Accident Tolerant Fuels Claddings and the Evolution of their Surface Characteristics under Critical Heat Flux

Enhanced Accident Tolerant Fuels (ATFs) systems for light water reactors require higher temperature oxidation resistance in steam environments than the current UO2 – zirconium fuel system. Investigating the ATF materials’ surface properties is needed as they have evidenced to affect the Critical Heat Flux (CHF), which is an important parameter of the thermal-hydraulic performance required to provide safety margins. In this dissertation, the surface properties of FeCrAl alloys (APMT and C26M), Zircaloy-2, Zircaloy-4, and Cr-coating on Zircaloy-4 substrates produced by Physical Vapor Deposition (PVD) and Cold Spray (CS) were investigated according to their chemistry, topography, and wettability. The static contact angle changed from 75º for as-received Zircaloy-4 substrates to almost 20º after the Cr coating deposition. However, a hydrophobic recovery was noticed with environmental surface aging, which was explained by correlating the surface chemistry with surface energy. The surface chemistry of the coated samples and both FeCrAl alloys after CHF was studied thoroughly to understand their evolution at the early stage when subjected to CHF testing. The in-depth surface chemistry analysis evidenced the formation of various oxides that elucidate the rapid response of ATF cladding toward accident scenarios. The evolution of the materials’ surface chemistry also led to a noticeable increase in their wettability, with a slight increase in roughness. The investigation of the materials’ mechanical properties indicated an increase in hardness by 10-15% and an increase in their yield strength, as evidenced by the microindentation and ring compression tests conducted before and after CHF testing