Identification of the Cathode Reaction Accompanied with Overpack Corrosion
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Identification of the Cathode Reaction Accompanied with Overpack Corrosion Ichiro Otsuka1, Yoshihisa Iida2, Tetsuji Yamaguchi2, Osamu Kato3, Tsuyoshi Tateishi4, and Tadao Tanaka2 1 Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195, Japan (Present Organization: Japan Nuclear Energy Safety Organization, Minato-ku, Tokyo, 105-0001, Japan.) 2 Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195, Japan. 3 Kobe Steel, Ltd., Nada-ku, Kobe, 657-0845 Japan. 4 Kobelco Research Institute, Inc., Nishi-ku, Kobe, 651-2271, Japan.
ABSTRACT Cathode reaction on overpack corrosion in the geological repository environments of radioactive waste was identified from corrosion experiments of carbon steel specimens. Carbon steel specimens were encapsulated in degassed glass ampoules with various solutions which were prepared by distilled water degassed by Ar gas bubbling, and set in a thermostatic bath for several weeks. In the X-ray diffraction and X-ray photochemical spectroscopy, crystalline Fe3O4 and Fe2(OH)2CO3, and amorphous Fe(OH)2 were mainly detected on specimens which were immersed into distilled water, high concentration of sodium hydrogen carbonate solution, and low concentration of sodium hydrogen carbonate solution and sodium sulfate solution, respectively. Only hydrogen gas was detected in a gas phase analysis, indicating that hydrogen generation reaction was the dominant cathode reaction in the anoxic condition expected in geological repository environments.
INTRODUCTION Carbon steel is considered to be the most promising candidate material of the overpack container for the vitrified high-level radioactive waste [1]. After emplacement of the waste container in a deep underground repository, corrosion of carbon steel with pore water of engineered buffer materials strongly influences redox potential (Eh) of the pore water and the solubility of redox-sensitive radionuclides such as selenium-79 [2]. The corrosion of carbon steel can be associated with reduction of sulfate, carbonate and hydrogen ions and potentially controls the Eh. As a result of preliminary Eh analysis by the authors, reductions of sulfate ion, carbonate ion and hydrogen ion occurred in order [3]. However, there is possibility that some reactions might not proceed because of the slow reaction kinetics in the natural systems. For the evaluation of Eh, it is important to clarify whether the reduction of sulfate, carbonate and hydrogen ions actually occurs. In this study, corrosion experiments of carbon steel were conducted to identify the cathode reaction with overpack corrosion in the anoxic condition expected in geological repository environments.
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EXPERIMENT Carbon steel specimens (SM400B[3], 2mm×3mm×60mm: 16 pieces and 2mm×3mm×20mm: 8pieces) were set in a glass ampoule and the ampoule was welded with a stopcock. Ampoules were transferred into an Ar circulated glove box, in which the partial pressure of O2 was controlled less than or equal to 10-2 Pa, and filled with test solutions. Test solutions were distilled water, sodium hydrogen carbonate so
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