EH-90-3 Accumulation of Fissionable Material in Exhaust Ventilation Systems
ENVIRONMENT, SAFETY & HEALTH
BULLETIN
Assistant Secretary for U.S. Department of Energy
Environment, Safety & Health Washington, D.C. 20585
DOE/EH-0157 Issue No. 90-3 August 1990
ACCUMULATION OF FISSIONABLE MATERIAL IN EXHAUST VENTILATION SYSTEMS
BACKGROUND
This bulletin is to alert operators of DOE facilities of the possibility of
accumulation of undesirable quantities of fissionable materials in ventilation
systems which could lead to a criticality problem or an exposure problem to
workers. Although this deals basically with plutonium-239, uranium-235 and
other fissionable materials create a similar need for the information.
A Criticality Safety Assessment, completed July 1989 at the DOE Rocky Flats
facility, revealed plutonium from gloveboxes adhering to exhaust ventilation
system ductwork. Although some ducts in the ventilation systems had been
inspected, no quantitative tests were performed on a systematic basis on the
exhaust ductwork from gloveboxes.
To minimize the introduction of plutonium in the ducts, prefilters were
installed on gloveboxes after a fire occurred in the ducts in 1969. There
were several problems associated with the functioning of these filters.
1. The seal between the filter and exhaust duct was not always effective.
2. Glovebox prefilters were sometimes difficult to change, particularly those
located at the limit of the person's reach using gloves. Also, some of the
fixtures used to position prefilters were not sufficiently rigid to ensure
the filter was positioned properly.
3. Workers sometimes punctured the filter with a sharp tool or slipped the
filter aside to allow greater air flow through the glovebox when the filter
was plugged.
4. The bypass line with an automatic valve which opens when the pressure
differential drops within the glovebox does not have a prefilter. As a
result, the glovebox atmosphere can be drawn directly into the ductwork.
Dusty processes, such as sand, slag, and crucible milling and grinding, can
result in significant quantities of fissionable material being drawn into
the exhaust ductwork.
INCIDENTS AT OTHER DOE FACILITIES
During decommissioning in 1985, personnel at a Hanford facility detected
plutonium contamination inside radiologically controlled areas previously free
of contamination. The cause was the uncontrolled release of residual
plutonium located in the process piping and ventilation systems.
In 1989, personnel at a Savannah River Site facility detected a release of
plutonium to the environment. Contamination was found on the roofs of several
onsite buildings, the adjacent road, and other isolated spots within the
F-area. The particles were most likely lodged in the ductwork, fan housing or
stack of Building 772-F, and released from corroded ducting during
maintenance.
INCIDENTS AT FACILITIES OTHER THAN DOE
In 1984, the Nuclear Fuel Services Corporation reported to the Nuclear
Regulatory Commission a significant accumulation of fissionable material at
its Erwin, Tennessee, facility. In that case, uranium accumulated in the
plant uranium scrap recovery ventilation system and associated venturi
scrubber walls.
PRECAUTIONS
Personnel at facilities where fissionable materials (e.g., plutonium, uranium,
americium, curium, neptunium) are used should be aware that these products can
potentially accumulate in exhaust ventilation systems. Uncontrolled and
undetected accumulation of fissionable material in ventilation ducts can lead
to a criticality incident, potential for release and worker exposures. Exhaust
ducting immediately connected with gloveboxes in most cases is considered
"intrinsically safe" from a critical geometry point of view due to the small
diameter. However, accumulations of fissionable material in exhaust ducting
are not always in a stable configuration. The material in small ducts may
move into ducts with larger diameters where there is potential for a
criticality incident. This potential is increased if flooding of a duct can
occur from fire sprays or other sources of water. Any sudden movement of such
an accumulation, especially if it is wet or in slurry form, to a location
having an unsafe geometry is of concern.
RECOMMENDATIONS
To ensure unsafe quantities of fissionable material do not accumulate in
exhaust ventilation systems:
1. Establish nuclear criticality safety limits.
All DOE nuclear facilities handling fissionable materials in gloveboxes or
similar enclosures served by a ventilation system should evaluate and
establish appropriate nuclear criticality safety limits for the amount of
fissionable material allowed to be present in the ventilation system.
2. Perform routine, quantitative, in-situ measurements of fissionable material
holdup in exhaust ducting.
Measurements should be made on a frequency which, along with appropriate
operational constraints and actions, preclude the accumulation of unsafe
quantities of fissionable material.
Two instruments have been found suitable for this effort at the Rocky Flats
facility. One instrument is a collimated and shielded small sodium iodide
detector. Jomar JHH.02 is a production version of a background- updating,
spectral-selective instrument originally developed at Los Alamos National
Laboratory for use in special nuclear material searches. Because of its
small active volume and small collimator opening, it is ideal for "duct
contact" measurements. The entire instrument weighs less than five pounds,
is hand-held and battery-powered (8-hour operating time between charges).
Readings taken with this instrument sense plutonium in grams per square
inch in the immediate vicinity of the point of contact. This type of
measurement maximizes the signal-to-background ratio and serves to pinpoint
the plutonium distribution geometrically along the ducting.
The second instrument is a battery-powered, high purity, high resolution
HPGe unit, with a battery-powered 4096 channel pulse height analyzer. This
instrument provides gamma spectral resolution, and the measured spectra can
be recorded on magnetic tape. Measurements taken at a stand-off position
(30 - 36 inches directly under the line of ducting to be measured) provide
data for about 4 feet of ducting. These measurements, with proper
calibration and analysis, yield the overall plutonium distribution in the
ducting in grams per 4-foot length of ducting.
3. Remove fissionable material from exhaust ducts to the maximum extent
practicable.
Removing fissionable material from glovebox ducts is under study and review
at a mock-up facility in Broomfield, Colorado. Vacuuming is successful
when the material is in dust form. For the dried slurry type, chipping is
necessary. Until tools and techniques are available to clean ductwork, it
may have to be removed and replaced.
The accumulation of fissionable material in ventilation ducts has been a
subject of concern expressed by the Defense Nuclear Facilities Safety
Board(1) and Secretary of Energy James D. Watkins.(2)
The Secretary issued the following policy associated with the removal of
plutonium from the ductwork at Rocky Flats.
All lines of ductwork containing more than 400 grams of
plutonium shall be cleaned to remove plutonium to the
maximum extent practicable, but in no case to leave a
residue exceeding 400 grams of plutonium in any one
system of ducts. This will physically rule out the
possibility of a criticality in even the most unlikely
series of events.
Appropriate steps shall be taken to implement a method of
monitoring and operational constraints to insure that
future operation of any line will not allow the level of
plutonium in the associated exhaust ductwork to exceed
400 grams in the future.
If, based on attempts or evaluation, it is impractical to
accomplish the above, or if attendant risks to personnel
during replacement of the ductwork appear unacceptable,
continued operation of such ductwork will require full
justification and resultant Secretarial approval on a
case-by-case basis.
All DOE facilities handling fissionable materials should assure that
fissionable material does not accumulate in ventilation systems that could
cause a criticality incident or undue hazard to the worker or public.
(1)Federal Register Notice, Vol. 55, No.112, page 23584, dated June 11, 1990.
(2)Federal Register Notice, Vol. 55, No.144, page 30499, dated July 25, 1990.
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This publication is one of several bulletins published to share information
with DOE program managers and contractors concerning potential occupational
safety and health problems. For more information or additional copies,
contact Eleanor Crampton, Performance Assessment Division, Assistant Secretary
for Environment, Safety & Health, U.S. Department of Energy, Washington, DC
20545; Telephone FTS 233-3732, Commercial (301) 353-3732.
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