Effects of powder and process parameters on the ductility of cold spray copper coatings for the corrosion protection of used fuel storage containers.

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The Nuclear Waste Management Organization (NWMO) was established in 2002 for the long term management of the Canada's used nuclear fuel. The organization is undertaking a major effort to develop an underground Deep Geologic Repository (DGR) for used nuclear fuel. One of the key components of the DGR is the Used Fuel Container (UFC), which is a copper-clad steel vessel designed and fabricated to ensure the containment of the fuel for many thousands of years. Cold Spray is an emerging thermal spray technology where metal powders are sprayed at supersonic velocities onto the surface to be coated, whereupon they consolidate and bond instantly on impact, enabling the rapid production of thick and dense coating, while minimising oxidation and microstructural alteration of the deposited material. These characteristics are the main reason for the selection of cold spray method by NWMO for the production of the thick corrosion resistant copper coating. Since 2011 the NWMO has partnered with the Canadian National Research Council (NRC) for the development of cold spray for UFC application. The coating under development has specific requirements in terms of thickness, composition, adhesion, density, integrity and ductility. For the specific case of the NWMO's UFC, spray parameters have already been optimized to maximise the in-flight velocity of the powders, enabling dense coatings to be produced with a deposition efficiency of virtually 100%. On the powder side, the effect of the particle size distribution on coating adhesion has been systematically studied in order to cope with application requirements. However, it is only recently that we observed that mechanical properties of copper coating can vary significantly, from lot to lot of the powder feedstock. For instance, using the same spray parameters, a variation of ductility from 0.7 to 39 % has been recently observed, without any 'significant' change in the powder characteristics. Thus, the main goal of this project is to understand the effect of powder feedstock characteristics and cold spray process parameters on the ductility in order to define powder and process parameters to impart optimized and reproducible properties to the copper coating.