Exhaust Systems

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

C.5.1.4 Exhaust Systems

The number of required exhaust-filtration stages from any area of the facility

should be determined by analysis to limit quantities and concentrations of airborne

radioactive or toxic material released to the environment during normal and

accident conditions. Materials released should be in conformance with applicable

standards, policies, and guidelines. In general, each exhaust filter system should

consist of a minimum of two HEPA filters for room air and three HEPA filters in

series for glove-box or hood-exhaust air. Only two stages of glove-box or hood-

exhaust filters need to be equipped for in-place testing.

The filtration system should be designed to allow for reliable in-place testing of the

HEPA filters and ease of filter replacement to the extent practicable.

The exhaust system for a glove box or hood should be separate from the exhaust

system for room air. The hood exhaust system need not be separate from room-

exhaust ventilation if ventilation is once-through. Exhaust air should be drawn

through a HEPA filter at the glove-box or hood-exhaust point to maintain primary

control at that point and minimize contamination of ductwork. This filter should

not be counted as a confinement barrier unless it is testable in place. Additional

HEPA filters in series should be separated at a sufficient distance to permit in-place

testing of each stage of the filters.

Dampers should be installed in the glove-box, hood, and room-exhaust ducts so

that required air-pressure differentials can be maintained. Automatic backflow

dampers should be installed in series with the exhaust dampers. Manual controls,

or automatic controls with manual override, should be provided as needed for

ventilation systems or their components for flexibility of operation.

Integral fire-suppression equipment should be provided as needed within each

ventilation system to ensure that a safety-basis fire could not degrade the integrity

of the high-efficiency air cleaning system. Where appropriate, a cool-down

chamber with water sprinklers, a prefilter demister, and a spark arrestor screen

should precede the first stage of the final HEPA filtration system. The water spray

from a cool-down chamber should be automatically actuated by appropriate

temperature- and smoke-sensing devices as determined by the accident analysis.

All potentially contaminated air should be exhausted through a common stack.

Continuous monitoring and a representative, redundant sampling capability should

be provided on exhaust stacks that may contain radioactive or toxic materials. The

ventilation exhaust stack should be located as far away from any air intake as is

reasonably possible. Design criteria for effluent monitoring and sampling and

elements for consideration in effluent radioactivity measurement are described in

DOE/EH-0173T, Environmental Regulatory Guide for Radiological Effluent

Monitoring and Environmental Surveillance (DOE, 1983). ANSI N42.18 provides

specifications and performance of onsite instrumentation for continuously

monitoring radioactivity in effluent (ANSI, 1974a).

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