Investigation on two-phase flow stability in a natural circulation system

A research program on thermal-hydrodynamic stability of the two-phase flow, simulating the behavior in the primary loop of a nuclear heating reactor d eveloped by the Tsinghua University Institute of Nuclear Energy Technology (INET) in China has been executed for several years In the integrated prima ry loop of the NHR heating reactor the natural circulation of the coolant w ater 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 ke ep the reactor operating under stable conditions with enough safety margins . The program was aimed at: (1) accumulating experimental data for verifica tion of the models and codes used in the design and safety analysis of this type of reactor; (2) understanding the unstable behavior, its physical mec hanisms and parameter effects. The results of the study show that under cer tain geometric conditions and operating parameters a self-sustaining, low f requency, even amplitude mass flow oscillation may be excited at very low s team qualities. Stability maps under different conditions are provided. An unstable region, which exists between the single-phase stable region and th e low steam quality bulk boiling stable region, was experimentally demonstr ated.