A ventilation system operating method for a service personnel-accessible operations room or control room in a nuclear plant or nuclear power plant enables a supply of decontaminated fresh air at least for a few hours in the event of serious incidents involving the release of radioactive activity. The content of radioactive inert gases in the fresh air supplied to the operations room should be as low as possible. Therefore, an air supply line is guided from an external inlet to the operations room, a first fan and a first inert gas adsorber column are connected into the air supply line, an air discharge line is guided from the operations room to an external outlet, a second fan and a second inert gas adsorber column are connected into the air discharge line, and a switchover device interchanges the roles of the first and second inert gas adsorber columns.