Patent Publication Number: US-2022215973-A1

Title: Lower end fitting locknut for nuclear fuel assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Division of U.S. patent application Ser. No. 16/144,648 filed on Sep. 27, 2018, now U.S. Pat. No. ______, which is a divisional of U.S. patent application Ser. No. 13/447,655 filed on Apr. 16, 2012, now U.S. Pat. No. 10,124,472, the entire disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     The following relates to the nuclear power reactor arts, nuclear fuel assembly manufacturing and deployment arts, and related arts. 
     In nuclear reactor designs of the integral pressurized water reactor (integral PWR) type, a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel. In a typical design, the primary coolant is maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. Primary coolant flows upward through the reactor core where it is heated and rises through the central riser, discharges from the top of the central riser and reverses direction to flow downward back toward the reactor core through a downcomer annulus defined between the pressure vessel and the central riser. In the integral PWR design, at least one steam generator is located inside the pressure vessel, typically in the downcomer annulus. Some illustrative integral PWR designs are described in Thome et al., “Integral Helical Coil Pressurized Water Nuclear Reactor”, U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety. Other light water nuclear reactor designs such as PWR designs with external steam generators, boiling water reactors (BWRs) or so forth, vary the arrangement of the steam generator and other components, but usually locate the radioactive core at or near the bottom of a cylindrical pressure vessel in order to reduce the likelihood of air exposure of the reactor core in a loss of coolant accident (LOCA). 
     The nuclear reactor core is built up from multiple fuel assemblies. Each fuel assembly includes a number of fuel rods. Spaced vertically along the length of the fuel assembly are grid assemblies which provide structural support to the fuel rods. At the top and bottom of the fuel assembly are an upper end fitting and a lower end fitting, respectively, providing structural support. The lower end fitting, sometimes called a nozzle plate, may be supported by a lower core support plate, support pedestals, or the like. 
     The lower end fitting is the entrance for coolant flow into its fuel assembly. The fuel assembly also includes guide tubes interspersed amongst the fuel rods. Control rods comprising neutron absorbing material are inserted into and lifted out of the guide tubes of the fuel assembly to control core reactivity. The guide tubes in a conventional assembly are rigidly attached to the grid assemblies, and the upper and lower end fittings are secured to ends of the guide tubes using fasteners to form the structural framework of the fuel assembly. 
     A dashpot is optionally disposed at the lower end of the guide tube to slow the descent of the control rod during a scram. The dashpot may be formed as a narrowed-diameter lower portion of the guide tube, or as a small-diameter dashpot tube disposed coaxially inside the lower end of the guide tube. A fastening arrangement, typically including a threaded fastener, connects the lower end fitting to the guide tube. The fastening arrangement optionally also includes a locking element to prevent the threaded connection from working its way out over time. Some illustrative fastening arrangements of this type are described, for example, in Walton, U.S. Pat. No. 4,036,692 which is incorporated herein by reference in its entirety, and in John Jr. et al., U.S. Pat. No. 5,068,083 which is incorporated herein by reference in its entirety. 
     BRIEF SUMMARY 
     In accordance with one aspect, a fuel assembly includes a plurality of fuel rods comprising fissile material, a plurality of guide tubes interspersed amongst the fuel rods, an upper end fitting connected with upper ends of guide tubes, a lower end fitting, end plugs connected with the lower ends of the guide tubes and having threaded male shafts extending from the lower ends of the guide tubes and passing through openings of the lower end fitting, and female lock nuts threaded onto the threaded male shafts of the end plugs and locked with the lower end fitting to secure the lower end fitting to the lower ends of the guide tubes. The female lock nuts may have locking portions that are outwardly deformed into recesses of the lower end fitting to lock the lock nuts with the lower end fitting after tightening. Dashpot tubes may be disposed in the lower ends of the guide tubes and connected to the end plugs. The female lock nuts do not engage the dashpot tubes. 
     In accordance with another aspect, a nuclear reactor is disclosed, comprising a pressure vessel containing a reactor core comprising an array of fuel assemblies as set forth in the immediately preceding paragraph. 
