nuclear weapons weapons of mass destruction powered by atomic, rather than chemical, processes. Nuclear weapons produce large explosions and hazardous radioactive byproducts by means of either nuclear fission or nuclear fusion. Nuclear weapons can be delivered by artillery, plane, ship, or ballistic missile (ICBM); some can also fit inside a suitcase. Tactical nuclear weapons can have the explosive power of a fraction of a kiloton (one kiloton equals 1,000 tons of TNT), while strategic nuclear weapons can produce thousands of kilotons of explosive force. After World War II, the proliferation of nuclear weapons became an increasing cause of concern throughout the world. At the end of the 20th cent. the vast majority of such weapons were held by the United States and the USSR; smaller numbers were held by Great Britain, France, China, India, and Pakistan. Israel also has nuclear weapons but has not confirmed that fact publicly; North Korea has conducted a nuclear test explosion but probably does not have a readily deliverable nuclear weapon; and South Africa formerly had a small arsenal. Over a dozen other countries can, or soon could, make nuclear weapons. In addition to the danger of radioactive fallout , in the 1970s scientists began investigating the potential impact of nuclear war on the environment. The collective effects of the environmental damage that could result from a large number of nuclear explosions has been termed nuclear winter . Treaties have been signed limiting certain aspects of nuclear testing and development. Although the absolute numbers of nuclear warheads and delivery vehicles have declined since the end of the cold war, disarmament remains a distant goal. See atomic bomb ; cold war ; disarmament, nuclear ; guided missile ; hydrogen bomb ; nuclear energy ; nuclear physics .

Bibliography: See L. Martin, The Changing Face of Nuclear Warfare (1987).

Nuclear Weapons. 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.

Impact of Nuclear Weapons.

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.

Sidney Drell

Nuclear Weapons. The history of nuclear weapons began well before the United States entered World War II.Spurred by the German discovery of nuclear fission announced early in 1939, scientists at several universities had confirmed the feasibility of an unimaginably powerful chain‐reacting bomb and suggested how to build one. In August 1939, the émigré physicist Albert Einstein wrote a letter to President Franklin Delano Roosevelt (drafted by another émigré physicist, Leo Szilard) reporting these developments; in response, Roosevelt authorized a modest research program. In the Summer of 1941, the federal Office of Scientific Research and Development transferred this small, scattered research program to the Army Corps of Engineers. Taking charge of what became known as the Manhattan Project, General Leslie R. Groves organized a crash program of expanded research, industrial production of fissionable materials, and bomb development.

Research was consolidated at the University of Chicago's new Metallurgical Laboratory, where Enrico Fermi and his team achieved the first controlled nuclear reaction in December 1942. Chicago also developed the health and safety measures adopted throughout the project. Construction of production facilities for enriched uranium at Oak Ridge, Tennessee, and for plutonium at Hanford, Washington, proceeded in parallel. Groves picked J. Robert Oppenheimer to direct bomb development in a new laboratory at Los Alamos, New Mexico, managed by the University of California. Buildings were still going up when scientists began work in April 1943.

By 1945, Oppenheimer's team had designed and built two fission bombs. One used enriched uranium in a gun‐type assembly, a design deemed so reliable as to need no proof‐testing before deployment. The other depended on the newly discovered fissionable element plutonium assembled by implosion, a much less certain technique that did demand testing. A secret test, code‐named Trinity, took place at Alamogordo, New Mexico, on 16 July 1945, producing energy equivalent to 21,000 tons (21 kilotons) of high explosives.

On 6–9 August 1945, the United States launched its nuclear attack on Japan, dropping the uranium bomb on Hiroshima, the plutonium bomb on Nagasaki. At Bikini Atoll in July 1946, two more plutonium bombs furnished the firepower for a test series called Operation Crossroads. Part public spectacle intended to demonstrate America's nuclear might, part attempt to assess the effect of nuclear weapons on ships, Operation Crossroads became the Manhattan Project's last hurrah.

After heated congressional debate, the Atomic Energy Act of 1946 settled responsibility for developing future nuclear weapons on a civilian agency, the Atomic Energy Commission (AEC). Civilian control of nuclear weapons remained intact when the AEC gave way in 1974 to the Energy Research and Development Administration, itself succeeded in 1977 by the Department of Energy. The Manhattan Project officially transferred its facilities to the AEC on 1 January 1947. Its remaining, specifically military components merged under the new Department of Defense as the Armed Forces Special Weapons Project (after two subsequent name changes, it eventually became the Defense Special Weapons Agency).

