Development of a multiphysics Best‐Estimate approach for LWR reference calculation

The aim of the work presented here is to produce a best-estimate multiphysics calculation scheme to predict the power distribution at a fine scale for steady-state conditions. By best-estimate, it is meant that several modelling options are explored in order to find the best trade-off between quality of the results and computing time. In respect of the multiphysics, the simulation should simultaneously take into consideration neutronics, thermal-hydraulics and thermal-mechanics. The test case selected for the analysis is a 3D cluster of 5x5 PWR fuel assemblies, all of 4% enriched UO2, but at different burnups. Two reflector isotopic compositions are considered, 50% water, 50% stainless steel on one side, and 5% water, 95% stainless steel on the others. Neutronics simulations are done at the pin cell level, following the classical two-steps approach (fuel assembly calculation in fundamental mode and fuel 3D core calculation using Diffusion, SPN or SN resolution). Discrete Ordinates of order 8 (S8) at 30 energy groups is used as reference neutronics method. Its results are compared with S8 at 20 energy groups, Simplified Spherical Harmonics of order 3 (SP3) at 8 energy groups and 2 groups diffusion calculations. Thermal-hydraulics is simulated with a 3D four equations model at the quarter of assembly level. 1D heat conduction is solved for an average fuel rod per quarter of assembly. For both analyses (isothermal and coupled calculations), the SP3 with 8 groups predicts the power distribution within a relative error within a root mean square of 0.45% over the entire core, lasting 1.65% of the computing time. This performance makes it the best-estimate calculation scheme, within the considered study context. Moreover, the analysis shows that the composition of the reflector, which strongly affects the power distribution, has no impact on the results produced by the SP3 solver compared to the reference solution.