Corrosion products from carbon steel formed in compacted bentonite under reducing conditions

圧縮ベントナイト中の鉄腐食生成物の存在形態

石寺 孝充 ; Xia, X.*; 出光 一哉*; 菊池 芳郎*

Ishidera, Takamitsu; Xia, X.*; Idemitsu, Kazuya*; Kikuchi, Yoshio*

オーバーパック腐食生成物が圧縮ベントナイト中の放射性核種の収着挙動,拡散挙動へ及ぼす影響を調べるため、あらかじめ電解腐食させた炭素鋼を圧縮ベントナイトに接触させて鉄腐食生成物をベントナイト中へ移行させ、圧縮ベントナイト間隙中の鉄腐食生成物の存在形態を調べた。圧縮ベントナイトをスライスし、スライス片の選択溶解抽出を行い鉄腐食生成物の結晶化の程度を調べ、鉄腐食生成物の価数を調べた結果、室温で数年の範囲内では、鉄腐食生成物は非晶質のFe(OH)及びFe(OH)として存在していることが推測された。XRD分析の結果より、鉄腐食生成物によってNa型スメクタイトのFe型化はほとんど起こっておらず、鉄腐食生成物はスメクタイト粒子間に存在していることが示唆された。また、あらかじめ大気下で生成した3価の鉄腐食生成物は、圧縮ベントナイト間隙中では容易に2価に還元されないものと考えられた。

For safety assessment of high-level radioactive waste disposal, it is important to identify the forms of corrosion products migrating from overpack material into compacted bentonite. In this study, the carbon steel, which was previously corroded electrochemically under aerobic condition, was in contact with compacted Kunigel V1 and Kunipia F under reducing condition for 3-4 years at room temperature. The corrosion products migrating from carbon steel into compacted bentonites were investigated by the selective dissolution analysis, which can estimate the crystallinity of Fe-bearing compounds. The valence of iron in corrosion products was investigated spectrophotometrically. Furthermore, the alteration of smectite contained in compacted bentonite to Fe-smectite was analyzed by X-ray diffractometry (XRD). The results of selective dissolution and valence analysis suggested that the corrosion products in compacted bentonite were amorphous, non-crystalline or poorly ordered Fe(OH) and Fe(OH). In the XRD profiles, no diffraction peak suggesting the existence of Fe-smectite in the compacted bentonite was found. Therefore, the corrosion products in compacted bentonite were considered to have no effect on the alteration of smectite contained in Kunigel V1 and Kunipia F to Fe-smectite.