Large-scale backfilling simulations at ÄSPÖ Laboratory Sweden and applicability of results to repository concepts considered for Canada

As part of the Nuclear Waste Management Organisation's (NWMO's) Adaptive Phased Management (APM) approach to development of deep geological repository concepts for the permanent disposal of used nuclear fuel, effort is being put into reviewing, assessing and where possible participating in international activities associated with repository sealing in a variety of geological host media. Of considerable interest are developments related to backfilling and closure of the repository openings following placement of the used fuel containers (UFCs). In a host medium such as granite, the potential exists for localized inflow of water or combining of small localized inflows into larger features if flow is not managed. Under conditions of high inflow these could form channelled flow features (piping features) through backfilled rooms or tunnels. This could cause erosion of the backfill and complicate ongoing construction and installation activities within the affected room. While UFCs would not be installed in locations where unsuitable hydraulic conditions exist (i.e., high inflow rates), tunnels may pass through such hydraulic features and hence the backfill installed in those locations must be able to deal with the accumulated inflow from these and other smaller features. Posiva and SKB have been examining how water enters and passes through backfill materials at laboratory- to nearly full-scales (1/12^th to 1/2-scale placement room cross-section) in order to define when water inflow begins to become disruptive to the backfill. Additionally, studies to determine the effects of varying the composition of the backfill blocks and pellets on system performance have also been examined as part of the process of design optimization. Work by SKB and Posiva has demonstrated that backfilling of placement rooms in crystalline rock environments can be achieved using a combination of precompacted clay blocks and clay pellets using modified conventional equipment. There are hydrogeological conditions that could be potentially disruptive to newly placed backfill, in particular the inflow of water to the room and its subsequent movement through the unsaturated backfill. Tests have evaluated a number of backfill materials options, the effects of intersecting fracture features, rate of water influx and how isolated pockets of unsaturated backfill may respond to subsequent water influx. (author)