Introduction to Nuclear Weapons

Nuclear Weapons, explosive weapons that gain their explosive power from nuclear reactions. Nuclear weapons generate much more energy than weapons using chemical explosives such as TNT. Nuclear weapons can be in the form of aerial bombs, artillery shells, torpedoes, mines, or missile warheads. The nuclear weapons are much more dangerous than any conventional weapon. ( section “Military Missiles.”) Nuclear devices can be exploded in the atmosphere or on or under a land or water surface.

Nuclear weapons are of two types, the fission weapons, also known as atomic bombs or atomic weapons, and the thermonuclear weapons, also called as hydrogen bombs, hydrogen weapons, or fusion weapons. When the nuclei of certain kinds of uranium or plutonium atoms get split, its matter converts into energy in fission weapons. In thermonuclear weapons matter converts into energy when pairs of certain kinds of hydrogen nuclei combine to form single nuclei. Most nuclear weapons are thermonuclear, and they are much more powerful than fission weapons.

A nuclear explosion immediately creates a luminous fireball consisting of ionized matter. The explosion also creates a powerful blast wave. In an atmospheric explosion, the fireball rapidly rises and forms a mushroom-shaped cloud. The blast wave moves away from the fireball at supersonic speed and can demolish buildings over large areas. Heat emitted by the fireball can cause serious skin burns and even start fires from a great distance. The explosion also produces highly penetrating nuclear radiation that can cause serious illness or death. Radioactive matter created during the explosion can leave a region virtually uninhabitable for some time.

The devastating power of nuclear weapons has twice been used in warfare. On August 6, 1945, during World War II, a United States B-29 bomber, Enola Gay, dropped a 9,000-pound (4,082-kg) nuclear device on Hiroshima, Japan. The explosion of this atomic bomb (as the weapon was called) resulted in a huge number of deaths—the exact figures are unknown, but estimates range from 68,000 to 200,000 persons. The city was largely destroyed. On August 9, another B-29, Great Artist, released a 10,000-pound (4,536-kg) nuclear device over the Japanese city of Nagasaki, with similar results. Five days later Japan surrendered to the United States and its Allies, bringing World War II to an end.

Many of the nuclear weapons that have been tested since World War II have hundreds of times the explosive power of the atomic bombs dropped on Japan. After the United States' monopoly on nuclear weapons ended in the summer of 1949, when the Soviet Union exploded its first nuclear device, the world was faced during the Cold War with the awesome possibility of a nuclear exchange between the United States and the Soviet Union that conceivably could have destroyed civilization owing to their extreme enmity. It is believed that the fear of nuclear war helped in maintaining peace during this period. Many nations have since sought ways to control such weapons and reduce the risk of nuclear war.

The Cold War ended in 1991, with the collapse of the Soviet Union, but during the previous decades both the United States and the Soviet Union had built up their nuclear armaments to such an extent that each had enough capacity to destroy all the major cities of the other. Each had also maintained a second-strike capacity—that is, no nuclear attack could destroy enough of its weapons to prevent an effective retaliatory strike. This situation was referred to as mutual assured destruction, or MAD, because a nuclear attack by one country on the other would likely lead to the devastation of both countries. Small, potent arsenals or supplies of nuclear weapons were developed by United Kingdom and France during the Cold War.

During the late 20th century, the United States worked on a defensive missile system called the Strategic Defense Initiative (SDI), popularly called “Star Wars.” The system was conceived as a means to destroy all incoming ballistic missiles, and thereby end reliance on mutual assured destruction. The end of the Cold War and the growing difficulties in developing such a system virtually put an end to the program.

The United States and Russia have the greatest number of nuclear weapons Other countries that possess nuclear weapons are the United Kingdom, France, China, India, and Pakistan. In addition, Ukraine, Belarus, and Kazakhstan, which together with Russian were part of the Soviet Union, possess nuclear weapons. Several other countries, including Israel and North Korea, may also have nuclear weapons.

Although the end of the Cold War lessened concern over a major nuclear exchange, a new concern of the world community is the possibility of proliferation of nuclear weapons among small nations, such as North Korea, that have aggressive foreign policies. In 2006, North Korea tested a small nuclear device, and it believed that Iran is also planning to build nuclear weapons. South Africa also had produced nuclear weapons, which they voluntarily destroyed later during the late 20th century.

