Research On Natural Circulation Cooling Abilities Of Double Barriers Containment

The double-barrier containment is applied to AP1000.The inner barrier is a steel containment and the outer barrier is a concrete guard.It relies on water falling by gravity from the top of the containment water tank and natural circulation heat transfer of air outside,realize the cooling and pressure control of inner-barrier.After the incident of Fukushima,the safety of nuclear power plant when the water tank on the top fails is one of the issues people care about.So the research about natural circulation cooling for double barriers containment is important.The reactor scrams when there is an accident,and the steam is generated in the steel containment due to decay heat.The steam condenses when it touches the cold inner surface of steel containment.Then the heat is conducted to the outer surface of steel containment and the outer surface is cooled by air flow which is driven by natural circulation and flows into the annular channel between containment.So the decay heat is carried by the air flow.Based on the structure and operational principles of double barriers containment,this research establishes a physical model to simulate the working state of containment cooling system after a double-end cold/hot leg rupture accident.This model can describe natural circulation of steam mixture inside,film condensation heat transfer of steam mixture with non-condensable gases,conduction through wall of containment,natural circulation of air flow out of containment,and convection between air outside and outer surface of the steel containment.After establishing the mathematical model,designing how to solve the model,writing UDF of program,mesh division and Fluent simulation,this thesis calculates the physical state in the containment in 3 days after an accident shutdown,only depending on the natural circulation of air.Comparing these results with the calculation of passive simulation program WGOTHIC approved by U.S.NRC validates the correctness of this simulation program.The results indicate that it is impossible to take away the decay heat entirely in 3 days only relying on air natural circulation;the capacity of air natural circulation continuously enhances,but when all the heat dissipation balances the decay heat,pressure in the containment has exceeded the limit;the concentration of non-condensable gases is the highest on the wall near the bottom of the containment,where also has the lowest concentration of water steam.It also proves that it is right to install the hydrogen elimination device unit in that position.Meanwhile,the influences of height and radius of the containment,and the height of chimney to natural cooling were analysed in this thesis.