Nuclei which are fissile under bombardment by thermal neutrons are named fissile nuclei. These are the only ones which are able to sustain a chain reaction, even when fast neutrons are used. Fertile nuclei give birth to fissile nuclei after neutron absorption, followed by one or two decays. The only fissile nucleus found in nature is Uranium 235, present at a concentration of 0,7 % in natural uranium, the main component of which is Uranium 238. This fertile nucleus produces, after neutron capture and 2 beta decays Plutonium 239, which is fissile. The pair Uranium 238-Plutonium 239 forms the Uranium-Plutonium cycle. Thorium is about four times more abundant in nature than Uranium. Only one Thorium isotope, Thorium 232 is found in nature. This fertile nucleus produces, after neutron capture and two beta decays the fissile Uranium 233. The cycle is then known as the Thorium-Uranium cycle . Present PWR reactors burn essentially Uranium 235. The corresponding energy reserves, at market prices, are not more abundant than those of oil. This is why, very soon, the use of breeding reactors using the Uranium-Plutonium cycle was considered. The possibility to obtain breeding depends on the hardness of the neutron spectrum. Indeed, for the Uranium-Plutonium cycle only ''fast'' reactors with an average neutron energy of several hundreds of KeV allow breeding. These fast reactors use liquid metals as coolant, more precisely, liquid sodium. An example of such a reactor is the French Superphenix, which although it worked only a small fraction of the expected time, was able to demonstrate breeding. From the few months of operation it could be deduced that the number of fissile nuclei should be multiplied by two after 4 years. This amount of time is known as the doubling time. While current reactors like the PWR can only use about 1% of natural Uranium, breeding reactors would allow a full use of it. It would, then, be economical to exploit minerals with a very small concentration of Uranium, maybe, even, to extract Uranium from sea water. Uranium reserves would, then, be close to 10000 times more important than those available for present PWR.

The Thorium-Uranium cycle allows breeding even with thermal neutron spectra. However, in this case, the doubling time reaches 20 years. Using fast spectra would shorten this time, although it would still be longer than for the Uranium-Plutonium cycle. This is one of the reasons why the Thorium-Uranium cycle has not been as popular as the Uranium-Plutonium cycle. Furthermore, only the Uranium-Plutonium cycle could be started from scratch, due to the presence of Uranium 235 in natural Uranium. Thus the only fissile species available for building the first atom bombs were Uranium 235 and Plutonium 239. Accelerator technology would not have allowed, at that time, the production of enough Uranium 233 for building an atom bomb. We all know how much nuclear energy production has largely been determined by military nuclear applications.

Information provided by: http://isnwww.in2p3.fr