Numerical Study On Natural Circulation Characteristics Of External Cooling In Pressure Vessels

External reactor vessel cooling(ERVC),as an important severve accident mitigation measurements of advanced Pressurized Water Reactors(PWRS),is an safety system used to prevent the molten from going through the bottom head of Reactor Pressure Vessel(RPV)after the core melting down.It is widely applied because of high reliability and low cost due to it’s passive safety feature and simplified structure.ERVC is implemented by natural circulation occurring in the gap between outside wall of the reactor vessel and the thermal insulation structure.In the process of circulation flow,cooling water will remove the decaying heat which is generated by core molten.The effectiveness of ERVC mainly depends on the capability between natural circulation and the capability of heat transfer of the reactor vessel outside wall and water.A lot of works have been done aiming at critical heat flux of RPV wall so far,including theoretical analysis,experimental and numerical calculation research.However,there are still some inadequacies in the field of ERVC,namely the two phase flow model used now can’t represent the real characteristics of two-phase flow accurately.Based on the mass,momentum and energy conservation equations of the fluid in ERVC system.This problem is solved by establishing the mathematic models describing the ERVC natural circulation flow with reasonable assumptions.The homogeneous flow mathematic model and separated flow mathematic model are established separately in this thesis.The mathematical model of ERVC two-phase flow is programmed with Matlab code and preliminarily verified by existing experimental data.Based on the Matlab program and typical engineering parameters,the calculations and analysis are conducted with parameters such as heat power,inlet water temperature,gap clearance and submersed depths.The characteristics of natural circulation in ERVC are then investigated by contrasting the predicting results of different models and sensitivity analysis of key parameters.The result shows that the established homogeneous flow model and separated flow model of ERVC can be used to estimate the natural circulation characteristic of ERVC,and the accuracy is accepted.Parameters such as heat power,inlet water temperature and clearance gap have significant influence on the natural circulation process of ERVC.In the range of 90℃～100℃,the circulation flow rate increases as the inlet water temperature,the heat power,the inlet area and the gap clearances increase.The circulation flow rate decreases with the increase of submersed depths.When the inlet temperature is different,the effects of heat power,gap clearance and submersed depth on circulation flow rate will be different.The predicting accuracy of the separated flow model is higher than that of homogeneous flow model under t low subcooled boiling.The reason is that the assumptions of the separated flow model take the interactions between vapor phase and liquid phase and subcooled boiling effects into consideration.The investigation in this thesis is mainly based on engineering parameters.As a result,the obtained results and conclusions can provide reference and basis for the design,optimization and numerical simulation of external reactor vessel cooling system of pressurized water reactor.