Abstract:
A method for working on a defective heating element of a pressurizer comprising a tank lying flat along a longitudinal axis and elongate heating elements extending within the tank, and at least one spacer plate through which the heating elements pass and which is capable of maintaining a transverse spacing between the heating elements is provided. The method includes cutting at least one spacer plate around the heating element so as to detach said heating element from said spacer plate, and then extracting the heating element from the tank in one single piece.

Description:
The invention relates to a method for working on a defective heating element of a pressuriser comprising a tank elongated along a longitudinal axis and elongate heating elements extending within the tank, and at least one spacer plate through which the heating elements pass and which is capable of maintaining a transverse spacing between the heating elements. 
     BACKGROUND 
     Pressurised water nuclear reactors have a primary circuit for circulation of the cooling water of the reactor and a pressuriser disposed on a branch of the primary circuit in order to maintain the water pressure in the primary circuit between certain predetermined limits. The pressuriser also carries out the function of compensating for variations in the volume of water in the primary circuit during the operation of the reactor. 
     The pressuriser comprises a water tank, elongate heating elements or “heating rods”, extending in a lower portion of the tank, and a spray nozzle for spraying cold water in the upper portion of the tank. The pressuriser contains water in its lower portion connected to the primary circuit and a vapour bubble in its upper portion. Controlling of the water pressure is done by evaporating the water by means of heating rods and then condensing the vapour by spraying so as to control the volume and pressure of the vapour bubble. 
     The tank is elongated along a longitudinal axis, oriented vertically. The heating rods pass through a bottom wall of the tank and extend into the interior of the tank parallel to the longitudinal axis. 
     The pressuriser includes at least one spacer plate, generally two spacer plates in order to hold the heating rods in place transversely. Each spacer plate is fixed in the interior of the tank. 
     While in operation, a heating rod could become faulty, such that it becomes necessary to proceed with its extraction and possibly its replacement. 
     In order to extract a heating rod, it is normally required that a downwards traction (pull) be exerted on the heating rod to extract it through the bottom wall. 
     However, there are cases in which the defective heating rod has a deformed section making it impossible to perform its extraction through the bottom wall of the tank by exerting downwards traction. 
     Document FR 2666679 discloses a method for the extraction of a defective heating rod having a bulge, wherein the heating rod is cut and then the sections of the heating rod are extracted through an inspection—service opening provided in a top wall of the tank. 
     However, this method has several drawbacks which make the implementation thereof long, difficult and risky. 
     On account of the geometry of the spacer plates, accessibility of robotic devices and their handling with no direct visibility is very difficult, which is similarly true for the extraction of the heating rod sections. The existing risks include the generation of debris that would be difficult to recover, as well as loss of the robot and damage to adjacent rods. Furthermore, this process does not provide for the restoration of the surface condition of the openings for passage of the heating rods in the spacer plate or plates, which prevents the subsequent replacement of the damaged heating rods by new rods. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method of intervention enabling the extraction of defective heating rods with significant damage, regardless of the location of the rod in the pressuriser, without risk to the adjacent rods, and with the possibility to replace the extracted rod with a new rod. 
     To this end the invention provides a method wherein it involves cutting at least one spacer plate around the heating element in a manner so as to detach this heating element from said spacer plate, followed by the extraction of the heating element from the tank. 
