Fundamental Study on Transport Model for Radionuclides under Unsaturated Condition around Near-Surface Underground
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.54
Fundamental Study on Transport Model for Radionuclides under Unsaturated Condition around Near-Surface Underground Takenori ozutsumi1, Masayuki kogure1, Yuichi niibori1, Taiji chida1 1
Tohoku University, Aobayama 6-6-01-2, Sendai 980-8579, JAPAN
ABSTRACT
The low-level nuclear wastes such as decontamination waste from Fukushima are disposed in near-surface underground, where the intermittent recharge of rain and groundwater causes spatial distribution of water content. Therefore, pores of soils are not filled with water, that is, an unsaturated zone will be formed. In such a condition, since the water flow path are detoured by clogged gas in pores of soil in the unsaturated zone, the migration path of radionuclide would be different from the saturated zone. So far, the one-dimensional advection-dispersion equation (ADE) model has been widely used in order to explain experimental results under an unsaturated condition. However, the detouring of local flowpaths remarkably affects the mass transfer. The one-dimensional ADE evaluates such a detouring effect by using Peclet number and retardation coefficient as fitting parameters. In other words, the one-dimensional ADE model is difficult to explain mass transfer under an unsaturated condition. Therefore, the purpose of this study is explaining such complicated transport of radionuclides using a multi-path model based on phenomena in underground. The proposed multi-path model considering both water saturation and permeability distributions showed good agreement with the experimental data under an unsaturated condition.
INTRODUCTION: For assessing the performance of the near-surface disposal system for low-level radioactive wastes, it is important to also estimate the migration behaviour of radionuclides under an unsaturated condition. At a shallow depth in underground, pores of soil are not always filled with a liquid phase completely, because a porous medium in near-surface underground is affected by mainly intermittent recharge of rainwater. In such an unsaturated zone, it is reported that the migration behaviour of radionuclides 223
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may be changed, because the water flow detours due to a gas phase clogged in flowpaths. So far, the one-dimensional, advection-dispersion equation (1D-ADE) has been widely used in order to compare experimental results with calculating results (e.g., [1]), while the conventional 1D-ADE model cannot accurately describe the complex transportation of radionuclides in an unsaturated zone. Especially, as shown in Fig.1, in the experimental results implying such a detouring effects of liquid phase due to the existence of gas phase (i.e., local permeability distribution), the 1D-ADE model may overestimate the dispersion coefficient and/or the r
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