Abstract:
A pressurizer is for a pressurized water nuclear power plant and it includes an upper cap provided with a tube; an end piece connected to the tube using a weld; and a sleeve protecting the weld, disposed inside the tube. The protective sleeve is mounted in a removable manner, such that the thermal sleeve is removed.

Description:
FIELD OF INVENTION 
     The present invention relates to a pressurizer for a pressurized water nuclear power plant. 
     BACKGROUND INFORMATION 
     A pressurizer for a pressurized water nuclear power plant is a pressure vessel that is part of the main primary system of the plant. It is composed of 3 vertical cylindrical shells and two hemispherical caps at the two ends. It is constructed in ferritic steel with a coating in austenitic stainless steel over all inner surfaces in contact with the primary fluid. The pressurizer is equipped, on its lower part, with vertical heaters and a connecting branch with an expansion line. On its upper cap, it comprises a nozzle equipped with a thermal sleeve and a spray head. The thermal sleeve allows the nozzle to be protected against excessive thermal changes, thus reducing as much as possible damage due to fatigue. The nozzle is made of ferritic steel and is covered on its inner face with a stainless steel coating. The nozzle is connected to a spray pipe via a safety end piece in austenitic stainless steel. The weld between the nozzle and the end piece is thus made in the form of a bimetallic weld while the weld between the safety end piece and the spray nozzle in austenitic stainless steel is made by using a homogeneous weld. 
     In general, document FR0158544 describes a thermal sleeve welded to the inside of the conduit in which it is located. Replacing this sleeve involves cutting the conduit twice before the sleeve can be removed by eliminating the weld bead of the sleeve on the conduit. 
     SUMMARY INVENTION 
     In this context, the present invention aims to provide a pressurizer for a pressurized water nuclear power plant particularly facilitating the inspection and maintenance of the bimetallic weld mentioned above as well as the stainless steel coating of the nozzle. 
     For this purpose, the invention proposes a pressurizer for a pressurized water nuclear power plant comprising:
         an upper cap equipped with a nozzle,   an end piece connected to said nozzle through a weld,   a sleeve for protecting said weld disposed inside said nozzle,
 
the pressurizer being characterized in that said protective sleeve is mounted removably such that the removal operation of said thermal sleeve is done from inside said pressurizer.
       

     Thanks to the invention, removal of the protective sleeve allows the weld between the spray nozzle and the end piece to be inspected and maintained. 
     In addition, as the sleeve makes the inner stainless steel coating of the nozzle inaccessible, the inspection and maintenance of this coating will be considerably facilitated by removing the sleeve. 
     In addition, it will be noted that the sleeves used in the prior art were welded on an end piece: the replacement of one sleeve would therefore involve cutting at the level of the end piece or the spray pipe to remove the sleeve and have access to the weld. The act of making the sleeve removable thus also facilitates the replacement of this sleeve by the inside of the pressurizer without cutting the end piece or the spray pipe. 
     The pressurizer may also present one or more of the characteristics below, considered individually or according to all technically possible combinations: 
     Advantageously, the protective sleeve comprises on its external surface a plurality of regularly distributed antivibration pads. 
     According to a preferential embodiment, the nozzle is covered on its inner face with a coating and the pressurizer comprises a support piece fixed on said coating and presenting an internal thread, the protective sleeve being screwed on the internal thread. 
     In addition, the support piece presents an external thread, the pressurizer comprising a spray head screwed on the external thread. 
     In a particularly advantageous manner, the lower end of the protective sleeve is such that, when the sleeve is in place, said end is positioned between the support piece and the spray head. 
     Preferentially, the pressurizer comprises means for locking in rotation the spray head that may be fixed to the support piece through a weld. 
     The present invention also relates to a method for removing a thermal sleeve in a pressurizer according to the invention and comprising the following steps:
         removing the weld between the rotational locking means and the support piece,   removing the rotational locking means,   unscrewing the spray head,   unscrewing the thermal sleeve.       

    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Other characteristics and advantages of the invention will emerge clearly from the description that is given below, for indicative and in no way limiting purposes, with reference to the attached figures, among which: 
         FIG. 1  is a simplified schematic representation of the primary system of a pressurized water nuclear reactor, comprising a pressurizer according to the invention; 
         FIG. 2  is a partial cross sectional view, enlarged, of the spray system  19  such as represented in  FIG. 1  showing the protective sleeve disposed inside the nozzle. 
     
