Nuclear Weapons From Grolier's The New Book of Knowledge

Atomic bomb

Nuclear weapons are the most powerful weapons in the world. The explosion of a nuclear bomb produces destruction on a scale far vaster than the explosion of a conventional bomb. And nuclear weapons can produce harmful effects that last far longer, and cover a far larger area, than the immediate effects of the explosion itself.

Nuclear weapons have been used in combat only twice, by the United States against Japan at the end of World War II. Along with the United States, Russia, Ukraine, China, France, Britain, India, and Pakistan acknowledge having nuclear weapons. Together these countries have enough weaponry to threaten all of civilization.

The world has never experienced a nuclear war, in which countries attack each other with nuclear weapons. Based on what they know about these weapons, however, scientists think that the results of such a war would be devastating. Thus nuclear weapons have had a deep and troubling effect on the world.

How Nuclear Weapons Work

The power of nuclear weapons comes from nuclear energy--energy released from the nucleus, or center, of an atom. The energy may be released in two ways: through the splitting of the nucleus, or nuclear fission, and through the joining of two nuclei, or nuclear fusion.

Fission Weapons. Fission weapons (also called atomic weapons) make use of nuclear fission to produce an explosion. In these weapons, many nuclei undergo fission so rapidly that, in effect, they split at the same time and release an enormous amount of energy. This is accomplished through a chain reaction.

A chain reaction begins with the fission of a few nuclei. Each nucleus is made up of particles called protons and neutrons. When the nucleus splits, most of these particles go to form lighter nuclei. Several neutrons are also released, along with a great deal of energy. The free neutrons strike other nuclei nearby, causing them to split and release neutrons. These neutrons in turn produce still more fission, and the chain reaction quickly builds.

Only a few materials will support a chain reaction. Fission weapons contain materials such as uranium 235 and plutonium 239. These are fissionable forms of uranium and plutonium--that is, forms that readily undergo fission. The materials used in weapons are produced in special factories.

A chain reaction also requires a certain amount of fissionable material. (Scientists use the term critical mass to describe the amount that is needed.) A fission bomb contains separate pieces of fissionable material, each too small to support a chain reaction. The bomb also contains a conventional explosive. When this explosive is set off, it pushes the pieces of fissionable material together with great force, starting the chain reaction.

Some of the materials used in weapons, especially uranium 235, are also used to produce fission in nuclear reactors. But the material in a reactor is far less concentrated than the material in a bomb. It is also arranged so that the reaction takes place in a slow, controlled way. In a bomb, the entire chain reaction takes place in less than a millionth of a second. Huge amounts of energy are released in an enormous explosion.

Fusion Weapons. Fusion bombs are often called hydrogen bombs because they depend on the fusion of many hydrogen nuclei. Fusion releases even more energy than fission (for the same weight of nuclear material). It is also more difficult to achieve. For fusion to take place, the hydrogen nuclei must be confined and heated to temperatures of millions of degrees. Because of the heat, fusion weapons are also known as thermonuclear weapons.

The conventional explosives that are used to trigger atomic bombs are not powerful enough to set off thermonuclear explosions. Instead, thermonuclear weapons use fission explosions to trigger the fusion.

A thermonuclear weapon depends on materials such as deuterium and tritium, which are forms of hydrogen that will undergo fusion more readily than ordinary hydrogen. The weapon also contains a fission bomb. The fission bomb is set off first, and its explosion produces extremely high temperatures and pressures. These forces make the deuterium and tritium nuclei fuse almost all at once, producing a massive explosion.

Hydrogen Bomb

Kinds of Nuclear Weapons

People often speak of nuclear weapons in terms of kilotons (thousands of tons) and megatons (millions of tons). These terms refer not to the weight of the weapons but to their strength (or yield) as compared to the strength of a well-known conventional explosive, TNT (trinitrotoluene). A one-kiloton weapon produces an explosion equal to the explosion of 1,000 tons of TNT. The explosion of a one-megaton bomb releases the same amount of energy as a million tons of TNT.

The first nuclear weapons were atomic bombs with forces of about 20 kilotons or less. Today a single missile can carry a thermonuclear warhead with a yield of 25 megatons. And weapons with more than twice that yield have been built. Powerful weapons such as these are strategic nuclear weapons. They are designed to destroy cities, industrial centers, and large military installations. Strategic weapons are made to be dropped from planes or delivered by missiles, some of which can travel halfway around the world.

