Uranium ore contains approximately 0.15% equivalent of U308. This is processed and concentrated in a mill situated near the mine. After precipitation in the form of sodium diuranate or magnesium diuranate the product, because of its appearance, is called “yellow cake”. In order to be enriched uranium has then to be converted in UF6 which is a gaseous product. The natural uranium which contains 0.71% 235U is then enriched to 93% for use in HTRs with Th cycle, or to 5-8% for use in HTRs with low enriched U cycle.

Fuel fabrication takes place over a period of several months before loading into the reactor. The in-core residence time may vary between 3 to 6 years; after a cooling period the spent fuel is then shipped to a reprocessing plant where the fissile materials are separated from the other isotopes.

A typical fuel cycle scheme is shown in Fig. 10.1.<M) Figure 10.2<l4) shows the time sequence of a typical fuel segment. The name fuel segment is given to a fraction of the core which is loaded and discharged together (sometimes the term batch is used to indicate a segment).