On-line extraction of the variance caused by burn-up in in-core three-dimensional power distribution

In most of PWRs, the ex-core ion-chambers are the sole real-time sensors to respond to in-core power and its axial offset. However, the calibration co efficient of the ion-chambers depends on the (3D) power distribution and va ries with the burn-up. People expect to know the variance in distribution c aused by burn-up directly from the signals of ion-chambers. This expectatio n is not realized as yet, because an ion-chamber almost only responds to it s nearest fuel assemblies. The authors then developed a two-step method for burn-up characteristic extraction: the harmonics synthesis method and harm onics' burn-up grouping. Using the extracted burn-up characteristics, the r elationship between the readings of the ex-core ion-chambers and the in-cor e 3D power distribution is set up. Through the simulation on the heating re actor, the method of burn-up characteristic extraction is verified under en gineering conditions. It is possible to on-line extract the variance caused by burn-up in 3D power distribution. (C) 2001 Elsevier Science B.V. All ri ghts reserved.