The examples of neutron capture just described are concerned with the disintegration of nuclei into two parts of unequal size. In 1939, Hahn and Strassman investigated the action of neutrons on uranium-235 and found that it was split into two roughly-ecuct pieces, one of which proved to be barium and the other krypton. Otto Frisch coined the expression nuclear fission to describe such cases as this. The importance of fission is that it is accompanied by the release of about ten times as much energy as in the case of ordinary disintegration, where only a small piece is broken off the nucleus. The uranium fission reaction was first used successfully to produce heat energy on a large scale by the Italian physicist, Enrico Fermi, at the University of Chicago in 1942.
In chapter 7 we have already described the fission process which goes on inside a commercial nuclear reactor used to produce heat for the production of electric energy. The reactions which go on inside these reactors are, of course, highly complex, but the one responsible for the bulk of the heat production is