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.