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Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package〔The physics package is the nuclear explosive module inside the bomb casing, missile warhead, or artillery shell, etc., which delivers the weapon to its target. While photographs of weapon casings are common, photographs of the physics package are quite rare, even for the oldest and crudest nuclear weapons. For a photograph of a modern physics package see W80.〕 of a nuclear weapon to detonate. There are three existing basic design types. In most existing designs, the explosive energy of deployed devices is derived primarily from nuclear fission, not fusion. *Pure fission weapons were the first nuclear weapons built and have so far been the only type ever used in warfare. The active material is fissile uranium (uranium with a high percentage of U-235) or plutonium (Pu-239), explosively assembled into a chain-reacting critical mass by one of two methods: * *Gun assembly: one piece of fissile uranium is fired at a fissile uranium target at the end of the weapon, similar to firing a bullet down a gun barrel, achieving critical mass when combined. * *Implosion: a fissile mass of either material (U-235, Pu-239, or a combination) is surrounded by high explosives that compress the mass, resulting in criticality. :The implosion method can use either uranium or plutonium as fuel. The gun method only uses uranium. Plutonium is considered impractical for the gun method because of early triggering due to Pu-240 contamination and due to its time constant for prompt critical fission being much shorter than that of U-235. *Boosted fission weapons improve on the implosion design. The high pressure and temperature environment at the center of an exploding fission weapon compresses and heats a mixture of tritium and deuterium gas (heavy isotopes of hydrogen). The hydrogen fuses to form helium and free neutrons. The energy release from this fusion reaction is relatively negligible, but each neutron starts a new fission chain reaction, speeding up the fission and greatly reducing the amount of fissile material that would otherwise be wasted when expansion of the fissile material stops the chain reaction. Boosting can more than double the weapon's fission energy release. *Staged thermonuclear weapons are essentially a chain of fusion-boosted fission weapons, usually with only two stages in the chain. The secondary stage is imploded by x-ray energy from the first stage, called the "primary." This radiation implosion is much more effective than the high-explosive implosion of the primary. Consequently, the secondary can be many times more powerful than the primary, without being bigger. The secondary can be designed to maximize fusion energy release, but in most designs fusion is employed only to drive or enhance fission, as it is in the primary. More stages could be added and conceptual designs incorporating up to seven have been produced, but the result would be a multi-megaton weapon too powerful to serve any plausible purpose.〔. Article on the Soviet Tsar Bomba test. Because explosions are spherical in shape and targets are spread out on the relatively flat surface of the earth, numerous smaller weapons cause more destruction. From page 35: ". . .five five-megaton weapons would demolish a greater area than a single 50-megatonner."〕 (The United States briefly deployed a three-stage 25-megaton bomb, the B41, starting in 1961. Also in 1961, the Soviet Union tested, but did not deploy, a three-stage 50–100 megaton device, Tsar Bomba.) *Pure fusion weapons have not been invented. Such weapons, though, would produce far less radioactive fallout than current designs, although they would release huge amounts of neutrons. Pure fission weapons historically have been the first type to be built by a nation state. Large industrial states with well-developed nuclear arsenals have two-stage thermonuclear weapons, which are the most compact, scalable, and cost effective option once the necessary industrial infrastructure is built. Most known innovations in nuclear weapon design originated in the United States, although some were later developed independently by other states;〔The United States and the Soviet Union were the only nations to build large nuclear arsenals with every possible type of nuclear weapon. The U.S. had a four-year head start and was the first to produce fissile material and fission weapons, all in 1945. The only Soviet claim for a design first was the Joe 4 detonation on August 12, 1953, said to be the first deliverable hydrogen bomb. However, as Herbert York first revealed in ''The Advisors: Oppenheimer, Teller and the Superbomb'' (W.H. Freeman, 1976), it was not a true hydrogen bomb (it was a boosted fission weapon of the Sloika/Alarm Clock type, not a two-stage thermonuclear). Soviet dates for the essential elements of warhead miniaturization – boosted, hollow-pit, two-point, air lens primaries – are not available in the open literature, but the larger size of Soviet ballistic missiles is often explained as evidence of an initial Soviet difficulty in miniaturizing warheads.〕 the following descriptions feature U.S. designs. In early news accounts, pure fission weapons were called atomic bombs or A-bombs, a misnomer since the energy comes only from the nucleus of the atom. Weapons involving fusion were called hydrogen bombs or H-bombs, also a misnomer since their energy comes mostly from fission. Insiders favored the terms nuclear and thermonuclear, respectively. The term thermonuclear refers to the high temperatures required to initiate fusion. It ignores the equally important factor of pressure, which was considered secret at the time the term became current. Many nuclear weapon terms are similarly inaccurate because of their origin in a classified environment. ==Nuclear reactions== Nuclear fission splits heavier atoms to form lighter atoms. Nuclear fusion bonds together lighter atoms to form heavier atoms. Both reactions generate roughly a million times more energy than comparable chemical reactions, making nuclear bombs a million times more powerful than non-nuclear bombs, which a French patent claimed in May 1939.〔.〕 In some ways, fission and fusion are opposite and complementary reactions, but the particulars are unique for each. To understand how nuclear weapons are designed, it is useful to know the important similarities and differences between fission and fusion. The following explanation uses rounded numbers and approximations.〔The main source for this section is Samuel Glasstone and Philip Dolan, ''The Effects of Nuclear Weapons'', Third Edition, 1977, U.S. Dept of Defense and U.S. Dept of Energy (see links in General References, below), with the same information in more detail in Samuel Glasstone, ''Sourcebook on Atomic Energy'', Third Edition, 1979, U.S. Atomic Energy Commission, Krieger Publishing.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nuclear weapon design」の詳細全文を読む スポンサード リンク
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