Burn-up credit in criticality safety of PWR spent fuel

•Designing spent fuel wet storage using WIMS-5D and MCNP-5 code.•Studying fresh and burned fuel with/out absorber like “B4C and Ag–In–Cd” in racks.•Sub-criticality was confirmed for fresh and burned fuel under specific cases.•Studies for BU credit recommend increasing fuel burn-up to 60.0 GWD/MTU.•Those studies require new core structure materials, fuel composition and cladding.

The criticality safety calculations were performed for a proposed design of a wet spent fuel storage pool. This pool will be used for the storage of spent fuel discharged from a typical pressurized water reactor (PWR). The mathematical model based on the international validated codes, WIMS-5 and MCNP-5 were used for calculating the effective multiplication factor, keff, for the spent fuel stored in the pool.The data library for the multi-group neutron microscopic cross-sections was used for the cell calculations. The keff was calculated for several changes in water density, water level, assembly pitch and burn-up with different initial fuel enrichment and new types and amounts of fixed absorbers. Also, keff was calculated for the conservative fresh fuel case.The results of the calculations confirmed that the effective multiplication factor for the spent fuel storage is sub-critical for all normal and abnormal states. The future strategy for the burn-up credit recommends increasing the fuel burn-up to a value >60.0 GWD/MTU, which requires new fuel composition and new fuel cladding material with the assessment of the effects of negative reactivity build up.