This paper presents a methodology for conducting a simplified gas-explosion analysis when there are uncertainties about the amount of fuel involved and the mode of combustion. The methodology is illustrated by a case study of an explosion of a cloud of hydrogen-selenide (H2Se), nitrogen and air. Hydrogen-selenide (H2Se) diluted with N-2 is used in a reactor vessel to produce solar cells. An explosive mixture could be created if the reactor vessel failed and its contents mix with ambient air. Mixtures of 20% or 6% H2Se in N-2 were considered as feedstock into the reactor. It was determined theoretically that an explosion involving either mixture would challenge the reactor room＇s integrity. However, it is unlikely that a local ignition will propagate in the dilute 6% H2Se mixture, because its adiabatic flame temperature is only 850 K; the 20% mixture is borderline flammable. Because of the proximity of personnel to the reactor room and the high toxicity of H2Se, any damage to the room boundary is considered unacceptable. To prevent accidental mixing of H2Se with air in the reactor, a nitrogen buffer was installed between the reactor vessel and the ambient air.