Direct energy transfer by fission fragments near the wall of a cavity containing fissioning gas is studied in plane and cylindrical geometries. Analytical formulas are derived for the fission fragment energy flux. Heat transfer equations are solved for optically thick fissioning gases by taking into account the fission fragment energy transport effect. The results are applied to a heat transfer analysis of the fuel assemblies of a heterogeneous gas core reactor. The energy transfer mechanism in the fissioning gas is essentially nonlinear. Thus, the cooling effect due to direct fission fragment energy loss to the container walls does not become significant until the stopping range considerably exceeds the characteristic dimensions of the container. For example, when the ratio of the stopping range to the container dimension λ/δ is equal to 3, 45% of the energy flux at the container walls is due to the fission fragments; yet the maximum fuel temperature decreases by only 10%. If the ratio λ/δ is approximately 100, fission fragments account for 95% of the energy flux to the walls, and the gas temperature decreases by 50%.