Modeling And Numerical Study On Natural Circulation Flow Instabilities

Owing to its inherited safety properties,natural circulation is widely used in the design of modern nuclear reactors.However,the problem is that instability may occur in two-phase flow and the coupled effect between reactor power and thermal-hydraulic parameters also may influence the system.Therefore,it is essential to study the instabilities of a nuclear coupled natural circulation system.In this thesis the simulation of a coupled nuclear circulation is studied.Calculation models are set up with some simplification of the system.The homogeneous model is adopted as governing equations of two phase flow in the system and power is calculated by point reactor neutron kinetics model.The discrete governing equations are solved by self-developed code.The static flow excursion and density wave instabilities are simulated and parametric effects are analyzed.The results indicate that there may be more than one operating flow rates under some given operating condition if there is a negative gradient region of the operating characteristic curve.The flow rate of the system may drift from the original state to a new one with small perturbation,where flow excursion may occur.By increasing pressure,inlet resistance coefficient and reducing inlet sub-cooling,outlet resistance coefficient the instable region will shrink.As a response of inlet flow perturbation,variation of pressure drop is delayed because the fluid needs time to travel in the heated section and riser.Under some operating condition,the density wave instability will happen.The oscillation period is about twice of the time that the fluid travels through the pipe.In the studied range of parameter,the oscillation amplitude is increasing by increasing heated power,the length of riser and outlet resistance and reducing pressure,inlet sub-cooling and inlet resistance respectively.The oscillation period is relative longer with the parameters of higher power,longer riser,larger inlet resistance,smaller inlet resistance and lower pressure and lower inlet sub-cooling.When the nuclear coupled effect is considered,the oscillation amplitude becomes larger.Increasing absolute value of fuel temperature coefficient,coolant temperature coefficient and void fraction coefficient are benefit for reducing oscillation amplitude.