A lower internals nuclear reactor structure having a tubular core barrel with an upper and lower open end, coaxially supported therein. A reflector having an outside curvature that substantially matches the curvature of the inside surface of the core barrel and substantially contacts the inside surface substantially over an axial length of the core, is fixedly connected to the inside surface of the core barrel at a plurality of axial and circumferential locations to be substantially supported by the inside surface of the core barrel.