Underground radioactive waste repositories: assessment of post-closure safety under extreme events

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The proposed research is concerned with a long-term safety assessment of Deep Geological Repository (DGR) for low and intermediate-level nuclear waste. DGRs are being considered in many countries with a nuclear power production. Those include Switzerland, France, Sweden, USA, Germany, UK and Japan among others. The first project to enter the construction stage is Onkalo repository in Finland, which obtained regulatory approval in November 2015. In Canada the site-selection process is currently under way and the efforts have recently focused on argillaceous formations of southern Ontario (Cobourg limestone) as a possible location for DGR. ***The primary objective of this project is to examine the gaps in the existing approaches and to establish some new methodologies for evaluating post-closure safety of DGR constructed in argillaceous rocks. The study will include four major research themes, viz. (i) description of mechanical and hydraulic properties of Cobourg limestone; (ii) coupled hydro-mechanical analysis of flow in fractured media; (iii) case studies related to near field assessment of excavation damaged zone (EDZ), and (iv) evaluation of post-closure safety of DGR under extreme events.***In part (i), the fundamental issue addressed will be a correlation between the strongly heterogeneous fabric of the limestone and its mechanical/hydraulic properties. The quantification of fabric will employ an explicit tensorial measure, based on stereology, that will provide an orientation dependent information. For hydraulic properties, the conductivity tensor will be defined as a function of geometry of the pore network and an evolution law will be postulated to correlate it with the stress field. In part (ii), the research on the coupled hydro-mechanical analysis of flow in fractured media will be conducted. A new formulation will be developed that incorporates an averaging of the field operators within the referential volume adjacent to macrocrack. The last two parts, i.e. (iii) and (iv), will deal with a numerical implementation and verification of the proposed approaches. The emphasis will be placed on assessing the long-term safety of DGR in case of extreme events, such as strong earthquakes as well as climatic changes, viz. glaciation and/or permafrost. The mechanical analysis will be supplemented by the assessment of the radionuclide transport times through the geosphere and the possibility of escape to the environment. ***The primary outcome of this project will be an enhanced understanding of the physico-mechanical processes associated with long-term underground storage of nuclear waste and development of reliable numerical procedures for assessing the safety margins in case of extreme events. *****