Localization of an excessively vibrating control rod using neutron noise analysis at a pressurized water reactor

The purpose of this paper is to discuss some details of a successful localization of a control rod vibration occurring during full power operation of a 440 MWe PWR type reactor. The identification of the vibration and the localization of the vibrating control rod were accomplished as a result of routine in-core neutron noise measurements using a commercial noise analysis system developed in the Central Research Institute for Physics in Budapest and installed permanently at both Units of the Paks Nuclear Power Plant. Additional noise measurements at full power with different control rod configuration were consequently performed to examine the conditions under which the vibration prevailed. The effect of flow-induced vibration on the neutron noise field was investigated utilizing the spectral functions of the noise signals from standard self-powered neutron detectors. A recently developed localization procedure was used in the localization of the excessively vibrating control rod, demonstrating the capability of the method. This method is incorporated in the Reactor Internals Vibration Monitoring System, being under development at the Central Research Institute for Physics in Budapest. The vibration which occurred near to the end of the fuel cycle, was estimated to be well tolerable. During the refuelling period the suspected control rod was thoroughly examined, evidence of the vibration was found and suspected parts have been replaced. Since then no vibration has been observed