Abstract: A spacer grid for a nuclear reactor fuel assembly, comprising an assembly of two families of thin partitions which are respectively parallel in each family and perpendicular from one family to the next and which bound pockets of square section through each of which a fuel rod of the cluster extends, the walls of the pocket being formed with bearing points to retain the fuel rods of the cluster, wherein it comprises bearing bridges formed by profiled bosses produced in pairs on a sheet metal tongue obtained by making two longitudinal parallel slots in the thin partition on either side of such bosses, the bosses being oppositely directed in two adjacent pockets, and the reaction of the fuel rods on the bosses after they have been positioned in the grid produces a torque on the resilient sheet metal tongue, the deformation of the tongue thus enabling the assembly tolerances of the fuel rods to be absorbed in the grid. Application to nuclear reactor assemblies.

Abstract: A fuel assembly for a nuclear reactor is provided with two access plugs in the adapter plate of the top nozzle whereby selected fuel rods can be removed and replaced without disassembling the assembly and while the assembly is in place in its operating position in the core region.

Abstract: Device for the dismantlable fixing of a guide tube in a recess provided for this purpose in the end piece of a fuel assembly of a nuclear reactor, particularly of the water-cooled type, wherein the guide tube has an end fitting, whose ferrule-like end has an inner cylindrical profile slotted over its entire length into a certain number of flexible sectors which can be engaged in an annular recess, or on a bore having circular engagement threads on its surface, under the pressure of an internal cylindrical locking ring, means also being provided for braking the locking ring in axial translation with respect to the end piece and for remotely ensuring the dismantling of the ring.

Abstract: Upper internal equipment of a pressurized water nuclear reactor, comprising guide tubes containing guide plates and sleeves adapted to two different types of charge of the reactor core, one for obtaining a thermal neutron spectrum and the other for obtaining an epithermal spectrum. The plates (20) and the sleeves (33, 34) have, for each of the guide tubes, a first set of perforations (31, 36) and a second set of perforations (30,35) for guiding the control rods in either case. The invention applies to a pressurized water reactor operating either as a conventional reactor or as an undermoderated reactor.

Abstract: In a method of inhibiting or avoiding intermixing of the atmosphere which exists in a closed nuclear reactor containment space with a gaseous substance present in the same space a change in density of the atmosphere is induced in either lower or upper area of the space such that a stable atmosphere stratified into superimposed layers is formed in that space due to the change in density.

Abstract: A restraint assembly for use in restraining lateral movement of a reactor core relative to a reactor vessel wherein a plurality of restraint assemblies are interposed between the reactor core and the reactor vessel in circumferentially spaced relation about the core. Each lateral restraint assembly includes a face plate urged against the outer periphery of the core by a plurality of compression springs which enable radial preloading of outer reflector blocks about the core and resist low-level lateral motion of the core. A fixed radial key member cooperates with each face plate in a manner enabling vertical movement of the face plate relative to the key member but restraining movement of the face plate transverse to the key member in a plane transverse to the center axis of the core.

Abstract: It has two groups of thin partitions, which are parallel in one group and perpendicular from one group to the next, which are grooved and nested to define square cavities. Support points are made in the walls of the cavity to hold in place the fuel elements. In each group, a thin partition is constituted by two superimposed sheets. The height of the first sheet is equal to double the height of the second sheet. A first sheet is positioned above the second sheet in one of the groups and below the latter in the other group. Each second sheet is nested into the first sheet of the other group.

Abstract: Device for the emergency removal of the heat dissipated by a fast breeder nuclear reactor when shut down, comprising a liquid-liquid heat exchanger immersed in the vessel of the reactor and a liquid-air exchanger for the cooling of the secondary liquid of the latter exchanger. The liquid-liquid comprises a casing composed of a sleeve (18,19) having openings (21,22) for the natural circulation of the cooling fluid of the reactor in contact with the tubes (25) of a nest (20) disposed in the casing of the exchanger. Each of the tubes (25) includes a straight input section and a long straight return section, separated by a helically wound section forming a loop with a low number of turns, inclined to a small extent with respect to the horizontal plane. The coil diameter of the tubes (25) is constant and identical for all the tubes and very slightly less than the interior diameter of the sleeve (18,19).The invention applies in particular to liquid sodium-cooled fast breeder nuclear reactors.

Abstract: The invention comprises a core barrel plug capable of being remotely installed in a port of a core barrel of a pressurized water nuclear reactor for converting the reactor from a by-pass downflow configuration to a by-pass upflow configuration. The plug comprises a body having an expandable cylindrical portion with a movable mandrel disposed in the body. Remote fluid pressurization causes the mandrel to be advanced thus expanding the body into contact with the port. The plug also comprises a locking mechanism to prevent inadvertent release of the plug.

Abstract: A fuel bundle for a boiling water reactor contains a plurality of fuel rods, some of which contain a moderating material rather than nuclear fuel or contain nuclear fuel in the lower section of the fuel rod and moderating material in the upper section thereof, for improving both the axial and radial power shapes of the fuel bundle. The moderating material in the upper section of the fuel rods increases the neutron moderation in this section of the bundle, thereby compensating for the poor water moderation of the boiling water in this section, increasing the power generated in this section and flattening the axial power shape of the bundle. Fuel rods in the interior section of the bundle contain moderating material which also flattens the radial power shape. Caps containing moderating material are also placed on the upper end of each fuel rod in order to reflect neutrons thereby reducing the top axial fast-neutron leakage.

