Abstract: A device for supplementing structural integrity of a spent nuclear fuel assembly during handling is provided. The spent nuclear fuel assembly includes spacer grids for supporting fuel rods, guide tubes, and top and bottom nozzles for supporting upper and lower ends of the guide tubes. The device includes a housing, a rod member and fixing members. The housing includes a plurality of free end portions, stopper protrusions formed at an end of each of the free end portions, a first male thread formed on an outer surface, and a female thread formed on an inner surface. The rod member includes a pressurization protrusion at one end and a second male thread for engaging with the female thread of the housing. The fixing members are configured engage with the first male thread to secure the device to the top nozzle of the spent nuclear fuel assembly.

Abstract: An entire nuclear fuel core comprising a plurality of fuel assemblies is preassembled in a nuclear fuel cartridge having a self-supporting unitary support structure. During a refueling operation, the unitary support structure is moved into a nuclear reactor vessel. The unitary support structure may be formed by top and bottom core plates coupled together by vertically-oriented connecting rods which compress the fuel assemblies therebetween. A plurality of reflector wall segments circumscribe the core and are the coupled together between the core plates by the connecting rods which are coupled to the core plates. The connecting rods may extend through the wall segments.

Abstract: Refueling of a nuclear reactor (40) includes removing a fuel assembly (10). The removal method includes lowering a lifting tool (80) of a crane (44) onto a top of the fuel assembly. The lowered lifting tool including a plurality of downwardly extending elements (82) that surround and vertically overlap a portion (74) of a control rod assembly (70) extending above the top of the fuel assembly. The downwardly extending elements are locked with corresponding mating features (26) at the top of the fuel assembly to connect the lifting tool with the fuel assembly. The connected fuel assembly is moved into a spent fuel pool (42) using the crane, and the lifting tool is disconnected from the top of the fuel assembly by unlocking the downwardly extending elements from the corresponding mating features at the top of the fuel assembly.

Abstract: One embodiment provides a multi-segment rod that includes a plurality of rod segments. The rod segments are removably mated to each other via mating structures in an axial direction. An irradiation target is disposed within at least one of the rod segments, and at least a portion of at least one mating structure includes one and/or more combinations of neutron absorbing materials.

Abstract: A system for passively cooling nuclear fuel in a pressurized water reactor during refueling that employs gravity and alignment of valves using battery reserves or fail in a safe position configurations to maintain the water above the reactor core during reactor disassembly and refueling. A large reserve of water is maintained above the elevation of and in fluid communication with the spent fuel pool and is used to remove decay heat from the reactor core after the reaction within the core has been successfully stopped. Decay heat is removed by boiling this large reserve of water, which will enable the plant to maintain a safe shutdown condition without outside support for many days.

Abstract: A method for joint configuration of nuclear power plant fuel includes the following steps: (S1) for at least one operating unit, based on an equilibrium cycle or transition cycle reactor core design, at least one new fuel element is added to at least one operating unit; (S2) after running a combustion cycle, and on basis of the new fuel elements in step (S1), more first spent fuel elements are obtained from the at least one operating unit than are obtained from the equilibrium cycle or transition cycle reactor core design, and said first spent fuel elements are kept in reserve; (S3) for at least one new starting unit, a scheduled number of new fuel elements, as well as the first spent fuel elements obtained for reserve in step (S2), are set in the first reactor cores of at least one new starting unit.

Abstract: A storage container is carried into a nuclear-reactor containment, and then, the core structure is housed in the storage container. In this process, a bottom cover is carried to a storage-container body by using a moving device. The storage-container body and the bottom cover are positioned by adjusting the position of the bottom cover on the moving device while the bottom cover is supported by free bearings. The bottom cover is then attached to the storage-container body.

Abstract: Example embodiments are directed to apparatuses and methods for producing radioisotopes in instrumentation tubes of operating commercial nuclear reactors. Irradiation targets may be inserted and removed from instrumentation tubes during operation and converted to radioisotopes otherwise unavailable from nuclear reactors. Example apparatuses may continuously insert, remove, and store irradiation targets to be converted to useable radioisotopes.

