Two-phase heat transfer phenomena in a natural circulation loop during LOHS

Abstract

The secondary loop (SL) of a Natural Circulation (NC) System serves as the coolant supply channel to the Heat Exchanger (HX) for effective passive heat removal by the NC system. Insufficient coolant or total loss of coolant along the cold leg of the SL will lead to a loss of heat sink accident (LOHS). This phenomenon negatively affects the heat removal capability of the HX. The experiment has now been carried out to investigate the loss of secondary loop coolant as it affects flow characteristics in the primary system of a natural circulation system under two-phase conditions carried out in a well-scaled down dimensioned NC loop having a single heated channel. Induced power of 9.0 KW and a 0.5 MPa system pressure were predetermined and maintained. Transient was activated by shutting down the inlet of the secondary loop (cold leg) and then NC flow behavior was observed in each test section along the flow centered on the predetermined conditions. When HX outlet temperature upsurges to around its inlet temperature, data recording is halted, and the experiment is brought to an end. The acquired data were analyzed at the inlet and outlet of both the heated sections and the HX in various flow regimes. While the transient scenario was activated, there exists transient stability in the NC loop with temperature increasing steadily and consequently descents into the instability region. Thereafter, NC flow drips into a stable two-phase flow, and subsequently, outlet temperature surges, and flow loses stability and purges into the two-phase instability region leading to CHF occurrence. This experiment is useful for more understanding of NC fluid behavior and further exploitation of the close NC rectangular loop in the event of LOHS accident.