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DOE-STD-1128-98
The design of the ventilation system should include an analysis to
demonstrate that the system is capable of operating under the safety-basis
conditions. To the maximum extent practicable, the system should be
designed to ensure that the products of combustion are not spread beyond
the room of origin unless directed through appropriate ventilation
channels. The exhaust system should be designed to provide cleanup of
radioactive material and noxious chemicals from the discharge air and to
safely handle the products of combustion.
Provisions should be made for independent shutdown of ventilation
systems where this could be an advantage to operations, maintenance, or
emergency procedures such as firefighting. In assessing the desirability
of providing for shutdown of a ventilation system under such conditions,
full consideration should be given to all possible effects of the shutdown
on air flows in other interfacing ventilation systems. It may be more
appropriate to provide for drastically reduced flow rather than for system
shutdown. For example, reducing air supply to 10% and exhaust flow to
20% of operating values would minimize ventilation and maintain
negative pressure. Positive means should be provided for controlling the
backflow of air, which might transport contamination. The ventilation
system and the associated fire-suppression system should be designed for
fail-safe operation.
The ventilation system should be appropriately instrumented and
alarmed, with readouts in continuously occupied control rooms. A listing
and the function of required and recommended instrumentation are given
in ANSI N509-1989, Table 4-1 (ANSI, 1989b).
Building penetrations for ventilation ducts should be kept to a minimum
and should be designed to protect the critical systems against postulated
accidents. No penetrations should be permitted if the barrier around the
process area is the outside wall of the building.
Room air in controlled and process areas may be recirculated if the
recirculating air system is provided with two HEPA filter banks in series.
One of the filter banks should be in the exhaust duct leading from the
rooms where airborne activity might be introduced. An air monitor
should be located between the two filters and set to alarm when the air
concentration reaches a preset point. Airflow should then be diverted
either manually or automatically to a once-through system using the air
monitor alarm indication to trip the system. Recirculation from a zone of
higher contamination to a zone of lower contamination should be
prohibited.
C-23


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