Numerical Simulation Of Flow Field Inside Upper Plenum Of PWR With Two Outlets

The flow field in the upper plenum has a significant effect on the safe operation of the reactor,as the most popular technology of the third generation,AP1000 pressure vessel has two outlets instead of three.In this thesis,the numerical simulation of the flow field on the PWR with two outlets is carried out,and APR1400 is chosen for comParasion.Through this study,the coolant mixing effect inside the upper plenum and the thermal stratification inside the outlet nozzles are analyzed.The fluid characteristics of the two outlets reactor upper plenum with different internal structures are comPared,this study could be regarded as an important reference for the optimization of the internal components and relevant studies.In this Paper,the simulation is carried out on the linux system,the Salome is adopted for the pre-processing,to establish calculation model and to generate mesh,and the pre-processing results are saved with python script.The code_saturne is applied to carry out the calculation,and Paraview is chosen for the post-processing.According to the simulation results,the flow field characteristics in the upper plenum with two outlets were explored,several flow Patterns related to the coolant mixing phenomenon in the upper plenum are analyzed.The effect of the guide tube on the mixing effect of the fluid is obtained.Changes in the flow Patterns of the upper plenum with internal components are studied.And the thermal stratification in the upper plenum and outlets is analyzed under the low-leakage fuel management.The calculation results show that there are mainly three flow channels inside upper plenum.After entering upper plenum,the coolant from central area will firstly flow to the top plate,and then turn to outlet nozzles to leave the upper plenum,while the coolant from peripheral region flow directly to outlets.the fluid of lower temperature will generate a vortex beside the baffle which prolongs the duration of flow in the upper plenum.The transverse flow near the outlet nozzles is pretty strong,and it leads to a large pressure drop which has an imPact on the dropping velocity of control rods near hotlegs.As for APR1400,two vortices get entrained in the edge area of upper plenum,resulting in a large pressure drop there.The diamond arrangement of the internal components is proved to be better than the square arrangement.In addition,the coolant mixture mainly appears in the initial section of the outlet pipeline.The results of thermal characteristics show that the outlet has an important influence on the flow temperature distribution in the upper chamber.The temperature distribution in the central area does not change much along the axial direction,while the temperature distribution in the edge area gradually increases when the elevation moves up.In the radial direction,the temperature in the central area fluctuates little over time,while the fluctuation is stronger in the peripheral area.Axially,the central area has a more obvious temperature fluctuations on the top of upper plenum.The mixing effect inside the upper chamber is not sufficient.The maximum temperature difference(40~oC)at the core outlet reduces to 15~oC in the outlet pipeline.The maximum temperature difference inside AP1000 outlet pipeline is less than the maximum temperature difference of the APR1400.Therefore,the coolat mixing effect in AP1000 is better that that in APR1400.The main reason for this phenomenon is that the flow area of APR1400 upper core plate is smaller and the control rods arrangement is in square.