Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555
February 21, 1992
NRC INFORMATION NOTICE 92-14: URANIUM OXIDE FIRES AT FUEL CYCLE FACILITIES
Addressees
All fuel cycle and uranium fuel research and development licensees.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for fires involving uranium
dioxide (UO2) powder at various stages of transfer and conversion. It is
expected that recipients will review the information for applicability to
their facilities and consider actions, as appropriate, to avoid similar
problems. However, suggestions contained in this information notice are not
new NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances
In licensed fuel-fabrication facilities, there have been one recent and
several past incidents of fires involving uranium at various stages of
oxidation. The circumstances of two of them are described below in some
detail.
Incident 1:
In the most recent incident, a fire was discovered, in a
fuel-fabrication facility, involving a hood, hopper, and feed-screw
assembly, which was being used to transfer calciner drop powder
(uranium oxide) to a nitric acid dissolver tank. (See Figure 1.)
According to a report submitted by the licensee, an operator had
started to feed a batch of the powder into the dissolver tank when the
feed-screw of the Model 608 Accu-Rate feeder stopped. The operator
reversed the screw and tapped on the tube-shaped nylon screw-housing,
to free the screw. At this time, he observed smoke and sparks coming
out of the equipment below the hood. A small crack in the vinyl side
of the feeder hopper, apparently the result of contact with the hot
powder inside the hopper, was also noticed. The operator and other
employees then donned full-face respirators and removed approximately
18 kg of the powder, leaving about 2 kg of powder that could not be
removed, in the screw-housing. Meanwhile, the small crack on the side
of the hopper had developed into a baseball-sized hole, spilling some
powder onto a platform below. The employees cleaned up the spilled
powder. Assuming that the incipient fire had been extinguished, the
employees then left the area.
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Approximately one hour later, fire alarm bells sounded throughout the
plant, and the source of the fire was determined to be the same hood
and feeder assembly that the employees had been working on. When plant
emergency team members, dressed in protective clothing and using
self-contained breathing apparatuses, entered the room, they found the
visibility reduced to about 1 or 2 feet, because of the heavy smoke.
Using portable dry chemical and carbon dioxide fire extinguishers, they
extinguished the fire within 15 minutes of the alarm bells sounding.
All components of the hood and the Accu-Rate feeder that were made of
combustible material (e.g., the nylon feed tube, vinyl hopper, rubber
parts of a valve, and "Lexan" faces of the hood) were consumed by the
fire. The primary stage of the high-efficiency particulate air (HEPA)
filter for the room was loaded with soot, and the pre-filter was burnt.
The fire alarm bells stopped ringing after about 3 minutes, as the
alarm circuitry in the room was damaged by the fire. This confused
some employees, who could not tell whether the emergency was over.
The incident exposed a weakness in the emergency communications system
between the licensee and the local city fire department. Shortly
before the incident, the facility had tested a newly installed
extension of its fire alarm system, in conjunction with the fire
department. Even though the facility had notified the fire department
that the test was over, the fire department mistook the alarm, which
came in about 18 minutes later, to be merely a continuation of the
test. A 911 call was needed to alert them of the real emergency.
Precious minutes were lost. Fortunately, by the time the fire
department arrived, the plant emergency team had suppressed the fire.
In other observations, some employees thought that the alarm bells in
some areas were not loud enough. Voice communications over the public
address system were misunderstood by some employees and not heard by
others, especially in the office areas.
The cause of the fire is believed to be the oxidation of the calciner
drop powder consisting principally of uranium dioxide (UO2), but also
including other unstable oxides of uranium, which could further oxidize
at elevated temperatures. The friction of the feed-screw sliding on
the powder or on the nylon tube, which could have been warped, could
conceivably have contributed to heating the powder.
Incident 2:
In another incident, at another nuclear fuel-fabrication facility, a
fire was reported to have occurred in a slugger press containment
housing. In this configuration, uranium oxide powder, following a
blending process, was gravity-fed from a second floor hammermill
baghouse through a 4-inch diameter x 6-foot long "Viton" hose to a
first floor slugger press. The Viton hose was connected to the slugger
press shuttle by a "Neoprene" boot. The slugger press shuttle area,
including the Viton hose and the
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IN 92-14
February 21, 1992
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Neoprene boot, was enclosed by the containment housing, which had two
Lexan panels for access to the shuttle area. Containment ventilation
was provided through primary and secondary HEPA filters and a water
scrubber, before exhausting to the environment.
