Thermal diffusion of Helium and volatil fission products in UO 2 and zirconolite nuclear ceramics
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Thermal diffusion of Helium and volatil fission products in UO2 and zirconolite nuclear ceramics Danièle Roudil1 ; Xavier Deschanels1 ; Patrick Trocellier2 ; François Jomard3; Annick 3 Boutry ; Christophe Jégou1; Sylvain Peuget1; Dominique Gosset4, Pierre Nivet1 1 CEA VALRHO DEN/DTCD/SECM/LMPA, BP 17171 30207 Bagnols sur Cèze cedex, France 2 CEA SACLAY DEN/DMN/SRMP 91191Gif-sur-Yvette cedex, France 3 CNRS Meudon-Bellevue, Place A.Briand, 92195 Meudon , France 4 CEA SACLAY DEN/DMN/SRMA 91191Gif-sur-Yvette cedex, France ABSTRACT The behaviour and diffusion mechanisms of helium in nuclear ceramics, such as uranium dioxide spent fuel matrix and zirconolite for the specific conditioning of minor actinides, significantly impact the possible evolution of those matrices in interim storage or disposal conditions. In the framework of spent fuel storage studies, the additional diffusion of gas and fission products in uranium dioxide matrix is also an essential aspect of the R&D. Specific experimental studies have been conducted, devoted to thermal diffusion under 1000 C. Data processing methods, lead to helium diffusion coefficient and associated activation energy of 1.05 eV in the zirconolite and 2 eV in UO2. Comparatively with the uranium dioxide matrix, the helium diffusion coefficient in zirconolite is 1 to 100 million times higher; this parameter will have to be taken into account to dimension the waste form. Diffusion coefficients measurements between 800 C and 1000 C, investigated by SIMS, showed a very slow diffusion of volatile fission products Xe, I, Te and Cs, with coefficients two or three order of magnitude lower than for helium. INTRODUCTION The French waste management law dated 30 December 1991 calls for extensive research in three areas: minimization of the quantity and toxicity of waste by partitioning and transmutation or specific conditioning, feasibility of deep geological disposal, whether reversible or irreversible, packaging and conditioning, for safe long lasting containment and also studying long term surface storage.The conditioning of long-live radionuclides is one of the aspects of the enhanced separation strategy (first research area of the law). Zirconolite is a suitable matrix for accommodating rare earth elements and minor actinides. For long term interim storage and geological disposal of spent nuclear fuel (third research area), one of the major operational R&D questions concerns the monitoring of spent fuel packages in storage and the applicability of potential reconditioning processes. One of the consequences arising from the presence of actinides in these materials is the formation of a large quantity of helium produced by α disintegration. This amount of gas with very low solubility in many ceramics is to be added, for UO2, to insoluble fission gas (Xe, Kr) created under radiation in reactor.This phenomenon leads to physical modifications in the matrix accompanied by a significant variation in the source term that must be taken into account to evaluate the radionuclide balance lia
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