To cope with the shortcomings of nuclear fuel design exposed during the Fukushima Nuclear Accident, researchers around the world have been directing their studies towards accident-tolerant fuel (ATF), which can improve the safety of fuel elements. Among the several ATF cladding concepts, surface coatings comprise the most promising strategy to be specifically applied in engineering applications in a short period. This review presents a comprehensive introduction to the latest progress in the development of Cr-based surface coatings based on zirconium alloys. Part I of the review is a retrospective look at the application status of zirconium alloy cladding, as well as the development of ATF cladding. Following this, the review focuses on the selection process of ATF coating materials, along with the advantages and disadvantages of the current mainstream preparation methods of Cr-based coatings worldwide. Finally, the characteristics of the coatings obtained through each method are summarized according to some conventional performance evaluations or investigations of the claddings. Overall, this review can help assist readers in getting a thorough understanding of the selection principle of ATF coating materials and their preparation processes.

In continuation from the earlier Part I, this paper presents the status on the investigations of Cr-coated zirconium alloys under both normal operating and hypothetical accident conditions. This paper is aimed to provide sufficient knowledge to understand the service performance and research focus of accident-tolerant fuel (ATF) coatings.

Cr-coated Zr alloys are widely considered the most promising accident-tolerant fuel (ATF) cladding materials for engineering applications in the near term. In this work, Cr coatings were prepared on the surfaces of 1400 mm long N36 cladding tubes using an industrial multiple arc source system. Orthogonal analyses were conducted to demonstrate the significance level of various process parameters influencing the characteristics of coatings (surface roughness, defects, crystal orientation, grain structure, etc.). The results show that the arc current mainly affects the coating deposition rate and the droplet particles on the surface or inside the coatings; however, the crystal preferred orientation and grain structure are more significantly influenced by the gas pressure and negative bias voltage, respectively. Then, the underlying mechanisms are carefully discussed. At last, a set of systemic methods to control the quality and microstructures of Cr coatings are summarized.

The corrosion behavior of alumina-forming austenitic (AFA) stainless steels with different Nb additions in a supercritical carbon dioxide environment at 500 °C, 600 °C, and 20 MPa was investigated. The steels with low Nb content were found to have a novel structure with a double oxide as an outer Cr₂O₃ oxide film and an inner Al₂O₃ oxide layer with discontinuous Fe-rich spinels on the outer surface and a transition layer consisting of Cr spinels and γ’-Ni₃Al phases randomly distributed under the oxide layer. Oxidation resistance was improved by accelerating diffusion through refined grain boundaries after the addition of 0.6 wt.% Nb. However, the corrosion resistance decreased significantly at higher Nb content due to the formation of continuous thick outer Fe-rich nodules on the surface and an internal oxide zone, and Fe₂(Mo, Nb) laves phases were also detected, which prevented the outward diffusion of Al ions and promoted the formation of cracks within the oxide layer, resulting in unfavorable effects on oxidation. After exposure at 500 °C, fewer spinels and thinner oxide scales were found. The specific mechanism was discussed.

Coating technology to modify the property of zirconium (Zr) alloy is a potential method for accident-tolerant fuel claddings. In this work, Cr-coated and CrN-coated are prepared on the surface of Zr alloy by using multi-arc ion plating technology. The effect of coatings on the high-temperature oxidation performance at 800, 1000, and 1200 °C has been investigated. The oxidation behavior, microstructure, and phase composition of the samples were characterized by scanning electron microscopy, atomic force microscope and x-ray diffraction analysis. The mechanical properties of the coatings before and after oxidation were examined by ring compressive and hardness tests. It is shown that all the Cr and CrN coating can effectively protect the substrate from oxidation corrosion in air due to the formation of Cr ₂ O ₃ , which can effectively reduce the penetration of oxygen. The thickness of the oxide layer from the side of the coating doesn’t exceed to 5 μ m at 1000 °C, and from the uncoated side reaches to 70 μ m with pores and rough structure. Bubbles appeared on the surface of the coated samples after oxidizing at 1200°C. These bubbles are located at the Cr and Cr ₂ O ₃ layers. The high-temperature oxidation resistance of Cr coating at 1200 °C is better than that of CrN, the latter appears to crack and spall on the oxide layer. Due to the higher fracture toughness, ductility of the Cr coating and more suitable deformation compatibility, the Cr coating possesses better crack resistance than the CrN coating under mechanical loading.