In this incident, the operator noticed that the granulator downstream
of the slugger was not discharging powder. Apparently, this was not an
unusual occurrence, and he started to arrange for replacement of the
granulator screen, as was the normal practice. He then noticed a fire
in the slugger housing. The ventilation system smoke detector had by
that time sensed the fire and alarm bells were sounding. Employees
extinguished the fire within minutes using portable carbon dioxide fire
extinguishers. All of the combustible elements in the containment
between the hammermill and the slugger press (e.g., the Viton hose and
the Neoprene boot, as well as the Lexan parts of the containment
housing) were consumed by the fire. The primary HEPA filters were
extensively damaged. The secondary filters, however, were intact. In
this case, also, heat generated by oxidation of the powder, which
ignited the Neoprene boot, was judged to be the cause of the fire.
In other incidents, dating back to 1977, several fires involving calciner
discharge lines and at least one involving a hammermill hood have been
reported. In all cases, the oxidizing uranium powder was believed to be the
source of ignition, and combustible materials, such as transfer hoses and
boots, provided the fuel. All the fires were promptly extinguished.
Discussion
It has been common experience that unstable uranium oxide feed material
(comprised mostly of UO2, with a few other oxide forms present) in
granulated form and in contact with oxygen undergoes exothermic oxidation
reactions. In some cases, the heat generated by the reactions ignites
combustible elements of the transfer passages or other powder-handling
equipment (e.g., hoses, boots, etc.), which then contribute fuel to the
fire. The fires described above have this commonality of cause and effect.
The fuel fabrication process generates several oxides of uranium. The final
and most stable oxide is UO2. The literature on uranium chemistry describes
oxidation reactions that are complex, with their rates, heat evolution, and
final products depending on several parameters, but most importantly on the
fineness of the powder and the temperature. Indeed, according to one
author*, normally stable UO2 may be pyrophoric or oxidize rapidly even at
room temperatures when in very fine powder form (specific surface area >10
sq.m/g). Coarser powders, as is more commonly the case, may require
elevated temperatures (>300øC) to oxidize. The account of the most recent
fire suggests that elevated temperatures may have been generated by the
Accu-Rate feed-screw binding on its nylon housing. Friction of the
granulated material in motion may also have generated heat that raised the
temperature.
* Cordfunke, E.H.P., The Chemistry of Uranium, Elsevier Publishing Company,
1969.
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Since, by the very nature of the manufacturing process, unstable uranium
powder must be handled, certain preventive measures should be taken in order
to reduce the potential for fires; and many of these have been adopted at
licensed facilities. They are:
1. Limit the type of feed to stable powder whenever possible.
2. Store unstable powder in closed metal containers.
3. Replace the combustible components of powder-transfer lines and of
equipment, such as the Accu-Rate feeder, with components made of
noncombustible materials, as far as practicable.
4. Require an operator to be present when a process is under way, and
improve visibility around vulnerable equipment.
5. Incorporate fire safety of vulnerable equipment in the operator
training program, including use of portable fire extinguishers.
6. Implement a preventive maintenance program for vulnerable equipment.
Periodic inspection may alert the operator to telltale signs of
overheating.
Additionally, the following measures for upgrading the fire detection,
alarm, and suppression systems may be considered:
1. Install fire detectors in hoods and equipment exhaust ducts. These
detectors should be connected to a central panel, which is continuously
supervised.
2. Check alarm system wiring for vulnerability to fire and reroute, if
necessary and feasible. Implement a manual restart procedure, if alarm
circuitry is partially disabled and the alarm stops.
3. Upgrade the alarm system and public address system for audibility, if
necessary.
4. Add visible alarm signals in noisy areas.
5. Install carbon dioxide total flooding or local application system in
equipment enclosure. For use and limitations of such systems, see
NFPA-12, "Standard on Carbon Dioxide Extinguishing Systems," published
by the National Fire Protection Association. This should not preclude
the availability of portable fire extinguishers of both carbon dioxide
and dry chemical types.
Some lessons on emergency communications may be learned from Incident 1
above. Some protocol should be established between the facility and the
offsite fire department so that emergency calls are not misunderstood.
Licensees should consider reviewing this information notice with their local
fire department. The public address system announcement of an emergency and
related directives
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should follow standard wording that is familiar to all employees and
minimizes misunderstandings.
Fuel cycle licensees should review the Branch Technical Position (BTP) on
Fire Protection for Fuel Cycle Facilities, published in the Federal Register
(54 FR 11590-98) dated March 21, 1989. Licensees should examine their
facilities, procedures, and records to assure that the stipulations of the
BTP are met or exceeded.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate regional
office.
Richard E. Cunningham, Director
Division of Industrial and
Medical Nuclear Safety
Office of Nuclear Material Safety
and Safeguards
Technical contacts: Amar Datta, NMSS
(301) 504-2536
Charles H. Robinson, NMSS
(301) 504-2576
Attachments:
1. Figure 1
2. List of Recently Issued NRC Information Notices
3. List of Recently Issued NMSS Information Notices
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