Gamma Dose Rates at Rongelap Atoll, 1954 - 1963

Page 1

GAMMA DOSE RATES AT RONGELAP ATOLL, 1964-1963 Laboratory of Radiation Biology University of Washington UNITED STATES ATOMlC ENERGY COMMISSION l DlVISlON OF TECHNICAL INFORMATION

Page 2

Thts report WBBprepared as an account of Government sponsored work. Neither the United States, nor the Commiesion, nor any person acting on behalf of the Commlselon: racy, completeness. or usefulnese of the information contained in We report, or that the utre of any informatton, apparatue, method, or process dtscloeed in this report may not Infringe privately owned rfghts; or B. Assumes any liabilities with respect to the use of, or for damages reilulttng from the uee of any information, apparatus, method, or process dtacloscd in thla report. As used in the above, “person acting on behalf of the Commission” includes any employee or contractor of tie Commtssion, or employee of such contractor, to the extent that ruch employee or contractor of the CommIeeion, or employee of such contractor preparea, disseminates. or provides accea8 to, any information pursuant to hfs employment or contract with the Commlaslon, or his employment wtth such contractor. This report has been reproduced directly. from the best Printed in USA. Price $3.00. Available from the Clearinghouse for Federal Scientific and Technical Information, National Bureau of Standards, LT.S. Department of Commerce,

Page 3

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.

Page 4

GM DOSE RXI'ES AT RONGELAP ATOLL, 1954-1963 Rongelap Atoll was contaminated with fallout from a thermonuclear device on March 1, 1954. Gamma-dose rates on D + 1 ranged from fission products from Until 1959 the decline of gamma-dose rates followed the theoretical decay of mixed u235 calculated by Miller and Loeb. The reduction of gamma dose rates to approximately half the predicted levels in 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 of radionuclides from fission of U 235 would tend to be equal to or greater than actual levels.

Page 5

GAMMA DOSE RATES AT RONGELAP ATOLL, 1954-1963 Rongelap Atoll, Marshall Islands, was accidentally contaminated on March 1, 1954 with radioactive fallout from a Yiiermonuclear device 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 inhabitants and its economy have been briefly characterized (1). 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 Lomuflal islet in the northern part of the atollt2), Fig. 1, These estimates were based on extrapolations of measurements made two days after initial fallout (2). 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 (3)

Page 6

in Figs. 2 and 3. The theoretical cme is based on fission z&j products from slow neutron fission and the assumptions that there is no fractionation and that the radionuclides are dis- ; tributed over an infinite plane, while at Rongelap measurable _=z amounts of activity remained on the trees. !Fhe actual measure- j2 ments fit closely to the theoretical decay curve for Rongelap islet and at least for the first four years after fallout at Kabelle islet in spi,te of the assumptions made in determining the theoretical curve and the variability of the field measure ments. 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 following 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 garmna 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 shortlived radionuclides present and by following the beta decay rates in plants collected in 1958, the total contribution was

Page 7

a fraction of one per cent of the fallout deposited in 1954. merefore the Redwing ant!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 within which 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/?lr. At Eniaetok the range of the average readings was 0.073 to 0.079 mr/hr and at Kabelle islet,

Page 8

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 one inch above the floor were lower than those at three feet above the ground and readings close to the walls and roof were highest of all. There were relatively high readings over some 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 first, as would be expected, in the intertidal zone, except for a few small areas of beach rock'covered with a film of alg Levels of LO.02 mr/'hr were measured in July, 1957 in the inte tidal 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 bulldozi of a considerable part of the area.

