Thus, the yield of a nuclear weapon can be increased by surrounding the device with uranium-238, in the form of either natural or depleted uranium, or with thorium-232, in the form of natural thorium. Uranium-238 and thorium-232 (and some other fissionable materials) cannot maintain a self-sustaining fission explosion, but these isotopes can be made to fission by an externally maintained supply of fast neutrons from fission or fusion reactions. The secondary typically contains a composite of fusion and fissile materials carefully tailored to maximize the yield-to-weight or yield-to-volume ratio of the warhead, although it is possible to construct secondaries from purely fissile or fusion materials. Boosted primaries in modern thermonuclear weapons contain about 3 to 4 kg (6.6 to 8.8 pounds) of plutonium, while less-sophisticated designs may use double that amount or more. In an efficient, modern two-stage device-such as a long-range ballistic missile warhead-the primary is boosted in order to conserve on volume and weight. Thus, the fission and fusion processes that take place in the secondary are generally much more efficient than those that take place in the primary. Compression of the spark plug results in a fission explosion that creates temperatures comparable to those of the Sun and a copious supply of neutrons for fusion of the surrounding, and now compressed, thermonuclear materials. With the fission primary generating an explosive yield in the kiloton range, compression of the secondary is much greater than can be achieved using chemical high explosives. Typically, contained within the pusher is some fusion material, such as lithium-6 deuteride, surrounding a “spark plug” of explosive fissionable material (generally uranium-235) at the centre. The total pressure in the cavity-the sum of the contribution from the very energetic particles and the generally smaller contribution from the radiation-is applied to the secondary capsule’s heavy metal outer shell (called a pusher), thereby compressing the secondary. Initial, reflected, and re-irradiated radiation trapped within this cavity is absorbed by lower-density material within the cavity, converting it into a hot plasma of electrons and ion particles that continue to absorb energy from the confined radiation. Subsequent radiation from the primary is largely confined between this boundary and the outer surface of the secondary capsule. Radiation absorption heats the inner surface of the case, turning it into an opaque boundary of hot electrons and ions. Some of the initial radiation from the primary explosion is absorbed by the inner surface of the case, which is made of a high-density material such as uranium. Radiation from the fission explosion of the primary is contained and used to transfer energy to compress and ignite the secondary. Both primary and secondary are contained within an outer metal case. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!Ī typical thermonuclear warhead may be constructed according to a two-stage design, featuring a fission or boosted-fission primary (also called the trigger) and a physically separate component called the secondary.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.Britannica Classics Check out these retro videos from Encyclopedia Britannica’s archives.Britannica Explains In these videos, Britannica explains a variety of topics and answers frequently asked questions.
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