Patent Number: 050646060
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to nuclear fuel handling apparatus and, more particularly, to an improved channel box removing apparatus which is adapted for use in removing the channel box from a nuclear fuel assembly placed on a fuel rack in a spent fuel storage pool in a nuclear power plant. This invention is particularly useful for nuclear fuel assemblies for a boiling water type nuclear reactor. 2. Background Conventional designs of fuel assemblies for use in a boiling water type nuclear reactor include a multiplicity of fuel rods held in an organized array by spacers spaced along the fuel assembly length. As illustrated in FIG. 1 of the accompanying drawings, such a fuel assembly 10 includes a tubular channel box 12 which is generally square in cross section having a fuel assembly proper 14 mounted therein in a telescopic relationship to each other. The channel box 12 serves to promote contact of the fuel rods with primary coolant flow, facilitate guidance of the fuel rods in the reactor and protect the fuel assembly during transfer to and from the spent fuel storage pool 41 (FIG. 3). The channel box 12 is fastened to the fuel assembly proper 14 by means of fastener bolts 16. A plurality, e.g., about 760 fuel assemblies are used in a light water reactor of a million KW class, and usually about 1/4 of the 760 assemblies, i.e., about 200 assemblies are replaced once a year during a refuelling operation. During refuelling and other operations such as periodically performed inspections, the channel boxes 12 are removed from the associated fuel assemblies 10. FIG. 2 shows a conventional tool used to remove such channel boxes 12, which comprises a suspension ring-like member 22 adapted for suspension from a crane (not shown), an operating rod 24, and a channel box grappling section 26 mounted to the lower end of the operating rod 24. The channel box grappling section 26 includes a pair of rigid guide members 28 extending downwardly at opposite sides thereof to slidably receive therein the outer surfaces of a pair of diagonally opposed corners of the channel box 12, a pair of releasable hooks 30 pivotally mounted at their upper ends on the grappling section and adapted to engage the undersurfaces of associated clips (not shown) provided on the top of the channel box 12, and a control mechanism 32 for controling the movement of the releasable hooks 30 in response to a manually operated knob 34 provided on the top end of the operating rod 24. Removal of the channel box from the fuel assembly using such a removing apparatus is conventionally carried out during a refuelling operation within a spent fuel storage pool. Such removal of the channel box will be described hereinafter with reference to FIG. 3 which is a perspective view of the spent fuel storage pool. (1) About 200 spent fuel assemblies previously transferred to a fuel rack 40 within the spent fuel storage pool 41 are lifted one by one by a main hoist 42 installed on a refuelling platform car 44 for transfer to a preparation machine 46. (2) At the preparation machine 46, the fastener bolts threaded through the clips of each channel box to secure it and the fuel assembly proper together are first removed using a bolt wrench (not shown). (3) Then, the removing apparatus 26 suspended from a jib crane 50 provided near the preparation machine 46 is lowered toward the fuel assembly and actuated to bring the releasable hooks 30 into engagement with the associated clips 86 of the channel box 12 to thereby hold the channel box 12 in a predetermined position. With the channel box 12 thus held, the fuel assembly proper 14 is lowered by the preparation machine 46 until it is fully removed from the channel box 12. The channel box 12 thus removed is transferred by the jib crane 50 to a channel box rack 52. (4) Following the step (3), the fuel assembly proper 14 is transferred by the main hoist 42 from the preparation machine 46 back to the fuel rack 40 for temporary storage. (5) When the channel box rack 52 becomes fully occupied by channel boxes transferred thereto, additional channel boxes 12 are transferred by a channel box transferring machine (not shown) attached to the lower end of the jib crane cable 50 to a waiting rack 54. When this waiting rack also becomes fully occupied, further channel boxes 12 are transferred again to the fuel rack 40 by a channel box transferring machine (not shown) which is connected to the lower end of a cable of an auxiliary hoist 56. Only flawless channel boxes placed on the fuel rack 40 are later reused. Generally, the conventional channel box removing apparatus such as one generally designated at 26 has the limitation that it can be used only with such a preparation machine 46 which is capable of removing fuel assemblies, one at a time. Specifically, numerous fuel assemblies 10 are compactly placed on the fuel rack 40, leaving almost no space between the assemblies, as shown in FIG. 4. And, a framework 60 of the fuel rack 40 has a height slightly less than the upper end of each channel box 12. Accordingly, removal of the channel boxes from the fuel assemblies placed on the fuel rack 40 is not possible with the use of the conventional removal apparatus 26, since the guide members 54 cannot be brought into an operative position around the associated corners of the channel box 12 where the releasable hooks 30 can be operated into engagement with the associated clips 62 even if the removal apparatus is of a pneumatic drive type. Thus, it has been common practice to transfer fuel assemblies to the preparation machine 46, one at a time, where the channel box 12 is removed by lifting it from the fuel assembly proper 14. However, this causes inconveniences as set forth below: (a) Separation of the channel box 12 from the fuel assembly proper 14 at the preparation machine 46 and individual transfer thereof, as described above, require each channel box 12 to be lifted and lowered many times by different hoists or cranes. Specifically, the channel box 12 is transferred by the jib crane 50 having a small radius of transfer to the channel box rack 52 or the waiting rack 54 for temporary storage, while the associated fuel assembly proper 14 is transferred by the main hoist 42 to the fuel rack 40. The channel box 12 is then transferred by the auxiliary hoist 56 to the fuel rack 40. Thus, many transfer steps are involved in an overall channel box removal operation, increasing the possibility of causing