Remobilization of Sellafield-derived radionuclides and transport from the north-east Irish Sea

Plutonium-239,240 activities in seawater from the north-east Irish Sea, collected in January 1992, were in the range of 0.17–4.30 mBq l−1 in solution and 7.2–62.3 Bq kg−1 in suspended particulates. The 238Pu:239,240Pu activity ratios for the soluble fraction were consistent with those of the underlying sediments but inconsistent with contemporary discharges from Sellafield, indicating that most of the plutonium in solution was derived from re-dissolution from the sediment. The activity ratios for the particle associated material were more variable, but again, on average, typical of the sediments. The annual loss of 239,240Pu from the north-east Irish Sea in solution was estimated to be 1.19 TBq and that associated with suspended particulates to be 0.11 TBq. From this, a halving time of 350 years for the loss of 239,240Pu activity from the sediment by dissolution was determined. A similar treatment of 137Cs data yielded an annual loss of 86 TBq in solution and a halving time of 23 years. Calculations using a numerical model of the Irish Sea yielded similar estimates of current loss rates and permitted the estimation of total cumulative losses from the Irish Sea by advection through the North Channel of 7.9 TBq of 238Pu, 48 TBq of 239,240Pu 2.6 × 104 TBq of 137Cs.