A study on the natural circulation flow through an annular gap between the reactor vessel and the insulation under external vessel cooling

The behaviors of the boiling-induced two-phase natural circulation flow in the insulation gap were observed using an 1/21.6 scaled experimental facility to simulate the APR1400 reactor and insulation system. The liquid mass flow rates driven by natural circulation loop were measured by varying the wall heat flux, upper exit slot area and configuration. Simple loop analysis has also been performed and compared with the experimental data on the natural circulation flow rates. By the flow observation, the intensive back flow through the water inlet was generated periodically under the conditions of high heat flux and small area of the upper exit slot. Especially, when there was no steam vented slot on the upper part of water level, the periodical processes of accumulated steam venting and fresh water incoming were observed. The natural circulation mass flow rate increased as the heat flux, coolant temperature, and outlet area increased. The heat flux and coolant temperature had a larger effect on the natural circulation flow than the outlet area. The simple loop analytical results had 70% average error bound compared with experimental ones