Evolution of the spent nuclear fuel during the confinement phase in repository conditions: Major outcomes of the French research

Abstract

ABSTRACTThe 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.