Multiphysics Finite Element Modeling of Irradiation and Thermal Behavior Demonstrated on a Fuel-Assembly Problem

TL;DR

We develop a multiphysics framework that blends a micromechanically grounded polycrystal model (VPSC) with a finite-element solver (Code_Aster) via CAFEM to simulate irradiation and thermal effects in Zircaloy-2 under PWR conditions. The method incorporates irradiation-induced growth and creep, thermal creep, and thermal expansion at the grain scale, coupled to a macro-scale FE description through a self-consistent homogenization and rotation-based transfer of stresses and strains. Numerical results on a spacer-grid–cladding assembly show irradiation-induced mechanisms dominate the long-term spacer-cladding clearance (CLR), while thermal effects mainly influence early-stage contact and stress relaxation, with thermal pre-loading altering initial gap stability. The approach demonstrates the value of explicit micromechanical texture and slip-system activity for predicting spacer-cladding interaction and provides a pathway to reduced-order models for reactor design and safety assessment.

Abstract

This work presents a modeling framework to represent the thermomechanical behavior of complex materials based on micromechanical dynamics. The framework is applied to nuclear fuel rod elements composed of Zircaloy-2 cladding tubes and spacer grids under typical Pressurized Water Reactor (PWR) conditions. Thermal expansion and thermal creep are incorporated through a VPSC-FEM coupling with the finite element solver Code_Aster, enabling analysis of in-reactor behavior under combined thermal, mechanical, and irradiation loading. The model 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 modifies the stress distribution and clearance evolution, with relaxation governed by prismatic slip. For fuel rod components, irradiation-induced mechanisms dominate the long-term clearance behavior, whereas thermal effects remain relevant in contact dynamics during thermal preloading. The stress-strain response is found to be more sensitive to micromechanical processes than to elastic constants. This high-resolution formulation enables predictive modeling of spacer-cladding interaction and provides a foundation for developing reduced-order models.

Figures (14)

• Figure 1: Axial strain as a function of axial load for the texture shown in Fig. \ref{['fig:ReducedTexture']}(a), considering either irradiation creep alone or the combined effects of irradiation creep and thermal creep (including thermal expansion for heating rate$=0.0018K/h$).
• Figure 2: (a) Cladding tube–grid assembly. (b) Front view of the grid with four supporting dimples. The bottom-left corner shows the coordinate system for the grid sheet. (c) Top view showing the spring contact force on the cladding tube.
• Figure 3: Basal pole figures of the reduced textures used in the simulation. (a) Cladding tube: 7 orientations aligned with axial (A), hoop (H), and radial (R) directions. (b) Spacer dimples: 13 orientations with rolling (Ro), transverse (Tr), and normal (No) directions.
• Figure 4: CLR corresponding to the slaves surfaces for non-linear contact for 20 dpa and 523K.
• Figure 5: Evolution of CLR distance under different physical mechanisms. The CLR is evaluated by isolating each phenomenon individually: irradiation growth (blue symbols), irradiation creep (orange dashed line), thermal expansion (green dash-dotted line), thermal creep (brown dash-dotted line), and the combined case with all active phenomena — full (red solid line).