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Migration Behaviour of Lanthanides in Compacted Bentonite with Iron Corrosion Product Using Electrochemical Method Kazuya Idemitsu, Daisuke Akiyama, Yoshihiko Matsuki, Yusuke Irie, Yaohiro Inagaki, Tatsumi Arima Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka, Japan ABSTRACT After the closure of a high-level waste repository, corrosion of the carbon steel overpack will occur. The corrosion products can then migrate into bentonite and affect the migration behavior of radionuclides in bentonite. Therefore, electrochemical experiments, with Fe2+ supplied by anodic corrosion of carbon steel, were carried out to study trivalent lanthanides in compacted bentonite. The interface between a carbon steel coupon and bentonite (dry density, 1.5 Mg/m3) was spiked with a tracer solution containing Nd(NO3)3, Eu(NO3)3, Dy(NO3)3, and Er(NO3)3. The carbon steel coupon was connected as the working electrode to a potentiostat and held at a constant potential between -550 and 0 mV (vs. Ag/AgCl reference electrode) for 7 days. A model using dispersion and electromigration could explain the measured profiles in the bentonite specimens. The best-fit electromigration velocity was related to the applied electric potential and was 1.0–3.8 nm/s for Nd, Eu, Dy, and Er ions. For these lanthanides, the best-fit dispersion coefficient was also related to the applied potential and was 0.8–1.6 μm2/s, and the dispersion length was calculated as 0.2 mm from the linear relationship between the dispersion coefficient and electromigration velocity. Finally, the apparent diffusion coefficient for these lanthanides was estimated as 0.6–0.9 ȝP2/s. INTRODUCTION Bentonite and carbon steel are respectively candidates for the buffer material and overpack of high-level waste disposal facilities in Japan [1]. After closure of the repository, corrosion of the carbon steel overpack will occur, and corrosion products can then diffuse into bentonite [2]. Corrosion products affect migration behavior, especially for redox-sensitive elements. Therefore, we have developed and implemented an electrochemical method where ferrous ions are supplied by anodic corrosion of a carbon steel coupon into compacted bentonite [3]. Nondiffusive Pu4+ has been reported to be reduced to diffusive Pu3+ at the interface between carbon steel and bentonite [4]. Accordingly, the main objective of this study is to elucidate the migration behavior of trivalent lanthanide cations in compacted bentonite under reducing conditions in the presence of steel corrosion products. EXPERIMENTAL A typical Japanese sodium bentonite, Kunipia-F, which contains approximately 99 wt % montmorillonite, was used in this experiment. The chemical formula of Kunipia-F is (Na0.3Ca0.03K0.004)(Al1.6Mg0.3Fe0.1)Si4O10(OH)2 [5].

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Bentonite powder was compacted into cylinders (diameter, 10 mm; height, 10 mm; dry density, ~1.5 Mg/m3; porosity, ~0.44). Each compacted bentonite specimen was inserted into an acrylic resin column and saturated with 0.01 M NaCl (aq) for 30 days. A c