The increasing number of countries with nuclear arms has increased the danger of destruction by nuclear weapons over regional conflicts. In 2002, India and Pakistan faced similar tensions over the Kashmir issue when both the counties equipped with nuclear weapons exchanged artillery fire, as the nations were on the verge of a war.

Nuclear Energy

Nuclear energy (also called atomic energy) results from the conversion of mass into energy according to Albert Einstein's formula E=mc². (This is read "E equals m c squared." E represents energy, m mass, and c the speed of light. If the mass is measured in kilograms and the speed of light in meters per second, the result is energy in joules.) The conversion of one kilogram (2.2 pounds) of any substance into energy would produce about 9 X 1016 joules, or 25 billion kilowatt-hours, of energy.

Nuclear energy is released when the particles that make up the nucleus (core) of an atom are rearranged in some manner. As the particles are rearranged, a small portion of the mass of the nucleus is converted into energy. Nuclear energy in large amounts has been produced by two processes -- fission and fusion. Fission refers to the splitting (fissioning) of a large nucleus into two or more smaller ones. Fusion refers to the building up of a nucleus by combining smaller nuclei or individual protons and neutrons.

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.

Effects of Nuclear Explosions

Factors like terrain, weather, the point of explosion in relation to Earths surface, and the weapons yield determine the effects of a nuclear explosion. The three effects of nuclear explosions are the blast effect; the thermal effect; and the nuclear radiation effect. Blast and thermal effects are associated with both chemical explosions and nuclear explosions, but only nuclear explosions produce nuclear radiation. The relative strength of each type of effect produced by the explosion of a nuclear weapon in the atmosphere depends on the weapon's construction. On the average, the energy of such an explosion is 50 per cent blast, 35 per cent thermal, and 15 per cent nuclear radiation. Cumulative data from United States atmospheric explosions are given in the table titled Effects of Atmospheric Nuclear Explosions.

In the event of a large-scale nuclear war, the explosions produced by the detonation of hundreds or thousands of nuclear weapons would blast a large amount of soil into the air. The explosions would also start wide-spread fires that would send a large amount of smoke high into the atmosphere with the formation of a fireball formed due to cloud of dust and hot gases under high pressure. The gases begin to expand forming a blast wave, or a shock wave, which is like a wall of highly compressed air moving rapidly away from the fireball. The blast wave created by a 1-megaton explosion travels about 12 miles from ground zero in only 50 seconds; ground zero is the point on the ground that lies directly below the explosion. As the wave moves forward, it creates overpressure because a 1-megaton explosion produces an overpressure that can destroy all buildings within 1 mile of ground zero. Moderate to severe damage can also be caused by this overpressure, within 6 miles of ground zero which are accompanied by strong winds reaching speeds of 400 miles per hour at 2 miles from ground zero. This blast wave along with the wind probably would kill majority of people within 3 miles of ground zero and some people between 3 and 6 miles from ground zero. Several other people within 6 miles of ground zero would face injury as well. Some scientific studies indicate that such a war could result in a phenomenon commonly referred to as nuclear winter. According to the studies, the dust and smoke might block out the sun for weeks or months, causing temperatures at the earth's surface to fall well below normal. Reduced temperatures, together with the lack of sunlight, could kill much of the plant life that animals need for food.

The ultraviolet, visible, and infrared radiation given off by the fireball forms the thermal radiation. The ultraviolet radiation gets rapidly absorbed by the particles in the air, thus causing little harm. However, the visible infrared radiation causes eye injuries as well as skin burns called flash burns, which caused 20 to 30 per cent of the deaths at Hiroshima and Nagasaki.

Thermal radiations can also ignite newspapers and dry leaves, causing huge fires. Scientists are also of the opinion that the smoke from such fire absorbs so much sunlight that it lowers the surface temperature of the earth for several months or years which could result in crop failure and famine. However, these effects can vary considerably, depending on several conditions. In clean air it could even be only one-hundredth as strong as in fog.

A person can be protected from the flash burns of thermal radiation by walls, buildings, trees, and rocks. Light-colored clothing also reflects heat, and so can help protect a person. However, thermal radiation from a 1-megaton explosion can produce second-degree burns up to 11 miles from ground zero. As the thermal radiation last only about 10 seconds, it would not completely burn, but only char heavy fabrics and thick pieces of wood or plastic.

The nuclear radiation resulting from a nuclear explosion is divided into two categories: (1) initial, or prompt, radiation; and (2) residual, or fallout, radiation. Prompt radiation is radiation that is emitted within one minute of the explosion. All subsequent radiation is termed fallout radiation.