Abstract: The present invention relates to a nuclear reactor, in particular a pool-type nuclear reactor cooled with liquid metal (for example, a heavy metal such as lead or lead-bismuth eutectic) or with sodium or molten salts, having a core formed by a bundle of fuel elements and immersed in a primary fluid circulating between the core and at least one heat exchanger; the fuel elements extend along respective parallel longitudinal axes and have respective bottom active parts immersed in the primary fluid to constitute the core, and respective service parts that extend at the top from the active parts and emerge from the primary fluid; the fuel elements are mechanically supported via respective top end heads anchored to supporting structures and can be operated via handling machines.

Abstract: Nuclear reactor systems and methods are described having many unique features tailored to address the special conditions and needs of emerging markets. The fast neutron spectrum nuclear reactor system may include a reactor having a reactor tank. A reactor core may be located within the reactor tank. The reactor core may include a fuel column of metal or cermet fuel using liquid sodium as a heat transfer medium. A pump may circulate the liquid sodium through a heat exchanger. The system may include a balance of plant with no nuclear safety function. The reactor may be modular, and may produce approximately 100 MWe.

Abstract: A system includes at least one body, a link for suspending the body for movement with gravity from a first elevation position to a second elevation position, and an electrical energy generator coupled with the body through the link to drive the generator to generate electricity upon movement of the body with gravity from the first to the second elevation position. The at least one body has a mass of at least approximately 100 tonnes; the first and the second elevation positions define a distance therebetween of at least approximately 200 meters; and/or the system further includes an operator configured to operate the link to controllably move the at least one body against gravity from the second to the first elevation position to increase a gravitational potential energy of the at least one body, and to maintain the gravitational potential energy of the at least one body.

Abstract: An embodiment of the present invention takes the form of a system that allows for simultaneously assembling or disassembling multiple segmented nuclear fuel rods (hereinafter “segmented rods”). An embodiment of the present invention, may receive, secure, and move the segmented rods into a position that allows for performing the tasks of either assembly or disassembly, allowing for an operator to use a tool to complete the aforementioned tasks.

Abstract: The present invention relates generally to nuclear reactors, and more particularly, to nuclear reactors having fuel assemblies that employ support grids. A method of reducing friction and physical contact between a fuel rod and support grid in a nuclear fuel assembly is provided. The method includes applying a lubricant composition to the outer surface of the fuel rod during fuel assembly fabrication and removing the lubricant composition afterward.

Abstract: A device and a method to replace an irradiated fuel assembly with a new fuel assembly in the vessel of a nuclear reactor. This device includes: means for installing the two fuel assemblies in two containers containing heat transfer fluid, where the first container is filled with the irradiated fuel assembly having just been extracted from the reactor core by a handling arm, and where the second container is filled with the new fuel assembly brought in by a transfer basket, means for positioning both these containers, where the irradiated fuel assembly is accessible by the transfer basket, whereas the new fuel assembly is in a position where it can be taken up by the handling arm, and means for positioning both the fuel assemblies.

Abstract: A method for selecting a loading map for a nuclear reactor core including the following steps: a) providing production data relating to the nuclear fuel assemblies, b) providing neutron data which are representative of the operation of the core, c) calculating the three-dimensional distribution of the local power in the core, d) calculating the extreme value reached by at least one thermomechanical parameter within the nuclear fuel assemblies, and e) selecting, in accordance with the extreme values calculated, a loading map from the loading maps envisaged. A system, computer program and storage medium for selecting a loading map for a nuclear reactor.

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: In a method for improving the energy generating output of a nuclear reactor containing one or more fuel rods in one or more fuel rod bundles while satisfying a maximum subcritical banked withdrawal position (MSBWP) reactivity limit, enrichments of individual fuel rods in an axial cross-section of a lattice being evaluated at the top of the fuel bundle are ranked, and the fuel pins of the highest ranked rod location in the lattice are replaced with pins containing natural uranium. A core simulation is then performed to determine whether there is any margin to a MSBWP reactivity limit. For each lower ranked candidate rod position, the pin replacing and core simulation functions are repeated until no rod location violates the MSBWP reactivity limit, so as to achieve a desired lattice design for the top of the fuel bundle.

