A study of the two-phase natural circulation flow through the annular gap between a reactor vessel and insulation system

Abstract

ACT The behavior of boiling-induced two-phase natural circulation flow in the insulation gap was observed using a 1/21.6 scaled experimental facility to simulate the APR(Advanced Power Reactor) 1400 reactor vessel and insulation system. The liquid mass flow rates driven by the natural circulation loop were measured by varying the wall heat flux, upper outlet slot area, and water head condition. The experimental data were also compared with analytical ones given by simple loop analyses. By the flow observation, local recirculation convective flows in the minimum gap region were detected. As the heat flux and the upper outlet area increased, the natural circulation mass flow rates increased. Under the conditions of high heat flux and a small area of the upper outlet slot, the intensive back flow through the water inlet was generated periodically. Especially, when there was no steam vented slot in the upper part of the water level, the periodical processes of accumulated steam venting and incoming fresh water were observed more definitely. The simple loop analytical results have shown that the natural circulation mass flow rates between the reactor vessel wall and the insulation qualitatively agreed with the experimental ones as the wall heat flux increased.