Abstract: Monitoring instruments set at the intake of main control room (MCR) are used for monitoring of concentrations of radio-nuclides or environmental dose rate to provide the switching signal to the ventilation system (VCL). During accidents, the VCL system will be switched from normal ventilation to emergency filtration ventilation according to the dose rate meter. One air intake with lower contamination will be selected and the other with higher contamination will be isolated according to the β activity detectors at emergency intakes. The settings of the monitoring instruments at the intake for MCR are very crucial to reduce the amount of radioactive substances entering the MCR and enhance the habitability of MCR envelop. This paper is devoted to the research on the settings of monitoring instrument for normal and emergency intakes of the MCR. The thresholds for switching from normal vent system to emergency system are calculated by applying one full year meteorological data of the site and considering typical accidental source term, VCL system functions and the construction features of the MCR in HPR1000 unit. The atmospheric relative concentrations and the doses of the workers in the MCR on the conditions of different switching time intervals (STI) are analyzed on basis of specific site meteorological condition and the response characteristic of monitoring instruments. The doses of the workers in the MCR are evaluated for every case and compared with the dose limits. Acceptable minimum STI of dual air intakes is recommended according to the analysis.

Key words: main control room, intake, monitoring instrument, switching threshold, switching interval