Abstract: A reactor core is immersed in a liquid metal coolant in a core barrel of a liquid metal cooled reactor. The reactor core includes a plurality of fuel assemblies contained in the core barrel, a neutron absorber that absorbs a neutron in the reactor core, and a neutron moderator that moderates a neutron therein so as to control a reactivity of the reactor core. The neutron absorber and the neutron moderator constitute a mixture contained in reactivity control assemblies of the reactor core in the liquid metal coolant prior to immersion of the reactor core. The neutron moderator is composed of zirconium hydride.

Abstract: Fuel bundles for a nuclear reactor are described and illustrated, and in some cases includes fuel elements each having a first fuel component of recycled uranium; and a second fuel component of at least one of depleted uranium and natural uranium blended with the first fuel component, wherein the blended first and second fuel components have a first fissile content of less than 1.2 wt % of 235U. Other fuel bundles are also described and illustrated, and include a first fuel element including recycled uranium, the first fuel element having a first fissile content of no less than 0.72 wt % of 235U; and a second fuel element including at least one of depleted uranium and natural uranium, the second fuel element having a second fissile content of no greater than 0.71 wt % of 235U.

Abstract: Fuel bundles for a nuclear reactor are described and illustrated, and in some cases include fuel elements each having a fissile content of 235U between about 0.9 wt % 235U and 5.0 wt % 235U, and wherein at least one of the fuel elements is a poisoned low-enriched uranium fuel element including a neutron poison in a concentration greater than about 5.0 vol %.

Abstract: Illustrative embodiments provide a reactivity control assembly for a nuclear fission reactor, a reactivity control system for a nuclear fission reactor having a fast neutron spectrum, a nuclear fission traveling wave reactor having a fast neutron spectrum, a method of controlling reactivity in a nuclear fission reactor having a fast neutron spectrum, methods of operating a nuclear fission traveling wave reactor having a fast neutron spectrum, a system for controlling reactivity in a nuclear fission reactor having a fast neutron spectrum, a method of determining an application of a controllably movable rod, a system for determining an application of a controllably movable rod, and a computer program product for determining an application of a controllably movable rod.

Abstract: A fuel assembly, comprising: a plurality of first fuel rods including uranium and not including a burnable poison; a plurality of second fuel rods including said uranium and said burnable poison; and a water rod; wherein said second fuel rods are placed at corners of an outermost layer of a fuel rod array; other second fuel rods are placed, in said outermost layer, adjacent to said second fuel rods placed at said corners; and other second fuel rods are placed adjacent to said water rod.

Abstract: The present invention provides a nuclear fuel assembly, where a boron-containing compound is used as a burnable poison and is distributed in a majority of the rods in the assembly. The assembly comprises a plurality of fuel rods, each fuel rod containing a plurality of nuclear fuel pellets, wherein at least one fuel pellet in more than 50% of the fuel rods in the fuel assembly comprises a sintered admixture of a metal oxide, metal carbide or metal nitride and a boron-containing compound.

Abstract: A reactivity control rod adapted to be used in a reactor core of a fast reactor and disposed at a substantially central portion of the reactor core for controlling a reactivity therein. The reactivity control rod includes a wrapper tube surrounded by a plurality of fuel rods in a reactor core, and a plurality of neutron absorber rods arranged in the wrapper tube. At least one of the plurality of neutron absorber rods includes a cladding tube and a mixture filled in the cladding tube. The mixture is composed of a neutron absorber that absorbs a neutron and a neutron moderator that moderates the neutron.

Abstract: A fuel assembly attains high burnup and increases reactor shut-down margin when loaded into a reactor core wherein a water gap width on a control rod side and a water gap width on a side opposite to the control rod side are almost equal to each other. The fuel assembly has a plurality of fuel rods arranged in a square lattice pattern, each fuel rod being filled with nuclear fuel pellets and also has at least one neutron moderator rod shifted toward one corner where a control rod is inserted, away from a cross sectional center of the fuel assembly.

