Explosives and Propellants
Explosions occur when gases in a confined space expand with a pressure and velocity that cause stresses greater than the confining structure can withstand. The gas expansion can be caused by rapid generation of gaseous molecules from a solid or liquid (e.g., an explosive) and/or rapid heating (as in a steam "explosion"). An explosion can be low-level, yet still dangerous, as in the deflagration (rapid burning) of a flour dust and air mixture in a grain elevator, or very intensive, as in the detonation of a vial of nitroglycerin.
Explosives and propellants are mixtures of fuel and oxidizer. The intensity of combustion is determined by the heat of combustion per pound of material, the material's density, the gas volume generated per volume of material, and the rate of deflagration or detonation. The latter, the most important variable, is determined by the speed at which fuel and oxidizer molecules combine.
The first explosive was black gunpowder, invented by the Chinese in the Middle Ages. In gunpowder, the fuel is powdered sulfur and charcoal, and the oxidizer is saltpeter (potassium nitrate). When heated, the oxidizer molecule decomposes to form potassium oxide (a solid), nitrogen and nitrous oxides (gases), and excess pure oxygen that burns the fuel to form more gases (carbon oxides, sulfur oxides). The gases generated by this rapidly burning mixture can explode (rupture its container), as in a firecracker, or propel a projectile, as in a rocket or a gun. Because it takes time for oxygen to diffuse to the fuel molecules the explosion of gunpowder is a rapid burning, or deflagration, not the high-rate detonation characteristic of a high explosive.
While propellants are formulated to burn rapidly and in a controlled manner, they can go from deflagration to detonation if mishandled. High explosives, on the other hand, are designed to detonate when activated. Here oxidizer and fuel are always situated in the same molecule, and in the right proportions, as determined by the desired end-products. Once initiated, gases are formed too fast to diffuse away in an orderly manner, and a shock wave is generated that passes through the explosive, detonating it almost instantaneously. This shock wave and the resultant high-velocity expansion of gases can cause great damage, even if there is no confining container to rupture.
A propellant typically burns at the rate of about 1 cm/sec, some 10,000 times the rate coal burns in air. However high explosives "burn" at a rate some 100,000 to 1,000,000 times faster than propellants because the reaction rate is controlled by shock transfer rather than heat transfer. Thus, although there is more energy released in burning a pound of coal than a pound of dynamite, the power output, which is the rate of doing work, can be 100 times larger for a propellant, and 100 million times larger for a high explosive.
Black gunpowder, was the only explosive (actually a propellant) known until 1847, when the Italian chemist, Ascani Sobrero, discovered nitroglycerin. This rather unstable and shock-sensitive liquid was the first high explosive. It was too dangerous to use until Alfred Nobel developed dynamite in 1866, a 50 percent mixture of nitroglycerin stabilized by absorption in inert diatomaceous earth. The much safer trinitrotoluene (TNT) was synthesized soon after, and military needs in the two World Wars led to a number of new high explosives. Almost all of these are made by nitration of organic substrates, and they are formulated to be simultaneously safer and more energetic (see Table 1). For commercial uses, low cost is of paramount importance, and it has been found that a mixture of ammonium nitrate, a fertilizer ingredient, and fuel oil (ANFO) gives the most "bang for the buck."
By designing an explosive charge to focus its blast on a small area, a so-called shaped or armor-piercing charge, gas velocities as high as 20,000 mph and at pressures of 3 million psi can be achieved. Such a force pushes steel aside through plastic flow, much as a knife cuts through butter. Under the high pressure, the metallic steel behaves like a viscous liquid, the same way plastics flow when extruded through dies to make various shapes.
The thrust of recent explosives research continues to emphasize increased safety and control. These properties are achieved by the use of primary and secondary explosives. The secondary explosive is the main ingredient in a charge, and it is formulated to be stable in storage and difficult to initiate. Some secondary explosives just burn slowly without detonating if accidentally ignited. The initiator or primary explosive is a small quantity of a more sensitive material fashioned into a detonator or blasting cap. It is attached to the main charge just before use and is activated by a fuse, by percussion (as in a gun), or by an electrical current.
The key to safety in explosives manufacturing is to use isolated high-velocity nitric acid reactors that have only a very small hold up at any one time (that is, only a small amount of dangerous material is "held up" inside the reactor at any time). Units are widely spaced, so any accident involves only small amounts of explosive and does not propagate through the plant. Fire and electrical spark hazards are rigorously controlled, and manpower reduced to the absolute minimum through automation.
The recent rise in the use of expolosives in terrorist activity poses new challenges to industry and law enforcement. This challenge is being met by the use of sophisticated chemical detection devices to screen for bombs and more rigorous explosive inventory safeguards and controls. Plans have also been proposed to tag explosives with isotopes to make them easier to trace if misused.
Nuclear Energy; Nuclear Fission; Nuclear Fusion.
Bibliography
Harding, G., ed. (1993). Substance Detection Systems, Vol. 2092. Bellingham, WA: Proceedings of the International Society for Optical Engineering.
Squire, D. R. (1991). Chemistry of Energetic Materials. San Diego, CA: Academic Press.
Urbanski, T. (1985). Chemistry and Technology of Explosives New York: Pergamon Press.
U.S. Bureau of Alcohol, Tobacco and Firearms. (1986). Firearms and Explosives Tracing. Washington, DC: Author.
U.S. Senate, Committee on the Judiciary. (1993). Terrorism in America. Washington, DC: U.S. Government Printing Office.
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