Abstract:
A method for preparing a built-in part for disposal including the steps of mounting a supporting element such that it can be displaced in an axially elastic manner by a spring on a head of the built-in part and being used for support on the grid plate of the head frame of a fuel element. The supporting element is fixed in a pushed-back position against the action of the spring, enabling a common disposal of the built-in part with the fuel element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuing application, under 35 U.S.C. § 120, of copending international application No. PCT/EP2003/007154, filed Jul. 4, 2003, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. 102 32 596.0, filed Jul. 18, 2002; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION  
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a method for preparing a built-in part of a fuel element of a pressurized water nuclear reactor, in particular, a control element, for disposal. In addition, the invention relates to a built-in part conditioned for disposal by this method.  
         [0003]     The built-in parts of a fuel element of a pressurized water nuclear reactor include, for example, control elements, absorber elements, neutron sources, and throttling elements. These parts are subject to wear, just like other parts of a nuclear plant, which makes it necessary for these to be replaced from time to time. The replaced built-in parts have to be sent for disposal, just like used fuel elements.  
       SUMMARY OF THE INVENTION  
       [0004]     It is accordingly an object of the invention to provide a method for preparing a built-in part of a fuel element of a pressurized water nuclear reactor for disposal, and corresponding conditioned built-in part that overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that can be carried out without difficulty and in which the processing effort required is reduced to a minimum.  
         [0005]     With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for preparing a built-in part of a fuel element of a pressurized water nuclear reactor for disposal having a supporting element mounted axially displaceable with a spring within the built-in part to support the built-in part in a sprung manner on the grid plate of a top frame of the fuel element, including the step of fixing the supporting element in a pushed-back position counter to action of the spring.  
         [0006]     According to the method, a supporting element that serves to support the built-in part on the upper grid plate of a fuel element and is mounted in a sprung manner such that it can be displaced axially on the top of the built-in part is fixed in a pushed-back position counter to the action of the spring. Such a measure makes it possible to dispose of the built-in part together with a fuel element because the projection of the control element beyond the hold-down springs and the top frame of the fuel element, determined by the axial deflection of the supporting element, is reduced to such an extent by utilizing the available spring travel that the package including the built-in part and fuel element can be introduced for disposal into containers that are originally provided for the transport and storage of fuel elements and are matched to their dimensions. The present invention is based on the finding that the available spring travel of the supporting element is large enough to reduce the projection to an extent that ensures that the clearance intrinsically present in storage and transport containers that are already available and approved is sufficient to accommodate the package, even taking into account growth of the package including fuel element and built-in part induced by radiation and longitudinal expansion caused by a temperature increase in the container.  
         [0007]     In accordance with another mode of the invention, the built-in part is inserted into a fuel element so that it rests in a sprung manner on the upper grid plate, at least indirectly. The built-in part is, then, pressed against the grid plate counter to the action of the spring so that the supporting element is pushed back counter to the action of the spring. Such a compressed or pushed-back position is, then, fixed. This measure makes it possible to process the built-in part on site, for example, in the fuel element storage pond, with the handling devices that are available there and supplemented by auxiliary devices in accordance with the method, in particular, the fuel element-loading machine. In such a case, the requirement for additional auxiliary devices is reduced to a minimum because the fuel element, itself, serves as a holder for the built-in part during the conditioning.  
         [0008]     In principle, it is possible to fix the entire built-in part in the top frame of the fuel element in this compressed or pushed-back position of the supporting element. However, a fixing is, preferably, provided in which the compressed position is fixed in the built-in part itself so that the built-in part can be removed from the fuel element with the compressed, axially locked supporting element.  
         [0009]     For such a purpose, in accordance with a further mode of the invention, before the insertion of the built-in part, a lower holding ring for the supporting element is placed on the upper grid plate of the fuel element (upper top plate) and, in the compressed position, is connected through an axial form fit (axial force fit by a form fit) to an upper holding ring placed on the built-in part. As a result, during the conditioning, no complicated processing operations that lead to material removal from the fuel element or from the built-in part are necessary. Thus, during the conditioning, no nuclear waste arises that would, likewise, again have to be sent for disposal.  
         [0010]     In accordance with an added mode of the invention, the supporting element is braced between the upper and lower holding rings.  
