PHASE EQUILIBRIUM BETWEEN METALLIC LIQUID AND OXIDE IN CE-ZR-FE-O SYSTEM AS A SURROGATE SYSTEM OF METALLIC DEBRIS

Abstract

In the in-vessel late phase of the severe accident (SA) at the Fukushima Daiichi Nuclear Power Station (FDNPS), fuel debris is considered to have relocated to lower plenum and then steel structures in the lower part of the Reactor Pressure Vessel (RPV) inside could have been severely damaged. To evaluate the phase status of the debris during the damaging process, it is important to understand phase equilibrium of the U-Zr-Fe-O system. In this study, phase equilibrium between metallic liquid and oxide solid of the Ce-Zr-Fe-O system was experimentally investigated as a surrogate for the U-Zr-Fe-O system in the temperature range from 1823 K to 1873 K. The experimental results indicated that oxygen solubility in metallic liquid decreases with decreasing the Zr concentration and increasing the Ce concentration. Considering the expected similarity with the U-Zr-Fe-O system, the U and Zr in metallic part of re-melted fuel debris may have been selectively oxidized to form a surface oxide layer.