n.

A device, such as a bomb or warhead, whose great explosive power derives from the release of nuclear energy.

Nuclear weapons derive their explosive power from the fission (splitting) and fusion (combining) of atoms. Fusion devices need to be combined with a nuclear fission weapon to generate the intense heat necessary to begin the still more powerful process of fusion. Fusion weapons—the ‘H’ (hydrogen) bomb—can be a thousand times more powerful than fission weapons and these opened the horrific possibility of global destruction through nuclear missile war. Many early ‘fusion’ weapons were in fact ‘boosted fission devices’, gaining most of their power from the fission explosion with a fusion component to enhance its efficiency. Military requirements have also led to enhanced radiation/reduced blast weapons, the so-called ‘neutron bomb’, in which the immediate radiation is multiplied in order to kill troops rather than destroy installations.

The idea of a source of enormous energy for motive power or weapons featured in the work of 19th-century science-fiction writers including Jules Verne, George Earle Bulwer-Lytton, and H. G. Wells who talks of atomic bombs in The World Set Free. By 1914 the Newtonian view that the universe consisted of lumps of indestructible matter had given way to the realization that matter could be transformed into energy. It was not until the eve of WW II that the practical possibility of a nuclear weapon was understood. On 2 August 1939, Albert Einstein signed a letter to Pres Franklin D. Roosevelt, saying that recent work in France and the USA had indicated the possibility of setting up a nuclear chain reaction in a large mass of uranium. This new phenomenon could ‘also lead to the construction of bombs and it is conceivable—though much less certain—that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port, together with some of the surrounding territory. However, such bombs might very well prove too heavy for transportation by air.’ He was wrong on the last point, and six years and four days later the US dropped the first atomic bomb over Hiroshima, the result of the Manhattan Project.

The first test bomb ever to be exploded, at Alamogordo in the New Mexico desert on 16 July 1945, ‘the Gadget’, was an ‘implosion’ device, with a hollow plutonium core weighing about 8.3 lb (3.8 kg), compressed to critical density by about 4, 866 lb (2, 270 kg) of high explosive. The ‘yield’—the size of the explosion—was 22 kilotons. Nuclear weapon yields are measured as kilotons (each 1, 000 tons of TNT) or megatons (one million tons of TNT).

The first bomb to be dropped on Japan on 6 August was of a different type—a ‘gun-assembly’ device called ‘Little Boy’. It was cruder than the Gadget—132.3 lb (60 kg) of highly enriched uranium in two pieces, one of which was fired at the other down a gun-type barrel, producing a far less efficient yield of 12 to 15 kilotons. The US B-29 bomber Enola Gay, named after the pilot's mother, carried the bomb to Hiroshima from its base at Guam, escorted by two other planes carrying observers and instruments. The bomb had been brought from the USA by the cruiser Indianapolis, which was to be sunk by a Japanese submarine on its return trip. The city was obliterated.

Although simple in principle, nuclear weapons are difficult to engineer in practice. The 'gun assembly' type is less efficient but more robust and favoured for artillery shells. The implosion type is more difficult to build and more fragile, but is more efficient and is essential to exploit the advantages of fusion
(Click to enlarge)

The second operational bomb, ‘Fat Man’, dropped on Nagasaki on 9 August, was of the same type as the Gadget. Implosion devices are more efficient than the gun type and do not require high-grade uranium, but they are more complex in their design and more delicate, requiring precise co-ordination of the timing for the detonation of the surrounding explosive charges, and are unsuitable to withstand the stress of being launched from a gun. For this reason, gun-assembly devices retain their importance as nuclear artillery shells, while implosion devices tend to be carried on rockets.

The detonation of a nuclear weapon produces an incandescent ‘fireball’ in which everything is vaporized. Being hot, the fireball climbs upwards, generating the characteristic ‘mushroom cloud’ as it pulls debris from the ground up with it. Nuclear weapons have five main effects. The first is heat, or ‘flash’, causing burns and eye damage far from the immediate source of the explosion. The second is blast, caused by a shock wave moving outwards from the fireball at about the speed of sound. Third is immediate radiation travelling at the speed of light-neutron radiation at shorter distances, and gamma radiation further out. Most of the latter is generated after the explosion, so a dive for cover may still be worth while. Fourth is residual radiation: the neutrons from the explosion make other material radioactive, which falls back to earth as fallout. Fifth is electromagnetic pulse (EMP), which burns out electronic apparatus. A multi-megaton weapon burst high above the earth, outside the atmosphere, would not have perceptible flash-and-blast effects but could knock out power grids and electrical equipment over a wide area.

After the explosion of the bombs over Japan, some thought the nuclear weapon would render traditional forms of war obsolete. Others were sceptical about the bomb's power, although some used this to try to discourage its development. As early as October 1945, Sir George Thompson, participating in the revision of the British Tizard report on future warfare, foresaw that the bomb's very potency might preclude its use. Whereas the tendency had been to wage war more and more unrestrictedly, it was possible that the nuclear weapon might reverse this trend. Therefore Britain should keep ‘pre-atomic defence forces’. The first atomic bombs were very large, could only be delivered by large aircraft, and were also very rare. Therefore, like other new weapons in military-technical revolutions, they were only suitable for use against strategic targets. Tizard's revised report noted ‘the bombing of towns and industry now give a far greater return for war effort expended and may therefore become the most profitable type of war’.

By contrast, the effect of a Hiroshima- or Nagasaki-type bomb on military targets appeared to offer little return on investment. In 1946 Tizard estimated that when detonated at 300 feet (91 metres) above the ground such a bomb would disable tanks and guns out to 400 yards (366 metres) and soft-skinned vehicles out to 800 yards (732 metres), while in battlefield conditions it would disable troops in the open out to 1, 200 yards (1, 097 metres) immediately and 1, 800 yards (1, 646 metres) with delayed action. A 1953 assessment drew similar conclusions. All that was likely to be accomplished was the destruction of 120 to 140 vehicles over a length of 4 miles (6.4 km) or so. Atomic weapons would tend to create obstacles in front of advancing columns, rather than destroy them. Whereas fighting troops, dug in, were very robust in the face of nuclear weapons, large staging areas and ports were vulnerable. Amphibious operations like D-Day, would clearly be a lucrative target for nuclear strikes.