         [0011]     In accordance with an additional mode of the invention, the upper and lower holding rings are screwed to one another by at least one axial threaded bolt.  
         [0012]     In accordance with yet another mode of the invention, the threaded bolt is pre-mounted in the upper holding ring before the upper holding ring is placed on the built-in part.  
         [0013]     In accordance with yet a further mode of the invention, the threaded bolt is secured against working loose when the threaded bolt is mounted.  
         [0014]     In accordance with yet an added mode of the invention, security against the threaded bolt working loose with a form fit is provided between a head of the threaded bolt and the upper holding plate.  
         [0015]     In accordance with yet an additional mode of the invention, the built-in part is pressed against the grid plate with a compression device anchored to the top frame of the fuel element.  
         [0016]     In accordance with again another mode of the invention, the built-in part is a control element of the nuclear reactor.  
         [0017]     With the objects of the invention in view, there is also provided a method for preparing a built-in part of a fuel element of a pressurized water nuclear reactor for disposal, including the steps of mounting an axially displacing supporting element with a spring on top of a built-in part to support a top frame in a sprung manner on the grid plate of a fuel element and fixing the supporting element in a pushed-back position counter to action of the spring.  
         [0018]     With the objects of the invention in view, in a built-in part of a fuel element of a pressurized water nuclear reactor conditioned for disposal, there is also provided a disposal device including a top frame, a grid plate, a spring, and a supporting element mounted by the spring to displace axially in a sprung manner on top of the built-in part and supporting the top frame on the grid plate, the supporting element having a fixing assembly fixing at least one of the top frame and the supporting element in a pushed-back position counter to action of the spring.  
         [0019]     In accordance with again a further feature of the invention, there are provided an upper holding ring placed on the built-in part and a lower holding ring for holding the supporting element, the lower holding ring, in a compressed position thereof counter to action of the spring, being connected by an axial form fit to an the upper holding ring.  
         [0020]     In accordance with again an added feature of the invention, there is provided at least one axial threaded bolt, the upper and lower holding rings being connected to one another by being screwed to the at least one axial threaded bolt. Preferably, the threaded bolt is secured against working loose.  
         [0021]     In accordance with again an additional feature of the invention, the threaded bolt has a head and is secured against working loose by a form fit connection between the head and the upper holding ring.  
         [0022]     In accordance with still another feature of the invention, the threaded bolt has an anti-rotation safeguard device preventing the threaded bolt from loosening from the upper and lower holding rings.  
         [0023]     In accordance with still a further feature of the invention, the threaded bolt has a head and the anti-rotation safeguard is a form fit connection between the head and the upper holding ring.  
         [0024]     In accordance with still an added feature of the invention, the built-in part of the fuel element is a control element of the fuel element of the pressurized water nuclear reactor.  
         [0025]     With the objects of the invention in view, there is also provided a package conditioned for disposal, including a fuel element of a pressurized water nuclear reactor and a built-in part connected to the fuel element, the built-in part including a top frame, a grid plate, a spring, and a supporting element mounted by the spring to displace axially in a sprung manner on top of the built-in part and supporting the top frame on the grid plate, the supporting element having a fixing assembly fixing at least one of the top frame and the supporting element in a pushed-back position counter to action of the spring.  
         [0026]     In accordance with a concomitant feature of the invention, the built-in part is a control element of the fuel element.  
         [0027]     Other features that are considered as characteristic for the invention are set forth in the appended claims.  
         [0028]     Although the invention is illustrated and described herein as embodied in a method for preparing a built-in part of a fuel element of a pressurized water nuclear reactor for disposal, and corresponding conditioned built-in part, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
         [0029]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a fragmentary partially cross-sectional and partially side elevational view of a control element inserted into the top frame of a fuel element before its conditioning;  
         [0031]      FIG. 2  is a fragmentary partially cross-sectional and partially side elevational view of a control element inserted into the top frame of the fuel element and conditioned in accordance with the invention;  
         [0032]      FIG. 3  is a plan view of the fuel element of  FIG. 2  provided with the conditioned control element;  
         [0033]      FIG. 4  is a cross-sectional view through an auxiliary device according to the invention for the conditioning of the control element;  
         [0034]      FIG. 5  is a fragmentary plan view of an upper holding ring according to the invention in the region of the passage hole of the threaded bolt; and  
         [0035]      FIG. 6  is a fragmentary, cross-sectional view through a threaded bolt according to the invention in the region of the head of the threaded bolt.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     Referring now to the figures of the drawings in detail and first, particularly to  FIG. 1  thereof, there is shown a built-in part. A control element  1  in the example, having control rods  2  is inserted into a fuel element  3 , of which only the top frame  4  is illustrated in the figure for reasons of clarity. The control element  1  is inserted to the maximum and rests on the grid plate  6  of the top frame  4  by a spring-mounted, sleeve-shaped supporting element  5 , having its weight reduced by the buoyancy in water.  
