Aspects temporels de l' impact de rejets radioactifs effectues en mer, sur les eaux d' une station littorale de la Manche

Time-dependent impact of seaward radioactive discharges on waters of one coastal station of the English Channel. The disposai at sea of liquid waste from the nuclear fuel reprocessing plant at La Hague (CEA/Cogema) causes modifications to the radioactivity of marine waters in both space and time. Fluctuations in this radioactivity are controlled by many different parameters; to study these parameters, samples were taken, in particular, from a coastal site situated 5 km from the outlet pipe. The overall impact of these releases on the radioactivity of sea-water depends on various factors, including the amount and frequency of discharges, the hydrodynamic regime of water masses in the region and local meteorological conditions. In order to assess this impact, use is made of numerous samples collected over the period 1985-1989. The radioactivity of these waters is expressed in terms of the activity level (in Bq.l-1) normalized to a standard release rate of 1 Bq per day. This overall value may be considered as a satisfactory indicator of the degree of effluent dispersal as well as the general quality of the environment. However, it is not always strictly applicable to the radioecological interpretation of phenomena which occur at a given moment in time. The present study shows that the effect of discharges on sea-water radioactivity may manifest itself on several time-scales. Two different stages are identified here; the first is short-term in nature (of the order of two weeks after release into the environment), whereas the second is clearly delayed and associated with movements of water masses occurring over an extended period. The impact observed on the short term (< 10-13 Bq.l-1 per Bq day-1 discharges) is in close agreement with the forecasts made on the basis of tracer-dye studies before the construction of the plant. By contrast, the long-term impact may turn out to be greater than predicted; this effect is even more pronounced when the radionuclide under consideration has a long half-life (e. g. 4 x 10-13 Bq.l-1 per Bq day-1 of 137Cs discharged; half-life: thirty years). In order better to understand the mechanisms controlling the delayed environmental impact, it is necessary to establish the role of the different parameters involved, particularly the long-term recycling of water masses. An approach is proposed here with the object of drawing up a plan to conduct the required research.