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 Pu-238:Pu-239,Pu-240 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 Pu-239,Pu-240 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 Pu-239,Pu-240 activity from the sediment by dissolution was determined. A similar treatment of Cs-137 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 Pu-238, 48 TBq of Pu-239,Pu-240 and 2.6 x 10(4) TBq of Cs-137. (C) 1997 Elsevier Science Limited.