Thermal impact on geological disposal of hull and end piece wastes resulting from high-burn-up operation of LWR and introduction of MOX fuels into LWR

The thermal impacts of hull and end piece wastes from high-burn-up UO ₂ and MOX fuels on a conventional disposal system were investigated. The heat generation rates in the canister containing these wastes were obtained using burn-up calculations of PWR fuels. For wastes from spent MOX fuel, the heat generation rates increase to 3.2-4.5 times that from present-day burn-up spent UO₂ fuel when these canisters are disposed of. The temperature distributions in the area around the disposal galleries for these wastes were obtained using two-dimensional thermal analyses by assuming a maximum 80°C temperature exposure of the cement mortar. For wastes from spent MOX fuel, the temperature of the surrounding rock remains at about 60-70°C after disposal, even after 1,000 years. In this case, the number of canisters loaded in a waste package must be decreased from four to around one. This increases the number of waste packages to contain the required number of canisters. It will be important to apply alternative approaches to increase the amount of wastes in a waste package by reducing the amounts of FPs and actinides adhering to hulls and to optimize the layout design of galleries, which may be done by significantly increasing the distance separating neighboring galleries.