Abstract: In a control rod for a nuclear reactor having a cruciform shape with four flat planar members, an improved membrane for safeguarding against crevice cracking corrosion is disclosed. Each of the four flat planar members of the control rod is fabricated from side-by-side tubular members containing sealed neutron absorbing poisons. Each of the tubular members has square outside sections. These square outside sections are welded together to form the flat planar members of the control rod. The improvement includes a protective membrane wrapped over the structural flat planar member. The protective membrane surrounds the member and is maintained to the planar member under compression by ambient reactor pressure. The membrane provides an additional margin against crevice corrosion cracking by preventing the water of the reactor from coming into contact with the welded side-by-side square sectioned tubular members.

Abstract: A reconstitutable control rod spider assembly includes a spider structure, control rods, and attachment joints for detachably fastening the control rods to the spider structure. Each attachment joint includes a connecting finger of the spider structure having a central bore extending therethrough and a pair of opposing flat segments formed on the interior thereof, and an elongated upper end plug of each control rod inserted through the finger bore and having a pair of opposing flat sectors formed on the exterior thereof beng interfitted with the opposing flat segments on the connectng finger to retain the control rod in a predetermined angular position relative to the finger. The upper end plug also has an externally-threaded portion formed on the exterior thereof above the flat sectors and being disposed above the connecting finger, and a pilot aligning portion formed on an outer terminal end thereof above the externally-threaded portion.

Abstract: In a control rod construction for moderating and controlling a reaction of a nuclear reactor, a control rod structure is disclosed in which hafnium is welded to stainless steel. Typically, the control rod is confined for movement axially along its length into and out of the reactor. The hafnium is surrounded in the control rod structure by a sheath of stainless steel to inhibit movement relative to the control rod structure other than in the axial direction of control rod movement. The hafnium is friction welded to the stainless steel of the control rod so as to stress the weld in either tension or compression due to dynamic loading occurring from control rod movement. The stainless steel to which the hafnium is welded is in turn conventionally welded or mechanically connected to the remaining steel components of the control rod.

Abstract: The mechanical spectral shift reactor comprises a reactive core having fuel assemblies accommodating both water displacer elements and control rods for selectively changing the volume of water-moderator in the core. The fuel assemblies are arranged in alternating fashion so that one drive mechanism may move displacer elements in more than one fuel assembly without interfering with the movement of control rods or the corresponding control rod drive mechanisms.

Abstract: A burnable absorber cluster assembly includes a support plate, a plurality of burnable absorber rods, and an improved attachment joint for detachably connecting an upper end of each of the absorber rods to the support plate. The attachment joint includes a plug insert defined on the end of each of the rods, a hollow annular wall extending outwardly from each plug insert, holes defined through the support plate for receiving the plug inserts, and recesses formed in the support plate and connected with the holes therein. Each recess is conical-shaped and extends in flared fashion outwardly from one hole to a maximum diameter greater than the diameter of the hole. The annular wall on each plug insert is deformed into conformity with the shape of one of the recesses for providing an interengaging connection between the insert plug and the support plate in which the plug insert is disposed within the support plate hole.

Abstract: A nuclear reactor includes a plurality of upstanding guide thimbles and a plurality of control rods received in the guide thimbles and supported for movement relative to the thimbles between inserted and withdrawn positions. The control rods each include a tubular cladding member and an end plug attached to a lower end of the member. The improvement relates to the end plug having a stabilizing configuration which reduces lateral vibratory motion and contact between the control rod and its respective guide thimble. The end plug has an asymmetrical shape which places its terminal tip end in a position offset to one side of the central axis of the control rod and its end plug. As a result of interaction of the asymmetrical plug tip with coolant flowing along the control rod, a lateral steady-state force is imparted to the end plug which maintains the control rod end plug pressed against the wall of tis respective guide thimble. Several different asymmetrical configurations can be employed on the end plug.

Abstract: A nuclear reactor includes a plurality of upstanding guide thimbles and a plurality of control rods received in the guide thimbles and supported for movement relative to the thimbles between inserted and withdrawn positions. The control rods each include a tubular cladding member and an end plug attached to a lower end of the member. The improvement relates to an wear sleeve disposed on the end plug so as to provide a contact interface between the control rod and its respective guide thimble. The sleeve is composed of material similar to that of the guide thimble and attached to the end plug by an interlock connection. The interlock connection includes a circumferential groove formed in the end plug and a circumferential protuberance swaged or mechanically roll formed in the sleeve so as to extend into the end plug groove.

