B.D.SMITH, G.J.HUNT and W.C.CAMPLIN CEFAS Lowestoft Laboratory Lowestoft NR33 0HT United Kingdom Revision 1

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1 CHANGES IN IMPACTS OF LIQUID RADIOACTIVE WASTE DISCHARGES FROM SELLAFIELD TO THE IRISH SEA DURING THE 1990s AS A RESULT OF NEW PLANT AND AUTHORISATION REVISION. B.D.SMITH, G.J.HUNT and W.C.CAMPLIN CEFAS Lowestoft Laboratory Lowestoft NR33 0HT United Kingdom INTRODUCTION British Nuclear Fuels plc (BNFL) and its predecessors have been reprocessing nuclear fuel for nearly five decades at Sellafield in west Cumbria, UK. Operations generate low-level liquid radioactive wastes which are discharged into the Irish Sea, such discharges requiring authorisation under UK law. For the period considered by this paper (1990 to 1998), two authorisations have been in force. The first was a variation in January 1990 to the authorisation granted in 1986 under the Radioactive Substances Act 1960, and the second authorisation came into effect in January 1994 under the Radioactive Substances Act Both were issued by the Ministry of Agriculture, Fisheries and Food (MAFF) and Her Majesty s Inspectorate of Pollution (HMIP). Under the terms of the Environment Act 1995, authorisations in England are now granted by the Environment Agency (EA) which subsumed HMIP, and MAFF is a Statutory Consultee. Table 1 shows the annual limits in these authorisations in simplified form. Radionuclide 1990 Variation Limit (TBq year -1 ) 1994 Revision 1 Limit (TBq year -1 ) 3 H C Co Sr (48) 95 Zr / 95 Nb (9) 99 Tc Ru (63) 129 I Cs Cs Ce (8) Pu-alpha 7 4 (0.7) 241 Pu (27) 241 Am (0.3) total alpha (1) total beta Figures in parentheses applied from 1 January 1995, after commissioning of EARP. Table 1. Authorised annual liquid discharge limits for BNFL Sellafield. 1

2 More details of the 1994 revision of the Sellafield liquid authorisation have been published (2), but in summary it took account of the new radiological protection criteria (1) and a number of new developments on the Sellafield site. These developments included: (a) (b) (c) (d) (e) proposed start-up of the Thermal Oxide Reprocessing Plant (THORP) to reprocess oxide fuel from the UK s advanced gas-cooled reactors (AGRs), and UK and overseas water-cooled reactors; operation of the Enhanced Actinide Removal Plant (EARP) for improved treatment of routine and decaystored waste streams mainly from the older, Magnox reprocessing plant. diversion of waste containing 14 C from a gaseous discharge stream to a liquid waste stream in order to reduce its overall radiological impact; proposals for greater throughput of Magnox fuel and at higher levels of irradiation in reactors; and proposals to continue decommissioning and waste management strategies particularly aimed at waste accumulations and redundant plants. The 1994 authorisation reduced discharge limits of nuclides of greatest radiological significance (such as 106 Ru, 137 Cs and the transuranics) which it was anticipated EARP would remove, and increased limits for some nuclides of low radiological significance to allow the other developments. Thus, additional 3 H and 129 I would be produced by THORP, additional 90 Sr and 99 Tc from processing of decay-stored liquid waste concentrates through EARP, additional 14 C discharges from the diversion scheme described in (c), and additional 60 Co from decommissioning activities. It was anticipated that the new authorisation would reduce the overall impact of discharges compared with the previous authorisation. CHANGES IN DISCHARGES FROM 1990 TO 1998 The trends in discharges of selected radionuclides from are shown as the histograms in Figures 1-8. Radiocaesium discharges (Figure 1) decreased over the period, partly because EARP began commissioning in 1994, but also because of improved performance of the site ion-exchange effluent plant (SIXEP) which treats effluents from fuel element storage ponds. Typically, annual discharges are now less than 10 TBq whereas they were up to 5000 TBq in the mid-1970s. Carbon-14 discharges (Figure 2) increased, as expected, with the rerouting from a gaseous waste stream into a liquid one. A maximum of 12 TBq was discharged in 1995, but since then, annual discharges have been tending back towards those before the diversion at less than 4 TBq. Discharges of 90 Sr (Figure 3) have increased from less than 5 TBq to a maximum of 38 TBq in 1997, particularly as a result of processing decay-stored liquors in EARP from 1994; discharges also depend on SIXEP performance. Discharges of 106 Ru (Figure 4), 239 Pu Pu (Figure 5) and 241 Am (Figure 6) all decreased as a result of EARP operations from 1994, those of the transuranics by at least an order of magnitude. Increased discharges of 60 Co (Figure 7) from about 0.1 TBq y -1 for to 2.5 TBq in 1998 have been due primarily to operations in the THORP fuel feed pond. Processing of stored wastes through EARP have led to substantial increases in discharges of 99 Tc from a pre-existing level of less than 10 TBq y -1 to a maximum of 192 TBq in Since then, discharges have reduced (to 53 TBq in 1998). Figure 8 shows the monthly variations in 99 Tc discharges for to allow more detailed comparison with the results of environmental monitoring. CONCENTRATIONS IN BIOTA On behalf of MAFF, CEFAS carries out a comprehensive environmental monitoring programme for radioactivity in the marine environment of the British Isles. This programme is kept under review to take account of any factors such as changes in discharges, environmental conditions and the habits of critical groups. The results of this monitoring are published annually (3), and selected results from near Sellafield are presented with the discharge data in figures 1-8. Sampling and analytical uncertainties are described in the annual reports; for the purposes of this paper, the concentration data have representative ±10% error bars. Figure 1 presents annual mean concentrations of 137 Cs in plaice (Pleuronectes platessa) and cod (Gadus morhua). These species are of significance to the critical group of fish and shellfish consumers. Figure 1 shows that 137 Cs concentrations in fish are continuing a reducing trend. In the early 1990s, concentrations reductions appear more rapid than discharge reductions, probably reflecting continuing reductions in concentrations from the late 1970s and early 1980s when discharges were much higher. Later in the 1990s, concentrations reflect 2

