Explosion

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An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive". Subsonic shock waves are created by low explosives through the slower burning process known as deflagration.

Natural
Explosions can occur in nature. Most natural explosions arise from volcanic processes of various sorts. Explosive volcanic eruptions occur when magma rising from below has much dissolved gas in it; the reduction of pressure as the magma rises causes the gas to bubble out of solution, resulting in a rapid increase in volume. Explosions also occur as a result of impact events. Explosions can also occur outside of Earth in the universe in events such as supernova. Explosions frequently occur during Bushfires in Eucalyptus forests where the volatile oils in the tree tops suddenly combust.

Chemical
The most common artificial explosives are chemical explosives, usually involving a rapid and violent oxidation reaction that produces large amounts of hot gas. Gunpowder was the first explosive to be discovered and put to use. Other notable early developments in chemical explosive technology were Frederick Augustus Abel's development of nitrocellulose in 1865 and Alfred Nobel's invention of dynamite in 1866.

Nuclear
A nuclear weapon is a type of explosive weapon that derives its destructive force from the nuclear reaction of fission or from a combination of fission and fusion. As a result, even a nuclear weapon with a small yield is significantly more powerful than the largest conventional explosives available, with a single weapon capable of completely destroying an entire city.

Electrical
A high current electrical fault can create an electrical explosion by forming a high energy electrical arc which rapidly vaporizes metal and insulation material. This arc flash hazard is a danger to persons working on energized switchgear. Also, excessive magnetic pressure within an ultra-strong electromagnet can cause a magnetic explosion.

Vapour
Boiling liquid expanding vapour explosions are a type of explosion that can occur when a vessel containing a pressurized liquid is ruptured, causing a rapid increase in volume as the liquid evaporates.

Astronomical
Among the largest known explosions in the universe are supernova, which result from stars exploding, and gamma ray bursts, whose nature is still in some dispute. Solar flares are an example of explosion common on the Sun, and presumably on most other stars as well. The energy source for solar flare activity comes from the tangling of magnetic field lines resulting from the rotation of the Sun's conductive plasma. Another type of large astronomical explosion occurs when a very large meteoroid or an asteroid impacts the surface of another object, such as a planet.

Mechanical
Strictly a physical process, as opposed to chemical or nuclear, eg, the bursting of a sealed or partially-sealed container under internal pressure is often referred to as a 'mechanical explosion'. Examples include an overheated boiler or a simple tin can of beans tossed into a fire. A boiling liquid expanding vapor explosion (BLEVE) (see above) is one type of mechanical explosion, but depending on the contents of the container, the effects can be dramatically more serious - consider a propane tank in the midst of a fire. In such a case, to the limited effects of the simple mechanical explosion when the tank fails are added the chemical explosion resulting from the released (initially liquid and then almost instanteaously gaseous) propane in the presence of an ignition source. For this reason, emergency workers often differentiate between the two events.

Force
Explosive force is released in a vertical direction to the surface of the explosive. If the surface is cut or shaped, the explosive forces can be focused to produce a greater local effect; this is known as a shaped charge.

Velocity
Rapidity of reaction distinguishes the explosive reaction from an ordinary combustion reaction by the great speed with which it takes place. Unless the reaction occurs rapidly, the thermally expanded gases will be dissipated in the medium, and there will be no explosion. Again, consider a wood or coal fire. As the fire burns, there is the evolution of heat and the formation of gases, but neither is liberated rapidly enough to cause an explosion. This can be likened to the difference between the energy discharge of a battery, which is slow, and that of a flash capacitor like that in a camera flash, which releases its energy all at once.

Evolution of heat
The generation of heat in large quantities accompanies most explosive chemical reaction. The exceptions are called entropic explosives and include organic peroxides such as acetone peroxide It is the rapid liberation of heat that causes the gaseous products of most explosive reactions to expand and generate high pressures. This rapid generation of high pressures of the released gas constitutes the explosion. The liberation of heat with insufficient rapidity will not cause an explosion. For example, although a pound of coal yields five times as much heat as a pound of nitroglycerin, the coal cannot be used as an explosive because the rate at which it yields this heat is quite slow.