damage to channel boxes, prolonging the channel box removal time, and increasing the manpower required and the level of exposure for the operators. (b) Also, there is a likelihood of the separated fuel assembly proper 14 being damaged during transfer to the fuel rack 40 because of lack of protection provided by the channel box. It has been proposed to use a pneumatic drive to control the releasable hooks of the removing apparatus rather than manually control the knob 34 and also to place a pedestal, called "channel storage adapter" (not shown), in the fuel rack 40 to raise the fuel assembly so as to expose the upper portion of the channel box 12 above the framework 60 so that the removal apparatus 26 can be brought into an operative position with the channel box for transfer from the preparation machine 46. However, the use of such pedestals complicates and prolongs the removal operation and also makes it more difficult to place the guide members 28 of the removal apparatus 26 on the associated corners of the channel box 12 because the fuel rack 40 is located fairly distant from an operating deck. SUMMARY OF THE INVENTION It is an object of the invention to provide an improved channel box removing apparatus which overcomes the above-stated disadvantages of the prior art apparatus. It is another object of the invention to provide an improved channel box removing apparatus which enables an efficient and reliable removal of channel boxes from nuclear fuel assemblies placed on a fuel rack. The objects stated above and other related objects in this invention are accomplished by the provision of an apparatus for removing a channel box from a nuclear fuel assembly having a bail provided on the top thereof, the channel box being generally square in cross section having the fuel assembly mounted therein in a telescopic relationship to each other, the apparatus having a pair of clips provided adjacent a pair of diagonally opposed top corners thereof, the channel box comprising: a frame structure adapted for suspension from a lifting and transporting device; a pair of releasable hooks pivotally mounted to the lower portion of the frame structure and movable between a first position in which the releasable hooks are in lifting engagement with the clips of the channel box and a second position in which the releasable hooks are released from the lifting engagement with the clips; an operating mechanism for moving the pair of releasable hooks between the first and second positions; and, bail cap means mounted to the lower portion of the frame structure for vertical movement relative to the frame structure, the bail cap means being adapted to cooperate with the bail of the fuel assembly to guide the channel box removing apparatus into an operative position with the fuel assembly during lowering of the apparatus toward the fuel assembly with the pair of releasable hooks in the second position, the operating mechanism being activated to move the pair of releasable hooks into the first position after arrival of the channel box removing apparatus at the operative position. The bail cap means comprises a pair of spaced support members provided in opposed relationship to each other and having a vertically extending guide slot defined therebetween to slidably receive the bail, a connector means securely connecting the pair of support members in their middle portions and adapted for abutting engagement with the top of the bail, and a pair of bail guides extending downwardly and outwardly from the bottom ends of the respective support members and having their open lower edges adapted to abut the upper ends of the channel box adjacent a pair of diagonally opposed corners thereof different from the pair of diagonally opposed corners having the clips provided thereon. Each of the releasable hooks has an upper end pivotally mounted to the lower portion of the frame structure and a lower end extending downwardly from the bottom of the frame structure, and bracket means pivotally connected to the releasable hooks in a manner to straddle the hook pair, and the operating mechanism comprises a cylinder mounted to the frame structure, a piston rod for the cylinder and having its lower end operatively connected to the bracket means, and conduit means for supplying operating fluid to the cylinder. Activation of the operating mechanism causes vertical movement of the piston rod and accordingly the bracket means to move the releasable hooks between the first and second positions. A hand lever is provided having one end rotatably mounted to the frame structure and the other end adapted to be hooked for forced disengagement of the pair of releasable hooks from the channel box, the hand lever being pivotally connected adjacent the other end thereof to the top of the piston rod so that pivotal movement of the hand lever in a direction to cause upward movement of the piston rod moves the pair of releasable hooks to the second position. Further in accordance with the present invention, there is provided an apparatus for removing a channel box from a nuclear fuel assembly having a bail provided on the top thereof, the channel box being generally square in cross section having the fuel assembly mounted therein in a telescopic relationship to each other, the channel box having a pair of clips provided adjacent a pair of diagonally opposed top corners thereof, the apparatus comprising: a frame structure adapted for suspension from a lifting and transporting device; a pair of releasable hooks pivotally mounted to the lower portion of the frame structure and movable between a first position in which the releasable hooks are in lifting engagement with the clips of the channel box and a second position in which the releasable hooks are released from the lifting engagement with the clips; an operating mechanism for moving the pair of releasable hooks between the first and second positions; bail cap means mounted to the lower portion of the frame structure for vertical movement relative to the frame structure, the bail cap means being adapted to cooperate with the bail of the fuel assembly to guide the channel box removing apparatus into an operative position with the fuel assembly during lowering of the apparatus toward the fuel assembly with the pair of releasable hooks in the second position, the operating mechanism being activated to move the pair of releasable hooks into the first position after arrival of the channel box removing apparatus at the operative