Assessment of a MOX Fuel Assembly Design for a BWR Mixed Reload

Abstract

Reprocessing benefits are still being debated from the standpoint of economy. However, it is a clear option to reduce the amount of depleted fuel assemblies and a reduction of the reactor plutonium inventories. Several mixed-oxide (MOX) fuel concepts have been considered as an option for mixed-fuel reload assemblies in boiling water reactors in the past. In this work, a new MOX fuel assembly design is proposed. The design is based on the use of a proportional fissile ratio between equivalent fissile plutonium (239Pu + 241Pu) and fissile uranium (235U). This is referred to as the PUF ratio. Furthermore, the moderation ratio will be increased in the assembly as a way to reduce the possible impact of using MOX fuel on the reactivity control systems. The design and performance of the MOX fuel assembly and the mixed core are presented and discussed. The new design, for the cases considered, can increase the MOX batch reload up to 52 MOX assemblies, in comparison with the 24 assemblies from a design that does not increase the moderation ratio. The use of the combined PUF ratio and increased moderation ratio for the MOX assembly allows for a reduction in the average enrichment of fissile plutonium to 4.68 wt%, instead of the 6.75 wt% necessary without increasing the moderation ratio. Both MOX designs produce the same amount of energy during the proposed cycle length and satisfy the same thermal limits. Some comparisons are performed between the core with this MOX fuel assembly and the core that uses only standard uranium assemblies.