For none of these cases do fuel or clad manufacturing parameters have any significant influence. Rather, failure probability is a function of assembly fabrication, and, for the case of debris fretting, to reactor operation and maintenance. The outer surface of the cladding is progressively oxidised by the primary cooling water. High levels of oxidation can reduce performance and lead to clad failure. This occurs when such a reduction in the wall thickness has taken place, due to loss of metal to the oxide phase, that the clad can no longer withstand the stresses and strains which are imposed upon it.
Clad failure is prevented by limiting the clad oxide thickness.
This in turn limits the loss of wall thickness due to clad oxidation and thus is consistent with the loss of wall thickness allowed for as a result of manufacturing defects. The magnitude of clad corrosion is determined mainly by the fuel duty and by reactor primary coolant chemistry, with fuel and cladding manufacturing parameters having only minor influence. The waterside oxidation of Zircaloy generates hydrogen, which may either escape into the primary coolant, or else be absorbed into the clad to form platelets of zirconium hydride. Such platelets, which may also be formed by uptake of hydrogen on the inner surface of the clad, lead to embrittlement of the material, making it more susceptible to failure. A limit is set for the hydrogen uptake.