In the past few decades, zirconium alloy cladding has been successfully applied to light water reactors （LWR）, showing good radiation and corrosion resistance. However, a major problem in the application of zirconium alloy stacks is that it reacts strongly with water vapor at high temperatures, and emits a large amount of hydrogen and heat when the temperature is greater than 1200 °C. After the nuclear accident in Fukushima, Japan, the safety of nuclear power once again appeared in front of all nuclear workers. How to further improve the safety and reliability of nuclear fuel components in light water reactors under accident conditions has become an urgent problem to be solved.
The challenge for light water reactor nuclear fuel components is the development of accident-resistant fuels to meet the higher safety margin requirements of reactor design for fuel performance. The research and development of accident-resistant fuels that scientists have proposed include accident-resistant fuel cores and accident-resistant cladding materials. The accident-resistant cladding material is dedicated to improving the reaction kinetics of zirconium and water vapor, reducing the hydrogen release rate, and the cladding should have good thermodynamic properties. The development of accident-resistant cladding materials is mainly reflected in two aspects: one is to improve the high-temperature oxidation resistance and strength of the zirconium alloy cladding; the other is to develop non-zirconium alloys with high strength and oxidation resistance. This paper discusses the study of zirconium alloy cladding surface coatings for the former.
The main benefit of coated zirconium cladding applications is economics because of the sustainable use of existing equipment and the ease of commercialization of zirconium-based coating cladding. The technical challenge of coated zirconium cladding is to meet the various performance requirements of the fuel cladding and components, and the coating cladding does not change the size of the fuel cladding, which is critical to the performance of the reactor, especially during normal operation. Under conditions. During long-term operation, the coating should have certain stability under corrosion, creep and abrasion conditions. Therefore, it is necessary to continuously explore and optimize the preparation technology of zirconium alloy surface coating.
The new technology should be easier to control the quality of the coating, especially the thickness of the coating. The surface coating of the zirconium coating should be stable for a long time in the reactor environment.
At present, the international research on the surface coating of zirconium alloy cladding is still in the early stage of exploration. A series of coating candidate materials and coating process screening work have been carried out, and the coating performance characterization has also been carried out, and some results have been achieved. The United States mainly focuses on MAX phase and ceramic coating materials. South Korea and France mainly focus on metal Cr coating materials. The research on surface coatings of Chinese zirconium alloy cladding is still in its infancy.
1. Research status of surface coating of zirconium alloy cladding
Zirconium alloy surface anti-oxidation coating technology is a major method to improve the oxidation resistance of zirconium cladding surface. By coating a layer of material on the outer surface of the zirconium alloy to enhance the wear resistance and high temperature oxidation resistance of the cladding, the accident resistance of the zirconium cladding under normal conditions and accident conditions can be improved. At present, some preliminary screening results have been obtained in the international research on the surface coating of zirconium alloy cladding. The coating materials mainly involve MAX phase and metal Cr.
1.1 MAX phase coating
The US Department of Energy highlighted the application advantages and research recommendations of MAX phase materials in the 2014 Young Water Pile Enclave Structural Materials Research Recommendation. Benjamin et al. of the University of Wisconsin in the United States selected Ti2AlC material in the MAX phase as the zirconium cladding surface coating material. The coating process was cold sprayed and the coating thickness was about 90 μm. The test results show that the adhesion between the coating and the zirconium matrix is ??greater than 50 N, and the wear resistance of the MAX phase coating surface is better （Fig. 1）. After 700 °C, 60 min high temperature oxidation experiment, no oxide layer was observed between the coating and the interface of the substrate. Only the upper surface of the zirconium alloy coating was slightly oxidized, and the thickness of the oxide film of Zr-4 alloy reached 10 under the same conditions. Μm, because the coating surface forms a dense and stable protective film. The high temperature oxidation test results under simulated accident conditions show that the coating has a protective effect on the zirconium matrix.
Darin J. Tallman et al. studied the reactivity of MAX phase materials Ti3SiC2 and Ti2AlC with Zr-4 alloy at temperatures ranging from 1100 to 1300 °C. The results show that the diffusion thicknesses of Si and Al are in accordance with the parabolic law, and both Zr-Si and Zr-Al intermetallic compounds are formed, but the diffusion rate of Si to Zr-4 alloy is one order of magnitude lower than that of Al diffusion.
