Abstract:
A method and apparatus for reducing the volume of a cylindrical tube for disposal by crushing flat and then shearing into small coupons. Shearing is accomplished by opposed dies having a checkerboard grid of cutting edges. The resulting coupons are preferably substantially square in shape. The invention is particularly useful for reducing the storage volume of irradiated radioactive pressure tubes from a nuclear reactor.

Description:
The present invention relates to a method and apparatus for the volume reduction and disposal of material, the primary application involving irradiated radioactive material. In particular, the present invention relates to method and apparatus for the removal, processing and disposal of pressure tubes and calandria tubes from CANDU® nuclear reactors. 
     BACKGROUND OF THE INVENTION 
     In order to extend the operating life of CANDU® nuclear reactors, it may be necessary to undertake large scale fuel channel replacement. One of the key processes of large scale fuel channel replacement is the removal of the highly radioactive pressure tubes and calandria tubes from the reactor core. 
     The conventional process involves the removal of the approximately 6 meter long pressure tubes and calandria tubes whole, or cut in half at their midpoint. The reactor vault, on the side that the tubes are removed to, must be evacuated of personnel, which prevents parallel activities from occurring and thereby prolonging the work schedule. A very large and heavy lead-filled flask is used to transport the pressure tubes out of the reactor vault to disposal. It is a difficult and time consuming task to move this size of flask through the containment structure of a CANDUO® type nuclear reactor, requiring cranes or other heavy material handling equipment and personnel evacuation from the work area. This interrupts material, equipment and personnel movement for the whole reactor outage and is a major detriment in scheduling and critical path considerations. Thus, it is desirable to make the method of pressure tube and calandria tube removal and disposal more economically attractive. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for reducing the volume of a cylindrical tube for disposal by the process of crushing flat and then shearing into small coupons. Shearing is preferably accomplished on a checkerboard grid of multiple intersecting planes and the resulting coupons are preferably substantially square in shape. 
     In accordance with the present invention there is provided an apparatus for reducing the disposal volume of an elongated cylindrical tube comprising a pair of inwardly opposed die blocks, means for moving said die blocks between an open position and a closed position, and feeder means for positioning an end portion of said cylindrical tube between said die blocks in said open position, each of said die blocks comprising an array of raised cutters and recessed pockets, each of said cutters of one of said die blocks adapted to be closely received into an opposed pocket of the other of said die blocks when said die blocks are moved from said open position to said closed position to sequentially crush said end portion to a substantially flat configuration and sever it into a plurality of coupons. 
     In accordance with another aspect of the invention, there is provided an apparatus for reducing the disposal volume of irradiated radioactive nuclear reactor cylindrical tubes comprising a movable mounting base adapted to be operatively positioned adjacent a reactor face at selected tube positions; a feeder unit mounted on said base for engaging and advancing said selected tube out of said reactor; a press assembly mounted on said base comprising a pair of inwardly opposed die blocks and means for moving said die blocks between an open position and a closed position, said die blocks being positioned to receive therebetween in said open position the end portion of said selected tube, each of said die blocks comprising an array of raised cutters and recessed pockets, each of said cutters of one of said die blocks adapted to be closely received into an opposed pocket of the other of said die blocks, whereby when said die blocks are moved from said open position to said closed position, said end portion is sequentially crushed to a substantially flat configuration and severed into a plurality of coupons. 
     In accordance with another aspect of the invention, there is provided a method for reducing the disposal volume of an elongated cylindrical tube comprising: (a) positioning the end portion of said cylindrical tube between a pair of inwardly opposed die blocks movable between an open position and a closed position, each of said die blocks comprising an array of raised cutters and recessed pockets, each of said cutters of one of said die blocks adapted to be closely received into an opposed pocket of the other of said die blocks when said die blocks are moved from said open position to said closed position; (b) moving said die blocks from said open position to said closed position to sequentially crush said end portion to a substantially flat configuration and sever it into a plurality of coupons; and (c) repeating steps (a) and (b) until said cylindrical tube is severed into coupons. 
     In accordance with another aspect of the invention, there is provided a method for reducing the disposal volume of irradiated radioactive nuclear reactor cylindrical tubes comprising: (a) engaging the end of a selected tube at the reactor face and advancing a portion of said selected tube out of said reactor; (b) positioning the end portion of said cylindrical tube between a pair of inwardly opposed die blocks movable between an open position and a closed position, each of said die blocks comprising an array of raised cutters and recessed pockets, each of said cutters of one of said die blocks adapted to be closely received into an opposed pocket of the other of said die blocks when said die blocks are moved from said open position to said closed position; (c) moving said die blocks from said open position to said closed position to sequentially crush said end portion to a substantially flat configuration and sever it into a plurality of coupons; and (d), repeating steps (b) and (c) until said cylindrical tube is severed into coupons. 