Abstract: A spacer grid with hybrid flow-mixing devices for nuclear fuel bundle is made up of an intersection of a plurality of thin straps at right angles to form a plurality of cells for receiving and supporting fuel rods. Each strap is composed of two types of strap units, called a primary strap unit and a secondary strap unit, which are alternately arranged along the strap. The primary strap unit is a strap section having a primary vane set, and a secondary strap unit is a strap section having a secondary vane set. The straps intersect such that, by primary and secondary strap units, each intersection forms a hybrid flow-mixing device around the top of each junction. The primary vane set, consisting of a trapezoidal primary vane stand and two bent primary mixing vanes on both sides, protrudes upwardly from the strap and is primarily for generating cross flow between channels.

Abstract: In a boiling water fuel assembly, some of the fuel rods are shortened. It is necessary to establish a sufficiently high maximum power for transition to boiling. To optimize this power, spacers are at a constant distance at a bottom and are at a shorter distance at a top. The spacers belonging to upper group have turbulence-generating vanes which, however, do not project above the shortened fuel rods.

Abstract: A fuel assembly attains high burnup and increases reactor shut-down margin when loaded into a reactor core wherein a water gap width on a control rod side and a water gap width on a side opposite to the control rod side are almost equal to each other. The fuel assembly has a plurality of fuel rods arranged in a square lattice pattern, each fuel rod being filled with nuclear fuel pellets and also has at least one neutron moderator rod shifted toward one corner where a control rod is inserted, away from a cross sectional center of the fuel assembly.

Abstract: A control element for a nuclear reactor includes an absorber and at least three absorber enclosures for receiving the absorber. The absorber enclosure is constructed in such a way that the received absorber can be subjected to local relative burn up of more than 90% without the burnt-off absorber material getting into the reactor coolant. The control element is constructed in such a way that there are at least three absorber enclosures and a predetermined spacing between the enclosures, so that each absorber enclosure forms a mechanical resistance for the absorber and is removable from the starting position in the event of expansion of the absorber.

Abstract: A fuel assembly attains high burnup and increases reactor shut-down margin when loaded into a reactor core wherein a water gap width on a control rod side and a water gap width on a side opposite to the control rod side are almost equal to each other. The fuel assembly has a plurality of fuel rods arranged in a square lattice pattern, each fuel rod being filled with nuclear fuel pellets and also has at least one neutron moderator rod shifted toward one corner where a control rod is inserted, away from a cross sectional center of the fuel assembly.

Abstract: An alloy of zirconium and niobium that includes erbium as a consumable neutron poison, its method of preparation and a component comprising said alloy are provided. This invention relates to an alloy of zirconium and niobium that includes erbium as a consumable neutron poison. The invention also relates to a method for the preparation and conversion of said alloy and a component comprising said alloy. Such an alloy is particularly intended for the manufacture of cladding and/or other elements or structural components of fuel assemblies for nuclear reactors using water as coolant.

Abstract: A fuel assembly for a boiling water reactor comprising a plurality of fuel units (3a, 3b, 3c, 3d), stacked on top of each other, each one comprising a top tie plate (5), a bottom tie plate (6) and a plurality of fuel rods (4a, 4b, 4c) arranged between the top tie plate and the bottom tie plate. The fuel units are surrounded by a fuel channel (9) with a substantially square cross section. At least some of the fuel units comprise fuel rods with different diameters and different fuel quantities. The fuel rods are adapted such that fuel quantity and lattice space are optimized laterally and axially in the fuel assembly.

Abstract: Nuclear fuel assembly configurations are provided for mixed oxide fuels. Neutron poisons are provided within the MOX of certain fuel rods in the fuel assembly, those fuel rods being preferentially grouped towards the periphery of the fuel assembly. In this way, optimized reduction of reactivity is provided during the initial part of the fuel cycle but the neutron poison is burnt out so as not to interfere with the reactivity during subsequent parts of the cycle.