As nuclear weapons became more plentiful, the possibility of using them on the battlefield grew, and by 1955 both the USSR and NATO, now arrayed against each other in Europe, were contemplating their use in this way. In 1954 the Soviet army conducted winter manoeuvres in the Carpathians, in which an ‘attacker’ succeeded in penetrating the high mountain passes near Czernowitz. Both sides were assumed to be using nuclear weapons and to be about equal in strength. Each side concentrated its strikes on the opponent's lines of communication and supply centres. The umpires declared that, because of this, operations had ground to a standstill.

The talented military thinker Ferdinand Miksche, a Czech émigré, believed that nuclear weapons would, like other increases in firepower, lead to a ‘bogging down of the fighting’. He was also aware of the effect of electromagnetic pulse. If all high-frequency apparatus within 4 miles of a nuclear blast burned out, he reasoned, what were the implications for land forces? Nuclear weapons might give rise to a development ‘entirely different from the one foreseen by armies at present’. Simpler weapons and equipment would be more dependable than complex systems, which would be powerless without the extensive, rear services. It might be that in nuclear warfare ‘only material and tactical methods of the simplest kind will retain their value’.

Nonetheless, the opposing blocs had to reckon with nuclear weapons on the battlefield as well as strategically. In 1953 the USSR and the USA exploded the first fusion bombs, which, combined with the development of ballistic missiles, appeared to shift the emphasis away from armies towards a new form of war. During the late 1950s the USSR concluded that a ‘revolution in military affairs’ had taken place, expressed in Sokolovskiy's Strategy. By 1970, when Sidorenko's The Offensive was published, the vision of the future battlefield was truly apocalyptic. ‘A new characteristic of the offensive in nuclear war’, he wrote, ‘is the conduct of combat actions under conditions of the presence of vast zones of contamination, destruction, fires and floods.’

This vision led western powers, whose territory was likely to be the scene for such vast zones of destruction, to try to refine the effects of nuclear weapons. In an enhanced radiation/reduced blast weapon, such as the US W-79 8 inch artillery round, the burst of prompt nuclear radiation (neutrons and gamma rays) is enhanced by minimizing the fission yield relative to the fusion yield. Small enhanced radiation weapons can kill by radiation well outside the range of the blast: with ordinary nuclear weapons, the blast and heat usually kill first. Because there is little residual radiation, friendly troops could then operate in the area very soon afterwards.

The Soviet emphasis on armoured warfare may have owed as much to the characteristics of nuclear weapons as to the lessons of the eastern front in WW II. Tanks and armoured fighting vehicles were heavy and low and did not blow away easily in a nuclear storm, they could be hermetically sealed against fallout, and their armour was good protection against immediate radiation.

Similar considerations influenced Israel, India, and Pakistan. Israel probably had nuclear weapons by the time of the 1973 Arab-Israeli war, and might have used them if overrun, but has never admitted it publicly. India and Pakistan were the foremost ‘threshold’ nuclear powers until 1998, when each conducted multiple nuclear tests, although India had set off a ‘peaceful’ nuclear explosion back in 1974. The Indian and Pakistani armies both placed great emphasis on armour as the arm of choice on a nuclear battlefield, and discussed nuclear warfare extensively. The use of nuclear weapons exploded below ground as demolition charges in the Himalayas has also been discussed.

The development of precision-guided munitions and improved conventional area munitions has, arguably, made battlefield nuclear weapons obsolete. The first are conventional bombs able to strike their targets so accurately they could perform the same military tasks as nuclear weapons. The second distribute mines or a fuel-air explosive cloud over an area similar to that seared by a nuclear device, but far more efficiently. Nuclear weapons are wasteful, and it is almost impossible to imagine any circumstances in which their use could possibly be justified. But to some Third World dictators, the bomb is still the ultimate status symbol.

A number of ‘third-generation nuclear weapons’ are under development in the USA and Russia, however. These include diverting the rays from a nuclear explosion and pumping it out in a beam, or very low-yield nuclear weapons to destroy missiles or electronics with radiation alone. Over the past half-century, nuclear weapons have had enormous influence on the design of conventional military forces. But, apart from the two bombs on Japan which ended WW II, they were seen as too terrible to use. Clausewitz said that war ‘tended to the extreme’, but that there were always limitations in practice. With nuclear weapons, it seems the limitations became overriding; let us hope that they remain so.

Bibliography

• Cochran, Thomas, Arkin, William, and Hoenig, William, Nuclear Weapons Databook (5 vols., Cambridge, Mass, 1984).
• Glasstone, S., and Dolan, P. J., The Effects of Nuclear Weapons (Washington, 1977).
• Grace, Charles, Nuclear Weapons: Principles, Effects and Survivability (London, 1994).
• McNaught, L. W., Nuclear Weapons and their Effects (London, 1984).
• Miksche, Ferdinand Otto, Atomic Weapons and Armies (London, 1955).
• Sidorenko, Col A. A., The Offensive (Moscow, 1970); trans. and ed. USAF (Washington, 1976).
• Sokolovsky, Vasily, Soviet Military Strategy, ed. Harriet Fast Scott (3rd edn., London, 1975).
• Tizard, Sir Henry, Examination of the Possible Development of Weapons and Methods of War, PRO Defe II TWC 1252 (46)3(Revise), 30 January 1946

