Primary pipe rupture is an important design base accident in the high temperature gas cooled reactor (HTGR). When a primary pipe of the HTGR ruptures, helium coolant gas in the reactor blows out into the reactor confinement structure and the reactor primary system depressurizes. After the pressures of the reactor and the confinement equalizes, air is expected to enter the reactor core from the breach. Consequently, the core graphite structures may be oxidized by the air and the complicated natural convection of multi component gas mixtures with chemical reactions would take place inside the reactor. Hence, it is necessary to investigate the air ingress process, the natural convection of multi component gas mixtures in order to understand the effect and develop mitigation of the air ingress. JAEA has performed analysis and fundamental experiments about air ingress from the rupture of one or more main coolant pipes on the lower body of the RPV. These studies showed the air ingress phenomena in the depressurized reactor and proposed a new passive mechanism of sustained counter air diffusion (SCAD) that has been shown effective in preventing major air ingress through natural circulation in the reactor. In the present plan, JAEA will construct an experimental reactor mockup including reactor core, the SCAD system, pressure vessel, coaxial pipe and so on. The core is made of graphite or ceramics and heated by electric heaters to allow for test operation up to 1200°C. Present status of these activities will be presented. Based on the analytical results and know-how obtained through the bench test, a 1/8 scale air ingress mockup test, which intends to simulate the accident condition of GTHTR300, is being planned with a conceptual design as the next step of the air ingress experiment evaluation in JAEA. In the design of mockup experimental facility, it is important to reproduce flow phenomena in a reasonable scale from the viewpoint of construction cost. We designed the internal structure to reproduce mixing performance of multi-component flow involving ingress phenomena especially in the guillotine breaks of primary coolant pipe. Complex flow pattern with gas oxidized chemical interaction in the graphite porous structure of the HTGR core will be characterized. Preliminary analytical results especially natural circulation flow patterns induced by density and concentration difference obtained with a CFD model agreed well with that measured by bench experiments, which showed natural circulation pattern in a simplified reactor.

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