The search for fuel assemblies with defective fuel rods by sipping tests involves expensive prolongations of the refuelling periods in nuclear power plants. It is attempted to reduce the number of fuel assemblies to be checked by sipping during the refuelling periods by means of rough localization of the defects on the basis of the fission product concentration ratio134Cs/137Cs in the primary coolant already during reactor operation. First results obtained in two examples of application at VVER type reactors are encouraging. The burnup of the defective assemblies could be correctly predicted from the cesium ratio in these two cases, which was confirmed by sipping tests after reactor shutdown.
Rough techniques for pinpointing defective fuel pins during actual reactor operation were developed for nuclear power plants. These techniques are based on various fission product concentration ratios. Here, a new cesium concentration ratio,134Cs/136Cs, was tested in combination with the more usual cesium ratio134Cs/137Cs. This new cesium ratio confirmed the conclusions drawn from the ratio134Cs/137Cs and provided some additional information on the location of the defective fuel rods. Application of this second cesium ratio improves the reliability of the rough localization method.