Tritium along the French coast of the English Channel

Within the scope of a joint IRSN/EDF research program, it is possible to present a first status of free water tritium and organically bound tritium levels in the French coastal marine environment, from Concarneau to Gravelines. Seasonal sampling was conducted over two years, 2001 and 2002. The matrices selected for this specific survey include water, seaweed, molluscs, crustaceans and fish. The background concentration, estimated from the results obtained at two sites, Concarneau and Roscoff, remote from industrial sources, is close to the detection limit of 1,2 Bq.l -1 . Along the Channel coast and the entrance of the North Sea, tritium is released in seawater by four nuclear power plants (14 reactors) and mainly by the reprocessing plant of La Hague. The concentrations measured are in good agreement with activities calculated with the hydrodynamic model of dispersion TRANSMER. The results confirm a dilution factor of two between the north Cotentin area and the Straits of Dover for soluble radionuclides. The concentrations are in the range of 2 to 20 Bq.l -1 for free and organically bound tritium in biota. Although isotopic fractionation is theoretically expected to be slight, the available results indicate a ratio between tritium bound to organic matter and tritium in free water greater than one.