Abstract:
A method of repairing a vessel bottom head penetration of a nuclear reactor is presented, said penetration comprising a tube fastened to the internal wall of the vessel bottom head by an internal weld bead and emerging outside the vessel bottom head, forming a groove. The method consists inter alia in preparing a mock-up representative of the vessel bottom head penetration to be repaired and in depositing this mock-up on site in an irradiation-free zone, in fastening two half-inserts to this mock-up in the groove, each half-insert having a profile corresponding to the half-profile of the groove, in forming, again on this mock-up, automatically and by remote control, a weld bead in the groove, in carrying out a dimensional check and, after validation of this weld bead, in carrying out the steps of fastening the two half-inserts and forming a weld bead automatically, directly in the groove of the vessel bottom head penetration to be repaired.

Description:
This application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 06 05401 filed in France on Jun. 16, 2006, the entire contents of which is hereby incorporated by reference. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of repairing a vessel bottom head penetration of a nuclear reactor and in particular a pressurized-water nuclear reactor. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A pressurized-water reactor comprises a vessel filled, when the reactor is in operation, with a cooling fluid consisting of pressurized water. The core of the reactor consisting of fuel assemblies is entirely immersed in the cooling water. 
     When operating such reactors, it is necessary to take measurements in the core while the reactor is operating. In particular, it is necessary to take neutron flux measurements at various points over the height of the core in order to know the neutron flux distribution and power distribution along the axial direction of the core, which generally corresponds to the vertical direction. 
     The fuel assemblies constituting the core include, in their central part, an instrumentation tube into which a sealed thimble may be introduced over the entire height of the core, in which thimble a neutron flux measurement probe is placed during operation of the reactor. 
     The thimbles associated with each of the fuel assemblies in which the flux measurements are carried out must be able to be extracted from these assemblies, for example when the core is being refuelled. These thimbles are therefore mounted so as to move and slide inside the guide tubes joining the lower portion of the vessel to a measurement or instrumentation room in which the opposite ends of the thimbles from the ends introduced into the core are accessible in order to move the probes, so as to collect measurement signals from said probes, and in order to move the thimbles inside the guide tubes. The thimbles are moved inside the guide tubes by simply pushing or pulling them, a sufficient clearance being provided so as to limit the forces to be exerted on the thimbles, despite the fact that the guide tubes are curved over most of their path. 
     To allow the various thimbles associated with the fuel rods to penetrate into the reactor core, a number of tubes called vessel bottom penetration tubes pass through the vessel bottom head. 
     The tube of each vessel bottom penetration has, on the one hand, an upper end emerging inside the vessel in an instrumentation column for the passage of a thimble and, on the other hand, a lower end emerging outside this vessel and connected to the corresponding thimble guide tube. Each tube, made of “Inconel” alloy is fastened to the internal wall of the vessel bottom head, made of ferritic steel coated with a layer of stainless steel, by a weld bead made of “Inconel” alloy, which is intended in particular to seal between the inside and the outside of this vessel. 
     Now, it may turn out that, after the reactor has been operated for a certain time, microcracks form in the internal weld bead of a tube of a vessel bottom head penetration, which thus causes weeping on the tube resulting in a whitish deposit on the external surface of said tube, coming from the boron contained in the coolant of the reactor as neutron flux moderator of this reactor. 
     This is because the boron is dissolved in the coolant and, when this coolant weeps along the vessel bottom head penetration tube, it evaporates owing to the pressure and temperature prevailing inside the vessel. The boron, in the form of a powdery deposit, is therefore present on the external surface of the tube and also both over the thickness of the vessel bottom head wall and along this tube to the outside of the vessel. 
     One repair method, which naturally comes to mind to those skilled in the art for remedying this problem, consists in depositing, in the space located at the intersection of the external wall of the vessel and the external wall of the tube of the vessel bottom head penetration to be repaired, which space is denoted hereafter by the term “groove”, a weld bead. 
     However, many problems arise. 
     Firstly, the zone in which the work has to be carried out, beneath the reactor vessel, is highly irradiated, thus precluding the presence of humans. Secondly, the groove has a changing profile following a warped curve, so that to deposit a weld bead comprising several layers in said groove requires the use of an automatic welding tool capable of carrying out the work in a highly irradiated zone without human presence. 