— Christopher Bellamy

The possibility of creating nuclear weapons of almost unimaginable destructive power was first realized in the 1930s as physicists developed a fundamental understanding of the nucleus of the atom. A nuclear explosion is created when heavy nuclei are split—or fissioned—into several of their component parts that are smaller and more stable. Nuclear fission is a fundamentally different process from chemical explosions that occur in conventional high‐explosive or incendiary bombs. In chemical explosions, larger molecular structures are broken apart and rearranged into smaller parts, but the individual atomic nuclei remain untouched. A chemical explosion produces a sudden release of energy that generates an explosive blast, whose resulting high air pressures and strong winds can crush and knock down nearby structures and people. In the case of early nuclear weapons based on the fission process, the energy release, which occurs in microseconds, is enormously larger because the nuclear bonds that hold nuclei together and are broken during fission are so much stronger than the chemical bonds that bind atoms into molecules. Since the nuclear forces are typically 100,000 to 1 million times stronger than the electrical ones responsible for molecular structures, the resultant energy releases are correspondingly larger.The nuclear blast is so powerful that it can crush objects many miles away with high winds in excess of 150 mph generated at distances greater than a mile. The release of the enormous energy in a nuclear explosion leads to extremely high temperatures, comparable to those that occur at the center of the Sun, causing massive and deadly fires. As a measure of comparison, the temperatures generated by nuclear weapons are hundreds to thousands of times higher than the temperatures on the surface of the Sun, which heats the surface of the Earth from a distance of more than 90 million miles. Dangerous radioactive fallout is also spread over large distances by the resulting nuclear radiation emerging with the nuclear debris.

The ability to release such enormous energy from single weapons, on a scale unparalleled in human history, profoundly alters the very nature of war, as well as its consequences. An appreciation of the consequences of a nuclear explosion can be learned from the experience of the only nuclear weapons used in war, the atomic bombs dropped by U.S. air forces on Hiroshima and Nagasaki in 1945. These two weapons devastated two entire cities. They had yields of 15–20 kilotons. That measure simply means that the energy release was the same as that from detonating 15,000–20,000 tons of TNT (TNT is an acronym for the chemical formula of dynamite). By way of comparison, the largest conventional bombs used in World War II—the so‐called blockbusters used by the Royal Air Force (RAF)—detonated 10 tons (20,000 pounds) of TNT.

Those fission bombs of 1945 are no more than primitive versions of the first stage, or triggers, of modern nuclear weapons, whose yields range into the megatons, or millions of tons of TNT equivalent, and whose deadly devastating impact ranges over many miles. (One kiloton is equivalent to 2 million pounds of TNT; 1 megaton is equivalent to 2 billion pounds of TNT.) In modern nuclear weapons, such fission triggers are known as the primaries. They ignite a secondary stage by creating very high temperatures in order to generate still larger quantities of energy by driving together, or fusing, light nuclei into more stable ones. This is known as fusion. Such modern weapons are commonly referred to as thermonuclear weapons—or, more simply, H‐bombs.

The effect of a 1‐megaton thermonuclear weapon has an energy release 100,000 times greater than the largest 10‐ton blockbusters of World War II; the area destroyed by blast would be several thousand times larger than that leveled by such blockbusters. Collateral destruction and casualties due to fires and radioactive fallout would extend even further than the area destroyed by blast.

Soon after World War II, it was realized that the existence of nuclear weapons posed a new and fearsome threat to modern civilization and that it was vital to treat them differently from “conventional”—nonnuclear—weapons. Serious initiatives during the decade immediately following WWII tried to bring these terrifying new weapons under international control. These efforts failed as the confrontation between the Western powers and the Soviet Union and its allies grew into a cold war. Fueled by this dangerous competition during the 1960s, the individual nuclear arsenals of the United States and the Soviet Union accumulated to tens of thousands of warheads. In addition, France, England, and China acquired their own, albeit much smaller, nuclear arsenals. Furthermore, the newly developed delivery systems of intercontinental‐range, and in particular, land‐based intercontinental ballistic missiles (ICBMs)—and long‐range ballistic missiles on submarines (SLBMs) moving about invisibly under the surface of the oceans—brought the threat of nuclear annihilation very close to home, less than thirty minutes away from a nation's borders.

Difficulty of Protection Against Nuclear Weapons

It also became clear before long that there was no known or prospective technology that could provide a defense against a determined nuclear attack. In contrast to previous wars, essentially nothing would be left of a large urban “target”—its population and industry—if just one, or at most a few, nuclear warheads exploded over it. Witness the bombings of Hiroshima and Nagasaki.

A defense would have to be essentially perfect to provide protection against nuclear weapons, and that is neither a realistic standard of performance today nor a prospective one for future military systems. In World War II, during the Battle of Britain, the RAF defense system managed to destroy no more than one in ten of the attacking planes. At such a rate, the German Air Force was reduced faster than it could replace its losses. At the same time, cities like London could put out the fires and rebuild after the damage. Human defenselessness is a basic fact of the nuclear age. It is also troubling since it denies one of the most basic instincts of the human race: to defend ourselves, our families, our friends, our vital interests. Recognition of the ineffectiveness of defenses against the almost unimaginable destructive potential of a massive attack by nuclear bombs led the United States and the former Soviet Union to acknowledge that their very survival was based on mutual deterrence—ensuring that nuclear weapons were not used.

Basic Physical Processes in Nuclear Weapons

The first step in detonating a thermonuclear weapon is to ignite the high explosive that causes a shock wave to travel inward and compress the nuclear material the explosive surrounds, known as the pit. At the same time, a strong source of neutrons is activated to flood the compressed pit.

If the material in the compressed pit reaches a condition known as criticality, the neutrons initiate a strong fission chain reaction. This is the fission, or primary, stage of a thermonuclear explosion. In a chain reaction, an incoming neutron splits the nucleus of fissile material (either an isotope of uranium, U²³⁵, that occurs in nature, or of plutonium, Pu²³⁹, that is man‐made), releasing at least two neutrons, which then run into other fissile material, producing more neutrons, which then run into other fissile material, and so on. Thus, in successive steps, or “generations,” of fission, the neutrons will multiply: 2, 2 × 2, 2 × 2 × 2, … After very roughly 100 generations, if the fissile material can be held together long enough, (i.e., for microseconds), enough nuclei will have fissioned and enough energy will have been created to generate an explosive equivalent to 10 kilotons or so of TNT.

