The long-term decrease of 90Sr availability in the environment and its transfer to man after a nuclear fallout

Due to its long physical half-life, and the fact that its long-term mobility in the environment as well as its radiotoxicity is higher than that of Cs-137, the long-term bio-availability of Sr-90 in the environment is of importance with regard to the long-term population exposure after fallout from nuclear weapons detonations or a severe reactor accident. It will also substantially influence the time-span required until re-utilisation of highly contaminated territory is possible again. An assessment of the longs term decrease of the activity concentration in all foodstuffs relevant for internal exposure after severe 90Sr fallout was performed. The observed effective half-lives were approximately 1.8-2.1 years in the first 2-3 years after the end of fallout and 8-10 years in the following three decades. This is equivalent to a biological half-life of about 13.2 years and results in a total 50 year dose of 6.2 times the first year exposure. Due to this decline in Sr-90-availability, the average annual activity intake of 90Sr in Austria has decreased from 840 Bq at the climax of the nuclear weapons tests to about 42 Bq in 1997 for adults, and from 500 Bq to about 35 Bq for 1 year old infants. This is equivalent to a 90Sr ingestion dose of 1.2 mu Sv for adults and 2.5 mu Sv for 1 year old infants in 1997 or less than 0.4% of the ingestion dose by natural radionuclides in the diet.