Advanced Modelling of ATF Chromium-Coated Zr- Based Cladding High Temperature Oxidation

Currently, the use of special low-corrosion coatings on nuclear reactors claddings made of zirconiumbased alloys is considered as one of encouraging ways to strengthen the reliability and performance of nuclear fission generation. This milestone direction of development is an evolutional way in nuclear power plants (NPPs) development. The chromium seems to be one of the best materials for protective coating. The chromium has excellent characteristics of corrosion and oxidation resistance compared to zirconium both for the NPP normal operation temperatures and high-temperature conditions. It is very important for the nuclear safety including the resistance to beyond-design-basis and design-basis accidents at NPPs. However, recent experimental data showed that in the temperature range close to upper limit of designbasis accident (1200°C) and higher there is a considerable worsening of Zr/Cr cladding protective properties. In particular, a role of Cr-Zr interdiffusion with subsequent influence on degradation of protective properties is revealed. In this paper, the new advanced model of high-temperature oxidation of Zr/Cr cladding is developed based on simultaneous solution of oxygen and zirconium diffusion equations in different layers of the cladding. In particular, a very important role of Zr outward diffusion to the interface between chromium oxide and metallic chromium resulting to severe degradation of protective properties is discovered recently. This phenomenon is taken into account in the model. The model is implemented to newly developed numerical module. The predictions of the model are compared with some experimental results on Zr/Cr claddings high-temperature oxidation available in the literature. The comparison shows that the basic phenomena are modeled with reasonable accuracy.