Prompt Radiation

consists primarily of gamma rays (radiant energy of the same nature as light but of much higher frequency) and neutrons, which are a form of radiation similar to X rays. Some of the gamma rays and neutrons are emitted immediately, and the rest come out as a massive mushroom-shaped cloud of radioactive material.

Effects of Atmospheric Nuclear Explosions
The data for blast damage are based on the explosion height that gives maximum blast effect, and the ranges are measured from ground zero, the point on the ground directly below the burst. Ranges for the last two columns are measured from the explosion center.
Range within which Stated Effects Will Occur
Blast EffectThermal EffectNuclear Radiation Effect
EXPLOSION YIELDDeath to 50 per cent of exposed persons Severe damage to wooden-frame structures Second-degree burns to all exposed persons; ignition of combustible materials Death to 50 per cent of exposed persons
1 kiloton 0.3 mi 0.5 km 0.6 mi 1.0 km 0.5 mi 0.8 km 0.5 mi 0.8 km
10 kilotons 0.8 1.3 1.4 2.3 1.4 2.3 0.8 1.3
100 kilotons 1.9 3.1 2.8 4.5 2.5 4.0 1.1 1.8
1 megaton 4.8 7.7 6.4 10.3 10.0 16.1 1.5 2.4
10 megatons 12.0 19.3 13.7 22.0 23.0 37.0 2.3 3.7

By destroying cells or parts of cells of the body, gamma rays and neutrons can cause severe illness; in large doses they render a person helpless and cause death within hours or days. The cells that carry the factors governing heredity are especially susceptible to radiation damage. Thus, a person may become sterile as the result of radiation, or the heredity factors may be so altered that children may be born with serious birth defects.

A neutron bomb is a low-yield fusion bomb that produces a proportionately large amount of prompt radiation (primarily in the form of neutrons). The bomb is designed as a tactical weapon; the prompt radiation would incapacitate or kill most soldiers within the limited area of a battlefield, yet there would be relatively little fallout or destruction of buildings or other property.

Fallout Radiation

comes from the radioactive atomic nuclei formed from the materials inside the bomb and the bomb casing. Residual nuclear radiation is given off later than one minute after the explosion. Residual radiation created by fission consists of gamma rays and beta particles or electrons. Residual radiation produced by fusion consists mainly of neutrons. It strikes rock, soil and water particles and other materials that make up the mushroom-shaped cloud. Thus, these particles become radioactive, fall back to earth and are known as fallout. The closer an explosion occurs to Earths surface, the more is the fallout produced. Fallout radiations are of two types: early fallout and delayed fallout. When a nuclear explosion occurs on or near the ground, most of the radioactive debris attaches itself to particles of dirt carried into the air and falls to earth within a few days, killing or severely injuring living things as it reaches the ground within 24 hours of the explosion with heavy and highly radioactive particles. This is called early fallout. Some of the radioactive debris rises high into the atmosphere and can remain aloft for months or years, causing long-term radiation damage to living things as it reaches the ground from 24 hours to many years after the explosion has taken place. It has tiny, invisible, particles that fall in small amounts over large areas. This is called delayed fallout.

The principal hazards from fallout are gamma radiation from local fallout (fallout that reaches the earth within 24 hours of the explosion) and radiation from strontium 90, a radioactive isotope of the element strontium.

Strontium 90 is one of the more than 200 different isotopes that are formed during the fissioning of uranium or plutonium. Strontium is chemically similar to the element calcium, which is a major constituent of bone. For this reason strontium 90 will become permanently fixed in the bones of growing children when taken internally (in dairy products or vegetables, for example). Strontium 90 gives off beta particles (electrons emitted from the nucleus), which can cause bone cancer and leukemia. It takes 28 years, on the average, for half of the strontium 90 assimilated to become stable, or nonradioactive.

The isotope cesium 137, which emits gamma rays, can also be assimilated internally. The amount of cesium 137 is carefully checked in fallout measurements, along with the amount of strontium 90. Cesium 137 passes from the body after a few months.

The casing of a so-called “clean” bomb contains little or no material that can be made radioactive. A “dirty” bomb, on the other hand, has a container composed of one or more metals that can be made to emit nuclear radiation. A thick casing of cobalt, for example, around a fusion bomb (a combination called a “cobalt bomb”) could result in the creation of huge quantities of radioactive cobalt 60 that theoretically could make large areas of the earth uninhabitable for years.

