Rootcause analysis for the violation of surveillance Q(z)

At the beginning of UCN Unit 2 Cycle 11, there was a violation of surveillance Q(z) limits specified in the Technical Specification, which led to reduction in power level and the reduction of RAOC AFD band. In this study, the cause has been analyzed by reviewing of the measured power distribution and design data. It is found that the major cause was the increase in axial power at top half of the core resulted from the AOA(Axial Offset Anomaly) of previous cycle. Also, in spite of the higher peak power at top half of the core(6∼8ft) during rod insertion which is caused by the shallower insertion of the lead bank rather than Westinghouse 17x17 plants, the lower Q(z) limit compared with other plants is another cause