Migration Behavior of Potassium and Rubidium in Compacted Bentonite Under Reducing Condition With Iron Corrosion Product

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Migration Behavior of Potassium and Rubidium in Compacted Bentonite Under Reducing Condition With Iron Corrosion Product Kazuya Idemitsu1, Hirotomo Ikeuchi2, Daisuke Akiyama1, Yaohiro Inagaki1, Tatsumi Arima1 1 Dept. of Applied Quantum Physics and Nuclear Engineering, Kyushu Univ., Fukuoka, JAPAN 2 Japan Atomic Energy Agency, Tokai, JAPAN ABSTRACT Carbon steel overpack will corrode by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The iron corrosion products can migrate in bentonite as ferrous ion through the interlayer of montmorillonite replacing exchangeable sodium ions in the interlayer. This replacement of sodium with ferrous ion may affect the migration behavior in the altered bentonite not only for redox-sensitive elements but also the other ions. Therefore the authors have carried out electromigration experiments of potassium or rubidium with source of iron ions supplied by anode corrosion of iron coupon in compacted bentonite. Five to fifteen micro liter of tracer solution containing 3.3 M of KCl or 2.2 M of RbCl was spiked on the interface between an iron coupon and bentonite, which dry density was around 1.4 Mg/m3, before assembling. The iron coupon was connected as the working electrode to the potentiostat and was held at a constant supplied potential between - 600 and 300 mV vs. Ag/AgCl reference electrode for up to 8 days. Potassium could migrate faster and deeper in bentonite specimen than iron in each condition. On the other hand rubidium could migrate slower than iron. Migration velocity was a function of applied electrical potential and 8 to 14 nm/s for potassium, 5 to 10 nm/s for iron and 3 to 5 for rubidium, respectively. Dispersion coefficient was also a function of applied potential and 10 to 14 x 10 -12 m2/s for potassium, 4 to 8 x 10-12 m2/s for rubidium and 2 to 4 x 10 -12 m2/s for iron, respectively. Diffusion experiments were also carried out for comparison. Potassium and rubidium might migrate slightly slower in the altered bentonite by iron corrosion than in ordinary compacted bentonite.

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]. Carbon steel overpack is corroded after closure of the repository. Corrosion products diffuse into buffer materials and then maintain the reducing environment in the vicinity of the repository [2]. Corrosion products may affect migration behavior for not only redoxsensitive elements but also other elements. Authors have developed and carried out electromigration experiments with source of iron ions supplied by anode corrosion of iron coupon in compacted bentonite [3]. It is reported in the previous study that the larger amount of ferrous ion was introduced into bentonite as the higher potential is supplied to iron coupon [4]. The main objective of this study is to accumulate