Abstract: A double lock joint structure is used in a reconstitutable fuel assembly for attaching the adapter plate of the assembly top nozzle in releasable locking engagement upon the upper end portions of the assembly guide thimbles. The joint structure includes axially extending slots defined in each guide thimble upper end portion permitting inward elastic collapse thereof to a compressed position and outward elastic return thereof to an expanded position. The joint structure also includes axially spaced upper and lower annular bulges formed in the upper end portion. The upper annular bulge has an outside diametric size greater than the inside diametric size of the adapter plate passageway when the guide thimble upper end portion is at its expanded position and less than the inside diametric size of the adapter plate passageway when the upper end portion is collapsed to its compressed position allowing insertion and withdrawal of the upper end portion into and from the adapter plate passageway.

Abstract: An integral reusable locking arrangement for a top nozzle of a reconstitutable fuel assembly includes a thin-walled tubular section on the upper end portion of each guide thimble of the fuel assembly above an externally threaded section thereon and an axial section on ecah alignment sleeve of the top nozzle above its lower internally threaded section. The tubular section has an annular circumferential protrusion defined thereon having an external diametric size which is greater than that of the remainder of the tubular section. The axial section on the sleeve has an internal diametric size which is greater than that of the tubular section but less than that of the protrusion.

Abstract: In a reconstitutable fuel assembly, a top nozzle attaching structure, which has an outer socket formed by a circumferential groove defined in the top nozzle adapter plate passageway wall and an inner socket formed by a circumferential bulge defined in the guide thimble upper end portion being capable of seating within the circumferential groove, incorporates an improved reusable tube disposable at a locking position for releasably locking the inner socket in locking engagement within the outer socket when the circumferential bulge is seated within the circumferential groove. The reusable locking tube includes an elongated hollow metal tubular body and at least a pair of small dimples preformed on the tubular body exterior prior to initial insertion of the locking tube to its locking position.

Abstract: The hold-down plate of the top nozzle of a fuel assembly is interconnected to the upper extension members of the fuel assembly guide thimbles by an improved joint structure associated with each of a plurality of passageways through the hold-down plate which receives an upper end portion of each one of the guide thimble upper extension members. The improved joint structure includes an internal annular ledge on the hold-down plate within the passageway at a lower portion thereof so as to surround the upper end portion of the extension member, an annular recess on the upper end portion of the extension member, and an annular spring member fitted on the upper end portion of the extension member within the recess.

Abstract: In a reconstitutable fuel assembly, a top nozzle attaching structure for mounting the top nozzle adapter plate in releasable locking engagement upon the guide thimble upper end portions includes several improved features. First, each locking tube is mounted within the guide thimble upper end portion for movement between an upper locking position wherein the adapter plate and guide thimble upper end portion are maintained in locking engagement and a lower unlocking position wherein the adapter plate is releasable from the guide thimble upper end portion. Also, cooperating means are defined on the locking tube and the guide thimble for retaining the locking tube at either of its upper or lower positions. The guide thimble cooperating means includes a pair of upper and lower circumferential bulges defined on the guide thimble upper end portion below the adapter plate.

Abstract: A reconstitutable fuel assembly includes an improved attaching structure which has a plurality of coupling sleeves interfitting the upper hold-down plate, lower adapter plate and a plurality of hold-down springs of the top nozzle of the assembly. The coupling sleeves insert over a plurality of extensions extending upwardly from the main bodies of the guide thimbles. The sleeves rest on annular ledges formed on the bottom portions of the respective extensions. A removable cap is attached to the top portion of each of the thimbles extensions and applied to the upper portion of each of the coupling sleeves to releasably lock the coupling sleeves on the guide thimble extensions. In view that the coupling sleeves are fixed to the adapter plate at their lower portions, the sleeves and hold-down plate, adapter plate and hold-down springs disposed between the plates and encircling individual sleeves can be removed together from the fuel assembly as a unitary subassembly upon removal of the removable caps.

