Deposition of actinides in the vicinity of Sellafield, Cumbria: Accounting for historical discharges to atmosphere from the plant

Soil core samples from 95 locations within a few kilometres of the Sellafield nuclear complex have been used to assess the cumulative deposition of 238Pu, 239,240Pu, 241Pu and 241Am in the vicinity of the plant. The deposition pattern is characterised by high levels of 239,240Pu (up to 20 000 Bq m−2) within a few hundred metres of the site, declining rapidly to much lower levels (∼1000 Bq m−2) within 3 km. The activity ratio of 238Pu to 239,240Pu associated with the highest levels of deposition is in the range 0.02–0.05, indicating that the plutonium originated from uranium of low irradiation and suggesting that the deposition occurred during the early years of operation of the plant. 241Am deposition levels correlate strongly with 239,240Pu and the average activity ratio of 241Am to 239,240Pu, at 0.2, is consistent with ingrowth from deposited 241Pu. Deposition of 238Pu and 239,240Pu at distances >2 km from the plant is consistent with that calculated from the cumulative recorded emissions to atmosphere from the plant, together with sea to land transfer and weapons testing fallout. To explain the high levels of deposition close to the site, it is necessary to assume an additional cumulative release of ∼70 GBq with a high deposition velocity. The spatial pattern of deposition indicates that some of this additional release is associated with a known incident involving the Calder cooling towers in 1961; a further source in the northern part of the chemical separation area is also indicated.