Optimization Of Nonlinear Coast-down Model Of Nuclear Main Pump Under Accident Conditions

Compared with traditional energy,nuclear energy is higher in efficiency,cleaner and more sustainable.Therefore,stepping up efforts to research nuclear power is closely related to the background of rapid development and greater environmental pressure in China.The prevention of nuclear accidents accounts for a great percentage in current research work,especially the nuclear safety under power failure.As one of the core equipment of nuclear power station,nuclear main pump plays an important role in the cooling loop system of nuclear reactor.When the power failure occurs,after the nuclear main pump loses external power,the inertia energy stored in the unit could maintain the rotation of the nuclear main pump.If the coast-down time cannot reach the safety standard,the system cooling cycle become invalid,which result in the reactor core cooling work not in time and rapid accumulation of hydrogen.A severe condition leads to nuclear explosion and leakage.Thus the research of coast-down process of the nuclear main pump establishes the foundation of nuclear safety research.As the important part of the hydraulic components in the nuclear main pump,the design of diffuser affects the performance of the pump.The effects of different diffuser on the hydraulic performance and coast-down characteristics of the nuclear main pump are studied in this paper by analysis,simulation and test.The test-bed of model pump is built to verify the optimization results.Unsteady simulation analysis of nonlinear coast-down transients has been carried out.A mathematical model for the inertia of the diffuser structure has been established.Also,the model has been verified by dynamic simulation system.The main contents and conclusions are as follows:1.Based on the analysis of the energy conversion in the coast-down transition process,the method of optimizing the diffuser to enhance the hydraulic performance of the pump is proposed to reduce the energy loss during the coast-down transition process,so as to achieve the purpose of prolonging the coast-down time.Based on a large number of references,the inlet angle,outlet angle,wrap angle,thickness,outlet width of blade and the clearance between the diffuser and the impeller are chosen for orthogonal test.Combined with grey correlation analysis,the optimal parameters of the diffuser are obtained.2.The optimizations of adding baffle plate in diffuser flow passage is put forward for the first time.Through the Pro/E and CFX,the three-dimensional model and external characteristic calculation of the nuclear main pump of before and after optimization schemes are completed.Compare different optimization model,it is found that after optimization the efficiency and head under small flow condition are improved,and the high efficiency area of optimization model ofthe diffuser with the baffle plate move to the small flow lightly.3.In order to verify the optimization reliability,a hydraulic prototype test-bed for nuclear main pump has been built.After the external characteristic test and coast-down transient process test of model pump with different diffuser,it is found that the optimization of the diffuser model has a certain improvement on the hydraulic performance compare with before.Under the same conditions,the decline speed of flow rate,head and rotation of the optimization model are slower than before,and the coast-down performance of the diffuser with baffle plate is better.The different tube resistance has little effect on the rotation speed of the process,but has a great influence on the flow and head.4.Based on the experimental results,unsteady numerical simulation has been carried out for the coast-down transient phase of the main pump with different diffuser.Wavelet analysis is used to compare the signal fluctuation of pressure pulsation in different channels.The different stages of coast-down process are compared.The results show as follow: The time domain diagram of pump pressure pulsation in different models is similar in different frequency bands.After optimization,the amplitude of signal fluctuation is reduced,and the flow process is stabilized.The low frequency signal reflects the effect of tube resistance on pressure fluctuation.The pressure fluctuation signal of intermediate frequency affects the change of flow.The high frequency signal is less affected by the different model.Based on the analysis of the different stages of coast-down transient pressure pulsation signals,it can be seen that optimization makes the fluctuation amplitude of pressure fluctuation signal smaller,and takes advantage to eliminating the influence of abnormal fluctuation.Before the half flow,the rotor-stator interaction is related to intermediate frequency signal.After the half flow,the fluctuation amplitude decreases further due to the decrease of flow rate.5.Based on the principle of the main pump torque balance,the balance equation of coast-down condition is established.Relationship between hydraulic performance and hydraulic parameters of diffuser is established by the Matlab.The mathematical model of the coast-down rotation speed of the nuclear main pump is derived.Build the dynamic simulation system to verify the mathematical model.After the comparison,the reliability is proved.