Solidification tests on the U-Zr-Gd-O molten corium

Sudo, Ayako 

In order to obtain the knowledge on the re-distribution of various elements under the solidification process of molten corium, the fundamental experiment on the U-Zr-Gd-O melt was performed. As the core-melt constituents, a powder mixture of wt.70%UO+30%ZrO with adding of wt.2%GdO was prepared with mass of 655.5g. These powders were melted in a cold crucible using induction heating technique in the air atmosphere with metallic zirconium (6g) as an initiator. For the corium microstructure study, sample pieces from solidified ingot were subjected to phase identification and elemental analysis by XRD and SEM/EDX. For the evaluation of the characteristics of corium debris under the various solidification processes, two heating tests with different cooling rates were performed. During quick-solidification test (J1) ingot of 4.8cm was obtained. The near-bottom region of J1 sample was crystallized and not melted. The structure of slow-solidification sample (J2) was similar to J1, however, it was formed big cavity under the upper crust of ingot. This cavity was formed during slow-solidification process due to volume shrinkage. Both of samples were found that Gd was concentrated in the inner region where the later crystallization of the melt was occurred.