Depletion calculations for PWR assemblies including burnable absorbers with lattice code PARAGON

Most of the lattice physics codes, used for routine core design calculations, are based on the spatial flat-flux assumption representation to solve the neutron transport equation. Consequently for regions (like fuel or control rods) with strong flux gradient, a fine computational mesh becomes required for better accuracy of the predictions. In the case of a PWR assembly, this situation particularly occurs with Gadolinia fuel (UO₂-Gd₂O₃) or Erbia fuel (UO₂-Er₂O₃) rods. The aim of this study is to determine, for UO₂-Gd₂O₃ and UO₂-Er₂O₃ pins, the optimal number of computational fuel mesh rings that preserves good accuracy and at the same time is consuming minimum computational running time. This study will be carried out using PARAGON lattice physics code. PARAGON is a two-dimensional neutron/gamma transport code used mainly to generate the group constants for core simulator codes such as ANC. PARAGON (with its new module SDDM) can treat the multi-region resonance self-shielding effect for all resonant isotopes in the fuel rods micro-regions including Gd spatial concentration variation due to burnup depletion. With this capability, PARAGON does not use the pre-tabulated Gd effective cross-sections that are usually generated with a supercell calculation model. The power distribution used during the fuel integrity evaluation of the fuel rods with burnable absorbers will be also discussed in this paper. By comparing the power profiles of UO₂ and absorber rods, it is found that the UO₂ values are more limiting and consequently they can be used for conservative evaluations. (author)