A high temperature gas-cooled reactor (HTGR) is now being constructed at Japan Atomic Energy Research Institute. It is a graphite moderated high temperature gas-cooled reactor of 30 MW thermal power and 95°C outlet helium coolant temperature. Once the primary coaxial tube rupture accident occurs, helium in the reactor vessel are blown out to the containment vessel initially. After the pressure in the reactor depressurizes to that in the containment vessel, the air in the containment vessel flowed into the reactor vessel. The air corrodes high temperature graphite structures in the reactor, then blows up to the top cover by natural circulation. Since helium is still remains in the top cover under these conditions, the injected heavy air and light helium forms stably stratified field. The flow of the injected heavy air is called the negatively buoyant jet, because buoyancy force opposes the flow. It is important to the safety study because the stably stratified field delays the time that the heated air reaches the steal pressure vessel inner surface.