Cycle du combustible nucléaire et milieu marin - Devenir des effluents rhôdaniens en méditerranée et des déchets immergés en atlantique Nord-Est.
Thèse de Doctorat d'Etat "es-sciences" de l'Université Aix-Marseille II Soutenue le 4 Septembre 1998
Man-made radionuclides released into the marine environment by the installations from the nuclear fuel cycle are used as tracers of various bio-geochemical processes. Several installations belonging to the whole nuclear fuel cycle, except the uranium mining, are set up on the Rhône River banks. The sea disposal of low and intermediate level radioactive waste has never been authorized in the Mediterranean sea but several sites have been used in the North-East Atlantic especially in abyssal waters.
Radionuclides released by the Rhodanian installations are used in order to study the dynamics of the Rhone inputs into the Mediterranean Sea, In the river, freshwater samples reflect quite accurately the discharge composition with a predominance of 106Ru, a radionuclide mostly released by the spent nuclear fuel reprocessing plant in Marcoule. Conversely, at the Rhône mouth, in the sediment compartment 106Ru yields to caesium isotopes (134Cs and 137Cs) in importance. As these two isotopes demonstrate very different half lives (30,2 and 2,1 years respectively), the temporal evolution of their ratio acts as a chronometer enabled to date sediment accumulation near the river mouth. Mean accumulation rates greater than 35 cm.y-1 have been determined in the prodeltaic zone near the Roustan buoys over the period 1983-1991. Accumulation rates decrease rapidly with distance from the mouth and therefore most of the 137CS inventory in this part of the Gulf of Lions is limited to the prodeltaic area. A first study about the part the different 137CS sources in the Mediterranean Sea play in this inventory has been carried out. Direct (atmospheric) and indirect (fluviatile) inputs due to fallout from both past nuclear tests and the Chernobyl accident could contribute to this inventory at the highest to 40% while the industrial releases could contribute at the lowest to 60%.
The last site used for the dumping of low and intermediate level radioactive waste in the North-East Atlantic has been the subject of studies especially as regards the biological compartment. Indeed, the occurrence in abyssal waters of a very motile fauna, i.e. fish (Coryphaenoides armatus) and amphipods (Eurythenes gryllus), able to undertake important vertical migrations into the water column, represents a possible pathway towards the surface for the dumped radionuclides. The ignorance as regards populations inhabiting this site has led to a study of the population structure for the amphipod Eurythenes gryllus. This species is cosmopolite since it is found at all the latitudes in the world ocean both in bathyal and abyssal waters. Well known for its necrophagy and ability to consume baits, itis supposed to feed on carcasses. However, study of a natural radionuclide, i.e. 210Po, in this species has shown that it is possible that it feeds also on particle fluxes.
In order to verify the validity of the data used for the modelling of the radiological assessment of these operations, in situ labelling experiments have been realized on these amphipods as well as on polychaetes from food labelled with radionuclides characterizing the dumped wastes. Though these results are still too few to acknowledge this validity, as expected from studies on coastal species the tranfert factors food organisms are quite low.
Analyses carried out for surveillance purposes both on the fish Coryphaenoides armatus and the amphipod Eurythenes gryllus show that in these deep waters the main source of man made radioactivity is the global fallout due to the atmospheric nuclear testings carried out mainly in the sixties. A transfert via the particle fluxes is necessary to explain the concentrations found for 3H 137Cs, 238Pu and 239+240Pu. However, 238Pu/239+240Pu isotopic ratios in some fish samples suggest an influence from the dumped wastes and underline the possible part taken by these necrophagous organisms in the dispersion of radionuclides released from the low and intermediate level radioactive waste dumped in the North-East Atlantic.