    
    
     In all figures, common elements bear the same reference numbers. 
     DETAILED DESCRIPTION 
       FIG. 1  represents a primary system  1  of the pressurized water nuclear reactor comprising three or four primary loops.  FIG. 1  is a simplified schematic representation and only one primary loop  10  is represented. This primary system  1  comprises a vessel  2  in which nuclear fuel assemblies, 3 or 4 steam generators  4  (one per loop) equipped with primary and secondary sides, 3 or 4 primary coolant pumps  6  (one per loop), and a pressurizer  8  are found. Each primary loop  10  allows the vessel  2 , a steam generator  4  and a pump  6  to be connected. The pressurizer  8  is connected to one of the 3 or 4 primary loops  10  through an expansion line  11 . For illustrative purposes, the pressurizer  8  is connected to the primary loop  10  represented in  FIG. 1 . This primary loop  10  contains primary water, this water being discharged by the pump  6  to the vessel  2 , traverses the vessel  2  by undergoing heating in contact with fuel assemblies, then traverses the primary side of the steam generator  4  before returning to the suction of the pump  6 . The primary water heated in the vessel  2  gives up its heat in the steam generator  4  to secondary water traversing the secondary side of this generator. The secondary water circulates in a closed loop in a secondary system, not shown. The water evaporates by traversing generator  4 , the steam thus produced driving a steam turbine. 
     The pressurizer  8  is mounted in parallel on primary loop  10  by expansion line  11  stuck on the section of the primary loop  10  connecting vessel  2  to generator  4 . It is disposed at an elevation higher than that of pump  6  and vessel  2 . The pressurizer  8  comprises an external forged vessel  12  that is substantially cylindrical and with a vertical axis, equipped with a hemispherical cap  13  and a lower bottom  14 . The lower bottom  14  comprises a central opening  16  connected to conduit  11  by a branch  18 . 
     The pressurizer  8  also comprises a spray system  19  equipped with a spray head  21  disposed inside the vessel  12 , a pipe  22  in austenitic stainless steel connecting the spray system  19  to the primary loop  10 , at the level of the pump  6  discharge, and means (not shown) for selectively allowing or prohibiting circulation of primary water in pipe  22  up to spray head  21 . The spray system  19  will be described in further detail with reference to  FIG. 2 . 
     Primary system  1  also comprises a safety circuit  23  comprising a relief tank  24 , 3 pipes  25  (only one is represented) connecting the tank  24  to the cap  13  of the pressurizer and 3 safety valves  26  (only one is represented) interposed on pipes  25  between tank  24  and pressurizer  8 . 
     The inner space of pressurizer  8  is in communication with primary system  1  through branch  18  and expansion line  11 . Pressurizer  8  is permanently partially filled with primary water, the water level inside the pressurizer being a function of the current operating pressure of the primary system. The crown of the pressurizer  8  is filled with water vapor, at a pressure substantially equal to the pressure of water circulating in primary pipe  10  connecting generator  4 . 
     In case of excess pressure in the pressurizer, valve  26  opens and the water vapor is evacuated to tank  24 , in which it condenses. 
     Pressurizer  8  is equipped with several dozen electric heaters  28 . These heaters are disposed vertically and are mounted on the lower bottom  14 . They traverse the bottom  14  by openings provided for this purpose (in the sleeves), sealing means being inserted between the heaters and their sleeves. 
     The function of pressurizer  8  is to control the water pressure in the primary system. Because it communicates by expansion line  11  with the primary pipes, it plays the role of an expansion vessel. Thus, when the volume of water circulating in the primary system increases or decreases, the water level inside pressurizer  8  will rise or drop, depending on the case. 
     This variation in water volume may result from, for example, an injection of water in the primary system, or a variation in the operating temperature of the primary system. 
     Another function of pressurizer  8  is to increase or decrease the operating pressure of the primary system. 
     To increase the operating pressure of the primary system, the heaters  28  are powered electrically, such that they heat the water contained in the lower part of the pressurizer and bring it to its boiling temperature. Part of this water evaporates, such that the pressure in the pressurizer  8  crown increases. Because the vapor is constantly in hydrostatic balance with the water circulating in primary system  1 , the operational pressure of this primary system  1  increases. 
     To cause the operating pressure of primary system  1  to decrease, spray head  21  disposed in the pressurizer  8  crown is operated by allowing water circulation in pipe  22  by using means provided for this purpose. Water removed in primary pipe  10  at the pump  6  discharge is projected into the pressurizer  8  crown and causes the condensation of part of the water vapor found there. The water vapor pressure in the pressurizer  8  crown drops, such that the operating pressure of the primary system  1  also drops. 
     Thus, pressurizer  8  allows the water pressure of the primary system to be maintained at 155 bar to prevent the water that is heated to more than 300° C. from boiling. 
       FIG. 2  is a partial cross sectional enlarged view of spray system  19  such as represented in  FIG. 1 . 
     The spray system  19  comprises:
         a spray nozzle  29  in SA 508 Gr3 type ferritic steel covered on its inner face with a coating  30  in stainless steel,   a protective sleeve  31 ,   a support piece  32 ,   a spray head  21 ,   an end piece  34  in 316 LN type austenitic stainless steel.       

