The purpose of the reactor ventilation system is to control the concentration of radioactive gases produced from normal reactor operation (primarily Ar-41) within the building or room, maintain a controlled, monitored air flow path to limit or evaluate the accidental release of any radioactive material, and to dehumidify the building air for personnel and equipment requirements. The ventilation system of a research reactor building is composed of an inlet system and an outlet (often called effluent) ventilation system. In addition, the reactor control room and auxiliary areas may have separate ventilation and air-conditioning system.

The inlet air has to pass coarse filters, electrostatic filters and fine filters before the air is distributed into the reactor hall on both sides of the building. For a reactor hall with a total volume of 8000 m³ the average total blower volume is 14000 m³/h. The outlet air passes again through coarse and fine filters. The capacity of the outlet blower should be higher than of the inlet blower or the flow rates should be balanced automatically to maintain a higher net flow out of the building. A typical air change rate for a reactor building is two air changes per hour for the building volume. In this case a slight under-pressure (about 1 - 2 mm water column) is maintained in the reactor hall during blower operation. The slight negative pressure of the reactor building prevents uncontrolled air leakage out of the building because air will naturally “leak” into the reactor building and out the effluent stack. This is generally considered “Confinement” control of the facility air. Normally, the neutron beam tubes and the surface of the pool have a separate ventilation system which feeds directly into the effluent stack. This auxiliary ventilation system helps reduce the concentration of Ar-41 in the overall building by sweeping the gas away from the largest sources in the building. The building effluent path is normally equipped with radiation detectors for aerosols, noble gases and for iodine, these monitors are connected on-line to the reactor control room for data storage and documentation. Measurement and control of building radioactive effluents is generally a requirement of the regulatory agency and the facility operating license.

An emergency shutdown and isolation of the building’s reactor ventilation system should be provided in case of unanticipated or accidentally release of radioactive material into the reactor building. The emergency ventilation shutdown should shut off the normal ventilation, shut down the inlet ventilation flow and reduces the outlet ventilation by at least a factor of ten. With these values, a slight under pressure is still maintained in the reactor hall and the effluent is released only after activity monitoring. The radioactively contaminated effluents may be guided (at some facilities) through another ventilation channel and across absolute filters (HEPA-filters) to prevent any release of particulate radioactive materials. It should be noted that the HEPA filters will not prevent the discharge of radioactive gases but an activated charcoal filter may provide some delay or holdup of these radioactive gases. Operators may choose to turn off all effluent air flow in the event of a very large release of radioactive gases to prevent the discharge the gases to the environment.