Uranium is the heaviest chemical element found in nature. It has essentially two nuclides, uranium-235 and uranium-238. Uranium-235 has 235 subatomic particles or nucleons in its nucleus, with 92 protons that are positively charged and 143 neutrons that have no charge. Uranium-235 makes up 0.7% of the uranium found in nature, with the bulk of the remainder being uranium-238 which has 3 more neutrons per atom.

The nuclear fuel in the commercial reactors operating today requires the fission of a heavy atomic nucleus to release energy for power generation, as opposed to the fusion of light atomic nuclei that is still in active research. The elements found in nature that may undergo fission are limited to thorium-232, uranium-235 and uranium-238. Of the three, only uranium-235 has a much higher likelihood of undergoing fission, since it has fewer neutrons and therefore a smaller need to act with the electrostatic repulsive force found in the protons to overcome the short range strong nuclear force that binds the nucleons together and therefore to break up the atomic nucleus. The other two elements, namely thorium-232 and uranium-238, need first to be converted through neutron absorption into uranium-233 and plutonium-239 respectively before they may undergo fission with ease, similar to uranium-235.

A large number of the commercial nuclear reactors operating today make use of uranium fuel with a uranium-235 enrichment of 0.7 - 4.5%, with the remaining fuel material being uranium-238. While uranium-238 makes up of the bulk of uranium and thorium-232 is several times more abundant than uranium, the need to convert them into readily fissionable fuel has rendered them as nuclear fuel for the future, to be accompanied with the current development of a more advanced type of reactor so that these two resources may later be used.

Based on its “identified recoverable reserves” and the current rate of usage, uranium resources will be available for about 130 years, giving it a similar duration as iron and copper though somewhat shorter than aluminium. However, by also using uranium-238 and thorium for nuclear fission, the existing resources may be reasonably expected to provide energy for another few centuries.

This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.