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.