     In addition, when the welding is being carried out in the groove, the boron deposited on the external wall of the tube, between this wall and the vessel bottom head wall, may mix with the weld metal and therefore run the risk long term of causing cracks in the weld bead, precluding the desired sealing. 
     A known method consists in first plugging, via the inside of the vessel, the tube of the vessel bottom head penetration, then in cutting, substantially at mid-thickness of the vessel wall, the vessel bottom head penetration tube and, after having extracted this tube portion, in replacing it with a new tube portion. 
     Next, to hold the tube in place and to seal it, a weld bead is deposited in the groove left between the external wall of the vessel bottom head and the external wall of the new tube portion. 
     The object of the invention is to propose a method of repairing a vessel bottom head penetration that avoids the abovementioned drawbacks and, in particular, avoids the preliminary operation of plugging the tube, consequently reducing the time that operators have to be present in a highly irradiated zone. 
     The subject of the invention is therefore a method of repairing a vessel bottom head penetration of a nuclear reactor, said penetration comprising a tube fastened to the internal wall of the vessel bottom head by an internal weld bead and emerging outside the vessel bottom head, forming with the external wall of said vessel bottom head, a groove, characterized in that it consists of the following steps:
         after having observed, on the tube and on the outside of the vessel bottom head, a deposit of boron indicating a leak through the internal weld bead, a mock-up representative of the vessel bottom head penetration to be repaired and comprising a vessel bottom head portion and a tube forming, with the said portion, a groove, is prepared and this mock-up is placed on site in an irradiation-free zone;   two half-inserts are fastened to this mock-up in the groove, each half-insert having a profile corresponding to the half-profile of the groove;   a weld bead is formed, again on this mock-up, automatically in the groove and the good distribution of the welding passes is verified;   a dimensional check of the weld bead is carried out on the mock-up and after this weld bead has been validated;   the boron on the tube of the vessel bottom head penetration to be repaired is removed, preferably by brushing it off; and   the step of fastening the two half-inserts and then the step of forming a weld bead, automatically and by remote control, directly in the groove of the vessel bottom head penetration to be repaired are carried out.       

     According to other features of the invention:
         the weld bead is formed on the vessel bottom head penetration to be repaired, automatically, as several layers, each layer comprising at least one pass;   the various layers are formed by means of an automatic welding torch having four axes of movement, said torch being remotely controlled from a radiation-free zone;   a liquid penetrant test is carried out after the first layer and the last layer of the weld bead have been formed on the vessel bottom head penetration to be repaired, and preferably such a test is carried out on every third layer;   at least one intermediate liquid penetrant test is carried out between forming the first layer and the last layer of the weld bead on the vessel bottom head penetration to be repaired;   at least one welding pass is carried out automatically for each layer so as to fill the groove;   each half-insert has a triangular cross section; and   the angle of the triangle of the cross section of each half-insert placed in the bottom of the groove includes a flat.       

     The features and advantages of the invention will become apparent over the course of the description that follows, given by way of example and with reference to the appended drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic view, in section and in elevation, of the lower portion of a vessel of a pressurized-water nuclear reactor; 
         FIG. 2  is a schematic view, in axial section, of a vessel bottom head penetration; 
         FIG. 3  is a schematic view, in axial section, of a tube of a mock-up for a vessel bottom head penetration to be repaired, showing some of the various steps of the repair method according to the invention; and 
         FIG. 4  is a schematic view, on a larger scale, of the tube of the mock-up representative of the tube of the vessel bottom head penetration to be repaired, in which the two half-inserts have been shown. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows schematically a bottom head  1   a  of a pressurized-water nuclear reactor vessel  1  containing a reactor core  2  (not shown) consisting of fuel assemblies. The vessel bottom head  1   a  has the form of a spherical dome. Referring to  FIG. 2 , the internal wall of this vessel bottom head is covered with a stainless steel coating  8  and its external wall is covered with an Inconel coating  1   b.    
     While this type of nuclear reactor is operating, measurements are made in the core  2  in order in particular to determine the neutron flux distribution at various points over the height of this core. To do this, the fuel assemblies constituting the core  2  include, in their central part, an instrumentation tube  5  ( FIG. 2 ) into which may be introduced, over the entire height of the core, a sealed thimble  6  inside which a neutron flux measurement probe is moved during operation of the reactor. 
