The possibility of obtaining power from nuclear sources rests on two atomic properties: First the atoms of certain isotopes near the heavy end of the atomic scale are instable and when struck by neutrons are capable of fissioning — that is, of dividing into two roughly equal parts consisting of the nuclei of atoms in the midrange of the atomic scale. Second, atoms at the light end of the atomic scale, particularly hydrogen and its heavier isotopes deuterium and tritium, are capable of being fused under extreme temperature conditions into the heavier element helium.

In each case, a very large amount of energy is released. The energy released by a single fission of 236U amounts on the average to 3.20 X 10_1J thermal joules. The fissioning of 1 gram of this isotope releases 8.19 x 1010 joules of thermal energy, which is equivalent to the heat of combus­tion of 2.7 metric tons of coal, or to 13.7 barrels of crude oil. The energy released per fission of other heavy isotopes is substantially the same as that for 235U.

For fusion, several different reactions are possible, with slightly dif­fering amounts of energy released. For the fusion of deuterium to helium, the energy released per deuterium atom amounts to 7.94 x 10 13 joules.

At present, large power plants based on controlled fission are already in operation, and a number of others with capacities up to 1,000 mega­

watts are under construction. As yet, however, controlled steady-state fu­sion has not been achieved.