How Nuclear Weapons Work

Explosive devices that utilize the fission process were originally called atomic weapons, while those that depend on fusion were known as hydrogen weapons or thermonuclear weapons. These terms are still used occasionally, but the term “nuclear weapons,” which designates both fission and fusion weapons, is used more frequently.

Fission Weapons. Only certain types of atoms have nuclei that can be readily fissioned. Of those that do have a fissionable nucleus, the three most easily produced in quantity for nuclear weapons are the two isotopes of uranium 235 (whose nucleus contains a total of 235 protons and neutrons) and U-238 and one isotope of plutonium 239. Isotopes are the different kinds of atoms for the same element where each atom has a different mass. In a fission reaction only about 0.1 per cent of the mass of the atom is converted into energy. Nevertheless, the fissioning of all the atoms in 1 kilogram (2.2 pounds) of either uranium 235 or plutonium 239—a chunk about the size of a golf ball—yields as much energy as would be released by 17,000 tons of TNT.

When a sufficient amount of either uranium 235 or plutonium 239 is brought together, a spontaneous, self-sustaining chain reaction occurs. In a chain reaction, the splitting of atomic nuclei causes the emission of particles called neutrons that, in turn, cause other nuclei to split. A neutron is a subatomic particle carrying no electric charge which continues to split the nuclei of other atoms resulting in the rapid splitting of nuclei, thus producing a combined energy called fission explosion. The amount of uranium or plutonium required, called the critical mass, depends on the composition and shape of the material. Generating a chain reaction requires the least amount of fissionable material known as the critical mass. In general, plutonium 239 has a smaller critical mass than uranium 235. The critical mass for a solid sphere of plutonium 239 is only about 35 pounds (16 kg). A fission weapon is detonated by very rapidly bringing together more than enough fissionable material to form a critical mass. The fission reactions proceed through the material at an uncontrolled rate, leading to the release of a tremendous amount of energy within a very short period of time—less than a millionth of a second. A subcritical mass is a mass too small to support a self-sustaining chain reaction.

There are at least two basic methods used to make a fission bomb explode. In one method, used in the atomic bomb dropped on Hiroshima, two masses of uranium 235 are driven together by a chemical explosive charge. (The two masses must be brought together quickly to prevent the material from blowing apart before most of the uranium undergoes fission.) This is called the gun-type method, two sub critical pieces of uranium are placed in a device similar to the barrel of a gun where one piece rests on one end of the barrel and the other is some distance away from the first, with a powerful conventional explosive behind it. Both ends of the barrel are sealed. When the fuse of the weapon is triggered, the conventional explosive propels the second sub critical mass at high speed into the first and the resulting combined mass immediately becomes supercritical thereby causing a rapid, self-sustaining chain reaction and explosion. In the second method, used in the bomb dropped on Nagasaki, a number of high explosive charges are used to crush a hollow sphere of plutonium into a dense ball. It is called the implosion method, where a subcritical mass is converted into supercritical by compressing it to reduce its volume. The subcritical mass is in the center of the weapon, and it is surrounded by conventional explosives. When the fuse of the weapon is triggered, all the conventional explosives go off simultaneously. The explosions compress the mass into a high-density supercritical mass thus leading to a self-sustaining chain reaction causing nuclear explosion.

A major difficulty in constructing a fission weapon lies in the preparation of a supply of fissionable material of adequate purity. Many nations possess the technical ability to develop a fission bomb, but only a few have the necessary resources.

Fusion Weapons

Fusion, the uniting of two lightweight nuclei to form a heavier one, accounts for most of the energy produced by the sun and other stars. Fusion reactions can be readily produced in large number only at temperatures of many millions of degrees, far in excess of the temperatures occurring naturally on earth. It is equal to, or greater than, 27,000,000 F or 15,000,000 C that is found in the suns core. Such temperatures are momentarily attained, however, during the explosion of an implosion-type fission bomb. For this reason, a fission explosion is used as a “trigger” to detonate a fusion weapon.

Because of the enormously high temperature that is required to initiate fusion reactions, fusion weapons are often called “thermonuclear weapons.” (The prefix “thermo” means “heat.”) Fusion bombs are commonly known as hydrogen bombs, or H-bombs, because they derive most of their explosive energy from the fusion of hydrogen atoms.

The hydrogen used in fusion bombs consists of two rare forms of the element—deuterium and tritium. Ordinary hydrogen has a nucleus composed of just one proton. Deuterium, on the other hand, has a nucleus composed of one proton and one neutron, and tritium has a nucleus composed of one proton and two neutrons.

The nuclei fused in thermonuclear weapons are of the hydrogen isotopes deuterium (H-2) and tritium (H-3). The fusion reaction easiest to produce is the one in which a nucleus of deuterium (symbolized by D) combines with a nucleus of tritium (T) to form helium 4 (He4), which has two protons and two neutrons in its nucleus:

D + T — He4 + neutron + energy

As indicated by this equation, a neutron is released together with a quantity of energy. The combined mass of the deuterium and tritium nuclei is slightly greater than the combined mass of the helium 4 nucleus and the neutron. The “lost” mass is converted into energy in accordance with Einstein's equation (E = mc²).

