Artificial radionuclides in the Irish Sea from Sellafield: remobilisation revisited

Measured concentrations of Cs-137, tritium, Tc-99, Pu-239+240 and Am-241 in representative materials from the Irish Sea were investigated with reference to continuing remobilisation from sediments. Long time series of monitoring data since the 1960s were employed. Cs-137 in sea water and fish shows peaks in concentrations normalised to discharge rate (NACs) from 1985 to 1989. This is consistent with the time needed for dispersion in sea water following the preceding reductions in discharges; continuing enhancements of NACs above pre-1970s levels follow, consistent with the effect of activity remobilised from sediment. It is estimated that about 300 TBq of Cs-137 was remobilised from the immediate tidal area around Sellafield from 1989 to 2009. The enhancements in concentrations continue to this day, with the effect of remobilisation at present being ∼6 TBq y−1, approximately doubling the effect of direct discharges. To provide an indication for the future, the rate of Cs-137 remobilisation is decreasing with a half-time of ∼6 years. The data for tritium and Tc-99 were examined in view of the interest in these radionuclides. The concentrations broadly reflect the levels of discharges and the need for dispersion. As expected, there is no evidence of sustained remobilisation of tritium, due to its mobility (or low Kd). The same lack of evidence was found to apply for Tc-99 despite known sorption of a small proportion of the discharged activity by Irish Sea sediments. Pu-239+240, by contrast, shows much evidence of the effect of remobilisation; concentrations in sea water near Sellafield have reduced much more slowly than discharges. At Southerness, ∼50 km away, there was no significant reduction in sea water concentrations from 1985 to 1996, and winkles showed an increase then decrease in concentrations over this period, consistent with a spreading of activity. This effect was replicated in mud at Garlieston, ∼70 km from Sellafield. For Am-241, the rate of grow-in from Pu-241 has dominated direct discharges since the late 1970s. Grow-in continues today in the Irish Sea at the rate of ∼8 TBq y−1, ∼200 times the rate of direct discharge. Winkles at Southerness show evidence of a spreading effect of Am-241, with an increase then decrease from 1985 to 1996. At Garlieston there was an increase in concentrations in mud from 1985 to 1997, and at Carlingford in Northern Ireland the concentration of Am-241 in mud appears to be increasing still. This effect of the spread of activity away from Sellafield may continue, at least in the near future. (paper)