The Ningbo Institute of Materials of the Chinese Academy of Sciences also conducted research on the coating of MAX phase materials, and carried out preliminary exploration experiments on different coating materials and different coating processes. The Institute is more focused on the coating mechanism, pointing out that the essence of the MAX phase coating is the dressing effect, the key to the problem is to solve the diffusion of oxygen atoms into the zirconium matrix. ZongjianFeng of the Chinese Academy of Sciences, Ningbo Institute of Materials Science and Technology, prepared the Ti2AlC coating by DC magnetron sputtering, and studied the composition control of the coating. The base material was selected from 316L austenitic stainless steel with a coating thickness of about 10 μm. The high temperature oxidation test of Ti2AlC coating samples in 750 °C, air or pure water vapor environment was carried out, and the oxidation stratification and oxidation mechanism were observed. Discussed. The oxidation test of Ti2AlC coating in air showed that the sample formed four layers: the outermost layer was a thick mixed oxide of Al2O3 and TiO2, followed by a thin layer of α-（Al,Cr）2O3 with a thick Fe2O3 in the middle. Mixed with TiO2 oxide, the inner layer is a thin Al2O3 rich layer. Oxidation in pure steam water showed oxidation inside the sample, and the Ti2AlC coating did not form a clear oxide layer, which may be related to coating quality control. Therefore, the preparation of alloy surface coating by magnetron sputtering has yet to be further studied.
E.N. Hoffman et al.  analyzed the core application and neutron enthalpy performance of MAX carbide materials for future nuclear power plants. The commercially available MAX phase materials were placed in fast neutron reactors and thermal neutron reactors for 10, 30, and 60 years, respectively, and their neutron activities were simulated. The simulation results show that the activity of the MAX phase material is similar to that of SiC and three orders of magnitude lower than that of the 617 alloy, either in the fast neutron reactor or in the thermal neutron reactor.
The neutron irradiation test results of Ti3SiC2, Ti3AlC2 and Ti2AlC three kinds of MAX phase materials also verified the rationality of the neutron irradiation simulation analysis results. Ian Younker et al. evaluated the neutron properties of coating candidates for accident-resistant fuels. The results show that the thickness of the MAX phase coating should be controlled at 10 to 30 μm to limit neutron loss. Darin J. Tallman et al. studied the defect evolution behavior of Ti3SiC2 and Ti2AlC materials during neutron irradiation, indicating that Ti3SiC2 has a better prospect than Ti2AlC as a candidate for MAX phase coatings for high-temperature nuclear energy applications. Qing Huang et al.  also studied the anti-irradiation properties of MAX phase materials Ti3SiC2 and Ti3AlC2. The results show that the anti-irradiation performance of Ti3AlC2 is better than that of Ti3SiC2 at room temperature, and the two MAX phase materials are spoke at 600 °C. Photo stability is better than room temperature.
It has been reported that the MAX phase is a promising candidate material for the accident-resistant cladding coating material, but its coating preparation process needs further screening and optimization. Research on the application performance of the MAX phase-coated cladding has yet to be further developed.
1.2 Metal Cr coating
In order to reduce the oxidation rate of zirconium-based alloys in high-temperature steam environments, Hyun-Gil Kim of Korea Institute of Atomic Energy （KAERI） explored related coating materials and coating technologies. A Cr coating on the surface of the zirconium alloy was prepared by a 3D laser coating technique with a coating thickness of 90 μm. The adhesion of the surface coating of the zirconium alloy was examined, and a high temperature oxidation test was carried out. The results show that the Zr-4 alloy has excellent adhesion to the Cr coating due to the formation of the intermediate diffusion layer. The Cr-coated cladding did not show cracks until 4% strain （Fig. 2）, meeting the 1% strain requirement of the fuel cladding. Oxidation test data show that the high temperature oxidation resistance of the coated zirconium alloy is significantly better than that of the Zr-4 matrix.
Jung-Hwan Park et al. prepared the Cr coating on the surface of Zr-4 alloy by arc ion plating. The purity of the metal Cr target was 99.9%, and the deposition temperature was controlled at 473 K during the preparation. The oxidation test results in water vapor environment at 1200 °C for 2000 s show that the high-temperature oxidation resistance of the coated zirconium alloy is stronger than that of the zirconium alloy matrix （Fig. 3）, and the Cr-coated zirconium cladding has better ductility.
France, J.C. Brachet et al. prepared a metal Cr coating on the surface of zirconium alloy by PVD method. The latest test results show that the prepared Cr coating is very dense and has no defects （Figure 4）. The Cr coating prepared by the process optimization improves the high temperature oxidation resistance of the zirconium shell, and retains some residual ductility after oxidative quenching under accident conditions （Fig. 5）, providing an important accident reaction time for remedial measures. .
Studies have shown that metal Cr coatings have good resistance to high temperature oxidation and can be used as coating candidates for accidental zirconium alloy cladding.
At present, the research on surface coating of zirconium alloy in the United States mainly focuses on MAX phase materials, including coating process, high temperature oxidation performance of coated zirconium alloy and irradiation performance of coating materials. South Korea and France are mainly concerned with the preparation process of metal Cr coating and high temperature oxidation performance research. Domestically, the preparation process and some properties of MAX phase materials have also been explored in the early stage. Since the research on the surface coating of zirconium alloy cladding is in the feasibility exploration stage, the reports on the application performance of coatings mainly focus on the high temperature oxidation and corrosion performance, and the research on other application properties needs further development.