     According to other modes of implementation, the method includes one or more of the following characteristic features, considered individually or in accordance with any technically possible combinations:
         the heating element is extracted in one single piece;   the heating element is extracted by pulling on an upper end portion of the heating element and/or by pushing on a lower end portion of the heating element, preferably with a lower end portion extending through a wall of the tank and out to the exterior of the tank ;   the or each spacer plate is cut by means of an electrical discharge cutting tool comprising of an electrical discharge head and an electrical discharge electrode ;   the electrode is slipped around the heating element and then the electrode is lowered in a manner so as to cut the or each spacer plate around the heating element;   the cutting tool is fastened on to a spacer plate by positioning the cutting tool while using as reference points the heating elements adjacent to the heating element to be extracted and/or the water circulation openings for the circulation of water through the or each spacer plate ;   the electrode is tubular;   the heating element is detached from a through passage sleeve of the heating element through a wall of the tank on the exterior of the tank;   a replacement heating element is positioned in place at the location of the extracted heating element;   the heating element is fixed to said spacer plate by means of a section of plate that is attached and fastened to the spacer plate and includes a through opening for passage of the replacement heating element;   the section of plate is positioned by using as reference points the heating elements adjacent to the replacement heating element, and/or the openings for circulation of water through the or each spacer plate;   the extracted heating element is disposed in a handling container inside the tank prior to extracting the heating element from the tank.       

     The invention also relates to a tool apparatus for extracting a heating element of a pressuriser held in place in a tank of the pressuriser by at least one spacer plate, comprising of an electrical discharge cutting tool including a support frame and a tubular electrical discharge electrode carried by an electrode holder mounted in a slidable manner on the support frame. 
     According to other embodiments, the tool apparatus includes one or more of the following characteristic features, considered individually or in accordance with any technically possible combinations:
         a gripping device capable of gripping a heating element extending into the interior of the electrode holder;   a base for positioning and fastening the cutting tool on to a spacer plate, making it possible to position the cutting tool while using as reference points the heating elements adjacent to the damaged heating element and/or the openings for the circulation of water through the spacer plate;   a fastening member for fastening of the cutting tool on to a spacer plate, the fastening member comprising of a body adapted to be inserted into the aligned openings of two parts, a support flange and a retractable clamping finger movable between a retracted position in the interior of the body and an extended position projecting out to the exterior of the body under the effect of a clamping element in a manner so as to clamp together the parts through which the body passes between the flange and the clamping finger.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and its advantages will be better understood upon reading the description which follows, given solely by way of example and with reference being made to the accompanying drawings, in which: 
         FIG. 1  is a cross sectional view through a vertical plane of a pressuriser of a pressurised water nuclear reactor; 
         FIGS. 2 and 3  are views similar to that in  FIG. 1 , illustrating the bottom part of the pressuriser during the steps for extraction of a defective heating rod according to the method of the invention; 
         FIG. 4  is a partial schematic cross sectional view of a base for positioning of a cutting tool apparatus attached to a top spacer plate of the pressuriser; 
         FIG. 5  is a partial cross sectional view of the pressuriser illustrating a step for replacing of the defective heating rod by a replacement heating rod. 
     
    
    
     DETAILED DESCRIPTION 
     The pressuriser  2  illustrated in  FIG. 1  comprises of a tank  4 , elongate heating elements or heating rods  6  extending into the interior of the tank  4 , a lower spacer plate  8  and an upper spacer plate  10  for holding the heating rods  6  inside the tank  4  and a spray nozzle  11 . 
     The tank  4  is elongated along a longitudinal axis L, oriented vertically. The tank comprises of a tubular cylindrical shell  12  extending along the longitudinal axis L, closed off at its axial ends by a top convex wall  14  and a bottom convex wall  16 . 
     The top wall  14  is penetrated through by an inspection—service opening  20 , allowing access to the interior of the tank  4 . The spray nozzle  11  is secured to the top wall  14  in a manner so as to spray water downwards. 
     The bottom wall  16  is penetrated through by an opening for water circulation  22  allowing for connecting the tank  4  by fluid means to the primary circuit of the nuclear reactor. 
     The heating rods  6  pass through the bottom wall  16  and extend into a lower portion of the tank  4 . The heating rods  6  are elongated and extend parallel to each other and to the longitudinal axis L. The heating rods  6  are distributed around the longitudinal axis L along several concentric circular rows, in three rows herein. 