Abstract: In a reconstitutable fuel assembly, a top nozzle attaching structure for mounting the top nozzle adapter plate in releasable locking engagement upon the guide thimble upper end portions includes several improved features. First, a locking tube is mounted within the guide thimble upper end portion for movement between an upper locking position wherein the adapter plate and guide thimble upper end portion are maintained in the locking engagement and a lower unlocking position wherein the adapter plate is releasable from the guide thimble upper end portion. Also, a plurality of protrusions are attached on the locking tube so as to extend outwardly thereof and through axial slots defined in the guide thimble upper end portion. The protrusions are yieldable for engaging and releasing from a groove in the passageway of the adapter plate. The purpose of the adapter plate passageway groove is to receive a bulge on the guide thimble upper end portion when the guide thimble is engaged with the adapter plate.

Abstract: A fuel assembly includes top and bottom nozzles, elongated hollow control rod guide thimbles extending longitudinally between and rigidly interconnecting the top and bottom nozzles such that together they form a rigid structural skeleton of the fuel assembly, transverse grids being supported on the guide thimbles at axially spaced locations therealong between the top and bottom nozzles, fuel rods extending through and being supported by the grids between the top and bottom nozzles so as to extend in generally side-by-side spaced relation to one another and to the guide thimbles, and elongated solid tie rods extending longitudinally between the top and bottom nozzles and through and rigidly interconnected with the grids so as to extend in generally side-by-side spaced relation to one another, to the fuel rods and to the guide thimbles so as to bolster the stiffness of the structural skeleton of the fuel assembly.

Abstract: A method of straightening an irradiated fuel assembly having a plurality of control rod guide thimbles in which some of the guide thimbles are bowed and thus greater in length than other of the guide thimbles comprises the steps of determining the length adjustments required for shortening the respective bowed guide thimbles in order to make their respective lengths generally the same as the other guide thimbles, and then forming expansions in the bowed guide thimbles so as to shorten their respective lengths by the amounts of the length adjustments. Usually, a plurality of expansions are made in most of the bowed guide thimbles at spaced locations between opposite ends thereof. Some of the expansions differ in the amount of shortening from other of the expansions. Also, the total amount of shortening of the bowed guide thimbles is controlled by the number of the expansions and the amount of shortening which results from each expansion.

Abstract: The invention relates to a fuel assembly with cooled guide tubes for a nuclear reactor.It consists of a bundle of parallel fuel tubes (1) held in a structure comprising spacer grids, guide tubes (6) and end plates. At least a part of the guide tubes (10) comprises a set of guiding surfaces (16) projecting towards the interior of the tubes (10) over the whole of their length. The projections (16) are formed by embossing of the wall of the tube (10) and permit the guidance of the rod of absorbing material which is moved in the tube and the provision of circulating spaces for the cooling fluid between the tube and the rod of absorbing material.The invention applies in particular to pressurized water reactors and to reactors with neutron spectrum variation.

Abstract: The nuclear fuel assembly comprises a bundle of fuel elements some of which contain a burnable neutronic poison, held in position at the nodes of a regular lattice by a skeleton having end pieces connected together by guide tubes occupying some of the nodes of the lattice. All the poison-containing elements are disposed so as to act on the reactivity of the adjacent assemblies in the heart of the reactor.

Abstract: A method for reconstituting a nuclear reactor fuel assembly having its top nozzle adapter plate welded to its control rod guide thimble sleeves. Circumferentially cut the sleeves from the adapter plate below the weld. Separate the top nozzle from the cut sleeve 5. Obtain a modified top nozzle refashioned with a groove in its adapter plate control rod passageways. Insert the cut sleeves in the passageways of the modified top nozzle. Circumferentially bulge the sleeves into the grooves. A system for attachment of the top nozzle to the sleeves employing the above-described circumferential bulge and groove attachment.

Abstract: A nuclear reactor fuel assembly having an improved attaching structure for removably mounting the top nozzle on the upper ends of the control rod guide thimbles. The attaching structure includes an outer socket defined in the adapter plate of the top nozzle, an inner socket formed on the upper ends of the guide thimbles, and a removable locking tube that is inserted in the inner socket to maintain it in locking engagement with the outer socket. The outer socket is in the form of a complement of special shaped bores having an annular groove. The inner socket is in the form of a sleeve having its lower end attached to the upper end of the guide thimble and with its upper end provided with a circumferential bulge which seats in the annular groove. Three elongated slots are provided in the upper end of the sleeve to permit inward elastic collapse of the slotted portion so as to allow the larger bulge diameter to be inserted through the bore of the adapter plate and expansion into the annular groove.

