Impact of thermal conductivity models on the coupling of heat transport and oxygen diffusion in UO2 nuclear fuel elements

We study the coupled thermal transport, oxygen diffusion, and thermal expansion of a generic nuclear fuel element consisting of a UO2 fuel pellet and stainless steel cladding separated by a helium gap for the purpose of evaluating the impact of various thermal conductivity models on the predictions of the temperature profile and deformation. Using a series of steady-state and time-dependent finite-element simulations with a variety of initial- and boundary-value conditions, thermo-mechanical response of the fuel element is evaluated. The results show that including the deviation from stoichiometry, x, in the thermal conductivity model is paramount for obtaining accurate predictions in the centerline temperature and the extent of the radial deformation of the fuel pellet. In a surprising result, the coupling between the heat transport and the oxygen diffusion is relatively strong for small values of the fixed composition boundary conditions xb, whereas the coupling becomes weaker for large values of xb.