No abstract

Canada is currently considering Cu-coated carbon steel containers for the long-term storage of used nuclear fuel in a deep geological repository. The Cu coating provides a corrosion-resistant barrier, protecting the underlying steel from coming into contact with groundwater. However, galvanically accelerated corrosion of steel is possible if there is a defect through the Cu coating. To investigate this scenario, the progression of steel corrosion at the base of a simulated though-coating defect was imaged using synchrotron X-ray micro-computed tomography. Results show that coatings produced using different methods (cold spray, annealed cold spray, electrodeposition) lead to different corrosion propagation geometries. These findings can be used for modelling steel corrosion at a though-coating defect under deep geological repository conditions.

Carbon steel (CS) vessels coated with ∼3 mm of Cu have been proposed for the permanent disposal of used nuclear fuel in a deep geological repository (DGR) in Canada. In the event of an undetected defect in the Cu coating that exposes the underlying CS to groundwater, the possibility of galvanically accelerated corrosion of CS arises. In this work, the impact of O 2 availability, NaCl solution concentration, and cathode:anode area ratio on the galvanic corrosion behavior of Cu/CS couples was evaluated by monitoring the galvanic potential of the couple and the galvanic current passing between Cu and CS. The corrosion products and surface damage were analyzed using Raman spectroscopy and SEM/EDX. Varying the Cu:CS area ratio from 1:1 to 2500:1, the [Cl − ] from 0.001 to 3.0 M, the sparging gas from air to Ar, and monitoring the resulting changes in the galvanic current, galvanic potential, corrosion products, and surface damage showed that the galvanic corrosion of CS was most severe when it was exposed to air-sparged solution with a moderate [Cl − ] (0.1 M) as part of the couple with the largest Cu:CS area ratio.

The Nuclear Waste Management Organization is evaluating the safety and feasibility of the permanent disposal of used nuclear fuel in a deep geological repository. Their current design concept utilises copper-coated steel used fuel containers to isolate the waste from the environment. Immediately following repository closure, a finite quantity of O 2 will be trapped inside the repository and could cause some amount of oxic corrosion to the outer copper layer of the containers. On a per container basis, 13 mol of O 2 will be trapped in the repository rooms at the time of closure, based on reference design dimensions. This corresponds to a maximum depth of copper corrosion of 81 μm, assuming a uniform distribution. This work also considers the sensitivity of this oxic corrosion allowance to various hypothetical design changes to the repository that may occur before or during construction.

Within the multi-barrier system proposed for the permanent disposal of used nuclear fuel, the primary engineered barrier is the sealed metallic container. The present Canadian container design utilizes a carbon steel vessel coated with Cu for corrosion protection. In the event of a defect in the Cu coating that exposes the steel substrate, galvanically accelerated corrosion of steel is, in principle, possible. In this work, the progression of corrosion at a simulated through-coating defect in 3.0 mol/L NaCl solution containing dissolved O2 was monitored using electrochemical measurements and imaged non-destructively using synchrotron X-ray micro computed tomography (micro-CT). The damage volume at the base of the simulated defect was measured from the 3D micro-CT data and used to calculate the amount of O2 used to drive steel corrosion. The results demonstrate that the availability of O2 determines the rate and overall extent of corrosion, while the coatings produced using different deposition and treatment methods (cold spray deposition, heat-treated cold spray deposition, electrodeposition) lead to different corrosion propagation geometries, with the distribution of damage depending on the quality of the Cu/steel interface.