Energy

The cycle of nuclear fuel

Nuclear fuel is subject to a cycle throughout its life. Obvious preliminaries are all the mining operations, which are followed by a long and complex series of various purification processes, with the primary aim of eliminating the elements that absorb neutrons. Neutrons are particles capable of starting the fission process by breaking the 235U nucleus with subsequent release of energy: if there are elements that absorb neutrons, these cannot produce fission reactions (“neutron poisons”). The operations undertaken in this first part of the fuel cycle are mainly of a chemical nature and lead to the production of a gaseous compound of uranium (uranium hexafluoride, UF₆) that allows the enrichment process of the isotope 235U. This phase is necessary since the majority of nuclear reactors uses fuel made of enriched uranium: on average the enrichment is around 3% of 235U, against 0.72% of 235U in the uranium found in nature. If we send the gaseous compound of uranium hexafluoride to a centrifuge it is possible to discriminate the different mass of the isotope 235U compared to the isotope 238U and it is possible to concentrate an isotope compared to another. Gaseous ultracentrifuges constitute the enrichment plants: other enrichment processes are possible through the gaseous diffusion plants or the laser selective isotopic separation.
The enriched hexafluoride is successively converted in uranium dioxide (UO₂) powder, which is assembled in pellets that, appropriately canned, will constitute the fuel element.
Nuclear fuel is thus inserted in nuclear reactors and produces energy until the end of its life. At this stage the fuel element has become radioactive and it is put into pools, usually near the reactor, in order to reduce the radioactivity level.
Exhausted fuel can have two different endings: the definitive deposit in areas with appropriate geological characteristics or reprocessing.
During its time inside the reactor not all 235U is burned (about 1% is left) and in the meantime, because of nuclear reactions, other nuclids have been born that can produce a nuclear fission reaction: fissile nuclei such as plutonium, 239PU, born from 238U through the “fertilisation” process. These can be used in turn as nuclear fuel, while the remaining fuel must be stocked in definitive deposits.
The reprocessing alternative, which is used by some countries like France and the UK, has some advantages: first of all it allows a more rational exploitation of fuel, allowing not only the recovery of the left over 235U but also the newborn 239Pu  that represents an extremely important resource because it descends through fertilisation from238U and represents the great majority of the uranium found in nature.
Secondly, the reprocessing allows to substantially reduce the volume of highly radioactive products that require long term stocking. Finally reusing already irradiated reduces considerably the risk of proliferation by making material treated twice unsuitable for the production of nuclear weapons.