Plutonium from Mayak: Measurement of isotope ratios and activities using accelerator mass spectrometry

Accelerator mass spectrometry (AMS) has been used to measure Pu activities and Pu-240/Pu-239 isotope ratios in samples contaminated by releases from t he Mayak nuclear installation. Determination of Pu isotopes in high-level s amples indicated that the ratio of Pu-240/Pu-239 in waste has increased tow ard the present. The lowest Pu-240/Pu-239 atom ratios, 0.012-0.024, were fo und at the Asanov Swamp, where the primary source of contamination was disc harge of intermediate-level radioactive waste between 1949 and 1951. The hi ghest ratios, 0.06-0.29, were found in industrial reservoirs contaminated b y various sources of waste up to the present day. Measurement of Pu isotope s in low-level samples collected from the Techa, Iset, and Ob Rivers showed that while activity levels decrease with distance from Mayak-from 2000 Bq/ kg at 7 km downstream to less than 1 Bq/kg sediment at 250 km-Pu-240/Pu-239 isotope ratios increase. Results suggest that most of the plutonium in the Upper Techa River originates from the early waste discharges, although enh anced atom ratios in surface sediments downstream (0.035-0.099) indicate a contribution from other sources. On the basis of procedural blanks, detecti on limits for AMS were below 1 fg of Pu.