A properly designed fallout shelter, or even a modified basement in a home, will provide good protection against all airborne nuclear radiations. A fallout shelter offers no protection, however, against internally assimilated fallout, such as that contained in radioactive food and drink.


Physicists had known of the tremendous energy contained in the atom ever since Lord Rutherford's investigations of radioactivity early in the 20th century. It was not until 1938, however, that it became apparent that it might be possible to make an explosive weapon that would utilize this energy. Late that year the German scientists Otto Hahn and Fritz Strassmann succeeded in fissioning a small number of uranium atoms in the laboratory.

News of this achievement spread rapidly, and physicists in many nations realized that a “uranium bomb” might now be feasible. Europe at that time was on the brink of World War II, and many of Germany's scientists had fled that country because of the Nazi persecution of Jews. Among these displaced scientists was Albert Einstein, who was living in the United States. A group of scientists, upon hearing of the Hahn-Strassmann experiments, persuaded Einstein to write a letter to President Franklin D. Roosevelt.

Manhattan Project

In 1939, just months before the start of World War II, the physicists in the United States had become aware of the military applications of nuclear energy. They were worried about Nazi Germany who might develop a nuclear weapon. German-born physicist Albert Einstein's letter, to U.S. President Franklin D. Roosevelt, written in August, 1939, suggested that a nuclear bomb might be feasible, and that the United States should investigate the possibility of developing such a bomb. World War II began on Sept. 1, 1939. President Roosevelt appointed a committee to look into the matter, and in August, 1942, a program to develop a nuclear bomb was established under the code name “Manhattan Engineer District,” with Brigadier General Leslie R. Groves in charge. The program soon became known as the Manhattan Project.

In December, 1941, the United States had entered World War II, and many of its top scientists and engineers were assigned to the Manhattan Project. The first definite indication of ultimate success came on December 2, 1942. On that date a self-sustaining chain reaction was achieved with a primitive uranium reactor (or “pile”) set up under the west stands of Stagg Field at the University of Chicago.

The Italian-American physicist Enrico Fermi directed this operation, which ushered in the nuclear age. The enormous technical difficulties involved in isolating a sufficient quantity of uranium 235, which constitutes only 0.72 per cent of natural uranium, had to be overcome before this experiment could be attempted.

Early in 1943, a special laboratory to develop the nuclear bomb was set up at Los Alamos, New Mexico, with the physicist Robert Oppenheimer as director. On July 16, 1945, a 22-kiloton plutonium bomb was exploded on a steel tower in the desert near Alamogordo, New Mexico. It was an implosion-type fission device that convinced U.S. leaders that fission weapons could be built.

Just 21 days later, on August 6, 1945, a 13-kiloton uranium bomb was dropped on Hiroshima, Japan from an American B-29 aircraft. Three days later, a 22-kiloton plutonium bomb identical to the one tested at Alamogordo was dropped on Nagasaki, Japan, again from another American B-29 aircraft. The uranium used in the Hiroshima bomb was produced at Oak Ridge, Tennessee and the weapon was a gun-type fission bomb. The plutonium used in the Alamogordo and Nagasaki bombs was manufactured in a reactor at Hanford, Washington.

Postwar Projects

At the conclusion of World War II in the summer of 1945, many United States scientists and political leaders favored ending the manufacture of nuclear weapons for moral and other reasons. Production and testing continued, however, under the direction of the U.S. Atomic Energy Commission, which was established by Congress in 1946 to replace the Manhattan Engineer District.

A cold war had started between the Soviet Union and the U.S following the Second World War. In September, 1949, President Truman announced that the Soviet Union had exploded its first nuclear device. This news caused a considerable amount of concern in the United States, and the American nuclear weapons program was accelerated. Emphasis was placed on the development of a fusion bomb.

Some scientists thought it was technically impossible to develop a fusion bomb; others believed that the large amounts of radioactivity that it would produce could endanger humanity. However, a group led by the Hungarian-American physicist Edward Teller prevailed, and work on the bomb began in 1950. In November, 1952, during the Korean War, the United States detonated the world's first fusion weapon on Eniwetok Atoll in the Pacific. The Soviet Union exploded its first fusion bomb in 1955. During the mid 20th century, the Soviets built their first nuclear missiles equipped submarines. This was followed by the test launch of their first land-based intercontinental ballistic missile (ICBM) in 1957. The first U.S. ICBM came into being in 1959 which was immediately followed by the commissioning of their first submarine equipped with ballistic missile.

