Analysis Of Counter Current Flow Limitation Phenomenon In PWR Nuclear Power Plant Based On RELAP5 Program

Loss of Coolant Accident,LOCA,is getting high attention because when this accident happens,it is possible to release radiative coolant directly into the external environment,and while the accident occurred at the same time the fuel rod is still producing heat due to the continuous nuclear reaction.The LOCA will reduce the coolant flow rate and cooling capacity,which may eventually cause the core to burn down and emit a large amount of radiation into the environment.Therefore,it is necessary to study the process of accidents and the mechanism of the phenomena,so as to meet the design requirements and provide greater margin for the design of nuclear power plants as far as possible.In the event of LOCA of PWR,due to the decrease of core coolant flow rate and insufficient pressure,reverse flow coolant is needed to cool down the core,and there is a Counter Current Flow Limitation,CCFL,that is,when the coolant flows downward under the action of gravity,it will be blocked by upward flow of steam or other gases,resulting in the phenomenon that some or all of the coolant is taken away by the gas phase,and no further increase in the flow rate of the backward coolant,limiting the heat transfer effect.In this paper,the phenomenon of CCFL in the case of SB-LOCA occurred in the main pipeline of the first loop is analyzed on the platform of the best estimation program RELAP5/mod3.4.First,this paper establishes the UPTF experimental bench model with the best estimation program relap5/mod3.4.Under the condition that the model parameters and convergence conditions are determined,the sensitivity analysis of the numerical simulation ability of the CCFL model of relap5 program is carried out according to the pressure,the value of m and the node partition scheme respectively.The results show that when the relap5 program is used to analyze the system,the simulation results are closer to the actual conditions under the high pressure conditions,and the RELAP5 program can not reflect the real situation under the low pressure conditions well.The value of m has obvious influence on the result of calculation,and a suitable value of m should be selected according to the parameters and properties concerned by the user.The finer the node partition is,the more accurate the calculation result is,so that in the acceptable calculation time,a more precise node partition scheme should be chosen.On the basis of discussing the numerical simulation ability of relap5 program of CCFL phenomenon,this paper takes Siddiqui's experiment as the foundation of modeling.The effects of tube length L,pipe diameter D and inclination angle ? are discussed respectively.The result shows that the smaller the pipe length L,the greater the diameter D,result in the higher safety margin of CCFL,and there is no obvious relationship with the inclination angle ?under such situation.