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Investigation of the influence of some key parameters in the groundwater flow and solute transport modelling for in-situ decommissioning projects

Abstract of the technical paper/presentation presented at:
International Conference on Decommissioning Challenges: Role and importance of innovations (DEM 2024)
May 27–29, 2024

Prepared by:
Cédric Androuët
Canadian Nuclear Safety Commission


Building on international experience in decommissioning experimental reactors through in-situ decommissioning (ISD) techniques, two in-situ disposal facilities from two experimental reactors that have been in storage with surveillance mode for the past three decades are proposed in Canada (namely Nuclear Power Demonstration, or NPD, and Whiteshell Reactor No. 1, or WR-1). The proposed approach is to encase the below grade structures in two specially formulated grouts, with the above grade structures dismantled and either removed or grouted below grade, and to install a concrete cap and a low permeability engineered cover to achieve closure.

One area of interest for both projects is that the grouted structures would be below the water table. The long-term behaviour of the specially formulated grouts and existing structures with regards to groundwater flow and solute transport therefore needs to be adequately assessed and understood, in a long-term assessment of human health and ecological risk assessment perspective.

The transport of water and aggressive agents into cementitious materials occurs mainly by three distinct modes: hydraulic conductivity (or permeability), absorption and diffusion. Cracks within cementitious materials constitute the preferential path for aggressive agents and fluids to flow and have the ability to dramatically influence the transport properties of fluids in cementitious materials. Hydraulic conductivity of cementitious materials increases with the cube of the crack opening displacement (COD), the diffusion coefficient of cementitious materials increases linearly with the COD up to a threshold of around 80 μm (after which diffusion is similar to diffusion in water), and absorption increases with the number of cracks (crack density).

Based on a literature review, the impact of cracks on the transport of liquids through cementitious materials will first be discussed for the purpose of establishing orders of magnitude for the above-mentioned properties under cracked conditions. The paper will then present the Canadian Nuclear Safety Commission’s (CNSC) staff ongoing experimental research project aimed at assessing some of the properties of both grouts. In particular, in addition to characterizing some of the usual properties of the grouts (such as workability, air content, temperature, unconfined compressive strength, etc.), the experimental program will characterize both grouts’ bleeding, static segregation, adiabatic behaviour, Young’s modulus, tensile strength as well as density, absorption and voids. The shrinkage will be assessed, with a particular focus on the early-age strain behaviour of both mixes. Finally, the (relatively) long-term behaviour of the grouts will be assessed through a characterization of their hydraulic conductivity at different ages and under different crack opening conditions. Findings from the literature review and from the research project will enable to develop a science and evidence based regulatory approach with regards to the two ISD projects.

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