     In accordance with another aspect, an apparatus comprises: an end plug configured to connect with the lower end of a guide tube of a nuclear reactor fuel assembly, the end plug having a threaded male shaft that extends away from the lower end of the guide tube when the end plug is connected with the lower end of the guide tube, the threaded male shaft being sized to pass through an opening of a nuclear reactor fuel assembly lower end fitting; and a female lock nut configured to thread onto the threaded male shaft of the end plug, the female lock nut having a deformable side wall configured to be swaged into a recess of the nuclear reactor fuel assembly lower end fitting to lock the female nut with the nuclear reactor fuel assembly lower end fitting. 
     In accordance with another aspect, a method comprises: inserting a threaded male shaft of an end plug connected with the lower end of a guide tube of a nuclear reactor fuel assembly through an opening of a nuclear reactor fuel assembly lower end fitting; installing a female lock nut onto the inserted threaded male shaft of the end plug by rotating the female lock nut in a tightening direction to secure the nuclear reactor fuel assembly lower end fitting to the lower end of the guide tube; and deforming a locking portion of the female lock nut into a recess of the nuclear reactor fuel assembly lower end fitting to lock the female lock nut with the nuclear reactor fuel assembly lower end fitting. The method may further include, after the deforming, removing the female lock nut from the threaded male shaft by rotating the female lock nut in a loosening direction using rotational force sufficient to break the lock with the nuclear reactor fuel assembly lower end fitting provided by the deformed locking portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention 
         FIG. 1  diagrammatically shows a side view of a fuel assembly with a lower end fitting. 
         FIG. 2A  shows a perspective view of operations performed in connecting the lower end of a guide tube of fuel assembly frame to a lower end fitting. 
         FIG. 2B  shows an end view of the fuel assembly frame with the lower end fitting installed. 
         FIG. 3  is an enlarged portion of  FIG. 2B . 
         FIG. 4  is a cross-sectional view taken along the line  4 - 4  in  FIG. 3 . 
         FIGS. 5 and 6  show end and perspective views, respectively, of a female nut used in the connection shown in  FIG. 3 . 
         FIG. 7  is a cross-sectional view taken through Section A-A indicated in  FIG. 5 . 
         FIGS. 8 and 9  show perspective and end views, respectively, of a hand tool for installation/removal of the female nut of  FIGS. 5-7 . 
         FIG. 10  shows an exploded perspective view of an alternate embodiment of a hand tool for installation/removal of the female nut of  FIGS. 5-7 . 
         FIGS. 11 and 12  show perspective views of the hand tool of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Existing end fitting fastening arrangements have certain disadvantages recognized herein. In some such arrangements, a threaded male shaft inserts into the bore of the guide tube, which is a blind operation. The installer has no feedback as to whether the shaft has fully engaged, and any error in this connection may not be discovered until after the fuel rods have been loaded into the fuel assembly frame formed by the rigid assembly of guide tubes and spacer grids. Fastening arrangements that include a locking element have an advantage in reliability, but at the cost of additional component cost and increased complexity of the fastening arrangement. 
     An improved connection disclosed herein employs an end plug for the guide tube which has a male threaded shaft that passes through the lower end fitting. A female nut then engages the male threaded shaft to complete the connection. The installer can visually observe the position of the male threaded shaft passing through the opening of the lower end fitting, and the position of the female nut, and accordingly can visually verify complete engagement and proper connection. Moreover, the female nut can be a locknut with an integral deformable locking mechanism to enable the connection to be locked without welding and without the use of a separate locking element. In some embodiments, a designated combination tool provides both installation and swaged locking of the female nut. 