Tactical nuclear weapons have much shorter ranges and much lower yields. They are mostly battlefield weapons, such as anti-aircraft missiles and artillery shells that are designed for use against enemy troops and tanks. The yield of a battlefield nuclear weapon may be as little as a tenth of a kiloton or as much as several kilotons.

Most nuclear weapons are designed to explode in the air just over a target. (The explosion point is sometimes called point zero, and the target area beneath it is called ground zero.) Space-based nuclear weapons have been proposed, although such weapons have not been built. In one type, a nuclear explosion would be used to trigger an X-ray laser--a powerful, focused beam of X rays. This beam could be used to destroy enemy satellites and missiles over a wide area.

Effects of Nuclear Weapons

The effects of a nuclear explosion are so awesome that they are difficult to imagine. In 1945, a single atomic bomb destroyed most of the city of Hiroshima, Japan, and killed or injured more than half the people. A second bomb caused similar destruction in the Japanese city of Nagasaki. Most of what people know about the effects of nuclear weapons was learned in these explosions and in tests conducted in isolated areas where no people live.

The effects of a nuclear explosion vary with the yield of the weapon. Scientists are still unsure of all the effects, but they know that a powerful nuclear explosion causes destruction in several main ways.

Blast. The enormous release of energy in a nuclear explosion heats the air very quickly. The hot air expands rapidly, creating a shock wave (or blast) that travels out from the site of the explosion. This blast accounts for half the energy released by the explosion. It can flatten buildings near the explosion site and cause damage for miles around. The force of the explosion can also produce a large crater in the ground.

Heat. A nuclear explosion creates a fireball with temperatures of millions of degrees. The temperatures are high enough to instantly destroy almost anything near the site of the explosion. Heat accounts for about a third of the energy released in the explosion. The heat from the fireball can set off raging fires over a wide area. Meanwhile, dust and dirt are sucked into the fireball. Because hot air rises, the dust and dirt rise with the fireball, creating a huge, mushroom-shaped cloud.

Radiation. Along with the blast and heat, large amounts of harmful radiation are released in a nuclear explosion. About half of this radiation is called prompt radiation and is mostly in the form of neutrons and gamma rays (high-energy rays that, like X rays, are very penetrating). The radiation can kill people and animals near the site of the explosion. Even several miles away, people can become sick from exposure to radiation. And radiation can produce diseases such as cancer that take years to develop.

The delayed radiation effects of a nuclear explosion can be widespread and long-lasting. The explosion produces vast quantities of tiny particles that are radioactive--that is, they continue to give off radiation. These particles coat everything within range of the explosion. They also coat the dust and dirt that are drawn into the mushroom-shaped cloud. In this way the particles enter the atmosphere, where they may be carried for many miles.

Eventually the particles fall back to earth as radioactive fallout. The fallout from a nuclear explosion could harm people hundreds of miles away from the explosion site. It could contaminate soil, water, and food supplies over large areas. And the particles would remain radioactive for a very long time, even centuries.

The radiation produced by a nuclear weapon can vary with the design of the weapon. For example, some thermonuclear weapons are considered "cleaner" than others because they produce less radioactive fallout. A type of weapon called a neutron bomb (or enhanced radiation weapon) releases less of its energy as blast and more in the form of neutron radiation. Thus a neutron bomb does less damage to buildings and to tanks and other military equipment. But its effects on people and other living things are deadly.

Effects in Space. The height at which a nuclear explosion takes place is a factor in its effects. For example, a bomb that exploded high above the ground would draw up less dirt and dust and thus would produce less fallout. If a nuclear weapon were exploded in space, there would be no air or other material to produce shock waves, fires, or radioactive dust. The main effect of the explosion would be an enormous burst of radiation (mostly X rays and gamma rays). The radiation could destroy satellites many miles away.

The explosion would also produce an electromagnetic pulse--a powerful burst of electric current that is one of the most curious nuclear effects. The pulse is produced as gamma rays enter the atmosphere and interact with it. If a nuclear weapon were exploded just above the atmosphere (or very high in the atmosphere), an electromagnetic pulse would travel to the ground.

