There are two reasons. First, the world's supply of fossil fuels is finite and eventually man will have to find another source of energy. Second, and this seems to have been the compelling reason for the present rash proliferation of nuclear power reactors, the cost of nuclear electrical power was projected to be cheaper than that from conventional plants. The first reason is justifiable, although as we shall indicate later, it is not of immediate importance. The second reason and the basis of the present rash proliferation was wrong.
Nuclear power is more expensive.
When a uranium atom absorbs a neutron and undergoes fission, in addition to producing the two lighter elements, it also emits two or three neutrons. These neutrons can, in turn, react with other uranium atoms, which will undergo fission producing more neutrons. Since more neutrons are produced than consumed, it is possible, under the right circumstances for this reaction to proceed at an ever increasing rate and produce a tremendous amount of heat in a very short period of time. In other words, an uncontrolled nuclear reactor could literally blow itself up. Controlling a nuclear reactor therefore, means controlling the multiplication of neutrons in the reactor core.