Neutron signal characterization of the power instability event at Laguna Verde BWR

On January 24, 1995, a power instability event occurred in Laguna Verde, a BWR/5 commercial plant. Recorded power oscillations were studied from the point of view of noise analysis. The 723-s long recorded signal comes from average power range monitors and was bad conditioned for noise analysis practice; it was neither stationary in mean, nor in variance. The signal first stage corresponds to the stable reactor; the third stage, to the unstable reactor. There was a second intermediate stage regarded as a transition one. The signal was preconditioned and divided in small blocks. Noise was analysed within each block in the amplitude, frequency and time domains. The analysis was aimed at on early recognition of instability by using the noise to discriminate between stable, transition and unstable state, regardless of the domain chosen for analysis. The experience obtained from studying real events, not depending on any physical model, are the ground for making safer operation procedures.