     The lower spacer plate  8  and the upper spacer plate  10  hold in place the heating rods  6  transversely across the interior of the tank  4 . Each heating rod  6  passes through the lower spacer plate  8  and the upper spacer plate  10 . The lower spacer plate  8  and the upper spacer plate  10  are each attached at their periphery to the tank  4 . 
     The lower spacer plate  8  and the upper spacer plate  10  are spaced apart along the longitudinal axis. The lower spacer plate  8  is in the shape of a disc. The upper spacer plate  10  is annular and includes a central opening with a large diametre. 
     The lower spacer plate  8  and the upper spacer plate  10  each include openings for heating rods  24  through which the heating rods  6  extend and openings for water circulation  26  for the circulation of water through the lower spacer plate  8  and the upper spacer plate  10 . 
     The tank includes four tubular sleeves  28  penetrating through the bottom wall  16 . Each sleeve  28  is welded to the bottom wall  16  in a sealed tight manner. Each heating rod  6  extends through a respective sleeve  28 . Each heating rod  6  is soldered to the associated sleeve in a sealed tight manner. 
     The heating rods  6  are for example electric heating rods. Each heating rod  6  comprises a sheath and heat dissipating elements that are electrically conductive extending within the heating sheath. Each heating rod  6  comprises at its end located on the exterior of the tank  4  an electrical connector  30  for connection to an electric power supply. 
     A heating rod  6  may become defective such that it becomes necessary to proceed with its extraction and possibly its replacement. 
       FIGS. 2 and 3  illustrate the steps for extraction of a defective heating rod  6  based on a process according to the invention. 
     According to the method of the invention, at least one spacer plate is cut around the defective heating rod  6  in a manner so as to detach the defective heating rod  6  from the or each spacer plate that is cut ( FIGS. 2 and 3 ). 
     In addition, the defective heating rod  6  is detached from the bottom wall  16 . 
     In order to do this, the lower end of the defective heating rod  6  exiting from the sleeve  28  out to the exterior of the tank  4 , is detached in a manner so as to allow the defective heating rod  6  to slide in the sleeve  28  towards the interior of tank  4 . 
     Then, the defective heating rod  6  is extracted by pulling exerting upwards traction on the upper end portion of the heating rod and, if this is necessary, a push on the lower end portion of the defective heating rod  6  projecting outside of the tank  4  (Arrow F in  FIG. 3 ), in a manner so as to raise the defective heating rod  6  towards the inside of the tank. 
     Then, the defective heating rod  6  is removed from the tank  4  through the inspection -service opening  20 , the defective heating rod  6  preferably having been previously disposed in a retaining container, given that the defective heating rod  6  is contaminated and may likely no longer be intact. 
     According to the invention, at least the spacer plates located above the deformed section of the defective heating rod are cut, due to the fact that the deformed section prevents the sliding of the defective heating rod  6  through the spacer plates located there above. 
     Depending on the specific circumstances, it is possible to not cut all of the spacer plates. If the deformed section is located below the lowermost spacer plate, all of the spacer plates are cut. If the deformed section is located between two spacer plates and the defective heating rod  6  is able to slide through the or each spacer plate located below the deformed section, one need cut only the spacer plates situated above the deformed section, or all of the spacer plates. 
     The method of the invention can be implemented on any defective heating rod, including a heating rod having no apparent deformation, but the extraction of which may prove to be difficult or impossible by exerting downward traction, due to the friction being too high between the heating rod and the spacer plates and/or the sleeve  28 . 
     A tool apparatus  40  for extraction of heating rods is illustrated in  FIGS. 2 to 4 . 
     As illustrated in  FIGS. 2 and 3 , the tool apparatus  40  comprises of a cutting tool  42  and a base  43  for fastening the cutting tool  42  on the upper spacer plate  10 . 
     The cutting tool  42  comprises of a support frame  44 , an electrical discharge head  45  mounted in a slidable manner on the support frame  44  along a sliding axis A, an electrical discharge electrode  46  carried by an electrode holder  47  fixed to the electrical discharge head  45 , and a gripping device  48  capable of gripping the top end of a heating rod  6  so as to pull it upwards with the cutting tool  42 . 
