Chemical interactions between pre-oxidized Zircaloy-4 and 304 stainless steel-B4C melt at 1300 °C

During severe nuclear accidents, control rods rapidly liquefy at temperatures above 1250 °C due to eutectic reaction, forming a 304 stainless steel (304SS)-B4C melt. The melt will relocate and attack surrounding fuel rod claddings made of Zircaloy-4 (Zry-4). To understand to what extent ZrO2 oxide scale formed on Zry-4 will protect Zry-4 claddings against 304SS-B4C melt attack, we studied the chemical interactions between pre-oxidized Zry-4 and 304SS-B4C melt at 1300 °C. Bare Zry-4 was completely dissolved in 304SS-B4C melt within 60 min. The presence of ZrO2 oxide scale on Zry-4 significantly delayed the interactions, especially when ZrO2 oxide scale was dense. Typically, the reaction zone consists of ZrB2, Zr6(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, ZrO2 oxide scale becomes thermodynamically unstable. Dissolution of dense ZrO2 oxide scale can be described in three stages with different dissolution rates. Dissolution of ZrO2 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 ZrO2 dissolution rate and reaction zone growth rate are much slower in the reaction couples of pre-oxidized Zry-4 and 304SS-B4C melt. The corresponding reasons were discussed.