A colloidal-gas core nuclear reactor having a high tensile strength outer semispheroidal pressure shell enclosing a moderator body member which surrounds a central cavity with an expanding exit nozzle at one end thereof. The cavity wall has a gradually increasing radius from the nozzle end in a direction upstream within the cavity. A hydrogen gas manifold is positioned at the maximum radius of the cavity and supplies gas to the cavity through a plurality of annularly positioned swirl vanes. Additional hydrogen gas is supplied to the cavity with a tangential component through nozzles positioned in a plurality of positions of the cavity wall adjacent the exit nozzle to prevent the flow of nuclear particles along the forward wall boundary layer to the exit nozzle.