A low capacity nuclear reactor with spherical fuel elements in a subterranean configuration. The reactor is characterized by a compact construction and extensive elimination of active operating devices, such as charging installation, gas purification devices, regulating systems and safety systems. It is particularly suitable for the generation of thermal energy for heating purposes. A core vessel contains a pile of fuel elements also contains a portion of the graphite bottom and side reflector. Channels are provided in the part of the side reflector located inside the core vessel. Trim and shutdown rods are contained in the channels. The roof reflector rests on the stationary pile of fuel elements. The entire core vessel together with the components contained therein is removed following burnoff of the fuel elements. The heat generated in the core is transferred to a cooling system mounted on the inside of a pressure vessel enclosing the nuclear reactor by a cooling gas which flows from top to bottom. The cooling system is separated from the primary loop by a gas tight jacket.