Fire and Explosion in METALS
A detailed explanation of characteristics and hazards of some modern industrial materials and recommended extinguishing practices
ROCKETS, MISSILES, jets and nuclear reactors have stimulated unprecedented research and development in new metals and alloys which demand increasing consideration in the field of fire protection. Accelerated activity, with the constant pressure of competition and the necessity for setting up production lines at the earliest possible moment, often creates a situation in which the solution to safety problems lags behind the pace of development. While it is true that the fire and explosion potentialities are not entirely overlooked, in some instances, the familiar pattern of catastrophe preceding a determined and complete study of the fire possibilities may be anticipated. In other situations, conditions arise as the project progresses that could not be foreseen in the initial stages. All of which points up the fact that it is important to know, at least, that which has been already learned about the hazards of the metals which are assuming greater importance with each passing day.
Sodium and sodium-potassium alloys
Sodium is among these metals which have been in use for many years but is now being explored for new applications. Some nuclear reactors make use of liquid sodium as a heat transfer agent. In other applications a sodium-potassium alloy is used. Sodium and potassium are solids at ordinary temperatures. Most alloys of these two metals, however, are liquid, following the general condition that an alloy has a melting point lower than its components in a pure state.
Sodium is employed industrially in the production of fatty alcohols, sodium methylate and tetraethyl lead. Now becoming more important as a reducing agent in the separation of metals such as titanium and potassium from their salts, it is shipped in a wide variety of packaging. These include from 1 to 12pound bricks, 3 ⅛-gallon pails, 55-gallon drums and 80,000-pound tank cars. With a low melting point of 207.5° F., the metal is easily liquefied for filling tank cars. To withdraw the sodium when the tank car has arrived at its destination, it is again melted by the application of hot oil and withdrawn under an atmosphere of nitrogen. The melting process may take from 8 to 16 hours.
Sodium and sodium-potassium alloys are combustible and will generally ignite at temperatures above 239° F. The exact temperature depends upon the amount of surface exposed and other factors such as humidity and dispersion. In practically every high temperature sodium system, a leak results in a sodium fire which can burn until the oxygen content is reduced to less than 5 per cent. Temperatures of 1500 to 1660° F. are observed in the earliest stage and continued burning liberates large quantities of white opaque smoke. Because no volatile product is given off from sodium, as compared with the mechanism of a volatile liquid, the burning is not accompanied by flame. Only a hot glowing mass is apparent. Once ignited, the sodium will even continue to burn in an atmosphere of carbon dioxide, reducing it to carbon monoxide. While it will not explode, a violent reaction results from the application of common extinguishing agents such as water, carbon tetrachloride, sodium bicarbonate or carbon dioxide.
Cooling is not feasible where alkali metals arc involved in fire and exclusion of oxygen is indicated. The most effective materials are dry alkali metal chlorides, such as sodium chloride, graphite and soda ash. Two extinguishing agents developed to treat this problem are the Pyrene G-1 powder and the Ansul MetL-X dry powder. The former is the graphite variety and the latter is an impregnated sodium chloride that will not absorb moisture and is free flowing from an extinguisher under pressure of a compressed gas. Graphite powder is applied from spreading tubes, by scoops or shovels.
Soda ash and metallic chloride agents require skilled application to prevent a rekindle. As this material is directed at the fire, a crust forms on the surface, but it is subject to fracture with subsequent reignition. If the material is applied inexpertly, in large quantities, it may sink below the surface without exerting its extinguishing effect. Formation of the most effective crust is accomplished by carefully applying the powder in repeated light quantities. Of course, complete coverage of the entire mass is essential to exclude the oxygen. The problem is more difficult when the fire is burning on a vertical or irregular surface but extinguishment can be achieved. Before the fire has been brought under control, the fundamental consideration of protecting exposures is most important. Yellow has been adopted as the standard color for alkali metal fire extinguishers.
Once the fire has been brought under control, the residue consisting of unburned metal, hydroxides, oxides and fire extinguishing medium must be carefully handled. It is necessary to remove the entire mass with as little disturbance as possible and place it in a container with a cover to prevent exposure to the atmosphere and the possibility of a rekindle. A device has also been designed for the application of Met-L-X powder through a fixed pipe system where conditions indicate that it is practical.
An effective fire prevention technique in the handling of alkali metals is to maintain an inert or low oxygen atmosphere. One test of the alkali metal spills in depleted oxygen atmospheres yielded the following results:
It is worthy of note that no safety clothing gives complete protection and that self-contained respiratory equipment with a full face piece is recommended. Of greatest concern to personnel is the reaction of alkali metal on the skin and eyes. Skin contact requires quick action to scrape off any clinging metal with a sharp knife or spatula. Acetic acid, 3-5 per cent is used to neutralize alkali metal burns and boric acid solution should be applied immediately where there is eye contact.
