Uranium-238, however, can be changed into an isotope of another element which will undergo fission and therefore "burn". Once again the neutron plays an essential part. When a neutron hits a uranium-238 nucleus it is absorbed; as a result, and after some internal rearrangement accompanied by the emission of particles, the nucleus is transmuted into a nucleus of an isotope of the element plutonium, plutonium-239. This new material is an even better nuclear fuel than uranium-235. In a similar manner a third nuclear fuel can be made by exposing the element thorium to neutrons; the thorium isotope of mass 232 absorbs neutrons and is transmuted into uranium-233, a fissile isotope of uranium which does not exist in nature.
These three then, uranium-235, plutonium-239 and uranium-233, are the fuels of the atomic age; the essential raw materials from which they are extracted or made by nuclear transmutation are natural uranium and thorium. The striking thing about these nuclear fuels, compared with ordinary chemical fuels, is the enormous amount of energy that is released for each pound of fuel burnt. Thus a pound of uranium, if all the atoms in it were made to undergo fission, would release as much energy as 3, 000,000 pounds (or 1,300 tons) of coal. Such complete utilization of uranium has yet to be realized in practice, though the fact that non-fissile uranium-238 can be transmuted into fissile plutonium implies that it is theoretically possible.
The energy released in fission is imparted in the first instance to the two fragments into which the nucleus is split, causing them to move apart with great speed. No way of using the energy of these fragments directly has yet been devised, but the motion through uranium heats the metal and this heat can be removed and converted to mechanical energy by a steam engine or gas turbine. In short, atomic energy is obtained by burning an uncommon fuel in an uncommon way and then using the heat obtained in a quite ordinary manner.
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