Unlike the conventional pressurized water reactor (PWR), helium is adopted as the coolant in the high-temperature gas-cooled reactor pebble-bed module (HTR-PM). Due to the leakage of the primary coolant, in certain process rooms and some important areas, the airborne radioactivity should be monitored for the purpose of personnel radiation protection and emergency control. The system executing above function in HTR-PM is called important area airborne radioactivity monitoring system. This system consists of five separate monitoring channels, in which online and/or sampling monitoring devices will be included. For radiation monitoring system of a reactor, the calculation and setting of alarm thresholds for online devices is an important issue. The set of alarm thresholds requires not only accurate calculation but also feedback from actual operational experience. The design of the high-temperature reactor is different from the PWR, so the set of the alarm thresholds need special considerations. Based on the calculation of source terms and production-removal model, the theoretical radioactive levels of the sampled gases of all the online monitors were calculated and the alarm thresholds adopted in the current design are presented in this paper. The set of the alarm thresholds takes the influences of the background and other factors into account. This paper provides a method for calculating and determining the alarm thresholds of the radiation monitoring system, which can provide references for other nuclear power plants.

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