The physical mechanism of core-wide and local instabilities at the Forsmark-1 BWR

During the last 15 years, the problem of BWR instabilities has attracted th e attention of a number of researchers. In 1996, an unusual instability eve nt occurred at Forsmark-1 in which superimposed on the classical, fundament al spatial mode oscillations, there were relatively large-amplitude, highly localised oscillations. Subsequent time-series analysis of the local power range monitor (LPRM) signals resulted in very different decay ratios for t he two sides of the core (from approximate to 0.2 to approximate to 1.0), a n inexplicable result. Furthermore, noise analysis-based localisation techn iques pointed towards the existence of two strong 'perturbation sources' in the two halves of the core, one of them coinciding with the radial positio n of an unseated bundle. Motivated by these findings and the fact that as w as found later, at least one bundle was unseated, the code RAMONA3-12 was u sed in order to try to simulate these experimental findings, by assuming th at in some bundles, there are relatively large amplitude thermohydraulic pe rturbations. These perturbations (which are in fact density wave oscillatio ns) were excited by modifying the inlet loss coefficients of the bundles in question. (C) 2001 Elsevier Science B.V. All rights reserved.