Experimental Study On Flow Instability During Gravity-driven Reflooding

Injecting cooling water into the core after a water loss accident is an important safety measure that allows the core to be submerged to cool the core and improve the safety of the nuclear reactor.In recent years,reactor types using passive safety technologies have continued to evolve.The passive safety technology relies on the natural forces such as gravity and density difference to drive the core decay residual heat,which improves the safety and reliability of the nuclear reactor under accident conditions.However,in the process of core re-submerging relying on the natural force as the driving force,since the gravity head of cooling water is small,and the cooling water is in contact with the high-temperature fuel rods,a large amount of steam is accumulated in the pressure vessel,which hinders the continuous injection of the cooling water.The flow instability during gravity-driven reflooding involves complex problems such as two-phase flow and vapor-liquid phase change.Therefore,related research needs to be further studied.In this paper,the experimental study and numerical calculation are combined to carry out the study on the flow instability during gravity-driven reflooding.The main research contents and conclusions are as follows:(1)An experimental research facility for the flow instability during gravity-driven reflooding is designed and built,and an experimental research plan is developed.The experimental facility can adjust the initial conditions according to the working conditions and observe the changes of parameters such as temperature,pressure/pressure difference and flow rate during the experiment in real time,and has the ability to carry out experimental research and influence law research on the flow instability during gravity-driven reflooding under different working conditions.(2)An experimental study on the flow instability during gravity-driven reflooding is carried out.The typical experimental phenomena are analyzed.On this basis,the type of flow instability is determined.The criterion for judging whether flow instability occurs is established.And the occurrence mechanism of typical flow instability phenomenon is studied.The effect of influencing factors on the flow instability phenomenon is analyzed.The results show that with the increase of the initial temperature of the heating rod,the decrease of the liquid level of the high water tank and the system pressure,the increase of the heating power of the heating rod and the opening of the cooling water inlet,and the decrease of the opening degree of the steam outlet,the flow instability happens more easily.Among them,it is further clarified that the initial temperature of the heating rod,the liquid level of the high water tank,the heating power of the heating rod,and the opening degree of the cooling water have a more obvious influence on the flow instability phenomena.Therefore,the above four key influencing factors are analyzed for their contribution to key indicators of water injection process.(3)A numerical calculation model is established for the experimental facility for flow instability during gravity-driven reflooding.The numerical analysis is carried out and the calculation results are compared with the experimental results.Through experimental research and numerical analysis,this paper has a deeper understanding of flow instability during gravity-driven reflooding.The results can provide support for the improvement of safety tools and practical engineering applications.