Identification of failed fuel rods

Another way to reduce the work needed for the identification of failed rods is to distinguish during plant operation failed fuel assemblies showing low burnup levels from those showing high burnup levels. In principle, such a differentiation should be possible by comparing the ratio of the source strengths of Cs and 

An alternative choice to the wet sipping techniques described above is the so-called dry sipping technique, for which the fuel assembly is placed into a test chamber filled with air after a short time, the fission product noble gases (in particular Kr and/or Xe) released from the defective fuel rod during heatup are measured in a gas counter. Reliability and detection limit of this technique are virtually identical to that of the wet sipping procedure. 

The techniques described result in the identification of the assemblies containing failed fuel rods however, the identification of the failed rod itself is not possible. Frequently, in a fuel assembly identified by one of the techniques mentioned above. 

In order to replace failed fuel rods in an assembly lattice, the defective rod has to be identified. Detection of the failed rod in the assembly usually is done by ultrasonic testing, a change in signal intensity indicating the presence of liquid water in the rod. The techniques currently available can be applied to both PWR and BWR assemblies without disassembling the fuel assembly under consideration and without rod withdrawal their sensitivity does not depend on fuel bumup or other parameters. 

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