What is an Explosion and what are its characteristics?

An explosion is a sudden and violent release of stored/ intrinsic/potential energy which manifests in the surrounding in different ways. If the surrounding contains a medium, solid/liquid/ gaseous; rapidly moving rise in pressure called a blast wave or shockwave is generated which can cause enormous damage. If the released energy appears in form of heat, high temperature may be generated which shall appear as flash with bright light. Large increase in volume shall generate shock/blast wave which can be heard as loud sound. Shockwave can cause extensive damage in the surroundings as it happens in case of high capacity lightning, when we witness blinding flash, thunderous sound and impacting shock waves causing huge damage to buildings, structures and equipment.

Potential energy may be in any of the following forms:
(i) Physical (ii) Chemical (iii) Nuclear

Physical Explosion

Common examples can be kid’s balloon, football bladder, vehicle’s tubes, etc. which, when being filled with compressed air, can burst causing huge sound. There is no chemical reaction or nuclear chain reaction at all. The explosion is purely physical in nature.

Rupture and subsequent explosion occurs when the internal pressure exceeds the yield point of the container, causing wave generation in the surrounding media. Thus, we hear the loud sound. It will also propagate shockwaves through the media. Boiler tube, containing high pressure, high temperature steam, on rupturing, shall also produce similar effects.

BLEVE (Boiling Liquid Expanding Vapour Explosion) also comes under the category of physical explosion, since there is lack of any reactions involved. If the tank, holding the liquid under pressure, gives way, part of the liquid vaporizes extremely rapidly and expands, forming a shock wave. This can be very destructive.

Nuclear Explosion

Nuclear explosions take place as a result of nuclear reactions, during which mass converts into energy as per Einstein’s well-known equation E=mc2, where E is the amount of energy produced (ergs), m is the mass in (gm) and c is the velocity of light (3x1010 cm/sec).

For example, 1 Megaton capacity bomb shall have mass consumption to the extent of only 46 gm.

Nuclear energy may be released in either of the two ways:

i) By disintegration or fission of a nucleus – as in atom bomb.
ii) By synthesis or fusion of lighter atoms – as in hydrogen bomb

e.g. fusion of hydrogen nuclei to form helium. This accounts for the heat of the sun and stars by the transformation of hydrogen into helium. Our nearest star and guardian of solar family, the Sun, is losing mass at the rate of nearly 1 million tons per minute and is capable of supplying energy to nearly 1 billion Earths at the present rate.

[Note: Reaction in neutron bomb is not dealt here]

The associated chain reaction in nuclear fission is very rapid in exponential form, taking only a few microseconds and the resultant energy production heats the surrounding air and causes it to expand in the form of a blast wave – causing disasters and holocausts. Hiroshima and Nagasaki in Japan were victims of nuclear bombs in 1945 during Second World War.

Controlled nuclear reactions have many useful applications – such as power generation; and the byproducts of fission and radio-isotopes have many-fold applications in industries, agriculture, chemistry, medical, sterilization, laboratory and other diverse fields. India has also setup several atomic reactors namely Apsara, Zerlina, Purnima, CIR etc. for research and isotope production; in addition to several large capacity power generating plants.

Chemical Explosions

Chemical explosions are caused by chemical reactions which are exothermic in nature and the energy released by the reaction is majorly converted to work. The process of reaction may be of decomposition or combination.

In combination reactions, the exothermic process is that of oxidation, mainly of hydrocarbons, converting them to water vapour (H20) and carbon oxides (both mono and di-). Chloride, fluoride, bromide, iodide or the halogen group is also exothermic in nature and they too are counted as oxidants. Oxidation process in large and violent form causes major explosions with extremely high potential of devastation.

For explosive range, there is a LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit) and between the two limits, lies the explosive range, which, when ignited by powerful ignition source, results in explosion. For example, explosive range of Hydrogen gas with air is (4.9% to 75%), that of Methane is (5% to 15%) and that of H2S is (4% to 46%).

Some hydrocarbons having no oxygen, such as acetylene (C2H2) can decompose explosively, since acetylene is a highly unstable compound.

There are examples of combination reactions where two components react together exothermically to produce explosions such as mixture of ammonium nitrate and fuel oil. The reactants that make up the explosive are to be mixed in a manner that ensures continued reaction.

In decomposition reactions, some oxygen-containing compounds like TNT and nitroglycerine, produce combustion gases. On ignition, high volume of gas is generated with the high pressure impact, in turn, forming shockwave and deadly explosion. It has more shattering effect.

Explosives are generally unstable compounds which must be transported and stored under close and careful supervision. Once exothermic reaction starts, temperature rises and reaction accelerates which can’t be stopped usually.

Hence, appropriate precautions and relevant laws should be followed as stipulated.

Vapour cloud explosions occur when some rich hydrocarbon fuel, like propane, leaks and mixes with atmosphere to form explosive mixture cloud. On ignition, it forms deadly shockwaves damaging the installations in the locality.

Detonation Vs Deflagration

When an explosive charge is detonated, hot gases are generated and their rapid expansion causes formation of compression wave, also called shock wave, which propagate through the media, all around, where it finds passage. In detonation, the combustion propagates at supersonic velocity and the principal heating mechanism of the mixture is shock compression.

In contrast, in deflagration, the combustion process is the same as in the normal burning of a gas mixture, the combustion zone propagates at sub-sonic velocity and the pressure build-up is slow.

Whether detonation or deflagration occurs in a gas-air mixture depends on various factors, including concentration of the mixture and the source of ignition. Unless confined or ignited by a high-intensity source like any detonator, most materials will not detonate.

To Conclude

This brief article on explosion science, in simple language, dealing with basics is presented to dear readers for common acquaintance only. Details have been purposely avoided.

Human ingenuity has made it possible to use explosions of various kinds for useful and peaceful purposes in mining, construction and building sectors for blasting of hard rock/soil, demolition purposes,--etc. Nuclear power has vast scope for human service.

- L.B Sinha