As-placed Density Study of Gap Fill Material as Part of the Full-Scale Emplacement Trial

Abstract

The Nuclear Waste Management Organization (NWMO) conducted a full-scale emplacement trial at the NWMO’s Proof Test Facility in 2022. The trial was designed to replicate the NWMO placement concept that consists of copper-coated Used Fuel Containers (UFCs) encapsulated in Highly Compacted Bentonite (HCB) placed in an underground room. Gap Fill Material (GFM) is then delivered into the space between the HCB buffer boxes and walls of the emplacement room to fill all remaining voids. The as-placed GFM is to meet a minimum target dry density of 1.40 g/cm³.

As part of the full-scale emplacement trial, density of the as-placed GFM was assessed with the focus on the local GFM density distributions and the pre- and post-installation particle size distributions of the GFM. Two field testing methods, sand-cone test and nuclear gauge test were used for measuring local density of the as-placed GFM. In the top region of the mock emplacement room, inhomogeneous density distribution of the as-placed GFM was observed in the range of 1.35 g/cm³ to 1.56 g/cm³ (an average dry density of 1.40 g/cm³). Similarly, inhomogeneity in dry density was observed across the sidewalls. Dry densities of the as-placed GFM in the sidewalls were higher than those measured at the top (an average dry density of 1.47 g/cm³). Despite inhomogeneity in dry density observed across the as-placed GFM, its overall dry density achieved was greater than the minimum target dry density of 1.40 g/cm³. The as-placed GFM experienced a discernible reduction in the coarse particle sizes up to 14 %. This reduction was likely due to the crushing of the particles during the delivery process.