抄録
A depressurization accident is the one of the design-basis accidents of a Very High Temperature Reactor (VHTR). It is an accident unique to a VHTR where air enters a large amount in the rector and oxidizes the graphite core structure, which may damage radioactive material emission and core shape. In order to predict or analyze the air ingress phenomena during the depressurization accident, it is important to develop the method for prevention of air ingress during the accident. We have conducted an experiment to obtain the mixing process of two component gases and natural circulation flow characteristics. The experimental apparatus consists of a cylindrical double coaxial vessel and a horizontal double coaxial pipe. The end of the double coaxial pipe are closed by a valve. The experimental method is as follows. This apparatus is filled with helium gas. Then, the inner cylinder is heated, and at the same time the outer cylinder is cooled. Finally, after confirming the steady state of the temperature of gas and wall in the apparatus, the valve of the horizontal double coaxial pipe are opened. From the experimental results, the gas mixing process and air ingress process (counter current flow, localized natural convection, molecular diffusion) occurring during the depressurization accident can be reproduced roughly. However, since the inner cylinder doesn't reproduce the thin flow path of the actual core, natural circulation flow occurred immediately after opening the valve.