Abstract: A fuel assembly for a boiling water reactor comprising a plurality of fuel units (3a, 3b, 3c, 3d), stacked on top of each other, each one comprising a top tie plate (5), a bottom tie plate (6) and a plurality of fuel rods (4a, 4b, 4c) arranged between the top tie plate and the bottom tie plate. The fuel units are surrounded by a fuel channel (9) with a substantially square cross section. At least some of the fuel units comprise fuel rods with different diameters and different fuel quantities. The fuel rods are adapted such that fuel quantity and lattice space are optimized laterally and axially in the fuel assembly.

Abstract: In order to use up a stockpile of weapons-grade plutonium, the plutonium is converted into a mixed oxide (MOX) fuel form wherein it can be disposed in a plurality of different fuel assembly types. Depending on the equilibrium cycle that is required, a predetermined number of one or more of the fuel assembly types are selected and arranged in the core of the reactor in accordance with a selected loading schedule. Each of the fuel assemblies is designed to produce different combustion characteristics whereby the appropriate selection and disposition in the core enables the resulting equilibrium cycle to closely resemble that which is produced using urania fuel. The arrangement of the MOX rods and burnable absorber rods within each of the fuel assemblies, in combination with a selective control of the amount of plutonium which is contained in each of the MOX rods, is used to tailor the combustion characteristics of the assembly.

Abstract: In order to use up a stockpile of weapons-grade plutonium, the plutonium is converted into a mixed oxide (MOX) fuel form wherein it can be disposed in a plurality of different fuel assembly types. Depending on the equilibrium cycle that is required, a predetermined number of one or more of the fuel assembly types are selected and arranged in the core of the reactor in accordance with a selected loading schedule. Each of the fuel assemblies is designed to produce different combustion characteristics whereby the appropriate selection and disposition in the core enables the resulting equilibrium cycle to closely resemble that which is produced using urania fuel. The arrangement of the MOX rods and burnable absorber rods within each of the fuel assemblies, in combination with a selective control of the amount of plutonium which is contained in each of the MOX rods, is used to tailor the combustion. characteristics of the assembly.

Abstract: A channel box for a nuclear-reactor fuel assembly (MOX fuel assembly) containing plutonium, capable of properly controlling the excess reactivity without mixing neutron absorber or burnable poison into a fuel rod, and a channel box for a nuclear-reactor fuel assembly which does not form any gap between the channel box and members containing burnable poison in the channel box and in which the burnable poison does not directly contact with reactor water.

Abstract: In order to use up a stockpile of weapons-grade plutonium, the plutonium is converted into a mixed oxide (MOX) fuel form wherein it can be disposed in a plurality of different fuel assembly types. Depending on the equilibrium cycle that is required, a predetermined number of one or more of the fuel assembly types are selected and arranged in the core of the reactor in accordance with a selected loading schedule. Each of the fuel assemblies is designed to produce different combustion characteristics whereby the appropriate selection and disposition in the core enables the resulting equilibrium cycle to closely resemble that which is produced using urania fuel. The arrangement of the MOX rods and burnable absorber rods within each of the fuel assemblies, in combination with a selective control of the amount of plutonium which is contained in each of the MOX rods, is used to tailor the combustion characteristics of the assembly.

Abstract: A nuclear fuel element for use in a gas cooled nuclear reactor, such as a Magnox reactor, has a plurality of fuel pin supports defining an element axis passing through the supports. Suspended between the supports are a plurality of fuel pins formed by a stack of oxide fuel pellets within a metal tube. Connected between the supports are a number of elongate members, for example tubular rods, which extend parallel to the axis and are disposed around the fuel pins. One or more of the tubular rods incorporates a doping material, such as gadolinia.

Abstract: A neutron absorbing apparatus which includes two adjacent neutron absorbing plates and a mounting assembly with latching means configured to be easily secured to fuel assemblies while the fuel assemblies remain under water in a fuel storage rack, thereby eliminating the need to remove the fuel assemblies or the fuel storage rack for installation. The two neutron absorbing plates are positioned orthogonally to form a chevron cross section which can be placed about the fuel assemblies by insertion in the existing space between the fuel assemblies and the cell walls of a fuel storage rack. A prescribed orientation of the chevron configured neutron absorbing plate in the cells of the fuel storage rack together with the selected use of a single neutron absorbing plate economically provides sufficient neutron absorption in all radial directions about the fuel assemblies to maintain safe storage condition sin closely packed fuel storage racks.

