Migration behavior of plutonium affected by ferrous ion in compacted bentonite by using electrochemical technique
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Migration behavior of plutonium affected by ferrous ion in compacted bentonite by using electrochemical technique Daisuke Akiyama1, Kazuya Idemitsu1, Yaohiro Inagaki1, Tatsumi Arima1, Kenji Konashi2, Shinichi Koyama3 1 Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 819-0395, Japan Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan
2 3
Japan Atomic Energy Agency, 4002 Narita, Oarai, Higashi-Ibaraki, Ibaraki 311-1393, Japan
ABSTRACT The migration behavior of plutonium is expected to be affected by the corrosion products of carbon steel in compacted bentonite at high-level waste repositories. Electrochemical experiments were carried out to simulate the reducing environment created by ferrous iron ions in equilibrium with anoxic corrosion products of iron. The concentration profiles of plutonium could be described by the convection -dispersion equation to obtain two migration parameters: apparent migration velocity Va and apparent dispersion coefficient Da. The apparent migration velocity was evaluated within 1 nm/s and was found to be independent of the experiment duration and the dry density of bentonite in the interval 0.8-1.4 Mg/m3. The apparent dispersion coefficient increased with the experiment duration at a dry density of 1.4 Mg/m3. The results for other dry densities also showed the same trend. These findings indicate that plutonium migration likely starts after ferrous ions reach the plutonium, in other words, the reducing environment due to ferrous ions could change the chemical form of plutonium and/or the characteristics of compacted bentonite. The apparent diffusion coefficient was estimated to be around 0.5 to 2.2 μm2/s and increased with decreasing the dry density of bentonite. INTRODUCTION Bentonite buffers and carbon steel overpacks will constitute important parts of multi-barrier systems for geological disposal of high-level waste in Japan [1]. After closure of a high-level waste repository, the carbon steel overpack will produce anoxic iron corrosion products containing ferrous iron, which can migrate into bentonite. Iron corrosion products may alter bentonite and maintain a reducing environment, which may affect the migration behavior, especially for redox-sensitive elements such as plutonium. Plutonium has a very low diffusivity in compacted bentonite under an oxidizing environment. There have been a few studies on
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plutonium diffusion in compacted bentonite, for example, in a concrete-bentonite system with an experiment duration as long as 5 years [2]. However, there have been few reports on its diffusion behavior under a reducing environment. Therefore, the authors have developed and carried out electrochemical experiments with ferrous ions supplied to compacted bentonite by anodic corrosion of an iron coupon [3,4]. EXPERIMENTAL A typical Japanese sodium bentonite, Kunipia-F, was used in this experiment. It contains approximately 95 wt % of montmorillonite. The chemical formula of Kunipia-F is estimated to be (Na0
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