       FIG. 1  illustrates a typical nuclear fuel assembly generally designated by the numeral  10 . Fuel assembly  10  is typical of that used in a pressurized water reactor (PWR), and includes a plurality of fuel rods  12 , spacer grids  14 , guide tubes  16 , an upper end fitting  18 , and a lower end fitting  20 . The upper and lower end fittings  18 ,  20  are sometimes also referred to as upper and lower end fittings. In the installed configuration the fuel rods  12  are generally vertically oriented, although some deviation from exact gravitational vertical is contemplated, for example in maritime nuclear reactors that may tilt with ocean currents or vessel maneuvers. Fuel rods  12  are maintained in an array spaced apart by the spacer grids  14 . The guide tubes  16  extend through the spacer grids  14  and are rigidly connected with the grids by welding or by a mechanical fastening arrangement to form a rigid fuel assembly frame. The guide tubes  16  are hollow tubes that, in addition to providing structural support for the fuel assembly, also serve as guides for control rods. Some guide tubes may also be used as conduits for instrumentation or sensors (elements not shown). The guide tubes  16  connect at their upper ends with the upper end fitting  18  using suitable fasteners (although welding is also contemplated for this connection). The lower ends of the guide tubes  16  connect with the lower end fitting  20  using a combination of an end plug with a male threaded shaft engaged by a female nut, as described herein. 
     Upper and lower end fittings  18 ,  20  provide structural and load bearing support to the fuel assembly  10 . The end fittings  18 ,  20  have openings, slots, grids, or the like to allow coolant to flow vertically through the fuel assembly  10 . The lower end fitting  20  may rest on a lower core support plate (not shown) of the reactor and directly above coolant inlet openings in the lower core support plate that direct coolant upward to the fuel assembly. Alternatively, in some embodiments upward primary coolant flow is sufficient to lift the fuel assembly during reactor operation, in which case the upper end fitting  18  (or springs built into the fitting, not shown) may press against an upper plate or other “stop”. The fuel assembly  10  shown in  FIG. 1  is merely an illustrative example, and the fuel assembly may have different numbers of fuel rods, non-square cross sections (e.g., a hexagonal cross section in some embodiments), different numbers and arrangements of guide tubes, and so forth. 
     With reference to  FIGS. 2A, 2B, and 3 , the lower end fitting  20  is a substantially planar square element with a plurality of flow channels  24  and guide tube bosses  26 . While the illustrative lower end fitting  20  is square, but more generally the lower end fitting is sized and shaped to match the cross section of the fuel assembly  10 . As best seen in  FIG. 2A , the lower end of each guide tube  16  is connected with an end plug  28  having a threaded male shaft  30 . (See leftmost guide tube  16  shown in  FIG. 2A  in which the end plug  28  is not yet attached with the guide tube). When the end plug  28  is connected to the lower end of the guide tube  16 , the threaded male shaft  30  extends away from the lower end of the guide tube  16 . (See middle guide tube  16  shown in  FIG. 2A , which has the end plug installed so that the threaded male shaft  30  is seen extending away from the lower end). The guide tube bosses  26  include openings or through-holes passing through the generally planar square structure of the lower end fitting  20 , and the connection of the lower end fitting  20  to the lower ends of the guide tubes  16  entails inserting the threaded male shafts  30  through the openings of the bosses  26  (see rightmost guide tube  16  shown in  FIG. 2A ) and installing female lock nuts  32  onto the threaded male shafts  30  (see  FIGS. 2B and 3 ). 
     With reference to  FIG. 4 , which shows Section A-A taken through the line A-A of  FIG. 3 , the connection is further described. The threaded male shaft  30  of the end plug  28  connected with the guide tube  16  is received in the opening through the guide tube boss  26  of the lower end fitting  20 . The female lock nut  32  is installed on the threaded male shaft  30  of the guide tube  16  by turning it in a tightening direction. This draws the female lock nut  32  and the end plug  28  together. The boss  26  includes a counterbore  34  that receives the female lock nut  32 , and the geometry of the boss  26  (e.g., the thickness of the boss  26  and the depth of the counterbore  34 , in the illustrative configuration) determines the extent of the final tightening of the female lock nut  32  onto the shaft  30 , and thus determines a penetration P of the tip of the threaded male shaft  30  outside of the female lock nut  32 . (It is contemplated in some embodiments for this penetration P to be zero or even negative, i.e. for the tip of the shaft to be flush with the surface of the nut or even to be slightly within the bore of the nut when fully tightened). From the extent of the observed penetration P, which is readily observed by the installer, it is visually verified that the female lock nut  32  is tightened over the threaded male shaft  30  by the proper amount. 
