Microstructural Characterization of the Critical Regions of Copper Coatings for Canada’s Used Nuclear Fuel Containers

For many decades, nuclear energy has been utilized worldwide as an efficient, stable, and sustainable source of electricity with low carbon footprint; however, with increasing amounts of used nuclear fuel, their proper management and disposal has become a challenging issue. In Canada, the Nuclear Waste Management Organization (NWMO) is responsible for the long-term management of Canada’s used nuclear fuel. NWMO is developing a deep geological repository (DGR) that consists of multiple engineered barriers to safely isolate the nuclear waste from the environment. In this design, used Canada Deuterium Uranium (CANDU) fuel bundles will be stored in used fuel containers (UFC), which will then be placed underground at a reference depth of approximately 500 m in a suitable rock formation and sealed with bentonite clay.<br/>The UFC is a core component of the multiple-barrier system. In the current design, the inner vessel of the UFC is fabricated from low carbon steel, which comprises a cylindrical shell welded to two hemispherical heads. A minimum 3-mm thick copper layer is applied to the exterior surface of the steel vessel as a corrosion barrier. Two coating techniques are jointly used to coat the entire UFC: the copper coatings on the cylindrical shell and hemispherical heads are applied via electrodeposition (ED), while after loading the fuel bundles into the container, the head is welded to the shell, and the closure-weld zone is copper-coated by the cold spray (CS) process followed by local annealing to impart ductility.<br/>The expected long service life of the UFCs (~one million years) placed high demands on the structural integrity and corrosion resistance of the copper coating. As two deposition techniques are used, it is essential to develop a deeper understanding of how the processing history affects the microstructure, and in turn, the macroscopic properties of the copper coatings. Specifically, the interfaces where ED-Cu and CS-Cu converge might perform differently from the rest of the coatings due to the inherent structural heterogeneity in these regions. In the current study, detailed characterizations including scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were conducted on the interfacial regions to reveal the microstructural characteristics of different copper coatings (ED, CS, different annealing temperatures), complemented with hardness measurements.