Of the major transferred facilities, only Los Alamos remained concerned primarily with nuclear‐weapons research and development. Its former engineering division, however, had grown rapidly after moving to Albuquerque, New Mexico, in 1945. In 1949, it became the independent Sandia Laboratories, its management transferred from the University of California to Bell Laboratories. Its primary function was providing the engineering support to turn Los Alamos designs into working weapons.

With the Cold War now well under way, nuclear‐weapons development became a high national priority. The AEC inaugurated its nuclear‐weapons testing program in the Spring of 1948 with Operation Sandstone. Supported by a joint army‐navy task force, Los Alamos scientists tested three new fission‐bomb designs at Enewetak Atoll. Part of the United Nations Trust Territory of the Marshall Islands administered by the United States, Enewetak officially became the Pacific Proving Ground, which expanded in 1951 to include Bikini. When the outbreak of the Korean War threatened to disrupt schedules for Operation Greenhouse, the next Pacific test series, the AEC selected a continental test site in Nevada, first used for Operation Ranger in January 1951.

During the 1950s, annual testing alternated between Nevada, where operations were cheaper but restrictions greater, and the Marshall Islands, which served as the site for testing very‐large‐yield thermonuclear weapons. A Soviet atomic‐bomb test in August 1949, decidedly sooner than many expected, had severely jolted American complacency. To meet the perceived challenge, Edward Teller (among others) vigorously advocated accelerated development of the hydrogen bomb, the so‐called Super, based on thermonuclear fusion, the main subject of Teller's research at wartime Los Alamos. Although no one yet knew how to design such a weapon, President Harry S. Truman in January 1950 authorized a crash program.

The conceptual breakthrough came a year later, in February and March 1951, from a suggestion by the Los Alamos mathematician Stanislaw Ulam, which Teller improved and extended. A fission first stage (primary) would provide the energy to ignite the thermonuclear fuel (deuterium and tritium, the heavy isotopes of hydrogen) in a second stage (secondary). In essence, the Ulam‐Teller idea was to couple the primary's energy to the secondary via X‐rays. Hydrogen bombs (H‐bombs) promised yields measured in megatons rather than the kilotons of fission bombs.

Although not based on the Ulam‐Teller principle, the Greenhouse George test in May 1951 showed that a fission detonation could indeed ignite small amounts of thermonuclear fuel. Teller still deemed H‐bomb progress too slow, however, and with air force support and backing from the cyclotron inventor and Nobelist Ernest O. Lawrence, he successfully lobbied the AEC for a second nuclear‐weapons laboratory. It opened in September 1952 as the Livermore branch of Lawrence's Berkeley Radiation Laboratory. Two decades later it became the independent Lawrence Livermore Laboratory, still under University of California management.

The new Livermore laboratory contributed little to early thermonuclear development, which remained largely a Los Alamos enterprise. The “Mike” test in Operation Ivy at Enewetak on 1 November 1952 demonstrated a full‐scale thermonuclear detonation. Sixteen months later at Bikini, on 1 March 1954, the Bravo test of Operation Castle proved the design of an aircraft‐deliverable H‐bomb. Twice as powerful as predicted, Bravo caused heavy fallout that injured Marshall Islanders and Japanese fishermen a hundred miles and more from ground zero. Public outcry led to the test moratorium of 1958–1961, then to the Partial Nuclear Test Ban Treaty of 1963 that ended above‐ground testing. Testing moved underground.

The peak of innovation in nuclear‐weapons design, with Livermore now playing a major role, came in the period 1955–1965. Despite the three‐year moratorium, at least two and as many as five new types of warheads entered the stockpile each year. Gravity bombs continued to improve; the introduction in 1955 of the long‐range jet‐powered B‐52 gave the air force a bomber that could plausibly deliver them on strategic targets. Intercontinental ballistic missiles (ICBMs) benefited even more from the trend toward efficient, lighter warheads. The first air force squadron of Atlas ICBMs became operational in 1958, followed in 1960 by the navy's nuclear‐powered, missile‐equipped Polaris submarine. Nuclear warheads for a variety of tactical missiles, artillery shells, torpedoes, and other munitions also proliferated.

