Morphology and phase distributions of molten core in a reactor vessel

•Experiments and thermodynamic analyses on morphology and phase equilibrium of corium.•NUCLEA analysis results on U-Zr-O-Fe system agreed well with the experimental findings.•U/(U+Zr) ratio in good agreement whereas some dispersions observed in oxygen and steel compositions.•Zirconium diboride phase trapped boron and mainly distributed in the metallic layer.•Additional data required to improve for chemistry of corium containing B4C. Investigations on the morphology and phase equilibrium characteristics of corium were performed by a series of experiments in parallel with thermodynamic phase equilibrium analyses. Melting and solidification experiments were performed using corium consists of U, Zr, ZrO2, SS, and B4C. TROI-49 and TROI-50 experiments with corium compositions representing Pressurized Water Reactor (PWR), whose compositions are similar to those of MA-3 and MA-4 of OECD MASCA (Material Scaling) Program while amount of mass is 5 times more, resulted in two-layered structure with oxide rich layer on top of metal rich layer. Experiments of FK-1 and FK-2 with a mixture of UO2, ZrO2, Zr, SS and B4C at a representative Fukushima Daiichi Nuclear Power Plant (FDNPP) corium composition resulted in a formation two-layered structure of corium with upper layer rich in metal and lower layer rich in oxide. Zirconium diboride phase trapped boron and mainly distributed in the metallic layer. Predictions by thermodynamic calculations for the equilibrium phase distribution and chemical compositions of each phase using the NUCLEA thermodynamic database with a focus on U-Zr-O-Fe system were in good agreement with the experimental results. Agreement in terms of U/(U+Zr) was quite close while there were dispersions in oxygen and steel components. The temperatures for the formation of two immiscible liquids calculated by the NUCLEA for FK-1 and FK-2 was 300 K higher than the experimental observation. This point has to be looked at for improving the NUCLEA models for corium containing B4C, by introducing a non-zero boron solubility in the ceramic phase. The agreement between the NUCLEA predictions and the results of experiments clearly indicate that thermodynamic equilibrium phases play an important role in governing the core damage progression, which is important not only for the severe accident management but also for decommissioning and defueling process for the Fukushima Daiich damaged reactors.