The first sobering realization was that less than one percent of the uranium (the U-235 isotope) is susceptible to the chain reaction. And to get it in high enough concentration to keep the nuclear fire burning, it has to be separated from its heavier isotope (U-238). This is a difficult and costly operation, and a fair fraction of the energy that is gained from the U-235 when it is burned has already been spent in separating it.

Then, too, the uranium does not burn completely to ashes. As it is consumed, the fission residues begin to eat up more and more of the neutrons that the uranium produces, and this quenches the chain reaction. Long before it is used up, the fuel has to be taken out and replaced with fresh uranium-235.

However, there is a way of greatly extending the present supply. It is based on the fact that while uranium-235 is being used up, uranium-238 is being transmuted to plutonium. This can be separated chemically from the used fuel, and it makes an even better source of energy than the U-235. In reactors fueled by plutonium, it is possible to regenerate the fuel faster than it is used up, so that ultimately nearly all the uranium, rather than just a fraction of one percent, becomes available.

But there is a risk that hangs like an ominous cloud over all present and future programs. The same uranium that is used in power plants can be diverted to the manufacture of bombs. For this reason governments have maintained a tight monopoly on the plants that separate uranium-235, and keep a strict account of where the product goes.