Estimate of the Radiation Dose to Thyroids of the Rongelap Children Following the Bravo Event, Castle Operation

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ESTIMATE OF RADIATION DOSE TO THYROfDS OF THE RONGELAP CHfLDREN FOLLOWKNQ THE BRAVO EVENT OFFICIAL USE ONLY ESTIMATE OF RADIATION DOSE TO THYROIDS OF THE RONGELAP CHILDREN FOLLOWING THE BRAVO EVENT An lettmete is made of the radiatitm dose to the ihyrcids of Rongeiep children following the Bravo event of March 1, 1954. The available expertmental data are used to estimate the doee under two alternate aeaurnptiona of mode of intake: (a) all of the Intake wae by inhalation. end (b) all of the intake was by oral ingestion, It is concluded that the meet probable doee to the thyroid of a 3- to 4-year-old giri is in the range 700 to 1400 red. The cloud arrival time ia givenl le H + 4 to 6 hours. The duration of the cloud paesege ie lees well known, but probably lies in the range of 8 to 16 hours. In atl calculations we will assume that cloud paeeage waa in the The residents of Rongelap were evacuated at H + 51 hours. Refiable dose-rate measurements were not obtained at that time, but the gamma dose rate 3 feet above the ground waa measured ls 37S mR/hour 7 days after the detonation. Assuming t-1.2 decay. the H + 24 hour dose rate wam tbeo M The eources of expoeure to the thyroid whtch must be conddered are: (2) Internal deposition of iodine isotopes. The whole-bo& ~ae waa estimatedl to be 17$.R. The exact method Of making this eetimate ie not gtven, lo an independent efimate is-~de below. In particular, it appears thet this estimaie doee not include the doee from the cloud but only from fallout.

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decay during this interval, and uae the reading of 375 mR/hour at 7 dsyo, the estimated dose from fsllout durtng cloud passage is 47 R. The dose from Sedan indicates that the doee from the cloud itself is approximately equal to the dose from fallout during cloud pamaca. The total estimated dose in then Withfn the error of the measurements and the lccuracy of the assumptions, this estimate, which does not contain any correction for the small effect of time indoors, does not differ significantly from the value of 17S R. 1 We will, therefore. take the lverage whole-body gamma dose ls 175 * 25 R. fnternal Deposition of Iodine Isotope~ Unfortunately, no direct measurement was possible on the radioactive iodine content of individuals from Ron clap. Urine mmples were taken from which the average thyroid burden of 11 ‘* has been estimated. The Los Alamos Scientific Laboratory callected pooled 24-hour samples 15 days peat- detonation and estimated2 the 1-day thyroid tontent as 11.2 pCi of 1131. USNRDL collected samples from each member of the exposed group 43 and 46 daya poet-detonation and, by an indirect method, estimated the average thyroid at 1 day. The LASL estimate of 11.2 pCi was 1 obtained by direct counting of 1131 in the urine end should be more reliable than the NRDL estimate. The value of 11.2 #Ci will be used aa a basis for all following considerations. This estimate was based on the assumption of — 0.1% of the maximum thyroid burden being excreted in the urine on the 15th day. Vartation fn the biological half-life and other factors indicate that a range of 0.05 to 0.2% should be piac ed on this numberS (see appendix). We, therefore, take 5.6 to 21.4 fJCi m the range of adult llS1 thyroid burden. The pooled mmplee repreeent all lge groofm. The number of individuals in these age gro~; lnd the volumo of urine from each age group is approximately as followw ‘ Number of Volume of ~ of Totat AThe urine samples are typical of adults and the calculate thyroid burdens are presumably also those of adults. Associated with this 1191 are the shorter-lived isotopes I132, 11S3 and I . ff the iodine entered by wayof inhalation, the time of intake was H + 6 to H + 18. On the other hand, if water (and food) were the principat source, the time of ingestion would be extended from H + 6 to H + 51. Three items contribute to the differences in dose from the various iodine isotopes. These are: (1) radioactive decay before inhalation or oral ingeetion, (2) differences in the fission yields of the chains, 6 and (3) the average energy deposited in the thyroid per disintegration. These factore are presented in Table I for I and 1135. fn the case of inhalation, uniform distribution in the cloud was assumed. For oral ingestion it was assumed that, on the average, one-third of the intake occurred at H + 10 and twoTable L Ratio of doses for tbe two modes of intake. Inhalation Oral Ingestion The dose to the thyroid in rads from all three isotopes fs thue 3.4 times the dome due to 1131 alone for inhalation and 2.6 times the I 131 doso for oref ingeetion. Delay in, reaching the thyroid &er inhalation or ingestion would lower these factor. somewhat. However, the 1132 daughter of the 7~-~ TG132 has been neglected and would approximately compensate for decay of 1133 and 1135 before reaching the thyroid. We can now pc#ceed to estimate the dose to the thyroids of 3- to 4year-old girls aes&ing (1) inhalation ls the mode of inteke end (2) orel

