Effects of Compaction and Temperature on Sorption and Diffusion of Cs and HTO in Compacted Bentonite Saturated with Sali
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0985-NN11-12
Effects of Compaction and Temperature on Sorption and Diffusion of Cs and HTO in Compacted Bentonite Saturated with Saline Water Satoru Suzuki, Masashi Haginuma, and Kazunori Suzuki Nuclear Chemistry and Chemical Engineering Center, Institute of Research and Innovation, 1201 Takada, Kashiwa, 277-0861, Japan
ABSTRACT The sorption and diffusion of Cs and tritiated water (HTO) in compacted bentonite was investigated at temperatures from 30 to 60°C. The apparent (Da) and effective (De) diffusion coefficients were determined by in-diffusion and through-diffusion experiments with a constant boundary concentration maintained. The temperature dependence of De and Da obeyed an Arrhenius-type equation, allowing determination of the activation energy for diffusion of Cs and HTO. The De value of Cs was three times the De of HTO, which is considered to be a result of surface-excess diffusion. Cs may be concentrated near the surface of the negatively charged clay, thus giving a large diffusive flux. The activation energies for Cs diffusion were 21.4±2.8 kJ/mol and 37.3±1.5 kJ/mol as determined based on De and Da, respectively. This difference was due to the temperature dependence of the distribution coefficient Kd of Cs.
INTRODUCTION Determining the sorption and diffusion of radionuclides in bentonite is of fundamental importance in performance assessments of buffer materials in high-level radioactive waste (HLW) disposal. Assuming that the repository is constructed under the seashore, the groundwater may be saline water, and the temperature of the repository may become moderately high, e.g. up to 60°C as determined for a performance assessment in Japan [1]. However, among the extensive studies done, sorption and diffusion experiments have mostly been performed under conditions at around room temperature and for solutions containing a small concentration of salt. In this study, the De, Da and Kd values of Cs were determined at elevated temperatures up to 60°C in compacted bentonite saturated with saline water, and the activation energy of diffusion was obtained. The linear sorption model has been generally applied to predict nuclide migration in the buffer [1, 2], and a preliminary performance assessment given in the reference [1] showed a high release rate of Cs. In JNC [1], the activation energy of 15 kJ/mol determined in bulk solution was assumed for the diffusion in bentonite. In a previous report, we determined the De, Da and Kd values of 137Cs for compacted crude bentonite at 60°C [3]. However, the temperature dependence of the diffusion was not investigated. Here we describe the temperature dependence and discuss the effects of both compaction and sorption on the diffusion of Cs in compacted bentonite.
EXPERIMENT Materials The compacted bentonite in this study was a mixture of silica sand and bentonite (with a mass fraction of 3:7) compacted to a targeted dry bulk density of 1.6 Mg/m3, as proposed for the buffer material in the H12 report [1]. The bentonite material was Kunigel V1®, which contains 46–49 w
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