Abstract: A liquid fluoride salt cooled, high temperature reactor having a reactor vessel with a pebble-bed reactor core. The reactor core comprises a pebble injection inlet located at a bottom end of the reactor core and a pebble defueling outlet located at a top end of the reactor core, an inner reflector, outer reflector, and an annular pebble-bed region disposed in between the inner reflector and outer reflector. The annular pebble-bed region comprises an annular channel configured for receiving pebble fuel at the pebble injection inlet, the pebble fuel comprising a combination of seed and blanket pebbles having a density lower than the coolant such that the pebbles have positive buoyancy and migrate upward in said annular pebble-bed region toward the defueling outlet. The annular pebble-bed region comprises alternating radial layers of seed pebbles and blanket pebbles.

Abstract: A method of aligning a nuclear fuel bundle and handling selected fuel rods within the fuel bundle located in a spent fuel pool of a nuclear power plant. The bundle includes water rods, full-length and part-length fuel rods extending through a plurality of fuel spacers provided between top and bottom ends of the bundle, each spacer having a plurality of cells accommodating corresponding fuel and water rods. The method includes insertion of a rod grapple tool into a top end of a fuel bundle and down to a desired location within the bundle. A guide pin retrieval tool inserts into a side of the fuel bundle to remove a guide pin from a distal end of a gripper of the rod grapple tool, allowing the guide pin retrieval tool to grip a part-length fuel rod within the fuel bundle.

Abstract: Among other things, an apparatus includes a combination of a fissionable material, a molten salt, and a moderator material including one or more hydrides, one or more deuterides, or a combination of two or more of them.

Abstract: Refueling of a nuclear reactor (40) includes removing a fuel assembly (10). The removal method includes lowering a lifting tool (80) of a crane (44) onto a top of the fuel assembly. The lowered lifting tool including a plurality of downwardly extending elements (82) that surround and vertically overlap a portion (74) of a control rod assembly (70) extending above the top of the fuel assembly. The downwardly extending elements are locked with corresponding mating features (26) at the top of the fuel assembly to connect the lifting tool with the fuel assembly. The connected fuel assembly is moved into a spent fuel pool (42) using the crane, and the lifting tool is disconnected from the top of the fuel assembly by unlocking the downwardly extending elements from the corresponding mating features at the top of the fuel assembly.

Abstract: Provided is a sliding support and a system using the sliding support unit. The sliding support unit may include a fulcrum capture configured to attach to a support flange, a fulcrum support configured to attach to the fulcrum capture, and a baseplate block configured to support the fulcrum support. The system using the sliding support unit may include a pressure vessel, a pedestal bracket, and a plurality of sliding support units.

Abstract: In various embodiments, the system comprises a system for storing radioactive material, wherein the system includes a storage pool for storing a plurality of radioactive objects submersed in a radiation shielding and cooling liquid. The system additionally includes an assembly building located above the storage pool for constructing one or more radioactive articles using the radioactive objects transferred from the storage pool. Furthermore, the system includes at least one transfer shaft connecting the storage pool and the assembly building. The transfer shaft(s) are used for transferring the radioactive objects directly from within the storage pool to an interior of the assembly building and directly from the interior of the assembly building into the storage pool.

Abstract: An outer filter removal tool for a boiling water reactor control rod drive that uses a mechanical advantage obtained through the use of lead screw threads to pull the outer filter off of the control rod drive. Fingers on the tool are closed around the upper flange of the outer filter by sliding a collar over the outwardly biased fingers. A shaft extending through the tool is rotated which in turn extends a push plate against the control rod drive index tube causing the fingers to pull against the upper flange on the outer filter until the filter is freed from the control rod drive. The tool will hold the filter in place until affirmatively released for proper disposal.

Abstract: Disclosed is a remote control system for a fuel handling system of a nuclear power plant, in which the system includes a remote control console; a programmable logic controller (PLC) unit; a servo motor drive; and a human-machine interface (HMI) unit.

Abstract: A method of loading nuclear fuel assemblies into a fuel rack in an underwater (or other submerged) environment that reduces the depth required for the pool to effectuate the fuel rack loading procedure. In one embodiment, the method comprises submerging a nuclear fuel assembly having an axis and a horizontal cross-section in a pool; providing a fuel rack in the pool, the fuel rack comprising a body structure comprising at least one elongated cell, a top, a bottom, a first lateral side, at least one elongated slot in the first lateral side that forms a lateral passageway into the cell; positioning the fuel assembly laterally adjacent to the elongated slot of the fuel rack so that the axis of the fuel assembly is substantially aligned with the elongated slot; and translating the fuel assembly in a lateral direction through the elongated slot and into the cell.

