The most common fuel failure mode in the UK mag- uo reactors has been body failure. This occurs near i-e position ot peak can temperature, the strains im — po^’d on the can by uranium swelling causing it to lail by creep cavitation at subgrain boundaries within — uee grains. Analysis of the failure frequency has in­dicated a strong correlation with element manufac­turing number (which is, in turn, probably indica­tive of minor variations in the heat treatment); thus, once failures begin to occur, other fuel at risk can be quickly identified and remedial action taken. As in other cases, this provides a lesson in the usefulness of complete historical records for each fuel element. Currently the failure rate is about one element per reactor year.

Another cause of fuel failure is coolant leakage through damaged or faulty end cap welds. Since cool­ant ingress to the fuel occurs as soon as the reactor is pressurised, this leads to progressive oxidation of uranium at a position remote from the weld with, at first, very little release of activity. Subsequently, the expansive forces generated by the uranium oxide cause

the can to fail and produce a high-burst can signal. However, such ‘fast-bursts’ have been shown to be less serious than was originally supposed since, even when oxidised right through, a failed fuel element is still capable of supporting the weight of the fuel stack above it. In addition, such failures are generally li­mited to low burn-up fuel since, at higher irradia­tions, creep down of the clad onto the fuel prevents extensive in-leakage of coolant gas.

As explained previously, the generation of inter­nal stresses in the uranium bar by irradiation growth leads directly to yielding creep. Because this is a low temperature phenomenon it can lead to bowing of the bottom elements of the fuel stack, where the temperatures are lowest and the axial loads highest. The elements bow until the splitters or lugs touch the channel wall, after which the bow shape becomes progressively more complex. However, little channel distortion has been found and, since it is limited to low rated positions, the heat transfer penalties of such bowing are minimal.

Magnox AL80 oxidises only slowly in CO2 forming a strong, adherent film. However, at the higher clad temperatures, the presence of certain impurities in the coolant — notably water or the higher hydrocarbons — can cause the film to produce tensile strains in the fins, especially the thinner sections. Since the fins taper towards their tips, the net effect is that they become wavy with considerable loss of heat transfer capability. Such strains are most severe in the thinner — finned polyzonal elements. In the UK this problem arose at about the same time that severe corrosion of

jld steel components was found in the reactors. This, ner problem led to an upper limit of 360°C being nut upon channel gas outlet temperature and a side effect of this was that it effectively eliminated tin

w a’dng.