Uranium and Plutonium

Whereas the U-235 atom is 'fissile', the U-238 atom is said to be 'fertile'. This means that it can capture one of the neutrons which are flying about in the core of the reactor and become (indirectly) plutonium-239, which is fissile. Pu-239 is very much like U-235, in that it fissions when hit by a neutron and this also yields a lot of energy. Because there is so much U-238 in a reactor core (most of the fuel), these reactions occur frequently, and in fact about one third of the energy yield comes from "burning" Pu-239. But sometimes a Pu-239 atom simply captures a neutron without splitting, and it becomes Pu-240. Because the Pu-239 is either progressively "burned" or becomes Pu-240, the longer the fuel stays in the reactor the more Pu-240 is in it

The significance of this is that when the spent fuel is removed after about three years, the plutonium in it is not suitable for making weapons but can be recycled as fuel.

From uranium ore to reactor fuel

Uranium ore can be mined by underground or open-cut methods, depending on its depth. After mining, the ore is crushed and ground up. Then it is treated with acid to dis­solve the uranium, which is then recovered from solution. Uranium may also be mined by in situ leaching (ISL), where it is dissolved from the orebody in situ and pumped to the surface. The end product of the mining and milling stages, or of ISL, is uranium ox­ide concentrate (U₃ 0₈). This is the form in which uranium is sold. Before it can be used in a reactor for electricity generation, however, it must undergo a series of processes to produce a useable fuel.

For most of the world's reactors, the next step in making useable fuel is to convert the uranium oxide into gas, uranium hexafluoride (UF^, which enables it to be enriched. Enrich­ment increases the proportion of the uranium-235 isotope from its natural level of 0.7% to 3 - 4%. This enables greater technical efficiency in reactor design and operation, particularly in larger reactors, and allows the use of ordinary water as a moderator. After enrichment, the UF₆ gas is converted to uranium dioxide (U0₂) which is formed into fuel pellets. These fuel pellets are placed inside thin metal tubes which are assembled in bundles to become the fuel elements for the core of the reactor. For reactors which use natural uranium as their fuel (and hence which require graphite or heavy water as a moderator) the U₃O_g concentrate simply needs to be refined and converted directly to uranium dioxide. Spent reactor fuel is removed, stored, and then either reprocessed or disposed of underground.