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1265-AA06-07

Migration Behavior of Alkali Earth Ions in Compacted Bentonite with Iron Corrosion Product Using Electrochemical Method Kazuya Idemitsu, Daisuke Akiyama, Akira Eto, Yoshihiko Matsuki, Yaohiro Inagaki, Tatsumi Arima Dept. of Applied Quantum Physics and Nuclear Engineering, Kyushu Univ., Fukuoka, JAPAN ABSTRACT Carbon steel overpack will corrode by consuming oxygen introduced during repository construction after closure of repository, that will keep the environment in the vicinity of repository reducing. The iron corrosion products can migrate in bentonite as ferrous cations (Fe2+) through the interlayer of montmorillonite replacing the exchangeable sodium ions in the interlayer. This replacement of sodium may affect the migration behavior in the altered bentonite not only for redox-sensitive elements but also the other ions. Therefore we have carried out electrochemical analysis, of calcium, strontium or barium with the ferrous ion supplied by anodic corrosion of iron coupons in compacted bentonite. Fifteen micro liters of tracer solution containing 8.6 M of CaCl2 or 3.0 M of SrCl2 or 1.5 M BaCl2 were spiked on the interface between the iron coupon and bentonite, for which the dry density was in the range of 1.4 to 1.5 Mg/m3, before assembling. The iron coupons were connected as working electrodes to the potentiostat and held at a constant supplied potential between - 500 to +300 mV (vs. Ag/AgCl reference electrode) for up to 7 days. Calcium and strontium could migrate faster and deeper into the bentonite than iron in each condition, while barium could migrate slower than iron. A model using dispersion and electromigration can explain the measured profiles in the bentonite specimens. The fitted value of electromigration velocity was a function of applied electrical potential and 10 to 23 nm/s for calcium, 11 to 19 for strontium, around 4 nm/s for barium and 5 to 15 nm/s for iron, respectively. Alternatively, the fitted value of the dispersion coefficient was not a function of applied potential, and the values were 3 - 8 x 10-12 m2/s for calcium, 2 - 4 x 10-12 m2/s for strontium, 5 - 10 x 10-12m2/s for barium and 3 - 9 x 10-12 m2/s for iron, respectively. 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. The 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 ions were introduced into bentonite as the higher potential is supplied to iron coupon [4]. T