Thermal explosion may occur in the storage process of reactive chemicals with exothermic decomposition reactions when heat removal is not sufficient. The inflection point on temperature versus concentration or time curve has been widely used to study the critical behavior for this type of system. Previous work of critical behavior analysis normally assumed uniform temperature distribution or only pure heat conduction. Hence, the results obtained were only applicable to the small-scale or solid systems. The self-heating effect of the decomposition reaction and reactant consumption often induce temperature and concentration gradients, respectively, which consequently induce natural convection. Therefore, a study of the influence of natural convection on the critical behavior for a large-scale system is necessary. In this work, the critical behavior of hydroxylamine nitrate (HAN) in a cylindrical tank stored in air is studied with the commercial CFD package, Fluent 6.3, by considering the air-film thermal resistance. it was found that the feasible storage region of the large-scale system is smaller than that of the small-scale system because the increase of heat generation with quantity increase overwhelms the heat transfer enhancement due to natural convection. (C) 2008 Elsevier Ltd. Ail rights reserved.