     As best seen in  FIG. 3 , the counterbore  34  has a portion thereof having a non-circular profile comprised of diametrically opposed recesses  36  (e.g., radially outwardly extending recesses). The female lock nut  32  includes a deformable portion which in the illustrative embodiment is an outboard annular wall  38  that can be deformed, e.g. swaged, to form a deformed locking portion  40  that is deformed into the recesses  36  to lock the female lock nut  32  against rotation. Said another way, the female lock nut  32  is threaded onto the threaded male shaft  30  of the guide tube  16 . After the threading, a swaging operation is performed to deform the annular wall  38  to engage the recesses  36  of the boss  26  in order to lock the female lock nut  32  against (further) rotation. In this swaging operation, the deformed locking portion  40  of the female lock nut  32  is deformed to form the swage (that is, the deformed locking portion  40  of the female lock nut  32 ) that engages the recesses  36  of the boss  26  of the lower end fitting  20 . In this manner, the female lock nut  32  is secured in the tightened configuration and prevented from backing out during operation of the nuclear reactor and the accompanying vibrational, fluid flow, and differential thermal expansion stresses. 
       FIGS. 5 and 6  show end and perspective views, respectively, of the female lock nut  32  prior to the swaging operation.  FIG. 7  shows Section B-B along the line B-B indicated in  FIG. 5 . These views show the threading  42  of the female lock nut  32 , as well as engagement features  44  by which a tool can engage and rotate the female lock nut  32 . The illustrative engagement features  44  are two recesses, but other engagement features can be employed. 
     With returning reference to  FIG. 4 , the guide tube  16  optionally includes a dashpot tube  50  disposed inside the lower end of the guide tube  16 . Alternatively, a dashpot can be formed integrally in the guide tube via a narrowed lower end of the guide tube. As yet another contemplated alternative, the dashpot can be omitted entirely. During a scram, the control rod is released from the control rod drive mechanism (CRDM) and falls toward the bottom of the guide tube  16 . Primary coolant in the dashpot tube  50  (or in a narrowed lower end of the guide tube that acts as a dashpot) is compressed by the descending control rod and acts to decelerate the control rod in a gradual fashion, and hence provides dampening of the falling control rod as it nears the bottom of the guide tube  16 . The dashpot tube  50  is not connected with the female lock nut  32  that secures the lower end of the guide tube  16  to the lower end fitting  20 . Rather, the dashpot tube  50  is connected to the end plug  28  of the lower end of the guide tube  16 . In the illustrative configuration of  FIG. 4 , the end plug  28  includes a cylindrical nub or protrusion  52  of smaller diameter than the maximum outer diameter of the end plug  28 . The dashpot tube  50  is connected with this cylindrical nub or protrusion  52  by a friction fit, or by welding, or by another bonding mechanism. This connection, and the installation of the end plug  28  into the lower end of the guide tube  16 , is suitably performed before the guide tube is connected with the lower end fitting  20 . The end plug  28  is attached to the lower end of the guide tube  16  by welding, or by a friction fit, or by another bonding mechanism. Typically the dashpot tube  50  will be attached to the end plug  28  first, followed by connection of the end plug  28  with the lower end of the guide tube  16 . 
     Turning to  FIGS. 8 and 9 , an illustrative hand tool  70  for installing the female lock nut  32  over the threaded male shaft  30  is illustrated. The tool  70  includes a body  72  having a locknut engaging end  74 , and handles  76  operatively coupled to the body  72  and configured to actuate a pair of swaging heads to deform (i.e., crimp or swage) the deformable portion  38  of the female lock nut  32  after the nut is threaded onto the threaded male shaft  30  of the end plug  28  of the guide tube  16 . The crimping or swaging action can employ substantially any mechanical configuration. Since there is a tendency to squeeze the handles  76  during rotation of the hand tool  70  when threading the locknut onto the guide tube, in some embodiments the crimping or swaging action is effectuated by pulling the handles  76  outward. 