Although few details on fusion weapons have been made public, it is probable that the reaction just described is one of a sequence of reactions that takes place within such a weapon. Most fusion weapons probably contain lithium deuteride, a solid compound of deuterium and lithium. When the fission device explodes, it instantly releases neutrons that bombard a compound called lithium 6 deuteride, inside the weapon. Lithium 6 deuteride consists of deuterium and lithium 6 (Li-6), an isotope of lithium. The fission explosion that detonates the fusion weapon causes the lithium to change into helium and tritium, which then undergoes fusion with the deuterium. A small amount of matter from each deuterium and tritium nucleus forms a large amount of energy, thus resulting in a thermonuclear explosion. The explosive power or yield of the thermonuclear weapon can be augmented by covering the lithium 6 deuteride with a uranium isotope U-238, which fissions during the hydrogen explosion.

The amount of energy released in an individual fusion reaction is very small. However, in a fusion weapon the energy from trillions upon trillions of nuclear reactions is released nearly simultaneously.

Weapon Yields

The total energy, or yield, of a nuclear weapon is expressed as the amount of chemical high explosive, usually TNT, that would produce the same energy. The units used are kilotons and megatons. A 1-kiloton nuclear weapon is equivalent in power to 1,000 tons of TNT. A 1-megaton nuclear weapon has the power of 1,000,000 tons or 907,000 metric tons of TNT.

The bomb dropped on Hiroshima had a yield of about 12.7 kilotons. Some older bombs had power of 20 megatons which is equivalent to 1,540 Hiroshima bombs. Most of the thermonuclear weapons are 8 to 40 times as powerful as the Hiroshima bomb. In October, 1961, the Soviet Union exploded in its Arctic test area a 58-megaton fusion weapon, the most powerful weapon ever detonated.

Strategic and Tactical Weapons

The terms “strategic” and “tactical” are often applied to nuclear weapons, which are basically of two different types owing to their role in the military. Strategic weapons have relatively large yields and are designed to destroy enemy bases and industrial centers far removed from combat areas. They can be delivered by bombers and long-range or intermediate-range missiles. The strategic weapons include bombs and missiles delivered by long-range bomber aircraft and missiles that can deliver explosive devices to targets up to 6,500 miles from the launch site. These missiles are based on land, and on submarines underwater. Some of these missiles even have several nuclear warheads, each of which carries explosive material to a different target. Strategic nuclear missiles comprise of intercontinental ballistic missiles (ICBM's), submarine-launched ballistic missiles (SLBM's), and cruise missiles. Tactical weapons have relatively low yields and are designed for use in or near combat areas. The theater nuclear weapons are mainly used within a military theater, which is an area where military operations are conducted. Theater nuclear weapons mainly comprise of medium-range ballistic and cruise missiles, short-range guided missiles, and unguided rockets. The nuclear artillery shells, mines, and torpedoes also form a part of theatre nuclear weapons.

Since the end of World War II, in 1945, nuclear weapons dominated the military planning of the United States and the Soviet Union, who were the worlds leading military powers. With the rise and fall of tensions between the nations, their national arsenals of nuclear weapons increased and decreased. But this has changed over time, and gradually many regional military powers have acquired nuclear weapons.

During the early 20th century, some U.S. military planners felt that airpower was the best way of ending a war. They preferred bombing against an aggressor's homeland which led the United States to drop bombs on Germany and Japan during World War II. However, the war seemed to last very long despite this practice. Only after allied ground forces defeated its army, Germany surrendered and Japan surrendered only after disastrous nuclear bombings at Hiroshima and Nagasaki. This further strengthened the belief of the supporters of airpower theory. As a result of it, nuclear weapons became vital to U.S. strategic military power during the Cold War.

In the mid 20th century, the United States adopted the policy of massive retaliation which stated that if Soviet Union attacked any area which was important to the United States or its allies, the United States might counterattack with a massive nuclear strike against the Soviet Union.

In the mid 20th century, this policy of massive retaliation was replaced by a flexible response policy, according to which the U.S. response to enemy hostility would match the nature of the enemys attack. The United States, in order to stop this, could use any type of weapon, including conventional forces or nuclear weapons.

In the late 20th century, a sweeping transformation of strategy took place which the military planners called as a revolution in military affairs (RMA). This included a move from large forces and massive bombing to precision strikes, using smart munitions, delivered by smaller and more responsive forces. The United States and many other nations having modern technology turned to new and advanced conventional weapons instead of nuclear weapons,. This proved beneficial against Iraq in the Persian Gulf War of 1991, the Iraq War, in 2003, the operations in Yugoslavia in 1999 and in Afghanistan in 2002. In 1990, the United States proclaimed that it would utilize nuclear weapons as a last option.

During the early 20th century, Soviet planners followed the deep battle theory according to which they emphasized the use of all available weapons in a fast, overwhelming, and surprise offensive that could even include the use of nuclear weapons. U.S planners were concerned by the Soviet Unions surprise offensives because in a first strike, a part of the Soviet nuclear arsenal could cripple the U.S. missile and bomber forces.