Effect of pH on Plutonium Migration Behavior in Compacted Bentonite
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Effect of pH on Plutonium Migration Behavior in Compacted Bentonite Ryo Hamada1, Noriyuki Maeda1, Kazuya Idemitsu1, Yaohiro Inagaki1, Tatsumi Arima1, Daisuke Akiyama2, Kenji Konashi3, Makoto Watanabe3, Shin-ichi Koyama4 1
Dept. of Applied Quantum Physics and Nuclear Engineering, Kyushu Univ., 744 Motooka, Nishi-ku, Fukuoka, Japan 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1 Katahira, 2-choume, Aoba-ku,Sendai, Japan 3 International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University, 2145-2 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki, Japan 4 Japan Atomic Energy Agency, 4002 Narita, Oarai, Higashi-Ibaraki, Ibaraki, Japan ABSTRACT In disposing of high-level radioactive waste, the drop in pH in the repository as the iron overpack corrodes must be considered. Plutonium migration behavior may be affected by the pH of pore water in compacted bentonite barriers in high-level waste repositories. To examine the effect of pH on migration behavior, H-bentonite was prepared by treating Japanese Na-bentonite, Kunipia-F, with hydrochloric acid. Diffusion experiments were performed with mixtures of Naand H-bentonites. The pH value in the pore water of the water-saturated bentonite mixtures decreased from 8 to 3 as the mixing ratio of H-bentonite increased. Diffusion experiments were carried out by using 238Pu then apparent diffusion coefficients were determined from the plutonium distribution in the specimens. The apparent diffusion coefficients were on the order of 10-13 to 10-12 m2/s at pH values lower than 4, whereas they were less than 10-14 m2/s at pH values higher than 6.5. These results indicate that plutonium diffused faster as Pu3+ or PuO22+ due to disproportionation at lower pH while plutonium could be retarded as Pu(OH)40 by sorption on bentonite at higher pH. INTRODUCTION In assessing the safety of geological disposal of high-level radioactive waste and spent nuclear fuel in Japan [1], understanding the barrier function of the bentonite buffer and carbon steel overpack that form the multi-barrier system is crucial. The transport of radionuclides in compacted bentonite is mainly controlled by diffusion, and thus it is important that the diffusion coefficients of radionuclides are determined. After high-level waste repositories are closed, the carbon steel overpack will form corrosion products, which can migrate into bentonite. Iron corrosion products may alter the bentonite and maintain reducing conditions, causing a pH drop by precipitation of ferrous or ferric hydrate in the repository that may affect migration behavior, especially for redox-sensitive elements such as plutonium. Plutonium, which is a radiochemical toxicant, has very low diffusivity in compacted bentonite. There are several studies of plutonium diffusion under oxidizing conditions in compacted bentonite, for example in a concrete-bentonite system over 5 years [2]. However, there are few reports of plutonium diffusion under reducing conditions. Therefore
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