In June 1978, the Meteorology Division at Lawrence Livermore National Labo- ratory was subcontracted to provide a computer simulation of the dispersion, transport, and deposition of fallout from the 1955 atmospheric nuclear test, BRAVO. A subcontract to provide neutron activation analysis of archival soil © samples was given to the Radiological Sciences Department, Battelle-Pacific Northwest Laboratory. Soil samples were provided by Seymour, Director of the University of Washington's Laboratory of Radiation Ecology. During 1980, members of BNL researched the protracted exposure to fallout at Rongelap and Utirik Atolls. The interval of interest was from the time each: population returned to their BOs, “ioe 2, bS2to 50 years later. The nuclides considered were 137¢s, 60¢5, and. 239pu. Thyroid absorbed dose from these sources was ne leitae” celnlve to " the thyroid dose committed during the first few days after the accidental exposure (Le84). The subject of this report is the estimation of thyroid absorbed dose due to fallout exposure of the inhabitants of Rongelap, Utirik, and Sifo Islands on March 1, 1954. To determine thyroid dose, the amount of fallout activity taken into the body was estimated by reexamining the 13ly excreted from persons who were at Rongelap. The other components of fallout taken into the body had to be inferred from studies on fallout composition. Initially, fallout composition was assumed and nuclide activity concentrations in air, water, and food were established on the basis of meteorological and archival soil study results. fFurther study led to dose estimates based on actual BRAVO fallout composition rather than hypothetical composition. Finally, knowledge was gathered about the intake pathway and the time post-detonation at which intake was likely to have occurred, and this was factored into the thyroid absorbed dose estimates. The report was prepared under the authorization of the Department of Energy's (DOE) Division of Biological and Environmental Research, which provided funding and review from 1978 until 1983. After organizational changes at DOE in 1983, funding and review were provided under the DOE Office of Military Application. The purpose of the study was to clarify or document further the relationship between thyroid absorbed dose and incidence of thyroid nodules or thyroid cancer. The high incidence of benign and cancerous thyroid lesions was very evi- dent (Co74). Our efforts were directed towards reevaluation of thyroid absorbed dose estimates upon which Conard's risk estimates were based. The limitations for applying the risk estimated here to other exposed groups include the following: 1) thyroid dose estimates have a large standard error, 2) thyroid dose estimates apply to a unique situation involving ingestion of fallout plus external irradiation, and 3) the medical observations quoted are mot infallible, that is, a reevaulation of medical results may result in reclassifications of thyroid lesions, or reveal other cancer sites, or addi- tional thyroid lesions. The sources of information were many and varied. Discussions with persons initially engaged in these studies, e.g., Stanton Cohn, Victor Bond, and Eugene Cronkite,’ led to review of documents which are cited in the references of this an, %oos : ad te. 7 Ae 4 wh 14 b Of tN 3 Re fi k s