Study Of Simulated Coupled Neutronic-Thermo-Hydraulic Low Pressure Natural Circulation Flow Instability

Natural circulation does not take advantage of pump to provide driving force and heat is removed by the buoyancy of hot fluid.Natural circulation has been widely used in the advanced nuclear power plants and marine nuclear power devices.In the water cooled reactor,change of water temperature and void fraction does not only have an influence on the natural circulation force but also the moderation of neutron,resulting in the multi-coupling of neutronic-thermo-hydraulic.Natural circulation is susceptible to flow instability.There are some differences between the stability of natural circulation under constant power and coupled neutronic-thermo-hydraulic conditions for the out-of-pipe experiment or numerical simulation.The relation between reactor kinetics and thermal-hydraulic are built in this paper.An experimental and therotical investigation of low pressure natural circulation flow instability is carried out under coupled neutronic-thermo-hydraulic conditions.A low pressure natural circulation experimental facility is designed and constructed.Natural circulation flow instability under constant power conditions is experimentally studied.Two kinds of flow instability are found in the natural circulation system under constant power conditions: flow boiling induced instability and natural circulation induced instability.Flow boiling instabilities are calssfied as subcooled boiling instability(xe,max < 0),saturated boiling instability(xe,max > 0)and periodic dryout instability(xe,max = 1)according to the exit vapor quality.The flow and heat transfer characteristics,flow pattern at the exit of test section,period and amplitude are studied respectively.Besides,the mechanisms of flow boiling instabilities are illustrated.Subcooled boiling instability is induced by the growth and condensation of bubbles at the exit of test section and flashing in the riser has little influence on subcooled boiling instability.Saturated boiling instability is classified as Pressure Drop Oscillations and caused by the interaction between flow boiling in the test section and compressible volume in the loop.Periodic dryout instability happens when periodic dryout type critical heat flux is reached.The violent bubble growth pushes the fluid back to the entrance of the test section when periodic dryout happens.Natural circulation induced instability is superimposed with subcooled boiling instability,saturated boiling instability and flow reversal.Natural circulation induced instability is classified as Natural Circulations Oscillations and caused by the imbalanced heat transfer in the test section and condenser.The existence of compressible volume has a great influence on the flow instability.Flow instability is suppressed in the stiff system.Lastly,the onset of flow instability is predicted and the boundaries between different flow instabilities are given.The reactor kinetics is modeled by a point neutron kinetics model.The point reactor kinetics equations are solved by a Fully Implicit Runge-Kutta method based on Gausss-Legendre Quadrature nodes(GLFIRK).The results of GLFIRK algorithm are compared with Ganapol benchmark.The applicability of GLFIRK algorithm is validated under positive/negative step reactivity insertion,ramp reactivity insertion and Zig-zag reactivity insertion.Heat conduction in the fuel rod is modeled by a one-dimension transient heat conduction model and a lumped parameter model.The relation between nuclear power and thermal power is built.The results from two methods agree well when a right fuel time constant is chosen.Saha-Zuber correlation is used to predict void fraction in the test section.Coupled neutronic-thermo-hydraulic algorithm is developed considering fuel temperature reactivity feedback and void reactivity feedback.The applicability of coupled neutronic-thermo-hydraulic algorithm is validated as well.Natural circulation flow instability under sinusoidal power oscillations are experimentally investigated to analyze the effects of coupled neutronic-thermo-hydraulic on the stability of low pressure natural circulation system.The characteristics of power oscillations under coupled neutronic-thermo-hydraulic conditions are studied by reference to those under sinusoidal power oscillations.The characteristics of power oscillations and stability of natural circulation system are experimentally studied under different reactivity feedback coefficients and fuel time constants.The results indicate that coupled neutronic-thermo-hydraulic effects have littile influence on the stability of natural circulation system when feedback coefficients(absolute value)are small.Natural circulation flow changes from stable state to flow instability when feedback coefficients(absolute value)are increased to a certain value.The system gets more unstable by increasing fuel temperature reactivity coefficient.The system is more unstable under larger void reactivity coefficient.The system is more stable under larger fuel time constant.One-dimensional two-phase flow in the test section is modeled by a four-equation drift flux model.Subcooled boiling,flow pattern transition and dynamic of Nitrogen tank pressurizer is taken into consideration.A MATLAB code is developed to simulate the coupled neutronic-thermo-hydraulic natural circulation system based on the natural circulation facility.Experimental data developed by Bartolomei is used for the Verification and Validation of numerical simulation code.The stability of natural circulation system under constant power and coupled neutronic-thermo-hydraulic conditions are therotically investigated using the numerical simulation code.The numerical simulation code can well predict the Marginal Stability Boundary of natural circulation system under constant power conditions.The parametric effects on the natural circulation system under coupled neutronic-thermo-hydraulic conditions from the numerical simulation code are in agreement with the experimental data.