Sorption Behavior of Radionuclides on Calcium-Leached Mortar
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Sorption Behavior of Radionuclides on Calcium-Leached Mortar Nobuo Ashikawa1, Takatoshi Tajima2, Hiroshi Saito2 and Ai Fujiwara3 1 Kyushu Environmental Evaluation Association, 1-10-1 Matsukadai, Fukuoka 813-0004, Japan 2 Obayashi Corporation, 640 Shimokiyoto 4-chome, Kiyose-shi Tokyo 204-8558, Japan 3 Radioactive Waste Management Center, No.15 Mori Bldg. 2-8-10 Toranomon, Minatoku Tokyo 105-0001, Japan ABSTRACT Low-level radioactive waste (LLRW) is disposed of by shallow land burial. Reinforced concrete is used as the radioactive waste repository. However, the concrete structure is in contact with water and will gradually degrade over an extended period of time due to leaching. It is important to investigate the interaction between radionuclides and degraded concrete in the safety assessment of nuclear waste disposal. The authors measured the distribution coefficients (Kd) of various radionuclides for calcium-leached mortars. The calcium-leached mortars were prepared by an accelerated leaching test for mortar based on the electrical potential gradient. These degraded conditions are similar to that of degraded concrete in contact with water for a long period of time. The degradation degree of calcium-leached mortar is evaluated by the CaO/SiO2 molar ratio (Ca/Si ratio) of calcium silicate hydrate (C-S-H). As a result, the relationship between Kd and the Ca/Si ratio in C-S-H can be roughly grouped into the following three types : 1. 137Cs and 85Sr – Kd decreases with an increase in the Ca/Si ratio. 2. 95mTc and 110mAg – There is no correlation between Kd and the Ca/Si ratio. 3. 14C, 241Am and 125I – Kd increases with an increase in the Ca/Si ratio. INTRODUCTION Materials based on hydraulic cements have many potential applications in the field of radioactive waste disposal. LLRW generated at nuclear power plants is disposed of by shallow burial. A schematic of a shallow burial of LLRW in Japan is shown in Figure1 [1]. Reinforced concrete is used for the radioactive waste repository. The buried concrete structure is in contact with water and will gradually Figure 1. Schematic of shallow burial of LLRW in Japan [1].
degrade over an extended period of time through leaching and chemical attack. To estimate disposal safety, it is important to investigate the interaction between radionuclides and engineered barriers (e.g. concrete, bentonite). Many laboratory experiments concerning the diffusion and sorption of radionuclides in engineered barriers have been carried out [2-4]. In addition, various authors have investigated the degradation mechanism of cementious material in a radioactive waste repository environment [5-7]. In this study, accelerated leaching test for mortar, based on the electrical potential gradient, was carried out to prepare calcium-leached mortar. These mortars that have had Ca and other ions leached out are similar to the degraded concrete in contact with underground water for a long time. After that, we measured the Kd between seven kinds of radionuclides and these mortars. EXPERIMENTAL Materials The
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