抄録
A depressurization accident is one of the design-basis accidents of a very high-temperature reactor, such as the Gas Turbine High Temperature Reactor 300 for Cogeneration (GTHTR300C). When a primary pipe rupture accident occurs, air is expected to enter into a reactor pressure vessel from a breach and oxidize in-core graphite structures. Therefore, it is important to understand the air ingress processes and the mixing processes of air and helium gas. In this study, we investigated the air ingress process by an experimental apparatus simulating the flow path of the GTHTR300C at the time of horizontal double coaxial pipe breaks. The experimental apparatus consisted of a double coaxial cylinder and a horizontal double coaxial pipe. The outer pipe of the horizontal double coaxial pipe was 34 mm in outer diameter and 27.2 mm in inner diameter. The inner pipe was 21.7 mm in outer diameter and 16.1 mm in inner diameter. The outer cylinder of the double coaxial cylinder was 410 mm in height and 283.4 mm in diameter. The inner cylinder was 255 mm in height and 139.8 mm in diameter. Four cartridge heaters were installed in the inner cylinder. There was a helical waterway in the outer cylinder, and the outer cylinder was cooled by flowing water. Immediately after opening the valves, the flow velocity at a horizontal double coaxial pipe increased rapidly, because air entered into the apparatus by the countercurrent flow. Subsequently, the flow velocity decreased. It is considered that a stable density stratified fluid layer was formed, and the flow velocity decreased. Then, because the natural circulation flow that circulates through the apparatus occurred, the flow velocity increased. The results obtained show that a stable density stratified fluid layer of twocomponent gas is formed when the primary pipe breaks. However, the natural circulation flow that circulates in the reactor pressure vessel occurs afterwards. The mixing process of two-component gases during double coaxial pipe breaks is described and discussed.
