A nuclear steam supply system includes an elongated reactor vessel having an internal cavity with a central axis, a reactor core having nuclear fuel disposed within the internal cavity, and a steam generating vessel having at least one heat exchanger section, the steam generating vessel being fluidicly coupled to the reactor vessel. The reactor vessel includes a shell having an upper flange portion and a head having a head flange portion. The upper flange portion is coupled to the head flange portion, wherein the upper flange portion extends into the internal cavity, and the head flange portion extends outward from the internal cavity. The flanges have a space saving design which are configured to minimize outward extension from the cavity while still providing desired leak protection at the interface between the shell and the head.