The main control room (MCR) ventilation system has been designed to maintain habitability of the control room envelope both under normal condition and accident condition. The system adopting dual air intakes adds one more air intake for accidents at suitable position. During accidents, the air intake with lower contamination will be selected and the other with higher contamination will be isolated, to reduce the amount of radioactive substances entering MCR extremely and enhance the habitability of MCR envelop. This paper is devoted to research on the impact of switching time interval (STI) for dual intakes on workers in main control room during accidents. As the contamination condition varies, the switching action will be happened. Switching time interval (STI) referred in this paper means the time between two switching actions. When accidents occur, the air intake will operate and switch between two intakes automatically. The action of switching will be influenced by several parameters: the meteorological conditions of the site, the response features of the monitoring instruments and the source term released to the environment after accidents. Analysis of these parameters and their sensitivity analysis are performed, which show that the ventilation system cannot afford too frequent switching actions resulted from instantaneous sudden changes of intake’s activity. That’s the reason why it is necessary to set a minimum STI which means the contamination of one intake have to be lower than the other intake and this dominant position should be kept longer than the minimum STI, if not, the switching action will not be happened. As it is essential to set a minimum STI to prevent frequent switching of system, the analysis of its impact on the atmospheric relative concentrations and the doses of the workers in main control room are performed on basis of specific site meteorological condition and the response characteristic of dose monitoring instruments. Three kinds of accident release conditions are considered, which are relief valve release, containment leakage and elevated funnel release. The atmospheric relative concentrations and the doses of the workers in MCR are evaluated for every case and compared with the dose limits. Finally an acceptable minimum STI of dual air intakes is recommended.

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