Nonenriched uranium is a radioactive metal that is also combustible. Its radioactivity does not affect its combustibility, but can have a bearing on the amount of fire loss. Most metallic uranium is handled in massive forms that do not present a significant fire risk unless exposed to a severe and prolonged external fire. Once ignited, massive metal burns very slowly. In the absence of strong drafts, uranium oxide smoke tends to deposit in the immediate area of the burning metal. Unless covered with oil, massive uranium burns with virtually no visible flame. Burning uranium reacts violently with carbon tetrachloride, 1,1,1-trichloroethane, and the Halons. For power reactor purposes, uranium fuel elements are always encased in a metal jacket (usually zirconium or stainless steel).
Uranium in finely divided form is readily ignitable, and uranium scrap from machining operations is subject to spontaneous ignition. This reaction can usually be avoided by storage under dry (without moisture) oil. Grinding dust has been known to ignite even under water, and fires have occurred spontaneously in drums of coarser scrap after prolonged exposure to moist air. Because of uranium's thermal conductivity, larger pieces generally have to be heated entirely to their ignition temperature before igniting. Moist dust, turnings, and chips react slowly with water to form hydrogen. Uranium surfaces treated with concentrated nitric acid are subject to explosion or spontaneous ignition in air.
The pyrophoric characteristics of uranium are similar to those of plutonium except that uranium forms do not ignite as easily as those of plutonium. Both form pyrophoric oxides and hydrides. Both react violently with water and are best stored in their oxide form (UO{sub 2}, PuO{sub 2}) in dry, inert atmospheres. Uranium metal (U) releases hydrogen gas (H{sub 2}) when allowed to react with water. The hydrogen may then react with the metal to form uranium hydride (UH{sub 3}) which may in turn react with oxygen in the air to form stable uranium oxide (UO{sub 2}) and hydrogen gas (H{sub 2}). This sequence of events is given by the equations
U + 2H{sub 2}O {yields} UO{sub 2} + 2H{sub 2} + {Delta}Q
U + 3/2H{sub 2} {yields} UH{sub 3} + {Delta}Q
4UH{sub 3} + 7O{sub 2} {yields} 4UO{sub 2} + 6H{sub 2}O {Delta}Q.
Note, in all instances, heat ({Delta}Q) is liberated, which increases the rates of reaction.
Under a dry, slightly oxidizing atmosphere, however, uranium corrodes quiescently according to the equation
U + O{sub 2} {yields} UO{sub 2} + {Delta}Q .
The heat generated from slow corrosion is not sufficient to ignite the uranium.
The storage of the many forms of uranium is beyond the scope of this primer. For a complete discussion of uranium storage, the reader should refer to the draft Assessment of Uranium Storage Safety Issues at Department of Energy Facilities (referenced).
Uranium fires should be extinguished with the same techniques and precautions as plutonium fires (see corresponding paragraph on plutonium).
A variety of metals burn, particularly those in finely divided form. Some metals burn when heated to high temperatures by friction or exposure to external heat; others burn from contact with moisture or in reaction with other materials. Because accidental fires may occur during the transportation of these materials, it is important to understand the nature of the various fires and the hazards involved.
The hazards involved in the control or complete extinguishing of metal fires include extremely high temperatures, steam explosions, hydrogen explosions, toxic products of combustion, explosive reaction with some common extinguishing agents, breakdowns of some extinguishing agents with the liberation of combustible gases or toxic products of combustion, and, in the case of certain nuclear materials, dangerous radiation. Some agents displace oxygen, especially in confined spaces. Therefore, extinguishing agents and methods for their specific application must be selected with care. Metal fires should not be approached without suitable self-contained breathing apparatus and protective clothing, unless the fire is enclosed in a glove box.
Numerous agents have been developed to extinguish combustible metal (Class D) fires, but a given agent does not necessarily control or extinguish all metal fires. Although some agents are valuable in working with several metals, other agents are useful in combating only one type of metal fire. Despite their use in industry, some of these agents provide only partial control and cannot be considered actual extinguishing agents. Certain agents that are suitable for other classes of fires should be avoided for metal fires, because violent reactions may result (e.g., water on sodium; vaporizing liquids on magnesium fires).
