CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to Provisional Patent Application Ser. No. 61/294,514, filed Jan. 13, 2010 and claims priority thereto. 
    
    
     GOVERNMENT INTEREST 
     This invention was made with Government support under Contract No. DE-FC07-07ID14779 awarded by the Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains in general to pressurizers for pressurized water nuclear reactor power generating systems and more particularly to the thermal liners attached to the interior of the surge nozzles for such pressurizers. 
     2. Related Art 
     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. 
     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. Typically, a plurality of the above-described loops are connected to a single reactor vessel  10  by reactor coolant piping  20 . The primary side is maintained at a high pressure in the order of 155 bars by means of a pressurizer  22  that is connected to one of the loops of the primary side. 
     The pressurizer makes it possible to keep the pressure in the primary circuit between predetermined limits either by spraying the primary coolant fluid when the pressure tends to exceed the permissible upper limit or by electrical heating of the primary fluid when the pressure tends to fall below the permissible lower limit. These operations are carried out inside the pressurizer which comprises a generally cylindrical casing arranged with its axis vertical and having its lower and upper parts closed by means of domed ends. The lower domed end has sleeves passing through it in which electrical heaters are introduced into the pressurizer. The lower domed end also has a combined inlet and outlet surge nozzle that communicates directly with the primary loop piping  20  to maintain the pressure within the primary circuit within design limits. 
     As can be appreciated from  FIGS. 2 ,  3  and  5 , the surge nozzles  24  of the pressurizers  22  include thermal sleeves or liners  26  to reduce the effect of thermal transients on the fatigue of the nozzles. These thermal sleeves have typically been welded to or explosively expanded into the nozzle  24 .  FIG. 2  shows the thermal sleeve  26  welded at one axial location  28  along the interior of the nozzle. A spacer  29  is positioned between the thermal sleeve  26  and the nozzle  24 , proximate an inner end to minimize vibration of the sleeve, to keep the sleeve centered in the nozzle during welding, and to maintain a radial gap between the nozzle and the sleeve as a thermal barrier.  FIG. 3  shows the thermal sleeve  26  explosively expanded at the expansion zone  30 , into the interior surface of the surge nozzle  24 . Both of these installation techniques have drawbacks. Welding the thermal sleeve to the nozzle occurs only over a portion of the circumference, since welding over the entire circumference would result in unacceptable stresses in the thermal sleeve during certain transients. This results in non-uniform by-pass behind the thermal sleeve and bending in the nozzle. More particularly, the welding occurs on the interior of the nozzle typically over a 45° arc length. During cold water in-surge transients, the thermal sleeve contracts relative to the nozzle, and the asymmetric welding pattern results in a gap between the thermal sleeve and nozzle opposite the weld. Explosive expansion can also result in non-uniform expansion, and residual stresses in the sleeve material. The thermal sleeve is tightly fit to a groove machined into the cladding. There is no feature to center the thermal sleeve in the nozzle so contraction of the thermal sleeve during cold in-surge transients will result in non-uniform radial gaps, and hence additional thermal and bending stresses in the nozzle. In addition, explosive expansion is not always a well controlled process, and requires special permitting and handling which creates difficulties for the manufacturers. 
     Accordingly, an improved means for attaching the thermal sleeve to the nozzle is desired that will keep the thermal sleeve centered in the nozzle and not create non-uniform gaps between the sleeve and the interior of the nozzle. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved by this invention which provides a pressure vessel, and more particularly a pressurizer pressure vessel having a surge nozzle with a thermal sleeve mechanically attached to the interior thereof. The surge nozzle has an axial dimension and a first opening adjacent an interior of the pressure vessel at one end of the axial dimension and a second opening adjacent an exterior of the pressure vessel at a second end of the axial dimension. The thermal sleeve lines at least a portion of the interior of the surge nozzle along the axial dimension with a first end of the thermal sleeve proximate the first opening and a second end of the thermal sleeve proximate the second opening. A plurality of mechanical couplings connect the interior of the surge nozzle and thermal sleeve proximate the first end and the first opening and supports the thermal sleeve in the axial direction, with the first plurality of mechanical couplings being circumferentially spaced around the interior of the surge nozzle. A second plurality of mechanical couplings connect the interior of the surge nozzle and the thermal sleeve proximate the second end and the second opening and secures the thermal sleeve from rotation, with at least some of the second plurality of mechanical couplings circumferentially spaced around the interior of the surge nozzle. Preferably, each of the first plurality of mechanical couplings and the second plurality of mechanical couplings are equally spaced circumferentially around the thermal sleeve. 