Abstract: In a fast breeder reactor provided with a core comprising a core region packed with a fissile material and a blanket region which surrounds the outside of said core region and most of which is formed of a fertile material, and a plurality of control rods which are put in and out of the above core region by a control rod driving device, the fast breeder reactor of the present invention is characterized in that each of said control rods is constructed of a neutron absorber region packed with a neutron absorber and a gas region disposed in the end portion on the side further separated from said control rod driving device than said neutron absorber region. With this invention, the spectral shift effect can be produced by utilizing said gas region, thereby increasing the core reactvity of the fast breeder reactor.

Abstract: A method and apparatus is disclosed for reducing the exposure of pressure vessel welds to fast neutron fluxes. Localized peripheral core areas are provided with rod assemblies which may be made up of nuclear absorbing materials, nuclear reflecting materials, or any combination thereof, and of any desired length so as to reduce the exposure of the welds to such fast neutron fluxes. The rod assemblies are precisely tailored consistent with nuclear calculations to provide the desired effect without substantially reducing core ratings or adversely affecting reactor shutdown margins.

Abstract: A control rod for a nuclear reactor comprises a number of elongated absorber plates (13-16) which are each provided with a plurality of channels (18b), said channels extending substantially perpendicularly to the longitudinal direction of the absorber plate, containing powdered boron carbide or other powdered absorber material which gives off gas and swells upon irradiation, and being hermetically separated from the surroundings of the control rod by an edge portion (30), which is arranged at an edge (31) running in the longitudinal direction of the absorber plate and comprises a gas-tight edge (31) and a longitudinal space (32) arranged inside said edge and being in open communication with and permitting a gas flow between the different channels in the absorber plate.

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 quenching element of the size and shape of a graphite nuclear fuel ball is introduced to the stacked balls of a stacked ball nuclear reactor core to release a neutron absorbing substance upon being heated to a predetermined temperature, thereby controlling the reactivity of the reactor by causing this substance, which may be a gadolinium compound, to deposit upon the graphite surfaces of the fuel element balls.

Abstract: A cluster of standardized reduced length burnable absorber rods include a plurality of middle rod sections having a multiplicity of different axial lengths and containing burnable absorber material in sealed chambers therein, a plurality of upper end spacer sections having a multiplicity of different axial lengths and each defining an empty space, and a plurality of lower end spacer sections having a multiplicity of different axial lengths and each defining an empty space. Each of the rods is formed of one middle rod section tandemly arranged between and interconnecting one upper end spacer section and one lower end spacer section. The three interconnected sections which form each rod being selected so as to provide the same standard combined axial length for each of the rods in the cluster thereof although the axial lengths of the middle rod sections can vary from rod to rod and thereby the axial location of the burnable absorber material along the rod can also vary from rod to rod.

Abstract: Slender water displacer rods for use in water reactors are provided with rings of a wear resistant coating spaced along the length of the rod. Each coating contains Cr.sub.2 C.sub.3 and is metallurgically applied and bonded to the zirconium base alloy forming the outer portion of the rod by electrospark-deposition (ESD) technique.

Abstract: A coolant-displacement rod for a nuclear reactor including a stack of zirconium-oxide pellets in cladding of ZIRCALOY-4 alloy. The outer surfaces of certain of the pellets spaced at intervals along the stack are depressed. The cladding grows permanently when exposed to neutron flux but the zirconium oxide is dimensionally stable. Under the hoop stress impressed by the coolant on the cladding, the part of the cladding encircling each of the pellets with the depressed outer surface engages the outer surface compartmentalizing the pellets into sub-stacks. The formation of a long unsupported gap under the cladding which might collapse under the pressure and at the temperature coolant is prevented by forming a plurality of short unsupported gaps instead of one long gap.

Abstract: A mechanical spectral shift reactor comprises apparatus for inserting and withdrawing water displacer elements having differing neutron absorbing capabilities for selectively changing the water-moderator volume in the core thereby changing the reactivity of the core. The displacer elements may comprise substantially hollow cylindrical low neutron absorbing rods and substantially hollow cylindrical thick walled stainless rods. Since the stainless steel displacer rods have greater neutron absorbing capability, they can effect greater reactivity change per rod. However, by arranging fewer stainless steel displacer rods in a cluster, the reactivity worth of the stainless steel displacer rod cluster can be less than a low neutron absorbing displacer rod cluster.

