Updated Probabilistic Failure Analysis for Wound Composite Ceramic Cladding Assembly

Abstract

There is sustained interest in advanced ceramics and ceramic matrix composite assemblies based on silicon carbide (SiC) for nuclear fuel cladding applications in light water reactors (LWRs). SiC is known to possess high strength, reasonable thermal conductivity, and low chemical reactivity at high temperatures. These combined good properties are especially attractive when compared to those of traditionally used zirconium alloys such as ZircaloyTM for the aforementioned applications. It is therefore expected that based on these properties, SiC composite assemblies could act as nuclear fuel cladding which would remain intact and safe during and after long periods of time at very high temperatures, such as what is seen in loss-of-cooling accident (LOCA) events at nuclear reactors or spent fuel pools. Additionally, the low chemical reactivity of SiC is expected to mitigate oxidizing reactions that break down water molecules producing free hydrogen gas that is not only potentially explosive, but may embrittle tubes (such as the formation of zirconium oxide and hydride in zirconium alloys). Proposed concepts for SiC-based fuel cladding include a monolithic SiC cylinder surrounded by carbon-coated SiC fibers woven into a tubular form and infiltrated (by chemical vapor deposition) with SiC, infiltrated tubular woven carbon-coated SiC fibers surrounded by thin monolithic layers of SiC on both sides, and hybrid assemblies consisting of both metallic elements and ceramic matrix composite elements potentially containing a monolithic SiC layer. Additionally SiC coatings on the outermost surface of these assemblies are also being considered to prevent hydrothermal corrosion of the fibrous structure [1]. For any of these concepts, there is a need to come up with the understanding of microcracking/failure behavior and an adequate method to evaluate the probability of failure. Thus, which such an approach offers the promise of higher burn-up rates and safer behavior in the case of LOCA events, the reliability of such ceramic-based structures still needs to be critically examined, in particular its ability to reliably retain fission products under normal operation. Therefore, probability failure analysis studies are needed to prove the feasibility of these design concepts.