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Also known as: H-bomb, hydrogen bomb

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Last Updated: Jan 17, 2025 • Article HistoryAlso called: hydrogen bomb, or H-bombKey People: Edward TellerIgor Vasilyevich KurchatovRelated Topics: thermonuclear warheadTeller-Ulam configuration

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What is the difference between a thermonuclear bomb and an atomic bomb?

Which is more powerful, a thermonuclear bomb or an atomic bomb?

Who developed the first thermonuclear bomb?

thermonuclear bomb, weapon whose enormous explosive power results from an uncontrolled self-sustaining chain reaction in which isotopes of hydrogen combine under extremely high temperatures to form helium in a process known as nuclear fusion. The high temperatures that are required for the reaction are produced by the detonation of an atomic bomb.

A thermonuclear bomb differs fundamentally from an atomic bomb in that it utilizes the energy released when two light atomic nuclei combine, or fuse, to form a heavier nucleus. An atomic bomb, by contrast, uses the energy released when a heavy atomic nucleus splits, or fissions, into two lighter nuclei. Under ordinary circumstances atomic nuclei carry positive electrical charges that act to strongly repel other nuclei and prevent them from getting close to one another. Only under temperatures of millions of degrees can the positively charged nuclei gain sufficient kinetic energy, or speed, to overcome their mutual electric repulsion and approach close enough to each other to combine under the attraction of the short-range nuclear force. The very light nuclei of hydrogen atoms are ideal candidates for this fusion process because they carry weak positive charges and thus have less resistance to overcome.

The hydrogen nuclei that combine to form heavier helium nuclei must lose a small portion of their mass (about 0.63 percent) in order to “fit together” in a single larger atom. They lose this mass by converting it completely into energy, according to Albert Einstein’s famous formula: E = mc². According to this formula, the amount of energy created is equal to the amount of mass that is converted multiplied by the speed of light squared. The energy thus produced forms the explosive power of a hydrogen bomb.More From Britannicanuclear weapon: Thermonuclear weapons

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Deuterium and tritium, which are isotopes of hydrogen, provide ideal interacting nuclei for the fusion process. Two atoms of deuterium, each with one proton and one neutron, or tritium, with one proton and two neutrons, combine during the fusion process to form a heavier helium nucleus, which has two protons and either one or two neutrons. In current thermonuclear bombs, lithium-6 deuteride is used as the fusion fuel; it is transformed to tritium early in the fusion process.

In a thermonuclear bomb, the explosive process begins with the detonation of what is called the primary stage. This consists of a relatively small quantity of conventional explosives, the detonation of which brings together enough fissionable uranium to create a fission chain reaction, which in turn produces another explosion and a temperature of several million degrees. The force and heat of this explosion are reflected back by a surrounding container of uranium and are channeled toward the secondary stage, containing the lithium-6 deuteride. The tremendous heat initiates fusion, and the resulting explosion of the secondary stage blows the uranium container apart. The neutrons released by the fusion reaction cause the uranium container to fission, which often accounts for most of the energy released by the explosion and which also produces fallout (the deposition of radioactive materials from the atmosphere) in the process. (A neutron bomb is a thermonuclear device in which the uranium container is absent, thus producing much less blast but a lethal “enhanced radiation” of neutrons.) The entire series of explosions in a thermonuclear bomb takes a fraction of a second to occur.

A thermonuclear explosion produces blast, light, heat, and varying amounts of fallout. The concussive force of the blast itself takes the form of a shock wave that radiates from the point of the explosion at supersonic speeds and that can completely destroy any building within a radius of several miles. The intense white light of the explosion can cause permanent blindness to people gazing at it from a distance of dozens of miles. The explosion’s intense light and heat set wood and other combustible materials afire at a range of many miles, creating huge fires that may coalesce into a firestorm. The radioactive fallout contaminates air, water, and soil and may continue years after the explosion; its distribution is virtually worldwide.

Thermonuclear bombs can be hundreds or even thousands of times more powerful than atomic bombs. The explosive yield of atomic bombs is measured in kilotons, each unit of which equals the explosive force of 1,000 tons of TNT. The explosive power of hydrogen bombs, by contrast, is frequently expressed in megatons, each unit of which equals the explosive force of 1,000,000 tons of TNT. Hydrogen bombs of more than 50 megatons have been detonated, but the explosive power of the weapons mounted on strategic missiles usually ranges from 100 kilotons to 1.5 megatons. Thermonuclear bombs can be made small enough (a few feet long) to fit in the warheads of intercontinental ballistic missiles; these missiles can travel almost halfway across the globe in 20 or 25 minutes and have computerized guidance systems so accurate that they can land within a few hundred yards of a designated target.

