Diffusion of corrosion agents through an engineered barrier system

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Many countries are studying long-term solutions for safe storage of used nuclear fuel. One such solution,selected by the Canadian Nuclear Waste Management Organization (NWMO), is the use of deep geologicalrepositories (DGRs) that would be located several hundred metres below ground level. DGRs would store usedfuel canisters (UFCs) surrounded by an engineered barrier system (EBS), composed of copper coatings to act asa corrosion barrier and highly compacted bentonite (HCB) that surrounds the UFC. HCB swells upon contactwith water and can therefore seal gaps within the repository and provide a moderately thermally conductivemedium. It can also slow groundwater infiltration and retard the movement of radionuclides in the event of aUFC breach. In addition, the HCB can suppress microbial activity near the container by restricting space andwater availability to microbes that may be present. However, diffusion of sulphide can result in microbiallyinfluenced corrosion (MIC), potentially causing early failure of the barrier. The proposed research willinvestigate the factors that can affect sulfide diffusion to the UFC surface. The project will include diffusionexperiments performed under a range of potential repository conditions, based on current Canadian designspecifications and predictions for DGR conditions. The results of the experimental studies will be used tofurther expand and validate a three-dimensional computer model. The resulting model will be capable ofparametric studies to refine and optimize the design of the Canadian DGR. The results of this study willidentify key parameters that may result in higher corrosion rates, leading to a refinement of NWMO's EBSdesign and providing Canadians with a safe and sustainable method of used nuclear fuel disposal. This studywill address significant scientific questions regarding diffusion of corrosive species through the engineeredbarrier system in sub-surface environments, and will also enhance predictive modelling capabilities tostrengthen the sponsoring agency's long-term DGR design/performance criteria.