The Nuclear Fuel Cycle : Generalities and Front End of the Cycle

The energy delivered by nuclear reactors is based on the fission of atomic nuclei by neutrons. This fission phenomenon is only possible for certain very specific heavy nuclei, known as fissile nuclei. Nuclear reactors must therefore be supplied with these fissile nuclei, packaged in various forms in an object called nuclear fuel. This term is in fact an extension of the one generally reserved for carbon-rich materials burned to produce energy in conventional plants, known as fossil fuels (essentially coal, peat, gas and oil). The analogy ends there, as the basic phenomena are totally different in the two cases: chemical reactions for fossil fuels, involving the electronic forces of atoms, and nuclear reactions for nuclear fuels, involving the forces contained in atomic nuclei.

This is an opportunity to underline the enormous gap that exists in quantitative terms between these two forms of energy. Remember that the fission of an atomic nucleus releases a quantity of energy roughly equal to 200 MeV, whereas the energy released by any chemical reaction is at most of the order of a few dozen electron volts. Consequently, in the field of nuclear energy, and on an industrial scale, we express ourselves in terms of tonnes or at most thousands of tonnes of fuel, whereas for fossil fuels the quantities involved run into millions or even hundreds of millions of tonnes.

Nuclear fuel is therefore basically made up of fissile atomic nuclei. However, in nature there is only one nucleus with this property: uranium 235 (noted ²³⁵ U), which is one of the two isotopes making up natural uranium (noted U _nat ), the other being uranium 238 (noted ²³⁸ U). Thus, it is U _nat that constitutes the basic natural element used to manufacture the fuels used in nuclear reactors. This natural element, like most other natural materials used to manufacture industrial products, has to be searched for on the surface of the globe to locate economically profitable deposits, and then extract the desired material. As this material is never pure in its natural state, it has to be purified and then transformed using a variety of physical and chemical processes to obtain an elaborate product that meets the desired needs and standards. Finally, this elaborated product must be packaged in the desired form, in this case, a nuclear fuel directly loaded into the core of a nuclear reactor, to produce energy. All these stages are called the "upstream fuel cycle", and will be discussed in section