Nuclear Engineering and Design 196 (2000) 327 – 336 Neutron signal characterization of the power instability event at Laguna Verde BWR Juan Bla ´zquez, Jorge Ruiz * Department of Nuclear Fission, CIEMAT, A. Complutense 22, 28040 Madrid, Spain Received 25 January 1999; accepted 7 November 1999 Abstract 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. © 2000 Published by Elsevier Science S.A. All rights reserved. www.elsevier.com/locate/nucengdes 1. Introduction In a commercial BWR, power instability during the start-up process is an unusual event, although it occurs often compared with other anticipated transients. Nevertheless, some cases have been reported in the past (Murphy, 1988; Bergdahl et al., 1990; Castrillo et al., 1991) without apparent damage to the Plant. The BWR owner group, Licence Authorities and Vendors have encouraged the research community to understand better the events and to design operational procedures in order to prevent new cases. Two specific meetings (International Work- shop, 1989; Stability Symposium, 1989) devoted to the topic were organized as a consequence of LaSalle County Station Unit 2 event (Araya et al., 1989; Cheng, 1988). Stability monitors — work stations measuring power decay ratio from neu- tron noise — have been developed and installed in nuclear power plants (NPP) (Lorenzen et al., 1991; Anegawa et al., 1995). Noise analysis of BWR/6 neutron signal during a real instability event (Bla ´zquez and Ballestrı ´n, 1995) reported that fluctuations were the dominant behaviour of the signal prior to instability; but in a few min- * Corresponding author. Present address: National Institute of Nuclear Research, Me ´xico. Tel.: +34-91-346-6123; fax: +34-91-346-6233. E-mail address: blazquez@ibm1.ciemat.es (J. Ruiz) 0029-5493/00/$ - see front matter © 2000 Published by Elsevier Science S.A. All rights reserved. PII:S0029-5493(99)00299-X