Fission-type nuclear bombs

Today nuclear war is probably the most awesome threat facing civilization. Only two rather primitive fission-type atom bombs were used to destroy the Japanese cities of Hiroshima and Nagasaki and bring World War II to an early end. The threat of nuclear war is increased by the fact that the number of nations possessing nuclear weapons is steadily increasing. 

This year there were completed the first three atomic (or nuclear) bombs. The first bomb was successfully exploded in the New Mexico desert on July 16, 1945, and a second over Hiroshima, Japan on Aug 6, 1945, and the third over Nagasaki on Aug 9, 1945. The bombs were of Fission Type and of tens of kilotons (thousands of tons of TNT equivalent) (Vol 1 of Encycl, p A499-L)... 

Atomic (or Nuclear) Bomb. A weapon invented during WWII and developed in the United States as a joint effort with the British and Canadian governments. It utilizes for its destructive effect the energy of an Atomic or Nuclear Explosion (qv). Since atomic explosions are of two types, fission and fusion, atomic bombs are of. corresponding types. However, it has been necessary to first initiate an atomic explosion with a nuclear fission reaction in order to bring about the conditions under which a nuclear fusion(thermonuclear) reaction can occur. 

Perhaps you are familiar with the terms nuclear fission and nuclear fusion. Nuclear fission is the process by which a relatively massive nucleus is divided into smaller nuclei and one or more neutrons. Nuclear fission is the process that generates so much power in nuclear power plants and in certain types of nuclear weapons, such as the bombs that were dropped on Japan in 1945. The following equation shows an example of nuclear fission (the nuclear fission of uranium-235) ... 

Notice that the reaction consumes one neutron, but the reaction releases three neutrons. Those three neutrons are then free to initiate additional fission reactions. This type of situation in which there is a multiplier effect is a chain reaction. We can use isotopes that undergo chain reaction in both the production of bombs and in nuclear power plants. U-235 is fissionable, but U-238 is not. There is a certain minimum quantity of fissionable matter needed to support a chain reaction, the critical mass. 

The distinction between these two types of weapons is blurred because they are combined in almost all advanced modern weapons. For example, a smaller fission bomb is first used to create necessary conditions of high temperature and pressure which are required for fusion. Similarly, fusion elements may also be present in the core of fission devices as well because they generate additional neutrons which increase efficiency of the fission reaction. Further, most of the fusion weapons derive substantial portion of their energy from a final stage of fissioning which is facilitated by the fusion reactions. The simplest nuclear weapons are pure fission bombs. They were the first type of nuclear weapons built during the American Manhattan Project and are considered as a building block for all advanced nuclear weapons. 

Now, much of the emphasis, in terms of national security, has shifted to the use of unconventional weapons unleashed on civilian targets by terrorists. Sometimes referred to as superterrorism, this includes the possible use of nuclear (that is, a fission-reaction explosion), radiological (as in the so-called dirty bomb), biological, or chemical weapons. In the context of terrorism, this chapter concerns itself with the latter two types. 

The destructive power of nuclear weapons derives from the core of the atom, the nucleus. One type of nuclear weapon, the fission bomb, uses the energy released when nuclei of heavy elements such as plutonium fission (split apart). A second even more powerful type of nuclear weapon, the fusion or hydrogen bomb, uses the energy released when nuclei of hydrogen are united (fused together). 

The hydrogen bomb uses a similar type of fusion reaction as its source of energy. A conventional nuclear fission bomb is used as the heat source to start the fusion (thermonuclear) reaction. It may some day be possible to ignite a thermonuclear bomb reaction without a fission bomb, but at this time, no one has a practical notion as to how it might be accomplished. As a result, fusion reactors cannot lead to the production of hydrogen bombs. 

Plutonium can also be used to power a nuclear fission reactor. Plutonium-239 undergoes a chain reaction that eventually produces more pluto-nium-239 from uranium-238, as well as heat that is used to generate electricity. This type of fission reactor, which produces fissionable material as it operates, is called a breeder reactor. A breeder reactor can actually produce more fissionable material than it uses. The construction of this type of reactor has been discouraged in many countries because of the health hazard that plutonium presents, as well as the fact that the plutonium-239 generated can be used to make powerful fission bombs. 

In 1940 he and his colleagues Edwin McMillan, Arthur Wahl, and Joseph Kennedy succeeded in isolating plutonium (Pu) as a product of the reaction of uranium with neutrons. We will talk about reactions of this type, called nuclear reactions, in Chapter 21. We wUl also discuss the key role that plutonium pla) in nuclear fission reactions, such as those that occur in nuclear power plants and atomic bombs. 

It should be stressed that the explosion that happened at Chernobyl was a physical steam explosion, not a nuclear explosion. Nuclear explosions are the result of the controlled reaction (fission or fusion) of highly enriched, bomb-grade nuclear material. The fuel used in nuclear power plants is not enriched enough to generate a nuclear explosion, While radioactive material can easily be released during a nuclear power plant accident, it is simply not possible for power plants to generate bomb-type, Hiroshima-like nuclear explosions. 

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