Certain combustible metal extinguishing agents have been used for years, and their success in handling metal fires has led to the designations "approved extinguishing powder" and "dry powder." These designations have appeared in codes and other publications where it was not possible to employ the proprietary names of the powders. These terms have been accepted in describing extinguishing agents for metal fires and should not be confused with the name "dry chemical," which normally applies to an agent suitable for use on flammable liquid (Class B) and live electrical equipment (Class C) fires. Class B extinguishing agents may not be safely applied to come combustible metal (Class D) fires. Other extinguishing agents discussed herein have been used only experimentally in limited areas or at specific installations, and require much judgment in application.
The successful control or extinguishment of metal fires depends heavily upon the method of application, training, and experience. Practice drills should be held on the particular combustible metals on which the agent is expected to be used. Prior knowledge of the capabilities and limitations of agents and associated equipment is always useful in emergency situations. Fire control or extinguishment will be difficult if the burning metal is in a place or position where the extinguishing agent cannot be applied in the most effective manner. In industrial plant locations where work is performed with combustible metals, public fire departments and industrial fire brigades have the advantage of fire control drills conducted under the guidance of knowledgeable individuals.
A number of proprietary combustible metal extinguishing agents have been submitted to testing agencies for approval or listing. Others have not, particularly those agents developed for special metals in rather limited commercial use. Those extinguishing agents described as follows have been approved or listed for use on fires involving magnesium, aluminum, sodium, potassium, and sodium-potassium alloy. Information on extinguishing agents was obtained from the 17th Edition of the NFPA Handbook.
"Pyrene" G-1 powder is composed of screened graphitized foundry coke to which an organic phosphate has been added. A combination of particle sizes is used to provide good packing characteristics when applied to a metal fire. The graphite acts as a heat conductor and absorbs heat from the fire to lower the metal temperature below the ignition point, which results in extinguishment. The closely packed graphite also smothers the fire, and the organic material in the agent breaks down with heat to yield a slightly smoky gas that penetrates the spaces between the graphite particles, excluding air. The powder is nontoxic and noncombustible.
G-1 powder is stored in cardboard tubes or metal pails, and can be stored for long periods of time without deterioration or caking. It is applied to the metal fire with a hand scoop or a shovel. The packing characteristics of the powder prevent its discharge from a fire extinguisher.
The powder is applied by spreading it evenly over the surface of the fire to a depth sufficient to smother the fire. A layer at least 12.5 mm (1/2 in.) deep is recommended for fires involving fines of magnesium and magnesium alloys. Larger chunks of metal require additional powder to cover the burning areas.
Where burning metal is on a combustible surface, the fire should be extinguished by (a) first covering it with powder, (b) shoveling the burning metal onto another 25 or 50 mm (1 or 2 in.) layer of powder that has been spread out on a nearby noncombustible surface, and (c) adding more powder as needed.
G-1 powder is effective for fires in magnesium, sodium, potassium, titanium, lithium, calcium, zirconium, and hafnium, and has been recommended for special applications on powder fires in aluminum, zinc, and iron. It is listed by Underwriters Laboratories Inc. (UL) for use only on magnesium and magnesium alloys (dry fines and moist fines that are not moistened or wetted with water or water soluble cutting oils) and is approved by the Factory Mutual System (FM) for use on fires in magnesium, aluminum, sodium, potassium, and sodium-potassium alloy. When plans call for use of G-1 powder on those metals mentioned, practice fire drills should be held in advance. For fires of thorium, uranium, beryllium, or plutonium, G-1 powder can be effective in covering the fire to prevent its spread, but will not extinguish these fires. The products of combustion of thorium, uranium, beryllium, and plutonium can be a health hazard, and precautions should be observed consistent with the usual procedures in combating fires in radioactive material.
MetalGuard powder is identical to G-1 powder in composition, and is simply a trade name variation.
This dry powder, with its particle size controlled for optimum extinguishing effectiveness, is composed of a sodium chloride base with additives. The additives include tricalcium phosphate to improve flow characteristics and metal stearates for water repellency. A thermoplastic material is added to bind the sodium chloride particles into a solid mass under fire conditions.
Met-L-X powder is noncombustible, and secondary fires do not result from its application to burning metal. No known health hazard results from the use of this agent. It is nonabrasive and nonconductive.
Stored in sealed containers or extinguishers, Met-L-X powder is not subject to decomposition or a change in properties. Periodic replacement of extinguisher charges is unnecessary. Extinguishers range from 14-kg (30-lb) portable hand units (carbon dioxide cartridge propellant), through 68- and 160-kg (150- and 350-lb) wheeled units, to 900 kg (2,000 lb) for stationary or piped systems. The wheeled units and piped systems employ nitrogen as the propellant.
The powder is suitable for fires in solid chunks (such as castings) because of its ability to cling to hot vertical surfaces. To control and then extinguish a metal fire, the nozzle of the extinguisher is fully opened and, from a safe distance (in order to prevent blowing the burning metal into other areas), a thin layer of agent is cautiously applied over the burning mass. Once control is established, the nozzle valve is used to throttle the stream to produce a soft, heavy flow. The metal can then be completely and safely covered from close range with a heavy layer. The heat of the fire causes the powder to cake, forming a crust which excludes air and results in extinguishment.
Met-L-X extinguishers are available for fires involving magnesium, sodium (spills or in depth), potassium, and sodium-potassium alloy (NaK). In addition, Met-L-X has been successfully used where zirconium, uranium, titanium, and powdered aluminum present serious hazards.
Based upon their past usage and known value as extinguishing agents for metal fires, the two agents previously discussed (G-1 and Met-L-X powders) are the most notable. Continuous experience with these agents has provided sufficient information to list, in Table 3, the capabilities and limitations of each when applied to certain metal fires.
This powder was developed to satisfy the need for a low chloride content agent that could be used on sodium metal fires. Na-X has a sodium carbonate base with various additives incorporated to render the agent nonhygroscopic (does not absorb moisture) and easily fluidized for use in pressurized extinguishers. It also incorporates an additive which softens and crusts over an exposed surface of burning sodium metal. Na-X is noncombustible, and does not cause secondary fires when applied to burning sodium metal above temperatures ranging from 649 to 816 degrees C (1,200 to 1,500 degrees F). No known health hazard results from the use of this agent on sodium fires, and it is nonabrasive and nonconductive.
Stored in 23-kg (50-lb) pails, 14-kg (30-lb) hand portables, and 68- and 160-kg (150- and 350-lb) wheeled and stationary extinguishers, Na-X is listed by UL for fires involving sodium metal up to a temperature of 649 degrees C (1,200 degrees F). Na-X has been tested on sodium metal (spills and in depth) at fuel temperatures as high as 816 degrees C (1,500 degrees F). Stored in the supplier's metal pails and extinguishers, Na-X is not subject to decomposition, so periodic replacement of the agent is unnecessary.
In magnesium foundry operations, molten magnesium is protected from contact with air by layers of either molten- or crust-type fluxes. These fluxes, which are also used as molten metal cleaning agents, consist of various amounts of potassium chloride, barium chloride, magnesium chloride, sodium chloride, and calcium fluoride. The fluxes are stored in covered steel drums. When applied to burning magnesium, these fluxes melt on the surface of the solid or molten metal, excluding air. The thin layer of protection can be provided by properly applying relatively small amounts of flux.
Fluxes are valuable in extinguishing magnesium spill fires from broken molds or leaking pots and in controlling and extinguishing fires in heat treating furnaces. In open fires, the flux is applied with a hand scoop or a shovel. Areas of furnaces that are difficult to reach can be coated by means of a flux throwing device similar to those used to throw concrete onto building forms.
While fluxes would rapidly extinguish chip fires in machine shops, such use is not recommended. The fluxes are hygroscopic and the water picked up from the air, combined with the salt, causes severe rusting of equipment.
Advances in the science of alternative propulsion systems have led to the development of copper powder as a viable extinguishing agent for combustible metals. Work sponsored by the Naval Sea Systems Command was conducted to evaluate the adequacy of existing lithium fire suppression agents and to develop new agents should deficiencies exist.
