Flow stability and linear disturbance analysis of single-phase natural circulation loop under different thermal boundaries

Abstract

Natural circulation is a common flow state that occurs in the pressurized water reactor’s normal operation conditions and accident scenarios. Essentially, the driving force between a lower-level heat source and a higher-level cold source will cause the flow. During the pressurized water reactor design process, the natural circulation concept is applied intelligently in the passive system. Compared to active driving, passive driving is equipped with the characteristics of a low driving force and is easily affected by other factors, like flow resistance, and impurities in the working medium, increasing the system’s uncertainty. Flow instability is an important phenomenon that affects the passive system function. Flow oscillations will cause the components mechanical fatigue and departure from nucleate boiling, which further triggers power oscillations due to thermal–hydraulic and neutron coupling. Nyquist Criterion and Root-Search methods have been proposed by previous researchers for solving rectangular loop stability boundaries. On this basis, this paper proposes an Identifiable-Parameter Stability Criterion (IPSC). A one-dimensional mathematical analytical model of the simplified rectangular loop under different thermal boundary conditions was developed based on the linear perturbation analysis method. This paper removes the restriction of symmetry conditions in previous work, expanding the applicability of thermal boundary conditions. Based on the mesh discrete-independence calculations, the effect of size proportions in non-uniformly arranged loops on stability is analyzed.