A COMPARISON OF THERMOMECHANICS COUPLING STRATEGIES IN FUEL PIN AND PRESSURE VESSEL SIMULATIONS

Nuclear fuel operates in an extreme environment that induces complex nonlinear multiphysics phenomena. This multiphysics behavior is often tightly coupled, a well-known example being the thermomechanical interaction as the gap between fuel and clad closes in LWR fuel rods. Fuel performance simulation codes must obtain solutions for the thermal and solid mechanics physics and the coupling between them. We define solution strategies for solving systems of coupled equations as loosely-coupled, where the individual physics are solved separately, keeping the solutions for the other physics fixed at each iteration, and tightly coupled, where the nonlinear solver simultaneously drives down the residual for each physics, taking into account the coupling between the physics in each nonlinear iteration. In this paper, we compare the computational performance and results of loosely and tightly coupled solution algorithms for a single fuel pin including thermal and mechanical contact, which, due to the relationship between gap size and fuel centerline temperature, is the source for strong interdependence between thermal and mechanics solutions. The results of these comparisons indicate that loosely coupled simulations require significantly more nonlinear iterations and may lead to convergence issues as the gap between fuel and clad closes. We also show how loosely coupled solution strategies perform better in problems that do not have a strong two-way connection between thermal and mechanical response using a comparison of tightly and loosely-coupled thermomechanics simulations of a reactor pressure vessel.