3 discharges more closely. Throughout this period, there may also be a contribution from remobilisation of 137 Cs adsorbed on sediments (4). Figure 2 presents annual mean concentrations of 14 C in plaice and cod. A natural background component of 26 Bq kg -1, estimated by comparison with 14 C concentrations in these species at distance from known sources, has been subtracted. In contrast to a 6-fold increase in discharges over the period, concentrations have only doubled, and in the late 1990s are now reducing with the discharges. This suggests a much slower rate of uptake by the fish than the rate of discharge increases. A similar conclusion could be drawn for 90 Sr in plaice and cod (Figure 3) where an almost ten-fold increase in discharges appears to give only a relatively small increase in concentrations. However, the observation may be linked to a residual effect of much higher 90 Sr concentrations in the Irish Sea environment in the 1980s when discharges were higher. Figures 4, 5 and 6 present concentrations of 106 Ru, 239 Pu Pu and 241 Am in winkles (Littorina littorea) from Nethertown, 5 km north of Sellafield. Like plaice and cod, winkles are of significance to the critical group of seafood consumers. In each case, reductions in discharges and concentrations have occurred over the period. However, whilst concentrations of 106 Ru have reduced mostly in line with the discharges, reductions in concentrations of the transuranic nuclides have been much less than the relevant discharge reductions. These observations are consistent with the known adsorption of transuranic radionuclides on sediments and the effect on concentrations near Sellafield of higher discharges in the 1970s and 1980s. Concentrations of 60 Co in winkles from Nethertown are plotted in Figure 7. The effect of the increased discharges in the late 1990s is apparent, concentrations generally reflecting discharge increases. Discharges during the early 1990s were much lower and fairly stable, but concentrations, though also low, reduced progressively until This reduction is likely to have been the residual effect of higher 60 Co discharges during the 1980s. Figure 8 shows quarterly concentration data for 99 Tc in lobsters (Homarus gammarus) chosen on the basis of radiological significance, and in the seaweed Fucus vesiculosus because of its strong uptake of 99 Tc and indicator potential. This seaweed is not eaten but it can be used as a fertiliser. As would be expected because of dilution effects, seaweed at St Bees, 10 km north of Sellafield, shows lower concentrations than seaweed close to Sellafield. Generally though, the trends in concentrations at the two locations mirror each other. The trend in lobster concentrations is dissimilar to that in seaweed, peak concentrations in one occurring at different times to the other. However, concentrations of 99 Tc in lobster appear to be declining following the decreases in discharges since Detailed interpretation of the results is complicated by the varying discharge rates, particularly after 1996, and the observed patterns may also be influenced by local hydrographic conditions and variations in seasonal uptake rates. RADIOLOGICAL SIGNIFICANCE Radiation doses to members of the public from Sellafield discharges are kept under close review. Potential critical pathways are high-rate consumption of local fish and shellfish, and external exposure through bait digging or living on boats which settle on sediments at low tide (3). The critical group (i.e. the most exposed) has changed in recent years, depending upon the habits of the people comprising the groups and upon the radioactivity concentrations or external dose rates. Recent changes in discharge patterns are usefully studied through doses from high-rate consumption of fish and shellfish. Figure 9 shows effective doses (1) for with the contributions from different radionuclides. Total doses have remained well within the 1 msv dose limit. Variations in doses are due to changes in consumption rates as well as changes in radioactivity concentrations. The former are reviewed annually and summarised in Table 2. The increase in effective dose in 1998 was due to increased consumption rates, and it can be seen that the contribution from actinides has fluctuated but remains high despite the reductions in concentrations shown in figures 5 and 6. The significance of 99 Tc has also increased in line with concentration increases though that trend is now reducing with the reduced discharges in the late 1990s. Doses from 60 Co are too small to show on Figure 10. Monitoring continues so that the situation may be kept under review. 3

