Physical and chemical properties

The limitations of uranium fuel naturally led to the~ search for a suitable substitute. A number of com­pounds of uranium have been tried in various ex­perimental reactors but the only one in widespread commercial use is uranium dioxide, UO2. Since the density of UO2 is less than that of pure uranium (ll. O compared to 18.5 g/cc), it is normal to increase the concentration of fissile atoms by enrichment. The dioxide is black/brown ceramic with a melting point of 2800°C; it has a cubic (fluorite) crystal structure and, because uranium can take any of a number of valance states, it has a variable composition capable of existing with an excess or a dearth of oxygen atoms the hyper and hypostoichiometric forms respectively. This variable stoichiometry is important in that it affects diffusion, including self-diffusion, which in turn controls such processes as sintering and creep. Chemically, UO; shows little reaction with stainless steel or zirconium cladding.

Uranium dioxide pellets are generally made from powder by cold pressing followed by sintering at high temperatures, typically 1600-1800°C. The choice of route and the presence or absence of binder material at the pressing stage are likely to affect pellet dimen­sional tolerance, grain size and density. The finished pellet is usually in the form of an approximately right cylinder with less than 5% porosity; PWR pellets have a diameter of about 9 mm whilst CAGR pel­lets are hollow with an outside diameter of 14.5 mm and a bore diameter of 5-7 mm. Most designs re­quire pellets to have slightly dished ends since this ensures that the thermal expansion of the pellet stack is controlled by their surface, rather than their centre, temperature.