Research On The Hydraulic Characteristics In The Density Lock

The density lock is an important component to realize the inherent safety of the reactor. The"closure"and"open"of the density lock completely depend on the operating characteristics of the reactor without intervention of active component and operators. The application of the density lock in passive residual heat removal systems of current dispersed PWRs will significantly improve the inherent safety of next generation reactors. Whether the density lock can be applied successfully or not depends mainly on the following two conditions: A. whether the stable stratification between cold and hot fluids can be formed in the density lock; B. whether the hydraulic balance conditions can be met at the interface.With the methods of experimental study and theoretical analysis, this dissertation make researches on the two conditions which can ensure the density lock can be applied successfully, including stratified characteristics, fluid stratification mechanism and hydraulic balance characteristics in the density lock.Firstly, visualized experiments are conducted on the forming process of stratification in the density lock. On the basis of the experimental study, the heat transfer mechanism model is built and the heat transfer characteristics between hot and cold fluids are analyzed. The results indicate that working fluid in the density lock can be divided into three zones from top to bottom: mixing layer, interfacial layer, constant temperature layer. Interfacial layer works as a valve and can separate the hot fluid from cold fluid without mixture. The stratification mechanism is that because of the existence of the transition points in the heat transfer modes, the differences in the rates of temperature increase appear. It is these differences which give the appearance of fluid stratification. The density lock with a structure of variable cross section grids can effectively control the position of the transition points of the heat transfer modes. In addition, the research on the stratified stability indicates that there are two kinds of instabilities—high amplitude oscillation and low amplitude oscillation. High amplitude oscillation will bring vigorous heat transfer between stratified fluids.Then, the structure of the density lock is optimized according to the research on the stratified characteristics. The optimized density lock is applied in the passive residual heat removal system and a simulating experimental loop is built. Moreover, characteristics of hydraulic balance is studied. The results show that the hydraulic balance in the density lock is characterized as dynamic balance and self-stability in a certain range. Namely, by virtue of the change of the hot-cold interfacial position in the density lock, the hydraulic balance in the density lock can be recovered again to make the density lock closed without the help of active components. Based on the characteristics of dynamic balance and self-stability, a method (self-balance startup) to establish the hydraulic balance in the density lock is proposed. The requirements to realize the self-balance startup are given through theoretical analysis. Moreover, experimental verification on this method is done. The result shows that hydraulic balance in the density lock can be established successfully with the method of self-balance startup, when the start-up flow is met with the conditions of self-balance startup.Lastly, on the basis of the experimental study, a computational model for the analysis of hydraulic balance in the density lock is developed. With this model, start-up flow which can ensure the establishment of the hydraulic balance is forecasted, and transient characteristics in the process of building the hydraulic balance are analyzed. The balanced flow calculated by this model coincides with the experimental results. And the calculated results are verified by the experiment of self-balanced start-up. The factors which influence the ability of self-balance start-up are analyzed, and this analysis indicates that the increase of the height of the density lock properly is beneficial to improve the ability of self-balance start-up.