The NWMO has developed a concept for containment and isolation of CANDU used fuel bundles in a Deep Geological Repository. The design is similar for emplacement in either a crystalline or sedimentary rock geosphere. The design consists of a 2.51 m long, 0.56 m diameter hemi-head metallic used fuel container (UFC), placed in a bentonite over-pack referred to as a Buffer Box. The Buffer Boxes are placed transversely in a stacked configuration in a drill and blast excavated placement room, and are separated by bentonite Spacer Blocks to limit the maximum temperature to 100° C at the UFC surface.

A critical component of this concept is the buffer, which consists of 100% Wyoming bentonite fabricated into two products:

(i) Highly Compacted Bentonite (HCB) (minimum dry density 1.7 g/cm3); and

(ii) Gap Fill Material (GFM) (a well graded granular material with a maximum particle size of 8 mm, with ? 10 % by mass particle size of less than 75 ?m, and with a minimum as-placed dry density of 1.41 g/cm3).

Similar bentonite materials are used as a buffer by several international nuclear waste management organizations in their repository designs. However, the placement concept is unique to the NWMO and the production and placement of the buffer needs to be demonstrated to build confidence in the design.

The objectives of this program were to fabricate full scale HCB blocks and to conduct a full scale GFM placement demonstration. The HCB block concept has progressed from the fabrication of 0.3 m by 0.1 m by 0.1 m bricks to the cold isostatic pressing of the full scale 3 m by 1 m by 0.5 m blocks. Test programs have confirmed the uniformity of the dry density (? 1.75 g/cm3) and moisture content (? 20 %) of the blocks, and have shown that the HCB blocks can be pressed to predicted densities that are consistent with small scale trials.

The GFM placement program has progressed from placing Gap Fill like material (crushed limestone with a similar particle size distribution), using commercially available agricultural augers in a 1 m by 1 m by 0.15 m test frame, to placing MX-80 bentonite GFM in a full scale, smooth wall representative gap using a purpose built screw conveyor. The full scale GFM demonstrations achieved dry densities of between 1.5 to 1.59 g/cm3. The GFM placement demonstration along with the full scale block pressing program demonstrated that the minimum required dry density for the two buffer components can be consistently exceeded.