Multiphysics finite element modeling of irradiation and thermal behavior demonstrated on a fuel-assembly problem

Abstract

This work presents a modeling framework to represent the thermomechanical behavior of complex materials, based on micro mechanical dynamics. This tool is then applied to fuel rod elements, consisting of Zircaloy2 cladding tubes and spacer grids, under typical Pressurized Water Reactor (PWR) conditions. The model incorporates thermal expansion and thermal creep through a VPSC–FEM coupling with the finite element method (FEM) solver Code_Aster, enabling analysis of in-reactor behavior under combined thermal, mechanical, and irradiation loading. The framework captures anisotropic deformation driven by crystallographic texture and prismatic slip activity under radial loading. Thermal creep, being stress-sensitive, contributes to early-stage stress relaxation and strain accumulation, leading to higher strain compared to the irradiation-only case. The interaction of thermal creep with irradiation mechanisms was found to modify the stress distribution and clearance (CLR) evolution, with relaxation governed by prismatic slip. For fuel rod elements, irradiationinduced mechanisms dominate the long-term CLR behavior, whereas thermal effects remain relevant in contact dynamics during thermal preloading. Furthermore, the stress–strain response was found to be more sensitive to micromechanics than to elasticity. This high-resolution formulation enables predictive modeling of spacer–cladding interaction and provides a basis for the development of reduced-order models.