Copper powder was found to be superior to known lithium fire extinguishing agents in extinguishing capacity. The dry powder is of uniform particle size and extinguishes a lithium fire more quickly and efficiently than existing agents. The process of extinction is by formation of a copper-lithium alloy, which is nonreactive and forms preferentially on the surface of the molten lithium. The alloy becomes an exclusion boundary between air and the molten metal, preventing reignition and promoting cooling of the unreacted lithium.
Copper powder can be applied from hand-portable extinguishers. The nominal charge for each extinguisher is 14 kg (30 lb); it is 68 and 160 kg (150 and 350 lbs) for wheeled units, as well as fixed systems. Argon is used as the propellant. The method of application is similar to that of other metal fire powders, in that the fuel surface is coated with the copper powder in an initial pass, with a throttled application following once control is achieved. Typical application densities are 3.6 kg (8 lbs) of copper powder per pound of lithium for complete extinguishment of the lithium. An 18-kg (40-lb) lithium fire can be fully controlled in 30 seconds and completely extinguished in 9 minutes. Copper powder also has been used to extinguish magnesium and aluminum fires.
This dry powder is composed of a special graphite base with additives. The additives render it free flowing so it can be discharged from an extinguisher. The technique used to extinguish a metal fire with this agent is the same as that used with Met-L-X. Lith-X does not cake or crust over when applied to burning metal. It excludes air and conducts heat away from the burning mass to effect extinguishment. It does not cling to hot metal surfaces, so it is necessary to completely cover the burning metal.
Lith-X will successfully extinguish lithium fires and is suitable for the control and extinguishment of magnesium and zirconium chip fires. It will extinguish sodium spill and sodium fires in-depth. Sodium-potassium alloy spill fires are extinguished, and fires in-depth are controlled.
TMB is the chemical abbreviation for trimethoxyboroxine. The agent contains methanol to render it free flowing. It is classed as a flammable liquid for shipping purposes. The liquid is colorless and hydrolyzes readily to form boric acid and methanol. Contact with moist air or other sources of water must be avoided to prevent hydrolysis.
This agent is applied with a specially adapted 9.5 L (2-1/2 gal) stored-pressure extinguisher which delivers either spray or a straight stream. Typical application of TMB to a metal fire yields a heat flash because of the breakdown of the chemical compound and ignition of the methanol. A molten boric oxide coating on the hot metal prevents contact with air. A stream of water may be used to cool the mass as soon as metal flames are no longer visible; this should be done cautiously to avoid rupture of the coating. Indoor application (such as in machine shops) is not recommended because of the large volume of boric oxide smoke produced. Boric oxide is only slightly toxic.
While TMB has been used primarily on magnesium fires, it has shown value in application to fires in zirconium and titanium. Although TMB applied as a spray has been used to control small sodium and sodium-potassium alloy fires, it is not recommended for fires in sodium, sodium-potassium alloy, and lithium. TMB reacts violently with lithium and sodium- potassium alloy. It will extinguish sodium in-depth, but the protective coating formed by the TMB absorbs moisture very rapidly and in time may penetrate through to the sodium, resulting in a violent reaction. Field experience has been limited to aircraft fires.
Pyromet powder is composed of specially processed sodium chloride, diammonium phosphate, protein, and a waterproofing and flow promoting agent. The powder is discharged under pressure provided by a carbon dioxide gas cartridge. The unit contains 11 kg (25 lb) of powder. The applicator consists of a tubular extension from the control valve, terminating in a cone-shaped nozzle. A mechanism in the nozzle absorbs the discharge pressure by swirling the powder as it is expelled. This enables the operator to let the powder fall gently on the burning metal rather than to scatter burning material under the blast of a jet of powder.
Pyromet has proven effective in handling fires involving sodium, calcium, zirconium, and titanium, as well as magnesium and aluminum in the form of powder or chips.
TEC (ternary eutectic chloride) powder is a mixture of potassium chloride, sodium chloride, and barium chloride that is effective in extinguishing fires in certain combustible metals. The powder tends to seal the metal, excluding air. On a hot magnesium chip fire its action is similar to that of foundry flux. In tests reported in Fire Technology, TEC powder was the most effective salt for control of sodium, potassium, and sodium-potassium alloy fires. TEC should not be used on plutonium, uranium, and alkali metal fires because it is hygroscopic.