     The thimbles  6  must be able to be removed from the assemblies, for example when the core is being refuelled. 
     These thimbles are therefore mounted so as to be able to be moved and slide inside the guide tubes  7  (FIGS.  1  and  2 ) that join the lower portion of the tube  10  of the vessel bottom head penetration to a measurement or instrumentation room (not shown) in which the opposite ends of the thimbles from the ends introduced into the core are accessible in order to move the probes, to collect the measurement signals and to move the thimbles inside the guide tubes  7 . 
     As shown in  FIG. 1 , to allow the various thimbles associated with the fuel assemblies to penetrate inside the reactor core  2 , the vessel bottom head  1   a  is penetrated by vessel bottom head penetration tubes  10 . 
     As shown more particularly in  FIG. 2 , the tube  10  of each vessel bottom head penetration has an upper end  10   a  emerging in an instrumentation tube  5  and a lower end  10   b  emerging outside the vessel bottom head  1   a  and connected to the guide tube  7  for the corresponding thimble  6 . 
     Each tube  10  is fastened to the internal wall of the vessel bottom head  1   a  by a weld bead  11 . 
     As is apparent in  FIG. 1 , the vessel bottom head  1   a  therefore includes a plurality of tubes  10  that are approximately parallel to the longitudinal axis of the vessel and are distributed at various points over the spherical dome of the vessel bottom head. The tube  10  for each vessel bottom head penetration defines, with the external wall  1   b  of the vessel bottom head  1   a , a groove  15  having, owing to the spherical dome shape of this vessel bottom head  1   a , an intersection along a warped curve. This is because, and as may be seen in  FIG. 2 , the opening of the groove  15  around the tube  10  varies according to the position of the penetration on the vessel bottom head, on the one hand, and according to the radial orientation around the tube, on the other. 
     If, during a maintenance check, the presence of boron in the form of a deposit is detected on the external wall of the tube  10  of a vessel bottom head penetration, on the outside of the vessel bottom head  1   a , meaning that the weld bead  11  of this tube  10  is cracked, a repair must be carried out on said tube  10 . 
     The method according to the invention allows this type of repair to be performed by carrying out, beforehand, all the necessary operations on a mock-up representative of the vessel bottom head penetration to be repaired, said mock-up being located on site in an irradiation-free zone and then by reproducing the operations directly on the vessel bottom head penetration to be repaired, thereby making it possible for the work time and the presence of operators in a highly irradiated zone to be considerably reduced. 
     Thus, as shown in  FIG. 3 , a mock-up is constructed and this mock-up, which is representative of the vessel bottom head penetration to be repaired, comprises a portion of a tube  20  that passes through a vessel bottom head portion  100   a  that has the same characteristics and the same dimensions as a portion of the vessel bottom head  1   a . The tube  20  defines, with the external wall  100   b  of the vessel bottom head  100   a , a groove  25  identical to the groove  15  of the tube  10  of the vessel bottom head penetration to be repaired. The external wall  100   b  of the vessel bottom head  100   a  of this mock-up is also covered with an Inconel coating  100   c  identical to that coating  1   c  deposited on the external wall  1   b  of the vessel bottom head  1   a.    
     Once this mock-up has been produced, an insert  30  is prepared, this insert  30  consisting, as is apparent in  FIG. 4 , of two half-inserts,  30   a  and  30   b  respectively, each having a profile corresponding to the half-profile of the groove  25 . The two half-inserts  30   a  and  30   b  each have a triangular cross section and have, like the groove  25 , a profile that changes along a warped curve. 
     The two half-inserts  30   a  and  30   b  are positioned in the groove  25  and fastened to the external wall  100   b  of the vessel bottom head portion  100   a  of the mock-up and also to the external wall of the tube  20  of this mock-up. These two half-inserts  30   a  and  30   b  are for example fastened by manual spot welding or by any other method. 
     Next, a weld bead denoted by the reference  35  is formed in the groove  25  of the mock-up. 
