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An assessment of the availability of Tc-99 to marine foodstuffs from contaminated sediments

Project Code: R01062

31/03/2008

Centre for Environment, Fisheries & Agriculture Science (CEFAS) Lowestoft
Leonard, K ; McCubbin, D; Jenkinson, S; Bonfield, R; McMeekan, I

Technetium-99 is a long-lived (t 2.13 x 105years) beta-emitting radionuclide and is an increasingly important component of high-level radioactive wastes. It is also present in the liquid effluent that has been discharged, under authorisation, into the north-east Irish Sea from the British Nuclear Fuels (BNF) reprocessing plant at Sellafield (Cumbria, UK) for over 50 years.

The enhanced ⁹⁹Tc discharges to the Irish Sea between 1994 and 2003 have been the subject of national and international controversy, resulting from the perceived contamination of regional fisheries. The impact upon the Norway lobster, Nephrops norvegicus, is of particular concern given i) the relatively high bioaccumulation by this species, and ii) that it constitutes a most valuable fishery in the Irish Sea. Results from previous Food Standards Agency (FSA) projects (FSA, 2001) have provided preliminary indications of measurable ⁹⁹Tc uptake in Irish Sea sub-tidal sediments. The aim of the present project was to more fully investigate seabed accumulation and to assess the implications of seafood contamination. This was achieved via i) a program of sediment core sampling to estimate the inventory of ⁹⁹Tc currently residing in the sub-tital sediments of the Irish Sea, and ii) tuning, validating and applying a multi-compartmental model to estimate the rate and extent of re-dissolution of ⁹⁹TC in the overlying seawater, and hence the availability of ⁹⁹Tc to marine foodstuffs.

The inventory of ⁹⁹Tc currently residing in the sub-tidal sediments of the Irish Sea is estimated to be of the order of 37 TBq. The multi-compartmental model was optimised and validated using our detailed long-term datasets for ¹³⁷Cs, given that remobilisation is known to provide the major contribution towards present levels of this radionuclide in seawater and seafood. Further effort (outside the scope of the present study) was however identified as being necessary to improve the simulation of vertical mixing processes within the seabed. Technetium-99 seawater data, derived from model simulations, have subsequently been used to estimate levels in seafoods (and hence the dose to seafood consumers) using the concentration factor (CF) method. The impact of ⁹⁹Tc remobilisation from sediments upon levels in the water column is predicted to be minimal in the short to medium term (up to 2012). CF values for accumulation between individual species of shellfish are highly variable and complicate the comparison of environmental data and model simulations. Although predicted doses are slightly lower than those provided in recent RIFE reports, the disparity is relatively small (< 3 fold) given the errors associated with both sets of values. It seems reasonable to conclude that discharges from Sellafield (hence levels in fish and shellfish in the Irish Sea) are likely to remain broadly similar to those in 2005 in the short to medium term (up to 2012). Consequently, levels in Nephrops are likely to persist at ~100 Bq/kg wet weight and ~10 Bq/kg wet weight in the eastern and western Irish Sea, respectively.

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