     As best seen in  FIG. 9 , the locknut engaging end  74  is generally cylindrical and includes diametrically opposed protrusions  80  configured to mate with the engagement features  44  of the female lock nut  32 . The cylindrical shape of the locknut engaging end  74  is sized to be received coaxially within the outboard deformable annular sidewall  38  of the female lock nut  32 . Also provided at the locknut engaging end  74  are diametrically opposed crimping or swaging heads  82  that are movable between the position shown in  FIG. 9  where they are disposed within the circumference of the cylindrical locknut engaging end of the body  72 , to an expanded position where they protrude radially outwardly from the cylindrical shape of locknut engaging end  74  of the body  72 . The handles  76  are configured to move the diametrically opposed crimping or swaging heads  82  to the expanded position when drawn apart, i.e. drawn outward, by the lower end fitting installer. Alternatively the swaging or crimping can be performed when the handles  76  are squeezed, or another mechanical actuation mechanism can be used to cause the swaging or crimping heads  82  to expand outward to perform the swaging or crimping. It is also contemplated for the rotational tightening and swaging operations to be performed robotically, in which case the handles  76  would be replaced by appropriate robotic actuators. 
     With reference to  FIGS. 10-12 , another illustrative tool  100  for installing the female lock nut  32  over the threaded male shaft  30  is illustrated, which is suitably used in place of the hand tool  70  of  FIGS. 8-9 . The tool  100  is configured for mounting on a ratchet wrench (not shown).  FIG. 10  shows an exploded perspective view of the tool  100 , while  FIG. 11  shows an assembled view of the tool  100  configured for applying torque to the lock nut  32  and  FIG. 12  shows an assembled view of the tool  100  configured to perform a “punch” operation that swages the outboard annular sidewall  38  outward at two deformation locations located 180° apart around the lock nut  32 . The tool  100  includes a cylindrical sleeve  102  that coaxially surrounds a cylindrical nut engaging element  104  that in turn coaxially surrounds a punch element  106 . The three elements are locked against relative rotation by a holding pin  108  that engages slots  110 ,  112  in the nut engaging element  104  and the punch element  106 , respectively. (Slots  110 ,  112  are visible only in the exploded view of  FIG. 10 ). The slots  110 ,  112  provide defined allowable axial travel for the nut engaging element  104  and the punch element  106 , respectively, respective to the sleeve  102 . A holding screw  114  can be tightened to lock the nut engaging element  104  in either the extended position (i.e., the torqueing configuration shown in  FIG. 11 ) or in a retracted position (i.e., the punch configuration shown in  FIG. 12 ). In the extended or torqueing position, diametrically opposed protrusions  120  are configured to mate with the engagement features  44  of the female lock nut  32 . These protrusions  120  are functionally equivalent to the protrusions  80  of the hand tool of  FIGS. 8-9 . In the retracted or punch position, the protrusions  120  are retracted into the sleeve  102 . The punch element  106  includes wedges  122  that are functionally equivalent to the diametrically opposed crimping or swaging heads  82  of the hand tool of  FIGS. 8-9 , i.e. they swage portions of the outboard annular sidewall  38  outward to lock the lock nut  32 , although they operate somewhat differently. 
     With particular reference to  FIG. 11 , in the extended position, i.e. the torqueing configuration, the diametrically opposed protrusions  120  extend out from the sleeve  102  to mate with the engagement features  44  of the female lock nut  32 . A square recess  126  at the top of the punch element  106  (visible only in  FIG. 12 ) mates with a conventional square fitting (optionally with spring-loaded ball detent) of a ratchet wrench (elements not shown), so that the ratchet wrench can apply torque to the female lock nut  32  via the protrusions  120  in order to rotate the lock nut  32  onto the threaded male shaft  30  of the end plug  28  of the guide tube  16 . Note that in  FIG. 11  the wedges  122  of the punch element  106  protrude from between the nut-engaging protrusions  120  of the nut-engaging element  104 . However, because the nut-engaging element  104  is free to travel axially within the limits defined by the slot  112 , these wedges  122  will be readily pushed into the sleeve  102  when the protrusions  120  are brought to bear against the lock nut  32 . On the other hand, the holding screw  114  serves to ensure that the nut engaging element  104  does not get pushed back into the sleeve  102 . (Note, the holding screw  114  can engage the nut engaging element  104  frictionally, or can insert into a bored recess in the nut engaging element  104 ). As is conventional, the ratchet wrench is typically configured to ratchet in only one direction (possibly selectable), i.e. in the nut-tightening direction. The ratchet wrench may optionally also include a torque setting such that it will only tighten the lock nut  32  to a specified torque. 