Edward Teller, Stanislaw M. Ulam, and other American scientists developed the first hydrogen bomb, which was tested at Enewetak atoll on November 1, 1952. The U.S.S.R. first tested a hydrogen bomb on August 12, 1953, followed by the United Kingdom in May 1957, China (1967), and France (1968). In 1998 India tested a “thermonuclear device,” which was believed to be a hydrogen bomb. During the late 1980s there were some 40,000 thermonuclear devices stored in the arsenals of the world’s nuclear-armed nations. This number declined during the 1990s. The massive destructive threat of these weapons has been a principal concern of the world’s populace and of its statesmen since the 1950s. See also arms control.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Meg Matthias.

nuclear fusion

Table of Contents

• Introduction
• The fusion reaction
• Methods of achieving fusion energy
• History of fusion energy research

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Also known as: atomic fusion, fusion, thermonuclear fusion

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Article HistoryKey People: Yevgeny Konstantinovich ZavoyskyHans BetheIgor Vasilyevich KurchatovLyman SpitzerGersh Itskovich Budker(Show more)Related Topics: fusion reactornuclear energyproton-proton chainnucleosynthesisCNO cycle

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nuclear fusion, process by which nuclear reactions between light elements form heavier elements (up to iron). In cases where the interacting nuclei belong to elements with low atomic numbers (e.g., hydrogen [atomic number 1] or its isotopes deuterium and tritium), substantial amounts of energy are released. The vast energy potential of nuclear fusion was first exploited in thermonuclear weapons, or hydrogen bombs, which were developed in the decade immediately following World War II. For a detailed history of this development, see nuclear weapon. Meanwhile, the potential peaceful applications of nuclear fusion, especially in view of the essentially limitless supply of fusion fuel on Earth, have encouraged an immense effort to harness this process for the production of power. For more detailed information on this effort, see fusion reactor.

This article focuses on the physics of the fusion reaction and on the principles of achieving sustained energy-producing fusion reactions.

The fusion reaction

Fusion reactions constitute the fundamental energy source of stars, including the Sun. The evolution of stars can be viewed as a passage through various stages as thermonuclear reactions and nucleosynthesis cause compositional changes over long time spans. Hydrogen (H) “burning” initiates the fusion energy source of stars and leads to the formation of helium (He). Generation of fusion energy for practical use also relies on fusion reactions between the lightest elements that burn to form helium. In fact, the heavy isotopes of hydrogen—deuterium (D) and tritium (T)—react more efficiently with each other, and, when they do undergo fusion, they yield more energy per reaction than do two hydrogen nuclei. (The hydrogen nucleus consists of a single proton. The deuterium nucleus has one proton and one neutron, while tritium has one proton and two neutrons.)

Fusion reactions between light elements, like fission reactions that split heavy elements, release energy because of a key feature of nuclear matter called the binding energy, which can be released through fusion or fission. The binding energy of the nucleus is a measure of the efficiency with which its constituent nucleons are bound together. Take, for example, an element with Z protons and N neutrons in its nucleus. The element’s atomic weight A is Z + N, and its atomic number is Z. The binding energy B is the energy associated with the mass difference between the Z protons and N neutrons considered separately and the nucleons bound together (Z + N) in a nucleus of mass M. The formula isB = (Zm_p + Nm_n − M)c²,where m_p and m_n are the proton and neutron masses and c is the speed of light. It has been determined experimentally that the binding energy per nucleon is a maximum of about 1.4 10⁻¹² joule at an atomic mass number of approximately 60—that is, approximately the atomic mass number of iron. Accordingly, the fusion of elements lighter than iron or the splitting of heavier ones generally leads to a net release of energy.

Two types of fusion reactions

Fusion reactions are of two basic types: (1) those that preserve the number of protons and neutrons and (2) those that involve a conversion between protons and neutrons. Reactions of the first type are most important for practical fusion energy production, whereas those of the second type are crucial to the initiation of star burning. An arbitrary element is indicated by the notation ^A_ZX, where Z is the charge of the nucleus and A is the atomic weight. An important fusion reaction for practical energy generation is that between deuterium and tritium (the D-T fusion reaction). It produces helium (He) and a neutron (n) and is writtenD + T → He + n.

To the left of the arrow (before the reaction) there are two protons and three neutrons. The same is true on the right.

The other reaction, that which initiates star burning, involves the fusion of two hydrogen nuclei to form deuterium (the H-H fusion reaction):H + H → D + β ⁺ + ν,where β ⁺ represents a positron and ν stands for a neutrino. Before the reaction there are two hydrogen nuclei (that is, two protons). Afterward there are one proton and one neutron (bound together as the nucleus of deuterium) plus a positron and a neutrino (produced as a consequence of the conversion of one proton to a neutron).

Both of these fusion reactions are exoergic and so yield energy. The German-born physicist Hans Bethe proposed in the 1930s that the H-H fusion reaction could occur with a net release of energy and provide, along with subsequent reactions, the fundamental energy source sustaining the stars. However, practical energy generation requires the D-T reaction for two reasons: first, the rate of reactions between deuterium and tritium is much higher than that between protons; second, the net energy release from the D-T reaction is 40 times greater than that from the H-H reaction.Load Next Page