Used nuclear fuel poses significant risks to human health and the environment, necessitating its safe and permanent disposal. The universally proposed plan for this is to bury the fuel-containing containers in a multi-barrier system known as a deep geological repository (DGR) at least 500 metres underground. These containers, crucial for withstanding long-term mechanical loads and a corrosive environment, vary in design across countries, with some featuring copper (Cu) shells on nodular cast iron insert (Sweden, Finland) or Cu-coated carbon steel vessels (Canada). Upon emplacement, the containers undergo evolving conditions underground, transitioning from warm, humid, and oxidizing to cool, dry, and anoxic environments over time.
During the initial oxidizing phase, oxygen entrapped upon sealing the DGR and water-radiolysis will lead to the formation of an oxide/hydroxide film on the Cu container surface. Subsequently, as anoxic conditions prevail, bisulfide (SH−) ions produced by the action of sulfate-reducing bacteria (SRB) remote from the container will become the primary oxidant.
While considerable efforts have been devoted to investigating exclusively either the oxic or anoxic periods, the current study has addressed the gap in understanding how early oxide growth impacts later stages, particularly in the presence of SH− ions. In a series of experiments, various methods were employed to create copper oxide/hydroxide layers with known compositions and structures to investigate their role in bisulfide-induced corrosion of the Cu substrate under de‑aerated conditions. The morphology of the oxide film and the concentration of bisulfide species influence potential interaction mechanisms, including chemical conversion, galvanic coupling, and direct corrosion of Cu by bisulfide species.
Our findings have shown that regardless of the composition or structure of the oxide film, it underwent partial conversion to copper sulfide via chemical and/or galvanic processes. Moreover, an unreacted remnant of the oxide layer detected on the surface was non-protective and permitted direct Cu corrosion by bisulfide species.
Electrochemically- and radiolytically-grown oxides exhibited quick conversion to copper sulfide, whereas hydrothermally-grown oxides, thicker in nature, underwent slower conversion, with regions remaining unreacted. These results highlight the importance of electrochemical pathways in facilitating rapid oxide-to-sulfide conversion, contrasting with slower chemical pathways