The United Kingdom exploded a fission bomb in 1952. In 1957 that country exploded its first hydrogen bomb. France became the fourth member of the “nuclear club” in 1960 by detonating a fission device. In 1964 China exploded a small uranium bomb, and in 1967 it detonated its first hydrogen bomb. France exploded its first fusion bomb in 1968. In 1974 India became a nuclear power by detonating a fission device.

Between 1989 and 1992 political developments in eastern Europe and the Soviet Union ended the Cold War. The focus was on reducing nuclear arsenals and properly controlling them. After the terrorist attacks of Sept. 11, 2001, a new threat from terrorists has taken shape who might gain access to nuclear arms, with the support of hostile nations.

Control of Nuclear Weapons

The strategies of deterrence and the limitations on the testing, numbers, and proliferation of nuclear weapons have been the main approaches to control nuclear weapons. The theories of deterrence may be offense-based or defense-based. According to the offense-based deterrence theory, possession of a strong nuclear force by two opposing nations best prevents nuclear war. A defender would retain enough weapons to devastate an aggressor even if attacked first. The theory was widely called the doctrine of mutually assured destruction (MAD).

As early as 1946 the United Nations had become concerned about the possible proliferation of nuclear weapons throughout the world as well as about the effects of radioactive fallout from uncontrolled test explosions. In 1958, the United States, the Soviet Union, and United Kingdom began an informal moratorium on testing. It was ended, however, by a Soviet test series that began in 1961.

After many more tests and repeated negotiation, the United States, the Soviet Union, and United Kingdom signed and ratified in 1963 a limited test-ban treaty, which was the first test limitation treaty. It forbade all tests except those conducted underground, which create little or no fallout. Open to all nations for signature, the treaty was signed by more than 100 countries. France and China did not sign it, however, and continued above-ground testing. ( 1963.)

In 1967 the United States, the Soviet Union, United Kingdom, and other nations signed a pact banning the use of nuclear weapons in outer space. The Nuclear Nonproliferation Treaty, intended to halt the spread of nuclear weapons to nonnuclear nations, went into effect in 1970 after being ratified by 43 nations.

Limiting proliferation involves preventing the spread of nuclear weapons to nations that do not possess them. The United Nations approved the Treaty on the Non-Proliferation of Nuclear Weapons in 1968. Since then, the treaty has been ratified by almost all countries.

Attempts to limit the number of U.S. and Soviet nuclear weapons began about 1970. In 1971, the Soviet Union and the United States signed a treaty providing for a system of consultations that would prevent nuclear accidents from turning into war. The two powers in 1972 signed SALT I, which included strict limits on each countrys defenses against nuclear missiles. This treaty consisted of two accords, the ABM treaty and the Interim Offensive Agreement. The ABM treaty limited the number of each nation's anti-ballistic missilesto one missile site and required that the site have no more than 100 missiles. The Interim Offensive Agreement limited for a five-year period the size of offensive ballistic missiles of each nation. United States and the Soviet Union agreed not to test explosive devices with yields above 150 kilotons in 1974. This agreement, contained in the Threshold Test Ban Treaty, which finally took effect in 1990, the year it, was agreed by both countries though both the nations had followed its guidelines long before its endorsement. SALT II was signed by the two powers in 1979, placing limits on the number of warheads, launch vehicles, and other elements in nuclear strength. Although the United States did not ratify the treaty, the country abided by its terms for eight years, when finally U.S. Senate refused to ratify the 1979 SALT II treaty after Soviet forces invaded Afghanistan.

According to the defense-based deterrence theory, a giant nuclear power will not attack unless sure that it can destroy opponents ability to launch a nuclear counterattack. However, the theory also says that only an effective defense against a powerful first strike will protect a defenders ability to retaliate.

Military experts believe that in cases of threat from a limited nuclear attack, an effective defense will either deter or, at least, bound the chief destruction. However, no country today has an effective defense. From 1983 to 1993, the United States carried out research on the Strategic Defense Initiative (SDI), a space-based system of defense against missiles. It was originally intended as a complete protection against nuclear attack but later aimed at protection against limited nuclear attacks only.