         [0037]     The sleeve-shaped supporting element  5  is mounted such that it can be displaced axially by a spring  7  on a stud  10  fixed to the top part  9  of the control element and, in the unloaded state, is secured by a stop face  11  that is disposed at the end of the stud  10  and that corresponds to a corresponding annularly peripheral shoulder face  12  on the inner circumference of the supporting element  5 .  
         [0038]     The distance C between the upper side of the grid plate  6  of the top frame  4  and the end face  13  of the stud  10  corresponds to the maximum spring or braking travel that is available for the control element  1  falling into the fuel element  3 . The dead weight of the control element  1 , the spring constant, and the prestress on the spring  7  are dimensioned such that the dead weight of the control element  1  does not lead to a gap between the stop face  11  and the shoulder face  12 .  
         [0039]     The top part  9  of the control element  1  forms a spider-like carrier having loadbearing arms  14 . The control rods  2  are fixed by screw connections to the loadbearing arms  14 .  
         [0040]     In the state illustrated in  FIG. 1 , the control element  1  projects by the projection A beyond the top frame  4  and by the projection A′ beyond the hold-down springs  15 . This projection A, A′ is, then, too great to dispose of or to store the fuel element  3  together with the control element  1  in a container provided and approved for the fuel element  3 . The present invention is based on the thought that the spring travel C is sufficient to shorten the projection A, A′ to such an extent that the fuel element  3  provided with the control element  1  can be introduced into such a container.  
         [0041]      FIG. 2  now shows a situation in which the control element  1  is pressed against the grid plate  6  of the top frame  4  with the force illustrated symbolically by arrows F so that the supporting element  5  is pushed back axially counter to the action of the spring  7 . In such a pushed-back or compressed position, the supporting element  5  is fixed so that this position is maintained even if no more force is exerted on the control element. For such a purpose, use is made of a lower holding ring  20  and an upper holding ring  22 . The lower holding ring  20  simultaneously forms a seat for the supporting element  5  by a shape matched appropriately to the supporting element  5 .  
         [0042]     For such a purpose, in the exemplary embodiment, the inner surface of the lower holding ring  20  is shaped conically in the region of the contact face and is matched to the conically shaped outer face  23  of the supporting element  5 . The lower holding ring  20 , thus, forms a stop for the annular supporting element  5  so that the supporting element  5  can no longer be displaced axially under the action of the spring  7  when the load is relieved if the lower holding ring  20  is fixed relative to the top part of the control element  1 . For such a purpose, the lower holding ring  20  is screwed to the upper holding ring  22  resting on the loadbearing arms  14  by threaded bolts that are not visible in the figure.  
         [0043]     Instead of a conical seating face, a shoulder, which engages under the supporting element  5 , can also be provided on the holding ring so that the end face of the latter rests on the shoulder.  
         [0044]     The projection A of the control element  1  beyond the top frame  4  is, then, shortened by the spring travel C in the unstressed state.  
         [0045]     In the plan view according to  FIG. 3 , the upper holding ring  22  resting on the loadbearing arms  14  can be seen. With the aid of two threaded bolts  24 , the holding ring  22  is screwed to the lower holding ring  20 , which is visible only at its edge. Additionally, the holding rings  20 ,  22  are each provided with a mounting hole  30 , which has an internal thread and a conical insertion chamfer  31 . A mounting rod, which makes it possible to handle the holding rings  20 ,  22  with the aid of a rod-like tool during their mounting, can be screwed into the mounting hole  30 .  
