Thermomechanical analysis of SiC-based duplex claddings with varying thickness ratio for accident-tolerant nuclear fuel systems

SiC-based duplex claddings, consisting of monolithic SiC and SiC/SiC fiber composite, are emerging as a promising candidate for accident-tolerant fuel (ATF) systems in nuclear reactors. To analyze the performance of ATFs with SiC-based duplex claddings, a comprehensive computational analysis framework is presented that captures the essential properties and behaviors of the UO₂-SiC fuel system. Utilizing a previously developed continuum damage model, the pseudo-ductile behavior of SiC/SiC fiber composites is accurately modelled, connecting damage evolution parameters to instantaneous stiffness matrix degradation. This framework is used to investigate the performance of UO₂-SiC fuel rods under normal operating conditions and a typical Loss of Coolant Accident (LOCA) scenario. We assess the effects of the thickness ratio of the monolithic SiC and SiC-based composite layers, as well as pellet-clad cold gap thickness on the failure and leakage probabilities of the cladding. These claddings, with a thickness ratio ranging from 0.25 to 0.75, demonstrated minimal failure and leakage probabilities for both the original and reduced pellet-clad gap thickness (82.5/70 µm). When the gap thickness was further reduced to 57.5 µm, pellet-cladding mechanical interaction was observed and this greatly elevated the failure probability of the MSiC layer, thus giving rise to a loss of hermeticity. This research underscores the significant role of varying individual layer thicknesses in shaping fuel rod safety and offers potential for optimization across diverse operational conditions.