Abstract: Exemplary embodiments provide automated nuclear fission reactors and methods for their operation. Exemplary embodiments and aspects include, without limitation, re-use of nuclear fission fuel, alternate fuels and fuel geometries, modular fuel cores, fast fluid cooling, variable burn-up, programmable nuclear thermostats, fast flux irradiation, temperature-driven surface area/volume ratio neutron absorption, low coolant temperature cores, refueling, and the like.

Abstract: A liquid fluoride salt cooled, high temperature reactor having a reactor vessel with a pebble-bed reactor core. The reactor core comprises a pebble injection inlet located at a bottom end of the reactor core and a pebble defueling outlet located at a top end of the reactor core, an inner reflector, outer reflector, and an annular pebble-bed region disposed in between the inner reflector and outer reflector. The annular pebble-bed region comprises an annular channel configured for receiving pebble fuel at the pebble injection inlet, the pebble fuel comprising a combination of seed and blanket pebbles having a density lower than the coolant such that the pebbles have positive buoyancy and migrate upward in said annular pebble-bed region toward the defueling outlet. The annular pebble-bed region comprises alternating radial layers of seed pebbles and blanket pebbles.

Abstract: A system, apparatus and method of processing and/or removing radioactive materials from a body of water that utilizes the buoyancy of the water itself to minimize the load experienced by a crane and/or other lifting equipment. In one aspect, the invention is a method comprising: a) submerging a container having a top, a bottom, and a cavity in a body of water having a surface level, the cavity filling with water; b) positioning radioactive material within the cavity of the submerged container; c) raising the submerged container until the top of the containment apparatus is above the surface level of the body of water while a major portion of the container remains below the surface level of the body of water; and d) removing bulk water from the cavity while the top of the container remains above the surface level of the body of water and a portion of the container remains submerged.

Abstract: In a spent fuel pool of a nuclear power plant, there is provided a system for aligning a nuclear fuel bundle and handling selected fuel rods within the fuel bundle. The bundle includes water rods, full-length and part-length fuel rods extending through a plurality of fuel spacers provided between top and bottom ends of the bundle, each spacer having a plurality of cells accommodating corresponding fuel and water rods. The system includes a bundle alignment system for aligning the fuel rods and water rods, a rod grapple tool to extract selected part-length rods from the bundle, and a fuel rod guide block slidable onto the top end of the bundle for protecting an uppermost fuel spacer of the bundle, and for aligning fuel rods within individual cells of all the fuel spacers in the fuel bundle.

Abstract: Nuclear reactor systems and methods are described having many unique features tailored to address the special conditions and needs of emerging markets. The fast neutron spectrum nuclear reactor system may include a reactor having a reactor tank. A reactor core may be located within the reactor tank. The reactor core may include a fuel column of metal or cermet fuel using liquid sodium as a heat transfer medium. A pump may circulate the liquid sodium through a heat exchanger. The system may include a balance of plant with no nuclear safety function. The reactor may be modular, and may produce approximately 100 MWe.

Abstract: A method and apparatus for the permissive control of a mast and fuel grapple to be used in the movement of reactor fuel components, including fuel assemblies, single blade and double blade guides, to be used in a Boiling Water Reactor (BWR) nuclear reactor. The Permissive Control System reduces the chance of human error associated with the movement of reactor components by assisting in controlling the location (plant coordinate) of the mast for picking-up and dropping-off reactor components, the sequence of reactor component movements, the orientation (angular rotation) of the mast and fuel grapple, the raising and lowering of the grapple, and the opening and closing of the fuel grapple.

Abstract: A fuel element handling system having a gripping device, the structure of which can be pushed in the horizontal position and uncoupled from the handling system if a failure occurs, uncoupling releasing the load formed by the fuel elements and their gripped support and putting them in a safe situation.

Abstract: Disclosed embodiments include nuclear fission reactor cores, nuclear fission reactors, methods of operating a nuclear fission reactor, and methods of managing excess reactivity in a nuclear fission reactor.

