Abstract:
A pressurizer whose heater sheaths are conditioned to reduce the residual stresses resulting from cold working during manufacture. After material conditioning, the heater sheath undergoes a surface conditioning treatment to add outer surface compressive stresses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Provisional Application Ser. No. 60/992,153, filed Dec. 4, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention pertains generally to pressurized water reactor systems and more particularly to a pressurizer heater employed in such systems. 
         [0004]    2. Description of the Prior Art 
         [0005]    The primary side of nuclear reactor power generating systems which are cooled with water under pressure, comprises a closed circuit which is isolated and in heat exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side. 
         [0006]    For the purpose of illustration,  FIG. 1  shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel  10  having a closure head  12  enclosing a nuclear core  14 . A liquid reactor coolant, such as water is pumped into the vessel  10  by pump  16  through the core  14  where heat energy is absorbed and is discharged to a heat exchanger  18 , typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump  16 , completing the primary loop. At least one of the loops is also connected to a pressurizer pump  22  for maintaining the pressure of the system. Typically, a plurality of the above described loops are connected to a single reactor vessel  10  by the reactor coolant piping  20 . 
         [0007]    As a result of the harsh environment findings experienced in a pressurized water reactor system the pressure vessels, their welds and the components within the pressure vessels may degrade as a result of micro-cracking otherwise known as stress corrosion cracking, or other degradation/failure mechanisms during plant operation and/or plant transient conditions. Depending upon time, temperature, pressure and the corrosive nature of the contained fluid, which is borated water, these degradations may eventually develop into pathways through which fluids may leak from the pressure vessels or their internal components may fail. Thus, for example, after decades of operation at temperatures of up to approximately 600° Fahrenheit (316° Celsius) or more and pressures of up to 2200 PSI (15.2 MPa) or more, indications of cracking have been detected in the course of non-destructive examinations of pressure vessels in light water nuclear reactor systems designed to generate commercial electric power. In some cases, small leaks have been detected in the sleeves extending through the heads of pressure vessels such as in the sleeves that carry the powers cables through the pressure vessel walls of the pressurizers, that are employed to energize the resistance heaters used for raising the pressure within the pressurized water reactor system. In addition, resistance heater failures have been noted due to stress corrosion cracking of their sheaths that are designed to isolate the resistance heaters from the surrounding coolant in the pressurizer pressure vessels. patent application Ser. No. 11/075,494 filed Mar. 9, 2005 and published as U.S. Patent Application Publication 2005/0199591 addresses the repair of the sleeves in a manner that will minimize the potential for further leaks in the area. It is desirable to also provide an improved heater design that will minimize the potential for heater sheath failures due to stress corrosion cracking in the future to avoid the need for additional repairs and personnel exposure to radiation. 
         [0008]    Accordingly, it is an objective of this invention to provide a pressurizer heater for a pressurized water reactor system that has an improved operating life. 
       SUMMARY OF THE INVENTION 
       [0009]    This invention achieves the foregoing objectives by replacing the pressurizer heater sheaths with sheaths that have received a material conditioning treatment to reduce residual stresses that were originally introduced after cold working (swaging) during manufacture, followed by a surface conditioning treatment that adds outer surface compression stresses to the region of the sheath adjacent its outer surface. 
         [0010]    In one preferred embodiment, both material conditioning and surface conditioning treatments may be applied to existing heater sheaths during servicing of the pressurizer, to spare heater sheaths that are maintained in inventory or to newly manufactured sheaths, such that crack initiation is less likely to occur over extended plant operation. The preferred method for material conditioning is a heat treatment. The surface conditioning applied after heat treatment is preferably a method such as centerless burnishing or shot peening. In addition, laser peening may also be employed for the surface conditioning step. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0012]      FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
           [0013]      FIG. 2  is an elevational view, partially in section, showing a pressurizer made in the coordinates of this invention; 
           [0014]      FIG. 3  is a partial sectional view of a heating element for the pressurizer of  FIG. 2 ; and 
           [0015]      FIG. 4  is a schematic view of a material conditioning and surface conditioning treatment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Referring again to the drawings,  FIG. 2  shows a pressurizer  22  for a pressurized water reactor nuclear power plant system. The pressurizer  22  comprises a vessel having a vertically oriented cylindrical shell portion  24 , a first or upper hemispherical head portion  26  and a second or lower hemispherical head portion  28 . A cylindrical skirt  30  extends downwardly from the lower head portion  28  and has a flange  32  fastened thereto by welding or other means to form a support structure for the vessel. The upper head portion  26  has a manway  34 , one or more nozzles  36  in fluid communication with safety valves (not shown) and a spray nozzle  38  disclosed therein. The spray nozzle  38  is in fluid communication with a supply of relatively cool fluid and has means cooperatively associated therewith (not shown), which controls the flow of the relatively cool fluid to the pressurizer  22 . 
