Numerical Simulation On The Characteristics Of Filtered Containment Venting System With Wet Scrubber

Filtered containment venting system(FCVS)is a severe accident mitigation system for preventing the overpressure of the containment,it has been widely used in nuclear power plants around the world.The most researches focus on the filtration efficiency of the FCVS and the depressurization of the containment,but the researches on the thermal-hydraulic characteristics and the long-term operation behavior of FCVS are still rare.In this thesis,the thermal-hydraulic analysis code RELAP5 was used to perform detailed modeling of a FCVS with a wet scrubber,and we focused on the thermal-hydraulic and long-term operation characteristics of FCVS under a severe accident caused by Station Blackout(SBO).Meanwhile,in order to get more information of the two-phase flow field of FCVS and optimize the local structure,the CFD software was also used to simulate the local flow field of small facility “mini-FCVS” and the bubble column with a special nozzle.Firstly,the CCI FCVS was selected as the research object,and it was modeled by the RELAP5/MOD3.3 code.The long-term operation thermal-hydraulic characteristic in the system was analyzed when the “periodically-open-and-close-strategy” was adopted under SBO,the results showed that the scrubbing water inventory fluctuated periodically,the FCVS could operate normally for up to 250 hours.Based on the model,a simplified containment model was further developed as a boundary condition to simulate the thermal-hydraulic characteristic of the system with “open-and-non-close-strategy”.The results showed that the scrubbing water increased and then decreased gradually.The water might be evaporated to dry-out in the late of the venting period,which led to the failure of filtration.It was found that key factors such as environment temperature,initial liquid level,decay heat power and opening/closing pressure had important effects on the system's operation behavior.When the initial liquid level was selected in the range of 3~4 m,the long-term operation state of the FCVS was better,but it might need to be appropriately increased at high temperature and be decreased at low temperature.The decay heat power should be controlled between 150~260 kW,which could consider to be adjusted by auxiliary water supply and drainage.It was feasible for the opening pressure to be increased from 450 kPa to 500 kPa,and it was suitable for the closing pressure between 250~300 kPa.Secondly,the RELAP5 model for the small facility “mini-FCVS” was established,the simulation results were compared with the experimental results to verify the rationality of the RELAP5 model,and the influence of internal structural parameters on the two-phase liquid level was explored as well.In addition,the CFD software was used to model and numerically simulate the mini-FCVS and to verify the rationality of results compared with the experimental results,and the detailed gas-liquid two-phase behavior in system was analyzed.The results revealed that the nozzle formed the gas into a gas column,the mixing element enhanced the mixing between gas and the liquid,and the recirculation zone facilitated the gas circulation.Finally,a detailed 3D-simulation of the gas-liquid two-phase flow in the bubble column with baffled nozzle was carried out by CFD.The focus was the influence of the baffles on the flow field parameters such as the gas holdup,the axial flow velocity and the bubble diameter.The results showed that the addition of the baffle facilitated the decrease of gasliquid flow velocity near the wall,which led to the increase of the bubble residence time near the wall.And it also facilitated the decrease of the overall bubble diameter and the increase of the central local gas holdup and the local interface area.However,too many baffles would lead to a high-speed jet,which was not conducive to the gas dispersion and the improvement of overall gas holdup and the interfacial area.In summary,it was suitable to choose the number of baffles to as 2.In this thesis,the study on the thermal-hydraulic characteristics and long-term operation behavior of FCVS under different venting strategies can provide the reference criteria for the engineering design and operation of FCVS.The detailed research on the 3D gas-liquid twophase flow field inside the system can provide guidance for the optimization of the local structure.