Atomic bomb - The world is not ready ...

Atomic bomb - A device for suddenly producing an explosive neutron chain reaction in a fissile material such as uranium-235 (235U) or plutonium-239 (239Pu). In a wider sense, any explosive device that derives its energy from nuclear reactions, including not only the foregoing fission weapon but also a fusion weapon, which gets its energy largely from fusion reactions of heavy hydrogen isotopes, and a fission-fusion weapon, which derives its energy from both fission and fusion.

Because an atomic bomb derives its energy from nuclear reactions, it is properly called a nuclear explosive or nuclear weapon.
of the two principal fissile materials, the cheaper but less potent 235Uispresent in natural uranium usually in the proportion of 1 part to 139 parts of 238U and is separated from it by various enrichment processes. Weapons-grade plutonium is manufactured from 238Uina special military production reactor that has enough excess neutrons for the reaction below.

U + n - 239U (23-min half-life) 239Np(2.3-day half-life) 239Pu

Weapon cores are made of very high fractions of fissile materials: highly enriched 93% uranium-235 or weapon-grade 94% plutonium-239.

A fission bomb before ignition consists of a mass of fissile material and surrounding tamper—beryllium oxide or other reflector of neutrons intended ultimately to improve the neutron multiplication factor k—arranged in a geometry so favorable to neutron leakage that k is less than 1. These materials are suddenly compressed into a geometry where k substantially exceeds 1. This is done with chemical explosives that either implode a spherical subcritical mass of fission material or else drive two subcritical sections together in a gun-barrel type of arrangement. At the same time, enough neutrons are artificially introduced to start an explosively divergent (expanding) chain reaction. Fission-explosive devices intended for military application are highly sophisticated combinations of pure materials, precise design, and reliable electronics.

The explosive energy (yield) of a nuclear weapon is usually expressed in kilotons or megatons. A kiloton is the amount of energy liberated in the explosion of 1000 tons of TNT (1012 calories or 4.18 x 1012 J), and a megaton is a thousand times as large. The fission bombs that destroyed Hiroshima (gun-barrel type) and Nagasaki (implosion type) had estimated explosive yields of 13 and 22 kilotons, respectively. Fractional kiloton yields can be obtained (tactical nuclear weapons). Fission weapons have been tested up to approximately 500 kilotons, overlapping the yield of multistage fusion explosives (strategic nuclear weapons).

The nuclear explosive energy is communicated by mechanical shock and radiative transport to the surrounding water, earth, or air, ionizing it out to a radius which, in the case of explosions in air, is known as the fireball radius (150 yd or 140 m in about 1 s after a 20-kiloton nuclear explosion). Energy goes out from such a fireball into the surrounding relatively transparent air, in not very different orders of magnitude in the form of a shock wave and in the form of heat radiation that may continue for a number of seconds.


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