ネプツニウム含有硝酸溶液中でのステンレス鋼の腐食挙動

Corrosion behavior of stainless steel in nitric acid solutions including neptunium

本岡 隆文; 山本 正弘 

Motooka, Takafumi; Yamamoto, Masahiro

核燃料再処理工場でのステンレス鋼の腐食挙動を検討するため、放射性物質が取扱える実験室において、ネプツニウム(Np)を溶解させた硝酸溶液を用いて、常圧及び減圧下で腐食試験を実施した。減圧沸騰下では、単味硝酸溶液中に比べてNp含有硝酸溶液中においてステンレス鋼は加速腐食された。また、金属表面温度が溶液温度よりも高い伝熱条件で腐食が激しかった。常圧下での浸漬腐食試験でも、Np濃度増大とともにステンレス鋼の腐食速度は高くなった。溶液温度の上昇は、Np含有硝酸溶液中でのステンレス鋼の腐食速度を増大させた。溶液温度上昇に伴うステンレス鋼表面上でのカソード反応電流の増大が腐食速度増大にかかわっていると考えられた。ステンレス鋼の腐食電位は、硝酸がNpを含むと貴な方向に移動した。Np含有硝酸溶液では、Np(V)とNp(VI)の酸化還元反応速度が高いためNpの再生成が可能であると考えられる。硝酸溶液中でNp(VI)が支配的に存在することが、ステンレス鋼の加速腐食の要因と考えられる。

In order to investigate a corrosion behavior of stainless steel in the typical nuclear fuel reprocessing plant, corrosion tests using nitric acid solution including neptunium were conducted under atmospheric and reduced pressurized conditions in the laboratory where radioactive substance can be handled. Under reduced pressure condition, corrosion of stainless steel was accelerated in nitric acid solution with neptunium than in pure nitric acid solution. Corrosion was more aggressive in heating condition with boiling than in immersion in boiling solution. Corrosion rate of stainless steel was increased as increasing neptunium concentration. Increase of solution temperature accelerated the corrosion rate of stainless steel in nitric acid with neptunium. It is considered that the corrosion rate was accelerated by the increase of cathodic reaction current on stainless steel surface. The corrosion potential of stainless steel is also nobler in nitric acid solution with neptunium than without that. In nitric acid solution including neptunium, the high redox reaction rate of Np(VI)/Np(V) causes re-oxidation of Np. It is considered that the dominance of Np(VI) in nitric acid solution provides the acceleration of stainless steel in nitric acid solutions including neptunium.