Hydrogen explosion prevention system using SiC fuel cladding for large scale BWRs with inherently safe technologies

Abstract

One of the inherently safe technologies currently under development is a system to prevent hydrogen explosion during severe accidents (SAs). This hydrogen explosion prevention system consists of a high-temperature resistant fuel cladding of silicon carbide (SiC), and a passive autocatalytic recombiner (PAR). Replacing the zircaloy (Zry) fuel claddings currently used in LWRs with the SiC fuel cladding decreases the amount of hydrogen generated and thus decreases the risk of hydrogen leakage from the primary containment vessel (PCV) to the reactor building (R/B), including the operation floor. The PAR recombines the leaked hydrogen gas so as to maintain the hydrogen concentration at less than the explosion limit of 4 % in the R/B. The SiC fuel cladding being considered consists of an inner metallic layer, an SiC/SiC composite substrate, and an outer environment barrier coating (EBC). A thin inner metallic layer in combination with the SiC/SiC composite substrate functions as a barrier for fission products release. The EBC is introduced to provide both corrosion resistance in high temperature water environments during normal operation and oxidation resistance in high temperature steam environments during SAs. The advantages of using the SiC fuel cladding in the system were examined through experiments and SA analysis. Results of steam oxidation tests confirmed that SiC had 2 to 3 orders of magnitude lower hydrogen generation rates during oxidation in a high temperature steam environment than Zry. Results of SA analysis showed that the total amount of hydrogen generated from the SiC fuel cladding was reduced to one eightieth or less than that of Zry claddings. Calculation also showed that the lower heat and the lower reaction rate of the oxidation reaction of SiC moderated the hydrogen generation with time. It is expected this moderated generation will lessen the pressure increase in the PCV.

Keywords

• severe accident
• hydrogen generation
• accident tolerance fuels
• silicon carbide fuel cladding
• steam oxidation
• environmental barrier coat
• fission product containment