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

U-Zr-Gd-O系溶融コリウムに関する凝固試験

須藤 彩子 

Sudo, Ayako

溶融コリウムの凝固プロセスにおいて様々な構成元素のマクロな再分布に関する知見を得るため、U-Zr-Gd-O系コリウムの溶融試験を行った。炉心溶融物材料であるwt.70%UO+30%ZrOに2%のGdOを添加した粉末655.5gを、コールドクルーシブ誘導加熱炉に設置し、誘導材料として6gの金属Zrを添加後、空気中で溶融させた。加熱試験終了後、試料の詳細な凝固生成物の分析のため、XRDでの相同定およびSEM/EDXでの元素分析を行った。様々な凝固条件下での溶融コリウムの性状評価のため、試験は冷却速度を変えた2回行われた。炉冷条件で行った試料(J1)は固化後4.8cmの高さとなり、試料下部は結晶化しており加熱中も溶融していなかったと推測できる。徐冷条件で行った試料(J2)に関しては、構造はJ1とおおよそ一致しているものの、上部クラスト真下に引け巣の形成が確認できた。この引け巣は遅い凝固速度での固化時に容積の収縮により形成したと考えられる。また、J1,J2両方の試料中で、Gdは試験後期で凝固した試料中央部に濃縮していることが明らかとなった。

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.