position; and a locking mechanism for preventing accidental disengagement of the pair of releasable hooks from the associated clips of the channel box when the channel box is being lifted by the removal apparatus. The locking mechanism comprises a pair of swing plates capable of selectively doubling the range of pivotal movement of the releasable hooks, each swing plate being pivotally mounted adjacent its upper end to the frame structure and having a slit formed therein, a first pin projecting from each releasable hook and being loosely received in the slit of the swing plate to permit a swinging movement of the lower end of the swing plate beyond the pivotal range of the releasable hook, a second pin provided adjacent the lower end of each swing plate, and a stopper member integrally formed on each side surface of the bail cap means, each stopper member having a groove adapted to be engaged by the associated second pin when the associated releasable hook is in the first position, the second pin being in abutting engagement with the stopper member when the associated releasable hook is in the second position with the bail cap means slightly lowered relative to the removal apparatus. Still further in accordance with the present invention, there is provided an apparatus for removing a channel box from a nuclear fuel assembly having a bail provided on the top thereof, the channel box being generally square in cross section having the fuel assembly mounted therein in a telescopic relationship to each other, the channel box having a pair of clips provided adjacent a pair of diagonally opposed top corners thereof, the apparatus comprising: a frame structure adapted for suspension from a lifting and transporting device; a pair of releasable hooks pivotally mounted to the lower portion of the frame structure and movable between a first postion in which the releasable hooks are in lifting engagement with the clips of the channel box and a second position in which the releasable hooks are released from the lifting engagement with the clips; an operating mechanism for moving the pair of releasable hooks between the first and second positions; bail cap means mounted to the lower portion of the frame structure for vertical movement relative to the frame structure, the bail cap means being adapted to cooperate with the bail of the fuel assembly to guide the channel box removing apparatus into an operative position with the fuel assembly during lowering of the apparatus toward the fuel assembly with the pair of releasable hooks in the second position, the operating mechanism being activated to move the pair of releasable hooks into the first position after arrival of the channel box removing apparatus at the operative position; and a first indicator mechanism for providing a visual indication that the pair of releasable hooks of the removal apparatus are in the first position. The first indicator mechanism comprises a first indicator plate pivotally mounted to the frame structure for swinging movement between an upright position and a horizontal position, a first actuator link pivotally mounted to the frame structure adapted to be engaged by the bail cap means, and a control rod having upper and lower ends pivotally connected to the rear face of the first indicator plate and to one end of the first actuator link, respectively, the control rod comprising a lower portion and an upper portion adapted to be moved vertically relative to each other, the upper portion of the control rod being biased downwardly to move the first indicator plate to the horizontal position and adapted to move upwardly to move the first indicator plate to the upright position in response to movement of the pair of releasable hooks from the first to the second position, the lower portion of the control rod being biased upwardly and being in engagement with the lower end of the upper portion of the control rod when the first actuator link is out of engagement with the bail cap means. Lowering of the removal apparatus toward the fuel assembly after the bottom of the bail cap means comes into engagement with the top of the chanel box causes the bail cap means to move upwardly relative to the frame structure to bring the bail cap means into engagement with the lower end of the first actuator link to thereby rotate the first actuator link and to move the lower portion of the control rod downwardly, so that when the operating mechanism is activated in this condition, the upper portion of the control rod moves downwardly to move the first indicator plate to the horizontal position. A second indicator mechanism is provided for providing a visual indication that the pair of releasable hooks are in the first position and also that the channel box alone can be lifted separately from the fuel assembly. The second indicator mechanism comprises a second indicator plate pivotally mounted to the frame structure for swinging movement between an upright position and a horizontal position, a second control rod having an upper end pivotally connected to the rear face of the second indicator plate and a lower end extending downwardly through the frame structure into the lower portion thereof and adapted to be engaged by the bail of the fuel assembly, the second indicator plate being normally in the horizontal position due to the weight of the second control rod but adapted to be moved to the upright position by the upward movement of the second control rod caused by upward movement of the bail of the fuel assembly in abutting engagement with the lower end of the second control rod. Lifting of the removal apparatus with the pair of releasable hooks in the first position causes lowering of the bail cap means relative to the frame structure, disengaging the bail of the fuel assembly from the lower end of the second control rod to allow the second control rod to move downwardly thereby moving the second indicator plate to the horizontal position. A guide means is provided which is removably mounted to the bail cap means to facilitate guidance of the removal apparatus into the operative position with the fuel assembly. The guide means comprises a pair of conical guide rollers rotatable about a common horizontal axis and adapted to slidably and rotatably engage the pair of diagonally opposed corners of the channel box having no clips provided thereon, and support means removably mounted to the lower end of the bail cap means and supporting the pair of conical guide rollers.