Backfilling of Shafts in a Deep Geological Repository: Evaluation of Potential Materials

Abstract

The NWMO is investigating Sedimentary and Crystalline media as potential hosts for a Deep Geological Repository (DGR) for permanent isolation of used nuclear fuel. Access to the placement level of the DGR will be via vertical shafts. On completion of all placement and monitoring activities, the access tunnels, underground openings and the vertical shafts will be backfilled and sealed. The backfill installed in the shafts will consist of compacted bentonite-aggregate mixture(s), with plugs and seals installed in critical locations as required.

Backfilling operations are not strongly affected by the geosphere, but the swelling, hydraulic and other behaviours of backfill are strongly influenced by the backfill composition, its density and the composition (salinity) of the groundwater entering from the adjacent rock. The groundwaters encountered along the shafts will be site-specific but are anticipated to be mildly saline (~12 g/L TDS) in crystalline rock, to brine (~325 g/L TDS) in sedimentary rock.

Bentonite-aggregate mixtures containing from 50 to 90% by dry mass bentonite were tested to determine their compaction characteristics and following these initial results a comprehensive materials characterization program was undertaken. Parameters including fluid composition, aggregate-type, swelling pressure, hydraulic conductivity and gas transport were evaluated for selected bentonite-aggregate blends. This resulted in identification of backfill formulations that could provide the required performance for a shaft backfill (swelling pressure of >100 kPa and hydraulic conductivity of <10^-10 m/s, respectively).