     The support frame  44  and the electrical discharge head  45  are shown partially shaded in  FIGS. 2 and 3 , while the base  43 , the electrode  46 , the electrode holder  47  and the gripping device  48  are shown in cross section for the purposes of illustration. 
     The support frame  44  includes a bottom nozzle  50  and a top nozzle  52  spaced out along the sliding axis A and connected by upright posts (not shown). The lower nozzle  50  is annular. 
     The support frame  44  comprises of sliding rails  54  extending between the top nozzle and the bottom nozzle  52  along the sliding axis. The electrical discharge head  45  is mounted in a slidable manner on the sliding rails  54 . 
     The electrode  46  is tubular and circular and centred on the sliding axis A. It has a circular cutting edge. The electrode  46  is electrically powered by the electrical discharge head  45 . 
     The electrode holder  47  is tubular extending along the sliding axis A. The electrode is fixed to the lower end of the electrode holder  47 . The other upper end of the electrode holder  47  is connected to the electrical discharge head. 
     During movement of the electrical discharge head  45 , the electrode  46  and the electrode holder  47  slide through the bottom nozzle  50 . 
     The gripping device  48  is disposed on the electrode holder  47  at a distance from the electrode  46  and in the proximity of the electrical discharge head  45 . It includes a jaw  60  comprising of the clamping elements  62  that are movable radially under the effect of actuators  64  towards the centre of the support tube  54  so as to clamp the end of a heating rod  6  extending into the interior of the electrode holder  47 . The jaw  60  herein comprises two clamping elements  62  that are diametrically opposed. 
     The base  43  is adapted to be positioned and fastened on the upper spacer plate  10  it being positioned relative to the defective heating rod  6  and bearing upon the upper spacer plate  10  between adjacent heating rods  6 . 
     The base  43  is adapted to accommodate the cutting tool  42  supported on the base  43 . The base  43  has a central passage allowing for the movement of the electrode holder  47  through the base  43 . 
     The base  43  comprises a lower support plate  66 , an upper annular seat  68 , and the upright posts  70  extending between the lower support plate  66  and the upper seat  68 . 
     The upper seat  68  is provided to serve as support for the bottom nozzle  50  of the cutting tool  42 . The upper seat  68  and the bottom nozzle  50  are provided to abut against each other via corresponding conical bearing surfaces enabling the centring of the cutting tool  42 . The bottom nozzle  50  includes means for fastening on the upper seat  68 , for example clamping screws. 
     As illustrated in  FIG. 4  which shows the lower part of the base  43  in cross section in a sectional plane different from that in  FIGS. 2 and 3 , the support plate  66  comprises a central opening  72  for the passage of upper end portion  6  of the defective heating rod. 
     The heating rods being close to each other, the support plate  66  also has openings for the passage of the rods adjacent to the rod that has been removed. 
     The upright posts  70  are of a height greater than that of the end portions of the heating rods  6  protruding out above the upper spacer plate  10 . The upper seat  68  thus is located above the upper ends of the heating rods  6 . 
     The support plate  66  comprises fastening members  80  provided in order for each to be engaged in a respective opening for water circulation  26  of the upper spacer plate  10  so as to position the support plate  66  relative to the defective heating rod  6  and fasten it on to the upper spacer plate  10 . 
     Each fastening member  80  comprises a tubular body  82  having a support flange  83  projecting radially outwards, a pivoting gripping finger  84  mounted to be rotatable inside the tubular body  82  and movable through a lateral window of the tubular body  82  between a retracted position in the interior of the tubular body  82  (on the left in  FIG. 4 ) and an extended position projecting out to the exterior of the tubular body  82  (on the right in  FIG. 4 ). 
     Each fastening member  80  is disposed in an opening of the support plate  66  with its support flange  83  bearing against an upper face of the support plate  66  at the periphery of the opening. The support flange  83  is fastened on the support plate  66 , for example by means of screws. 
