Abstract: In order to ensure that radioactive substances in the external steam supply of AP1000 unit meet the relevant standard requirements, real-time online monitoring of the γ radioactivity in the steam generator blowdown pipeline is carried out. The online γ radiation monitoring device consists of four NaI (Tl) γ spectrometers, an on-site radiation processing unit, and a lead shielding bracket. The simulation results indicate that the detection efficiency of the monitoring device is less affected by the energy of γ rays. By preparing pipelines with different activity concentrations of ¹³⁷Cs and calibrating the monitoring device, a detection efficiency of 7.96×10^-2 was obtained. At a 95% confidence level and a measurement time of 15 seconds, the minimum detectable activity concentration was 2.53×10⁴ Bq/m³. The precision of the monitoring device was 2.86%, and the accuracy was better than ±9%, meeting the measurement requirements.

Key words: low γ radiation measurement, minimum detectable activity concentration, online monitoring at the management site, AP1000