Abstract: Exemplary embodiments provide automated nuclear fission reactors and methods for their operation. Exemplary embodiments and aspects include, without limitation, re-use of nuclear fission fuel, alternate fuels and fuel geometries, modular fuel cores, fast fluid cooling, variable burn-up, programmable nuclear thermostats, fast flux irradiation, temperature-driven surface area/volume ratio neutron absorption, low coolant temperature cores, refueling, and the like.

Abstract: Provided is an isotope delivery system and a method for irradiating a target and delivering the target to an extraction point. The isotope delivery system may include a cable including at least one target for irradiation, a drive system configured for moving the cable, and a first guide configured to guide the cable for insertion and extraction from a nuclear reactor. The method for irradiating a target and delivering a target may include pushing a cable with an attached target through a first guide and into a nuclear reactor using a drive system, irradiating the target in the nuclear reactor, pulling the cable with the attached irradiated target towards the drive system, pushing the cable with the irradiated target towards a loading/unloading area using the drive system, and placing the irradiated target into a transfer cask, wherein the cable is pulled and pushed by the drive system.

Abstract: Detecting diversion of spent fuel from Pressurized Water Reactors (PWR) by determining possible diversion including the steps of providing a detector cluster containing gamma ray and neutron detectors, inserting the detector cluster containing the gamma ray and neutron detectors into the spent fuel assembly through the guide tube holes in the spent fuel assembly, measuring gamma ray and neutron radiation responses of the gamma ray and neutron detectors in the guide tube holes, processing the gamma ray and neutron radiation responses at the guide tube locations by normalizing them to the maximum value among each set of responses and taking the ratio of the gamma ray and neutron responses at the guide tube locations and normalizing the ratios to the maximum value among them and producing three signatures, gamma, neutron, and gamma-neutron ratio, based on these normalized values, and producing an output that consists of these signatures that can indicate possible diversion of the pins from the spent fuel assembl

Abstract: Exemplary embodiments provide automated nuclear fission reactors and methods for their operation. Exemplary embodiments and aspects include, without limitation, re-use of nuclear fission fuel, alternate fuels and fuel geometries, modular fuel cores, fast fluid cooling, variable burn-up, programmable nuclear thermostats, fast flux irradiation, temperature-driven surface area/volume ratio neutron absorption, low coolant temperature cores, refueling, and the like.

Abstract: A nuclear fission reactor fuel assembly and system configured for controlled removal of a volatile fission product and heat released by a burn wave in a traveling wave nuclear fission reactor and method for same. The fuel assembly comprises an enclosure adapted to enclose a porous nuclear fuel body having the volatile fission product therein. A fluid control subassembly is coupled to the enclosure and adapted to control removal of at least a portion of the volatile fission product from the porous nuclear fuel body. In addition, the fluid control subassembly is capable of circulating a heat removal fluid through the porous nuclear fuel body in order to remove heat generated by the nuclear fuel body.

Abstract: In a spent fuel pool of a nuclear power plant, there is provided a system for aligning a nuclear fuel bundle and handling selected fuel rods within the fuel bundle. The bundle includes water rods, full-length and part-length fuel rods extending through a plurality of fuel spacers provided between top and bottom ends of the bundle, each spacer having a plurality of cells accommodating corresponding fuel and water rods. The system includes a bundle alignment system for aligning the fuel rods and water rods, a rod grapple tool to extract selected part-length rods from the bundle, and a fuel rod guide block slidable onto the top end of the bundle for protecting an uppermost fuel spacer of the bundle, and for aligning fuel rods within individual cells of all the fuel spacers in the fuel bundle.

Abstract: Systems, devices, and methods for filling containers with radioactive materials are described. In certain embodiments, the systems comprise a shielding material that substantially defines a chamber and, preferably, substantially blocks radioactivity, a conduit extending through the shielding material into the chamber, and a securing unit that is disposed in the chamber proximal to the conduit and is adapted to receive a container through the conduit.

Abstract: The invention refers to a device and a method for handling a fuel assembly (3), which comprises a number of fuel rods extending between a lower part and an upper part of the fuel assembly, a debris filter located in the lower part of the fuel assembly and a casing surrounding the fuel rods. The device comprises a lifting device (15) for engaging, during a lifting operation, a fuel assembly located in a reactor vessel (1) and lifting the fuel assembly upwards and out from the reactor vessel. A conduit member is connected to the upper part of the fuel assembly. A pump (32) creates a flow of water through the conduit member and the fuel assembly during the lifting operation. The flow has such a size that possible debris particles contained in and/or immediately beneath the debris filter at least are retained in and/or immediately beneath the debris filter during the iifting operation.

