Knowledge of long-term movements of water-masses in the English Channel has been substantially improved using hydrodynamic modelling coupled with radio-tracers studies ; nevertheless, the precision of results so obtained is still largely dependent on measurement precision.New tools are now available to make more accurate determinations of radio-tracer distribution : -Repositioning of station locations at the same tide reference-time, giving a homogeneous spatial data set, coupled with the possibility of interpolating and quantifying the amounts of dissolved radioactivity flowing through the English Channel ;-The first measurements of tritium ( 3 H) in seawater on a large scale in the English Channel demonstrate that this fully conservative radionuclide is a clearly identifiable marker of industrial releases ;-Recent campaigns carried out during the FluxManche II CCE (1994) programme show the general distribution of dissolved radionuclides 137 Cs, 134 Cs, 60 Co, 125 Sb, 106 Ru and 3 H in the English Channel and the Irish Sea ;-The re-utilisation of data from previous campaigns (1983, 1986 and 1988) provides indications, at any given location in the English Channel, about the average dilution and distribution of releases derived from the La Hague reprocessing plant.Excesses and losses of radionuclides are now quantified with respect to known source-terms ; estimates of losses are provided for non-conservative radionuclides, while an excess of 137 Cs was observed in the English Channel during the period 1983 -1994. This excess which has the same order of magnitude as the quantities released from La Hague plant in the English Channel, could be explained by about 1% of Sellafield reprocessing plant releases entering the Channel.These results confirm and give a more detailed picture of the previously known distribution of water masses in the English Channel. They lead to clear information about transit times and dilution at this scale, and provide directly comparable data for the validation of hydrodynamic models.
From 1986 to 1992, 125 Sb released by La Hague Fuel Reprocessing Plant, was used to study the transfert of waters from mid-Channel areas towards the Dover Strait. Since 1990 these investigations have been part of MAST 53 FLUXMANCHE programme with repeated measurements on the Dover Strait transect. The discussion of the results will take into consideration new informations given by the hydrodynamic model elaborated in the frame of FLUXMANCHE: knowledge of the monthly fluxes of water in the Strait of Dover and simulation of the 125 Sb activities, week by week, in the waters flowing in this aera. It is shown that solubles radionuclides introduced into the central Channel are transfered towards the Dover Strait in a time which varies between 4 and 7 months, depending on the route taken.
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