The ventilation system should be designed to confine dispersible
radioactive material within prescribed areas of the facility. It should also
be designed to limit airborne concentrations of radioactive material in
occupied areas of the facility and in effluents to levels that are less than
the applicable concentration guides and ALARA.
The ventilation system, which serves as an engineered safety and control
system, should be designed to remain operational or fail safely under all
operational and credible accident conditions. The failure of any single
component should not compromise the ability of the system to maintain
confinement of radioactive materials or control their release to the
environment. Specific response requirements of the system and its
components should be identified through a safety analysis.
Air Flow and Balance
The design of ventilation systems should ensure that, under all normal
conditions, the air flows toward areas of progressively higher radioactive
material inventory. Air-handling equipment should be sized
conservatively enough that minor fluctuations in air flow balance (e.g.,
improper use of an air lock, or occurrence of a credible breach in a
confinement barrier) do not result in air flowing from higher to lower
radioactive material inventory areas. To prevent the movement of
contamination from high radioactive material areas to low radioactive
material areas in case of a flow reversal, HEPA filters should be
provided at ventilation inlets in confinement area barriers.
A minimum of two negative-pressure zones should exist within a process
building. The first, the process confinement system should serve the
spaces within the glove boxes, conveyors, transfer boxes, and other
spaces that may contain plutonium during the course of normal
operations. The second should serve the process areas and other
potentially contaminated areas adjacent to the process-confinement
system. Controlled areas that are contiguous to process areas and
potentially free of contamination constitute a third zone. Some facilities
have a minimum of three zones and frequently four.
A minimum pressure differential of between 0.75-in. and 1.0-in. (1.9-cm
and 2.5-cm) WG, negative with respect to the room, should be
maintained in all process confinement systems. A negative pressure
differential of at least 0.1-in. (0.25-cm) WG should be maintained
between process and controlled areas and between controlled areas and
uncontrolled areas. Air locks between zones should be provided where
necessary to ensure that proper differential pressures are maintained.
Differential pressure between the containment enclosure and the outside
atmospheric pressure may be as great as 3 in. of water [ERDA 76-21,
Nuclear Air Cleaning Handbook (ERDA, 1976)].