Abstract: A fuel rack for supporting radioactive fuel assemblies in an underwater (or other submerged) environment that reduces the depth required for the pool to effectuate the fuel rack loading procedure. The fuel rack is specially designed to afford lateral loading. In one embodiment, the fuel rack comprises a body structure comprising at least one substantially vertically oriented elongated cell for receiving a nuclear fuel assembly, the body having a top, a bottom and a first lateral side; at least one elongated slot in the first lateral side of the body structure that forms a passageway into the cell through which a vertically oriented fuel assembly can be loaded; and means for supporting a fuel assembly within the cell in a substantially vertical orientation. In another embodiment, the invention is a method of laterally loading a fuel rack that utilizes rotation of the fuel assembly to secure the fuel assembly within its designated cell.

Abstract: A nuclear reactor and method for generating energy from fertile and fissile nuclear fuel material. The reactor may comprise a pressure vessel for housing a nuclear reactor core, the vessel having a lower vessel with an upwardly facing opening and a vessel closure head having a sealable access port. The vessel closure head may be positionable in differing positions relative to the lower vessel so that differing portions of the interior of the pressure vessel may be accessed through the access port. The lower vessel may include penetrations for lateral insertion of one or more reactor control blades into the interior of the lower vessel.

Abstract: An apparatus for loading and unloading a core of a pressurized water reactor exerts a force acting transversely to an axial direction of a fuel assembly against an upper region of a fuel assembly adjacent a fuel assembly unloading or loading position of the core. The apparatus is an immersion vehicle being freely maneuverable under water and on which an extendable carrier arm is disposed and at least indirectly positioned against the fuel assembly.

Abstract: The present invention related to a novel process for accelerating the breeding and conversion of fissile fuel in various types of nuclear reactors. In said process a movable nuclear fuel ball bed filled with a coolant creeps through the reactor core. The said process could provide higher specific power per unit fuel volume inside the reactor core and proceed on-line refueling, thus requires much less initial fissile fuel inventory per unit power output as compared with the traditional breeding or conversion reactors. The said process has full inherent safety characteristics and could follow the external power demand with the inherent negative temperature coefficient of reactivity. The said process, therefore, is a more efficient and economic approach to meeting the enormous demand of fissile fuel for the forthcoming “second nuclear era”.

Abstract: A device and method to move a fuel assembly of a nuclear reactor, wherein the device eliminates the potential for dropping the fuel assembly due to stress corrosion cracking of the upper guide thimble sleeves that attach the top nozzle to guide thimbles.

Abstract: An aligning device for fuel elements allows fuel elements of a pressurized water reactor to be aligned in a comparatively easy and quick manner. To this end, the aligning device for fuel elements includes an aligning body, which extends in a longitudinal direction and whose lateral faces formed from side metal sheets can be displaced out of a position of rest in a direction that is perpendicular to the longitudinal direction. The cross-sectional dimensions of the lateral faces of the aligning body in the position of rest are selected so that they are less than the corresponding cross-sectional dimension of a reference fuel element by a predetermined nominal value.

Abstract: The invention relates to a method and a device for loading a fuel assembly into the core of a nuclear reactor. The inventive method consists in: inserting a dummy assembly (4) into the loading location (3) of the fuel assembly (5), said dummy assembly essentially having the same shape and dimensions as the loading location (3) and comprising smooth side walls; fixing the position of at least one fuel assembly (5) that is adjacent to the loading location (3), in the presence of the aforementioned dummy assembly (4), in relation to at least one second assembly (5) of the core, using at least one fuel assembly support tool (11); removing the dummy assembly (4) from the loading location (3); introducing the fuel assembly (5) which is being loaded into the loading location (3); and removing the fuel assembly support tool(s) (11).

Abstract: A device and method to move a fuel assembly of a nuclear reactor, wherein the device eliminates the potential for dropping the fuel assembly due to stress corrosion cracking of the upper guide thimble sleeves that attach the top nozzle to guide thimbles.