Nuclear weapons consist of fission weapons, also called atomic weapons, and thermonuclear weapons, which are also known as hydrogen weapons or fusion weapons.  In fission weapons, matter is changed into energy when the nuclei (cores) of certain kinds of uranium or plutonium atoms are split.  In thermonuclear weapons, matter is converted into energy when pairs of certain kinds of hydrogen nuclei combine to form a single nucleus.  In general, thermonuclear weapons are much more powerful than fission weapons.  Today, a large majority of nuclear weapons are thermonuclear devices. 

The first nuclear weapons were two fission bombs used by the United States during World War II (1939-1945).  In the war, one was dropped on each of the Japanese cities of Hiroshima and Nagasaki.  Japan's surrender ended the war.  The Hiroshima bomb caused the greater destruction.  It killed from 70,000 to 100,000 people and destroyed approximately 13 square kilometres of the city.  About 12,000 metric tons of TNT would have been required to produce the same amount of damage.  Most of today's large thermonuclear weapons are about 8 to 40 times as powerful as the Hiroshima bomb. 

The bombs exploded at Hiroshima and Nagasaki are the only two nuclear weapons that have ever been used.  Since the end of World War II, however, nuclear weapons have dominated the military planning of the world's most powerful nations.  Until 1991, two countries--the United States and the Soviet Union--had most of the world's nuclear weapons.  In 1991, the Soviet Union was dissolved, and its republics became independent.  Most Soviet nuclear weapons were located in the former republics of Belarus, Kazakhstan, Russia, and Ukraine.  These republics agreed in 1992 that all nuclear weapons would be turned over to Russia.  Other countries that possess nuclear arms include China, France, and the United Kingdom (UK).  In 1993, President F. W. de Klerk of South Africa announced that his country had manufactured six nuclear fission bombs in the 1970's, but gave assurances that the weapons had been dismantled between 1990 and 1991.  Several other countries may be able to build nuclear weapons. 

The great destructive power of nuclear weapons has led to much disagreement about whether any nation should have them.  Most experts believe that, since the end of World War II, the threat of nuclear war probably has helped keep the peace between the world's major nations.  But the extensive use of large nuclear weapons in a war would cause heavy damage to many nations. 
 

Fission weapons get their destructive power from the fission (splitting) of atomic nuclei.  Only three kinds of atoms are known to be suitable for fissioning in such weapons.  These atoms are of the uranium (U) isotopes U-235 and U-238 and of the plutonium (Pu) isotope Pu-239.  Isotopes are different kinds of atoms of the same element.  See ISOTOPE. 

Nuclear fission occurs when a neutron--a subatomic particle with no electric charge--strikes the nucleus of a uranium or plutonium atom.  When the nucleus splits, a small amount of its matter is transformed into a large amount of energy.  In addition, two or three additional neutrons are released.  These neutrons may then split other nuclei.  If this process continues, a self-sustaining chain reaction forms in which each split nucleus provides the means of splitting other nuclei.  Such a chain reaction must occur for a fission explosion to take place. 

The formation of a self-sustaining chain reaction requires a certain minimum mass of fissionable material.  This mass is known as the critical mass.  A mass too small to support a self-sustaining chain reaction is called a subcritical mass. 

Fission weapons use either of two basic methods to create a critical mass: (1) the gun-type method or (2) the implosion method.  In the gun-type method, two subcritical pieces of material are placed in a device similar to the barrel of an artillery gun.  One of the pieces rests at one end of the barrel.  The other is put some distance from the first piece, with a powerful conventional explosive packed behind it.  The barrel is sealed at both ends.  When the weapon's fuse is triggered, the conventional explosive propels the second subcritical mass at a very high speed into the first.  The resulting combined mass immediately becomes supercritical (greater than critical), causing a rapid, self-sustaining chain reaction, and thus a nuclear explosion.  The United States used a gun-type fission bomb at Hiroshima. 

In the implosion method, a subcritical mass is made supercritical by compressing it into a smaller volume.  The subcritical mass, which is ball-shaped, is placed in the centre of the weapon.  The mass is surrounded by a spherical arrangement of conventional explosives.  When the weapon's fuse is triggered, all the conventional explosives go off at the same time.  The explosions compress the mass into a high-density supercritical mass.  A self-sustaining chain reaction then takes place, and the explosion occurs.  The United States used an implosion-type fission bomb at Nagasaki. 

Thermonuclear weapons get their power from the fusion (combining) of atomic nuclei under intense heat.  The nuclei fused in thermonuclear weapons are of the hydrogen isotopes deuterium and tritium. 

Fusion reactions require temperatures equal to, or greater than, those found in the sun's core--about 15,000,000 °C. The only practical way to achieve such temperatures is by means of a fission explosion.  Thus, thermonuclear explosions are triggered by an implosion-type fission device.  When the fission device explodes, it also releases neutrons that bombard a compound inside the weapon.  This compound, called lithium-6 deuteride, consists of deuterium and lithium-6, an isotope of the element lithium.  When struck by the released neutrons, the lithium-6 forms helium and tritium.  Then, pairs of tritium nuclei, pairs of deuterium nuclei, and pairs of one tritium nucleus and one deuterium nucleus each fuse to form helium nuclei.  A small amount of matter from each deuterium and tritium nucleus is converted into a large amount of energy, and a thermonuclear explosion occurs.  The yield (explosive power) of a thermonuclear weapon can be increased by surrounding the lithium-6 deuteride with a blanket of the uranium isotope U-238.  The U-238 undergoes fissioning in the course of the hydrogen explosion.