密度の異なる2成分気体の混合現象に関する研究

抄録

A depressurization accident is one of the postulated accident events in the Very High Temperature Reactor (VHTR). It is an accident unique to the VHTR, in which a large amount of air entering the reactor core oxidizes graphite, which can cause the generation of combustible gas and the release of fission products due to damage to the core geometry. When the main piping is connected horizontally, air immediately enters the lower part of the break due to the counter current flow, and a stable density stratification layer is formed. The infiltrated air promotes gas mixing by local natural convection and counter current flow in addition to molecular diffusion. The experimental apparatus consists of a coaxial double cylindrical vessel and a coaxial horizontal double circular tube. The horizontal double circular tube is closed by a ball valve. The experimental procedure is as follows. First, the inside of the experimental apparatus is evacuated and filled with helium gas. The inner cylinder is the heated with a heater and the outer cylinder is cooled with water to ensure that the gas and wall temperatures have reached a steady state. Then, the valve is opened to reproduce the decompression accident. The experimental results show the following. After the air enters the apparatus, a stable density stratification of air and helium is formed and the air ingress rate decreases. Afterwards, the buoyancy of the air in the inner cylindrical vessel increases due to the heating of the air by the heater, and the stable density stratification is eliminated. Finally, a natural circulation flow is generated, which circles around the machine.