Heat of explosion
When a bomb is formed from its constituents, heat may either be absorbed or released. The quantity of heat absorbed or given off during transformation is called the heat of formation. Heats of formations for solids and gases found in explosive reactions have been determined for a temperature of 15 °C and atmospheric pressure, and are normally given in units of kilocalories per gram-molecule. (See table 12-1). A negative value indicates that heat is absorbed during the formation of the compound from its elements; such a reaction is called an endothermic reaction. The arbitrary convention usually employed in simple thermochemical calculations is to take heat contents of all elements as zero in their standard states at all temperatures (standard state being defined as natural or ambient conditions). Since the heat of formation of a compound is the net difference between the heat content of the compound and that of its elements, and since the latter are taken as zero by convention, it follows that the heat content of a compound is equal to its heat of formation in such non-rigorous calculations. This leads to the principle of initial and final state, which may be expressed as follows: "The net quantity of heat liberated or absorbed in any chemical modification of a system depends solely upon the initial and final states of the system, provided the transformation takes place at constant volume or at constant pressure. It is completely independent of the intermediate transformations and of the time required for the reactions." From this it follows that the heat liberated in any transformation accomplished through successive reactions is the algebraic sum of the heats liberated or absorbed in the several reactions. Consider the formation of the original explosive from its elements as an intermediate reaction in the formation of the products of explosion. The net amount of heat liberated during an explosion is the sum of the heats of formation of the products of explosion, minus the heat of formation of the original explosive. The net difference between heats of formations of the reactants and products in a chemical reaction is termed the heat of reaction. For oxidation this heat of reaction may be termed heat of combustion. In explosive technology only materials that are exothermic—that have a heat of reaction that causes net liberation of heat—are of interest. Hence, in this context, virtually all heats of reaction are positive. Reaction heat is measured under conditions either of constant pressure or constant volume. It is this heat of reaction that may be properly expressed as the "heat of explosion."

Initiation of reaction
A chemical explosive is a compound or mixture which, upon the application of heat or shock, decomposes or rearranges with extreme rapidity, yielding much gas and heat. Many substances not ordinarily classed as explosives may do one, or even two, of these things. For a chemical to be an explosive, it must exhibit all of the following: (1) Formation of gases (2) Evolution of heat (3) Rapidity of reaction (4) Initiation of reaction A reaction must be capable of being initiated by the application of shock, heat, or a catalyst (in the case of some explosive chemical reactions) to a small portion of the mass of the explosive material. A material in which the first three factors exist cannot be accepted as an explosive unless the reaction can be made to occur when desired.

Fragmentation
Fragmentation is the accumulation and projection of particles as the result of a high explosives detonation. Fragments could be part of a structure such as a magazine. They could be part of an ordnance case such as a projectile body or a bomb body. High velocity, low angle fragments can travel hundreds or thousands of feet with enough energy to initiate other surrounding high explosive items, injure or kill personnel and damage vehicles or structures.

Chemical explosions

 * Nanaimo mine explosion 1887
 * Halifax Explosion 1917
 * Battle of Messines 1917
 * Oppau explosion, Ludwigshafen, Germany 1921
 * Bombay Explosion (1944)
 * Port Chicago disaster 1944
 * RAF Fauld explosion 1944
 * Texas City Disaster 1947
 * Nedelin catastrophe 1960
 * Soviet N1 rocket explosion 1969
 * Flixborough disaster 1974
 * PEPCON disaster, Henderson, Nevada 1988
 * Ryongchon disaster 2004
 * 2005 Hertfordshire Oil Storage Terminal fire 2005
 * Albania explosion Gerdec 2008
 * Cataño oil refinery fire 2009

Nuclear testing

 * Trinity test
 * Castle Bravo
 * Tsar Bomba

Use in war

 * Atomic bombings of Hiroshima and Nagasaki

Exploding volcanoes

 * Santorini
 * Krakatoa
 * Mount St. Helens
 * Mount Tambora
 * Mount Pinatubo
 * Yellowstone Caldera