Several years after the development of such first‐generation fission bombs, weapons designers concentrated on improving their performance by using the material more efficiently. U.S. and Soviet weapons technology advanced rapidly after the first Soviet nuclear detonation, “Joe 1,” in 1949. The biggest advance occurred when the process of fusion was introduced into the explosive process. Fusion, in contrast to fission, involves combining, or fusing together, several nuclei of the lightest elements, such as hydrogen isotopes, to form more stable heavy ones. High temperatures are required to ignite the fusion process effectively. This is because at high temperatures, individual nuclei acquire high speeds, and move sufficiently rapidly to push their way though their mutual electric repulsion and get near enough to each other to collide and “fuse” together. The new nucleus thus formed is generally more stable, leading to the release of a large energy, plus more neutrons. Fusion is the process fueling the Sun's burning.

Modern weapons with both fission and fusion stages are called thermonuclear or hydrogen bombs. In a thermonuclear weapon, the primary, or fission, stage creates the necessary high temperatures to ignite the fusion stage, which provides additional neutrons to initiate still more fission, thereby releasing much more energy. A thermonuclear weapon can be built with virtually no limit on the amount of fusion materials it contains. Such weapons generate explosions as large as tens of megatons of TNT, or the equivalent of billions of pounds of TNT. In thinking about the totality of destruction in a nuclear war waged with modern thermonuclear weapons of such enormous yield, it is well to keep in mind that many of the destructive effects of nuclear weapons were not anticipated, and were discovered with surprise by atomic scientists when they were used or tested. This calls for great humility when it comes to predicting the consequences of nuclear warfare.

Since 1945, the total number of known nuclear tests, worldwide, adds up to some 2,000. A major purpose of testing has been to validate and confirm appropriate performance specifications for new weapons types designed in response to military needs formulated during the Cold War. Starting in the mid‐1950s, U.S. weapons were designed and built “ready to go.” They conserved special nuclear materials (SNM)—the fissile materials Pu²³⁹ and U²³⁵—and were essentially maintenance‐free, ready to go at any time. “Ready” means that no physical changes or steps such as inserting the SNM had to be made in order to detonate a bomb. One merely had to launch and detonate the warhead by signal.

In response to growing worldwide concerns about radioactive fallout from continued nuclear testing, the United States, the Soviet Union, and the United Kingdom joined in 1963 in a Limited Test Ban Treaty that forbade testing aboveground, in the atmosphere, underwater, and in outer space. Only underground testing was allowed. A further restriction on testing was negotiated in 1974, limiting the yields of underground tests to a maximum of 150 kilotons, roughly ten times the yield of the Hiroshima bomb. This so‐called Threshold Test Ban Treaty was generally obeyed henceforth, though it was not ratified until 1990.

In 1992, progress in negotiated reductions in the nuclear arsenals, and further progress in reducing reliance on nuclear weapons after the end of the Cold War, led President George Bush to rule out nuclear weapons tests for new warheads and to declare a nine‐month moratorium on all nuclear testing. This moratorium was continued by his successor and has also been honored by Russia and the United Kingdom. On 11 August 1995, President Bill Clinton announced U.S. support for negotiating a comprehensive test ban treaty in 1996. The treaty would be of unending duration, and would include, as do all such tests, a “supreme national interest” clause should unanticipated circumstances present compelling arguments for renewed tests. Such arguments might arise if there were serious reversals from the present progress toward reducing nuclear danger in the world, or if unforeseen technical problems arose over time in the enduring nuclear stockpile.

By the best current technical judgment, U.S. weapons appear to be safe, reliable, age‐stable, and fully adequate for deterrence; but it will be a new challenge to maintain that confidence without being able to conduct tests that produce any nuclear yield. Under its recently formulated program for stockpile stewardship and management, the United States has accepted this challenge, following a comprehensive scientific review of prospects and needs for its nuclear arsenal. So have the United Kingdom, Russia, France, and China.

On September 1996 President Clinton was the first world leader to sign the Comprehensive Test Ban Treaty at the United Nations in New York. Soon thereafter the other declared nuclear powers—England, France, China, and Russia—also signed, and as of November 1998 150 nations have signed the Treaty and twenty‐one have ratified it. For it to go into effect it must be ratified by all forty‐four nuclear capable nations, i.e., nations with nuclear reactors for research or for civilian energy production, in addition to those with nuclear weapons. A Comprehensive Test Ban after more than 2,000 tests over a 50‐year period would be a tremendous achievement. Efforts to accomplish that goal are currently in progress, together with continuing efforts to reduce the size of the nuclear arsenals at the Strategic Arms Reduction Talks (START) underway between the U.S. and Russia.

[See also Arms Control and Disarmament: Nuclear; Cold War: External Course; Cold War: Domestic Course; War Plans; Weaponry; World War II: Military and Diplomatic Course.]

Bibliography

• Margaret Gowing, Britain and Atomic Energy, 1939–1945, 1964.
• Samuel Glasstone and Philip J. Dolan, eds., The Effects of Nuclear Weapons, 3rd ed. 1977.
• Richard Rhodes, The Making of the Atomic Bomb, 1986.
• Robert Serber, The Los Alamos Primer: The First Lectures on How to Build an Atomic Bomb, 1992.
• David Holloway, Stalin and the Bomb, 1994.
• Richard Rhodes, Dark Sun: The Making of the Hydrogen Bomb, 1995

A complete assembly (that is, implosion type, gun type, or thermonuclear type) in its intended ultimate configuration that, upon completion of the prescribed arming, fusing, and firing sequence, is capable of producing the intended nuclear reaction and release of energy.

See the Introduction, Abbreviations and Pronunciation for further details.

For more information on nuclear weapon, visit Britannica.com.

Nuclear Weapons derive their energy from the splitting (fission) or combination (fusion) of atomic nuclei. This category of weapons taken together may have finally fulfilled the wish of technologists throughout history for a weapon so terrible that it would make war between great powers obsolete. The twentieth century was the bloodiest in human history, yet no two nations possessing nuclear weapons fought a major war against one another.

