A computational approach to the solution of Navier-Stokes equations for the thermal and flow fields of very high temperature gas-cooled and gaseous core reactors is presented. An implicit-explicit, finite volume, MacCormack method, in conjunction with the Gauss-Seidel line iteration procedure, is utilized to solve axisymmetric, thin-layer Navier-Stokes equations. An enthalpy rebalancing scheme is implemented to allow the convergence solutions to be obtained with the application of a wall heat flux. The subsonic and supersonic flows of helium in a very high temperature gas-cooled reactor and uranium tetrafluoride (UF4) in a gaseous core reactor under variable boundary conditions (such as adiabatic, isothermal, and constant heat flux) are calculated. The numerical results are compared with other published results and experimental-based correlations. The good agreement with empirical correlations indicates the usefulness of the presented model for the prediction of the flow and temperature distribution under the convective and radiative heat transfer environment of very high temperature gas-cooled and gaseous core reactors.