Long-term Availability of Sr-90 in Foodstuff after Nuclear Fallout

INTRODUCTION Among the many radionuclides possibly released in a severe reactor accident or a nuclear weapon's detonation Cs and Sr are of particular importance for the long-term exposure. Due to their long physical halflife of 30 and 28.5 years, respectively, the exposure of the population by these nuclides will last for decades and has to be taken into account in estimating the total 50 year-exposure caused by the fallout and in evaluating countermeasures with regard to long-term land utilization. However, as already observed after the weapons' test fallout, their effective decrease in the environment with regard to the population exposure follows a much shorter effective half-life reducing the exposure of the public much quicker than estimated from the physical half-life (1,2). Sr contributed to more than 80 % of the exposure after the atmospheric nuclear weapons' tests and also to a significant extent in past accidents. The long-term exposure of a nuclear fallout by this radionuclide therefore, is of particular importance. However, while in the past the long-term bio-availability of Cs was investigated (3,4), the long-term decrease of Sr was not comparably investigated. It should be noted that the observation of the effective half-life after a short-term nuclear fallout was not easily obtainable after the weapons' tests due to the fact that repeated detonations occurred for a period of several years and the dispersion occurred to the upper atmosphere from where the fallout continued for a long period. The atmospheric deposition, therefore, occurred for several years and blurred the observation of the long-term decrease as caused by the much lower soil-plant transfer. The long-term availability of Sr in milk, therefore, was investigated as a main indicator of the environmental behavior of this radionuclide and as the most important foodstuff with regard to the Sr-intake of both the infant and the adult. An estimate of the dose to infants and the adult in central Europe 35 years after the end of atmospheric nuclear weapons testing and 14 years after the reactor accident in Chernobyl is presented and compared to the dose of other long-lived radionuclides in the environment. Since Cs is easier to determine and therefore often used as main indicator for the contamination of the environment, ratios of Sr to Cs are given for the period after the weapons tests and after the Chernobyl accident up to the present time. The variation in this ratio with time and its causes as well as the differences in environmental decay of these two radionuclides are discussed.