Multidimensional gap conductance model for thermo-mechanical simulation of LWR fuel behavior using a virtual link element

Abstract

In light water reactor (LWR) fuel, the modeling of the heat transfer across the gap between the fuel pellets and the protective cladding is essential to understanding the fuel behavior. Based on the Ross and Stoute model, the gap conductance that specifies the temperature gradient within the gap depends on the gap thickness, which is related to the mechanical behavior. A multidimensional gap conductance problem can be challenging in terms of convergence and nonlinearity. In this work, a virtual link gap (VLG) element has been proposed to resolve the convergence issue and nonlinear characteristic of multidimensional gap conductance. The elements that link the node of a pellet surface with the node of the cladding surface are virtually generated so as to transfer heat as a function of gap thickness at every iteration step. To evaluate the proposed methodology for the simulation of the gap conductance, a thermo-mechanical model has been established using ANSYS Parametric Design Language (APDL) for a preliminary study, and a 3D thermo-mechanical module using FORTRAN77 has been implemented. In terms of calculation accuracy and convergence efficiency, the proposed VLG model has been evaluated. As a result, the convergence criterion of the thermo-mechanical calculation considering the iteration characteristics of the VLG element has been proposed. To demonstrate the effect of the VLG model in a 3D simulation with the implemented thermo-mechanical module, the simulation results of a missing pellet surface (MPS) have been compared.