Evolution of the spent nuclear fuel during the confinement phase in repository conditions: Major outcomes of the French
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Evolution of the spent nuclear fuel during the confinement phase in repository conditions: Major outcomes of the French research. Ferry, C1, Piron, J.P.2, Poinssot, Ch.1 1
Nuclear Energy Division, Department of physics and chemistry, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France 2 Nuclear Energy Division, Department of fuel studies, CEA-Cadarache, 13108 Saint-Paul lez Durance, France ABSTRACT The effects of the temperature and high radioactivity on the chemical and physical state of the fuel during a confinement phase were evaluated in the framework of the PRECCI program. No significant evolution of the fuel chemistry is expected. The radionuclides mobility due to thermal diffusion can be neglected in the temperature range of disposal and diffusion enhanced by α self-irradiation appears to be limited. Microscopic swelling due to alpha self irradiation cumulated damage corresponds to a low volume increase of about 1%. Macroscopic swelling will depend on the helium behavior in the spent fuel. Literature data and results from the project indicate that helium solubility is low in UO2 and should be reached after a few centuries of cooling for MOX fuels. Although the thermal diffusion of helium is some orders of magnitude faster than the thermal diffusion of xenon, it remains low in the temperature range of disposal. Therefore, the first step of physical damage due to helium production should be the precipitation of He bubbles in grains, causing micro-cracks, which could enhance the He release to the grain boundaries. These processes and their consequences on the stability of grain boundaries still need to be quantified today. INTRODUCTION The 1991 Radioactive Waste Management Act has given 15 years of research to study the various potential options for managing long-lived radioactive waste in France. The Commissariat à l’Énergie Atomique initiated the PRECCI research project in 1999 to investigate the long-term behavior of spent nuclear fuel. The primary objective of this program jointly supported by the French electricity utility EDF was to address the operational questions of interim dry storage and deep geological disposal of spent fuel [1]. In particular, it aims at developing new radionuclides (RN) source term models for spent fuel in repository conditions. For dry storage and deep geological disposal of spent nuclear fuel, the radionuclides source term models classically distinguish two types of release: - The first one refers to a rapid release of activity and is quantified by the so-called Instant Release Fraction or IRF. IRF was classically measured by leaching tests in fresh fuel (e.g. [2]). However, it did not take into account any evolution of the spent fuel before water arrival and could not be extrapolated to high burnup fuels or other types of fuel. A new approach for the quantification of IRF values was proposed in [3]. In this approach, the IRF corresponds to the RN located within the zones of the rod with low confinement properties at the breaching of the canister. In this case, it depends on the dist
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