Ben Maier

Following the Fukushima accident in 2011, the nuclear industry has focused significant research efforts on the development of fuels that are more tolerant to a temporary loss of active cooling than are current designs. This requires the development of cladding materials that can withstand aqueous environments in excess of 1473 K, while avoiding the strong exothermic oxidation reaction that zirconium alloys undergo in those conditions. This has to be accomplished while maintaining or improving cladding performance during normal operating conditions. For this purpose, chromium coatings have been applied using cold spray to cladding tubes made of current standard zirconium alloys. Cold spray is a low-temperature process that allows deposition of metallic layers at room temperature and ambient conditions, by propelling powder particles at supersonic velocities directly onto the surface of substrates using pressurized gas. This paper discusses metallurgical phenomena observed during the development and testing of cold spray chromium coatings on zirconium alloys. Parameters of the solid-state cold spray process have been optimized to minimize the deformation induced on the zirconium substrate, while producing dense and adherent coatings. Coated tubes have been subjected to extensive testing including corrosion in aqueous environments and mechanical testing in a range of temperatures between room temperature and 1673 K. Phenomena observed include the effect of chromium on the electrochemical corrosion potential of the system, which is critical to simulate and predict the in-reactor corrosion and crud deposition under pressurized water reactor water chemistry conditions. Interdiffusion between chromium and zirconium at high temperatures in an oxidative environment, leading to the appearance of chromium precipitates or the formation of eutectics and melting when the temperatures exceed 1573 K, is also discussed. Finally, the effect of in-reactor conditions on the chromium-zirconium system is presented from characterization of samples irradiated at the Massachusetts Institute of Technology Reactor.