Thermal mechanical assessment of a SiC-SiC-composite clad fuel pin concept in a light water reactor environment

Accident-tolerant fuels (ATFs) are designed to increase coping time following an accident scenario while preserving or improving current steady-state reactor operational performance. A potential ATF concept is using silicon carbide (SiC)-SiC-composite claddings. Fuel-performance simulations were conducted on a SiC-SiC-based cladding concept utilizing a multilayered approach for improved performance. This cladding concept is referred to in this paper as “the duplex concept” as it is a duplex structure composed of a monolithic SiC layer placed on the outside of an inner SiC-SiC composite layer. The monolithic SiC layer is used to provide gas tightness to the rod and protect the SiC-SiC-composite layer from exposure to the coolant. A liquid metal is added to the fuel-cladding gap for improved thermal transport between the fuel and the cladding. In this work, the BISON fuel-performance code was used to conduct fuel-performance simulations on the cladding concept. Comparisons are made with a current prototypic fuel-rod design consisting of uranium dioxide (UO₂) fuel enclosed in Zircaloy-4 cladding under four relevant conditions. For condition I (normal operations) two representative steady-state cases were considered, one with a constant rod average heat rate, and one with an initially higher heat rate. For condition II events, a pellet-cladding interaction (PCI) ramp case was simulated to analyze potential anticipated operational occurrences. Condition III/IV transient responses during a loss of coolant accident (LOCA) and a reactivity-initiated accident (RIA) were also simulated. This computational study demonstrated that for normal operating conditions, the SiC concept cladding performed as well as the baseline for the standard-power cases evaluated. The ramping evaluations indicate potential for earlier fracturing of the SiC-SiC composite cladding compared to the Zircaloy-4 cladding due to the temperature gradient and the subsequent differential thermal conductivity degradation and swelling across the composite thickness. In condition III/IV events the SiC-SiC duplex concept remains intact after a 700 J/g RIA similar to Zircaloy-4 fuel systems. Under LOCA conditions, the duplex concept showed significantly improved performance, remaining intact in contrast to Zircaloy-4 which ballons and bursts.