Abstract: A new failure criterion of cladding oxide layer dissolved by molten zircaloy is given based on the kinetics of molten zircaloy dissolve UO₂ and ZrO₂ simultaneously and its load conditions. The criterion was successfully validated against FPT-0 test. For a given structure of the fuel rod in nuclear reactors, the failure criterion can be simplified by temperature, which is a function of oxide scale and heat-up rate of the cladding. The relocation of melt in the flow path will be influenced by the new criterion predictions concerning mass and composition of relocated U-Zr-O mixture.