         [0017]    A plurality of straight tubular electrical immersion resistance heating elements  40  are vertically disposed in the lower head portion  28  of the vessel. Lower head  28  has a plurality of nozzles  42 , which have an enlarged end and which receive the heating elements  40 . A seal is formed between the tubular heating elements  40  and the nozzles  42  by welding or other sealing means. 
         [0018]    To support the heating elements  40 , a plurality of support plates  44  are disposed transversely in the lower portion of the vessel. These support sheets or plates  44  have a plurality of holes  46  which receive the heating elements  40 . The holes  46  and the adjacent support plates are aligned with the nozzles  42 . 
         [0019]    A combination inlet and outlet nozzle  48  is centrally disposed in the lower head  28  and places the pressurizer  22  in fluid communication with the primary fluid of the pressurized water reactor nuclear power plant system. 
         [0020]    As shown in  FIG. 3 , the tubular immersion heating element  40  have a tubular metallic sheath  50  and a resistance heating coil  52  disposed within the sheath  50  and separated therefrom by dielectrically insulating material  54 . 
         [0021]    Two electrical leads  56  are brought out at one end, at the back end, of the heating element  40 . As shown in  FIG. 3 , the back end of the heating element  40  has heavy walls and is expanded outwardly forming a bulbous end. The leads  56  are electrically connected to an electrical supply (not shown), which when energized results in the coils becoming resistantly heated. Another end of the heating element  40 , the front or nose end, has a pointed nose portion  58 . The pointed nose portion  58  comprises a conical portion  60  having a base diameter generally equal to the outside diameter of the sheath  50  and a cylindrical portion  62 , smaller in diameter than the base of the conical portion  60 . The sheath  50  has a counter-bore  64  which receives the cylindrical portion  62  of the nose portion  58 . A seal weld  66  is provided between the sheath  50  and the base of the conical portion  60 . The pointed nose portion  58 , shown in  FIG. 3 , allows the heaters to be replaced, when they burn out, without having someone inside the vessel, which is slightly radioactive, even though the openings  46  in the support plates  44  and the nozzles  42  are slightly misaligned, thus reducing the amount of radiation to which maintenance people are subjected during the replacement procedure. Thus, it should be understood that the pointed nose portion  58  is an optional feature to facilitate maintenance. 
         [0022]    The operation of the pressurizer  22  is as follows; normally the pressurizer  22  is partially filled with primary fluid or water, the remainder of the vessel  22  is filled with steam; the combined inlet and outlet nozzles  48  is in fluid communication with the primary fluid in the pressurized water reactor system; and to increase the pressure of the primary fluid the heating elements  40  are energized thereby causing the water to boil and increase the amount of vapor in the pressurizer  22  to increase the pressure in the primary fluid system; to reduce the pressure of the primary fluid system, relatively cold primary fluid is sprayed though the spray nozzles  38  in the upper portion of the pressurizer  22  condensing some of the steam and thereby reducing the pressure within the pressurizer and in the primary fluid system. 
         [0023]    As previously noted, stress corrosion cracks have been found in the heater sheaths  50  compromising the interior of the heater elements  40  resulting in premature failure. In accordance with one embodiment of this invention both material conditioning and surface conditioning treatments are applied to the heater sheath  50  to reduce residual stresses in the heater sheath  50  such that crack initiation is less likely to occur. The preferred method for material conditioning is a heat treatment, figurally illustrated in  FIG. 4  which shows a heated sheath  50  being treated in a furnace  68 . The heat treatment is preferably at a temperature between 1800 and 1900° F. (980 and 1040° C.) for a period of from 5 to 15 minutes. The surface conditioning is preferably a centerless burnishing treatment, as figuratively indicated by the rollers  70  in  FIG. 4 , or shot peening. Alternatively, laser peening may be employed during the surface conditioning step to impart compressive forces to the outer surface of the sheath  50 . These steps may also be employed on existing heaters during periodic maintenance of the pressurizer  22 , or on spare heaters that are maintained in inventory and can be exchanged with the existing heaters during such periodic maintenance. Most preferably, new replacement heaters will be manufactured with this process. 
         [0024]    While specific embodiments of the invention have been described in detail, it will appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.