[en] A fissile package must be subjected to a fire test of 800 deg. C for 30 minutes. For a package that contains enriched uranium hexafluoride (UF₆), the temperature of the contents has to be kept below 121 deg. C in order to avoid rupture of a 30B cylinder. A previous study on heat transfer mechanisms in an enriched UF₆ package type 'DOT-21PF-1' found that decomposed gas with a temperature of around 100 deg. C generated from heated phenolic foam thermal insulation was a major heat source heating the cylinder in the initial period of the fire test, in addition to the heat conducted from the outer surface through the thermal insulator into the interior of the package. Mitsubishi Materials Corporation has developed a new packaging for enriched UF₆ 'MST-30' that also uses phenolic foam as the thermal insulator, and has performed a fire test of 800 deg. C for 30 min, which also indicated the temperature history of the package interior affected by heating from the decomposed gas. In this study, a laboratory test was carried out to identify the kind of gas generated from the heated phenolic foam. The test found that the decomposed gas was water vapour. Since water vapour has a large latent heat, it has a large heat transfer coefficient when it condenses on a cool surface. By taking the water vapour heating mechanism into account, an analytical model was developed for MST-30 to simulate the measured cylinder temperature history. Analytical models to evaluate the cylinder temperature under the 800 deg. C/30 min fire test considering conservativeness are also presented. (author)