2019 年 18 巻 3 号 p. 111-118
The cooling rate of molten core materials during solidification significantly affects the segregation of major constituents of fuel debris. To understand the general tendency of the segregation, liquefaction/solidification tests of simulated corium (UO2, ZrO2, FeO, B4C and sim-FP oxides) were performed. The simulated corium was heated up to 2,600℃ in Ar atmosphere and then cooled with two different cooling processes: furnace cooling (average cooling rate of approximately 744℃/min) and slow cooling (cooling from 2,600℃ to 2,300℃ at a rate of 5℃/min and from 2,300℃ to 1,120℃ at approximately 788℃/min). Element analysis detected three oxide phases with different compositions and one metal phase in both solidified samples. The solubility of FeO in these oxide phases was mostly fixed at 12±5 at% in both samples, which is in reasonable accordance with the value estimated from UO2–ZrO2–FeO phase diagrams. However, the significant grain growth of one oxide phase, rich in Zr-oxide, was detected only in the slowly cooled sample. The composition of this particular oxide phase is similar to the initial average composition. The grain growth is considered to be caused by the connection of remaining liquid agglomerates during slow solidification.