Effects of alpha self-irradiation on actinide-doped spent fuel surrogate matrix
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Effects of alpha self-irradiation on actinide-doped spent fuel surrogate matrix Danièle Roudil1, Christophe Jégou1, Xavier Deschanels1, Sylvain Peuget1, Caroline Raepsaet2, Jean-Paul Gallien2, Véronique Broudic1 1 CEA Valrho, DEN/VRH/DTCD/SECM/ LMPA 2 CEA Saclay, DSM/SAC/DRECAM/LPS ABSTRACT During the initial period after disposal, in which no mass transfer occurs with environmental materials, spent fuel undergoes modifications mainly arising from the effects of α self-irradiation damage. To simulate this aging phenomenon and estimate the consequences on the matrix, an experimental study was carried out in which old UO2 samples doped with short-lived actinides were characterized and analyzed. Changes that occur rapidly at the beginning of the aging process were identified in two lots of actinide-doped samples. Volume and microscopic swelling increased, reaching a maximum relative level of 1.5%. At the same damage level, microscopic swelling remained below 0.9%, suggesting that helium formation and atomic defects have a role in this regard. Similarly, the sample microhardness rapidly increased by up to more than 12%. Characterizing 238PuO2 sources showed at very high integrated and instantaneous doses, irradiation damage and helium accumulation at the grain boundaries resulted in grain decohesion or even powder formation. Helium mobility appeared to be modified by the presence of defects, and local implantation experiments were devised in the CEA’s Pierre Süe Laboratory at Saclay to quantify this thermally enhanced diffusion component. The physical condition of the sintered PuO2 pellets after 30 years of storage revealed a purely athermal diffusion mechanism, depending on the sample α activity. INTRODUCTION The French waste management act of 30 December 1991 calls for extensive research in three areas: minimizing the quantity and toxicity of waste by partitioning and transmutation or specific conditioning, assessing the feasibility of reversible or irreversible deep geological disposal, and packaging and conditioning for safe long-lasting containment and long-term surface storage. For long term interim storage and geological disposal of spent nuclear fuel, one of the major operational R&D issues involves monitoring spent fuel packages in storage and the applicability of potential reconditioning processes. During interim storage and in repository conditions, spent nuclear fuel will evolve initially within a closed system in which no mass transfer occurs with the surrounding media, subjected only to self-irradiation and to the ambient temperature. Alpha self-irradiation arising from actinide decay is the primary factor of change in a closed system, not only because of physical and structural modifications resulting from defects created mainly by recoil nuclei, but also due to the formation of a large quantity of helium that is sparingly soluble in the UO2 matrix. Volume and microscopic swelling, together with the accumulation of gas bubbles, can have major effects on the long-term mechanical stability of the grain boun
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