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II3.7.1

MIGRATION BEHAVIOR OF IRON ION IN COMPACTED BENTONITE UNDER REDUCING CONDITION BY USING ELECTROMIGRATION. Kazuya Idemitsu, Seiji Yano, Xiaobin Xia, Yoshiro Kikuchi, Yaohiro Inagaki, Tatsumi Arima, Kyushu Univ, Dept of Applied Quantum Physics and Nuclear Engineering, Fukuoka, JAPAN

ABSTRACT Carbon steel is one of the candidate overpack materials for high-level waste disposal and is expected to assure complete containment of vitrified waste glass during an initial period of 1000 years in Japan. The lifetime of the carbon steel overpack will depend on its corrosion rate. The corrosion rate of carbon steel is reduced by the presence of buffer material such as bentonite. Buffer material will delay the supply of corrosive materials and discharge of corrosion products through it. Carbon steel overpack will be corroded by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The reducing condition will be expected to retard the migration of redox-sensitive radionuclides by lowering their solubilities. Therefore, the diffusion of corrosion products of iron in buffer material is important to discuss the corrosion rate of overpack, properties of buffer material and migration of redox-sensitive radionuclides. Electromigration experiments have been carried out with source of iron ions supplied by anode corrosion of iron coupon. Iron ions migrated as fast as 2mm in 2 or 3 hours under electrical field of 100V/m to the direction of cathode. Because the iron ions displaced exchangeable sodium ions in bentonite at a ratio of 1 to 2 during the electromigration, migrating iron ion could be ferrous ion.

INTRODUCTION Carbon steel is one of the candidate overpack materials for high-level waste disposal and is expected to assure complete containment of vitrified waste glass during an initial period of 1000 years in Japan[1]. The lifetime of the carbon steel overpack will depend on its corrosion rate. The corrosion rate of carbon steel is reduced by the presence of buffer material such as bentonite[2,3] and seems to be affected by the diffusion of corrosive materials and corrosion products through the buffer material. Carbon steel overpack will be corroded by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The reducing condition will be expected to retard the migration of redox-sensitive radionuclides such as uranium[4], technetium[5,6] and so on[7] by lowering their solubilities and diffusivities. Therefore, the migration of corrosion products of iron in buffer material is important to discuss the corrosion rate of overpack, migration of redox-sensitive radionuclides and properties of buffer material. It is, however, difficult to keep a reducing experimental condition or to characterize the reducing condition. The purpose of this study is to control reducing conditions for migration experiments easily and to accumulate