The nuclear era began with the Manhattan Project, the secret American effort during World War II to construct an atomic bomb. On 6 July 1945 the world's first atomic explosion was created during a test in the New Mexico desert. On 6 and 9 August, respectively, the Japanese cities of Hiroshima and Nagasaki were devastated by atomic bombings, and on 10 August Japan offered to surrender. The wave of celebrations in the United States that followed the end of the war were tinged with an immediate sense of shock at the terrifying power of this new class of weaponry. In a world where the science fiction of

H. G. Wells had suddenly become a reality, anything seemed possible, and popular reactions to the bomb varied widely. Many feared that the next world war would result in the literal extinction of humankind, and to witnesses of two world wars in the space of three decades, a third world war seemed a virtual inevitability. Others searched for hope in the new "atomic world, " imagining the imminent creation of a world government, the abolition of war, or even a utopia where the atom eradicated disease and provided limitless electrical power. While no such utopia emerged, nuclear energy did eventually fight cancer and generate electricity. No aspect of American society escaped the cultural upheavals of the bomb. By the early 1950s even schoolchildren were instructed by a cartoon turtle that they "must be ready every day, all the time, to do the right thing if the atomic bomb explodes: duck and cover!"

Political, military, and intellectual elites within the United States also grappled with the implications of nuclear weapons. A group of academic nuclear theorists led by Bernard Brodie began developing theories of deterrence for a world where preventing war seemed to be more important than winning one. Military leaders hoped that the American monopoly on nuclear weapons would deter any potential aggressor for the time being, but even optimists did not expect this monopoly to last more than a decade. If war with the Soviet Union did come, and "war through miscalculation" as well as by conscious design was always a fear, planners did not believe that the use of tens or even hundreds of atomic bombs would necessarily bring victory. Expansion of the American nuclear stockpile continued at the maximum possible rate, and following the first Soviet atomic test (years before it was expected) in August 1949, President Harry S. Truman gave permission to proceed with the development of a whole new kind of nuclear weapon, the hydrogen bomb. Unlike an ordinary atomic bomb, no theoretical or even practical limit existed on the terrific energy released by the explosion of one of these new "thermonuclear" weapons. In 1957 the Soviet Union tested the world's first intercontinental ballistic missile (ICBM), and the United States soon followed suit. The potential warning each side might receive of an attack from the other was now reduced from hours to minutes. As a result of these and other technical advances, by the early 1960s political leaders on both sides had reached the conclusion that in any global nuclear war neither superpower could hope to escape unacceptable damage to its homeland.

This realization did not prevent the continuation of the nuclear arms race, however. Each side feared that a technological breakthrough by the other might yield an advantage sufficient to allow a preemptive "first strike" so powerful as to make retaliation impossible. To prevent this, each superpower had to secure its "second strike" capability, thus ensuring the continuation of the deterrent of "Mutual Assured Destruction" or MAD. To this end the United States constructed a "strategic nuclear triad" built around an enormous armada of intercontinental bombers, a force of approximately one thousand land-based ICBMs, and beginning in 1960, a fleet of submarines equipped with nuclear-tipped ballistic missiles. In the 1970s MAD was threatened by the creation by both sides of ICBMs that could deploy multiple warheads, each potentially capable of destroying an enemy missile while it was still in its hardened silo. Another potential threat to MAD was the advent of antiballistic missile (ABM) systems. Both sides had worked on these since the 1950s, but in recognition of the technical difficulty of "hitting a bullet with a bullet" and of the possibly destabilizing nature of a partially effective defense, in May 1972 the two superpowers signed the ABM Treaty, severely curtailing deployment of and future research on such systems. In the 1960s and especially the 1970s nuclear weapons had become so plentiful for both sides that they were deployed in large numbers in a tactical role as well. Relatively small ground units and even individual ships and aircraft were now potential targets of nuclear attack. This raised at least the realistic possibility for the first time in the Cold War of a successful defense of Western Europe against a Soviet ground assault.

The question remained, though, of just how many Europeans might be left after the radioactive smoke had cleared from such a "successful" defense. Advocates of a nuclear freeze swelled in number both in Europe and in the United States, and following the election of President Ronald Reagan in 1981, popular fears of nuclear war grew to a level not seen since the 1950s. Reagan also challenged the prevailing logic of MAD, renewing the ABM debate by calling in March 1983 for the creation of a vast new system of defense against nuclear attack through his "Strategic Defense Initiative" (derided by critics as "Star Wars"). This final round of the arms race was cut short, however, by the collapse of the Soviet economy in the 1980s and in 1991 of the Soviet Union itself.

In the years that followed the end of the Cold War nuclear fears, both public and governmental, rapidly switched from a general nuclear war to the possible acquisition of nuclear weapons by "rogue states," such as Iraq or North Korea, and whether or not to build a limited national missile defense system. After the attacks of 11 September 2001 on the Pentagon and the World Trade Center, nuclear terrorism became the greatest potential nightmare of all.

Bibliography

Boyer, Paul. By the Bomb's Early Light: American Thought and Culture at the Dawn of the Atomic Age. Chapel Hill: University of North Carolina Press, 1994. First published in 1985.

Bundy, McGeorge. Danger and Survival: Choices about the Bomb in the First Fifty Years. New York: Random House, 1988. Thoughtful combination of history and memoir.

Carter, Ashton B., John D. Steinbruner, and Charles A. Zraket, eds. Managing Nuclear Operations. Washington, D.C.: Brookings Institution, 1987. Standard reference work.

Federation of American Scientists. "United States Nuclear Forces Guide." Available http://www.fas.org.

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Nuclear weapons are explosive devices that utilize the processes of fission and fusion to release nuclear energy. An individual nuclear device may have an explosive force equivalent to millions of tons (megatons) of trinitrotoluene (TNT, the chemical explosive traditionally used for such comparisons), more than enough to completely destroy a large city. The destructive power of nuclear weapons derives from the core of the atom, the nucleus. One type of nuclear weapon, the fission bomb, uses the energy released when nuclei of heavy elements, such as plutonium, fission or split apart. A second even more powerful type of nuclear weapon, the fusion or hydrogen bomb, uses the energy released when nuclei of hydrogen are forced to fuse (join together).

