Physico-chemical characterisation of radionuclides discharged from a nuclear establishment

Low-level liquid radioactive wastes are routinely discharged, under authorisation, from nuclear establishments into UK coastal waters. Samples from two separate effluent streams (SIXEP and seatank) arising from the British Nuclear Fuels plc reprocessing plant at Sellafield have been collected on a number of occasions to compare differences in their physico-chemical composition. The most significant amounts of radionuclides in SIXEP and seatank effluents were observed in the solution and particulate phases, respectively. Laboratory experiments have been carried out to determine the colloidal size distribution of a suite of radionuclides in each of the effluent samples using the ultrafiltration technique. Results suggest that colloidal forms of individual radionuclides, originating from the solution phase, are more likely to occur in SIXEP rather than in seatank effluent. Additional size fractionation experiments were carried out to investigate the likely persistence of radiocolloids following dilution of effluents into seawater. The extent of radionuclides associated with colloids in SIXEP effluent appeared to be reduced following dilution. Particulate material originating from seatank effluent, upon dilution, was also identified as a source (albeit small) of colloidally bound radionuclides. Having accounted for differences between effluent streams, including discharge levels, the contribution of individual physico-chemical forms of radionuclides from the Sellafield source is discussed.