Evaluation of dry density of as-placed gap fill material in full-scale emplacement trials

Two full-scale emplacement trials were carried out at the Oakville Test Facility of the Nuclear Waste Management Organization (NWMO), one in April 2022 and one in March 2024, to assess the feasibility of the NWMO placement concept as part of Canada's deep geological repository for used nuclear fuel. The placement concept consists of copper-coated used fuel containers encapsulated in highly compacted bentonite (HCB) blocks being lowered into an underground tunnel known as an emplacement room. Gap fill material (GFM) is then augered into the annulus between the HCB blocks and the rock walls of the emplacement room to fill all remaining voids. One of the main objectives of these trials was to examine whether the as-placed dry density of the bentonite buffer could meet dry density requirements specified in the NWMO placement concept. Since the HCB boxes were previously manufactured and their dry density remained unchanged (i.e. ≥1.70 g cm −3 ), the focus was primarily on the dry density of the as-placed GFM, which was measured during these emplacement trials. The placement concept specifies a minimum average dry density of 1.40 g cm −3 for GFM. In the first emplacement trial, the mock-up emplacement room was filled with seven HCB box assemblies, and layers of GFM were deposited after the second, fourth, sixth and seventh HCB boxes were installed. In the second emplacement trial, two wall panels were removed from the mock-up emplacement room wall to simulate outbreak areas or spalls in the rock. The emplacement room was filled with five HCB dummy boxes and three layers of GFM, deposited after the second, fourth and fifth HCB boxes were installed. Various field-testing techniques were employed to measure local dry densities of the as-placed GFM. The sand-cone method and nuclear gauge were used in the first trial, while time domain reflectometry sensors were used in the second trial. The study results indicate that in the first emplacement trial the average dry density of the as-placed GFM was 1.44 g cm −3 , although individual measurements ranged from 1.35 to 1.56 g cm −3 . In the second emplacement trial no meaningful overall average for dry density was obtained, as the testing programme focused on the GFM within the spall regions, where the average dry density was 1.34 g cm −3 . As with the first trial, inhomogeneous density distributions were observed across the GFM-filled region. This inhomogeneity suggests that achieving density equilibrium in the bentonite buffer system will take significantly longer, emphasizing the need to install the GFM at the highest practical initial dry density.

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