HANFORD ANIMAL STUDIES OF RADIOIODINE

Very early in the Hanford nuclear project, I-131 was recognised as a k ey radionuclide whose release from the nuclear fuel cycle would need t o be closely controlled and monitored. Later, concern emerged for I-13 1 as a significant fallout radionuclide from nuclear weapons testing. To provide a scientific basis for controlling exposures, a major multi faceted research programme was conducted at Hanford. A major portion o f the research programme focused on evaluating the radiotoxicity of I- 131 to domestic sheep because of their likelihood of receiving radioio dine exposure via ingestion of contaminated forage. The sheep studies involved multiple exposure levels and multigenerational studies conduc ted over a period of years and defined the effects of I-131 from expos ures producing acute effects down to minimal-effect levels and no-obse rved-effect levels. Other studies examined the influence of route of e xposure (oral, inhalation, intravenous, subcutaneous, and dermal) on t he disposition of I-131. I, all cases, the patterns of uptake and dist ribution were quite similar, with rapid accumulation of I-131 in the t hryoid. Another study in which dairy cattle ingested I-131 established the relationship among levels in the cow's diet, cow's thyroid, milk, and the thyroid of humans drinking the milk. Although the research wa s conducted over 30 years ago, it was innovative in its experimental d esign judged by today's standards and yielded results that will contin ue to be of substantial value in assessing the risks to animals and hu mans of I-131 exposure as long as such concerns exist.