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 top nozzle of a nuclear reactor fuel assembly is removably mounted to the guide thimbles of the fuel assembly by mating and unmating of pluralities of outer and inner sockets defined in the respective top nozzle and guide thimbles. A bearing plate supported on the top nozzle and movable relative thereto between a lower, non-release position and an upper, release position supports a plurality of locking tubes which are inserted at their lower end portions into the outer sockets. When the bearing plate is at its non-release position, the lower end portions of the locking tubes are inserted into the inner sockets and prevent them from unmating from the outer sockets and thereby retain the top nozzle mounted to the guide thimbles. However, when the bearing plate is at its release position, the locking tube lower end portions are withdrawn from the inner sockets and allow them to unmate from the outer sockets and thereby allow lifting of the top nozzle off and removal from the guide thimbles.

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: A complementary shutdown device for an undermoderated nuclear reactor, comprising, inside certain arrays (3) of the core, guide tubes (7) of which the cross-section is at least equal to the sum of the cross-sections of three rods (6), and which are surrounded by at least one row of fertile rods (8), and a set of shutdown rods, the cross-section of which matches the cross-section of the guide tubes (7). The shutdown is obtained by allowing the shutdown rods, containing a neutron-absorbing material, to fall into the arrays (3) of the core, in the position of maximum insertion. The invention applies, in particular, to pressurized water nuclear reactors.

Abstract: A symmetric blanket fuel assembly forming a part of a radial blanket in a nuclear reactor core includes a first group of fuel rods which contain natural uranium and a second group of fuel rods which contain enriched uranium. The first group is surrounded by the second group in a symmetrical relationship in which the first group forms an inner, centrally-located, generally squared pattern having four sides and the second group forms an outer, peripherally-located generally squared annular pattern which surrounds the first group on all four sides thereof. Also, a plurality of guide thimbles are interspersed in spaced-apart relationship among both the first and second groups of fuel rods, preferably likewise in a squared pattern.

Abstract: A top nozzle subassembly having a hold-down device incorporated therewithin is provided for a nuclear reactor fuel assembly. The subassembly is slidably mounted on the upper ends of the control rod guide thimbles and is removably attached by internally threaded collars which threadably mate with external threads provided on the upper ends of the guide thimbles. Unthreading of the collars allows the subassembly to be removed from the fuel assembly to gain top access to the fuel rods within the fuel assembly. The invention also includes a special tool for threading and unthreading of the collars.

Abstract: A nuclear fuel assembly is described for producing tritium in a light water moderated reactor. The assembly consists of two intermeshing arrays of subassemblies. The first subassemblies comprise concentric annular elements of an outer containment tube, an annular target element, an annular fuel element, and an inner neutron spectrums shifting rod. The second subassemblies comprise an outer containment tube and an inner rod of either fuel, target, or neutron spectrum shifting neutral.

Abstract: Fuel assembly for a nuclear reactor comprising at least two guide tubes (6) made of a material metallurgically compatible with the material of the spacer grids (9, 10). The guide tubes (6) are fixed rigidly to the plates (8) and to the spacer grids (9, 10). The other guide tubes (7), made of a material with a low neutron capture cross section, are secured to only one of the end plates and are movably engaged in the cells of the spacer grids (9, 10) so as to be able to move with respect to these under the effect of expansion.The invention is particularly applicable to pressurized water nuclear reactors.

Abstract: A grid of a fuel assembly structure for a nuclear reactor comprises two sets of uniformly spaced straps perpendicular to each other for receiving and supporting cylindrical sleeves. The straps forming cells which receive the sleeves have notches at their edges, and the sleeves are joined to the straps by lap welding at the notches.

