Multiphysics modeling of UO2-SiC composite fuel performance with enhanced thermal and mechanical properties

The relatively poor thermal conductivity of the UO2 fuel is a major challenge for optimizing reactor operation and safety performance, despite its widespread use in the majority of power reactors. The already low thermal conductivity further degrades with burnup due to accumulation of defects, fission product precipitates and fission gas bubbles. The high thermal stresses cause significant pellet cracking, leading to more pellet expansion and causing pellet cladding interaction and the release of fission product gases. Mitigation of these phenomena is accomplished by limiting operating power and ramp rates at high burnup. This study presents the development of a model for UO2-SiC composite light water reactor fuel with enhanced thermal and mechanical properties to mitigate some of the material performance limitations. The model is constructed using self-defined physics modules which are fully coupled and solved using the COMSOL Multiphysics platform. Key fuel performance phenomena being considered include heat generation and conduction, species diffusion, thermomechanics (thermal expansion, elastic strain, densification, and fission product swelling strain), grain growth, fission gas production and release, gap heat transfer, mechanical contact, gap/plenum pressure with plenum volume, cladding thermal and irradiation creep and oxidation. All the equations are solved via finite-element method on a 2D axisymmetric geometry of a fuel pellet and cladding. The use of UO2-SiC will enhance thermal conductivity, which in turn will decrease nuclear fuel temperatures. This subsequently will improve fission gas retention and reduce pellet cladding interaction resulting from fuel cracking, relocation, and swelling.