Fig. 5-5 Air flow in the reactor vessel after the onset of the natural convection A large amount of air enters the reactor vessel by natural convection, driven by the temperature difference between the high-temperature central passage in the reactor core, and the medium-temperature peripheral annular passage surrounding the reactor core. However, this natural convection of air will not occur for several hours or several days because the rector vessel is filled with helium which is lighter than air.

Fig. 5-6 The Second measures for preventing a large amount of air ingress during an accident The natural convection of air during the accident can be stopped by introducing helium in the medium-temperature peripheral annular passage surrounding the reactor core. In other words, purging a small amount of helium in the reactor can prevent the onset of the natural convection of air after the main pipe rupture accident.

A large amount of graphite has been used in the reactor vessel of the High-Temperature Gas-cooled Reactor (HTGR). A potential main pipe rupture would cause massive air ingress into the reactor vessel which would greatly damage the graphite structures by an air-graphite combustion reaction. Two measures have been found to prevent this air ingress during such accidents. The first measure is to cool the graphite structures rapidly using a large amount of air introduced by natural convection. Air is driven by the buoyancy force arising from the temperature difference between the high-temperature central passage of the reactor core and the medium-temperature peripheral annular passage surrounding the reactor core (Fig. 5-5). However, natural convection of air will not occur for several hours or several days after the main pipe rupture because the reactor vessel is filled with helium which is lighter than air. If the reactor can be cooled before the onset of natural convection, then the large amount of air ingress will not occur. We found that the onset of natural convection can be avoided by cooling the reactor at a rate faster than 6 degrees cent./h. Additionally, cooling at a rate of 4 ~ 6 degrees cent./h will not result in a dangerous situation because even at this slower cooling rate, the temperature of the graphite structures in the reactor will be so low at the onset of the natural convection that an air-graphite combustion reaction will not take place. The second measure is to introduce a small amount of helium into the medium-temperature peripheral annular passage. As helium is lighter than air, introducing helium into the passage prevents the growth of the buoyancy force, that is, natural convection of air will not begin (Fig. 5-6).

Reference T. Takeda et al., Study on the Passive Safe Technology for the Prevention of Air Ingress During the Primary-Pipe Rupture Accident of HTGR, Proc. '96 JSME Symp. on Power/Energy Technology, B101, 82 (1996).

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