Numerical Simulation Research On Fluid Transport And Heat Transfer In Double Layer Containment

The reactor containment as the last safety barrier preventing radioactive material from leaking,its integrity has a great significance for nuclear safety.Therefore,the passive containment cooling system(PCCS)as the system which protects this last safety barrier against leak of radioactive material,its characteristics of heat transfer has a great research value.In the event of an accident such as Loss of Coolant or Main Steam Line Break,a large quantity of high temperature and high pressure steam flows into the containment vessel.In order to prevent the containment vessel from overheating and over pressurization,and prevent radioactive material from leaking out,PCCS is used to reduce the temperature and pressure of the containment vessel.This system does not need human intervention and any electric power facilities,which greatly enhances its safety and reliability.This paper studies on the fluid flow and heat transfer characteristics inside and outside the containment vessel after high temperature and pressure steam leakage and PCCS entering into operation.In the steady state condition simulation,AP1000 double layer containment was simplified reasonably.Pro-E software was used to established a three dimensional model of double layer containment,and FLUENT software was used to simulate the natural circulation of air inside the containment vessel under steady state condition.Temperature and density contours under normal conditions were obtained.In the transient state condition simulation,the AP1000 double layer containment vessel was simplified as a two dimensional model.FLUENT software was used to simulate fluid flow inside and outside the containment with added UDF(User-Defined Functions)for simulating steam condensation containing air on the inside and water evaporation on the outside.MpCCI(Mesh-based parallel code coupling interface)software was used to carry out coupled transient calculations on the fluid flow inside and outside the containment,allowing for heat transfer from the inside to the outside through the containment wall.In this paper,the decay heat of the core after reactor shutdown is considered.The primary circuit temperature and pressure curves of AP1000 Loss of Coolant Accident(LOCA)was used as the reference data,so the steam inlet boundary conditions was defined appropriately.The heat fluxes of convection,condensation and radiation heat transfers continuously increase with time.The heat flux of condensation heat transfer is about two times that of the convection heat transfer,and the heat flux of radiation heat transfer is very small which is not on the same magnitude as that for the convection and condensation heat transfers.Before 1200 seconds,heat fluxes of convection and condensation heat transfers decrease with decreasing height of the containment wall.After 1200 seconds,the water outside the containment wall starts to evaporate,heat fluxes of convection and condensation heat transfers increase rapidly,and at10 meter height,heat flux reaches the peak.After the evaporation of the outside water film,the temperature and pressure inside the containment vessel gradually stabilize and they are lower than the design temperature and pressure.Therefore PCCS plays an important role in the prevention of radioactive material leak.In the cavity of the AP1000 double layer containment vessel,cooling water from the PCCS spays from the top forming a layer of water film on the surface.The film thickness decreases down the containment wall.In the ring space between the two layers,the water film has a relatively higher temperature,and starts to evaporate when the temperature reaches the saturation temperature corresponding to the pressure.Water vapor fills the ring space,and flows upwards along the containment shielding structure to the chimney air outlet.When there is no cooling water,the temperature on the outside surface of the containment is highest,and decreases along the normal line.The temperature continuously increases with decreasing height,and this leads to a higher temperature in the cavity of the double layer containment vessel.