Effects of burnup on heat generation in vitrified waste from spent MOX fuel for geological disposal

In this study, we evaluated the amount of decay heat generated in vitrified waste from MOX fuel and the bentonite buffer temperature after disposal under different burnup conditions, assuming the waste loading in the vitrified waste was constant. The effect of burnup was calculated for two cases: (1) the initial fissile Pu content in MOX fuel was constant and (2) the initial fissile Pu content increased as the burnup increased. In case (1), the amount of decay heat generated in vitrified waste (kW/canister) decreased as the burnup increased considering the content of Am-241 in the vitrified waste decreased owing to the significant increase in the number of fission products (FP). Conversely, in case (2), the amount of heat generated in vitrified waste (kW/canister) increased as the burnup increased. Furthermore, the waste loading, for which the maximum temperature of the bentonite buffer after disposal still complies with the current upper limit of 100 degrees C in Japan, was found to be approximately 13 wt% (including Na2O 10 wt%).