Analysis Of Internal And External Characteristics Of Sodium Pump In The Primary Circuit Of The Fourth Generation Nuclear Fast Reactor During Coasting-down Transition Process

The sodium-cooled fast reactor nuclear power plant is one of the most advanced fourth-generation nuclear power plants.It has obvious advantages in nuclear fuel utilization and nuclear waste disposal,and can operate under high temperature and atmospheric pressure,with higher safety.The sodium pump in the primary circuit of fast reactor transports the cooling medium to the core,which is the key core component of the primary cooling circulation system directly connected with the reactor core.When the sodium pump runs in coasting-down process(when the power source is lost due to a power failure accident,it continues to rotate at a transient rotating speed under the action of inertial force),the stability and safety of the fast reactor nuclear power plant will be affected by the operating status.Especially when the coasting-down characteristics do not meet the requirements,the core heat will be retained,and the major safety accidents such as core fusion and nuclear leakage will be caused.In view of the above reasons,analyzing the operating parameters of the sodium pump during the coasting-down process is the basis for ensuring the safety of the fast reactor system.In this study,the fast reactor primary circuit sodium pump(Vertical double suction coaxial in and out submersible pump)with specific speed of150 was taken as the research object.On the basis of theoretical analysis and numerical simulation,the hydraulic performance,the flow state,the blade load and the pressure pulsation characteristics of the sodium pump when coasting-down at different rotating speeds were analyzed.The specific work and the main research results of this paper are as follows:1.The transient external characteristics analysis of primary circuit sodium pump of fast reactorThe coasting-down mathematical model was constructed according to the momentum equation,the unsteady numerical calculation of the coasting-down transition process of the sodium pump was realized by compiling UDF(User Defined Function),and the external characteristic parameters of sodium pump under five different working conditions(when coasting-down from 1.1n_r、n_r、0.8n_r、0.5n_r and0.19n_r)were analyzed.The results show that before coasting-down,the head and the shaft power are positively correlated with the rotating speed,and the peak efficiency appears at nominal rotating speed.In the process of coasting-down transition,when the pump starts coasting-down at nominal rotating speed,the time for the rotating speed to drops to 50%and 19%of the initial value meets the nuclear safety standards,and when the pump starts coasting-down at five different rotating speeds,the head for the rotating speed to drops to 19%of the initial value is also within the safety range.With the increase of coasting-down time,the head and the torque decrease with fluctuation.When the rotating speed and the flow rate decrease to 19%of the rated value,the head and torque decrease to 3.6%of the rated value,the decline rates increase significantly.In addition,at different working conditions,the decline rates of the flow,the rotating speed,the head and the torque slow down continuously with the increase of coasting-down time.In the whole process of coasting-down,compared with coasting-down at low rotating speed,the idling time when coasting-down at high rotating speed is increased by 84%.The research results show that the idling time can be effectively improved by increasing the initial rotating speed of coasting-down.2.The internal flow distribution analysis of primary circuit sodium pump of fast reactor under coasting-down conditionWhen analyzing the internal flow of sodium pump under different coasting-down conditions,it is found that before coasting-down,the velocity energy of the model pump is positively correlated with the rotating speed,the pressure energy is positively correlated with the square of rotating speed,and the flow uniformity of sodium pump is better at nominal rotating speed,which is proportional to the efficiency.In the process of coasting-down transition,with the increase of time,the circumferential distribution uniformity of the velocity ratio and the pressure ratio in the guide vane of sodium pump decreases significantly.When coasting-down at nominal rotating speed,when the time increases from 8.8 s to 73.5 s,the decline rates of the velocity energy and the turbulent kinetic energy decrease by 84.4%and 77%respectively,and the decline rates slow down gradually.At the same coasting-down time,the flow in the pump has a linear relationship with the initial rotating speed under different working conditions.The lower the initial rotating speed is,the earlier the separation vortex in the impeller and guide vane is generated,and the more chaotic the flow is.3.The blade lode and pressure pulsation analysis of primary circuit sodium pump of fast reactor underBefore coasting-down,the blade load distribution is uniform,the blade load and the pressure in the impeller channel increase linearly with the increase of rotating speed.At the impeller outlet,the pressure pulsation periodicity increases,and its pulsation amplitude is obviously higher than that in the guide vane.In the process of coasting-down transition,with the increase of time,the flow separation phenomenon is obvious,the pressure ratio distribution uniformity of the impeller blade is weakened,and the pressure pulsation amplitude of the measured points in the guide vane channel is also significantly reduced.At the same coasting-down time,the lower the initial coasting-down rotating speed is,the weaker the uniformity of the blade load is,and the smaller the pressure pulsation amplitude in the guide vane is.Through the above research,it is found that the external characteristic parameters of sodium pump under different coasting-down conditions all meet the nuclear safety standards.By increasing the initial rotating speed of coasting-down,the idling time can be prolonged effectively,the uniform distribution of vortices in the flow field can be improved and the coasting-down hydrodynamic performance of pump can be enhanced.