Abstract: The universal fuel assembly has a plurality of elongated corner posts extending longitudinally between and releasably and rigidly interconnecting top and bottom nozzles so as to form a rigid structural skeleton of the fuel assembly. Additionally, a plurality of transverse grids are supported at axially spaced locations along the corner posts and a plurality of fuel rods are supported by the grids. Certain groups of the fuel rods are spaced apart laterally from one another by a greater distance than the rest of the fuel rods so as to define a number of elongated channels extending between the top and bottom nozzles. A cluster assembly having a cluster plate with a plurality of elongated rods is adapted to be removably supported on the top nozzle with its rods extending through the channels. The rods can be a plurality of guide thimbles in the case of one cluster assembly, or a plurality of oversized fuel rods in the case of another cluster assembly.

Abstract: In a nuclear reactor including a core, a plurality of control rods, a support supporting the control rods and movable for displacing the control rods in their longitudinal direction between a first end position in which the control rods are fully inserted into the core and a second end position in which the control rods are retracted from the core, and guide elements contacting discrete regions of the outer surface of each control rod at least when the control rods are in the vicinity of the second end position, the control rods being longitudinally movable relative to the guide elements to thereby cause the outer surface of the control rods to experience wear as a result of sliding contact with the guide elements, there is provided a displacement device operatively coupled to the control rods for periodically rotating the control rods in order to change the locations on the outer surfaces of the control rods at which the control rods are contacted by the guide elements.

Abstract: The mechanical spectral shift reactor provides a method and apparatus for controlling a nuclear reactor comprising inserting a plurality of reactor coolant displacer members into the reactor at the beginning of the core life. The displacer members reduce the volume of reactor coolant-moderator in the core at the start-up. As the reactivity of the core declines with fuel depletion, a selected number of displacer members are withdrawn from the core at selected time intervals. The withdrawal of the displacer member allows reactor coolant water to enter the core which increases core moderation at a time when fuel reactivity is declining. Thus for a given amount of nuclear fuel the life of the core can be extended or for a given life of a core the uranium fuel requirements can be reduced.

Abstract: A control rod for use in a nuclear reactor core to provide xenon compensation includes an elongated inner cylindrical member and an elongated outer cylindrical member surrounding the inner member. Each of the members is composed of axially-extending, alternating black poison and nonpoison regions. Also, the inner member is axially movable relative to the outer member to adjust the degree to which the poison regions of the members overlap with the nonpoison regions thereof and thereby change the overall worth of the rod in an axially uniform manner. The inner cylindrical member has a solid cross-sectional configuration, whereas the outer cylindrical member has an annular cross-sectional configuration and concentrically surrounds the inner member. Each of the poison regions in the members is about the same axial height, while each of the nonpoison regions is about the same axial height.

Abstract: Gas-cooled nuclear reactors include circulation systems for cooling gas to remove the heat generated in the reactor core. An improved arrangement includes a plurality of steam generators in a horizontal plane located above the horizontal plane of the reactor core. The cooling gas circulates from the bottom of the reactor core to the top. Improved heat removal is accomplished in this manner as the heated cooling gas enters the steam generators after passage through the reactor core without undergoing further circulation in a horizontal or inverse vertical direction.

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: 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 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: 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 control rod for a nuclear reactor wherein plates of neutron absorbing material (e.g. hafnium) are used in the upper portion thereof and tubes containing a different neutron absorbing material (e.g. boron) are positioned beneath the plates. To relieve the tubes of the weight of the plates, the plates are attached to support members of the control rod frame, to prevent or reduce the area of a gap between the plates and the tubes, joints therebetween are provided.

Abstract: A nuclear fuel rod contains nuclear fuel pellets that have incorporated therein a hybrid burnable absorber that affects a moderation of the burn-out rate of the system containing the rod. The nuclear fuel pellets contain 1 to 20 percent by weight gadolinium oxide and 0.02 to 1.0 percent by weight of boron carbide particles of a size between 20 to 100 microns in diameter, the particles coated with a 0.5 to 10 micron thick coating of a helium gas-impervious coating.

