Predictive Methods for Stability Margin in BWR

Abstract

iii This dissertation is dedicated to my mother, wherever she is now, who always gave me encouragement, love and support, my father raised me with infinite patience and advised me to obtain a degree in physics, my sister Pilar, example to follow both in the study and teaching and, especially my family, because this work has been done during the time I had to spend with my wife Marina, and my children Cecilia and Jose Andrés.I also want to thank my supervisors, Alberto Escrivá and José Luis Muñoz-Cobo without them this thesis would not have been possible. I also thank this work to José María Izquierdo Rocha who taught me to be rigorous and convinced me of the need for a thorough knowledge of nuclear engineering. To all my colleagues who have made me what I am today.Thanks to all v SUMMARY Power and flow oscillations in a BWR are very undesirable. One of the major concerns is to ensure, during power oscillations, compliance with GDC 10 and 12. GDC 10 requires that the reactor core be designed with appropriate margin to assure that specified acceptable fuel design limits will not be exceeded during any condition of normal operation, including the effects of anticipated operational occurrences. GDC 12 requires assurance that power oscillations which can result in conditions exceeding specified acceptable fuel design limits are either not possible or can be reliably and readily detected and suppressed.If the oscillation amplitude is large, before the scram occurs the fuel rods may experience periodic dry-out and rewetting, or if the oscillation is larger enough, extended dry-out.The Decay Ratio (DR) is the typical linear stability figure of merit. For analytical estimation of DR frequency domain codes are very useful. These types of codes are very fast and their results are very robust in comparison with time domain codes, whose results may be dependent on numeric scheme and nodalization. The only drawback of frequency domain is that you are limited to the linear domain; however, because of regulatory requirements imposed by GDC-12, reactors must remain stable and, thus, reactors always operate in the linear domain.LAPUR is a frequency domain stability code that contains a mathematical description of the core of a boiling water reactor. It solves the steady state governing equations for the coolant and fuel, and the dynamic equations for the coolant, fuel and the neutron field in the frequency domain. Several improvements have been performed to the current version of the code, LAPUR5, in order to upgrade it for use with new fuel design types. The channel geometry has been changed from constant area to variable area. The local losses due to the spacers and contractions along the flow path have been upgraded to use industry standard correlations. This new version is LAPUR 6.In this work, in order to check the correct implementation of these changes, a two-fold LAPUR 6 validation has been performed: vi First, an exhaustive validation of the models implemented has been performed, comparing single channels LAPUR 6 outputs...

Resolving mode mixing in boiling water reactors instability analysis using variational mode decomposition

“…Nevertheless, not all the instabilities are connected to the same harmonics, that is, there are instabilities where there is a prevalence of the 1 Hz harmonic and, in other cases, is the 0.5 Hz one. Even, it is possible to find other harmonics at 1.5 or 2 Hz 36 . Therefore, discriminating which harmonics are the most important can help to classify the instabilities.…”

“…Even, it is possible to find other harmonics at 1.5 or 2 Hz. 36 Therefore, discriminating which harmonics are the most important can help to classify the instabilities. With that purpose the backbone plot will be utilized.…”