Two types of injuries result from exposure and contact with alkali metals; thermal burn and alkali bum. The thermal burn will respond to conventional treatment for burns but complications are involved in the alkali burn. Hydroxide, formed on contact with moisture from the atmosphere or body, penetrates the tissues. This must be neutralized or excised before healing can be effected. Because the alkali burn is self-cauterizing, it seldom bleeds.
Two types of treatment have been used in cases where personnel was splashed with burning alkali metal. One recommended technique indicates flooding with water as quickly as possible and neutralizing with acetic acid. The other specifies Hooding with mineral oil and removing the metal particles by scraping or with a pair of tweezers. Neither approach has been authoritatively proven superior to the other.
With tire increased use of uranium in the program of nuclear power development, a consideration of its characteristics and typical fire incidents reveal some valuable information. An evaluation of the fire problem in which uranium is concerned must take into account the fact that it is encountered in various states, ranging from fine powders to large solid castings.
The powder, such as may be produced by a grinding operation in machining the metal, is highly pyrophoric. Tests at the U. S. Bureau of Mines indicate that uranium powder with a particle size of 10.8 microns will ignite spontaneously at room temperature and can also be ignited in an atmosphere of carbon dioxide at 1040° F. The ease with which it ignites at room temperature is linked with a probable discharge of an electrostatic spark within the dust cloud and the possible contributing factor of frictional heat developing during its dispersion.
Uranium turnings, briquettes or scrap does not present a problem in the same degree of severity as the powder but also demands great caution in handling and storing. One typical uranium incident involved a concentration of 51 drums each containing 300 pounds.
The material consisted of uranium turnings, received as scrap from a machining operation, pressed into briquettes and stored awaiting a recovery procedure. When a small amount of smoke was observed, one barrel and a covering canvas tarpaulin was involved. An initial response of the fire department applied a fine water spray to control the fire. The personnel on hand removed 31 drums from the danger area before the fire increased in intensity. Following the removal, nine of the remaining drums were subjected to solid streams individually while its burning contents was agitated. Using this technique, a 20-minute application of water in each barrel was sufficient to extinguish the fire. The practice has since been established to maintain the storage at about 100 pounds per drum under oil or water.
As a result of an accident, solid pieces of uranium were involved in a fire caused by the rupture of a gasoline tank on a truck transporting the metal. The fire spread to wooden crates containing the uranium. A local fire department responded but did not use water, under the once practiced but discarded principle that it would cause a violent reaction. When a more experienced, technically trained fire force arrived, the members applied water and extinguished the fire, which had spread, by then, to most of the cargo.
While spontaneous ignition of powdered and scrap uranium has been observed more commonly, cases of larger solid masses bursting into flame have also been noted. In these instances, a flaking on the surface apparently converted that portion of the metal into a fine powder, which, in turn, ignited. This phenomenon requires and is receiving more study in the research laboratories.
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The basic fissionable materials used in the production of atomic energy are uranium and plutonium. Only about seven-tenths of one per cent of natural uranium is the U-235 variety, which can be utilized in the reactors. The balance, for all practical purposes, is U-238. Because of the scarcity of the U-235, a process was developed to convert the more abundant U-238 to plutonium. The transmutation of U-238 is accomplished in a nuclear reactor in which a neutron is first absorbed into its nucleus, becoming U-239. An unstable isotope, U-239 throws off a beta particle and is identified as neptunium. This intermediate product, again unstable, throws off another beta particle to become the useful element, plutonium.
One typical fire involving plutonium turnings occurred when they were placed into a combustible container and sealed in a plastic bag. About half way through the sealing cycle a growing brown spot appeared on the plastic bag and a white smoke generated within the bag to inflate it. The two operators using three small carbon dioxide extinguishers attacked the flames, now 2 to 3 feet high. The extinguishers had practically no effect on the fire which burned itself out in several minutes. Radioactivity was involved and complete decontamination proceedings were necessary. The cause of this particular fire was undetermined although it is definitely established that plutonium chips are pyrophoric and that the fire potential is present in all plutonium chip operations. Since this fire, combustible containers for plutonium have been eliminated and process changes were made to prevent a recurrence.
The concluding installment will deal with the more common metal powders and metal hydrides used in industry today. In addition, the action of TMB, an extinguishing agent which burns to exert its effect, will be discussed.