Nuclear devices have been fashioned into weapons of many shapes with many purposes. Bombs can be dropped from airplanes; warheads can be delivered by missiles launched from land, air, or sea; artillery shells can be fired from cannons; mines can be placed on the land and in the sea. Some nuclear weapons are small enough to destroy only a portion of a battlefield; others, as already mentioned, are large enough to destroy entire cities and more.

Unlike chemical explosives, nuclear weapons have had no peacetime uses, although in the 1950s the U.S. government briefly considered using them to blast artificial harbors in the Alaskan coastline. They are possessed by a number of nations, including the United States, France, Great Britain, China, India, Israel, Pakistan, and the Russian Federation along with several former Soviet Republics. Iran and North Korea, among other nations, are interested in building them. Since nuclear weapons were invented during World War II, they have been used only twice, both times against cities in Japan by the United States.

Development of nuclear weapons. German physicist Albert Einstein (1879–1955) did not know it at the time, but when he published his Special Theory of Relativity in 1905 he provided the world with the basic information needed to build nuclear weapons. Einstein said that the amount of matter of an object (i.e., its mass) is equivalent to a specific amount of energy. The exact amount of energy in an object equals its mass multiplied by the square of the speed of light. The speed of light is large—186,282 miles per second (300,000 km/sec)—so even a small piece of matter contains a vast amount of energy. A baseball-size sample of uranium-235, for example, can explode with as much energy as 20,000 tons of TNT—and this involves the conversion of only a tiny fraction of the uranium's mass into energy. One pound of explosive material in a fission weapon is approximately 100,000 times as powerful as one pound of TNT.

As World War II approached, two German chemists, Fritz Strassmann (1902–1980) and Otto Hahn (1879–1968), pointed a stream of neutrons at a sample of uranium and succeeded in splitting the nuclei of some of its atoms. This splitting of nuclei is termed nuclear fission. The energy released through nuclear fission was the source of power for the first atomic bomb, which was built in the United States by a large team of scientists led by U.S. physicist J. Oppenheimer (1904–1967). This secret research and development program was termed the Manhattan Project.

The first atomic bomb was detonated in a test at Alamogordo, New Mexico, on July 16, 1945. Three weeks later, on August 6, a bomber named Enola Gay dropped a four-ton atomic bomb containing 12 lb (5.4 kg) of uranium-235 on the Japanese city of Hiroshima. Seventy thousand people died as a direct result of the blast. Within two months, nearly twice that many were dead from blast injuries and radiation. Three days later, on August 9, a bomb containing several pounds of plutonium was dropped on Nagasaki. Thirty thousand people died in the seconds following the explosion, and more later. The Japanese surrendered the next day, ending World War II.

These first nuclear weapons were atomic bombs or A-bombs. They depended on the energy produced by nuclear fission for their destructive power. However, scientists like U.S. physicist Edward Teller (1908–) knew even before the first atomic bomb exploded that the fission weapons could be used to create an even more powerful explosive, now called a thermonuclear device, hydrogen bomb, or H-bomb. This weapon gets it power from the energy released when atoms of the hydrogen isotopes deuterium or tritium are forced together, a process called nuclear fusion. Starting a nuclear fusion reaction is even more complicated than setting off a fission atomic bomb; it requires such heat to initiate it that a fission bomb is used as a detonator to explode the fusion bomb. The United States tested the first hydrogen bomb on November 1, 1952. It exploded with the force of 10.4 megatons (millions of tons of TNT equivalent). Three years later, the Soviet Union exploded a similar device.

For the next 40 years, the United States, with its allies, and the former Soviet Union, with its allies, raced to build more nuclear weapons, with each side producing tens of thousands. The end of the cold war and the breakup of the Soviet Union in the early 1990s led to the elimination of a significant number of nuclear weapons; however, the U.S. and Russia still possess many thousands of nuclear weapons.

The physics and mechanics of nuclear weapons. Conventional, chemical explosives get their power from the rapid rearrangement of chemical bonds, the links between atoms made by sharing electrons. In chemical explosives, atoms dissociate from other atoms and form new associations; this releases energy, but the atoms themselves do not change. Nuclear weapons are based on an entirely different principle. They derive their explosive power from changes in the structure of the atom itself, specifically, in the core of the atom, its nucleus.

Atomic bombs use the energy released when nuclei of heavy elements split apart or fission. Uranium and plutonium are the two elements that can be used as fuel for this type of weapon. When nuclei of these atoms are struck with rapidly moving neutrons, they are broken into two pieces nearly equal in size. They also release more neutrons, which split more nuclei. This is called a chain reaction. If enough atomic nuclei split they will release enough neutrons to ensure that all the nuclei of all the atoms in a sample will be split. Enormous amounts of energy are then released in a fraction of a second. This release of energy is the power behind the atomic bomb.

Uranium and plutonium are termed fissile materials because they can support a fission chain reaction if enough material is concentrated in one place. Too small a sample would not generate enough neutrons to keep the fission process going; for example, a one-pound (.45-kg) sample of uranium-235, a sample about the size of a ping-pong ball, is not large enough to support a chain reaction. The atomic bombs used in World War II proved that 12 or so pounds (about 5.5 kg) of fissile material, larger than a ping-pong ball but still small enough to fit into your hand, is enough to maintain a chain reaction. The smallest amount of material that can support a chain reaction is called the critical mass.

The instant enough bomb material is gathered together into a critical mass, a chain reaction begins. (At higher density, less mass is required.) This means that fissile material cannot be assembled in a critical mass until it is meant to explode. Therefore, the sample of uranium or plutonium in an atomic bomb is separated into several pieces, each of which is below critical mass. To set the bomb off, the separated pieces of bomb material are rammed together to create a critical mass. One design for creating a critical mass involves firing a subcritical "bullet" of fissile material into a subcritical "target" of fissile material. Together, the bullet and the target create a critical mass that starts a chain reaction leading to a nuclear explosion.