Abstract: Nuclear fuel assembly having four bundles of fuel rods, at least one fuel bundle being made with twenty symmetrically arranged fuel rod positions, the number of fuel rod positions in each of the remaining fuel rod bundles being at least sixteen and at most twenty-five.

Abstract: A nuclear fuel assembly suitable for use in fast breeders. The fuel assembly has a wrapper tube and a plurality of fuel elements contained in the wrapper tube, each fuel element having a clad tube. The wrapper tube is made of a material which exhibits a larger rate of swelling by neutron irradiation than the material of the clad tube. For instance, the wrapper tube is made of an austenitic stainless steel, and the clad tube is made of an austenitic Ni alloy having a high Ni content. According to this arrangement, it is possible to avoid undesirable contact between the clad tube and the wrapper tube in the last period of the life time of the nuclear fuel.

Abstract: A fuel asembly with a fuel channel (2) which surrounds a plurality of fuel rods (1) and which is divided, by means of a stiffening device (3) of cruciform cross-section and four wings (5), into four sub-channels (4) each of which comprises a bundle of fuel rods.Each fuel channel side has a plurality of stamped, inwardly-directed projections (6) arranged vertically one after the other, said projections being welded to one and the same stiffening wing. Each one of the wall portions located between the projections (6) defines, together with two adjacently positioned projections and a portion of the stiffening wing, a communication opening between two bundles located on one side each of the stiffening wing.

Abstract: A manually controlled tooling system for the remote placement and removal of a load, such as a plug, (12) at a location (6) inaccessible to maintenance personnel. The tooling provides for both horizontal and vertical movement of the load, and, in the preferred embodiment, rotational restraint. The load is supported vertically by at least one line (21) wrapped on a small diameter drum (19) which is supported from a travelling trolley (11) fitted to a sloping track (10). On the same shaft as the load drum, a large diameter centrally located sheave or control drum (20) accepts a single control line (13) counterwrapped in direction with respect to the load support line and terminating at its other end on a hand operated winch mechanism (14). The tooling acts to first allow the trolley to descend the track without relative motion between trolley and load. Then, with the trolley bottomed against a mechanical stop (26), the load is allowed to descend vertically to its desired position.

Abstract: The fissile inventory required in operating a negative power coefficient nuclear reactor in an electric power generating system is reduced by cycling the load imposed on the system when 100% power can no longer be maintained at equilibrium due to xenon poisoning in order to induce an oscillation in the xenon concentration which is in antiphase wtih the power requirements so that 100% power can be maintained at least during part of the day. The load can be progressively reduced by a preset amount each night or a xenon suppression controller which forecasts the xenon reactivity at the time selected for a return to full power as a function reactor history, current reactor flux and an arbitrary load schedule can be used to determine the maximum reduced power level that will permit operation at full power at the selected time.

Abstract: A magnetic reed switch assembly for activating an electromagnetic grapple utilized to hold a control rod in position above a reactor core. In normal operation the magnetic field of a permanent magnet is short-circuited by a magnetic shunt, diverting the magnetic field away from the reed switch. The magnetic shunt is made of a material having a Curie-point at the desired release temperature. Above that temperature the material loses its ferromagnetic properties, and the magnetic path is diverted to the reed switch which closes and short-circuits the control circuit for the control rod electromagnetic grapple which allows the control rod to drop into the reactor core for controlling the reactivity of the core.

Abstract: A flexible control rod for a nuclear reactor is built up of first, second, third and fourth absorber wings which are arranged in a cruciform configuration. Each wing is divided into a number of wing elements by slots extending substantially perpendicularly to the crossing line of the wings. The first and the third wings are arranged in a first plane with the slots in these wings located pairwise in line with each other, and the second and fourth wings are arranged in a second plane and also with their associated slots located pairwise in line. Further, the slots in the first plane are diplaced in parallel in the direction of the crossing line relative to the slots in the second plane, and the portions of the wing elements adjoining the crossing line of the wings are provided with recesses. This arrangement gives good flexibility without undue loss of strength.

Abstract: A fuel assembly is disclosed which comprises a fuel channel (1) with square cross-section supported by an inlet sleeve (22) having at its lower part an inlet opening for a flow of coolant flowing through the fuel assembly. The inlet sleeve (22) is made of stainless steel and is mechanically connected to a transition sleeve (21) made of thick zirconium alloy sheet, which is welded to the more thin-walled fuel channel (1).

Abstract: A passive decay heat removal system in which gas pressurization of a gap between the double walls of heat exchanger tubes initiates heat removal by triggering an increase in tube heat conductivity.

Abstract: A radioactive waste disposal package comprising a canister for containing vitrified radioactive waste material and a sealed outer shell encapsulating the canister. A solid block of filler material is supported in said shell and convertible into a liquid state for flow into the space between the canister and outer shell and subsequently hardened to form a solid, impervious layer occupying such space.