Solid/solution interaction and matter transfers in a temperature gradient: application to the confinement of high level radioactive wastes

The initial stage of a high level nuclear wastes disposal will be characterized by a large heat release in the near-field environment of the canisters. This heat flux will lead to an increase of temperature and a subsequent thermal gradient between the 'hot' canisters and the 'cold' geological medium. What will be the consequences for the alteration of the different constituents of the near-field of such a thermal field depending both on space and time? In particular, what could be the effects on the radionuclides in the accidental case of an early breach of a canister during the thermal stage? This work brings significant answers to these questions in the light of a performance assessment study. This work supports a triple methodological approach involving experimental studies, modelling calculations and a natural analogues study. It demonstrate that a thermal gradient leads to a large redistribution of elements within the system: some elements are incorporated in the solid phases of the hot end, some other in those of the cold end. The confrontation of the results of simple experiments with a model built on equilibrium thermodynamics allo us to evidence the likely mechanisms: thermodiffusion processes coupled to irreversible precipitation. Moreover, the effects of the variation of temperatures with time is studied by the way of a natural system presenting a similar temperature evolution as a disposal: the Jurassic Alpine bauxites. The initial uranium in this system has been extensively re-mobilized during metamorphism leading to a definite mineralogical trapping during the apex stage without any migration. This work demonstrates that the initial thermal stage will lead to significant modifications of the near-field environment. These transformations are not antithetical with the confinement properties and have to be taken into account for evaluation the long-term behaviour of disposal. (author)