4 FUTURE TRENDS IN RADIOLOGICAL IMPACT The observations on discharges, concentrations in biota, habits and consequent doses described above enable some general predictions to be made about the future trends of impact through fish and shellfish consumption around Sellafield. For example, most of the annual dose over the period has been due to actinides, particularly through consumption of molluscs. However, despite very low actinide discharges, concentrations in biota are only gradually declining because of the legacy of past discharges absorbed onto sediments. Doses from these radionuclides can therefore only be expected to reduce slowly, and will be largely dictated by changing mollusc consumption rates. The next main contributor to dose has been radiocaesium, particularly from consumption of fish. Reducing discharges leading to reduced concentrations have been reflected in reducing contributions to dose, and this trend would be expected to continue. However, remobilisation of historic radiocaesium discharges from local sediments may distort this trend. In recent years, the contribution of 99 Tc to overall doses has increased, particularly through consumption of crustaceans, and reflects increased discharges. Discharges are now being reduced and a new lower discharge limit (90 TBq year -1 ) came into effect in January The contribution of 99 Tc to doses can be expected also to reduce, though any increases in shellfish consumption would counter this. Contributions to dose from the remaining nuclides mentioned are small and reflect changes in concentrations or habits. This feature seems likely to continue, nothing that increased discharges of 14 C and 90 Sr did not lead to comparable increases in concentrations. REFERENCES 1. ICRP Recommendations of the International Commission on Radiological Protection. Annals ICRP 21(1-3) (1991). 2. Smith, B.D., Hunt, G.J., Porter, I.T. and Tipping, J. Revision of Sellafield s Authorisation for Liquid Radioactive Waste Discharges: A Brief Account of Technical Experience. Proc. Regional. Congr. Int. Rad. Prot. Ass. Portsmouth Nuclear Technology Publishing, Ashford (1994). 3. MAFF/SEPA. Radionuclides in Food and the Environment, MAFF and SEPA, London (1999). 4. Hunt, G.J. and Kershaw, P.J. Remobilisation of Artificial Radionuclides from the Sediment of the Irish Sea. J. Radiol. Prot. 10(2) (1990). 4

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9 P-4b-240 Year FISH CRUSTACEANS MOLLUSCS Consumption Species rate, kg/year Consumption rate, kg/year Consumption rate, kg/year Species (25%:75%) (50%:50%) 28 (75%:25%) (60%:40%) crabs, lobsters & nephrops (50%:40%:10%) (85%:15%) Species winkles winkles molluscs molluscs molluscs molluscs molluscs (60%:40%) molluscs (40%:60%) molluscs (30%:70%) Data taken from MAFF/SEPA (3) reports Table 2 Summary of data on consumption rates by the group of high-rate fish and shellfish consumers near Sellafield. 9