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THE COMBINED IMPACT ON DOSES TO MAN OF MULTIPLE, AUTHORIZED, RADIONUCLIDE DISCHARGES FOR THE YEAR 1999 REACHING THE UPPER RIVER THAMES, UK

Hilton, J.; Harvey, M.; Simmonds, J.

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Before issuing a new or revised authorization to discharge liquid radioactive waste to a river, past and current practice has been to assess the impact of each discharge on a site-by-site basis. This paper reports an assessment of the combined impact of all the (year 1999) authorized discharges in the upper River Thames (UK) catchment. A modification of the PC-CREAM model, using parameters specific to the upper River Thames, was used to estimate “potential doses” to the population, using the authorized discharge activity for each of the discharges as the main driving variable. The main purpose of this study was to investigate the difference in doses between single and multiple discharges in the Thames. A partial validation of the model against measurements was also carried out as a secondary investigation. The source term of the upper Thames consisted of 90 authorized discharges for 86 sites, made up of three nuclear licensed sites, 36 industrial discharges, 25 research organizations, 20 hospitals and two miscellaneous organizations. Of the authorized discharges, nuclear sites constituted 67% of the total activity in authorizations to discharge; hospitals 16%, research organizations 13%, and industry only 3%. Three radionuclides made up 96% of the total authorized discharge activity: tritium—74%; 99mTc—15%, and 14C—7%. No other single radionuclide constituted more than 1.5% of the total activity. All of the discharges from non-nuclear sites were via a sewage works so that, in total, only 39 discrete discharges to the river were included in the model. For the purposes of the assessments, it was assumed that all discharges were at the authorized limits (rather than using actual discharge). Even with this maximizing assumption, all the calculated potential doses from the combined source terms were significantly less than the radiation dose limit of 1 mSv y−1 for a member of the public. The highest estimated potential doses were investigated further to identify the major pathways. Ingestion of fish was the main pathway in several river stretches and in the highest case constituted 97% of the dose. In a few stretches, external irradiation from the riverbank or ingestion of drinking water were the main pathways, but the potential contribution was a very small percentage (<1%) of the dose limit for members of the public. An investigation of the reasons for the relatively high potential doses resulting from fish showed that the radionuclide with the largest contribution was 32P. Three factors had a major effect on potential dose estimations in these circumstances: the fish consumption rate, the 32P concentration factor and the use of 32P in some cases to represent radionuclides authorized for discharge as “any other radionuclide, except alpha emitters.” Using the precautionary principle, all authorized activity in the latter designation was modeled as the radionuclide which would produce the highest dose to humans.

* Centre for Ecology and Hydrology, Dorset (formerly the Institute of Freshwater Ecology), Winfrith Technology Centre, Winfrith Newburgh, Dorchester, Dorset DT2 8ZD UK; National Radiological Protection Board, Chilton, Didcot, Oxfordshire OX11 0RQ UK.

For correspondence or reprints contact: J. Hilton, Centre for Ecology and Hydrology, Dorset Winfrith Technology Centre, Winfrith Newburgh, Dorchester, Dorset DT2 8ZD UK, or email at jhi@ceh.ac.uk.

(Manuscript received 2 June 2003; revised manuscript received 4 December 2003, accepted 21 March 2004)

© 2004 by the Health Physics Society