The internationally accepted plan for nuclear fuel waste disposal is to store used nuclear fuels within a multi barrier system in a deep geological repository (DGR). Similarly, the DGR concept proposed in Canada involves isolating the used nuclear fuel in copper-coated carbon steel vessels surrounded by bentonite clay buffer boxes and gapfill material. The repository environment is anticipated to evolve from early oxic conditions to later anoxic conditions. Likewise, degradation processes of the UFCs should transition from the initial oxygen-driven processes to those governed by the availability of sulfide as oxidant. This transition may involve the conversion of accumulated (hydr)oxide-type corrosion products produced during the oxic phase to copper sulfide compounds. During this conversion, sulfide species could interact with the UFC surface in several ways depending on the type of (hydr)oxide corrosion products present, the nature and concentration of sulfide species, and other factors:

1. Chemical conversion: in this process substitution reactions replace oxy-anions in the corrosion product layers with sulfide anions. This conversion doesn’t involve redox reactions; therefore, until the conversion is complete, the sulfide species would not drive further corrosion of the Cu surface. Over time, the (hydr)oxide corrosion product layer would be replaced by a sulfide layer.
2. Galvanically-coupled process: in this process sulfides react with Cu metal exposed at the base of pores or cracks in the accumulated corrosion product layer, causing the Cu in those locations to oxidize and copper sulfide to form, while simultaneously expelling oxy-anions and causing cupric or cuprous ions to be reduced elsewhere in the corrosion product layer. Although the conservation of charge in these reactions would mean that no net oxidation of the surface would occur, the spatial separation of the sites where copper is oxidized (on the metal surface) and the locations where reduction occurs (in/on the corrosion product layer) would result in further damage to the Cu layer on the UFC surface, possibly localized at the sites where Cu metal is exposed. Eventually the (hydr)oxide corrosion product layer would be replaced by a sulfide layer and any localization should cease.
3. Direct corrosion: in this process, sulfide species react with the Cu exposed at the base of pores or cracks in the accumulated corrosion product layer, causing further corrosion at these locations without the existing (hydr)oxide species being converted (i.e. process a) or galvanically reduced (i.e. process b). These sulfide species would drive further corrosion of the UFC surface, possibly localized at the sites where Cu metal is exposed. Since this process would not contribute to the removal of the (hydr)oxide corrosion product layer, it is possible that localization, if present, could continue.

The research work reported here uses electrochemical and surface characterization methods to evaluate the extent to which each of these possible conversion processes take place, and the timescale/rate of conversion of (hydr)oxide layers grown in different ways. Results so far indicate a fairly rapid and quantitative conversion of oxides according to processes a) and/or b), followed by direct corrosion of the Cu surface by sulfide.