By the mid‐1960s, with nuclear‐weapons development no longer posing major scientific challenges, the focus of innovation shifted from warheads to delivery systems. In 1967, the air force completed replacing its first‐generation ICBMs, which used cryogenic propellants, with technically safer and more reliable missiles using solid (Minuteman) or hypergolic (Titan II) propellants. Protected in underground silos, the new missiles were ready for immediate launch. When the last Polaris submarine went to sea, also in 1967, the strategic triad of manned bombers, land‐based missiles, and missile‐armed submarines was in place.

The next missile generation, fitted with MIRVs (multiple independently targetable reentry vehicles), followed quickly. The air force deployed the first Minuteman IIIs in 1970, the navy its first Poseidon fleet ballistic missile systems in 1971. With land‐based and sea‐launched MIRV missiles, the United States acquired a reliable and essentially invulnerable means of responding to, and so deterring, nuclear attack.

Delivery systems and guidance, like warheads, continued to improve, but not radically. Although the air force's Peacekeeper missile (first deployed in 1986) and the navy's Trident system (1979) marked advances in accuracy over their predecessors, their basic character remained unchanged. Efforts to develop an antiballistic missile (ABM) system in the late 1960s and early 1970s produced only a modest deployment and were limited by the ABM Treaty of 1972. The much more ambitious Strategic Defense Initiative (Star Wars), pursued in the 1980s, cost more and produced less.

The end of the Cold War brought reductions in nuclear stockpiles and, in 1992, a halt to U.S. nuclear‐weapons testing. The United States retained its nuclear arsenal at reduced levels, however, and the Department of Energy instituted a laboratory science–based “stockpile stewardship” program to insure that aging weapons would remain both safe in storage and reliable if ever required.

While the Cold War nuclear-arms race faded, nuclear weapons remained a major concern. In the 1990s, fearful that nuclear know-how might fall into dangerous hands, the Clinton administration sought to safeguard nuclear installations in the former Soviet Union. Pakistan's test of a nuclear weapon in 1998 (thereby matching India, which had exploded a nuclear device as early as 1974) stirred fears of a regional nuclear arms race.

After the September 11, 2001 terrorist attacks, the U.S. government focused on a possible nuclear attack by a rogue state or even a small terrorist band. Under President George W. Bush, the nuclear danger was subsumed into a larger preoccupation with weapons of mass destruction (WMD), including chemical and biological weapons. Allegations (later proven unfounded) that Iraqi dictator Saddam Hussein possessed WMD provided a rationale for the Iraq War of 2003. After that war, Iran and Libya agreed to UN inspections to verify that they were not developing nuclear weapons. Particular attention focused on North Korea, which in 1994 had pledged to halt nuclear-weapons research but in 2003 claimed that it possessed such weapons. While pressuring North Korea to halt its program, the Bush administration also proceeded with deployment of a missile-defense system, a limited version of President Ronald Reagan's grandiose Strategic Defense Initiative.

Fears of nuclear proliferation deepened in 2004, when Pakistan's chief nuclear scientist, Abdul Kahn, confessed that he had sold nuclear-weapons components to Iran, Libya, and North Korea. While the nuclear threat mutated into new forms, it showed no signs of disappearing as the United States moved into the twenty-first century.
See also Antinuclear Protest Movement; Civil Defense; Federal Government, Executive Branch: Department of Defense; Federal Government, Executive Branch: Other Departments (Department of Energy); Hiroshima and Nagasaki, Atomic Bombings of; Nuclear Arms Control Treaties; Nuclear Strategy.

Bibliography

Richard Hewlett et al. , A History of the United States Atomic Energy Commission, 3 vols., 1962–1989.
Herbert F. York , The Advisers: Oppenheimer, Teller, and the Superbomb, 1976.
Samuel Glasstone and Philip J. Dolan, eds., The Effects of Atomic Weapons, 3d ed., 1977.
Richard Rhodes , The Making of the Atomic Bomb, 1986.
Barton C. Hacker , The Dragon's Tail: Radiation Safety in the Manhattan Project, 1942–1946, 1988.
Chuck Hansen , U.S. Nuclear Weapons: the Secret History, 1988.
Donald Mackenzie , Inventing Accuracy: A Historical Sociology of Nuclear Missile Guidance, 1990.
Norman Polmar and Timothy M. Laur, eds., Strategic Air Command: People, Aircraft, Missiles, 2d ed., 1990.
Barton C. Hacker , Elements of Controversy: The Atomic Energy Commission and Radiation Safety in Nuclear Weapons Testing, 1947–1974, 1994.

Barton C. Hacker

Paul S. Boyer