Because of the steep temperature profile in a gas core fission reactor, various heat transfer processes are involved. The purpose of this work was to investigate the eventual existence of a gas blanket near the wall of the reactor, where kinetic heat transfer dominates. We define the gas blanket as the distance from the wall where the kinetic heat transfer equals the radiative one. We find that for a spherical reactor with a radius of 2 m, the gas blanket has a size of

0.8 m for a thermal power of 5 MW

0.5 m for a thermal power of 10 MW

0.2 m for a thermal power of 50 MW

For a reactor operating with a thermal power of 1MW the kinetic heat flux is always higher than the radiative one.