Oxidation resistance of coated Mo and Nb refractory alloys for accident resistant fuel cladding

Abstract

The entire commercial light water reactor (LWR) fuel cladding is fabricated using zirconium-based alloys due to its low neutron absorption cross section, acceptable corrosion resistance, and satisfying mechanical properties at operating conditions. A major problem of Zirconium alloy cladding is rapid steam oxidation during accident situation and it leads to hydrogen gas release. A massively released hydrogen gas can trigger hydrogen explosion by the interaction between zirconium and steam which occurred at Fukushima nuclear power plant in 2011. After the Fukushima accident, the development of accident resistant fuel becomes a critical concern in nuclear industry and has been widely studied. Many approaches have tried to modify or replace current zirconium alloy cladding. For instance, SiC-based ceramic composites, advanced stainless steel and refractory alloys are being considered as cladding materials. An alternative approach has tried by applying coating technology to current zirconium alloy cladding. Refractory alloy has been suggested as a candidate cladding material due to its significant high melting temperatures and substantially higher mechanical properties at high temperature compare to zirconium alloys. However, well-known issues with refractory alloy include rapid oxidation at elevated temperature. In this research, silicide and aluminide coating have been employed as a surface protection layer for mitigating rapid oxidation phenomena of the refractory alloy at an elevated temperature for accident resistant fuel cladding application. The pack cementation deposition method was applied to coat silicide/aluminide layers on the substrate and oxidation resistance improvement was evaluated by the thermogravimetric analysis in steam and air atmospheres comparison with the un-coated specimen. From the microstructure analysis, protective surface oxide formation by oxidizing silicide and aluminide layer was identified and it prevents the refractory alloy substrate from rapid oxidation at an elevated temperature.