Semi-analytical fluid-structure model for the analysis of fuel assembly bow in full PWR cores

Abstract

Fuel assembly bow is an intricate industrial issue occurring in PWR nuclear cores, which relies on multiphysics coupled phenomena based on mechanics, thermal hydraulics and neutronics.Amongst many causes, the hydraulic forces induced by in-core flow redistributions play an important role on the fuel assembly mechanical deflection.Because deformation spread out over days, weeks or months, fuel assembly bow is often studied as a quasistatic fluid-structure interaction with neutronics parameters as inputs only. In order to set up an efficient partitioned two-way coupling between mechanics and hydraulics representing a full PWR core, simplified models must be used.Regarding hydraulics, common strategies imply porous media, or even more time-saving 1D networks. In a previous study, we followed such a 1D approach to simulate the flow redistributions occurring near spacer grids, from fuel assemblies to the bypasses surrounding them or the other way around. In the current paper, we extend the latter local hydraulic networks to build up a full fuel assembly, which is then two-way coupled with an in-house CEA mechanical code. The simulations are compared with a CEA experiment. The lateral force exerting on spacer grids induced by the bypass-fuel assembly redistribution appears to play an important role on the shape of the fuel assembly.Finally, we propose further extensions to a row of fuel assemblies, and then to a whole core. Qualitative comparisons with the literature highlight proper bow patterns for the single row test case, with high sensitivity to the flow inlet and outlet conditions. At last, we demonstrate that hydraulic forces in a full core can indeed be estimated accurately through separated calculations on single rows in orthogonal directions, as suggested in the literature.