Abstract:

The long-term management of used Canada’s used CANDU fuel will conclude with its placement in a deep geological repository (DGR), at least 500 m below surface, in the Wabigoon Lake Ojibway Nation-Ignace region of Ontario. Central to this strategy is the encapsulation of fuel bundles within carbon steel containers coated with copper. For this container, copper serves as a corrosion barrier, as oxidant availability for species capable of degrading copper is very limited within the DGR. However, through mining and radiolysis processes, it is possible that oxidizing species such as nitrites (NO₂^–) may be present at early stages of the DGR lifetime. Nitrites are of concern due to their established ability to induce stress corrosion cracking (SCC) in copper under tensile stress. However, prior research has demonstrated that the nitrite concentrations anticipated within a DGR are significantly lower than the reported 0.3 M [NO₂^–] threshold for SCC initiation. Nevertheless, a comprehensive understanding of the underlying SCC mechanisms and the identification of early-stage degradation processes (i.e., crack precursors) remain critical.

To address these knowledge gaps, this study employs an electrochemical approach to evaluate the influence of nitrite concentration on SCC susceptibility of copper. Potentiodynamic polarization testing was conducted on copper specimens in 0.1 M, 0.5 M, and 1.0 M KNO₂ solutions to assess the concentration-dependent electrochemical behavior. Results revealed that while Cu in 0.1 M KNO₂ exhibited active behaviour, Cu in 0.5 M KNO₂ and 1.0 M KNO₂ exhibited two distinct active-passive transitions. Though a film was formed, due to the high current density, it was not effective at limiting mass and charge transfer. That said, since film formation can often precede SCC, Cu U-bend samples were subjected to potentiostatic holds at each transition potential to identify the most critical potential region for crack initiation and propagation. Characterization of corrosion mechanisms and possible SCC was supported with long-term U-bend exposure experiments in similar solutions of interest.

Post-exposure characterization was performed using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) to analyze crack morphology and to determine whether SCC followed an intergranular or transgranular propagation mode. Additionally, X-ray diffraction (XRD) was utilized to characterize oxide composition after various potentiostatic holds, to aid in assessment of the surface film’s role in SCC. The findings of this study will provide key insights into nitrite-induced SCC mechanisms in copper and contribute to the ongoing effort to mitigate long-term degradation risks in DGR environments.

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Dr. Yoel Emun