Abstract: A process is provided for the long term shutdown of a high temperature nuclear reactor comprised of a pile of spherical fuel elements in a core by means of a neutron absorbing absorber material wherein the absorber material consisting of spherical absorber elements is introduced in the core of spherical fuel elements in the form of a column, but wherein the intermixing of the absorber material with the fuel elements is prevented. An apparatus for practicing the process is also provided comprised of a graphite side reflector concentrically surrounding a circular cylindrical core filled with a pile of spherical fuel elements of a high temperature nuclear reactor, into which at least two nose shaped projections distributed uniformly about the circumference radially project, with each of the projections comprising a vertical cavity to contain the absorber material. The vertical cavity is located in the vicinity of the core in the area of the frontal side facing the core of each projection.

Abstract: A gas cooled nuclear reactor may have a stationary pile of spherical operating elements. The reactor may be controlled and shut down by absorber rods displaceable in channels of the side reflector. The neutron sources required for the safe start-up of such pellet pile reactors, which in the case of higher capacity pile reactors are installed in bores of the side reflector, are arranged in the stationary pile, where they are more effective and do not occupy positions in the side reflector. They may be located in one or more graphite elements having the same diameter as the operating elements. These elements may be introduced and removed into and from the core together with the operating elements and remain stationary in operation.

Abstract: Improvements in high temperature gas cooled nuclear reactors is effected by an arrangement of paired absorber elements, each pair comprising a rod shaped absorber element and a tubular absorber element generally surrounding the rod shaped element. With this paired combination in a reactor employing spherical fuel elements, the effective life of the absorber rods is significantly extended by protection of the rod shaped absorber element from excessive amounts of irradiation from thermal neutrons.