Neutron noise analysis of simulated mechanical and thermal-hydraulic perturbations in a PWR core

Abstract

KWU pre-Konvoi PWRs (SIEMENS design) are commonly exhibiting high neutron noise levels which canlead to costly operational issues (i.e. activation of SCRAM system, operation of unrated core power, etc.).The frequency region of interest of neutron noise is below 1 Hz, which is the typical frequency range ofthermal-hydraulic phenomena. This feature seems to indicate that, coolant flow and temperature oscil-lations can have a key impact on neutron noise phenomena. Moreover, an increasing neutron noise trend(in term of normalized root mean square) was recently observed in many KWU-PWRs. This increasingtrend has been speculated to be correlated with the introduction of a new fuel design type in theKWU-PWRs. This fact indicates that there should be some correlation between neutron noise spectralcharacteristics and fuel assemblies’ performance. In order to advance in understanding this phenomenon,the transient nodal code SIMULATE-3K (S3K) has been used to simulate mechanical vibrations of fuelassemblies and thermal-hydraulic fluctuations of the core inlet flow and temperature. The simulatedneutron detectors responses are analysed with noise analysis techniques and compared to real plant data.This analysis indicates that the cross-feedback between the mechanical and thermal-hydraulic distur-bances complicate the identification of the origin of the perturbation source. The simulated results indi-cate that the neutron noise spectral characteristics can be associated separately to different causes. In thissense, the results of this work seem to indicate that the spectral features of the neutron noise are a con-sequence of both mechanical perturbations and thermal-hydraulic fluctuations.