Chromia coating with nanofluid deposition and sputtering for accident tolerance, CHF enhancement

Abstract

In Fukushima accident, zirconium cladding was rapidly oxidized with high temperature steam, which ultimately led to hydrogen explosion. To overcome materialistic limitation, accident tolerant fuel (ATF) was suggested to improve safety response of nuclear power plants during accidents by modifying cladding surface with various coating materials. When chrome was coated on cladding surface, it showed fewer weight gain by high temperature oxidation compared to bare zirconium cladding. Chrome forms chrome oxide or chromia (Cr2O3) when oxidized, and this layer prevents further oxidation thus protecting inner material from oxidizing. However, previous studies indicated that implementation of chrome containing alloys have major drawbacks such as excessive coating thickness or degraded critical heat flux (CHF). Instead, direct coating of chromia was suggested in this study with the expectation of CHF enhancement compared to other chrome alloy coatings. Chromia nanoparticles were coated on nichrome wire surface with boiling deposition of chromia nanofluid. Another method was applying RF sputtering with chromia target. Chrome coating with DC sputtering were also tested for comparison. Verification of chromia coating was conducted by three steps: CHF measurement with wire pool boiling, high temperature oxidation in furnace to compare the oxidation resistance of specimens, and surface investigation. Surface characteristics investigation were conducted with measurement of contact angle by sessile drop method, capillary wicking height, and scanning electron microscope image. Experimental results show that chromia coating significantly increased CHF. Weight gain by oxidation indicate chromia nanofluid coating had improved oxidation resistance property.