1 The environmental impact of the Sellafield discharges Luis León Vintró * , Kilian J. Smith, Julie A. Lucey and Peter I. Mitchell Department of Experimental Physics, University College Dublin, Belfield, Dublin 4, Ireland 1. Introduction Low-level liquid radioactive wastes from the British Nuclear Fuels pcl (BNFL) Sellafield establishment (formerly known as the Windscale and Calder Works) have been discharged into the north-eastern Irish Sea since operations began in the early 1950s. Although, at first, the major source of these discharges was the processing of nuclear fuel for the production of nuclear weapons, operations since the late 1950s have been dominated by the reprocessing of fuel from commercial nuclear power programs. The most important low-level radioactive wastes from Sellafield arise both in water used to purge the cooling ponds in which spent fuel elements (Magnox) are kept, and from the reprocessing plant whose low-level liquid waste are collected and neutralised in 'sea tanks' before being discharged around high water (Kershaw et al., 1992). Discharges take place, under authorisation, through a series of pipelines extending 2.5 km seaward of the high water mark, with a very much smaller level of activity being discharged via the site's sewer (Jackson et al., 2000). Discharges from Sellafield have always been subject to controls by regulatory governmental departments or agencies in the UK. Currently, authorisation to discharge is granted to BNFL by the Environment Agency under the Radioactive Substances Act, 1993 (United Kingdom – Parliament, 1993). Over the years, the Sellafield discharges have acted as a large point source of a complex amalgam of α –, β – and γ–emitting radionuclides originating from both fission and neutron activation processes, and resulted in significant increases of artificial radionuclide concentrations in a variety of environmental compartments throughout the Irish Sea and beyond, with minute traces even detectable as far away as the Central Arctic Ocean. The aim of this overview is to present the current state of knowledge on the environmental impact of the Sellafield discharges. This, of course, mainly depends on the way in which discharged radionuclides are dispersed and accumulated on different key environmental compartments upon their introduction into the Irish Sea. The spatial and temporal distributions of Sellafield- derived radionuclides is governed by a number of complex mechanisms. Some important factors which influence their behaviour include the rate of the input from authorised discharges, their chemical speciation on the effluent and upon contact with sea water, prevailing hydrographic conditions, and interactions with suspended particles, sediments and biota. In this paper, the effects of these factors on the environmental impact of selected radiologically important radionuclides is emphasised, and the manner in which this knowledge can be used to predict future trends highlighted. Discussion is mainly limited to 137 Cs and transuranium nuclides, not only because they have been the subject of the most intensive study over the years, but also as representative of radionuclides exhibiting conservative and non-conservative behaviour. When appropriate, however, reference is made to other radionuclides. * Corresponding author: Tel.: +353–1–706 2220; fax: +353–1–283 7275; e-mail: luis.leon@ucd.ie