Chemical interactions between pre-oxidized Zircaloy-4 and 304 stainless steel-B 4 C melt at 1300 °C

Abstract During severe nuclear accidents, control rods rapidly liquefy at temperatures above 1250 °C due to eutectic reaction, forming a 304 stainless steel (304SS)-B 4 C melt. The melt will relocate and attack surrounding fuel rod claddings made of Zircaloy-4 (Zry-4). To understand to what extent ZrO 2 oxide scale formed on Zry-4 will protect Zry-4 claddings against 304SS-B 4 C melt attack, we studied the chemical interactions between pre-oxidized Zry-4 and 304SS-B 4 C melt at 1300 °C. Bare Zry-4 was completely dissolved in 304SS-B 4 C melt within 60 min. The presence of ZrO 2 oxide scale on Zry-4 significantly delayed the interactions, especially when ZrO 2 oxide scale was dense. Typically, the reaction zone consists of ZrB 2 , Zr 6 (Fe,Ni,Cr) 23 and Fe-Ni-Cr metallic phase at room temperature. Due to the presence of α -Zr and β -Zr in Zry-4 metal matrix, ZrO 2 oxide scale becomes thermodynamically unstable. Dissolution of dense ZrO 2 oxide scale can be described in three stages with different dissolution rates. Dissolution of ZrO 2 oxide scale provides Zr source for the growth of reaction zone. Generally, the thickness of reaction zone linearly increases with time. Compared with the reaction couples of pre-oxidized Zry-4 and solid 316 stainless steel, both ZrO 2 dissolution rate and reaction zone growth rate are much slower in the reaction couples of pre-oxidized Zry-4 and 304SS-B 4 C melt. The corresponding reasons were discussed.