Fuel element failure

Encyclopedia : F : FU : FUE : Fuel element failure

Because the actinide commonly used as nuclear fuel have low melting points and are prone to corrosion by water, the de facto standard nuclear fuel is uranium dioxide or a mixed uranium/plutonium dioxide. It is the case that uranium dioxide is very corrosion resistant and it provides a stable matrix for many of the fission products. However some of the fission products such as the noble gases can leave the uranium dioxide matrix and enter the coolant, to prevent this from occurring it is normal to place the solid fuel within a corrosion resistant metal tube (normally Zircaloy for water cooled reactors)

Those elements are then assembled into channels for the coolant to flow through. As the fuel fissions, the radioactive fission products are also contained by the cladding, and the entire fuel element can then be disposed of as nuclear waste when the reactor is refueled.

If, however, the cladding is damaged, those fission products which are not immobile in the uranium dioxide matrix can enter the reactor coolant or storage water and be carried out of the core, into the rest of the primary cooling circuit, contamination levels there.

In the EU some work has been done in which fuel is overheated in a special research reactor named PHEBUS. During these experiments the emissions of radioactivity from the fuel are measured and afterwards the fuel is subjected to Post Irradiation Examination (PIE) to discover more about what happened to it.

This is a false colour tomography picture of a bundle (FPT1) of 18 irradiated fuel rods (23 GWd/tU mean burn-up) degraded under steam as part of the PHEBUS set of experiments. The black and blue is for areas of low density while red is an area of high density

It can be seen that the fuel has failed mechanically and has formed a pool near the bottom of the bundle, it is interesting to note that the bottom of the bundle did not melt.

For further details of how nuclear fuel behaves under accident conditions see fuel under accident conditions

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