Abstract: A fuel assembly and a reactor core using same which are able to increase size of the fuel assembly with ensuring thermal margin and reactor shut down margin.A distance between centers of adjacent fuel assemblies is about 23 cm, which is enlarged about 1.5 times of conventional fuel assemblies. A thickness of water gap region is about 16 cm, which is relatively thinner than that of prior art. While, H/U ratio is about 5 as same as that of the prior art, and decreasing amount of non-boiling water in the water gap region is arranged in a channel box as water rods. Consequently, a ratio of transversal cross section area of the water rods to transversal cross section area of the fuel rods becomes about 0.6, and local power peaking factor can be decreased and thermal margin can be increased. Further, the transversal cross section area of the water rod is selected to be 15 cm.sup.2 so as to ensure the reactor shut down margin by reducing excess reactivity.

Abstract: A nuclear propulsion reactor. A pressure vessel is provided with a reactor core that is surrounded by a radial reflector. Nuclear fuel elements in the core are formed from a hexagonal housing made from a high performance moderator and having a plurality of axial bores that extend the full length of the housing. A stack of nuclear fuel compacts having axial bores for coolant flow is received in the central axial bore of the housing. Hollow lithium hydride slugs are received in the bores at the corners of the housing. A rocket nozzle is attached to one end of the pressure vessel. Coolant/propellant flows into a passageway around the rocket nozzle for cooling thereof, upward through bores in the reflector and through the hexagonal housings of the fuel elements, downward through the bores in the nuclear fuel compacts and then out the rocket nozzle where propulsive thrust is produced.

Abstract: A fuel assembly for a light water reactor comprising a plurality of fuel rods which contain plutonium as a primary fissile material when exposure is zero, and a light water reactor including such fuel assemblies. The fuel assembly has a structure in which at least one of moderator rods is provided at least in one of each corner portion of an arrangement of the fuel rods and a position adjacent to the corner portion in such a manner that the moderator rods are located in rotation symmetry, each of the moderator rods being filled with water or a solid coolant over a length at least corresponding to a fuel effective length, and the fuel rods are provided at positions in the second layer from the outermost periphery which are adjacent to those positions at which the moderator rods are located.

Abstract: A reactor core removable fuel assembly includes upper and lower tie plates having pluralities of fuel rods and hollow control rods extending therebetween. The lower tie plate includes a lower manifold therein joined in flow communication with a reservoir containing a neutron absorbing control liquid, with the reservoir being removable from the reactor core together with the fuel assembly. The control liquid is selectively pumped from the reservoir through the lower manifold and into the control rods for selectively varying the level of the control liquid therein for controlling reactivity.

Abstract: A nuclear fuel assembly has an array of fuel rods comprising a plurality of first fuel rods each containing nuclear fuel material but not containing burnable poison and a plurality of further fuel rods each containing both nuclear fuel material and burnable poison. The further fuel rods comprise second fuel rods and third fuel rods, and each second fuel rod has at a lower region of the fuel assembly a burnable poison concentration which is a minimum burnable poison concentration in the further fuel rods. To increase the effectiveness of the second fuel rods in controlling axial power peaking, as seen in plan view on the array, the first fuel rods are the nearest neighbors of each further fuel rod.

Abstract: A fuel assembly has a plurality of first fuel rods and a plurality of second fuel rods having a shorter length in an axial direction than the first fuel rods. The second fuel rod is loaded with natural uranium in full length of its effective fuel length portion. The fuel assembly has a water rod having a larger horizontal cross sectional area at the upper region than the area at the lower region. The second fuel rods are arranged downward of the upper region of the water rod and adjacent to the lower region of the water rod. The width of the horizontal cross sectional area of the lower region of the water rod is set so as to locate the minimum values of both thermal neutron flux and resonance neutron flux in the horizontal direction of the fuel assembly at an outer side with respect the location of the second fuel rod in the horizontal direction.

