Air Flow and Balance

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

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)].

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).

C-21