Understanding low-frequency neutron noise: Evidence of flow shifting in KWU reactors and separation of global and local components

Low-frequency neutron noise below 1 Hz has been repeatedly observed in pressurised water reactors (PWRs) of KWU design, although its physical origin remains only partially understood. Early studies suggested that this frequency range arises from the superposition of a global component and a local component. In this work, neutron-noise measurements from a commercial KWU three-loop PWR are analysed combining conventional spectral techniques and Operational Modal Analysis (OMA) applied to neutron detectors and core-exit thermocouples. The results identify two dominant spatial contributions below 1 Hz. The first exhibits coherent axial behaviour with an approximately linear phase progression along the coolant flow direction, consistent with global thermohydraulic transport. The second displays radial out-of-phase regions and an axial dipole structure, indicating spatially non-uniform core dynamics. Two characteristic spectral contributions, centred near 0.1 Hz and 0.7 Hz, show different spatial distributions and fuel-cycle evolutions. Comparison with thermocouple signals reveals similar radial phase asymmetries, supporting the interpretation that the local component is associated with slow coolant-flow redistributions (flow shifting). Through moderator-temperature feedback, these hydraulic perturbations generate structured neutron-noise fluctuations. The findings indicate a dual interpretation of neutron noise below 1 Hz in KWU reactors as the coexistence of global thermal transport and local hydraulic redistribution processes, and demonstrate the usefulness of OMA for reactor diagnostics.