Abstract: The apparatus of the present invention is a plate of neutron absorbing material. The plate may have a releasable locking feature permitting the plate to be secured within a nuclear fuel assembly between nuclear fuel rods during storage or transportation then removed for further use or destruction.The method of the present invention has the step of placing a plate of neutron absorbing material between nuclear fuel rods within a nuclear fuel assembly, preferably between the two outermost columns of nuclear fuel rods. Additionally, the plate may be releasably locked in place.

Abstract: A nuclear fuel assembly for a nuclear reactor has a plurality of vertically extending fuel rods arranged side by side in a square array and containing fissile material. The array has two adjacent first sides which are next to a control rod region of the core and two adjacent second sides which are next to a non-control rod region of the core.

Abstract: Water rod configurations, such as for a boiling water nuclear reactor, are provided. An efficiency parameter is defined which relate to how well-used the sacrificed fuel rod positions are. Four particular water rod configurations are described, which produce high efficiency, such as having a water rod efficiency greater than about 0.6, preferably greater than about 0.7. Desired moderation is achieved by providing for sacrifice of more than four and less than nine lattice positions. The first "peanut" configuration has a cross-section with two round-cornered triangular regions, integrally connected by a constricted portion. The second configuration has a substantially rectangular cross-section. The third "clover" configuration has a four-lobed shape. The fourth "figure 8" configuration has two substantially circular cross-sectional portions. A method for analysis and design, using a new efficiency parameter, is provided.

Abstract: A nuclear reactor core sub-assembly is provided with a removable lifting head (12)/absorber pin (20) assembly. The absorber pins (20) are connected to the lifting head (12) in such a way that they can be disconnected, inverted and reconnected the other way round, either collectively as a unit or individually. This allows the same absorber pins to be used at least twice over in different sub-assemblies since only the absorber material closest to the core tends to become depleted during irradiation.

Abstract: A fuel assembly including a plurality of fuel rods and water rods. The fuel rods are arranged with a predetermined pitch and two water rods are disposed adjacent to each other in a diagonal direction of the fuel assembly. More particularly, the fuel rods are arranged in a lattice of nine rows by nine columns and the two water rods are disposed inside a region corresponding to a space sufficient for disposing seven fuel rods at the center of the fuel assembly. Each of the water rods has an outer diameter greater than the pitch of the fuel rods.

Abstract: A self-latching reactivity-reducing device contains neutron absorbing material and is installable in a guide thimble of a spent fuel assembly for reducing the reactivity of the assembly to allow its storage in an on-site fuel storage facility. The device includes an elongated rod which contains the neutron absorbing material and has a central passage extending between oppposite ends of the rod. A self-latching mechanism is disposed on a leading end of the rod and engageable with a lower end portion of the guide thimble upon full insertion of the rod in the guide thimble so as to render the self-latching mechanism unlatchable from the guide thimble without the use of an independent tool inserted solely through the rod passage from a trailing end thereof. A closure plug is removably fastened to the trailing end of the rod for closing the passage therethrough.

Abstract: A fuel assembly of the present invention comprises a plurality of fuel rods which are held by an upper tie plate and lower tie plate at the ends thereof and a moderating rod which is arranged between the fuel rods and held by the lower tie plate at its lower end. The fuel rods are arranged in a lattice form having 9 rows and 9 columns, and the moderating rod contains a passage for a coolant and has a cruciate cross-sectional form. The ratio A.sub.M /A.sub.C of the area A.sub.M of a moderator region in the moderating rod in the cross-sectional plane in which the moderator is present to the area A.sub.C of the coolant passages in said fuel assembly is within the range of 0.07 to 0.11, and the area A.sub.M is 75% or more of the total area of the fuel lattice units in which none of the fuel rods is arranged, but the moderating rod is arranged.

Abstract: The core is constituted by detachable assemblies (21) disposed vertically and maintained in position by a support or bolster (22) receiving the lower part or foot (21a) of the assemblies (21) inside sleeves having a vertical axis (25). Each of the sleeves (25) includes at least one orientation configuration (31) for the assembly (21) and each of the assemblies (21) includes on its foot (21a) at least one orientation configuration (30) adapted to cooperate with the orientation configuration (31) of the sleeve (25). The charging of the first core of the reactor may be carried out in a simple operation without a prior charging of false assemblies in an air atmosphere.