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1. Inhalation. ‘I%a ratio of volume of air respired by a 3- to 4-yearold girl to that of en adult can be estiftmted in two ways: (a) from the respiration rate.’ Both methods give a ratio of lbout 0.9. The thyroid burden of these children would then bc about 3.4 uC1 with a range of 1.7 to Assuming the Rom@ap chfldren arw similar to those of New York children. the maas of tbe thyrotd of the children is 2.S * 0.6 grams. 9 The mo~”probable dose from 1131 i= then 150 rad ad the dose from ail ieotopes if 510 rd. Ifwe consider the range of thyroid burden (1.7 to 6.6 pCi) and the variation in thyroid weigM (1.9 to s.] gr~s), the dose is in the range of 200 to 1350 rad. 2. Orai Ingestion. At the time of the lvent, the Rongelep people were on l water ration of 1 pint per day. They were wwwed not to drink water after the event, but most of them admitted they drank water lnyway. 10 The method of collecting water by r,unoff from the roofs into cisterns makes it very likely that this waa the main source of oral ingestion. There are reports that it “rained a little” on the afternoon of March 1 (D-Day). tie village dactor reported that the “water turned yellow.” As far ls food is concerned. the moat likely source is dried fish. Fish were dried on open racks. However, in the interviews none of timomlisted dried fish ls having been eaten during the time before evacuation. Under these circumstances it is reasonable to asaume that chiidren drank the same amount of water and, therefore, had the same intake as adults; i. e., their thyroid burdens The most probable dose from I i.a than 490 rad and the total dose 1270 rad. Considering a range in the tii>~oid burden (5.6 to 22.4 PCi) and l thyroid weight range of 1.9 to 3.1 grams, the range of total dose ia 520 to Incidentally, LIASL. aammed this mode of intake end calculated l dose of 150 rad.2 The thyroid weight used waa not given, but waa probably 20 greme. We would calculate 160 rad, in very good agreement with the LASL Thyroid doee (rade) to Rongelap gi l lgee 3 to 4. Inhalation I Oral Ingewtinn The actual intake was undoubtedly l combination of the two modes of intake. The most probable dose is, therefore, in the range 700 to 1400 red. Fallout Material and Determination bf the Dose of Radiation, ” Chapter I Of TID - 5358, Some Effects of Ionizing Radiation on Human Beinge. 2P. Harris, personal communication. cited by both References 3 and 4. Deposition of Radionuclides in Human Beings and Animals,” Chapter V of 4Cohn, Rinehart, Gong, Robertson, Miine, Chapman, and Bond, “internal Radioactive Contamination of Human Beings Accidentally Exposed to Radio5Ng, Ynok, private communication (1964). 6The fission yields for U 238 with high- energy neutrona ae given by Weaver, Strom. and Kileen, NRDL-TR-633, were used. 10Sharp and Chapmao. “Exposure of Marshall Islanders end Americae Military Personnel to Fallout,” WT- 936 (1957).

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CALCULATION OF URINARY RADIOIODINE EXCRETION Radioiodine appearing in urlnq except for that during a relatively short period following exposuro, origlnatea from the thyroid. fn the calculation for urinary radioiodlna it wee asaumed that iodine is released from the thyroid onty aa thyroxine, and that the release of thyroxine and ita mdJaequent degradation in the extrathyroidal hormonal apace can be adequately half-life of iodine in the thyroid and normal turnover rate of extrathyroidel thyroxine were selected from the best available data in the literature. The uptake of radioiodlne waa lssumed to be exponential with a half-period of increase of 4,5 hours, end 60% of the iodine released when extrathyroidal thyroxine is degraded WWI assumed to be excreted in urine. Ratee of urinary radioiodine excretion were calculated from the resulting expression shown below. L = rate of turnover of extrathyroidal thyroxine K = rate constant for the release of iodine from the thyroid %-f = peak radioiodine content of the thyroid X = phyeiccd decay constant for 1131 J = rate constant for the uptake of radioiodine in the thyroid A eummary of the calculationfi made to determine the normal range of urinary radiobdtne excretion lt 15 days eppeam below. Urinary radiolodlne excretion at 15 daya. On the baeis of these calculations, the normal range of urinary ra~oiodine excretion at 15 days was estimated to be 0.05 to 0.2~ of the pea The problem outlined above is a practical application of an analytical study on the uptake and excretion of iodine in man which will be described more fully in a UCRL report entitled, “The Dynamics of Iodine in Man.’t