         [0046]     The auxiliary device, assembled from the lower holding ring  20 , the upper holding ring  22 , and the threaded bolt  24 , is illustrated in more detail in  FIG. 4 . The two holding rings  20 ,  22  are in each case provided with a pin  28  and  29 , respectively, which project into a corresponding recess in the lower grid plate  6  and respectively into a clearance between the loadbearing arms  14  and are used as an anti-rotation safeguard. The passage holes  25  for the threaded bolts  24 , disposed in the upper holding ring  22 , are stepped and each have an annually peripheral contact shoulder  26 , on which the head  27  of the threaded bolt  24  is seated when it is finally mounted. The part of the passage hole  25  having the smaller internal diameter is additionally provided with a thread so that the threaded bolt  24  can be screwed into the upper holding ring  22  to make mounting easier and, together with the upper holding ring  22 , can be placed jointly on the control element. As a result, the setting of the threaded bolts  24 , which is otherwise retrospectively required, is dispensed with. The lower holding ring  22  is also provided with a mounting hole  30 , which makes its handling easier with a mounting rod screwed into the hole.  
         [0047]     An advantageous configuration of the holder of the cylindrical head  27  of the threaded bolt  24  in the passage hole  25  in the upper holding ring  22  is illustrated in  FIG. 5 . Machined into the annular contact shoulder  26  of the passage hole  25  are a large number of radial recesses or grooves  33 , which correspond to corresponding moldings or lands  34  that are disposed on the underside of the head  27  of the threaded bolt  24  and can be seen in  FIG. 6 . Here, the threaded bolt  24  is screwed in as far as possible so that the grooves  33  are aligned with the lands  34  and, when the load is relieved, come into engagement with the latter as a result of the axial resilience due to the spring  7  (see  FIGS. 1 and 2 ), and, in this way, by a form fit between the head  27  of the threaded bolt  24  and the upper holding ring  22 , form an anti-rotation safeguard for the threaded bolt  24 .  
         [0048]     Instead of the screw connection between the upper and lower holding plate explained in the exemplary embodiment, other connections that permit axial fixing can also be provided. The fixing can also been carried out, for example, by a bayonet connection, which is simultaneously formed as an anti-rotation safeguard. In such a variant, the bayonet connection can be provided in the upper or in the lower holding ring.  
         [0049]     The conditioning of the control element  1  is carried out under water in accordance with the method sequence set forth in the following text.  
         [0050]     First, the lower holding plate  20  is inserted into the fuel element  3  located in the fuel element storage pond with the control element  1  removed, and is placed on the grid plate  6  of the top frame  4  so that the lower holding plate  20  engages with its pin  28  in a passage hole formed in the grid plate  6  and, as such, is secured against rotation. Then, with the aid of the fuel element-loading machine, the control element  1  is inserted into the fuel element  3 . Then, the upper holding plate  22 , together with the threaded bolts  24  pre-mounted in it, is placed on the top part  9  of the control element  1  with the rod-like tool. With the aid of a compression device, for example, a pneumatic device, which is supported on the upper frame part of the top frame  4 , the control element  1  is pressed downward until the end face of the stud  10  is resting on the grid plate  6 . In other words, the stud  10  and, therefore, the control element  1  are pressed directly against the grid plate  6 .  
         [0051]     In the compressed state, the pre-mounted threaded bolts are, then, screwed through the upper holding ring  22  and screwed with their threads into the threaded holes in the lower holding ring  20 . The grooves  33  machined into the contact shoulder  26  must, then, be aligned with the corresponding lands  34  on the head  27  of the threaded bolt  24 , with the minimum axial distance between holding ring  22  and head of the threaded bolt  24 . After the load has been relieved, the lower and upper holding ring  20 ,  22  are forced apart by the spring  7  by the clearance so that grooves  33  and lands  34  interengage and the threaded bolt  24  is secured against rotation. The stud  10 , then, also rises away from the grid plate  6  by this clearance. The threaded bolt  24  is reliably prevented from working loose by the anti-rotation safeguard.  
         [0052]     The invention has been explained above using the conditioning of a control element. In principle, however, it can be applied to all the built-in parts of a fuel element of which the top region is constructionally comparable with the control element, are supported in a sprung manner on the upper grid plate and, in the unloaded state, project beyond the hold-down springs.