Talc, which has been used industrially on magnesium fires, acts to control rather than extinguish fire. Talc acts as an insulator to retain the heat of the fire, rather than as a coolant. It does, however, react with burning magnesium to provide a source of oxygen. The addition of organic matter (such as protein) to talc assists in the controlling action, but does not prevent the reaction which releases oxygen to the fire.
Specially formulated extinguishing powders are generally used to suppress fires involving metals. Because of the reaction many metals have with water, sprinklers and the use of other water-based agents are not appropriate and, in some cases, quite dangerous. However, many of the special agents for metal fires are at times unsatisfactory because they are corrosive, applied manually rather than by an extinguishing system, capable of clogging extinguishing nozzles, and expensive.
Studies have been undertaken to examine the effectiveness of carbon microspheres or microspheroids to extinguish fires involving alkali metals, such as sodium, sodium-potassium, and lithium. These microspheres are petroleum-coke-based particles with a diameter of approximately 100 to 500 microns. The particles possess high thermal conductivity, chemical inertness, and excellent flow characteristics and are capable of being directed onto fires from dry-chemical-type extinguishers and conventional nozzles.
Tests have shown that carbon microspheres compare favorably in performance to other metal extinguishing agents. In particular, experiments with carbon microspheroids incorporating neutron absorbers have been effective in extinguishing fires involving nuclear fissionable materials, such as uranium metal powder. The excellent flow characteristics and noncoking properties of these microspheres suggest an effective way to extinguish radioactive metal fires within the inert atmosphere glovebox enclosures used in the nuclear industry.
Graphite powder (plumbago) has been used as an extinguishing agent for metal fires. Its action is similar to that of G-1 powder in that the graphite acts as a coolant. Unless the powder is finely divided and closely packed over the burning metal, some air does get through to the metal and extinguishment is not as rapid as with G-1 powder.
Dry sand has often been recommended as an agent for controlling and extinguishing metal fires. At times it seems to be satisfactory, but usually hot metal (such as magnesium) obtains oxygen from the silicone dioxide in the sand and continues to burn under the pile. Sand is seldom completely dry. Burning metal reacting with the moisture in the sand produces steam and, under certain conditions, may produce an explosive metal-water reaction. By laying the sand around the perimeter of the fire, fine dry sand can be used to isolate incipient fires of aluminum dust.
Cast iron borings or turnings are frequently available in the same machine shop as the various combustible metals. Clean iron borings applied over a magnesium chip fire cool the hot metal and help extinguish the fire. This agent is used by some shops for handling small fires where, with normal good housekeeping, only a few combustible metal chips are involved. Contamination of the metal chips with iron may be an economic problem. Oxidized iron chips must be avoided to prevent possible thermite reaction with the hot metal, and the iron chips must be free from moisture.
Alkali metal fires can be extinguished by sodium chloride, which forms a protective blanket that excludes air over the metal so that the metal cools below its burning temperature. Sodium chloride is an agent that is used for extinguishing sodium and potassium fires. It can also be used to extinguish magnesium fires.
Sodium carbonate or soda ash (not dry chemical) is recommended for extinguishing sodium and potassium fires. Its action is similar to that of sodium chloride.
Lithium chloride is an effective extinguishing agent for lithium metal fires. However, its use should be limited to specialized applications because the chemical is hygroscopic to a degree and may present problems because of the reaction between the moisture and the lithium.
This agent has been used successfully to extinguish lithium fires.
If zirconium or titanium in the form of dry powder becomes ignited, neither can be extinguished easily. Control can be effected by spreading dolomite (a carbonate of calcium and magnesium) around the burning area and then adding more powder until the burning pile is completely covered.
Boron trifluoride and boron trichloride have both been used to control fires in heat treating furnaces containing magnesium. The fluoride is considerably more effective. In the case of small fires, the gases provide complete extinguishment. In the case of large fires, the gases effect control over the flames and rapid burning, but reignition of the hot metal takes place on exposure to air. A combined attack of boron trifluoride gas followed by application of foundry flux completely extinguishes the fire. For details of gas application, see NFPA 480, Standard for the Storage, Handling, and Processing of Magnesium.
