| The passive containment cooling system(PCS)is an extremely important safety system in the reactor.The third-generation nuclear power plant Hualong-1independently developed by my country used thi s system.In the passive containment cooling system,natural circulation is usually used as a safe and effective way to discharge the heat in the containment after the accident to prevent the containment from overheating and overpressure.The natural circulation system has a simple structure and good inherent safety.However,studies have shown that two-phase instability flow may occur during the operation of the natural circulation system,which will cause damage to the system and even a boiling crisis.Th erefore,the in-depth study of the flow characteristics of the natural circulation system is of great significance to the design and operation of the passive containment cooling system.In this paper,the flow characteristics of the open natural circulatio n system are studied and analyzed by means of experiments and numerical simulations.Firstly,the start-up characteristics and steady-state operation characteristics of natural circulation single-phase flow were studied through a large number of experiment s.The effects of the steam discharge flow,the pressure of the mixed gas in the pressure vessel,the inlet water temperature of the heating section and the wall subcooling on the flow characteristics of natural circulation are analyzed in detail.Then,th e flow phenomena of different flow modes and the generation mechanism in the open natural circulation two-phase flow are studied and analyzed through experiments.The flow patterns at the exit of the ascending section that are covered by the liquid level o f the water tank(the case of high liquid level in the water tank)and those at the exit that are not covered by the liquid level of the water tank(the case of low liquid level in the water tank)are compared and analyzed.And the flow instability distribution map of the heat flux density and the degree of subcooling are summed up when the water tank is full and the water tank is not full.Then,the open natural circulation single-phase flow and the open natural circulation two-phase flow are modeled and analyzed by the RELAP5 program.The corresponding experimental results are compared and analyzed,the simulation results of natural circulation single-phase flow are in good agreem ent with the experimental results.When simulating the two-phase flow of natural circulation,the simulation results and experimental results of the three working conditions of geyser unstable flow,pure flash unstable flow and pure flash stable flow are still more consistent with the experimental results.However,the simulation results of the unstable flow conditions where boiling and flashing coexist are in poor agreement with the experimental results.But in the simulation,it was found that the unstead y flow dominated by boiling that did not appear in the experiment.Finally,the approximate location of flashing and boiling in the two-phase unstable flow is also found through the simulation of the RELAP5 program.That is to say,the first position of fl ash evaporation is the highest part of the ascending section,and it moves to the lower part with the change of the gravity pressure drop.Boiling first occurs near the outlet of the heating section and moves towards the inlet of the heating section over t ime. |
PDF Full Text Request