Patent Publication Number: US-2005117684-A1

Title: Reactor head with integral nozzles

Description:
FIELD AND BACKGROUND OF INVENTION  
      The present invention relates to nuclear power plant systems and more particularly to a nozzle penetration arrangement for a nuclear reactor pressure vessel closure head, such as a control rod drive mechanism (CRDM) guide tube nozzle penetration, and methods of making them.  
      A pressurized water nuclear reactor (PWR) includes a lower reactor vessel with a reactor core and an upper control rod assembly, part of which can be lowered into the reactor vessel for controlling the reaction rate of the nuclear reactor. The control rod assembly contains a plurality of vertical nozzles which penetrate the upper cover of the vessel, or closure head, and houses extensions of a control rod, that can be lifted or lowered by a control rod drive mechanism (“CRDM”), which generally operates by some combination of electrical, electromechanical, hydraulic, or pneumatic motors or drivers. For further details of the design and operation of pressurized water reactors the reader is referred to Chapters 47 and 50 of  Steam/its generation and use,  40 th  Edition, Stultz and Kitto, Eds., Copyright ©1992, The Babcock &amp; Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.  
      As shown in  FIG. 1 , designs for a PWR closure head assembly  5  presently used throughout the industry include a reactor head flange  10  which surrounds and may be integral with closure head  20  that forms a hemispherical, dome-shaped pressure boundary. Control Rod Drive Mechanism (CRDM) guide tubes  30 , also referred to as the CRDM housing, CRDM nozzles, mech housing or Control Element Drive Mechanism (CEDM) nozzles, pass through and are attached to closure head  20 . A stainless steel flange or adaptor  40  is situated at the upper end of the guide tube  30  attachment of each CRDM or CEDM, via full penetration weld  70  shown in  FIG. 2 . A PWR closure head assembly  5  is a large, heavy structure, typically about 17 feet in diameter and weighing about 90 tons, and includes between 30-100 CRDM guide tubes  30 .  
      Referring to  FIG. 2 , guide tube  30  is manufactured separately from closure head  20 , and then installed in bore hole  22  extending through closure head  20  from concave inner surface  24  to convex outer surface  26 . As shown in  FIG. 2 , guide tube  30  protrudes beyond inner surface  24  and outer surface  26 . Closure head  20  is typically fabricated from low-alloy steel and provided with a corrosion resistant cladding  80 , such as 308/309 stainless steel, at inner surface  24 .  
      Guide tube  30  is attached to closure head  20  by welding the guide tube  30  to closure head  20  with a partial penetration weld  50  referred to as a ‘J’ groove weld. Guide tube  30  is typically fabricated from Inconel Alloy 600 or Inconel Alloy 690, in which case weld  50  is made using Inconel weld consumables. Partial penetration J groove weld  50  is made between guide tube  30  and a J groove weld preparation profile  52  formed at inner surface  24  and typically covered with a previously heat treated Inconel overlay, in what is known as J groove buttering  60 . The previously heat treated J groove buttering  60  allows welding of the guide tube  30  to the buttering  60  without subsequent heat treatment of the J groove attachment weld  50 .  
      J groove attachment weld  50  and the associated guide tube  30  have experienced life limiting degradation in the vicinity of the J groove attachment region attributed to stress corrosion cracking (SCC). This has forced the repair, replacement or inspection of the Inconel J groove weld  50  and guide tubes  30 . This degradation has become a commercial and safety concern for all operating PWR stations. A reactor closure head assembly which eliminates the J groove attachment welds between the guide tubes and the inner surface of the reactor closure head would therefore be welcomed by industry.  
     SUMMARY OF INVENTION  
      The present invention is drawn to method and apparatus for eliminating degradation mechanism classified as stress corrosion cracking on the ‘J’ groove weld, and consequently eliminates the inspection and potential repair on the ‘J’ groove welds as commonly occurring in many PWR stations.  
      Accordingly, one object of the invention to minimize stress corrosion cracking of a reactor closure head assembly.  
      Another object of the invention is to eliminate nozzle welds exposed to reactor coolant.  
      In one embodiment, the invention comprises a closure head assembly for a reactor pressure vessel. The assembly includes a closure head which has a concave inner surface and a convex outer surface and is made of a first material. The assembly has plurality of nozzles integral with the closure head. Each nozzle terminates in a nozzle tip and has a bore therethrough defining a bore surface extending from the inner surface of the closure head to the nozzle tip. A corrosion-resistant second material is established adjacent to each bore surface.  
      In another embodiment, the invention comprises a closure head assembly for a reactor pressure vessel. The assembly includes a closure head which is made of first material and has a concave inner surface and a convex outer surface. The closure head inner surface is clad with a corrosion-resistant second material. The assembly also includes a plurality of control rod guide tube nozzles. Each nozzle is integral with the closure head and terminates in a nozzle tip. Each nozzle also has a bore therethrough defining a bore surface extending from the inner surface of the closure head to a nozzle tip. A control rod guide tube flange is attached to each nozzle end tip with weld buttering therebetween. A corrosion-resistant third material is established adjacent the bore surfaces.  
      In yet another embodiment, the invention comprises a method of making a reactor closure head assembly. The assembly has a reactor closure head with a plurality of nozzles arranged about the closure head. Each nozzle is integral with the closure head and has a bore therethrough. The bores of the outermost nozzles define a maximum bore length. The method includes providing a dome-shaped forging having a concave surface and a thickness greater than the maximum bore length. A plurality of nozzle protrusions are machined from the forging and an associated plurality of bores are formed therethrough. Each bore has a bore surface extending from the concave surface and terminating in a nozzle tip.  
