Numerical Study On External Reactor Vessel Natural Circulation Cooling For A Large-size Advanced PWR

External reactor vessel cooling (ERVC) has been adopted widely as a part of in-vesselretention in the severe accident management strategy. ERVC method is implemented bysingle or two-phase flow between thermal insulation structure which is located outside thereactor vessel and the reactor vessel, in the process of cooling water circulation flow coolingwater will remove the heat which is generated due to the molten relocation to the lower head.Therefore, the effective of ERVC treatment mainly depends the natral circulation capabilityof cooling water and the heat transfer between cooling water and the reactor vessel wall.This paper utilizes RELAP5system safety analysis code to simulate and evaluate thethermal-hydraulic behavior of ERVC in the large-size advanced PWR. Firstly, theapplicability of RELAP5code to ERVC phenomenon is evaluated by using availablethermal-hydraulic experimental data. And the modeling process of ERVC system in alarge-size advanced PWR by RELAP5code is described, where the description of mainthermal-hydraulic components is represented. Secondly, steady calculation of a large-sizeadvanced PWR is carried out, some flow and geometry parameters which have importantinfluence on ERVC natural circulation capability, e.g. inlet and outlet area of flow path,inlet temperature of cooling water, heating power at the lower head of RPV and gap size offlow path, are selected for sensitivity analysis. The effect of these parameters on ERVCnatural circulation capability is studied qualitatively. Moreover, based on the steady stateresults achieved, transient calculation is performed to investigate the transient response process and effectiveness evaluation of ERVC during severe accident (in this study largebreak LOCA is selected). Again, sensitivity analysis about some flow parameters, boundaryconditions and and RELAP5two-phase flow model, like inlet and outlet area, gap size offlow path, water level in the reactor cavity and the selection of interphase friction model, isincluded in this transient calculation. Finally, preliminary instability analysis of ERVCnatural circulation flow is carried out, and instability boundaries of different pressures andriser section structures are obtained.The results obtained in this thesis can provide important information to numericalsimulation of ERVC phenomenon and design work of external reactor vessel flow path.