By 2000, people in the United States supported the idea of building a limited defense against attacks by few nuclear weapons which could be launched by terrorist or rogue nations, which are small or medium-sized nations that ignore international law and support terrorism. There were objections against this stating that developing such a national missile defense system would violate the ABM Treaty. After the Sept. 11, 2001, attacks on the United States, they withdrew from the ABM Treaty in June 2002 and increased funds on a defensive system. They also started developing a basic ground-and sea-based ballistic missile defense system for the west coast.

In 1987 the Soviet Union and the United States signed the Intermediate-range Nuclear Forces (INF) treaty. All Soviet and American ground-launched nuclear missiles with ranges of 310 to 3,420 miles (500 to 5,500 km) deployed in Europe were destroyed.

In 1990, the United States Senate ratified two treaties limiting underground nuclear explosions to 150 kilotons or less—the Threshold Test Ban Treaty, signed 16 years earlier, in 1974, and the Peaceful Nuclear Explosions Treaty, signed in 1976.

In 1991, the Soviet Union and the United States signed the Strategic Arms Reduction Treaty (START I), which U.S. President Ronald Reagan had initiated in 1982. It aimed at deep reductions in offensive strategic weapons that took approach in 1987, when Reagan and Soviet President Mikhail S. Gorbachev signed the Intermediate-Range Nuclear Forces (INF) Treaty. This treaty called for the elimination of all U.S. and Soviet ground-launched nuclear missiles with ranges of 500 to 5,500 kilometers (310 to 3,420 miles). It took effect in 1988. According to START, both sides agreed to steeply reduce their nuclear armaments. The Soviet Union broke up in late 1991, and in 1992 the United States and four successor nations of the Soviet Union that possessed nuclear weapons—Russia, Belarus, Kazakhstan, and Ukraine—signed a protocol to START I in May, 1992, that required Belarus, Kazakhstan, and Ukraine to eliminate their nuclear arsenals. The START I treaty came into force in 1994, when the last signatory, Ukraine, ratified it.

Efforts to limit the use of nuclear weapons took pace following the INF Treaty. Major political reforms in the Soviet Union and eastern Europe, including the fall of most Communist governments, led to reduced tensions and general arms reductions between 1989 and 1992. In 1990, the United States, the Soviet Union, along with 20 other nations signed the Treaty on Conventional Armed Forces in Europe (CFE) which took effect in 1992, and led to the destruction of a large numbers of tanks and other conventional weapons in Europe.

With much of the international tensions reduced, and the INF and CFE treaties in place, George H. W. Bush, U.S. president and Gorbachev, Soviet president signed the first treaty to significantly reduce existing numbers of strategic nuclear weapons in July 1991. The START I, aimed to reduce the number of U.S. and Soviet long-range nuclear missiles and bombers from 23,500 to 15,400 for both countries officially took effect in 1994. Soon in September 1991, United States announced the destruction of all its ground-based tactical nuclear weapons and many of those which were carried by ships and aircraft. The Soviet Union announced similar steps in the following month.

Meanwhile, in 1993 Russia and the United States signed START II, under the leadership of President George H. W. Bush and Russian President Boris Yeltsin, which called for further reductions in nuclear armaments, to between 6,000 and 7,000 for all of the countries combined. In 1995 representatives of 174 nations voted to renew the Nuclear Nonproliferation Treaty.

Ukraine, Belarus, and Kazakhstan had turned over all their nuclear weapons to Russia by late 1996. U.S. President George W. Bush and Russian President Vladimir Putin signed an agreement in 2002, which aimed at further cutting down their strategic nuclear arsenals to between 1,700 and 2,200 warheads for each country by 2012. This agreement, which was commonly known as the Treaty of Moscow, came into effect in 2003.

The United Nations sponsored the Geneva Conference on Disarmament in 1994. The conference drafted the Comprehensive Test Ban Treaty, which bars all nuclear tests. By mid-1998 it had 150 signatories including the United States and Russia. The Comprehensive Nuclear Test Ban Treaty was approved in 1996, by the United Nations, which would end all testing of nuclear weapons. Before it becomes effective, it has to be accepted by all countries having nuclear reactors, which are devices for producing nuclear energy. Till date, about three-fourths of those nations have signed the treaty.

START II was ratified by the United States Senate in 1996, but by late 1998 had yet to be ratified by the Russian Duma. In 1997 Presidents Clinton and Yeltsin signed an agreement to begin negotiations on START III. Non-proliferation efforts suffered a setback in 1998 when both India and Pakistan tested nuclear weapons. Neither country had signed the Comprehensive Test Ban Treaty, but began discussions with the UN after their tests.