Degradation behavior of chromium-coated zirconium cladding under 1200 oC steam oxidation according to the coating microstructure

In 2011, the Great East Japan Earthquake and tsunami caused a hydrogen explosion at the Fukushima Daiichi nuclear power plant, which exposed radioactive materials to the atmosphere and had a very negative impact on the nuclear power industry. Since then, efforts have intensified around the world to make nuclear power safer. Accident-tolerant fuel (ATF) is being developed to prevent the rapid oxidation of zirconium cladding, which directly causes hydrogen explosions. ATF research can be divided into two main approaches: changing the cladding material, or coating the surface of the cladding. Coatings are easier to commercialize and apply to existing nuclear power plants, so most vendors have focused on this approach. Chromium is a popular coating medium because of its superior properties such as a low oxidation rate and excellent adhesion. Therefore, research has been focused on suppressing the rapid oxidation of zirconium cladding in the event of an accident by adding a chromium coating with an appropriate thickness in terms of both economy and effectiveness. Even if the thickness of the coating is fixed, the penetration of oxidizing substances can be further delayed by improving the microstructure of the chromium coating, such as by reducing the grain boundary area. In this study, chromium-coated zirconium cladding tubes were fabricated by the arc ion plating process. The microstructure of the chromium coating was adjusted by varying the negative voltage (0–125 V), which in turn controlled the incident energy at which the chromium ionic particles hit the surface of the cladding tube. Experiments were then performed in 1200 °C steam environment to determine the optimal microstructure for high-temperature oxidation resistance. The development of various material degradation phenomena that occurred during 1200 °C steam oxidation was observed to identify the oxidation mechanism and the main factors of the microstructure that affect the zirconium oxidation rate.