It is found that, even if the plutonium is initially distributed uni­formly throughout the fuel, after only a short irradiation (less than 1% burnup) the relative concentrations of uranium and plutonium have changed, but the nature of the redistribution depends on x. Figure 2.14 shows experimental values of plutonium concentration at the centre of the fuel after irradiation, indicating that for x < -0.04 the plutonium moves outwards, whereas for x > -0.04 it moves inwards. In contrast the plutonium in the outer, cooler, part of the fuel is hardly affected. The distribution of plutonium in irradiated fuel with x = 0 is shown in Figure 2.15.

The mechanism for this redistribution seems to depend on the preferential evaporation and migration of different species. The stoi­chiometric ratio is important because if there is excess oxygen the mixture of vapours in equilibrium with the mixed oxide contains relat­ively more UO3, whereas if oxygen is deficient it contains more PuO, and for x < -0.03 or -0.04 PuO predominates. Thus for more positive values of x it is uranium in the form of UO3 that tends to evaporate from the hotter part of the fuel and condense in the cooler, whereas for more negative x plutonium in the form of PuO moves instead. This is not the whole story, however, because it does not explain why the segregation is so limited in extent. It may be that diffusion of vapour through the dense columnar grain region is in fact severely restricted.

If the segregation of plutonium and uranium were more severe it would have serious consequences. Because the plutonium generates most of the power its extensive migration to the centre would increase the fuel central temperature and possibly cause melting. It would also have the effect of delaying the Doppler feedback on reactivity (see section 2.3.2). For the limited segregation that is actually observed these effects are negligible.

The same mechanism also results in a tendency for the oxygen to migrate. For x > -0.04 the evaporation and recondensation of UO3 result in oxygen moving outwards so that the fuel at the outside is nearer stoichiometric while that at the centre has even less oxygen. The buildup of oxygen in the outer parts of the fuel tends to increase the partial pressure of UO3 there and so helps to oppose the outward movement of uranium.