Multi-ring Subgroup Method in Characterising Highly Self-shielded Gadolinia Burnable Poison Pins for the UK EPR Nuclear Fuel Assembly

Precisely modelling burnup behavior of Gadolinia burnable poison pins in a nuclear reactor is tricky, as it is a very strong absorber of thermal neutrons. The highly self-shielded burnable poison depletes largely from outermost zones inwards, presenting strong flux gradients around the pin. Classic modelling methods are based on equivalence theory, tracking pin-averaged cross sections, and collapsing all radial rings down. However, the subdivision of the whole pin into multiple radial zones is ineffective, as each zone is still represented by the same cross sections in 172 groups. To capture the self-shielding effect, a subgroup method is employed in this work to accurately account for the ring effect in Gadolinia-bearing pins. Deterministic code (WIMS) is used for producing homogenised cross sections for Gadolinia-zoning assemblies, obtaining lattice power distributions, the results of which are benchmarked against a Monte Carlo code (Serpent) for model verifications.