Abstract: To produce spherical fuel or absorber elements for high temperature reactors a mixture of coated nuclear fuel or absorber particles and graphite molding composition is molded into spheres, carbonized in a furnace having gas flushing and calcined in a vacuum. There are attained high throughputs without addition of transportation aides by employing as resin binders a thermosetting synthetic resin, hardening the resin at 110.degree. to 170.degree. C. and subsequently allowing the spheres to roll for 1 to 10 hours through an oven which is inclined around 2.degree. to 12.degree. to the horizontal. Thereby the oven must exhibit an increasing and decreasing temperature profile, the flushing gas introduced from both sides and be removed in a temperature zone of 400.degree. to 500.degree. C.

Abstract: A non-boron moderator filled rod for a nuclear reactor fuel assembly has a tubular body with a pair of end plugs sealing the opposite ends thereof and defining a chamber within the body. A liquid moderator, such as light water, is contained witin the chamber. One of the end plugs acts as a hydride sink and a hydrogen getter sponge material is disposed adjacent the other end plug.

Abstract: A control rod for a nuclear fuel assembly has structures formed of boron carbide mounted inside it. The boron carbide structures are provided deformable members between them which will deform at operating temperatures to accommodate the dimensional increase of the boron carbide structures and reduce the level of deformable stress on the rod cladding.

Abstract: A control rod assembly arrangement is disclosed for a light water nuclear reactor. Absorber rods are combined with stainless steel rods to achieve a more homogeneous poison distribution effect which reduces power depression within associated fuel assemblies and thereby flattens the overall power distribution within the core. In addition to improved load follow performance, safety analysis margins are improved due to the reduction in control rod reactivity worth.

Abstract: A soluble burnable absorber rod for a nuclear reactor fuel assembly has a tubular body with a pair of end plugs sealing the opposite ends thereof and defining a chamber within the body. A neutron absorber material in liquid form, preferably boric acid with an enhanced concentration of boron isotope B-10, is contained within the chamber. The tubular body has reinforcing convolutions formed therein for strengthening the body to withstand external pressure acting thereon. Also, one of the end plugs acts as a hydride sink and a hydrogen getter material is disposed adjacent the other end plug.

Abstract: A boiling water nuclear power reactor includes an improved control rod associated with each of its clusters of fuel assemblies. The control rod has an elongated central stem defining a longitudinally extending internal central gas plenum and a plurality of blades orthogonally arranged in pairs to provide a cruciform configuration in which the blades connect to and extend radially outward from the central stem. Each blade incorporates a series of internal cavities arranged in columns and rows across the length and width of the blade, and a neutron absorbing pellet is disposed in each cavity. The cavities communicate with the central plenum so that any gases generated by irradiation of the pellets can expand into the plenum. The cavities and pellets are arranged to form a longer, constant worth section in a body portion of each blade and a shorter, reduced worth section in an end tip portion of each blade.

Abstract: An annular burnable absorber rod which minimizes the displacement of cooling water from a nuclear reactor core is disclosed. The rod is constructed from a pair of concentric tubes providing a sealed annular space therebetween. A tubular support having a layer of zirconium diboride deposited by chemical vapor deposition is located within the annular space and protected against corrosion.

Abstract: A velocity limiting device providing greater resistance to motion through a fluid in one direction than in the opposite direction. The device includes a toroidally shaped control member having a smooth generally conical surface on one side for low resistance movement through the fluid in the one direction. On its opposite side the control member is formed with at least one groove providing a concave surface spaced from which is an arrangement of vanes forming a jet for directing a stream of fluid into the groove upon movement in the opposite direction whereby the resultant increase in fluid turbulence increases resistance to rapid movement in this opposite direction.

Abstract: An absorber plate designed to be located in a gap between fuel rod bundles in the core of a boiling water reactor is manufactured by anchoring a burnable neutron-absorbing material to at least one sheet by spraying the absorbing material in finely-divided melted or plastically-formable state onto the sheet, to create a solidified layer of the absorbing material, and by enclosing the absorbing material, thus anchored, in a protective casing.

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: The zirconium cladding of a coolant-displacement rod of a nuclear reactor is precollapsed in the zirconium oxide stack of pellets which supports the cladding. Current is conducted through the cladding in an atmosphere at reduced pressure containing residual oxygen, to heat the cladding to a temperature at which its yield strength is reduced. Then, while the rod remains at this temperature, it is subjected to isostatic pressure which collapses the cladding uniformly. The formation, by reason of exposure to neutron flux, of a long unsupported gap in the cladding which might be collapsed under the pressure of the coolant is precluded. In addition, the rod retains its symmetry. The outer surface of the cladding is oxidized, facilitating the movement of the rod into its thimbles of the core and improving the resistances of the cladding to reaction with the coolant.