     The end piece  34  presents a general truncated cone shape and vertical X axis. 
     The upper end of the end piece  34  is connected to pipe  22  such as represented in  FIG. 1  through a homogeneous weld. 
     The spray nozzle  29  comprises:
         a first part  35  with a general truncated cone shape and vertical X axis presenting an upper end welded to the lower end of the end piece  34  through a weld  36  constituting a bimetallic weld,   a second part  37  of hemispherical shape presenting a lower end welded on the hemispherical cap  13  (not represented in  FIG. 2 ).       

     It will be noted that nozzle  29  described here corresponds to a nozzle added to hemispherical cap  13 . Of course, the invention is also applicable to the case of a nozzle integrated to the cap. 
     The nozzle  29  thus delimits an internal channel  38  putting the inner space of casing  12  of the pressurizer in communication with pipe  22  such as represented in  FIG. 1  via end piece  34 . 
     The thermal protective sleeve  31  particularly protects weld  36 . In fact, this zone may be subjected to thermal shocks due to the fact that relatively cold (temperature that may be on the order of 15° C.) spray water may be injected into the pressurizer that contains a hot fluid having a temperature that may be on the order of 345° C. 
     Sleeve  31  presents a general cylindrical shape at the central axis X and is disposed in the inner channel  38 . The sleeve comprises:
         an upper free end covering weld  36 ,   a lower end equipped with a shoulder  39  on its inner face extended by a cylindrical part  40  threaded on its outer surface.       

     The cylindrical part  40  is itself extended by a flange  41 . 
     In order to prevent lateral vibrations induced by the passage of fluid, four antivibration pads  42  are regularly distributed on the periphery of the outer surface of the sleeve  31 . 
     The support piece  32  is of a general cylindrical shape presenting a shoulder boring with, on its lower end, an internal thread  44  and an external thread  45 . The support piece  32  is welded to the inner coating  30  of the pressurizer at the level of the spray nozzle  29  through a weld  46 . 
     The spray head  21  comprises, on its upper part, a nut  47  and on its lower part, water spraying means  48  in the pressurizer crown, not detailed here. 
     The cylindrical part  40  of the thermal sleeve  31  is screwed on the internal thread  44  of the support piece  32 . 
     The nut  47  of the spray head  21  is screwed on the external thread  45  of support piece  32 . 
     The spray head  21  is prevented from being unscrewed and possibly falling thanks to the use of a hook-shaped piece  49  allowing the nut  47  to be locked in rotation, said piece  49  being welded via a weld  50  to support piece  32 . 
     In the final position, flange  41  of sleeve  31  is wedged between spray head  21  and support piece  32 . 
     If needed, removal of the thermal protective sleeve  31  is simplified by removing the sleeve from inside the pressurizer. 
     The method allowing sleeve  31  to be removed from inside the pressurizer is as follows:
         the weld  50  between the rotational locking piece  49  and the support piece  32  is removed,   the rotational locking piece  49  is removed,   the spray head  21  is unscrewed,   the sleeve  31  is unscrewed.       

     Access to the bimetal weld  36  or to the coating  30  allowing the latter to be controlled or maintained is greatly simplified by removing sleeve  31 . 
     Of course, the invention is not limited to the embodiment that has just been described. 
     In particular, other means of fixing the sleeve to the support piece may be contemplated. The flange of the sleeve may, for example, be equipped with openings coinciding with the threaded bore in the support piece. Screws traversing the openings and screwed in the holes may ensure fixation.