Publication Abstract

Further studies of the distribution of 99Tc and 137Cs in UK and European coastal waters

D. McCubbin, K.S. Leonard, J. Brown. P.J. Kershaw, R.A. Bonfield and T. Peak

Data on the temporal and spatial variations of ⁹⁹Tc and ¹³⁷Cs in UK coastal waters are provided from surveys carried out over the period 1993–1998; prior to and post the discharge of elevated quantities of ⁹⁹Tc from the nuclear fuel reprocessing plant at Sella.eld (Cumbria, UK). ⁹⁹Tc seawater concentrations in the Irish Sea increased by more than an order of magnitude within the study period concomitant with the increase in discharges. ¹³⁷Cs discharges were relatively low (~0.3% of their peak values in the 1970s). Simplistically, the ¹³⁷Cs distribution re.ected varying hydrographic conditions superimposed upon the residual concentrations of a system approaching steady state. Differences between the ¹³⁷Cs/⁹⁹Tc ratio in Sella.eld discharges and seawater indicate that ¹³⁷Cs remobilisation, from sediments contaminated by large discharges in the 1970s, is presently the predominant (~90%) source term to the water column.

The ¹³⁷Cs/⁹⁹Tc ratio in the Irish Sea decreased signi.cantly within the period of these surveys (e.g. reduced from ~14 in 1993 to 0.4 in 1996, within the vicinity of Sellafield). However, it is dif.cult to extrapolate this information to assess the contribution from Sella.eld in distant waters because the low levels of ¹³⁷Cs are continuously perturbed by additional inputs during transit (e.g. remobilisation from sediments outside the Irish Sea). Examination of ⁹⁹Tc data for the North Sea indicates that the leading edge of the first EARP pulse, entering via the Scottish coastal current, may have migrated to the limit of the current .owing south along the British coastal margin within 9 months. The estimated transfer factor (TF) for Sellafield discharges in this current is 52 Bqm^-3/PBq annum^-1 with transit times from Sellafield to the Pentland Firth and Lowestoft of ~9 and ~24 months, respectively. These transit times, derived from the ⁹⁹Tc data, are signi.cantly shorter than previous estimates based on ¹³⁷Cs and ⁹⁰Sr data from the 1970s and 1980s.

Reference:

D. McCubbin, K.S. Leonard, J. Brown. P.J. Kershaw, R.A. Bonfield and T. Peak, 2002. Further studies of the distribution of 99Tc and 137Cs in UK and European coastal waters. Continental Shelf Research, 22(10): 1417-1445.