A different design was used to detonate the bomb dropped on Nagasaki. Plutonium was stored in one large but subcritical mass. It was compressed to a critical density by means of surrounding chemical explosives. When the chemical explosive detonated, the blast forced the bomb material into a density that reached criticality. In either type of design, once criticality is reached the explosion follows in a millionth of a second.

In order for nuclear fission to occur, a bomb must use heavy atoms for fuel. Heavy atoms have many nucleons—neutrons and protons—in their nuclei. When these heavy nuclei split apart they release energy (and neutrons, which may cause nearby heavy nuclei to split apart also). Another more powerful type of nuclear weapon uses forms of hydrogen as fuel. Hydrogen has few subatomic particles in its nuclei—usually only a proton, but the isotope deuterium has a proton plus a neutron, while the isotope tritium has a proton plus two neutrons. Instead of being split apart, these light atomic nuclei are forced together in high-speed collisions, a process called nuclear fusion. Energy is released when hydrogen nuclei fuse, forming helium. Fusion only occurs at temperatures of millions of degrees, such as exist in the hearts of stars. (The sun and other stars generate their energy primarily by fusing hydrogen into helium.) On Earth only an atomic bomb can raise kilograms of material to such a temperature, which is why atomic bombs are used as detonators for hydrogen fusion bombs.

Because hydrogen is lighter than uranium, more hydrogen atoms fit into a sample of the same weight. Thus, even though one fusion reaction releases less energy than one fission reaction, more hydrogen than uranium atoms can be packed into a nuclear weapon and many more fusion reactions can take place in the weapon than fission reactions can take place in a fission bomb. Fusion weapons, therefore, produce bigger explosions than fission weapons of the same physical bulk.

By 1954, a new feature had been added to the hydrogen bomb to create an even more dangerous weapon. Like earlier hydrogen bombs, this weapon was detonated with the explosion of an atomic or fission weapon. This raised temperatures enough to cause the hydrogen atoms in the bomb to fuse and explode like a regular hydrogen bomb. The designers also enclosed this new bomb in a shell of uranium-238. Neutrons released from the fusion of hydrogen caused the uranium-238 in the surrounding jacket to undergo fission, adding to the power of the blast. This new device was, in effect, a fission-fusion-fission bomb.

The power or "yield" of a nuclear weapon is expressed in terms of how much TNT would be required to equal the weapon's blast. Units of kilotons (thousands of tons) and megatons (millions of tons) of TNT are used to describe nuclear blasts.

Effects of nuclear weapons. Nuclear weapons produce two important effects that are also produced by conventional, chemical explosives: they release heat and generate shock waves, or pressure fronts of compressed air that smash objects in their paths. The heat released in a nuclear explosion creates a sphere of burning, glowing gas that can range from hundreds of feet to miles in diameter, depending on the power of the bomb. This fireball emits a flash of heat that travels outward from the site of the explosion (ground zero), the area directly under the explosion. This heat can cause second degree burns to bare human flesh miles away from the blast site if the bomb is large enough. (Although this heat can start fires, it seems that much of the fire damage in Hiroshima and Nagasaki following the nuclear explosions resulted from damaged electrical, fuel, gas, and other systems following physical damage caused by the shock or blast wave that accompanied the explosion.)

The shock wave produced when a nuclear weapon explodes creates a front of moving air more powerful than any produced by a natural storm. Destructive winds follow the front of displaced air, causing more damage to objects in their path. Many nuclear weapons are designed to be detonated high above their targets to take advantage of this shock effect. The more powerful the bomb, the higher in the sky it will be detonated. The fission bombs dropped on Japan (Hiroshima, 13.5 kilotons; Nagasaki, 22 kilotons) exploded between 1,500 and 2,000 feet (458–610 m) above their targets. A bomb with the power of 10 megatons is capable of destroying most houses within a distance of more than 10 miles from the blast site.

Unlike conventional explosives, nuclear devices can also release significant amounts of radioactivity and pulses of electromagnetic energy. Radioactivity is the release of fast particles and high-energy photons from unstable atomic nuclei. Besides the greater explosive power of nuclear weapons, radiation is the primary feature that most clearly distinguishes chemical from nuclear explosions. Radiation can kill outright at high doses and cause illnesses, including cancer, at lower doses. The initial burst of radiation during a nuclear explosion is made up of X rays, gamma rays, and neutrons. The energy of this radiation is so high that it can often penetrate buildings. Radioactive materials then contaminate the explosion site and often enter the atmosphere where they can travel thousands of miles before falling back to earth. This source of radiation is called radioactive fallout. Radioactive fallout can harm living things for years following a nuclear explosion. Fission bombs and fission-fusion-fission bombs produce more fallout than hydrogen bombs because the fusion of hydrogen atoms generates less radioactive byproducts than does fission of uranium or plutonium.

Electromagnetic pulses (EMPs) are also produced by nuclear weapons that are exploded at high altitudes, and are caused by the interaction of radiation from the explosion with electrons in the atmosphere and with the Earth's magnetic field. EMPs are essentially powerful radio waves that can destroy many electronic circuits.

The effects of fires and destruction following a largescale nuclear war could even change the climate of the planet. In 1983 a group of scientists, including U.S. astronomer Carl Sagan (1934–1996), published the "nuclear winter" theory, which suggested that particles of smoke and dust produced by fires caused by many nuclear explosions would, for a time, block the Sun's rays from reaching the surface of Earth. This, in turn, would reduce temperatures and change wind patterns and ocean currents. These climatic changes, according to the theory, could destroy crops and lead to the death by famine of many more animals and humans than were killed outright by nuclear explosions. Some scientists have challenged these predictions, but others, including some United States government agencies, support them. On the other hand, there is no controversy about whether a large-scale nuclear war could kill hundreds of millions of people and imperil the future of modern civilization, even apart from nuclear winter effects.