      In a still further embodiment, the invention comprises a method of making a reactor closure head assembly. The assembly has a reactor closure head with a plurality of nozzles arranged about the closure head. Each nozzle is integral with the closure head and has a bore therethrough. The bores of the outermost nozzles defining a maximum bore length. The method includes providing a dome-shaped forging having a concave surface and a thickness greater than the maximum bore length. A plurality of nozzle protrusions are machined from the forging and an associated plurality of bores are formed therethrough. Each bore has a bore surface extending from the concave surface and terminating in a nozzle tip. The concave surface is clad with a corrosion resistant layer, weld buttering is applied to the nozzle tips and the forging, including the corrosion resistant layer and the weld buttering, are heat treated. A control rod guide tube flange is attached to each nozzle tip adjacent the weld buttering. A protective layer is established adjacent each bore surface.  
      The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter, forming a part of this disclosure, in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In the accompanying drawings, forming a part of this specification, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same:  
       FIG. 1  is a schematic, sectional view of a known reactor pressure vessel closure head assembly.  
       FIG. 2  is an enlarged partial sectional view of a nozzle penetration arrangement used in a known vessel closure head assembly.  
       FIG. 3  is a partial sectional view of a forging used in manufacturing the nozzle penetration arrangement of the present invention.  
       FIG. 4  is a partial sectional view of a nozzle penetration arrangement according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The subject invention addresses the observed degradation of the prior art by eliminating the ‘J’ groove attachment weld  50  which creates detrimental residual stresses. The invention further eliminates the separate Inconel guide tube  30  which, along with the Inconel ‘J’ groove weld consumable, are materials susceptible to degradation by stress corrosion cracking.  
      Referring to  FIGS. 3 and 4 , the subject invention provides a closure head assembly  105  having guide tube nozzles  130  which are integral with reactor closure head  120 , thereby eliminating both separately installed guide tubes and associated attachment welds. As shown in  FIG. 3 , a dome-shaped, reactor head pressure boundary forging  100 , having a concave inner surface  24  is processed with extra thickness  102 , equal to or greater than the desired nozzle height. This allows machining of nozzle protrusions formed as an integral part of the forged, dome-shaped reactor closure head  120 . Convex outer surface  26  is formed as part of the machining process. A bore  132  is formed through each nozzle protrusion to form an integral guide tube nozzle  130  which extends beyond outer surface  26  and terminates in nozzle tip  136 . Bore  132  has a bore surface  122  extending from inner surface  24  to nozzle tip  136 . This complex forged shape is thermally treated in compliance with the forged material requirements, if needed.  
      As shown in  FIG. 4 , the inner surface  24  of the reactor closure head  120  is clad with a corrosion resistant cladding layer  80  of weld consumables, such as 308 and 309 stainless steel or a nickel-chromium alloy like an Inconel alloy, applied using weld cladding methods known in the art. Cladding layer  80  shields the carbon steel or low alloy forged closure head  120  from the borated reactor coolant fluid.  
      Nozzle weld buttering  90  is applied to nozzle tips  136  of integral guide tube nozzles  130  using a stainless or Inconel consumable. The partially completed closure head assembly  105 , including reactor closure head  120 , the cladding layer  80  on inner surface  24  and the nozzle weld buttering  90  at safe ends of integral guide tube nozzles  130 , is then heat treated in accordance with the requirements of the ASME code.  
      A guide tube flange or adaptor  40  is then attached to each integral guide tube nozzle  30  via a full penetration weld  70  at the end tip  136  adjacent nozzle weld buttering  90 . This attachment weld can be performed following the above-mentioned ASME code heat treatment, and advantageously does not require any further post weld heat treatments.  
      The bore surface  134  of the integral guide tube nozzle  130  is then covered with a protective layer  180 , designed to shield the carbon or low alloy steel forging material from the reactor coolant fluid. Protective layer  180  is applied to bore surface  122 , extending from cladding layer  80  on inner surface  24  up to full penetration weld  70 . Protective layer  180  can be applied by processes involving heating, for example via weld cladding methods known in the art, which require subsequent post weld heat treatment. Protective layer  180 , however, is preferably applied without heating, for example via electro-chemical deposition, thereby eliminating the need for subsequent post weld heat treatment. U.S. Pat. Nos. 5,352,266; 5,433,797; 5,516,415; 5,527,445; and 5,538,615 describe a pulsed electrodeposition process which is suitable for this purpose, and are incorporated herein by reference as though fully set forth. This pulsed electrodeposition process can be used to deposit, for example, a 0.020 inch thick protective metallic layer, such as nickel, on bore surface  122 . Other suitable materials for protective layer  180  include stainless steel, nickel-based alloys, and nickel-chromium alloys such as Inconel.  
      Alternatively, protective layer  180  could be established by introducing a sleeve of a corrosion resistant material into bore  132  adjacent bore surface  122 . As one example, a sleeve of corrosion resistant material having a diameter slightly greater than bore  132  is chilled to reduce the diameter of the sleeve, for example by exposure to liquid nitrogen, and the sleeve is inserted into bore  132 . The sleeve expands as it returns to room temperature, thereby forming an expansion-fit with bore surface  122 . Other means of establishing a protective layer  180  by way of a sleeve are also possible. The sleeve may or may not be bonded to bore surface  122 .  
      While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles.