Natural Analogue Studies in Support of Post-Closure Safety Assessment of Deep Geological Disposal

Natural analogues are systems that have evolved over geological timescales with features similar to one or several components of a deep geological repository (DGR). Natural analogues complement short-duration laboratory studies since they are existing reflections of many long-term processes that might affect the performance of a repository. Mathematical models are often used for the post-closure safety assessment of a DGR. Confidence in the models’ predictions is enhanced when the models successfully simulate the past evolution of a natural analogue. This paper summarizes the Canadian Nuclear Safety Commission’s (CNSC’s) recent research on natural analogues to inform on (1) glacial erosion, (2) engineered barrier system, and (3) uranium reactive transport in the context of DGRs for radioactive wastes. Glaciation and its erosion are prominent factors impacting the performance of future DGRs at high latitudes in the northern hemisphere. The authors have reviewed the field data from the Greenland Analogue Project, developed a conceptual and mathematical model for the simulation of the thermal conditions within the Greenland ice sheet, as well as the thermal-hydraulic conditions at its base and the ice sheet velocity, and eventually estimated the erosion rate at the site.

The Cigar Lake Analogue demonstrates the long-term radionuclide containment capability of the illite clay zone enveloping the ore body, serving as an analogy to the engineered clay barriers. The CNSC and University of Ottawa analyzed ¹²⁹I in the Cigar Lake core samples, and modeled and correlated the diffusion-dominated transport of radionuclides over the geological evolution of the Cigar Lake deposit. The results provide information on the mobility of fission products and significant radionuclides in conditions analogous to the source, engineered barriers, and near-field host rock of a DGR.

The reactive transport and geochemistry of the Kiggavik-Andrew Lake uranium deposit mineralization and remobilization was another natural uranium deposit analogue studied by the CNSC. A reactive transport model was established according to the conceptualized geochemical processes and run under specified boundary and initial conditions to validate the geochemical processes. The geometry, timing, geochemistry, and fluid composition were used as model constraints.