Abstract: Fuel assembly for a nuclear reactor having a bundle of fuel rods of circular cross section extending parallel to one another and disposed at positions forming a raster, including a guide tube for a control rod extending parallel to the fuel rods in the bundle and occupying the positions for and having a cross section covering the cross section of a plurality of mutually adjacent fuel rods in the raster the cross section of the guide tube being defined by circular arcs conforming with those of the circular cross section of the adjacent fuel rods covered thereby and being connected by concave arcuate sections.

Abstract: A fuel assembly for a nuclear reactor comprises a 5.times.5 array of guide tubes in a generally 20.times.20 array of fuel elements. The guide tubes are arranged to accommodate either control rods or water displacer rods. The fuel assembly also comprises a plurality of Inconel and Zircaloy grids arranged to provide stability of the fuel elements and guide tubes while allowing the flow of reactor coolant therebetween.

Abstract: A core construction for a nuclear reactor having a multiplicity of fuel cells forming a core in which each fuel cell includes four fuel assemblies arranged in adjacent relationship and one control rod insertable into a position surrounded by the four fuel assemblies. The fuel cells includes first fuel cells each having a relatively high cell mean infinite neutron multiplication factor and including one fuel assembly containing a burnable poison, and second fuel cells each having a relatively low cell mean infinite neutron multiplication factor and including no fuel assembly containing a burnable poison. The first fuel cells are arranged both in a central zone of the core disposed within about one-half the core radius from the center of the core and in a circumferential zone of the core outside the central zone. The second fuel cells are arranged only in the central zone and no second fuel cells are arranged in the circumferential zone.

Abstract: An improved control rod guide tube for use in a fuel assembly in a nuclear reactor. The guide tube extends the complete length of the fuel assembly and has its upper end fastened in a cylindrical housing by swaging the guide tube material into grooves formed in the housing walls. To eliminate wear on the guide tube inner walls caused by hydraulic induced vibratory forces on a control rod adapted to move therein, a thin-walled chrome plated sleeve is threaded into the top end of the guide thimble and extends downwardly a distance sufficient to be engaged by the control rod during reactor operation. The sleeve serves as a highly resistant wear surface between the control rod and walls on the guide tube in the fuel assembly.

Abstract: A reconstitutable nuclear fuel assembly is improved by the placement of a wear sleeve within the control rod guide tube and below the removable guide tube post. The improvement comprises the guide tube having an internal ledge at its upper end from which a flanged, thin-walled metal cylindrical sleeve is suspended. The cylinder is expanded into firm contact with the inner wall of the guide tube over substantially the entire length of the cylinder. The friction fit provided by the firm contact prevents the sleeve from being pushed upward by the flow of coolant through the tube, while the flange and ledge prevent any downward movement of the sleeve in the tube. The flange on the ledge is located in spaced vertical alignment with the lowermost internal portion of the post. Thus, as the sleeve thermally expands it moves upward into the space.

Abstract: A design of fuel assembly control rod guide thimble which includes insets or indentations which project inwardly from the upper end of the guide thimble and extend longitudinally of the thimble for about the same distance traversed by a control rod movable therein. These preferentially formed discrete indented areas or insets within the guide thimble provide for controlled localized control rod to guide thimble wear. The insets minimize wear at the control rod-guide thimble interface by lowering the normal load and encouraging the control rod into a line contact wearing mode. Additional insets may be remotely installed after irradiation if desired.

Abstract: The invention provides a hold-down device which maintains a net downward force on a fuel assembly in a nuclear reactor under all thermal and hydraulic conditions in the reactor core region. The hold-down device is particularly applicable in reactors having stainless steel support barrels and Zircaloy guide tubes.The hold-down device of the preferred embodiment of the invention is comprised of coil springs coaxially disposed about alignment posts which extend upwardly from the top end plate of each fuel assembly. A hold-down plate is slideably mounted on the alignment posts and the coil springs are interposed, in compression, between the hold-down plate and the top end plate. In use, the coil springs bias the hold-down plate upwardly against a core alignment plate to provide a downward force on the fuel assembly. The hold-down plate may also serve as a lifting surface for the fuel assembly during fuel handling.