Evaluation of potential shaft backfill formulations for use in Canada's deep geological repository

Abstract

The Canadian Nuclear Waste Management Organization (NWMO) is evaluating Sedimentary and Crystalline geosphere media as potential hosts for a Deep Geological Repository (DGR) for permanent isolation of used nuclear fuel and fuel wastes.Bentonite-aggregate mixtures containing from 50 to 90% by dry mass bentonite were tested. The aggregates used were relevant to the NWMO's DGR geological options (granitic sand (GS) and sand-sized crushed limestone (CRL)). Of importance is that the moisture conditioning fluids used are representative of those that will be encountered at the locations considered by NWMO for their aDGR. These materials, if moisture-conditioned with local groundwater will mean that the porewater of the backfill will be close to chemical equilibrium with its surroundings at the time of placement. The effectiveness of Standard and Modified compaction efforts in densifying these mixtures were evaluated, providing guidance regarding what materials and compaction options are likely to achieve the swelling pressure (Ps) and hydraulic conductivity (k) performance goals (>100 kPa and < 10−10 m/s, respectively) set for shaft backfill once it comes to density equilibrium and chemical balance with the surrounding rock mass.When Standard Compaction Effort was used, aggregate type did not affect densification, while aggregate content and compaction composition fluid did. Dry density increased with increasing aggregate content, while effective montmorillonite dry density (EMDD) showed a small increase as bentonite content increased from 50 to 90%.When Modified Compaction Effort was used, aggregate type and composition of the compaction fluid had little effect on the densification achieved. Dry density achieved steadily decreased with increasing bentonite content and EMDD achieved was little affected by bentonite contents >60% by mass.Hydraulic conductivity values measured were consistent with previously reported data for bentonite and bentonite-aggregate mixtures. The hydraulic conductivity of mixtures prepared using crushed limestone were slightly lower than determined for granitic sand.The swelling pressure and hydraulic conductivity requirements set for the SBF were met for all materials examined (50–90% bentonite blended with crushed limestone or granitic sand, using either standard or modified compaction effort at low salinity. Where highly saline conditions existed, the Ps and k requirements for the shaft backfill were not be met for materials compacted using standard compaction effort.