     As shown in  FIG. 3 , the weld bead  35  is formed automatically by means of a welding torch designed especially to be easily placed around the penetration tube and remotely controlled, as several layers  35   a ,  35   b ,  35   c , . . .  35   n , each layer comprising several passes  36   a ,  36   b ,  36   c , . . .  36   n , the number of which is determined according to the width of the groove  25  in order to fill the gap separating the external wall  100   b  of the vessel bottom head  100   a  of the mock-up from the external wall of the tube  20 . 
     In  FIG. 3 , various passes of each of the layers have been shown by sections of elliptical shape in order to make the figure easier to understand, although it is obvious that the weld metal making up the passes of each of the layers constitutes a homogeneous whole. 
     In the example shown in  FIG. 3 , the first layer  35   a  comprises one pass  35   b , this first layer filling the space separating the two side walls of the groove  25 . The second layer  35   b  comprises, in the zone where the groove  25  is narrower, two passes  36   b , while in the zone where the groove  25  is wider, it comprises three passes  36   b  so as to fill the space separating the two side walls of the groove  25 . The third layer  25   c  comprises, in the zone where the groove  25  is narrowest, three passes  36   c , whereas in the zone where the zone  25  is widest, this third layer  35   c  comprises five passes  36   c . These examples are given by way of indication and, depending on the depth of the groove  25 , the weld bead  35  comprises n layers and each layer itself comprises from 1 to n welding passes. 
     Each pass of each of the layers is performed automatically by means of said welding torch, starting from the lowest point of the groove  25  and rising over a half-path of this groove  25  towards the highest point. The second half-path of this same pass is always carried out by restarting from the lowest point and rising towards the highest point of the groove  25  so as to carry out a continuous welding pass over the entire perimeter of this groove  25 . 
     The passes of each layer of the weld bead  35  are carried out using an automatic welding torch having four axes of movement in order to follow the trajectory of the intersection along a warped curve, preferably of the TIG type (not shown), which comprises a system of tongs fastened to the tube  20  of the mock-up supporting a base for positioning the automatic welding torch, this system of tongs and this positioning base remaining in place throughout the operation of forming the weld bead  35 . 
     After this weld bead  35  has been formed, a dimensional check of this weld bead is carried out and if this weld bead is validated, the various operations of the repair method carried out on the mock-up are validated and authorization is given to carry out the repair on the tube  10  of the vessel bottom head penetration to be repaired. 
     Before carrying out the actual repair, the boron deposited on the external wall of the tube  10  of the vessel bottom head penetration to be repaired is removed, preferably by brushing it off or by any other appropriate means. 
     Next, the operations consisting in fastening the two half-inserts  30   a  and  30   b  of the insert  30  in the groove  15  and in depositing, in this groove  15  on top of the insert  30 , a weld bead  35  identical to the weld bead  35  formed on the mock-up and by means of the same tooling, are reproduced directly on the tube  10  of the vessel bottom head penetration to be repaired. 
     Preferably, liquid penetrant tests are carried out, especially after the first layer and the last layer of the weld bead  35  have been formed. At least one intermediate liquid penetrant test is also carried out between the formation of the first layer and the last layer of the weld bead, and for example after forming the fourth layer. 
     Preferably, a liquid penetrant test is carried out every three layers. 
     According to a variant, the angle of the triangle of the cross section of each half-insert  30   a  and  30   b  of the insert  30  placed in the groove bottom  15  may include a flat so that this insert is not in contact with the intersection of the external wall of the vessel bottom head and of the tube, since this junction zone may contain boron. 
     Preferably, a dimensional conformity check of the weld bead  35  is not carried out on the tube  10  of the vessel bottom head penetration after repair so as not to expose operators to a high level of irradiation, but a justification is produced by a dimensional check of the weld bead made on the mock-up, under these same conditions. 
     If a liquid penetrant test reveals a non-conformity, the defective zone is remelted by means of the welding torch and then another liquid penetrant test is carried out on this zone. Of course, all of the welding and test operations are monitored by cameras. 
     Positioning the insert before forming the weld bead makes it possible to prevent the boron present between the vessel bottom wall and the tube being mixed with the weld metal during formation of the weld bead. 
     In addition, by carrying out all the repair operations beforehand on a mock-up on site away from the irradiated zone, it is possible to guarantee the feasibility of the repair method without exposing operators to high levels of irradiation. 
     Such a method therefore minimizes the presence of operators in the irradiated zone.