The pulse could destroy or damage electric and electronic equipment--computers, television and radio equipment, communications networks, electrical circuits in homes and offices--for hundreds of miles around. Thus it would be possible for one country to use a nuclear weapon to disrupt communications and many other important aspects of life in another country.

Effects of Nuclear War. So far we have described the effects of a single nuclear explosion. The combined effect of many such explosions--as might be seen in a nuclear war--is unknown. But people can estimate how terrible a nuclear war would be.

For example, scientists have estimated what the results would be if several key U.S. military targets were attacked with nuclear weapons at about the same time. If the combined yield of the explosions were 400 megatons, the scientists think that half the country's population might be killed, mostly from the long-term effects of radiation.

A full-scale nuclear war would generate fires on an enormous scale and would reduce large areas to ashes. Huge quantities of smoke and dust would be sent up into the atmosphere. Some scientists think that this would lead to a severe change in the world's climate. The clouds of dust and smoke would circulate in the atmosphere for years, blocking sunlight. Without the warmth of the sun, much of the world would grow cooler. A nuclear winter lasting several years might affect as much as half the world.

Scientists agree that the massive use of nuclear weapons would produce vast amounts of radioactive material, which would also circulate in the atmosphere. Fallout might spread throughout the world and continue to rain down to earth for years.


The first nuclear weapons were built by scientists working in the United States during World War II. The U.S. Government supported their work under a secret program called the Manhattan Project. President Franklin D. Roosevelt gave the order to begin work on the first atomic bomb, and physicist J. Robert Oppenheimer directed the project. At the time the project was launched, the United States feared that scientists in Nazi Germany were developing a similar weapon. Their goal was to produce the weapon first, and they succeeded in doing so.

The first explosion of an atomic bomb was a test conducted on July 16, 1945, in the desert of New Mexico. Germany had surrendered two months before, but the United States was still at war with Japan. Less than a month after the test explosion, on August 6 and August 9, United States planes dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki. On August 10, Japan announced that it was ready to surrender. (The formal surrender took place on September 2.)

Because the Hiroshima and Nagasaki bombs hastened Japan's surrender, nuclear weapons have been credited with shortening the war. The decision to use the bombs has been questioned, and it is still debated. But in any case, the explosion of the first atomic bombs marked the start of what has become known as the nuclear age.

For a few years after World War II, the United States was the only country to have nuclear weapons. Then, in 1949, the Soviet Union exploded its first atomic bomb. The two countries were soon involved in an arms race, with each trying to build more (and more powerful) nuclear weapons than the other. The United States exploded the first hydrogen bomb in 1952. The Soviet Union tested its own hydrogen bomb about nine months later.

Soon other countries obtained nuclear weapons. But the number of these countries has remained small, chiefly because the knowledge and materials needed to produce nuclear weapons are difficult to obtain. Britain has had U.S.-made nuclear weapons since the early 1950's. France and China developed their own nuclear weapons in the 1960's. In the 1970's, India exploded what it called a "peaceful nuclear device." After India conducted more nuclear tests in the late 1990's, neighboring Pakistan exploded its own nuclear devices in several tests. South Africa had also developed nuclear weapons, but it dismantled them by the early 1990's. Belarus and Kazakhstan had nuclear weapons but gave them to Russia in the early 1990's after the breakup of the Soviet Union. Other countries suspected of developing the ability to produce nuclear weapons include Israel, Iran, Iraq, and North Korea.

The nuclear weapons that these countries have are only a fraction of those held by the United States, Russia, and Ukraine. As of the late 1990's, they had over 15,000 strategic warheads--fewer than the 50,000 U.S. and Soviet warheads in the late 1980's, but still enough to destroy each country several times.

Nuclear Arms and Policy

With each confrontation between countries that have nuclear weapons has come concern over the possibility of a nuclear war. If such a war should break out, the effects would reach around the world. Thus the question of how to reduce or eliminate the nuclear threat remains one of today's most important questions.

Nuclear Strategy. Some people believe that because of the terrible force of nuclear weapons, there will never be a full-scale nuclear war. This view is based on a theory called mutual assured destruction (MAD). According to the MAD theory, no country will use nuclear weapons if there is a strong chance that the other side will counter with a similar attack --because the only outcome would be complete destruction for both sides.

