Review of the NWMO copper corrosion program

Beginning in 2011, the Nuclear Waste Management Organization (NWMO) initiated a design change for their deep geological repository (DGR) system for the long-term storage of used nuclear fuel. In addition to significant changes to emplacement and clay buffer methods, a primary change was the adoption of a copper-coated used fuel container (UFC). To support this vessel, the NWMO initiated a series of copper corrosion related research programs, to compliment the many years of research that have been previously performed on copper for its use in DGRs. As much of this historic work has been previously reviewed [Scully & Edwards 2013], an emphasis within this current review was placed on the active copper corrosion programs, and the potential impacts of copper corrosion mechanisms to the copper-coated UFC. The review panel concluded that the NWMO has developed a well-integrated and world-leading research team to conduct this work, through a combination of academic institutions, industrial consultants and international collaborations. The research effort is extensive, and can be summarized in the following statements: The programs have been well designed to address the specific requirements of copper-coated UFCs in anticipation of the eventual licensing such a container. Future efforts and funding should remain within the existing topics: anoxic corrosion of copper; radiolysis related corrosion of copper; copper coating vs. wrought copper corrosion; localized corrosion of copper. Possible enhancements of research as proposed by the review team will fall into these categories. In addition to supporting the use of copper coatings, the programs are providing significant data for the generic use of copper as a DGR material within an eventual license application. Ongoing partnerships with other international organizations responsible for nuclear waste management have produced a mutually beneficial knowledge database that will continue to build confidence in this topic. The NWMO has developed a clear path forward with respect to continued research on copper corrosion as its proceeds to a site selection and the site-specific chemistry that will be produced in the future. The successful conclusion of the existing corrosion programs combined with the future programs that incorporate site-specific data are highly likely to support very long lifetime predictions for UFCs (i.e., 100,000 y), provided supporting assumptions with respect to geochemistry, environment, bentonite clay performance, as well as container/bentonite manufacturing can be substantiated. Information from parallel research in microbial processes affecting DGRs was also provided, but was not the scope of this review. Nonetheless, it was clear that the NWMO microbial and corrosion programs are well integrated, and supportive of each other. The review also focused on uncertainties within the prediction of copper corrosion, and a series of recommendations with respect to future work were derived. Using the same topic breakdowns as above, these are: Anoxic copper corrosion investigations should emphasize efforts for identifying the source and technical basis for hydrogen observed during experiments in pure water and brine. In addition, the effect of sulphide as a catalyst for anoxic corrosion should be specifically assessed; Radiolysis copper corrosion programs require an additional effort to integrate the experimental results that are providing enhanced understanding of mechanisms, etc., into applied results, such as rate laws or understanding of processes that contribute towards understanding the basis for estimating a corrosion allowance; Copper coating investigations should focus on ensuring a continuing characterization of samples that use the actual selected manufacturing processes, and the resultant corrosion behaviour as the NWMO develops a final manufacturing method. The technical basis for any differences in corrosion behavior between electrodeposited copper and wrought copper should be understood; this includes sulphide induced corrosion, radiolytic corrosion, and anoxic modes (i.e., any that contribute to the corrosion allowance calculation); Localized corrosion research should continue to define differences between active and passive conditions to ensure that corrosion via pitting does not occur. Long-term testing is encouraged. An additional recommendation from this review is that: The internal corrosion program should include specific investigations with respect to the evolved hydrogen, and its eventual fate (i.e., sources and sinks) inside a used fuel container. With respect to the related microbial programs not evaluated in this review, the NWMO is encouraged to conduct a future peer review, with the specific goals of assessing these programs, as well as their integration with the corrosion programs. (author)