Modern nuclear weapons. Today nuclear weapons are built in many sizes and shapes not available in the 1940s and 1950s, and are designed for use against many different types of military and civilian targets. Some weapons are less powerful than 1,000 tons of TNT, while others have the explosive force of millions of tons of TNT. Small nuclear shells can be fired from cannons. Nuclear warheads mounted on missiles can be launched from land-based silos, ships, submarines, trains, and large wheeled vehicles. Several warheads can be fitted into one missile and directed to different targets in the same geographic area upon reentry into the Earth's atmosphere. These multiple independently-targeted reentry vehicles (MIRVs) can release 10 or so individual nuclear warheads far above their targets, making enemy interception more difficult and increasing the deadliness of each individual missile.

In general, nuclear weapons with "low" yields (in the kiloton, rather than the megaton, range) are termed "tactical," and are designed to be used in battle situations against specific military targets, such as a concentration of enemy troops or tanks, a naval vessel, or the like. These weapons are termed tactical because the word tactics, in military jargon, refers to the relatively small-scale maneuvers undertaken to win particular battles. Larger nuclear weapons are classed as "strategic," because the word strategy, again in military jargon, refers to the large-scale maneuvers undertaken to win whole wars. Strategic nuclear weapons are targeted mostly at cities and at other nuclear weapons, and are generally designed to be dropped by bombers or launched on ballistic missiles; tactical nuclear weapons are delivered by smaller devices over shorter distances. However, one nation's "tactical" warhead may be another's "strategic" warhead: Russia, for example, maintains that U.S. tactical warheads in Western Europe are in fact strategic warheads, because they can strike targets inside Russia itself, while Russian "tactical" warheads in the same arena cannot strike the U.S. heartland.

In the summer of 2002, the George W. Bush administration sought and received permission from Congress to design a new class of nuclear weapons: "mini-nukes" are relatively low-yield tactical nuclear weapons for use against underground bunkers and other small battlefield targets. Also in 2002, the U.S. military—according to a secret Pentagon document leaked to the press—drew up an official set of contingency plans for attacking seven countries with nuclear weapons (China, Russia, Iraq, North Korea, Iran, Libya and Syria). Advocates of these new weapons point to the uniquely powerful, compact "punch" that can be delivered by a nuclear weapon; critics argue that even a small nuclear weapon may cause many civilian casualties, and, more important, that actual use of a nuclear weapon of any size would break the taboo on such use that has held since the end of World War II, making the use of larger, more destructive nuclear weapons more likely in future conflicts. Some analysts stressed that the Pentagon's explicit willingness to use nuclear weapons in a "first-use" fashion, that is, in response to "unexpected military situations" not involving attack on U.S. forces by nuclear weapons, or to use them on targets (e.g., deep bunkers) resistant to conventional explosives signaled a major shift in United States nuclear use doctrine.

Even the ability of nuclear weapons to release radioactivity has been exploited to create different types of weapons. "Clean bombs" are weapons designed to produce as little radioactive fallout as possible. A hydrogen bomb without a uranium jacket would produce relatively little radioactive contamination, for example. A "dirty bomb" could just as easily be built, using materials that contribute to radioactive fallout. Such weapons could also be detonated near Earth's surface to increase the amount of material that could contribute to radioactive fallout. "Neutron" bombs have been designed to shower battle fields with deadly neutrons that can penetrate buildings and armored vehicles without destroying them. Any people exposed to the neutrons, however, would die. (Neutron bombs also destroy with blast effects, but their deadly radiation zones extend far beyond the site of their explosions).

The United States and Russia signed a Strategic Arms Reduction Treaty in 1993 to eliminate two thirds of their nuclear warheads in 10 years. By 1995, nearly 2,500 nuclear warheads had been removed from bombers and missiles in the two countries, according to U.S. government officials. ("Elimination," in this context, does not necessarily mean dismantlement; many of the weapons that have been "eliminated" by the treaty have been put in storage.) Although thousands of nuclear weapons still remain in the hands of many different governments, especially those of the U.S. and the Russian Federation, recent diplomatic trends have at least helped to lower the number of nuclear weapons in the world. This has caused many people to assume that the danger of nuclear weapons evaporated with the end of the Cold War.

However, the number of nations possessing nuclear weapons continues to increase, and the possibility of nuclear weapons being used against human beings for the first time since World War II may be larger than ever. In May 1995, more than 170 members of the United Nations agreed to permanently extend the Nuclear Non-Proliferation Treaty, first signed in 1960. Under the terms of the treaty, the five major countries with nuclear weapons—the United States, Britain, France, Russia, and China—agreed to commit themselves to eliminating their arsenals as an "ultimate" goal. The other 165 signatory nations agree not to acquire nuclear weapons. Israel, which is believed to possess nuclear weapons (but officially denies doing so), did not sign the treaty. Two other nuclear powers also refused to renounce nuclear weapons: India and Pakistan, each of which probably possess several dozen nuclear weapons, have fought a number of border wars in recent decades, and in 2002 came close, as many observers thought, to fighting a nuclear war. As of 2003, North Korea had reactivated its nuclear-weapons-material production facilities and was engaged in a tense diplomatic standoff with the United States, which insisted that North Korea abandon its nuclear-weapons program.

Further Reading

Books

Rhodes, Richard. Dark Sun: The Making of the Hydrogen Bomb (Sloan Technology Series). New York: Simon & Schuster, 1995.

Sagan, Scott D. and Kenneth N. Waltz. The Spread of Nuclear Weapons: A Debate Renewed, 2nd ed. W. W. Norton & Co., 2003.

Walmer, Max. An Illustrated Guide to Strategic Weapons. New York: Prentice Hall Press, 1988.

Electronic

"U.S. Has Nuclear Hit List." BBC News. March 2, 2002. <http://news.bbc.co.uk/2/hi/americas/1864173.stm> (Feb. 26, 2003).

(DOD, NATO) A complete assembly (i.e., implosion type, gun type, or thermonuclear type), in its intended ultimate configuration which, upon completion of the prescribed arming, fusing, and firing sequence, is capable of producing the intended nuclear reaction and release of energy.

Any weapon that employs a nuclear reaction for its explosive power. Nuclear weapons include ballistic missiles, bombs (see atomic bomb and hydrogen bomb), artillery rounds, and mines.