Long-term analysis of ¹⁴CO₂ measurement using mid-IR cavity ring-down spectroscopy for nuclear power plants

Carbon-14 (C-14) is a radioactive isotope that is easily combined with living organisms and has a long half-life time, requiring continuous monitoring. In a nuclear power plant, it is necessary to monitor the concentration of C-14 generated during the power generation process, and strict management is required, especially in the case of carbon-14 dioxide (¹⁴CO₂) emitted to the outside as stack gas. In this study, a concentration measurement system of ¹⁴CO₂ using a mid-infrared (mid-IR) range cavity ring-down spectroscopy (CRDS) system was established. In the mid-infrared region, carbon dioxide (CO₂) has a strong absorption spectrum centered at a wavelength of 4.6 μm. This strong absorption allows for the measurement of CO₂ content with high sensitivity. To evaluate system performance, we measured ¹²CO₂ absorption spectrum that is close to ¹⁴CO₂ absorption. The measured sensitivity was ~ 50 ppt with 1000 s averaging based on noise equivalent sensitivity and long-term operation analysis was performed over 2 h for use in actual nuclear power plants. This technology will a be promising alternative to liquid scintillation counting (LSC) which is currently used for C-14 measurement in nuclear power plants.