Of the several hundred isotopes of the naturally occurring ninety-two chemical elements, only one, 235U, is capable of being fissioned. However, when 23SU or 232Th is subjected to neutron bombardment inside a nuclear reactor fired by 235U, these, after several nuclear transformations, end up as 289Pu and 233U, respectively. Both of the latter are fissionable. The iso­topes 235U, 289Pu, and 23SU are known accordingly as fissile isotopes. 238U and 232Th, which are not themselves fissile but are capable of being trans­formed into fissile isotopes, are said to be fertile materials.

Since the isotope 235U is the only naturally occurring fissile material, it follows that this isotope must be the initial fuel for any fission-power de­velopment. The source of 235U is natural uranium, which consists of the three isotopes, 23SU, 235U, and 234U, occurring in the fixed abundances of 99.238, 0.711, and 0.006 per cent, respectively. Since the abundance of 234U is negligible, natural uranium may be considered to consist of 238U and 236U in the ratio of 140 to 1.

In reactor technology, varying amounts of 238U or of 232Th may be converted into fissile isotopes which can be added to the initial fuel supply of 235U. The amount of this conversion is expressed by the conversion factor К defined by K — Q/Qo, where Q„ is the initial amount of fissile material contained in the reactor and its auxiliary equipment, and Q is the amount of fissile material remaining after the amount Q0 has been consumed. When К = 0, the reactor is said to be a burner; when 0 <K< 1, it is a con­verter; finally, when jK> 1, the reactor is a breeder.

Of these three types of reactors, the first two eventually exhaust any given initial amount of fissile material, and are capable of consuming only a fraction of the available fertile material. The breeder, however, is capa­ble of increasing the initial supply of fissile material at the expense of the fertile materials, and so is capable of completely consuming the latter. For this reason, only breeder reactors merit serious consideration in any long­term program of nuclear-fission power.

Historically, the first controlled fission reaction was that achieved at the University of Chicago on December 2, 1945. The first nuclear electric power was produced in 1951, and the first large-size nuclear power plant — that at Shippingport, Pennsylvania, with an initial capacity of 60 mega­watts—began operation in 1957. Since that time, nuclear power plants have been built in increasing sizes and numbers until by 1966 their total power capacity in the United States amounted to 1,800 megawatts. Re­cently it has been estimated by the Atomic Energy Commission that nu­clear power will reach 145,000 megawatts by 1980.

This would correspond to growth rate of 31 per cent per year, with a doubling period of but 2.4 years. For comparison, the total installed electric-power capacity in the United States in 1966 amounted to 233,000 megawatts, and this is estimated to increase at a rate of 6.5 per cent per year, to a figure of 579,000 megawatts by 1980.