Page 10

In September 1959, film badges sealed against moisture were__ _%a._~$exposed at Rongelap and Xabelle islets to measure gamma doses in 2 The badges were provided and set out by Radi- "3 ation Safety pe rsonnel 3 at the Pacific Proving Ground. The limit _$ of detection was an accumulated dose of 10 mr. Three badges were1 the ground attached rates measured with placed at each location, one suspended by strings three feet above the ground and away from tree trunks, one three feet Abe to a tree, and one on the ground. Gammadc 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 meter measurements and those explained by differences in sensitivity of the two methods to the gamma energies present in radium standard rather discrepancy does not 2 and 3 since the theodoses calculated from the survey obtained with film badges may be the field. Calibration was based on a than on actual fallout material. This invalidate the decline curves in Pigs. retical cume (solid line) is based on measurements with a surv 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

Page 11

radioactive sources having the same range and proportion of energies as the fallout material. Decline and fallout composition Ithas 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 with 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 U235 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 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 proportion of induced radionuclides to fission products

Page 12

and the decline of gamma dose rates will be more rapid. Therefore, 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. 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 origin. The various in fractionation are discussed in detail in the Hearings, 1959 (5) and with specific reference to the March 1, .~ 1954 explosion in "The Effects of Nuclear Weapons," 1962 (4). Redistribution of fallout fallout possibly occurred during the first storm after fallout =! (Dunning, 1957), but thereafter redistribution had very little eff& Sl on the gamma dose rates during the first four years after fallout. The exception, of course, is the relatively rapid decline of radio--?-- activity in the intertidal zone. Such rapid decline would also F' example is the in areas in which there is heavy erosion. An man-made erosion by bulldozers in the village area at Rangelap. The reduction of gamma dose rates following the storm could have been due to the washing of fallout material

Page 13

from the leaves of the vegetation and perhaps also to some shielding effect by additional moisture in the soil. It was certainly not due to rapid vertical movement of material in the soil. Analysis of soil leachates and soil cores shows that vertical movement of radionuclides in atoll soils is very slow. For example, cores taken in immature soil6 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 soil, Cesium-137 and Sb 125 are very slowly leached in the atoll the gamma-dose rate values at Rongelap islet in 1959-63 fall on the theoretical curve, it appears that the levels due to the.1954 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 approximated from the decay cme for

Page 14

$35 fission products in a local or intermediate fallout situation As instrumentation, tcchnigues 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. speaking, the reliability of such measurements will de the experience and judgement of the individuals making the measurements, the variety of environmental situations and the time available for making such measurements. reliability is further substantiated by the fact that of specific radionuclides in the various land organis ferent islands were roughly correlated with the g Errors in predicted levels will tend to be conservative, i.e., higher than actual levels. It would be useful in any future operations to h able known mixtures of radionuclides simulating the f radionuclides for a particular device, or better, a sample of the raw fallout material collected at each site to be studied. : This mixture could then be used to calibrate instruments, film badges and chemical dosimeters as time went on and as the sPeC_ trum of gamma-energies changed. Comparison of the decay Of

Page 15

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

Page 17

Gamma Dose Rates on Rongelap Island o University of Washington Theoretical decay of Uz3’ fission products OS measu radiac at 3 feet above an infinite plane (Miller and Days after March I,1954 and March1 of each year indicated Fig. 2. Decline of Gamma Dose Rates at Rongelap Island.

Page 18

Gamma Dose Rates on products as measured by the feet above an infinite plane Days after March 1.1954 and March 1 of each year indicated Pig. 3. Decline of Gamma Dose Rates at -belle Islet.

Page 19

E.E. Held, Qualitative Distribution of Radionuclides at Rongelap Atoll. In Radioecologv, 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 C.F. Miller and P. Loeb, Ionization Rate and Photon Pulse Rate of Fission Products from Slow Neutron Fission of U235. Atomic Energy Commission (1962) (See especially p. 460 and Fallout from Nuclear Weapons Tests, Hearings Before the Special Subcommittee on Radiation of the Joint Committee on Session, U.S. Government Printing Office (1959). (See Studies of Ion and Moisture Movement in Glacial Till and Coral Atoll Soils. Soil Sci--* sot. af Amer. Proc. 25, 4(1961).

Page 20

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)