Radionuclide Transport in a Water - Saturated Planar Fracture with Bentonite Extrusion
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1006-R07-03
Radionuclide Transport in a Water – Saturated Planar Fracture with Bentonite Extrusion R.A. Borrelli and J. Ahn Nuclear Engineering, University of California - Berkeley, 4155 Etcheverry Hall, MC 1730, Berkeley, CA, 94720 ABSTRACT This paper presents a radionuclide migration model that incorporates bentonite extrusion. The model consists of two parts: one for movement of water and bentonite in a planar fracture and the other for radionuclide transport by taking into account advection, diffusion, and sorption with moving bentonite particles. Numerical results indicate that strongly sorbing radionuclides are contained completely within the region of bentonite extrusion. This observation suggests importance of the region in the vicinity of buffer/rock interface in terms of impact on radionuclide release to surrounding host rock.
INTRODUCTION In the engineered barrier system, (EBS), for a water ñ saturated repository, the function of the buffer is to inhibit the mass-transfer processes around the waste package. Compacted bentonite will be utilized for the buffer in the saturated repository primarily due to three favorable characteristics: low permeability, high sorption capacity, and high swelling capacity. A material that demonstrates a low permeability will result in a negligibly slow groundwater flow. Bentonite particles possess an overall negative charge and a high surface area and thus exhibit a high sorption capacity. Bentonite also swells greatly beyond its initial volume when in contact with water. This swelling effect results in extrusion of the buffer into the fracture, which can effectively seal the fracture and reduce releases of radionuclides. The host rock containing fractures connecting the EBS to the far field represents an important domain. Due to the presence of fractures, infiltration of groundwater will form pathways for radionuclide release. Previous modeling studies of radionuclide transport in this region do not include effects of extruding bentonite on the transport of radionuclides. This paper presents a radionuclide migration model that considers the effects of bentonite extrusion by coupling an extrusion model with a radionuclide transport model. The bentonite extrusion changes temporally and spatially the fluid porosity in the fracture, resulting in variable hydraulic permeability and sorption retardation factors. In exchange with extruding bentonite, water comes into the bulk bentonite region. N*
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Figure 1. The model space for radionuclide transport modeling. See text for explanations.
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MODEL The model space is defined in Figure 1. A planar fracture assumed to be initially filled with water intersects saturated bulk bentonite. The bulk bentonite is cylindrical; extrusion occurs in a radial direction as indicated. A constant radionuclide (N*) source is located at the intersection between the bulk bentonite and the fracture (R0). Conditions for this study assume that a sufficient amount of time has elapsed so that short l
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