In this view, nuclear weapons serve as a deterrent, or method of preventing attack. Simply by having nuclear weapons, a country can prevent another country from staging a nuclear attack against it. Both the United States and the former Soviet Union relied on this theory. Each country continually sought to improve its nuclear force, so that the other side would not be able to overpower it.

The large-scale use of nuclear weapons would have disadvantages besides the threat of counterattack. For example, if a country were to succeed in conquering another country with a large nuclear attack, fallout and other long-lasting effects could make the conquered land of little use to the winners.

However, a limited nuclear war--one in which few nuclear strikes were made or only battlefield weapons were used--would not produce such widespread destruction. Some experts believe that if nuclear weapons are used in combat, it will be in a limited war. But there is debate over whether a limited nuclear war could be controlled. A small nuclear attack might be followed by a larger counterattack. Responses would follow one after another, and the war might soon escalate, or build, into a full-scale nuclear conflict.

Arms Control. To many people, the surest way to eliminate or reduce the threat of nuclear war is to eliminate the weapons, or at least to reduce their numbers. Almost from the time the first atomic bomb was exploded, there has been strong public pressure for the control of nuclear arms. Various groups at various times have called on governments to freeze (or halt) the growth of their nuclear arsenals, to reduce the numbers of nuclear weapons they have, or to ban nuclear weapons outright. Many government leaders have also pressed for limits on nuclear weapons. But progress toward these goals has been slow.

In the 1960's, the United States and the Soviet Union began a long series of negotiations on cutting back the nuclear arms each held. These Strategic Arms Limitation Talks (SALT) led to a treaty, signed in 1972, that set limits on the numbers of certain kinds of nuclear weapons. A second SALT treaty was signed in 1979. But the SALT II treaty was controversial. The U.S. Senate never approved it, and the U.S. Government terminated it.

In late 1987, the two countries signed the Intermediate-range Nuclear Forces (INF) Treaty, which banned nuclear missiles with ranges of about 300 to 3,400 miles (500 to 5,500 kilometers). Missiles in this range could travel from the Soviet Union to Western Europe but not to the United States.

The INF Treaty was a milestone. It was the first time that the two powers had agreed to reduce the size of their nuclear arsenals and destroy missiles. It also set up strict procedures for verification (making sure that both sides carried out the agreement).

In 1991, the United States and the Soviet Union signed the Strategic Arms Reduction Treaty (START), cutting long-range nuclear missiles. A second START treaty was signed by Russia and the United States in 1993. In 1996, the United Nations, including all the major powers, approved the Comprehensive Test Ban Treaty, banning all nuclear testing.

In 1972, the United States and the Soviet Union signed a treaty, agreeing not to build antiballistic missile (ABM) systems. An ABM system is designed to intercept and destroy enemy missiles that have been fired but have not yet reached their targets. The reasoning behind the treaty was that a country with an ABM system might believe that it could survive a nuclear attack and thus might be more likely to start a nuclear war. (In 1983 the United States proposed a space-based antimissile system called the Strategic Defense Initiative, but so far only short-range battlefield antimissile systems have been developed.) In 2002 the United States withdrew from the ABM treaty, citing the need for a missile defense system in the wake of the war on terrorism.

Fallout from test explosions became a worldwide concern in the 1950's. In 1963, an international treaty banned the testing of nuclear weapons in the atmosphere, in space, and under water. Since then most tests have been conducted underground. In addition, the United States and the Soviet Union agreed to limit the size of their underground tests to less than 150 kilotons. They were also among some 120 countries that signed a nuclear nonproliferation agreement. These countries pledged to work to prevent the spread, or proliferation, of nuclear weapons.

Despite the progress that has been made toward arms control, the agreements signed so far provide only limited protection against the threat of nuclear war. Such agreements are fragile and can easily be broken. The risk that more countries will obtain nuclear weapons remains. It is even possible that nuclear weapons might fall into the hands of terrorists outside government control.

Many people would like to see all nuclear weapons eliminated. There are obstacles to complete nuclear disarmament, however. Nuclear weapons cannot be "disinvented"--the materials and knowledge needed to make them are available. As long as countries feel threatened, they will not easily give up their right to protect themselves by any available means. Limiting nuclear weapons through arms control remains the best hope of avoiding nuclear war.

Benoit F. Morel
Carnegie Mellon University


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