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A configuration of molten core in the Fukushima Daiichi NPP (nuclear power plant) was investigated by a melting and solidification experiment. About 5 kg of a mixture, whose composition in terms of weight is UO 2 (60%), Zr + ZrO 2 (25%), stainless steel (14%), B 4 C (1%), was melted in a cold crucible using an induction heating technique. It was shown that the solidified melt consists of upper crust and lower solidified ingot. The solidified ingot was separated into two layers. A physical and chemical analysis was performed for the samples taken from the solidified melt to investigate the morphology and chemical characteristics. It was found that the solidified ingot consists of a metal-rich layer on the top and an oxide-rich layer at the bottom. In addition, the oxide layer at the bottom has composition close to the initial charge composition and surrounded by a thin crust layer. It turned out that B 4 C was more concentrated in the upper metal-rich layer. These findings provide important insights for understanding the core melt progression and taking proper post-accident recovery actions for the Fukushima Daiichi NPP.

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.

To simulate a relocation of molten core material and its interaction phenomenon with water during a severe accident in a nuclear reactor, a typical corium of UO 2 /ZrO 2 /Zr/Stainless steel mixed at a 62 wt%, 15 wt%, 12 wt% and 11 wt%, respectively, was melted and then cooled down to become a solidified ingot. It was shown that the molten corium was separated into two layers, of which the upper layer was oxide mixtures and the lower layer was metal alloys. The upper layer was UO 2 and ZrO 2 and the lower layer mostly consisted of metal mixtures such as uranium, zirconium and stainless steel. Iron content varied with the positions and about a half of it existed as an alloy such as Fe 2 U. Uranium metal was produced by reduction of UO 2 by zirconium metal. The average densities of the upper oxide layer and the lower metal layer were 8.802 and 9.411 g/cm 3 , respectively. In another test, metal-added molten corium was poured into water and it showed that a steam explosion could occur by applying an external trigger.

An interaction between molten fuel of a nuclear reactor, which is called corium and mainly consisted of UO 2 and ZrO 2 , and sub-cooled water may result in a steam explosion. It is one of the outstanding reactor safety issues. To investigate the fundamental mechanism behind the recent experimental observation that the composition of the material highly affected the strength of the steam explosion, a physical and chemical analysis for the fast quenched particles of UO 2 and ZrO 2 mixture at different compositions was performed. Six cases were selected for the study, in which the melt composition was changed, while other initial and boundary conditions of the molten fuel and water interaction tests were maintained the same. It was observed that the cases at eutectic composition resulted in a spontaneous steam explosion, while the cases at non-eutectic composition did not result in a spontaneous steam explosion. Electron probe microanalysis (EPMA) was performed for fast quenched particles along a cross-section. Results demonstrated that the UO 2 and ZrO 2 mixtures formed a solid solution of U 1− x Zr x O 2 . The mechanism for the hydrogen generation during the molten material and water interaction was examined by thermogravity analysis (TGA), X-ray diffraction (XRD) and hydrogen reduction analysis. It was demonstrated that the hydrogen generation was not directly related to the oxidation of UO 2 . Morphologies observed by scanning electron microscopy (SEM) indicated that the particles from the eutectic mixture had many holes, while the particles at non-eutectic mixture did not. The existence of mush phase for the non-eutectic mixture is suggested to be the reason for the non-explosive nature.