In a pressurized water reactor, the corrosion chemical reaction between zirconium alloy cladding and water will adversely affect the mechanical properties of the cladding, thus limiting the service life of the fuel elements. In order to slow down the oxidation rate of the cladding and prevent the potential risk of hydrogen explosion, a conceptual design of accident tolerant fuel was proposed. Chromium metal has excellent corrosion and oxidation resistance, and has been widely used as cladding coatings in the field of nuclear power. At present, the micro-mechanism of corrosion and oxidation resistance of chromium coating at high temperature is not clear, so it is urgent to carry out relevant research.

This study aims to investigate the diffusion behavior of oxygen and hydrogen in coating on fuel cladding.

The diffusion mechanism of oxygen and hydrogen in chromium crystals was investigated on the electronic scale by using the first principles method. The Arrhenius diffusion equation was employed to obtain the diffusion coefficients of O and H at different temperatures. In addition, the reaction-diffusion paths and migration energy barriers of oxygen and hydrogen were calculated by elastic band method.

Simulation results show that oxygen occupies the most stable position in the octahedral interstitial site (OIS), and hydrogen tends to occupy the tetrahedral interstitial site (TIS). The oxygen atoms diffuses from the reaction path TIS to TIS with the diffusion energy barrier 0.79 eV whilst the oxygen atoms diffusion along the TIS to OIS reaction path has the diffusion energy barrier 0.65 eV. It suggests that there is a preferential diffusion pathway from TIS to OIS for oxygen atom due to its lower diffusion energy barrier. Notably, hydrogen demonstrates comparable diffusion energy barriers (0.17 eV) when moving along the reaction path from TIS to TIS and from TIS to OIS, respectively. The diffusion coefficients of oxygen and hydrogen increase linearly with the increase of tempterature, respectively. And for, the diffusion coefficients of both oxygen atom and hydrogen atoms along the TIS to OIS reaction path are higher than that of TIS to TIS reaction path at the different temperatures.

The solubility of hydrogen is much lower than that of oxygen. The negative dissolution energy of oxygen in the interstitial site indicates that there is a strong mutual attraction between oxygen and the first nearest neighbor chromium. A fitted relationship established between temperature and diffusion coefficient in this study provides theoretical support for investigation coating corrosion properties at elevated temperatures.