Nuclear power plant instabilities analysis

We studied neutron power instability in BWRs of four different nuclear power plants (NPP). The methodology applied in this study is a properly structured, coherent and expanded use of conventional techniques in spectral analysis, Power Spectrum Density (PSD) analysis and Fast Fourier Transform (FFT). Analyses, based on time-frequency representation, allowed for characterization of time-varying spectral signal components, favoring BWR understanding and monitoring. Results show that in large amplitude events, (i.e. Laguna Verde Unit 1 and Fosmark) two coupled mechanisms are needed. The Forsmark NPP case, similarly to Laguna Verde NPP and Cofrentes NPP, confirms a frequency of 0.5316 Hz for in-phase instabilities and of 1.006 Hz for out-of-phase instabilities. The results of Ringhals NPP support the stated hypothesis that 1 Hz frequencies indicate out-of-phase power oscillations, which could point to the existence of an additional out-of-phase instability mode in Laguna Verde NPP.