Water migration through compacted bentonite backfills for containment of high-level nuclear waste

Abstract

Tests carried out with compacted sodium and calcium bentonites at room temperature indicate that bentonite backfills will effectively control water movement near a high-level nuclear waste package. Saturation tests indicate that water will rapidly diffuse into a dry bentonite backfill, reaching saturation in times on the order of tens of years. The apparent diffusion coefficient for sodium bentonite (-5 wt%a initial water content) compacted to 2.1 g/cm 3 is 1.7 × 10 −6 cm 2/sec. However, the hydraulic conductivities of saturated bentonites are low, ranging from approximately 10 −11 cm/sec to 10 −13 cm/sec over a density range of 1.5 g/cm 3 to 2.2 g/cm 3. The hydraulic conductivities of compacted bentonites are at least several orders of magnitude lower than those of candidate-host silicate rocks, indicating that most flowing groundwater contacting a bentonite backfill would be diverted around the backfill rather than flowing through it. In addition, because of the very low hydraulic conductivities of bentonite backfills, the rate of chemical transport between the containerized waste and the surrounding host rock will be effectively controlled by diffusion through the backfill. The formation of a diffusion barrier by the backfill will significantly reduce the long-term rate of radionuclide release from the waste package, an advantage distinct from the delay in release resulting from the sorptive properties of a bentonite backfill.