The Nuclear Waste Management Organization (NWMO) is responsible for implementing Canada's plan for the safe, long-term management of used nuclear fuel. This Canadian plan requires used fuel to be contained and isolated in a deep geological repository (DGR) in a willing and informed host community. The Canadian reference used fuel container (UFC) consists of a carbon steel inner structure to give the UFC strength, with a copper coating for corrosion protection. As the copper is directly adhered to the steel, its required thickness is based on its reactivity in the repository, and not its mechanical properties. The UFC is shown as an inset to one possible DGR layout in Figure 1.
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Within the DGR, the environment will evolve from warm, dry and slightly oxidizing conditions to benign, wet, reducing conditions. The buffer will swell and apply pressure to the container, but also seal the DGR from flowing groundwater conditions. While there is uncertainty on the exact timeframes over which the DGR will evolve (e.g. the duration of the oxic period), the design is tolerant to all conditions that a DGR will encounter. Extensive research programs1 into the UFC corrosion consider conditions such as: humid air2,3 and oxic3 atmospheres; effects of gamma radiation4; prediction of porewater chemistry as water enters a repository5; a range of geochemistry / groundwater alternatives for oxic pitting6, and including microbiological influences on long-term anoxic groundwater7. From these programs, as well as decades of work, including from other nuclear waste organizations, a conservative corrosion allowance of < 1.3 mm has been assigned to the NWMO UFC copper8, a value that has been reviewed extensively9,10. In addition work has examined possible upset conditions, such as assessing container performance where defects exist in the copper layer11,12, and there has been extensive work on identifying the nature of the internal UFC environment and the possible corrosion mechanisms that occur within the UFC13. In the former case, defective containers will have measureable lifetimes (i.e. » 1000 a), while no obvious degradation mechanism exists that will compromise the container due to internal corrosion.
Recent changes to the research approach have been focused on improving the understanding of synergistic effects within reasonable DGR conditions. To that end, many tests now include bentonite clays as part of the experimental matrix, and repository analogues such as underground research laboratories are being utilized more frequently. To augment this work further, the NWMO is in the early stages of exploring deep marine environments to experiment on container materials, to take advantage of the high-pressure, low-oxygen conditions, but within very biologically active conditions vs. a repository. In addition, we are in the planning stages of developing programs to emplace container materials at possible DGR locations, within boreholes developed by the NWMO site investigation team, and at repository depth (i.e. ~500 m).
References
1. Hall, D.S., and P.G. Keech, Corros. Eng. Sci. Technol. 52 (2017): pp. 2–5.
2. Ibrahim, B., “The Corrosion Behaviour of Cu in Irradiated and Non-Irradiated Humid Air,” University of Western Ontario, 2015, http://ir.lib.uwo.ca/etd/3315.
3. Hall, D.S., T.E. Standish, M. Behazin, and P.G. Keech, Corros. Eng. Sci. Technol. 53 (2018): pp. 309–315.
4. Ibrahim, B., D. Zagidulin, M. Behazin, S. Ramamurthy, J.C. Wren, and D.W. Shoesmith, Corros. Sci. 141 (2018): pp. 53–62.
5. King, F., D.S. Hall, and P.G. Keech, Corros. Eng. Sci. Technol. 52 (2017): pp. 25–30.
6. Qin, Z., R. Daljeet, M. Ai, N. Farhangi, J.J. Noël, S. Ramamurthy, D. Shoesmith, F. King, and P. Keech, Corros. Eng. Sci. Technol. 52 (2017): pp. 45–49.
7. Senior, N.A., R.C. Newman, D. Artymowicz, W.J. Binns, P.G. Keech, and D.S. Hall, J. Electrochem. Soc. 166 (2019): pp. C3015–C3017.
8. Kwong, G.M., “Status of Corrosion Studies for Copper Used Fuel Containers Under Low Salinity Conditions” (Toronto, Canada: Nuclear Waste Management Organization, 2011), https://www.nwmo.ca/en/~/media/Site/Reports/2015/09/24/08/15/1796_nwmotr-2011-14_status_of_corrosion_studiesfor_copper_containers_r0d.ashx.
9. Scully, J.R., and M. Edwards, “Review of the NWMO Copper Corrosion Allowance” (Toronto, Canada: Nuclear Waste Management Organization, 2013), https://www.nwmo.ca/en/~/media/Site/Reports/2015/09/25/08/02/2165_nwmo_tr-2013-04_review_copper_corrosion_allowance_.ashx.
10. Scully, J.R., D. Féron, and H. Hänninen, “Peer Review of the NWMO Copper Corrosion Program” (Toronto, Canada: Nuclear Waste Management Organization, 2016).
11. Standish, T.E., D. Zagidulin, S. Ramamurthy, P.G. Keech, J.J. Noël, and D.W. Shoesmith, Corros. Eng. Sci. Technol. 52 (2017): pp. 65–69.
12. Standish, T.E., D. Zagidulin, S. Ramamurthy, P.G. Keech, D.W. Shoesmith, and J.J. Noël, Geosciences 8 (2018): p. 360.
13. Wu, L., D. Guo, M. Li, J.M. Joseph, J.J. Noël, P.G. Keech, and J.C. Wren, J. Electrochem. Soc. 164 (2017): pp. C539–C553.
Copper Corrosion Nuclear Waste Cold-Spray
