Study On Pressure Fluctuation Under Break Loss Of Coolant Accident Of CAP1400 Nuclear Main Pump

Due to the late start and the lack of independent innovation capabilities,as one of the core equipment in nuclear power plants,the nuclear main pump relies on import severely.The continuous China-US trade friction reveals that only master the core technology in our own hands,can we master the right to speak and not be subject to others.Therefore,only by increasing the research of the nuclear main pump and nurturing capabilities of our country's independent design and manufacturing,can we get rid of the technical difficulties that are constrained by other countries and promote the development of Chinese nuclear power business.In the safety evaluation system of nuclear main pump,the safety performance of the nuclear main pump must be taken into account when a break loss of coolant accident(LOCA)occurs.When the break accident occurs at the inlet of the coolant main flow loop,the nuclear main pump is in different working conditions according to the size of the breakage.When a small break loss of coolant accident occurs in the coolant main flow loop of the nuclear power plants,the cavitation in the nuclear main pump is very likely to occur due to the reduction of the system circuit pressure.At this time,the nuclear main pump is in the forward pump condition.When a large break accident occurs in the coolant inlet pipeline,the inlet pressure of the nuclear main pump is rapidly reduced.Under the effect of the outlet high pressure,the main pump is turning forwards while the coolant is flowing backwards.At this time,the main nuclear pump is in the forward-reverse flow condition,and cavitation may also occur during this process.In order to comprehend the operating characteristics of the nuclear main pump in a break loss of coolant accident,this paper takes the CAP1400 nuclear main pump as the research object.Using numerical simulation and test methods,the characteristics of pressure pulsation in the flow field of a nuclear main pump are studied in detail.The main contents of the study include:(1)Numerical simulation are conducted to the operating state of the nuclear main pump under the design conditions.The pressure pulsation characteristics of different monitoring points in the nuclear main pump flow field are studied,and the related phenomenon is analyzed combined with the internal flow field.The study finds that the pressure distribution is uneven in the ball-like pump casing,which has a great impact on the pressure distribution of the impeller and the guide vane,directly leading to the periodic continuous oscillation of peak pressure fluctuation at the outlet.It is also found that the main frequencies of pressure fluctuation in impeller passage is not the same at different positions.(2)In the study of the effect of cavitation on the pressure fluctuation in the flow field of the nuclear main pump,it is found that the amplitudes of the axial frequency signal in the inlet monitoring point and the impeller monitoring point increase in different degrees in the initial cavitation stage,and then decrease rapidly.In severe cavitation stage,low frequency pulsation signals appear in the inlet of the main pump,impeller and guide vanes,and the intensity increases with the development of cavitation.The results of numerical simulation were verified by experiments,and the experimental results were consistent with the conclusion obtained by numerical calculations.(3)When studying the effect of flow rate on the pressure pulsation in the main pump under the forward-reverse flow condition,the different pressure pulsation characteristics of the main pump under the forward and forward-reverse flow conditions are compared and analyzed.The influence of the flow rate on the pressure pulsation in the main pump is also studied under the forward-reverse flow condition,combined with the internal flow field of the main pump.It is found that the pressure pulsation intensity in the flow field of the main pump under the forward-reverse flow condition is very large compared with that of the forward flow condition.It is also found that with the increase of counter-current flow,the pressure pulsation intensity of the main pump is greatly enhanced.In this process,the flow in the nuclear main pump becomes complicated and disordered,and signals of specific frequencies appear in the frequency spectrum of each monitoring point.With the analysis of internal flow field,it is found that the appearance of these signals is related to the irregular flow in the flow field.(4)With the study of the influence of cavitation on the flow field during the forward-reverse flow condition of the nuclear main pump,it is found that the producing positions of cavitation bubbles and cavitation development in the flow field during the forward-reverse flow condition are different from those of the forward flow condition.Comparing with the forward flow condition,The development speed of the cavitation in the nuclear main pump is very slow under the forward-reverse flow condition,and the head of the pump will increase continuously with the development of the cavitation in the pump under the forward-reverse flow condition,which sometimes even increase sharply.Severe cavitation causes a sudden drop in the head of the pump,which can be explained from the perspective of the internal flow field.At the same time,it was found that the pump head is more sensitive to the cavitation state than the gas flow rate under the forward-reverse flow condition.Therefore,the head lift percentage can be used to judge the cavitation state of the pump in the forward-reverse flow condition.(5)The study on the pressure pulsation characteristics of the nuclear main pump with cavitation under the forward-reverse flow condition found that: With the development of the cavitation,the main frequency peaks at the monitoring points of the main pump inlet and the impeller have gradually decreased.During this process,a large number of low-frequency ripple signals appeared.The rule is particularly evident at the impeller inlet monitoring point.Compared with other monitoring points,it is easier to capture the pulsation signal related to cavitation at the the pump inlet.(6)After verifying the reliability of the numerical simulation results through experiments,using the variation law of low-frequency signals and main frequency signals with cavitation in the spectrum of the pump inlet,a new method to determine the cavitation state based on the pressure fluctuation amplitude function is proposed.The feasibility of this method is verified,using the test data and simulation data under the forward pump operating condition.And at the same time,this method is used to discriminate the cavitation state of the nuclear main pump under the forward-reverse flow condition.