Mechanical Modeling of Porous Oxide Fuel Pellet A Test Problem

摘要

A poro-elasto-plastic material model has been developed to capture the response of oxide fuels inside the nuclear reactors under operating conditions. Behavior of the oxide fuel and variation in void volume fraction under mechanical loading as predicted by the developed model has been reported in this article. The significant effect of void volume fraction on the overall stress distribution of the fuel pellet has also been described. An important oxide fuel issue that can have significant impact on the fuel performance is the mechanical response of oxide fuel pellet and clad system. Specifically, modeling the thermo-mechanical response of the fuel pellet in terms of its thermal expansion, mechanical deformation, swelling due to void formation and evolution, and the eventual contact of the fuel with the clad is of significant interest in understanding the fuel-clad mechanical interaction (FCMI). These phenomena are nonlinear and coupled since reduction in the fuel-clad gap affects thermal conductivity of the gap, which in turn affects temperature distribution within the fuel and the material properties of the fuel. Consequently, in order to accurately capture fuel-clad gap closure, we need to account for fuel swelling due to generation, retention, and evolution of fission gas in addition to the usual thermal expansion and mechanical deformation. Both fuel chemistry and microstructure also have a significant effect on the nucleation and growth of fission gas bubbles. Fuel-clad gap closure leading to eventual contact of the fuel with the clad introduces significant stresses in the clad, which makes thermo-mechanical response of the clad even more relevant. The overall aim of this test problem is to incorporate the above features in order to accurately capture fuel-clad mechanical interaction. Because of the complex nature of the problem, a series of test problems with increasing multi-physics coupling features, modeling accuracy, and complexity are defined with the objective of accurate simulation of fuel-clad mechanical interaction subjected to a wide-range of thermomechanical stimuli.