Progress of Cold Spray Coating on the Surface of Zirconium Alloy Tubes for Accident Tolerant Fuels

Following the accidents at the Fukushima Daiichi Nuclear Power Station, an increasing number of countries have conducted research on zirconium alloy tube coatings for accident-tolerant fuel in light-water reactors. Among the various coating preparation techniques, cold spraying has emerged as a promising method for preparing zirconium alloy tube coatings, due to its low heat input, high deposition efficiency, and dense microstructure. This paper reviews research on cold-spray coating preparation for zirconium alloy tubes, focusing on the selection of materials for protective coatings on zirconium alloys, high-temperature oxidation resistance, and irradiation performance under loss-of-coolant accident conditions. First, the bonding mechanisms and microstructures of cold-sprayed chromium (Cr) coatings on Zr substrates are investigated. Cr coatings exhibit exceptional high-temperature oxidation and irradiation resistance due to their low neutron absorption cross-sections and stable high-temperature oxides. The factors influencing the deposition characteristics, such as powder preparation methods and pretreatment techniques, are discussed. The experimental results indicate that optimized Cr powders and post-treatments (e.g., annealing) significantly enhance coating density, reduce residual stress, and improve adhesion strength. The oxidation behavior of Cr coatings prepared by cold spraying was investigated under high-temperature steam conditions. However, interdiffusion between the Zr substrate and Cr coating affects the protective properties of the coating. To address this, an anti-diffusion layer, such as Nb or Mo coating, was successfully formed on the Zr substrate by cold spraying. Cr coatings were also successfully deposited on the anti-diffusion layer, resulting in significantly improved coating properties. Following the discussion of Cr coatings, the performance of FeCrAl coatings is also summarized. The excellent high-temperature stability and irradiation resistance of these coatings make them suitable for Zr tube applications. Under high-temperature and irradiation conditions, the surface of the FeCrAl coating evolves into a ZrC layer as the experiment progresses. This unique structure imparts self-repairing capability to the coating in irradiation environments. The formation of an Al₂O₃ layer on the coating surface during high-temperature exposure significantly enhances oxidation resistance. However, FeCrAl coatings exhibit interdiffusion with the Zr substrate, leading to the formation of low-melting-point eutectic phases that influence the protective properties of the coating. To address these problems, diffusion barrier layers such as Mo or Nb were deposited on the Zr substrate. These layers delay the interdiffusion process between the FeCrAl coatings and the Zr substrate, thereby extending the service life of the coatings. Additionally, MAX-phase coatings have shown potential for use in high-temperature oxidation and irradiation environments. However, the high porosity of these coatings compromises their protective properties. Furthermore, irradiation induces the formation of an amorphous layer and a disordered laminar structure, causing lattice damage and cracking in the MAX coatings. Therefore, further investigation is required on MAX-phase coatings prepared by cold spraying to maximize the protective benefits of the MAX phase. Finally, the research progress and challenges of zirconium alloy cold-spray coatings are reviewed, and future development directions are discussed. In summary, the progress in cold-sprayed coatings for zirconium alloy cladding tubes offers a comprehensive overview of their role in enhancing nuclear reactor safety. By addressing these technical challenges and exploring innovative solutions, cold spraying has the potential to advance the development of Zr alloy tube coatings, improve resistance to extreme operating conditions, and ensure the safe operation of light-water reactors.