Effect of Fallout on Measurement of Iodine-131 Around Nuclear Reactors

In early July 1974, ¹³¹I produced by detonation of a nuclear device by the People’s Republic of China appeared in fallout over New York State. Radioiodine levels in milk were measured using a ß-γ coincidence system with a sensitivity of 0.02 pCi/liter. Peak levels of 1.6 pCi/liter of milk in early July tapered off to ∼0.1 pCi/liter by early October. When fresh pasture growth ceased and supplemental feed was provided, radioiodine was no longer detectable. This episode shows that operators of light-water power reactors must analyze background samples collected some distance from the reactor site to meet the present U.S. Nuclear Regulatory Commission analytical requirements for ¹³¹I as expressed in Appendix I to 10CFR50 and Regulatory Guide 1.42.