that the early decline in gamma dose more rapid than the theoretical alone. As the art of producing rates at Rongelap would have been decay of mixed fission products the fission yield per kiloton will decrease. of induced radionuclides to There will be a

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,.., and the decline of gamma dose rates will be more rapid. ThereQ fore, in the future, long-range predictions of residual gamma dose rates based on the Rongelap experience would be likely to yield higher values than would actually occur. Fractionation of fallout In addition to the differences in composition of fallout from different devices and variations in measurements there is fractionation of the radionuclides, a change in with time or distance from orign. The various in fractionation are discussed in detail in the and with specific reference to the March 1, Reduction of gamma-dose rate levels due to redistribution of occurred during the first storm after fallout 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 be anticipated in areas example is the man-made in which there is heavy erosion. An erosion by bulldozers in the village area at Rcmgelap. The reduction the storm could have been due to of gamma dose rates following the washing of fallout material

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from the leaves of the vegetation and perhaps also to some effect by additional moisture in the soil. It was not due to rapid vertical movement of material in Analysis of soil Ieachates and soil cores shows that vertical movement of radionuclides in atoll soils is very slow. For 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 soils while other and Sb’2’ are very slowly leached in the atoll , and lh tend to remain at the surface (1,6,7) . Although the gamma-dose rate values at Rongelap islet in 1959-63 fall on the theoretical curve, it appears that the 1954 fallout have fallen below values 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 gamma-dose rates due to the fall~ut and due to background would be approximately twice background. The Rongelap experience has shown that the decline of gamma dose rates can be approximated from the decay curve 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 measurements for predicting the decline of gamma-dose rates. Practically speaking, the reliability of the experience and judgement measurements, the variety of such measurements will depend upon of the individuals making the and the time available for making such measurements. Their reliability is further substantiated by the fact that t!he levels of specific radionuclides in the various land organisms at different islands were roughly correlated with the gamma dose rates. Errors in predicted levels higher than actual levels. will tend to be conservative, i.e.~ It would be useful in any future operations to have available known mixtures of radionuclides simulating the fallout radionuclides for a particular device, or bettert a s~ple of the raw fallout material collected This mixture could then be used to at each site to be studied. calibrate instruments, film badges and chemical dosimeters as time went on and trum of gamma-energies changed. Comparison of the

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

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L- Estimated dose rate on Rongelap Island o University of Washington \ fission products as measu radiac at 3 feet above an infinite plane (Miller and Fig. 2. Decline of ~ DOSe Rates at Rongelap Island. Days after March 1,1954 and Marchl of each year indicated

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Gamma Dose Rates products as measured by the feet above an infinite plane Days after March 1,1954 and March 1 of each year indicated Fig. 3. Decline of Gamma Dose Rates at Kabelle Islet.

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E.E. Held, Qualitative Distribution of Radionuclides at Rongelap Atoll. In Radioecol ocw, Schultz and Klement 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 C.F. Miller and P. Loeb, Ionization Rate and Photon Pulse Rate of Fission Products from Slow Neutron Fission of U . Atomic Energy Commission (1962) (See especially p. 460 and Weapons Tests, Hearings Before the on Radiation of the Joint Committee on Session, U.S. Government Printing Office (1959). (See Studies of Ion and …