ウラン-ジルコニウム-鉄の混合溶融酸化物の凝固時偏析に関する基礎検討

Fundamental study on segregation behavior in U-Zr-Fe-O system during solidification process

須藤 彩子 ; 水迫 文樹*; 星野 国義*; 佐藤 拓未 ; 永江 勇二 ; 倉田 正輝 

Sudo, Ayako; Mizusako, Fumiki*; Hoshino, Kuniyoshi*; Sato, Takumi; Nagae, Yuji; Kurata, Masaki

炉心溶融物の凝固過程での冷却速度の違いは燃料デブリ構成成分の偏析に大きく影響する。偏析傾向を把握するため、模擬コリウム(UO, ZrO, FeO, BC, FP酸化物)の溶融/凝固試験を行った。模擬コリウムはAr雰囲気化で2600まで加熱し、2つの冷却速度での降温を行った。(炉冷(平均744C/min)および徐冷(2600C2300C:5C/min、2300C1120C:平均788C/min)元素分析により、炉冷条件および徐冷条件両方の固化後の試料中に3つの異なる組成を持つ酸化物相および1つの金属相が確認された。炉冷条件、徐冷条件ともにこれら3つの酸化物相へのFeO固溶度はおおよそ125at%であった。この結果はUO-ZrO-FeO状態図におおよそ一致している。一方、徐冷条件での試料中に、Zrリッチ相の大粒形化が確認された。この相の組成は液相の初期組成と一致しており、遅い凝固中で液滴の連結が起こり、凝集したと評価した。

Cooling rate of molten core materials during solidification significantly affects the segregation of major constituents of fuel debris. To understand general tendency of the segregation, liquefaction/solidification tests of simulated corium (UO, ZrO, FeO, BC and sim-FP oxides) were performed. Simulated corium was heated up to 2600C under Ar atmosphere and then cooled down with two different cooling processes; furnace cooling (average cooling rate is approximately 744C/min) and slow cooling (cooling rate in 2600C2300C is 5C/min and in 2300C1120C is approximately 788C/min). Element analysis detected three oxide phases with different composition and one metal phase in both solidified samples. Solubility of FeO in these oxide phases was mostly fixed to be 125at% in both samples, which is in reasonable accordance with the value estimated from UO-ZrO-FeO phase diagrams. However, a 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 comparable to the initial average composition. The condensation is considered to be caused by the connection of remaining liquid agglomerates during slow solidification.