     The construction and method of operation of the invention will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment of the disposal volume reduction system of the present invention. 
     FIG. 2 is a perspective view of the press assembly. 
     FIG. 3 is an elevation view of the press assembly. 
     FIG. 4 is a perspective view showing the checkerboard die block and cutter arrangement. 
     FIG. 5 is perspective view of the retraction unit. 
     FIG. 6 is a side view in cross-section of the retraction plug. 
     FIG. 7 is a perspective view of the feeder assembly. 
     FIG. 8 is an elevation view in part cross-section of the feeder assembly. 
     FIG. 9 is an elevation view in cross-section of the flask assembly during unloading. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the checkerboard shear volume reduction system of the present invention is shown. The system comprises retraction unit  100 , press assembly  200 , feeder assembly  300  and flask assembly  400 . The pressure tube volume reduction system is mounted on work platform  500 , which is capable of horizontal lateral motion to allow remote movement between lattice sites and inboard/outboard motion to allow movement toward and away from the lattice sheet, as well as vertical motion to allow movement up and down the lattice sheet. Cylindrical pressure tube  10  is shown in FIG. 1 in position for volume reduction. 
     Referring now to FIGS. 2 and 3, press assembly  200  is shown in greater detail. Press assembly  200  comprises base  202  and end plates  204 . End plates  204  are retained in fixed relation to one another by box frames  208  and bottom plate  201  and by parallel straps  210 . Straps  210  have a bolted connection at the four comers of box frames  208 . 
     Hydraulic cylinders  214  are fixedly mounted to end plates  204  with the cylinder rods  218  extending inwardly from end plates  204 . Cylinder rods  218  are connected at their distal ends to platens  220  by cylinder bolt  222 . Platens  220  have a bolted connection to dieblocks  226 . Dieblocks  226  are mounted on guide rods  212  through bores  224  at their four comers and are adapted for reciprocal horizontal sliding movement toward and away from one another under control of hydraulic cylinders  214 . Opposed die blocks  226  are carried on the inward facing surfaces of platens  220 . Die blocks  226  contain an interlaced set of tool steel shear blades, details of which can be seen in FIG.  4 . 
     Referring now to FIG. 4 details of die blocks  226  can be seen. Each die block  226  is machined to accept a checkerboard pattern of raised cutters  228  and recessed pockets  230  containing ejectors  232 . When brought into face-to-face relation, the raised cutters  228  of one opposed die block  226  are received in the pockets  230  containing ejectors  232  of the other opposed die  226 . While the checkerboard pattern shown in FIG. 4 comprises a 4×8 array of alternating raised cutters  228  and recessed pockets  230 , it will be appreciated that other patterns can be selected depending upon the size and shape of the cylindrical material to be processed. 
     Cutters  228  have inward facing surfaces  229  surrounded by cutting edges  231 . Surfaces  229  are profiled such that the midpoint of each top and bottom edge  231  is raised. The inward facing surfaces of ejectors  232  are angled to facilitate material ejection. 
     The distance that cutters  228  protrude inwardly from die block  226  reduces uniformly across the face of the die block. In particular, cutters  228  that are located closer to the side  260  of die block  226  that is farther away from the reactor face protrude inwardly a greater distance than cutters  228  located closer to the side  262  of die block  226  that at the edge closer to the reactor face. The result is that opposed cutters  228  and pockets  230  farther from the reactor face are in closer spaced relation than opposed cutters  228  and pockets  230  nearer to the reactor face. Accordingly, crushing and shearing will take place sequentially starting at the end of cylindrical tube  10  and proceeding toward the reactor face. 
     Referring again to FIG. 3, the ejectors  232  comprise rods  240  which extend through horizontal bores  242  in die blocks  226 . Pairs of ejector rods  240  that are vertically aligned are connected at their ends to an inverted ‘L-shaped’ ejector crossbar  238  which is carried in an elongated slot in die block  226 . Lower crossbar stops  244  are horizontally disposed below die blocks  226  on the upper surface of the base  202  and upper crossbar stops  246  are horizontally disposed on the inward facing surfaces end plates  204 . Ejector crossbars  238  are sized such that their ends engage crossbar stop members  244  and  246 . As die blocks  226  are draw away from one another when hydraulic cylinders  214  are retracted, ejector rods  240  and ejector crossbars  238  are carried toward end plates  204  until ejector crossbars  238  engage lower and upper crossbar stop members  244  and  246 . Continued retraction of hydraulic cylinders  214  will draw die blocks  226  between crossbar stop members  244  and  246 , while ejector crossbars  238  remain stopped against crossbar stop members  244  and  246 . This relative motion of die block  226  and ejector crossbars  238  causes ejectors  232  to be forced outward in pockets  230  in die blocks  226  to clear any undislodged cut material. Manual operation of ejectors  232  is possible to help remove jammed material from die blocks  226 . 