Abstract: A Local Power Range Monitor (LPRM) string is employed between BWR fuel assemblies arranged in side-by-side spaced positions. The string has a hollow tube and neutron detectors located therein at spaced axial locations and being adapted to provide local power monitoring information. The hollow string tube is adapted to receive a neutron flux sensitive probe for calibrating the detectors. Each fuel assembly has an outer hollow tubular channel surrounding fuel rods and spacers disposed within and axially along the channel and about the fuel rods. The spacers are composed of a material incapable of producing a localized change in neutron flux. Elements in the form of strips of a neutron absorber material are attached to the exterior of the channel of each fuel assembly at a respective corner thereon and located axially at different known positions therealong and adjacent to the hollow string tube.

Abstract: The fuel rods located in areas of a nuclear reactor core where water jetting damage to the rods can occur are secured together by a clip, in order to change the mass subject to fretting damage. The change of mass changes the amplitude of the induced fuel rod vibration, thus reducing possible fuel rod fretting an/or damage. To repair such damage, the combination of clip(s) and an inert rod are used to replace a damaged fuel rod.

Abstract: An apparatus and process utilizing a large water rod of a fuel bundle in a boiling water reactor fuel assembly as a radiation heat sink during a loss of coolant accident. Core cooling spray is collected at the top end of the bundle's large water rod by a collector and distributed by a distributor to coat the inside wall surfaces of the water rod. In the case of a water rod which is not bottom vented, the water rod body is modified so as to separate the downward flow of liquid water from the upward flow of steam. During normal plant operation, the large water rod serves as a water-filled moderator tube to provide a more uniform power distribution across the fuel bundle.

Abstract: A burnable, thermal neutron absorber element is provided with a zirconium alloy elongate container having sealed therein both a burnable absorber and the solid moderator material, zirconium hydride. The zirconium hydride is in a concentration and position to enhance the neutron capture efficiency of said thermal neutron absorber in a light water reactor neutron irradiation environment.

Abstract: A burnable poison rod for use in a nuclear reactor comprises a metallic tube with upper and lower closure means, a neutron absorber positioned within the cladding, and a neutron moderating spacer means positioning the neutron absorber in spaced relation to the lower closure means. The neutron moderating spacing means can comprise a solid neutron moderating material, or a liquid coolant moderator contained within the lower section of the cladding with a sealing plug positioned between the neutron absorber and the liquid coolant moderator.

Abstract: A fuel assembly having a plurality of fuel rods arranged in a lattice state and being charged into a core of a nuclear reactor using light water as a coolant, which comprises the fuel rods consisting of a first group of fuel rods and a second group of fuel rods, an average concentration of U.sup.236 in the first group being lower than an average concentration of U.sup.236 in the second group, and the first group and the second group being arranged unevenly in the fuel assembly, preferably, a fuel assembly wherein the first group is arranged at the periphery of the fuel assembly, and the second group being arranged at positions other than the periphery, has a low reactivity penalty due to U.sup.236 and can reduce the requirements for natural uranium.

Abstract: An improved spectral shift-producing rod in a fuel assembly of a nuclear reactor has an elongated hollow tubular body with a pair of end plugs attached to its opposite ends to hermetically seal the rod. A burnable poison material is contained in the hollow body. The material generates a gas within the body as operation of the reactor proceeds. Also, the material is soluble in moderator water when brought into contact with the same. The rod has a weakened structural region which is subject to rupture at a given level of internal pressure. Preferably, the weakened region takes the form of a thinned disc-like portion formed in at least one of the end plugs. The water soluble material within the rod depresses power initially by absorbing neutrons. Absorption of neutrons causes generation of helium gas which increases internal pressure within the rod. When the internal pressure exceeds the rupture strength of the weakened region of the rod, the hermetic seal is broken and water enters the rod.