     With particular reference to  FIG. 12 , after the lock nut  32  is tightened onto the threaded male shaft  30  of the end plug  28 , the tool  100  is switched to the retracted position, i.e. torqueing configuration, shown in  FIG. 12 . In this configuration the nut engaging element  104  is retracted such that the protrusions  120  are withdrawn into the sleeve  102 . The holding screw  114  is suitably loosened to enable the nut engaging element  104  to be slid back, as allowed by slot  110 , and then the holding screw  114  is tightened again to hold the nut engaging element  104  in the torqueing configuration. The wedges  122  are then brought to bear against the lock nut  32 . Doing so causes the punch element  106  to slide axially backward as permitted by the slot  112  in the punch element  106 . A mallet, hammer, or other tool is then brought down onto the top of the punch element  106  (i.e., onto the surface into which the recess  126  are formed) to force the wedges  122  downward so as to swage the outboard annular sidewall  38  of the lock nut  32  outward at the diametrically opposed locations of the wedges  122 . The downward punch travel is defined by the slot  112  as the travel distance d punch , which is preferably selected to ensure a slight remaining upward protrusion of the top of the punch element  106  above the top of the nut engaging element  104  when the punch is fully applied. 
     While operation of the tool  100  is described herein with reference to manual operation, it will be appreciated that the tool  100  is alternatively readily operated by a robotic apparatus, for example employing a robotic ratcheting mechanism and a pneumatic or hydraulic mechanism to deliver the punch force. The robotic approach may employ a different coupling than the recess  126  shown in  FIG. 12 , which is a conventional square shape. 
     In a typical lower end fitting installation procedure, the lower ends of the guide tubes  16  are first pre-assembled by attaching the dashpot tube  50  to the end plug  28  and then inserting the end plug  28  (with the dashpot tube  50  attached) into the lower end of the guide tube  16  and forming a connection, for example by welding or using a friction fit. The threaded male shafts  30  of the end plugs  28  of the guide tube  16  are then inserted into the openings of the bosses  26  of the lower end fitting  20  (best seen in  FIG. 2A ) and the female lock nuts  32  are installed. In a suitable approach, the lock nuts  32  are partially threaded onto the threaded male shafts  30  by hand. The hand tool  70 , or alternatively the tool  100 , is then used to tighten down each female lock nut  32  in turn. Once the female lock nut  32  is suitably tightened (as verified by visual assessment of the protrusion P of the shaft end as shown in  FIG. 4 ), the plier grips  76  of the hand tool are squeezed together to force the swaging heads  82  radially outwardly to deform the outboard annular side wall  38  into the recesses  36  of the counterbore  34  of the boss  26  of the lower end fitting  20 , so as to form the deformed locking portions. Alternatively, if using the tool  100  the most efficient approach is typically to tighten all the lock nuts  32  first using the tool  100  in the torqueing configuration ( FIG. 11 ), and then to switch the tool  100  to the punch configuration ( FIG. 12 ) and swage the lock nuts  32  to form the deformed locking portions  40 . 
     Typically, the lower end fitting  20  is not removed from the lower ends of the guide tubes  16  of the fuel assembly frame. The fuel rods are designed to be loaded (and, if necessary, unloaded) from the top after removing the upper end fitting  18 . However, under certain circumstances it may become necessary to remove the lower end fitting  20 . Such a circumstance could arise prior to loading the fuel rods, if some error occurs during construction of the fuel assembly frame. In this case, the hand tool  70  or the tool  100  can be used to remove the female lock nuts  32  from the threaded male shafts  30 . A nut is removed by rotating the female lock nut  32  in the loosening direction using rotational force sufficient to break the locks provided by the deformed locking portions  40  of the female lock nuts  32 . (In the case of the tool  100 , the direction of ratcheting is switched to the loosening direction). 
