Design Objectives cont'd - doe-std-1128-98

DOE-STD-1128-98

Because confinement systems are subject to component failures and other

accidents, differential air pressures are normally maintained so that a breach of

containment will not affect occupied areas or the environment. Glove-box lines are

at the lowest pressure, plutonium laboratories at a higher pressure, and other

occupied areas at the highest pressure but still negative with respect to the outside.

Because plutonium air-cleaning systems are usually expensive to service (requiring

workers to be dressed in multiple layers of protective clothing and respiratory

protection), and plutonium waste is expensive to dispose of, measures are taken to

protect the life of plutonium air-cleaning systems. Extraneous particulates are

eliminated by HEPA filtration of incoming air. (These HEPA filters may be

disposed of as sanitary waste.) Roughing prefilters are used to capture the bulk of

particulates and prolong the life of HEPA filters.

Care must be taken in designing HEPA filter installations for plutonium facilities

so that provisions are made to safely change the filters while maintaining

contamination control. Such measures normally include redundant banks of filters

(in parallel) that can be valved out for filter change, location of HEPA filter banks

in enclosed rooms that are themselves HEPA-filtered, and appropriate provisions

for filter bag-out.

New filters must be tested after they are installed to ensure proper gasketing, etc.

Once in place, they must be periodically retested to ensure that HEPA efficiency is

maintained. For this reason, HEPA filter installations must have ports for the

introduction of a challenge aerosol upstream of the filter and collection of a

representative sample in a region of laminar flow downstream of the filter. The

HEPA filters in plutonium use sometimes fail from mechanical fatigue and

vibration rather than plugging or being subject to some other mechanical failure.

The proper design of HEPA filtration systems and proper sampling provisions are

discussed in DOE Order 6430.1A, General Design Criteria (DOE, 1989b); ANSI

N510-1989, Testing of Nuclear Air-Cleaning Systems (ANSI, 1989d); ANSI/UL

586-1990, High-Efficiency Particulate Air Filter Units (ANSI, 1990); ANSI/ASME

N509- 1989, Nuclear Power Plant Air Cleaning Units and Components (ANSI,

1989b); DOE Implementation Guide, Workplace Air Monitoring (DOE, 1994g);

and ANSI N13.1-1969, Guide to Sampling Airborne Radioactive Materials in

Nuclear Facilities (ANSI, 1969b).

In addition to the above features of the air-handling system, there may be process-

selection features that will minimize the generation of airborne plutonium. If at all

possible, plutonium compounds should be handled in sealed containers or, in the

case of a metallic solid, the material encapsulated. Wet mechanical processes, such

as cutting and grinding, usually generate fewer particulates than dry ones, so they

are often preferred. However, it is also important to minimize the use of chemicals

that will attack the air-cleaning system or contaminate the filters with hazardous

chemicals, making them mixed waste. Even moisture will shorten the life of HEPA

filters, so wet processes should be enclosed to the extent practicable and demisters

and/or heaters used to pretreat the air from wet processes prior to HEPA filtration.

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