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Migration Behavior of Plutonium in Compacted Bentonite under Reducing Condition by Using Electromigration Kazuya Idemitsu, Xiaobin Xia, Yoshiro Kikuchi, Yaohiro Inagaki, Tatsumi Arima, T. Mitsugashira*, M. Hara*, Y. Suzuki* Dept of Applied Quantum Physics and Nuclear Engineering, Kyushu Univ., Fukuoka, JAPAN *The Oarai Branch, Institute for Materials Research, Tohoku University, Ibaraki, JAPAN $%675$&7 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. 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 presence of corrosion products of iron in buffer material is important to discuss the migration behavior of redoxsensitive radionuclides. Plutonium electromigration experiments in bentonite have been carried out with source of iron ions supplied by anode corrosion of iron coupon. Plutonium migrated from the iron anode toward cathode as deep as 1 mm of the interior of bentonite within 24 h. 2+ Thus plutonium chemical species would have positive charge and were estimated as PuOH or PuCl2+ by the thermodynamic calculation. ,1752'8&7,21  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]. Carbon steel overpack will be corroded by consuming oxygen introduced by repository construction after closure of repository. Corrosion products will diffuse into buffer materials and then will keep the reducing environment in the vicinity of repository[2]. The reducing condition will be expected to retard the migration of redox-sensitive radionuclides such as uranium[3], technetium[4,5] and so on[6] by lowering their solubilities and diffusivities. Rare study on plutonium diffusion in compacted bentonite is available. It has been reported that no movement could be measured for plutonium diffusion in a concrete-bentonite system under oxdizing condition as long as 5 ysars of experimental period[7]. On the other hand there is a report that plutonium has higher diffusion coefficient under the reducing condition with iron corrosion product than under oxidizing conditions[8]. 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 the knowledge about migration behavior of plutonium in compacted bentonite under the reducing conditions by electromigration technique[9].

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EXPERIMENTAL Materials A typical Japanese sodium bentonite, Kunipia-F, was used in this experiment. Kunipia-F c