Abstract: A neutron guide constituted by two superposed and separable tubular elements extends from the reactor core to the immediate vicinity of the bottom wall of the reactor vessel directly above a neutron detection chamber located outside the vessel. The upper element containing a neutron-emissive medium comprises a closed tube filled with inert gas and engaged coaxially with the lower element which is positioned within a support sleeve. The lower element is provided with another closed sleeve filled with inert gas and terminating near the internal surface of the bottom of the reactor vessel.

Abstract: The device comprises at least one compartment grid formed by tube elements of small height placed in juxtaposed relation and each traversed axially by one pin of the bundle. The tube elements are rigidly fixed to each other at successive points in the grid and relatively displaced along the height of the pins so as to be distributed in at least one helix.

Abstract: The safety device comprises an absorber element connected by a separable coupling to the lower end of a vertical operating stem. When released, the absorber element drops under gravity from a top position to a bottom position within an open-topped casing placed among wrappers containing fuel pin bundles. In the top position of the absorber element, the disconnectable coupling is placed at the top and partly within the interior of the casing while the absorber element extends within said casing into the peripheral blanket zone located externally of the central fuel zone.

Abstract: A supporting member for an array of parallel rails each carrying one row of slidably mounted pins of a fuel clister is placed coaxially at the lower end of a vertical fuel assembly wrapper. The ends of the rails are engaged in vertical slots formed in the supporting member and are secured by locking-rods mounted in open grooves located transversely to the slots in the outer faces of the supporting member.

Abstract: A supporting member for an array of parallel rails each carrying one row of slidably mounted pins of a fuel cluster is placed coaxially at the lower end of a vertical fuel assembly wrapper. Each parallel rail is provided at each end with a downward extension and terminal lug which engages in a lateral groove formed in the periphery of the supporting member in order to lock and maintain the rails and the fuel pins in uniformly spaced relation within the fuel assembly wrapper.