Search this website:


This web page location: NeutronNeutrons in Atomsnuclear chain reaction, deuterium, MeV, nuclear reactor, neutronsThe two key characteristics of nuclear fission important for the practical release of nuclear energy are both evident in equation (2). First, the energy per fission is very large. In practical units, the fission of 1 kg (2.2 lb) of uranium235 releases 18.7 million kilowatthours as heat. Second, the fission process initiated by the absorption of one neutron in uranium235 releases about 2.5 neutrons, on the average, from the split nuclei. The neutrons released in this manner quickly cause the fission of two more atoms, thereby releasing four or more additional neutrons and initiating a selfsustaining series of nuclear fissions, or a chain reaction, which results in continuous release of nuclear energy. Naturally occurring uranium contains only 0.71 percent uranium235; the remainder is the nonfissile isotope uranium238. A mass of natural uranium by itself, no matter how large, cannot sustain a chain reaction because only the uranium235 is easily fissionable. The probability that a fission neutron with an initial energy of about 1 MeV will induce fission is rather low, but the probability can be increased by a factor of hundreds when the neutron is slowed down through a series of elastic collisions with light nuclei such as hydrogen, deuterium, or carbon. This fact is the basis for the design of practical energyproducing fission reactors. In December 1942 at the University of Chicago, the Italian physicist Enrico Fermi succeeded in producing the first nuclear chain reaction. This was done with an arrangement of natural uranium lumps distributed within a large stack of pure graphite, a form of carbon. In Fermi's “pile,” or nuclear reactor, the graphite moderator served to slow the neutrons. Article key phrases: 

Search this website:
