The Study On The Instabilities In The Natural Circulation System With Multi-Parallel Channels Based On RELAP5

In recent years,under the background of boiling water reactor nuclear power plant,steam generator or other application flow instability in the parallel multi-channel natural circulation system gradually caught the attention of the researchers,.However,both experiment and numerical calculation of multi-channel flow instability mainly focused on the double parallel channel system.Such model which simplify various channels can not ensure reflecting the flow characteristics of each channel accurately.RELAP5 program has been widely applied to transient simulation analysis of light water reactor.The work of this paper is to study the applicability that RELAP5 calculates the multi-channel natural circulation system,and analyze some special unstable phenomenon of multi-channel system based on the result.Firstly,a single-phase multi-channel natural circulation system are studied using RELAP5.This system focus on the flow characteristics of power change conditions caused by uneven heat.Results show that the quasi steady state occurs when changing power in the single-phase multi-channel system.Only after power reaching a tipping point,the flow deflection phenomenon can be seen.In addition,the cross-flow can flatten the power.Then,this paper simulate flow instability under low pressure in double channel natural circulation system using RELAP5 code.Results show that the RELAP5/MOD3.4 program can simulate several kinds of unstable phenomenon accurately in 0.1MPa pressure,which are proved by analyzing mechanism.These phenomenon includes four kinds of unstable phenomenon: flash-inducing intermittent oscillation,irregular flow oscillation,sinusoidal oscillation under the condition of high dryness.The result are in good agreement with the experimental date in the period of oscillation.There is a inaccuracy in amplitude between calculation and the experimental results.The maximal difference of amplitude is nearly 28%.Based on the result,this section analyzes the pressure,thermal component's geometric parameters,throttle coefficient of heating channel's export which influence the unstable phenomenon.The conclusion is as follows: when the pressure is relatively low(less than or equal to 0.2 MPa),stability of the system is poor.When pressure is relatively high(greater than 0.5 MPa),system stability enhances with the pressure increases,as the amplitude reduces.With thermal component thickness thinning,flow oscillates more intensely,as the amplitude increases,the cycle reduces or even loss of cyclical.When throttle coefficient is low(less than 8),flow amplitude decreases with increasing the throttle coefficient.When the throttle coefficient is high(great than 8),the result is opposite.At last this paper research the flow instabilities of a four-channel natural circulation system,and compare the model with entrance compensation.The results showed that,in the four parallel channels,there are four kinds of typical natural circulation system instability phenomenon including: intermittent flash-inducing oscillation,in-phase oscillation,irregular oscillations,and the high-dryness sine wave type oscillation which have a phase difference.With increasing the number of channels,the system unstable phenomenon becomes more complex,as appearing in-phase oscillations.In addition,in the high-dryness sine wave type oscillation,there is complex phase difference between each channel.After considering entrance compensation,system is asymmetry causing that the instability phenomenon becomes more complicated.Main instability phenomenon includes:flash-inducing out-phase oscillation,saturated boiling and flash phenomenon coupled flow oscillation,in-phase oscillation in condition of high dryness.