Evaluating the Corrosion of Used Fuel Containers in Saline Environments for the Long Term Storage of Used Nuclear Fuel

Abstract

The internationally accepted best practice for managing used nuclear fuel (UNF) is to emplace it within a multiple-barrier system underground in a deep geological repository (DGR). In the proposed Canadian DGR design, used fuel bundles will be sealed in copper-coated carbon steel used fuel containers (UFC), encased in blocks of highly compacted bentonite clay, and emplaced approximately 500-800 metres below ground, depending on the characteristics of the host geology.

To simulate anticipated DGR conditions, copper materials representing the outer layer of the UFC, embedded within compacted bentonite clay, were exposed, while under pressure, to an environment with salinity similar to that of the groundwater found in a potential Canadian DGR host sedimentary rock site, containing oxygen at various concentrations. To achieve these conditions, open vessels, called "ocean modules" (OM), containing copper compacted in bentonite were placed in the Pacific Ocean, at depths of 90 and 980 metres, for six months and two years, respectively. We investigated the effect of the bentonite compaction density on the corrosion of copper, using dry densities of 1250 and 1450 kg/m³ under these conditions. Examination of the copper coupons exposed within the OM showed that uniform corrosion occurred on the embedded UFC material, with the dominant corrosion product being copper (I) oxide. Copper chloride species were detected moving into the bentonite clay. Overall, low average corrosion rates of the copper materials, ranging from 2 to 4 µm/year, were observed. At the higher bentonite compaction density, the corrosion rate and the amount of Cu₂S on the surface were lower, indicating that the increase in bentonite compaction suppressed microbial activity and/or slowed the diffusion of corrosive species to the copper surfaces.