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

Abstract During the last 15 years, the problem of BWR instabilities has attracted the attention of a number of researchers. In 1996, an unusual instability event occurred at Forsmark-1 in which superimposed on the classical, fundamental 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 the two sides of the core (from ≈0.2 to ≈1.0), an inexplicable result. Furthermore, noise analysis-based localisation techniques pointed towards the existence of two strong ‘perturbation sources’ in the two halves of the core, one of them coinciding with the radial position of an unseated bundle. Motivated by these findings and the fact that as was found later, at least one bundle was unseated, the code RAMONA3-12 was used in order to try to simulate these experimental findings, by assuming that in some bundles, there are relatively large amplitude thermohydraulic perturbations. These perturbations (which are in fact density wave oscillations) were excited by modifying the inlet loss coefficients of the bundles in question.