Air Flow and Balance cont'd

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