Diffusion Behavior of Organic Carbon and Iodine in Low-heat Portland Cement Containing Fly Ash
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1124-Q10-15
Diffusion Behavior of Organic Carbon and Iodine in Low-heat Portland Cement Containing Fly Ash Taiji Chida and Daisuke Sugiyama Central Research Institute of Electric Power Industry, 2-11-1, Iwado-Kita, Komae, Tokyo 2018511, Japan ABSTRACT The diffusion of radionuclides in cementitious materials used as an engineered barrier is an important parameter in the performance assessment of the sub-surface repository system used for low-level radioactive waste disposal in Japan. In particular, organic carbon-14 and iodine-129 would provide large contributions to the dose evaluation, because of their low ability to be adsorbed on cementitious materials. In this study, the diffusion of acetate and iodide in hardened cement pastes was examined by through-diffusion experiments. Low-heat Portland cement containing 30 wt% fly ash (FAC), which is a candidate cement material for the construction of the sub-surface repository, was prepared for the diffusion experiments. The effective diffusion coefficients, De, of the trace ions for hardened FAC cement pastes were estimated to be on the order of 10-13 m2 s-1 at the beginning of the diffusion experiments. Then, the rate of diffusion of the trace ions decreased over the experimental period of 1-15 months. This is probably due to the change in the microstructure of the FAC as the result of a pozzolanic reaction. After a few months, the values of De were estimated to be on the order of 10-14 m2 s-1. These results suggest that an engineered barrier made of FAC can act as an effective barrier inhibiting the diffusion of trace ions such as organic carbon and iodine. INTRODUCTION In the sub-surface repository system for the disposal of low-level radioactive waste in Japan, cementitious materials can be used as an engineered barrier to limit the migration of radionuclides [1]. In particular, organic carbon-14 [2] and iodine-129 would provide large contributions to the dose evaluation, because they are sorbed less strongly on cementitious materials [3]. Low-heat Portland cement containing 30 wt% fly ash (FAC) is a candidate cement material for the construction of the sub-surface repository because of its closely packed structure and its resistance to cracking. Therefore, the diffusion of radionuclides in cementitious materials is a very important parameter in performance assessment when considering the release of these radionuclides from waste and their migration in a cementitious repository environment. To clarify the barrier properties of FAC materials, in this study we examined the diffusion coefficients for organic carbon and iodine in FAC.
EXPERIMENT Materials Solid disk-shaped samples of FAC were prepared for the diffusion experiments. As a comparison, ordinary Portland cement (OPC), which is the most commonly used cementitious material, was also used in the experiments. The cements were mixed with deionized water in water/cement clinkers (w/c) with different w/c mixing ratios of 0.35 and 0.70, to produce different diffusivities in the solid. To prevent bleeding (the sepa
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