2. Several key issues in the study of zirconium alloy surface coating
2.1 Coating material selection
Considering the special application environment, the choice of cladding coating materials is mainly based on its physical properties. First, the coating material is to improve the high temperature oxidation resistance of the zirconium shell. Under the accident condition, the coated zirconium shell should exhibit a significantly low oxidation rate, and a dense and stable protective film can be formed on the surface to prevent or delay the further increase of oxidation, thereby preventing the zirconium shell from being damaged due to oxidation loosening. . When selecting the zirconium cladding coating material, in addition to considering the necessary high temperature oxidation resistance, it is also necessary to examine the melting point, thermal conductivity and mechanical properties of the candidate material under the melting point, and its neutron economy.
Considering the above factors, the two ceramic materials, Cr2O3 and Al2O3, are outstanding, and the growth rate is low and stable at high temperatures. Due to its brittleness, if a ceramic layer is formed directly on the surface of the zirconium alloy, it will be easily cracked during the mechanical preparation of the nuclear fuel. Considering the compatibility with the zirconium alloy, if the coating material can form a ceramic oxide film on the surface of the cladding at a high temperature oxidation reaction, it is more stable. Metal Cr and MAX phase materials can form a dense protective film after high temperature oxidation, and are promising candidate materials for zirconium coating.
2.1.1 MAX phase
Since 2000, the new MAX phase materials have combined with some excellent properties of metals and ceramics. The MAX phase material has good plastic deformation ability. This micro-plasticity and good thermal conductivity make the material have good thermal shock resistance, and its micro-plasticity also makes it have good damage resistance. Tables 1 and 2 show typical physical and mechanical properties of some common MAX phase materials.
It can be seen that the Ti2AlC, Ti3AlC2, Ti3SiC2 and Cr2AlC alloys have some excellent properties of metal and ceramic materials, and have good comprehensive performance.
It has been shown that the MAX phase material is an excellent high temperature resistant structural material. Table 3 shows the parabolic rate constants of some commonly used MAX phase materials when oxidized in air at 1000~1300 °C. It is known that these MAX phase materials are resistant. High temperature oxidation performance is good. In general, the oxidation resistance of Ti3AlC2 is better than that of Ti3SiC2, which is about 2~3 orders of magnitude higher. The reason for its good oxidation resistance is that a dense Al2O3 protective film is formed after surface oxidation. Ti2AlC also has good oxidation resistance and its oxidation mechanism is the same as that of Ti3AlC2. Cr2AlC has good high temperature oxidation resistance and is comparable to Ti3AlC2, but its thermal expansion coefficient （13.3×10-6 K） is too different from that of zirconium matrix （7.2×10-6K）, which is not conducive to coating quality control. The high temperature oxidation resistance of Ti3SiC2 material is relatively poor. With the increase of temperature, the oxidation rate increases obviously, and the oxide film is SiO2, which is a ceramic material with low thermal conductivity, which is not conducive to the heat dissipation inside the cladding. Ti3AlC2 and Ti2AlC are both oxidized to form a dense Al2O3 protective film. The difference between the thermal expansion coefficient （8.4×10-6 K） and the zirconium alloy （7.2×10-6 K） is small. Although Ti3AlC2 has better oxidation resistance than Ti2AlC, considering the neutron economy, Ti2AlC is a more ideal candidate for zirconium alloy cladding coating.
2.1.2 Metal Cr
Metallic Cr has good metallic luster and corrosion resistance and is commonly used to plate other alloy surfaces. High-purity metal Cr has the advantages of high temperature resistance, oxidation resistance, vibration resistance and creep resistance, and can be used as a target for various plasma and electron beam splashing, and has a wide range of applications.
Hyun-Gil Kim of Korea Institute of Atomic Energy studied the high temperature oxidation performance of coating candidate materials. The test results are shown in Table 4.
After oxidation in a water vapor atmosphere at 1200 °C, SiO2 has the best high temperature oxidation resistance among the four candidate materials, and Si is more effective than Cr in oxidation resistance. He also carried out corrosion tests under simulated operating conditions of 360 °C and 18.9 MPa. The results show that the metal Cr exhibits better corrosion resistance than the Zr-4 alloy, while the Si wafer and the SiO2 sample dissolve quickly under corrosive conditions. Materials with good oxidation resistance in high temperature steam environments do not guarantee corrosion stability under normal operating conditions in the reactor. Therefore, KAERI chose a metal Cr alloy as the coating material for the zirconium alloy cladding. French CAE is also working on metal Cr coating research, and has investigated more than 20 Zr-4-based coating materials, including ceramics and metals, and found that metal Cr coatings have the most potential for development. Studies have shown that metallic Cr is also a promising candidate for zirconium alloy coatings.
2.2 Coating process selection
In order to provide effective protection of the coated zirconium shell under accident conditions,