     Press assembly  200  further includes lower doors  250  mounted immediately below die blocks  226  and above flask assembly  400 . Doors  250  are actuated by hydraulic cylinders  252  and linkage  254 , and serve two main purposes. First, they keep all crushed/sheared material within the press area during the full crush/shear operation to ensure complete volume reduction. Secondly, they provide a means to close off the opening below press assembly  200  during flask changeout. This reduces the potential spread of contamination, eliminates the possibility of a small pressure tube piece falling from the system during flask changes, and allows a flask change to be performed with the pressure tube inside the volume reduction unit in a recovery-mode situation. For purposes of illustration, one door  250  is shown in the open position and the other in the closed position. It will be appreciated that in practice, both doors  250  are moved to the closed position during the shearing operation and both doors  250  are moved to the open position to permit discharge of sheared material. 
     Referring now to FIG. 5, the retraction unit assembly  100  is shown in greater detail. Retraction unit assembly  100  is used to initially draw the pressure tube into the volume reduction system for processing. Retraction unit  100  is mounted immediately outboard of press assembly  200  and comprises retraction plug  110 , chain magazine  160 , drive motor  170  and hydraulic hose reel  180 . Chain magazine  160  comprises a Serapid™ chain  162  which is powered in either direction by drive motor  170 . Serapid™ chain  162  is a machine chain that can only flex in one direction, allowing it to be used under either tension or compression. Because Serapid™ chain  162  can support both tension and compression loads, it may be used to pull the pressure tube out of the calandria tube, or push the pressure tube back into the calandria tube under certain back-out scenarios. 
     Retraction plug  110  is mounted at the end of serapid chain  162 . When Serapid™ chain  162  is extended by drive motor  170 , retraction plug  110  is advanced through gap  234  between opposed die blocks  226  of press assembly  200 , through feeder assembly  300  and into the end of the pressure tube to be removed. 
     As shown in FIG. 6, retraction plug  110  comprises nose  112  sized to be closely received inside the end of the pressure tube to be removed. Fingers  114  are fixedly mounted in radially disposed hydraulic pistons  116 . Radially disposed hydraulic cylinder bores  118  machined in nose  112  house the hydraulic pistons  116 . Hydraulic lines  126  extend from hose reel  180  to hydraulic cylinder bores  118 . When retraction plug  110  is engaged in the end of the pressure tube to be removed, hydraulic pressure is applied to hydraulic cylinder bores  118 , hydraulic pistons  116  and fingers  114  are extended radially outward into gripping engagement with the inside walls of the pressure tube to be removed. Ends  124  of fingers  114  can be bevelled, pointed or have other surface treatment to improve engagement with the pressure tube. Nose  112  is secured axially to retraction plug base  130  by means of thrust bearing  132  and nut  134 . Nose  112  is able to rotate about its axis with respect to retraction plug base  130  to allow rotation of pressure tube during retraction from reactor. Oil passage grooves  136  and O-ring seals  138  provide for hydraulic connection across rotary interface  140  between nose  112  and retraction plug base  130 . Adaptor block  150  connects serapid chain  162  to retraction plug base  130 . 
     Referring now to FIGS. 7 and 8, feeder assembly  300  is shown in greater detail. Feeder assembly  300  comprises carriage  302  which is slidably mounted for reversible longitudinal movement on linear rails  304 . Carriage  302  is driven over rails  304  by ball screw  306  turned by electric motor  308 . Grippers  310  are horizontally disposed above and below the longitudinal axis of the pressure tube being processed. Grippers  310  are driven by a double rack  312  with common single pinion  314 , actuated by hydraulical cylinder  318 . Pinion  314  is mounted on ball spline  316  to allow it to travel with carriage  302  under load. 
     Feeder assembly  300  functions to feed the pressure tube from the reactor face into press assembly  200  after each crush/shear cycle. This feeding motion is achieved by driving carriage  302  to its inboard position (i.e. toward the reactor face) by means of motor  308  and closing grippers  310  onto the outside of the pressure tube. Carriage  302  is then driven in the opposite direction which positions the end of the pressure tube into press assembly  200 . Grippers  310  operate vertically, which allows them to travel into the press assembly and maintain hold of the pressure tube until cutters  228  make contact. Manual actuation of the pressure tube feeder grippers  310  and carriage  302  via ball screw  306  and hydraulic cylinder bracket  322  is possible to aid in recovery. 
