Chapter 7 - Instabilities in natural circulation systems

Abstract

An introduction to instabilities in natural circulation systems (NCSs) is given in this chapter. Since a large number of instabilities excited by different mechanisms are reported for NCSs, there is a need to classify these instabilities. Different bases for classification of instabilities exist and the most commonly followed classification is based on the analysis method. Based on this, instabilities are classified as static and dynamic instabilities. Compound instabilities occur when more than one mechanism is at work. The instabilities observed in single-phase, two-phase, and supercritical NCSs are reviewed. In general, static instability involving multiple steady states is not reported for single-phase systems. However, density wave-type dynamic instability is possible to be observed in single-phase NCSs. Static and dynamic instabilities can occur in two-phase and supercritical NCSs. Two-phase systems often encounter two unstable regions known as Type I and Type II instabilities, whereas single-phase and supercritical systems have only one unstable region. Reported studies on natural circulation boiling water reactors (BWRs) also show the existence of Type I and Type II instabilities. Neutronically coupled thermal hydraulic oscillations are possible both in BWRs and supercritical water-cooled reactors (SCWRs). Both BWRs and SCWRs can have in-phase and out-of-phase oscillations.