Study On The Stability Of Natural Circulation Loop Under Supercritical Pressure And The Analysis With Dimensionless Parameters

Supercritical Water Cooled Reactor(SCWR)is one of the six GEN IV nuclear system recommended by Generation IV International Forum(GIF).It’s the one reactor cooled by water.So,it has a good technique continuity from the present systems of GEN II and GEN III.SCWR has some other advantages such as high thermal efficiency,good economy,simple construction,high safety and so on.However,because of the special properties of the fluid under supercritical pressure,there is a risk of instability in the system which threatens the safety of the system in some condition.Passive safety system is taken in some design of SCWR,so the study of stability of natural circulation loop is significant for the design of SCWR.There are mainly two kinds of instabilities in this study: static instability and dynamic instability.In this paper,a nonlinear numeric algorithm is used for the study of static instability and a frequency domain method is used for the study of dynamic instability.The results are analyzed with dimensionless parameters.The Thesis mainly include three aspects:(1)A typical natural circulation loop under supercritical pressure is simulated.We develop a nonlinear numeric algorithm for the dynamic instability and a program of frequency domain method for dynamic instability.Based on two-phase dimensionless governing equations,we give the dimensionless governing equations for supercritical pressure.And we choose 3 dimensionless parameters for the analysis of stability according the physical meanings.(2)According to a typical experimental natural circulation loop,calculate the static instability and find that there is a maximum mass flow rate Gm.Gm occurs when the fluid is near the pseudocritical points at the outlet of the heating section.We also find that there is a critical inlet temperature.There will be a static instability when the inlet temperature is lower than this temperature.The influence of different loop parameters is analyzed.The increase of diameter of heating section can dramatically increase the maximum mass flow rate and heating power and the decrease the critical inlet temperature;The higher the loop is,the higher maximum mass flow rate and heating power is;Local resistance at the inlet of heating section has little influence on the mass flow rate and heating power while it can improve the static stability;Local resistance at the outlet of heating section has little influence on the static stability while the mass flow rate and heating power is smaller.200 sets of parameters are chosen by random.Then the static stability of each set is calculated and presented in the dimensionless space.We find that stable and instable cases are located in different regions in the space.(3)The dynamic instability is calculated by a frequency domain program.We find there is a dynamic boundary for the loop.When the heating power is higher than the boundary,the loop is dynamically instable.For different inlet temperature,we can find an instable region in the dimensionless plane.The instable region has a big trans-pseudocritical number.We analyze the influence of different parameters.Length of heating section and diameters has little influence on dynamic stability;The influence of the height is complicated and is related to the heating length.