SUGGESTION OF TYPICAL PHASES OF IN-VESSEL FUEL-DEBRIS BY THERMODYNAMIC CALCULATION FOR DECOMMISSIONING TECHNOLOGY OF FUKUSHIMA-DAIICHI NUCLEAR POWER STATION

摘要

For the decommissioning of the Fukushima-Daiichi Nuclear Power Station (1F), the characterization of fuel-debris in cores of Units 1-3 is necessary. In this study, typical phases of the in-vessel fuel-debris were estimated using a thermodynamic equilibrium (TDE) calculation. The FactSage program and NUCLEA database were applied to estimate the phase equilibria of debris. It was confirmed that the TDE calculation using the database can reproduce the phase separation behavior of debris observed in the Three Mile Island accident. In the TDE calculation of 1F, the oxygen potential [G(O_2)] was assumed to be a variable. At low G(O_2) where metallic zirconium remains, (U,Zr)O_2, UO_2, and ZrO_2 were found as oxides, and oxygen-dispersed Zr, Fe_2(Zr,U), and Fe_3UZr_2 were found as metals. With an increase in zirconium oxidation, the mass of those metals, especially Fe_2UZr_2, decreased, but the other phases of metals hardly changed qualitatively. Consequently, (U,Zr)O_2 is suggested as a typical phase of oxide, and Fe_2(Zr, U) is suggested as that of metal. However, a more detailed estimation is necessary to consider the distribution of Fe in the reactor pressure vessel through core-melt progression.