Corrosion of the cladding by the fuel is discussed in section 2.4.7. It is strongly affected by the oxygen potential in the fuel and therefore by the stoichiometry. Figure 2.2 shows the variation of the oxygen potential of (U1-aPua)O2+x with x at constant temperature for various values of a. As expected the oxygen potential rises with x, but also it is higher for PuO2 (a = 1) than for UO2 (a = 0) because uranium adopts higher valency states than plutonium. The steep rise in the oxygen potential of UO2+x as x increases through zero is important.

The variation of oxygen potential with temperature is shown in Figure 2.3. Oxygen tends to migrate to the cooler parts of the fuel if x < 0, but if x > 0 the tendency is considerably reduced.

The effect of burnup is very complicated because of the wide range of elements formed as fission products. Each fission releases two

oxygen atoms, some of which go to oxidise the fission products, such as zirconium, strontium, barium and the rare earths, which have a strong affinity for oxygen. The number of oxygen atoms taken up by this oxidisation process depends on the yields of the various fission

products, and the yields differ for fission of different nuclides. Fis­sion of uranium yields more zirconium and strontium (which form oxides) and less ruthenium and palladium (which do not) than fission of plutonium. It so happens that as a result the average requirement to oxidise the fission products is for slightly more than two oxygen atoms per uranium fission and for slightly less than two per plutonium fission. The effect is shown in Figure 2.4.

In a typical reactor core fuel, far more fissions occur in 239Pu than in 238U, so oxygen is steadily released and the oxygen potential of the mixture of fuel and fission products rises. Just as in Figure 2.2 there is a particularly sharp rise at the point in burnup where the oxygen

content becomes stoichiometric, but the oxygen potential does not rise very high. An effective upper limit is set by molybdenum. The Mo/MoO2 system has an oxygen potential very similar to that of the fuel, and because the yield of molybdenum is high (about 0.06 atoms for each fast fission of either 239Pu or 238U) it forms a buffer. When the stoichiometric ratio is reached, oxygen released by further fissions is taken up by oxidising molybdenum.

To minimise oxidative corrosion of the cladding the fuel is usually manufactured a few percent sub-stoichiometric, with x = -0.02 or -0.03, but as explained in section 2.4.7 this does not prevent some corrosion taking place.