United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-14: Uranium Oxide Fires at Fuel Cycle Facilities

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                              February 21, 1992



All fuel cycle and uranium fuel research and development licensees. 


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 

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 

     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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     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. 


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 (>300�C) 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 

* Cordfunke, E.H.P., The Chemistry of Uranium, Elsevier Publishing Company, 

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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 

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 

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 

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 

                                   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

1.  Figure 1
2.  List of Recently Issued NRC Information Notices
3.  List of Recently Issued NMSS Information Notices
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