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 system for controlling the reactivity of a small fission reactor includes an elongated, flexible hollow tube in the general form of a helical coiled spring axially positioned around and outside of the reactor vessel in an annular space between the reactor vessel and a surrounding cylindrical-shaped neutron reflector. A neutron absorbing material is provided within the hollow tube with the rate of the reaction controlled by the extension and compression of the hollow tube, e.g., extension of the tube increases reactivity while its compression reduces reactivity, in varying the amount of neutron absorbing material disposed between the reactor vessel and the neutron reflector. Conventional mechanical displacement means may be employed to control the coil density of the hollow tube as desired.

Abstract: A sliding mechanism comprising two members maintained in sliding contact with each other, wherein one of the members (which may be a pin) is formed of an alloy consisting essentially of, by weight, less than 0.3% carbon, 7-40% nickel, 15-28% chromium, 2-8% silicon, 3-12% manganese and the balance essentially iron, and the other member (which may be a roller) is formed of an alloy consisting essentially of, by weight, less than 0.5% carbon, 13-22% chromium, less than 2% silicon, less than 2% manganese, 3-10% molybdenum, 10% of at least one of titanium and aluminum and the balance essentially nickel, the two members having a difference in Vickers hardness of below 200 at their contact surfaces. The sliding mechanism has particualr utility as means for guiding a control rod of a nuclear reactor by means of pins and rollers.

Abstract: A burnable poison rod for use in a nuclear reactor fuel assembly which includes concentrically disposed rods having an annular space therebetween which extends the full length of the rods. The inner rod is hollow to permit circulation of coolant therethrough. Annular burnable poison pellets are positioned in the annular space which is closed at both ends by plugs. A spring clip is located in the plenum space above the pellet stack in the rods. The spring clip is of cylindrical configuration having a gap in the material which provides two ends adapted to be squeezed toward each other. A cross section of the clip shows that its ends contain alternating flat and round edges, the round edges conforming to the outer rod inner surface to provide a retentive force which is releasably applied to the pellet stack as it grows during operation in a reactor.

Abstract: A self-actuated device, of particular use as a valve or an orifice for nuclear reactor fuel and blanket assemblies, in which a gas produced by a neutron induced nuclear reaction gradually accumulates as a function of neutron fluence. The gas pressure increase occasioned by such accumulation of gas is used to actuate the device.

Abstract: A control rod to be arranged in a nuclear reactor core comprises a body of a control rod, control rod guide rollers, pins connected to the guide rollers, and fixing members directly welded to the body of the control rod. The pins slidably support the respective guide rollers and the fixing members are engaged with the pins with a small clearance therebetween so as not to transfer welding heat from the fixing member to the pin.

Abstract: An elongated absorber plate, which is stationarily mounted in a gap between two fuel assemblies in a reactor core, comprises a plurality of absorber channels which are filled with a burnable absorber material. The channels (5b) are formed by welding to each other two confronting sheet elements (2b, 3b), of which at least one is corrugated.

Abstract: A technique is provided for engaging and disengaging burnable poison rods from the spider in a fuel assembly. The cap on the end of each of the burnable poison rods is provided with a shank that is received in the respective bore formed in the spider. In one illustrative embodiment, the shank is deformed to firmly secure the rod and attached shank in the spider bore. Pressing the shank in the direction of the bore axis overcomes the deformation to release the particular rod from the spider.

Abstract: A nuclear reactor includes an active portion with fissionable fuel and neutron moderating material surrounded by neutron reflecting material. A control element in the active portion includes a group of movable rods constructed of neutron-absorbing material. Each rod is movable with respect to the other rods to vary the absorption of neutrons and effect control over neutron flux.

Abstract: A control rod having a center of thermal neutron absorbing material is provided specifically for use within the core of a nuclear reactor, extending through the central zone of maximum flux density. The center is formed of a plurality of portions, normally three, arranged end to end and defining joins therebetween. A central one of said portions extends continuously through the zone of maximum flux density with the joins located outward of this zone.

Abstract: A technique is provided for engaging and disengaging burnable poison rods from the spider in a fuel assembly. The cap on the end of each of the burnable poison rods is provided with a shank that is received in the respective bore formed in the spider. A frangible flange secures the shank and rod to the spider. Pressing the shank in the direction of the bore axis ruptures the frangible flange to release the rod from the spider.