Study on Optimum Range of Zinc Concentration in Primary Circuit during Hot Functional Test of Nuclear Power Plant

The corrosion failure of key structural materials such as 316L and 690TT in the primary circuit of nuclear power plants will seriously threaten the safety and service life of nuclear equipment, and the risk cannot be ignored. Therefore, inhibiting or solving the corrosion failure problem has always been an important research topic in the field of nuclear power. Zinc injection in pressurized water reactor (PWR) nuclear power plant is a key technology to reduce the radiation dose of the primary circuit and inhibit the corrosion of materials. During the hot functional test (HFT), zinc injection helps to form a dense passivation film on the surface of the primary circuit material to alleviate the risk of corrosion failure. Although zinc injection has many advantages in the stage of HFT, there is still a lack of research on the effect of different zinc concentrations on the film formation properties of materials such as 316L and 690TT. Domestic nuclear power plants have not systematically carried out research on zinc injection technology in the primary circuit during HFT, so it is important and urgent to carry out research in this direction. In this study, a primary test bench simulating HFT conditions was set up to study the effects of different zinc concentrations on the passivation film formation of 316L and 690TT materials. By analyzing electrochemical impedance and corrosion potential, combined with scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS), the optimal concentration of zinc for film formation was efficiently screened. The results show that after zinc injection, a dense oxide film is formed on the surface of both materials, and the higher the concentration of zinc, the more dense and uniform the oxide films are. Moreover, zinc ions are detected in the metal oxide films of the two materials after zinc injection, and with the increase of zinc concentration, the zinc content on the surface of the materials shows an increasing trend. And the optimal film-forming zinc concentration during HFT is finally determined to be 75-125 µg/kg, with 100 µg/kg being the optimal value in the study conditions. In addition, the practical results show that, compared with the nuclear power units without or with low concentration of zinc, the injection of high concentration (100 µg/kg) of zinc in the primary circuit has a better passivation effect on the oxide film, and can greatly reduce the dose rate during the overhaul. This study provides theoretical support and practical guidance for the optimization of zinc injection process for new nuclear power units, which is conducive to improving the safety and economy of nuclear power plants, and the practical value is increasingly prominent.