Remediation Strategies to Facilitate Placement in Varied Local Geosphere Conditions

Abstract

The NWMO has characterized the regional geospheres for both the South Bruce and Revell sites, allowing repository designers to identify potentially suitable locations for the panels, each of which contain an array of placement rooms. In addition, designs have been established for the configuration of the Used Fuel Containers (UFC), and the Engineered Barrier Systems (EBS) in a placement room set in an idealized rock. As UFC placement will be carried out remotely, the room conditions will need to be well understood, so that the UFC and EBS design geometries are met. However, the local rock conditions can potentially vary and will only be finally characterized once the placement room itself is excavated and geologically evaluated in a rigorous mapping exercise.

Of concern with respect to rock conditions and their impact on UFC placement are the local fracture networks that can potentially provide water to the open excavations. This water can interact with the bentonite-based sealing, resulting in bentonite swelling or erosion prior to completing installation of all EBS components. This could result in the as-placed dry density of the bentonite-based components of the EBS not meeting the design requirements.

To facilitate placement operations in less-than-ideal geosphere conditions that were not identified during pre-excavation rock characterization, remedial strategies have been developed to accommodate a range of possible problematic environments. Depending on the water inflow, in combination with other factors, these remedial efforts could include, abandonment of a portion of, or a full room; grouting of the water-supplying feature(s); installation of a bentonite/concrete intermediate seal; or the use of simple floor drainage or localized water intercepting installations. Solutions could also include the use of modified Loaded Buffer Boxes, Spacer Blocks and Gap Fill Material placement techniques.