     Another possible circumstance calling for removal of the lower end fitting  20  could arise if a fuel rod breaks after loading. In this circumstance, a possible remedial procedure to be performed robotically and in a submerged state (in view of high radiation levels emitted by the loaded fuel assembly) is to move the fuel assembly into a spent fuel pool and perform fuel unloading by removing the upper end fitting  18  as per usual procedure. This leaves the fuel assembly frame (i.e., the guide tubes  16  rigidly secured together by spacer grids and still including the installed lower end fitting  20 ) which is empty of fuel except for the lower portion of the broken fuel rod. That broken lower fuel rod portion typically cannot be removed from the opening provided by removal of the upper end fitting  18 . Rather, removal of this remaining broken lower end is via the lower end. To accomplish this, robotic apparatus is employed to remove the lower end fitting  20 . The robotic apparatus suitably includes a nut removal tool such as the tool  100  operated by the robotic apparatus. The robotic nut removal tool is used to remove the female lock nuts  32  from the threaded male shafts  30 . Again, a nut is removed by rotating the female lock nut  32  in the loosening direction using rotational force sufficient to break the locks provided by the deformed locking portions  40  of the female lock nuts  32 . After removal of the female lock nuts  32 , the lower end fitting  20  can be pulled away from the lower ends of the guide tubes  16 , providing access to the lower portion of the broken fuel rod. (It should be noted that an alternative is to cask the entire fuel assembly frame including the lower portion of the broken fuel rod and the lower end fitting, without removing the lower end fitting. However, in some situations this may require a larger cask than is desirable). 
     Although shown and described with reference to the illustrative embodiment, it is to be understood that numerous variants are contemplated. For example, while the illustrative lock nut  32  includes two deformed locking portions  40  located at opposing sides of the female lock nut  32 , the number of swaged portions can be as few as one, or can be two, three, four, or more. In general, employing N swaged portions with N-fold rotational symmetry around the lock nut has advantages in terms of enabling the application of balanced forces around the lock nut during the swaging operation. In the illustrative example the two deformed locking portions  40  are located 180° apart around the lock nut and hence have two-fold rotational symmetry around the lock nut  32 . Three swaged portions at 120° intervals around the lock nut, or four swaged portions at 90° intervals around the lock nut, or so forth would also allow for advantageously balanced outward swaging forces. Moreover, the entire annular side wall  34  can be swaged, for example by deforming the annular sidewall  34  into an oval shape having two crimps at opposing sides along the major axis of the oval. Other approaches can be used by which the lock nut can lock with the lower end fitting. As another illustrative example, the annular side wall  34  can be replaced by one or more discrete deformable tabs that are deformed outward to provide the deformed locking portions. 
     As some illustrative further contemplated variations, the illustrative crimping handles or grips  76  can be replaced by another actuator, such as a rotational actuator disposed in the central cylinder of the hand tool that linearly drives a member between the crimping or swaging heads to force them outward, e.g. similarly to the linear operation of the punch element  106  of the tool  100 . Still further, while the illustrative hand tool  70  providing both rotational (for threading) and swaging actions is convenient, it is contemplated to employ separate hand or robotic tools: one tool for threading the lock nut onto the threaded end of the guide tube, and a second tool for performing the crimping or swaging. Also, as previously noted, the hand tool  70  or manual wrench operation of the tool  100  can be replaced by an equivalent robotic mechanism (and indeed, robotic tooling must be used for any removal operation performed after the fuel rods are loaded). Still further, it is contemplated to adapt the disclosed connection for use in the upper end fitting. In this case, the female locknut preferably engages a threaded outer diameter of the upper end of the guide tube, as an end plug is not an option (since the control rod passes through the upper end of the guide tube). 
     The preferred embodiments have been illustrated and described. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.