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Isotopic transformation of uranium

Study of the receiving/expedition/control building at the Georges Besse enrichment plant

Natural uranium is composed of 2 isotopes, uranium-238 and uranium-235, which is the only one that is fissile and usable in nuclear reactors. But the amount of uranium-235 in natural uranium is too low to drive a nuclear reactor. It is therefore necessary to enrich it to increase the proportion of uranium-235.

What is an isotope?

The nature of an atom is determined by the number of protons contained in its nucleus, from 1 for hydrogen to 92 for uranium. Its physical and chemical properties largely depend on this. Atoms are also classified according to this number, called the atomic number.

Two atoms with the same number of protons but a different number of neutrons are called isotopes of the element. For example, U₂₃₅ and U₂₃₈ are 2 isotopes of uranium. They each contain 92 protons, but U₂₃₅ has 143 neutrons and U₂₃₈ has 146 neutrons. Certain isotopes are present in nature and others are man-made.

The isotopes of an element can have different physical properties. For example, hydrogen (1 proton) is not radioactive. Its isotope deuterium (1 proton + 1 neutron) isn't, either, but its other isotope tritium (1 proton + 2 neutrons) is. Isotopes have the same chemical properties as their element.

Increasing the uranium-235 content within natural uranium

Between the 2 isotopes of natural uranium, uranium-238 and uranium-235, only the latter is fissile and can be used in nuclear reactors. Uranium-235 is present at a level of 0.7% in natural uranium, but this level is insufficient to permit the fuel reactivity needed for the function of nuclear reactors.

Enrichment thus consists of increasing the uranium-235 content in natural uranium to a level between 3 and 5%, in order to allow fuel reactivity adapted to reactor needs.

Starting from uranium hexafluoride (UF₆) originating at COMURHEX Pierrelatte, the enrichment plant uses isotopic separation to produce:

enriched uranium with a uranium-235 content raised to a level of 3 to 5% depending on customer requirements. Enriched uranium is used in 90% of nuclear reactors in operation in the world today.

depleted uranium, of which the uranium-235 content is between 0.2 and 0.5%. This is converted into uranium oxide (U3O₈) so that it can be stored safely. This represents a reserve of energy materials. This operation, called "defluorination", is carried out by the Chemistry BU.

The 2 processes of enrichment currently used on an industrial scale around the world are gaseous diffusion and centrifugation.