In some cases, inert gases (such as argon and helium) will control zirconium fires if they can be used under conditions that will exclude air. Gas blanketing with argon has been effective in controlling lithium, sodium, and potassium fires. Caution should be exercised when using the agent in confined spaces because of the danger of suffocation of personnel.
When burning metals are spattered with limited amounts of water, the hot metal extracts oxygen from the water and promotes combustion. At the same time, hydrogen is released in a free state and ignites readily. Since small amounts of water do accelerate combustible metal fires (particularly where chips or other fines are involved), use of common portable extinguishers containing water is not recommended except to control fires in adjacent Class A materials.
Water, however, is a good coolant and can be used on some combustible metals under proper conditions and applications to reduce the temperature of the burning metals below the ignition point. The following paragraphs discuss the advantages and limitations of using water on fires involving various combustible metals.
Water must not be used on fires involving these metals. Water applied to sodium, potassium, lithium, sodium-potassium alloys (NaK), barium, calcium, and strontium will induce chemical reactions that can lead to fire or explosion even at room temperatures.
Powdered zirconium wet with water is more difficult to ignite than the dry powder. However, once ignition takes place, wet powder burns more violently than dry powder. Powder containing about 5 to 10% water is considered to be the most dangerous. Small volumes of water should not be applied to burning zirconium, but large volumes of water can be successfully used to completely cover solid chunks or large chips of burning zirconium (e.g., by drowning the metal in a tank or barrel of water). Hose streams applied directly to burning zirconium chips may yield violent reactions.
Water is generally acceptable for use as an extinguishing agent for fires involving enriched uranium, plutonium, and thorium fires (fissionable materials). In rare cases where criticality safety considerations preclude the introduction of moderators such as water, suitable alternative fire protection measures need to be incorporated into the facility design. Limited amounts of water add to the intensity of a fire in natural uranium or thorium, and greatly increase the contamination cleanup required after the fire. A natural uranium scrap fire can be fought with water by personnel (wearing face shields and gloves and using long-handled shovels) shoveling the burning scrap into a drum of water in the open. The hydrogen formed may ignite and burn off above the top of the drum. The radioactivity hazard of nonenriched uranium is extremely low (in reality, uranium is a heavy metal poison, although considerably less toxic than lead). If ingested, plutonium is considerably more hazardous to humans than uranium.
Although water in small quantities accelerates magnesium fires, rapid application of large amounts of water is effective in extinguishing magnesium fires because of the cooling effect of water. Automatic sprinklers will extinguish a typical shop fire where the quantity of magnesium is limited. However, water should not be used on any fire involving a large number of magnesium chips when it is doubtful that there is sufficient water to handle the large area. (A few burning chips can be extinguished by dropping them into a bucket of water.) Small streams from portable extinguishers will violently accelerate a magnesium chip fire.
Burning magnesium parts such as castings and fabricated structures can be cooled and extinguished with coarse streams of water applied with standard fire hoses. A straight stream scatters the fire, but coarse drops (produced by a fixed nozzle operating at a distance or by use of an adjustable nozzle) flow over and cool the unburned metal. Some temporary acceleration normally takes place with this procedure, but rapid extinguishment follows if the technique is pursued. Well-advanced fires in several hundred pounds (100 lb equals 45 kg) of magnesium scrap have been extinguished in less than 1 minute with two 37.5 mm (1-1/2 in.) fire hoses. Water fog, on the other hand, tends to accelerate rather than cool such a fire. Application of water to magnesium fires must be avoided where quantities of molten metal are likely to be present; the steam formation and possible metal-water reactions may be explosive.
Water must not be used on fires in titanium fines and should be used with caution on other titanium fires. Small amounts of burning titanium (other than fines) can be extinguished and considerable salvage realized by quickly dumping the burning material into a large volume of water to completely submerge it. Hose streams have been used effectively on fires in outside piles of scrap, but violent reactions have been reported in other cases where water was applied to hot or burning titanium, resulting in serious injury to personnel. Additional information on the use of water on titanium fires can be found in NFPA 481, Standard for the Production, Processing, Handling, and Storage of Titanium.