The most commonly known nuclear weapon is the nuclear bomb. The first atomic bomb was tested in 1945. The explosion of the atom bomb is created by fission, or the splitting of uranium or plutonium atoms into two lighter atoms. A free neutron from one atom collides with another uranium atom. The collision causes the uranium atom to split into two smaller atoms. This fission releases two spare neutrons and 32 picowatts (32 million millionths of a watt) of energy. The two freed neutrons collide with two more atoms and undergo the same reaction. In this way 1lb (0.45kg) of U-235 can release over 36 million million watts of energy.
In a nuclear explosion there is immediately a blinding flash of bluish-white light and ultra-violent light. The surrounding air is then heated to 18,000,000 degrees fahrenheit (10,000,000 degrees celcius) and creates a fireball. The fireball generates radiant heat that travels at the speed of light. The wave of heat is followed by the blast, in the form of a pressure wave moving at 1150ft (350m) per second. Part of the wave is reflected up by the ground. Where the reflected wave catches up with the original wave, the pressure is doubled and the "match wave" is formed. The overpressure of the blast wave is followed by a negative pressure phase that draws winds up to 620mph (1078km/h) into the destroyed area. This corresponds with the upward movement of the fireball and hot gases. If the fireball has touched the ground, dirt and debris are sucked into the rising column of hot gases and smoke, and the "mushroom cloud" is formed. The fireball will not necessarily touch the ground, as nuclear bombs are most effective when exploded in an "air burst" from 2000 to 50,000ft (0.6 to 15.2km) above the ground. The distruction of the nuclear blast is total. Nothing in the vicinity of such an explosion is able to survive.
Since the 1950's, when nuclear weapons reached present overshadowing prominence in world politics, there has been a general trend away from single warheads of huge power and towards multiple warheads of increasing accuracy. More than any other type of weapon, the success of nuclear arsenals is to be judged primarily in the effectiveness of their threat as opposed to the overall dangers of their use. Nuclear weapons may be delivered three main ways.They may be dropped from an aircraft, used as an artillery shell, or be used as the payload of a self-propelled missile. Nuclear missiles may be launched from land, sea, or air. Earlier nuclear missiles carried only one warhead. In the 50's and 60's missiles were large in megaton range. Now the accent is on smaller warheads delivered more accurately. Multiple Re-entry Vehicles (MRVs) are designed to deliver several warheads to one large target, in a saturation effect. MRVs are already being replaced by the major powers with those of MIRV and MARV types. MIRV is a Multiple Independantly-targeted Re-entry Vehicle. It is a "bus" containing multiple warheads and usually some decoy devices to mislead enemy defenses. As the vehicle begins to descend, warheads and decoys are ejected toward seperate targets which can be scores of kilometers apart. MARV is a Maneuverable Alternative-target Re-entry Vehicle which differs from MIRV in that each warhead had its own rocket and computer and can change course to a preselected alternative target if enemy Anti-Ballistic Missile (ABM) defenses are encountered. MIRV and MARV have multiplied the threat posed by each delivery missile.
The four tactical roles for nuclear weapons are battlefield support, anti-ship weapons, nuclear depth charges, and anti-aircraft missiles. Apart from Britain and France, all NATO and Warsaw Pact members would need US or Soviet permission to use nuclear weapons. Third World countries now possess many nuclear capable launch systems, but for most of them reliable miniaturized nuclear warhead technology and accuracy is less easily acquired.
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