[en] Since a reactor containment building is the final barrier against the release of fission products to the environment, its design is very important. The containment building of a high-temperature gas-cooled reactor (HTGR) must be designed to withstand a design basis depressurization accident (DBDA), that is, rapid depressurization in the primary coolant system following failure of the penetration closure in a prestressed reactor containment vessel. Current depressurization calculations for HTGR's neglect layering phenomena of the primary coolant in the containment building on the assumption that a flammable mixture will not result even if the primary coolant contains combustible gases such as hydrogen or carbon monoxide. A previous analysis model that treats layering phenomena of helium gas in the containment building is extended to the case of flammable gas mixtures. In discussing the possibility and effects of combustion hazards in the containment building, this report presents an analysis that includes (1) layering calculation of helium, hydrogen, and carbon monoxide in the containment atmosphere (air) following a DBDA, (2) determination of the flammability of the above mixture, and (3) presentation of studies needed for more realistic analyses