     Each fastening member  80  includes an actuating mechanism  86  for actuating the clamping finger  84 . The actuating mechanism  86  comprises a control element  88  mounted in a slidable manner within the tubular body  82  and a connecting rod  90  connecting the control element  88  and the clamping finger  84  in a manner such that the axial movement of the control element  88  causes rotation of the gripping finger  84 . The actuating mechanism  86  includes a clamping screw  92  screwed into a threaded section of the tubular body  82  and bearing against the control element  88 , in a manner such that screwing of the clamping screw  92  moves the control element  88  so as to move the gripping finger  84  to the extended position. 
     The screwing of the clamping screw  92  makes it possible to apply the clamping finger  84  against the lower surface of the upper spacer plate  10 , such that the support plate  66  and the upper spacer plate  10  are clamped against each other between the support flange  83  and the gripping finger  84 . 
     In operation, the base  43  is placed on the upper spacer plate  10  in a manner such that each fastening member  80  is inserted into an opening for water circulation  26  of the upper spacer plate  10 . Each fastening member  80  is actuated so as to fasten the support plate  66  against the upper spacer plate  10 . 
     The cutting tool  42  is then positioned and fastened on the base  43  ( FIG. 2 ). Thereafter, the electrode holder  47  is moved downwards so as to cut the upper spacer plate  10  and then the lower spacer plate ( FIG. 3 ). 
     The circular tubular electrode  46  enables the cut to be executed in one single pass. By way of a variant, the electrode  46  has a cutting edge extending along one circular arc or two diametrically opposite circular arcs. In this case, the cutting of a spacer plate is executed in several passes, by causing the electrode to pivot between passes around the sliding axis A. In a general manner, the or each spacer plate  8 ,  10  is cut along a line, in a continuous manner in a single pass or in a discontinuous manner in several passes. 
     Once the heating rod  6  is free of each spacer plate  8 ,  10 , the gripping device  48  is then actuated so as to grip the defective heating rod  6 , and the electrode holder  47  is raised so as to exert an upwards traction pulling on the defective heating rod  6 . 
     According to the method of the invention, if the sleeve  28  of the defective heating rod  6  that is extracted is reusable, a replacement heating rod is fixed in place at the location of the extracted defective heating rod  6 . 
     In order to do this, as shown in  FIG. 5 , a section of plate  94  is attached on to the spacer plate. The section of plate  94  includes an opening for the heating rod  96  for passage of the replacement heating rod  6 . 
     According to the method, the section of plate  94  is positioned by using as reference points the water circulation openings  26  of the spacer plate  8 ,  10  and/or the adjacent heating rods  6 . 
     The section of plate  94  includes openings for fastening  97  designed to be facing the water circulation opening  26  of the spacer plate  8 ,  10 , for insertion of the fastening member  80  that is identical to those described with reference to  FIG. 5 . 
     The plate section  94  includes positioning notches  98 , adapted to receive the heating rods  6  adjacent to the replacement heating rod  6 . 
     Thereafter, the replacement heating rod  6  is inserted through the sleeve  28 , and through the heating rod opening  98  of the section of plate  94 . 
     Thanks to the invention, it is possible to easily extract a defective heating rod without having to cut the heating rod inside the pressuriser in order to extract it from the spacer plates and remove it from the pressuriser. It makes possible the extraction of a heating rod having significant deformations, which make the cutting of the heating rod difficult. If the heating rod has been broken, the cutting of the heating rod for example, could generate a lot of debris that would be likely to spread in the primary circuit and damage other elements of the nuclear reactor, such as the nuclear fuel assemblies. It is possible to easily carry out the replacement of a defective heating rod with a replacement heating rod. The extraction tool apparatus according to the invention is simple and allows for a controlled implementation that does not pose any risks to the integrity of the pressuriser and the restarting of the installation.