Experimental Study of Low Temperature Plasma Treatment of 14C Alkanes Compounds

Abstract

14C has become the nuclide that contributes the most to the annual effective dose to the surrounding public among the radioactive effluents during normal operation of nuclear power plants, but for a long time, domestic nuclear power units have not been equipped with airborne 14C treatment facilities, it is important and urgent to seek technical methods to reduce 14C emissions based on potential and actual needs under the requirements of the existing emission limits. Domestic nuclear power units under construction and in operation are mainly pressurized water reactors (more than 95% of the total), and the airborne 14C in this type of reactors mainly exists in the form of alkane compounds. Therefore, in this study, 14CH4, which accounts for the largest proportion of 14C alkane compounds and has the most stable chemical properties, was taken as the treatment target, and low-temperature plasma technology was introduced to investigate its discharge behavior and CH4 treatment performance. The results show that under the optimal conditions of normal temperature and pressure, output voltage of 17.89 kV, and gas flow rate of 0.83 cm/s, the plasma’s CH4 treatment efficiency can reach 99.37%, and the CO2 selectivity can reach 46.99%. The plasma’s CH4 treatment performance can be improved by increasing the output voltage, reaction temperature, and decreasing the gas flow rate. However, since increasing the reaction temperature would bring problems about energy consumption, safety, and equipment complexity, increasing the reaction temperature is not the first choice. In addition to CO2, there are more than thirty kinds of by-products produced in the process of plasma treatment of CH4, which are dominated by organic substances. The kinetics process of plasma treatment of CH4 is in accordance with the quasi-primary reaction kinetics model, and the corresponding rate constants are 1.104 8 m3/(kW·h). The above results indicate that plasma technology has a broad development prospect in the field of airborne 14C treatment and monitoring, especially in the treatment of 14C alkane compounds. The focus of subsequent research should be focused on optimizing the reaction pathway, lowering the reaction barriers, further increasing the proportion of CH4 directed oxidation to CO2, and significantly reducing the formation of by-products.

Keywords

• 14c
• plasma
• methane
• pressurized water reactor