Patent Number: 046735445
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a pushing device for sliding fuel rods out of a nuclear reactor fuel assembly. European Patent Application No. 83 10 7568.4 discloses a so-called dense-packing device which serves to compress, into a packing as dense as possible and preferably into a hexagonal array, fuel rods removed form a reactor fuel assembly and to transfer them into a storage container. To this end it is necessary to slide the fuel rods out of the spacers of the reactor fuel assembly by means of a pushing device and to introduce them into the dense-packing device via guide means, for instance, a guide tube bundle. After the fuel rods are subjected to a relatively high thermal stress during their operating phase and are also exposed to chemical influences, it may happen that individual fuel rods get jammed in the spacers. In such a case, the pushing device should be switched off altogether above a given pressure in order to avoid major damage. At least the force applied to the jammed fuel rods should be interrupted. An object of the present invention to provide a pushing device of the above-described type capable of sliding a desired number of fuel rods, in particular, all fuel rods simultaneously, out of the fuel assembly mounting and to introduce them into guidance means for the purpose of placing them in a dense-packing device. Another object of the present invention is to provide such pushing device in which the force applied to a fuel rod is interrupted or reduced upon a jamming of the fuel rod in its spacer mounting. Another, more particular, object of the present invention is to provide such a pushing device in which, upon a jamming of one or a small number of fuel rods, many or all of the remaining rods can still be removed. SUMMARY OF THE INVENTION In a nuclear reactor installation having a fuel assembly with spacers holding a multiplicity of nuclear fuel rods disposed parallel to one another in a geometric array, a pushing device in accordance with the present invention for simultaneously sliding from the spacers a plurality of fuel rods disposed in a geometric pattern comprises a plurality of axially movable push rods, a first mounting assembly, a second mounting assembly, a third mounting assembly, a drive and a locking assembly. The push rods are equal in number to the fuel rods to be simultaneously removed from the spacers and are disposed parallel to each other in the pattern in which the plurality of fuel rods is arranged. The first mounting assembly includes a pressure plate secured to the push rods at one end thereof for supporting the push rods and further includes a guide plate disposed at an opposite end of the push rods in a disengaged state of the pushing device for guiding the push rod during operation of the pushing device. The guide plate has a plurality of bores at least equal in number to the push rods and having substantially the same cross-sectional area and shape as the push rods. The bores in the guide plate are disposed in the same pattern as the push rods and are traversed thereby. The second mounting assembly includes a base plate on a side of the pressure plate opposite the guide plate and guide rods secured to the base plate and the guide plate for slidably securing the pressure plate for motion in a direction parallel to the push rods. The third mounting assembly is connected to the base plate and to the guide plate for supporting the same in a operating position juxtaposed to the fuel assembly. The drive is operatively connected to the base plate and to the pressure plate for moving the pressure plate relative to the base plate, thereby sliding the plurality of fuel rods from the spacers by means of the push rods. The locking assembly is connected to the push rods for preventing motion of at least any one push rod upon the exceeding of a threshold pressure by the force transmitted by that one push rod. In accordance with another feature of the present invention, the first mounting assembly includes a mounting plate slidably attached to the guide rods between the pressure plate and the guide plate, the mounting plate having a plurality of bores at least equal in number to the push rods and having substantially the same cross-sectional area and shape as the push rods. The bores in the mounting plate are disposed in substantially the same pattern as the push rods and are traversed thereby. In accordance with another feature of the present invention, the drive takes one of at least two alternative forms. The drive may include at least one threaded spindle traversing the base plate and threadedly engaging a threaded nut supported on the base plate. The spindle is secured to the pressure plate and a rotary power source is operatively connected to the threaded nut, whereby rotary power produced at the output of the power source is transformed into linear motion of the pressure plate. In an alternative form the drive includes a spindle rotatably mounted to the base plate and to the guide plate and extending substantially parallel to the push rods. The pressure plate is provided with a threaded bore traversed by the spindle, the spindle having an external thread operatively engaging the threaded bore of the pressure plate. Rotary motion of the spindle is thus converted into linear motion of the pressure plate and of the push rods connected thereto. Pursuant to a particular feature of the present invention, the locking assembly assumes one of at least two different embodiments. In the first embodiment a switch is operatively connected to the drive for disengaging the drive upon the exceeding of the threshold pressure by the force transmitted by a stalled push rod. The first mounting assembly includes at least one spring engaging the pressure plate and the push rods in part for transmitting compressive force from the pressure plate to the push rods and in part for determining the threshold pressure. In the other embodiment of the locking assembly, at least one shear pin is rigidly connected to the pressure plate and to the push rods and forms a component of the first mounting assembly as well as of the locking assembly. Upon the exceeding of the threshold pressure determined by the size, shape and material of the shear pin, the shear pin breaks, thereby preventing continued motion of the push rods. Preferably, each push rod is provided with its own shear pin rigidly connected to the pressure plate. A pushing device in accordance with the present invention is advantageous in that it allows removal of all or substantially all fuel rods from a nuclear reactor fuel assembly simultaneously. In the event that one or more fuel rods become jammed or caught in their respective spacers, the locking assembly in accordance with the present invention interrupts the continued application of force to these jammed fuel rods, thereby preventing damage to the reactor fuel assembly. If the shear pin type locking assembly is being used or if a plurality of pressure plates is included in a pushing device utilizing a locking assembly of the interrupter switch type, the pushing device may still be operative upon the jamming of a single fuel rod to remove some, or even all, of the other fuel rods of the fuel assembly. In accordance with a particular feature of the present invention, one or more mounting plates may be provided between the pressure plate and the base plate for supporting the push rods against bending during operation of the pushing device.