Abstract: A reusable fastener device includes an attachment nut and a retainer housing mounted to the adapter plate of the fuel assembly top nozzle and removable with the top nozzle upon reconstitution of the fuel assembly. The attachment nut has a central tubular stem and upper and lower flanges connected to and extending radially outwardly from opposite ends of the stem. The stem is internally threaded for mating with the threaded upper end plug extension of the strucutural member. The upper flange of the nut has a conical-shaped lower surface and a periphery adapted for engagement in order to rotate the nut for threading onto and unthreading from the structural member extension between fastened and unfastened positions. The lower flange of the nut is in the form of a plurality of radial segments extending outwardly from the stem and angularly spaced from one another so as to define a plurality of cutouts therebetween which alternate with the segments.

Abstract: A fuel assembly for use at non-control rod locations of a nuclear reactor core includes top and bottom nozzles and longitudinal structural members extending between and attached to the nozzles for forming an integral unitary structure. One or more of the structural members includes an elongated hollow cladding tube extending between the top and bottom nozzles and end plugs secured to opposite ends of the tube for hermetically sealing and attaching the tube to the top and bottom nozzles. The improvements in the structural member relate to features for reducing fuel assembly bow. Such features relate to a quantity of thermal or irradiation-induced creep resistant material and pretensioning means positioned within the tube. The creep resistant material is a ceramic material in stacked pellet form and coated with a burnable adsorber material.

Abstract: A fuel assembly with a top nozzle having an adapter plate, at least one guide thimble connected to the adapter plate, and a burnable absorber rod disposed within the guide thimble, includes a releasable latching structure for releasably interconnecting an end of the absorber rod to the adapter plate. The latching structure includes a recess defined in the adapter plate within a passageway through the plate, a mounting body attached to the end of the absorber rod and extending axially upward therefrom through the passageway and above the adapter plate, and a spring latch disposed about the mounting body above the adapter plate. The spring latch has circumferentially spaced latch fingers extending downwardly toward the adapter plate.

Abstract: A nuclear reactor in which control rods are inserted in the thimbles of only certain of the fuel assemblies (which may be called controlled assemblies) in the core and the remainder of the assemblies (which may be called non-controlled assemblies) are provided with hollow structural members containing burnable poison which form these non-controlled assemblies into integrated units. The hollow structural members are formed with end plugs welded to the lower end and are open at the top. The end plug of each member is secured to the bottom nozzle of the non-controlled fuel assembly. A skeleton is formed of the bottom nozzle, the structural members secured to it and a plurality of grids spaced along the structural members. A bulge tool is then inserted in each hollow member and it is on each side of each grid. Neutron absorbers are then inserted in each hollow member and an upper end plug is welded to it. The fuel rods are then inserted in the assembly and the upper end plugs are secured to the top nozzle.

Abstract: A nuclear fuel assembly has a plurality of fuel rods each containing a multiplicity of fuel pellets. The fuel rods include a plurality of first fuel rods and a plurality of second fuel rods. Each of the first fuel rods contains a burnable poison over substantially the entire axial length thereof. The first fuel rod also has a greater means enrichment in the upper axial region thereof than that in the lower axial region thereof. Each of the second fuel rods contains no burnable poison and has a uniform enrichment distribution over substantially the entire axial length thereof. In each cross-section of the fuel assembly perpendicular to the axis thereof, the outer peripheral portion has a greater mean enrichment than the central portion. The first fuel rods are disposed in the peripheral portion except the outermost peripheral portion of the cross-section of the fuel assembly.

Abstract: An arrangement for controlling the moderation of a nuclear reactor which includes multiple fuel assemblies each of which includes top and bottom nozzles with fuel rods and control rod guide thimbles located therebetween. Certain ones of said assemblies include at least one tube initially filled with a gas. One end of the tube rests on the lower nozzle upper surface while the upper end terminates at a point just below the lower plate in the upper nozzle. A spike immovably fixed in the lower plate of the upper nozzle is directed downwardly toward the end of the tube. As the tube expands or grows during operation in a reactor, and at a point where the criticality factor may proceed to less than one, the expanded tube will be pierced by the spike and thus permit the escape of gas from the tube which is then replaced by water coolant circulating through the reactor.