Effect of Carbonate on the Migration Behavior of Lanthanides in Compacted Bentonite
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.184
Effect of Carbonate on the Migration Behavior of Lanthanides in Compacted Bentonite Kazuya Idemitsu, Kazuyuki Fujii, Noriyuki Maeda, Yuki Kakoi, Noriya Okubo, Yaohiro Inagaki, Tatsumi Arima Dept. of Applied Quantum Physics and Nuclear Engineering, Kyushu Univ., 744 Motooka, Nishi-ku, Fukuoka, Japan
ABSTRACT The apparent diffusion coefficients of La, Nd, Eu, Dy, Er, and Lu in compacted bentonites were investigated at various bicarbonate concentrations. The apparent diffusion coefficients of these lanthanides tended to decrease with increasing dry density. At bicarbonate concentrations below 0.25 M, lanthanum had the largest diffusion coefficient (ca. 10-13 m2/s) at 1.0 Mg/m3, and the diffusion coefficient decreased with increasing atomic number. On the other hand, at bicarbonate concentrations above 0.25 M, lutetium had the largest diffusion coefficient, and the diffusion coefficient decreased with decreasing atomic number. In particular, lanthanum and neodymium had diffusion coefficients below 10-14 m2/s, even at 1.0 Mg/m3. The diffusion coefficient of europium was around 10-13 m2/s at 1.0 Mg/m3 and was influenced less by the bicarbonate concentration. The diffusion coefficient of lutetium increased from 2 × 10-14 to 10-12 m2/s as the bicarbonate concentration was increased to 1.0 M. The concentration of carbonate ion in the pore water of bentonite is estimated to be much lower than that in solutions in contact with bentonite from the viewpoints of solubility and chemical species of lanthanides. INTRODUCTION To assess the safety of the geological disposal of high-level radioactive waste and spent nuclear fuel in Japan [1], it is critical to understand the barrier function of the bentonite buffer and carbon steel overpack that forms the multi-barrier system. The transport of radionuclides in compacted bentonite is mainly controlled by diffusion, and thus it is important to determine the diffusion coefficients of these radionuclides. A trivalent cation has strong affinity with carbonate ion and generates a positively charged mono-carbonate complex, then a negatively charged di-carbonate complex. according to the carbonate concentration, as follows:
,
(1)
.
(2)
These complexes are expected to have quite different migration behavior in compacted bentonite because montmorillonite, which is the main mineral component of bentonite, has a negatively charged layer [2]. In this work, we examined the diffusion of lanthanides such as La, Nd, Eu, Dy, Er, and Lu at different carbonate concentrations in compacted bentonite to clarify
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their migration behaviors with carbonate ion. Th
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