Dissolution behavior and aging of iron-uranium oxide

鉄-ウラン酸化物の溶解挙動と経時変化

頓名 龍太郎*; 佐々木 隆之*; 岡本 芳浩  ; 小林 大志*; 秋山 大輔*; 桐島 陽*; 佐藤 修彰*

Tonna, Ryutaro*; Sasaki, Takayuki*; Okamoto, Yoshihiro; Kobayashi, Taishi*; Akiyama, Daisuke*; Kirishima, Akira*; Sato, Nobuaki*

原子炉構造材料のステンレス鋼成分である鉄とUOの高温反応により生成したFeUO化合物の溶解挙動を大気条件下で調べた。化合物はUOとFeOを出発原料として電気炉で調製し、X線回折、走査型電子顕微鏡-エネルギー分散型X線分光法、X線吸収微細構造分光法を用いて固体状態を分析した。FeUO化合物の浸出試験を最長3か月行い、水中に溶存する核種の濃度を調べた。FeUOの溶解の初期段階で3価のFeイオンと5価のUイオンの間で酸化還元反応が起こることが提案された。還元された2価のFeイオンは、溶存酸素の存在下で最終的に3価のイオンに酸化され、水酸化鉄がFeの溶解度を制限することが熱力学的に推論された。一方、6価のU(すなわちウラニルイオン)の濃度は、メタショエパイトやウラン酸ナトリウムなどの二次鉱物の存在によって制限され、その後、Fe酸化物などへの収着によって減少したと考えられる。RuやCeなどの核分裂生成物の多価イオン濃度も減少したが、これは上記の理由によるものと考えられる。一方、可溶性Csイオンの濃度は減少しなかった。この解釈の妥当性は、参照試料(Feを含まないUO)の溶解挙動と比較することで裏付けられた。

The dissolution behavior of FeUO compounds formed by a high-temperature reaction of UO with iron, a stainless-steel component of reactor structural materials, was investigated under atmospheric conditions. The compounds were prepared in an electric furnace using UO and FeO as starting materials, and their solid states were analyzed using X-ray diffraction, scanning electron microscopy energy dispersive X-ray spectroscopy, and X-ray absorption fine structure spectroscopy. The concentration of nuclides dissolved in water was examined by performing static leaching tests of FeUO compounds for up to three months. A redox reaction was proposed to occur between trivalent Fe and pentavalent U ions in the early stage of FeUO dissolution. It was thermodynamically deduced that the reduced divalent Fe ion was finally oxidized into a trivalent ion in the presence of dissolved oxygen, and iron hydroxide limited the solubility of Fe. Meanwhile, the concentration of hexavalent U (i.e., uranyl ion) was limited owing to the presence of secondary minerals such as metaschoepite and sodium uranate and subsequently decreased, possibly owing to sorption on Fe oxides, for example. The concentrations of multivalent ions of fission products, such as Ru and Ce, also decreased, likely for the reason above. By contrast, the concentration of soluble Cs ions did not decrease. The validity of this interpretation was supported by comparing the results with the dissolution behavior of a reference sample (Fe-free UO).