Contemplation of relative hydraulic conductivity for compacted bentonite in a high-level radioactive waste repository

Abstract

A deep geological repository has been suggested for the disposal of high-level radioactive waste (HLW) produced from nuclear power plants. The deep geological repository system can be divided into engineered barrier system (EBS) and natural barrier system (NBS). The engineered barriers are feature of the disposal system by humans, composed of a disposal canister, a backfill material, a gap-filling material, and a buffer material. Compacted bentonite buffer materials are one of the most important components as it protects the disposal canister from external impacts and the penetration of groundwater, and it retard radionuclide transport.

Since groundwater flows into the compacted bentonite buffer materials, it is important to investigate the unsaturated hydraulic properties of compacted bentonite buffer materials such as water retention curve (WRC) and relative hydraulic conductivity. The power law is usually used to describe the relative hydraulic conductivity, and it can be defined as unsaturated hydraulic conductivity divided by saturated hydraulic conductivity. The power coefficient (L) in the power law is mainly applied to values near 3 regardless of unsaturated soil properties. Therefore, this paper evaluated the relative hydraulic conductivity function of compacted bentonite buffer materials using the van-Genuchten model, which was converted to the potential law equation. Based on the process suggested in this research, the L values for MX-80 and FEBEX bentonites were derived. The L values obtained using the proposed process exhibited <5 ~ 10% difference from previously reported values. Furthermore, the L values for Korean compacted bentonite were calculated as 2.64 and 3.23 with dry densities of 1.59 g/cm3 and 1.73 g/cm3, respectively. This shows the importance of the proper estimation of the L value.