On irradiation a series of annular zones are formed depending on the radial temperature distribution, as shown in Figure 2.22. There is an outer а-phase zone in the cool periphery with a p-phase zone inside it and, if the temperature is high enough, a у-phase zone inside that. The redistribution of the constituents of the fuel alloy is also shown in Figure 2.22. The solubility of zirconium is different in the different phases and as a result it tends to migrate out of the p-phase both outwards to the а-phase and inwards to the у-phase. Uranium is displaced in the opposite directions into the middle annular zone while

the plutonium remains largely undisturbed. If the central temperature is lower, however, the central region may be depleted in zirconium. Table 2.3 gives typical local concentrations of the alloy components, based on electron microprobe data, for an element with a high central temperature.