Fissile Nuclides As a Renewable Resource

Uranium's two commonest isotopes are ²³⁸U and ²³⁵U, of which only the latter is easily fissile by neutrons. Another fissile isotope,²³³U, can be made from thorium. A fissile nucleus can be split when a neutron strikes it. Because of the weird rules of quantum mechanics, it behaves like a target of a certain subatomic radius which varies according to the speed or kinetic energy of the neutron. Strangely enough, the apparent cross-section of the target for the fission event is a maximum for relatively slow neutrons. But the rest of the uranium, ²³⁸U, can capture a neutron and for an instant becomes ²³⁹U, which decays to neptunium ²³⁹Np, and within days to ²³⁹Pu, a fissile isotope of plutonium. The apparent cross-section for this capture process goes up with the energy of the neutrons, i.e. the faster the better.

It follows that, if we are not pathologically afraid of plutonium, we can consider fissile uranium as an energy resource (of great compactness) and regard any technique that will produce plutonium from uranium as a renewable resource technology.

The technique for doing so is called a breeder reactor, which arranges that the neutrons which are produced in every fission event are likely either to produce another fission, or to perform the nuclear transformation that makes more fissile fuel from the non-fissile bulk of the uranium in the fuel rods. If operated efficiently for energy production, the plutonium also contains the next isotope, ²⁴⁰Pu, which spoils it for military use. So it is false to say that it is a proliferation-prone technology.

In principle, the same sort of thing can be done with the element thorium, which occurs as ²³²Th, and can be converted to the fissile ²³³U. India has high grade thorium ore, and is planning to use it for a sustainable nuclear power program.
Curiously enough, the amount of coal necessary to fuel a one gigawatt coal power station for a year produces ash containing enough thorium to fuel a nuclear plant of equal capacity for more than a year, if it were refined and neutron irradiated. Coal plant ash is not the best source of thorium, but this illustrates the grossness of the quantities of coal needed to match nuclear power plants. The waste products of coal burning are equally gigantic in quantity compared to nuclear waste.

Nuclear Engines

Compactness

A nuclear-powered submarine or aircraft carrier carries enough fuel for three years or so of seagoing duty, enough to visit every ocean on Earth, in a compartment that is far smaller than an adequate coal bunker or oil tank. A pound of concentrated fissile uranium serves the engines better than would a million gallons of oil for a non-nuclear vessel.