     Wiper  320  is mounted on lower gripper  310  and is used to sweep into the flask any debris that may collect in the feed assembly area. 
     Referring now to FIG. 9, flask assembly  400  is shown in greater detail. Flask assembly  400  comprises cylindrical flask  402  and cylindrical liner  404 . Top loading door  406  is mounted in top wall  408  of flask  402  for horizontal sliding movement. Top loading door  406  can be opened by sliding it by means of suitable control (e.g. hydraulic) to expose a longitudinal rectangular opening  410  directly below the area of die blocks  226 . Coupons that are cut by the action of dies  226  fall through opening  410  into liner  404 . 
     Liner  404  is a single-use (disposable) container formed of stainless steel. Once liner  404  is filled with coupons, liner  404  is removed from flask  402  for permanent disposal. Liner  404  is mounted within flask  402  and is retained by disposal door  412 . Disposal door  412  is mounted in bottom of flask  402  for horizontal sliding movement. Disposal door  412  can be opened by sliding it by means of suitable control (e.g. hydraulic) to expose circular opening  416  adjacent the underside of liner  404 . 
     Flask  402  is first removed from the volume reduction system of the present invention to the disposal area  430 . Liner disposal tool  420  comprises lifting rods  422  which are removably attached at their lower ends to the top wall  424  of liner  404 , by suitable means, for example by threading engagement. Lifting rods  422  pass through openings in top wall  408  and are connected at their uppers ends by cross-bar  426 . Liner  404  is disposed of by attaching lifting rods  422 , raising liner  404  by hoisting cross-bar  426  at lifting eye  428 , sliding open disposal door  412 , and lowering liner  404  into the disposal area. A fresh liner  404  can then be raised into flask  402  and the flask reassembled into the volume reduction system of the present invention. Flask size may be varied to meet shielding requirements and the available lifting capacity of the work platform. 
     The operation of the present invention is controlled by a PLC-based controller, programmed to run automated routines, with interlocks to prevent out-of-sequence events. The main control station is located in the reactor vault but away from the highest radiation fields. Remote manual control of all functions will be possible. A satellite control panel may be located near the volume reduction unit. 
     The operation of the pressure tube volume reduction system of the present invention will now be described. Prior to the start of the pressure tube volume reduction process, all end fittings and feeders are removed. Work may be performed on each reactor face in parallel. The volume reduction systems are installed on the work platforms and control stations are set up in the reactor vault. Lattice tube and bellows protective sleeves are installed on all channels with temporary lattice tube shield plugs (or equivalent). 
     An empty flask assembly  400  is loaded on the volume reduction unit. Once the channel location is determined, the volume reduction system is aligned with the channel and the lattice tube shield plug is removed. The volume reduction system is finally aligned with and locked on the channel using any standard mechanical latch to the lattice tube protective sleeve. Serapid™ chain  162  is driven forward by hydraulic motor  170  to advance retraction plug  110  and insert it into the end of the pressure tube. Fingers  114  are engaged to the end of the pressure tube and serapid chain  162  is driven in the reverse direction to withdraw the end of the pressure tube through gap  234  between die blocks  226 . Carriage  302  is driven to the forward inboard limit and pressure tube feeder grippers  310  are closed to engage the pressure tube. Lower press doors  250  are closed. Fingers  114  are retracted to disengage from the pressure tube and Serapid™ chain  162  is further driven in reverse direction to move the retraction plug  110  outboard of gap  324 . Die blocks  226  are driven together to the fully closed position. This causes a 14 {fraction (7/16)} inch length of pressure tube in gap  234  to first be crushed flat between inward facing surfaces  229  and then sheared by cutters  228  into 2 {fraction (1/16)} inch square coupons while the pressure tube remains centred on the reactor lattice site during the operation. The lower press doors  250  are opened and dies  226  are fully opened to eject all pressure tube coupons into flask  402  through opening  410 . 
     Carriage  302  is driven away from the reactor face until the end of the pressure tube is again positioned between die blocks  226 . Lower press doors  250  are closed and die blocks  226  are driven together to the 50% closed position. Pressure tube feeder grippers  310  are then opened and die blocks  226  are fully closed while carriage  302  is driven to the full inboard position. Pressure tube feeder grippers  310  are closed to engage the pressure tube and lower press doors  250  are opened. Die blocks  226  are fully opened to eject all pressure tube coupons into flask  402 . 
