The nuclear criticality calculations are performed on a two-region, spherically symmetric gaseous-core reactor. The core is fueled with U235 vapor and is externally moderated by various thicknesses of graphite at 4000K. A one-group thermal transport analysis with a fast effect correction, which accounts for above-thermal fissions and absorptions, is used to perform the criticality calculations. The thermal-group analysis is developed by applying the Sn transport approximation to the one-velocity, onedimensional, time-independent Boltzmann transport equation. Age-diffusion theory is used to estimate the fast effect correction. The criticality calculations are done on the IBM 7090 digital computer, and results are compared with results from a multi-group transport analysis for similar reactor configurations.