Investigation on two-phase flow stability in a natural circulation system / Untersuchung der Stabilität einer Zweiphasenströmung in einem System mit Naturumlauf

Abstract

A research program on thermal-hydrodynamic stability of the two-phase flow, simulating the behavior in the primary loop of a nuclear heating reactor developed by the Tsinghua University Institute of Nuclear Energy Technology (INET) in China has been executed for several years. In the integrated primary loop of the NHR heating reactor, the natural circulation of the coolant water was adopted with low system pressure, low steam quality at the exit of the core and a relatively long riser above the core. It is important to keep the reactor operating under stable conditions with enough safety margins. The program was aimed at: (1) accumulating experimental data for verification of the models and codes used in the design and safety analysis of this type of reactor; (2) understanding the unstable behavior, its physical mechanisms and parameter effects. The results of the study show that under certain geometric conditions and operating parameters a self-sustaining, low frequency, even amplitude mass flow oscillation may be excited at very low steam qualities. Stability maps under different conditions are provided. An unstable region, which exists between the single-phase stable region and the low steam quality bulk boiling stable region, was experimentally demonstrated.