Abstract: A thimble tube cap for replacing conventional thimble tube plugs (18) within thimble tubes (10, 110) of a nuclear reactor fuel assembly is fixedly mounted within a holddown assembly base plate (126) so as to cover the upper end of its associated thimble tube (110) fixedly mounted within the fuel assembly top nozzle adaptor plate (114). An axial through-bore (148) is provided within each cap for controlling the volume flow rate of coolant through the thimble tube (110). Each thimble cap includes a disc-shaped base portion (142) and an upstanding shaft portion (144) integrally formed therewith, an annular ledge or shoulder portion (146) being defined therebetween.

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

Abstract: An improved end plug is provided for a nuclear fuel rod assembly. The end plug is of an integral construction having an upper shoulder portion which abuts the top edge of a fuel tube and a circular body portion adapted to be inserted into the end of the fuel tube in a force fitting relationship. A flat is provided on the lower body portion so as to define a gap between the lower body portion and the inner wall of the fuel tube upon partial insertion of the plug into the tube, the gap providing an orifice for introducing an inert-gas under pressure into the tube. Upon total insertion of the lower body portion of the plug into the tube, the gap is eliminated and metal-to-metal contact is made between the lower body portion of the plug and the end of the fuel tube for sealing the pressurized gas and a nuclear fuel within the fuel tube.

Abstract: Elongated tubular elements containing burnable absorber material are clustered in an array to provide a burnable absorber assembly for insertion within one or more guide thimbles of a fuel assembly for controlling reactivity. The burnable absorber assembly, in alternative embodiments, include three tubular elements in triangular relationship and four tubular elements in rectangular relationship. The use of the burnable absorber assembly within the guide thimbles of a fuel assembly results in increased core thermal margin, minimization of end of cycle reactivity penalty, increase in operating cycle length before required refueling, and the ability to utilize burnable absorber assemblies having different strengths at different locations within the fuel assembly for controlling fuel assemblies having significant power gradients, as well as finer control of radial powered distribution and thus achieving lower peaking factors.

Abstract: A method and system for enclosing a gas at a preselected pressure in a nuclear reactor fuel rod having two ends open to its interior. Hermetically enclose each end in a separate airtight chamber. Introduce the gas into the first chamber and open the second chamber to purge the fuel rod interior and both chambers of residual atmosphere. After purging close the open second chamber and determine the gas pressure in the fuel rod interior. When the pressure reaches the preselected value, hermetically seal each fuel rod end while in its respective chamber. After sealing, remove the fuel rod from the chambers.

Abstract: This invention teaches an encapsulated fuel unit for a nuclear reactor, such as for an enriched uranium fuel plate of thin cross section of the order of 1/64 or 1/8 of an inch and otherwise of rectangular shape 1-2 inches wide and 2-4 inches long. The case is formed from (a) two similar channel-shaped half sections extended lengthwise of the elongated plate and having side edges butted and welded together to define an open ended tube-like structure and from (b) porous end caps welded across the open ends of the tube-like structure. The half sections are preferably of stainless steel between 0.002 and 0.01 of an inch thick, and are beam welded together over and within machined and hardened tool steel chill blocks. The porous end caps preferably are of T-316-L stainless steel having pores of approximately 3-10 microns size.

Abstract: An arrangement for crimping a malleable sleeve over a hole in a punctured nuclear fuel rod in order to prevent the escape of fission gases into the external environment. The arrangement operates underwater and includes jaws for crimping the sleeve onto the fuel rod, which are operated by a hydraulic cylinder. The jaws crimp circumferential sealing ridges onto the inner surface of the sleeve, which bear against the fuel rod.

Abstract: In a nuclear fuel assembly having a coolant conducting or water tube which also retains the spacers in axial position, the fuel rods experience greater axial growth with exposure than the water tube creating a risk that the water tube might become disengaged from the supporting tie plates. An arrangement for preventing such disengagement is described including lengthened end plug shanks for the water tube, a protective boss surrounding the lower end plug shank to protect it from flow induced vibration, a conical seat for the lower end plug and an arrangement for limiting upward movement of the water tube.

Abstract: An improved nuclear fuel assembly in which the lowermost grid is located close to the lower end fitting of the assembly, and a conventional Zircaloy-clad fuel rod is fitted with a stainless steel tip having a bevelled notch thereon for engaging resilient means on the grid within about one inch (2.5 cm) from the lower tip of the fuel rod. The notch mechanically restrains the fuel rod from lifting upwards during reactor operation, yet is shallow enough to permit removal of the fuel rod in the event reconstitution is required. The stainless steel tip is mechanically secured to the Zircaloy end cap of the fuel rod, and is located in a low importance flux region of the core so that the higher neutron poison absorption cross section of stainless steel is of little consequence. Since the lowermost grid secures each fuel rod within one inch of the lower end of the rod, there is little likelihood that the coolant flowing over the lower ends of the fuel rods will induce significant vibrations.

Abstract: Method for fixing a zircalloy guide tube of a nuclear reactor fuel assembly to the steel end plate of said assembly, wherein the following stages are successively formed:the previously expanded end of the guide tube is engaged around a sleeve until it abuts against a first outer shoulder of said sleeves;said end is covered with a force-fitted ferrule and the tube is squeezed against the sleeve, then the upper end of the ferrule is welded to the sleeve above the said first outer shoulder;the sleeve--tube--ferrule assembly is introduced into an opening provided for this purpose in the end plate and having a shoulder, until a second outer shoulder of the sleeve abuts against the said shoulder of the end plate opening;the upper part of the sleeve is welded to the upper part of the opening of the end plate of the assembly.The invention also relates to an apparatus for fixing the zircalloy guide tube to the steel end plate.

Abstract: A bottom end member (17b) on a retrievable fuel pin (13b) secures the pin (13b) within a nuclear reactor (12) by engaging on a transverse attachment rail (18) with a spring clip type of action. Removal and reinstallation if facilitated as only axial movement of the fuel pin (13b) is required for either operation. A pair of resilient axially extending blades (31) are spaced apart to define a slot (24) having a seat region (34) which receives the rail (18) and having a land region (37), closer to the tips (39) of the blades (31) which is normally of less width than the rail (18). Thus an axially directed force sufficient to wedge the resilient blades (31) apart is required to emplace or release the fuel pin (13b) such force being greater than the axial forces on the fuel pins (13b) which occur during operation of the reactor (12).

Abstract: A laser beam is used to puncture fuel cladding for release of contained pressurized fission gas from plenum sections or irradiated fuel pins. Exhausted fission gases are collected and trapped for safe disposal. The laser beam, adjusted to welding mode, is subsequently used to reseal the puncture holes. The fuel assembly is returned to additional irradiation or, if at end of reactivity lifetime, is routed to reprocess. The fuel assembly design provides graded cladding lengths, by rows or arrays, such that the cladding of each component fuel element of the assembly is accessible to laser beam reception.