     The press/gripper cycling is repeated until the complete pressure tube is processed into coupons. Thereafter, the system is disengaged from the channel, the lattice tube shield plug is replaced, the number of processed pressure tubes in the flask is confirmed and the volume reduction system is aligned with and locked on the next channel to be processed. 
     Once flask  402  is full, it is disengaged from the volume reduction system and is lifted off the work platform. An empty flask is then installed under the volume reduction system and the process is continued until all pressure tubes have been processed. 
     The interior space of the volume reduction system, including the press assembly and feeder assembly interior volume, is maintained under a slight negative pressure to prevent the spread of contamination by any oxide dust or other small particles generated. A shielded filter is used to collect the active dust. 
     The stepped process carried out by the method of the present invention allows the process to be stopped indefinitely at any time and restarted without problem. This is useful for dealing with repairs or malfunctions and also such incidences as power outages, shift changes or non-volume reduction related interruptions. 
     Retraction unit  100 , press assembly  200  and feeder assembly  300  are all of a modular design to allow quick field replacement of an individual sub-system rather than the complete unit, thereby minimizing contamination spread and lost time due to repair. Because all hydraulic cylinders, electric motors and other actuators are outside shielding, they may be repaired or replaced if required with radioactive pressure tube present in the volume reduction system. This allows repair or maintenance activities to be performed in the event a malfunction occurs while radioactive material is still present in the system. This greatly simplifies recovery scenarios as removal of the radioactive pressure tube by a back-out means is not required in order to effect repair or maintenance operations. Moreover, the compact modular design of the volume reduction system of the present invention permits the components thereof to be easily fitted with shielding. Top shielding elements can be seen in FIG. 1 over the press and feeder assemblies, but have been omitted from the balance if the figures for purposes of illustration. 
     As noted above, the volume reduction process of the present invention combines the crushing of the pressure tube flat and shearing on a checkerboard grid of multiple intersecting planes. Crushing generally causes the pressure tube to break cleanly along its sides perpendicular to the crushing force, separating the pressure tube into two complete halves that remain intact. A smooth transition from the crushed to uncrushed sections is produced immediately inboard of die blocks  226 . Materials subjected to high levels of irradiation undergo substantial material property changes. For Zr-2.5%Nb pressure tubes, this includes an increase in the ultimate tensile strength and a reduction in total elongation (ductility). 
     The surface profiling of cutters  228  produces a cutting edge that effects a progressive shear action which reduces the maximum shear force required. Moreover, the variation in the extent of the protrusion of the cutters  228  inwardly from die blocks  226  effects a sequential shearing action with closer spaced cutters near the end of the tube acting before more widely spaced cutters farther from the end of the tube. This also reduces maximum shear force. It will be understood that surface profiles other than that shown in FIG.  3  and variations in opposed cutter spacing can be used to reduce maximum shear force. In addition, cutters  228  need not be square but can be configured to produce coupons of other shapes. 
     The one-step crushing and shearing process of the present invention to reduce the pressure tubes into small flat coupons has a number of advantages. It has minimal material handling requirements so that no additional equipment is required to flask the irradiated waste and no subsequent material handling is required. The coupons may simply drop into the flask in random order, avoiding the complexity and potential risks of systems which require mechanisms to align or stack material. This simplifies the design, eliminates possible failure modes and maintains a compact overall size. Secondly, the form of the waste material does not constrain the subsequent shielded flask size or shape, so that an optimal flask size and shape can be chosen to receive the required amount of material to optimize material handling, weight, transportation, disposal and storage considerations. For example, the reduction in gross volume of the waste material allows the use of a smaller and lighter shielded flask for handling to the disposal site, thereby speeding up the overall removal/disposal task, making it more economically viable. The volume reduction system of the present invention is compact and light enough to be used in situ at various field locations while still being completely shielded to enable personnel to be present at or near the equipment. 
     While the volume reduction system of the present invention has been described with respect to use with pressure tubes, it can also be used to remove and process calandria tubes or other hollow cylindrical components of varied cross-sectional shapes. Removal of the calandria tubes may be done separately from pressure tubes, or simultaneously. Thus, the stroke of feeder grippers  310  and the stroke of dies  226  can be sized to accommodate the larger diameter calandria tubes. Further scaling is possible to adapt the system for processing other components. The checkerboard shear technique of the present invention is capable of processing components with thick cross-sections (up to 10 mm has been successfully tested) and can be readily scaled up or down to match the requirements of the application. The volume reduction system of the present invention can have other commercial applications such as the refurbishment and/or decommissioning of radioactive sites as well as volume reduction applications for non-irradiated components in nonnuclear industries such as waste management industries.