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[link] shows a simple diagram of how a thermonuclear bomb is constructed. A fission bomb is exploded next to fusion fuel in the solid form of lithium deuteride. Before the shock wave blows it apart, γ size 12{γ} {} rays heat and compress the fuel, and neutrons create tritium through the reaction n + 6 Li 3 H + 4 He size 12{n+ rSup { size 8{6} } "Li" rightarrow rSup { size 8{3} } H+ rSup { size 8{4} } "He"} {} . Additional fusion and fission fuels are enclosed in a dense shell of 238 U size 12{ {} rSup { size 8{"238"} } U} {} . The shell reflects some of the neutrons back into the fuel to enhance its fusion, but at high internal temperatures fast neutrons are created that also cause the plentiful and inexpensive 238 U size 12{ {} rSup { size 8{"238"} } U} {} to fission, part of what allows thermonuclear bombs to be so large.

The bomb is in the shape of a capsule whose outer cover is made of uranium and inside it there are two sections, one of plutonium and uranium together in cylindrical shape covered with lithium deuteride and the other of plutonium beryllium, and both sections are immersed in Styrofoam.
This schematic of a fusion bomb (H-bomb) gives some idea of how the 239 Pu size 12{ {} rSup { size 8{"239"} } "Pu"} {} fission trigger is used to ignite fusion fuel. Neutrons and γ size 12{γ} {} rays transmit energy to the fusion fuel, create tritium from deuterium, and heat and compress the fusion fuel. The outer shell of 238 U size 12{ {} rSup { size 8{"238"} } U} {} serves to reflect some neutrons back into the fuel, causing more fusion, and it boosts the energy output by fissioning itself when neutron energies become high enough.

The energy yield and the types of energy produced by nuclear bombs can be varied. Energy yields in current arsenals range from about 0.1 kT to 20 MT, although the Soviets once detonated a 67 MT device. Nuclear bombs differ from conventional explosives in more than size. [link] shows the approximate fraction of energy output in various forms for conventional explosives and for two types of nuclear bombs. Nuclear bombs put a much larger fraction of their output into thermal energy than do conventional bombs, which tend to concentrate the energy in blast. Another difference is the immediate and residual radiation energy from nuclear weapons. This can be adjusted to put more energy into radiation (the so-called neutron bomb) so that the bomb can be used to irradiate advancing troops without killing friendly troops with blast and heat.

The figure shows three pie charts. The first shows the energy distribution of a conventional chemical bomb as ten percent thermal and ninety percent blast. The second shows fifty percent blast, thirty five percent thermal, ten percent delayed radiation, and five percent prompt radiation in the case of conventional nuclear bomb. The third shows forty percent blast, thirty percent prompt radiation, twenty five percent thermal, and five percent delayed radiation in the case of neutron bomb
Approximate fractions of energy output by conventional and two types of nuclear weapons. In addition to yielding more energy than conventional weapons, nuclear bombs put a much larger fraction into thermal energy. This can be adjusted to enhance the radiation output to be more effective against troops. An enhanced radiation bomb is also called a neutron bomb.

At its peak in 1986, the combined arsenals of the United States and the Soviet Union totaled about 60,000 nuclear warheads. In addition, the British, French, and Chinese each have several hundred bombs of various sizes, and a few other countries have a small number. Nuclear weapons are generally divided into two categories. Strategic nuclear weapons are those intended for military targets, such as bases and missile complexes, and moderate to large cities. There were about 20,000 strategic weapons in 1988. Tactical weapons are intended for use in smaller battles. Since the collapse of the Soviet Union and the end of the Cold War in 1989, most of the 32,000 tactical weapons (including Cruise missiles, artillery shells, land mines, torpedoes, depth charges, and backpacks) have been demobilized, and parts of the strategic weapon systems are being dismantled with warheads and missiles being disassembled. According to the Treaty of Moscow of 2002, Russia and the United States have been required to reduce their strategic nuclear arsenal down to about 2000 warheads each.

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Source:  OpenStax, Physics 101. OpenStax CNX. Jan 07, 2013 Download for free at http://legacy.cnx.org/content/col11479/1.1
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