     In one embodiment, each of the first plurality of mechanical couplings are substantially at a first axial location and each of the second plurality of mechanical couplings are substantially at a second axial location. In the one embodiment, the first plurality of mechanical couplings are a key and slot coupling wherein the keys extend through a slotted opening in the thermal sleeve and into a groove formed in the interior of the surge nozzle. Preferably, the keys and slots are elongated and extend in the circumferential direction. Desirably, a head of the key is larger than the slot in the thermal sleeve through which the key extends and the head of the key is mated to and welded to an interior surface of the thermal sleeve and a portion of the key that fits within the slot on the interior of the surge nozzle is welded to the thermal sleeve. 
     In another embodiment, the second plurality of mechanical couplings are key and slot couplings wherein the slots are formed in the second end of the thermal sleeve and the keys protrude radially inward from the interior of the surge nozzle. Desirably, the slots in the second plurality of mechanical couplings are open ended and are elongated and extend in the axial direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
         FIG. 2  is a sectional view of a portion of a surge nozzle with a welded thermal sleeve; 
         FIG. 3  is a sectional view of a surge nozzle with an explosively expanded thermal sleeve; 
         FIG. 4  is a partial sectional view of a pressurizer; and 
         FIG. 5  is a sectional view of a surge nozzle with a mechanically attached thermal sleeve in accordance with this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 4 , there is shown a pressurizer  22  for a pressurized water nuclear power plant system. The pressurizer  22  comprises a pressure vessel having a vertically oriented cylindrical shell  32 , a first or upper hemispherical head portion  34  and a second or lower hemispherical head portion  36 . A cylindrical skirt  38  extends downwardly from the lower head portion  36  and has a flange  40  fastened thereto by welding or other means to form a support structure for the vessel. The upper head portion  34  has a manhole or man-way  42  for servicing the interior of the vessel, one or more nozzles  44 , respectively, in fluid communication with a safety valve (not shown) and a spray nozzle  46  disposed therein. The spray nozzle  46  is in fluid communication with a supply of relatively cool primary fluid and has means associated therewith (not shown), which controls the flow of the relatively cool fluid to the pressurizer. 
     A plurality of nozzles  48  are vertically disposed in the lower head  36  and a plurality of straight tubular electrical immersion heating elements  50  extend through the nozzles  48  and into the pressurizer  22 . The heating elements  50  have a metal sheath covering the outer surface thereof and seal welds are formed between the metal sheaths and the nozzles  48 . To support the heating elements in the pressurizer, a single or a plurality of support plates  52  are disposed transversely in the lower portion thereof. The support plate(s)  52  have a plurality of holes  54  which receive the heating elements  50 . 
     A combined inlet and outlet nozzle  24 , commonly referred to as a surge nozzle is centrally disposed in the lower head  36  and places the pressurizer in fluid communication with the primary fluid system of the pressurized water nuclear reactor power plant. 
     As previously mentioned, the surge nozzles of pressurizers include the thermal sleeves or liners previously discussed with regard to  FIGS. 2 and 3 , which are employed to reduce the effect of thermal transients on the fatigue of the nozzle. In accordance with this invention, the thermal sleeve is attached to the bore of nozzle by means of a mechanical attachment. The attachment means of this invention allows for the sleeve to fully expand in the longitudinal direction as well as radially, which is necessary to address thermal transients experienced by the surge nozzle. The sleeve attachment is accomplished by the inclusion of annular grooves in the nozzle bore, which receive supporting keys to provide axial support for the sleeve. To prevent rotational movement, slots are provided in the lower end of the sleeve which receive keys machined in the bore of the nozzle  24 . 
       FIG. 5  illustrates a sectional view of surge nozzle  24  having a thermal sleeve  26  lining the interior surface thereof, which is attached to the nozzle in accordance with this invention. The thermal sleeve  26  is supported at the top end of the nozzle  24  by radial keys  58 . The radial keys fit through openings  60  in the thermal sleeve  26 , and are welded around their perimeter  62  on the bore of the thermal sleeve  26 . Preferably, on one side the radial keys  58  have enlarged heads that are captured by the inner surface of the thermal sleeve around the opening  60  and at the other end are received into an annular groove  66  machined in the cladding  64  that lines the surface of a nozzle  24 . 
     The lower end of the thermal sleeve includes axially extending slots  68 . Small keys  70 , machined in the bore of the nozzle  24 , are received in the slots  68 , to maintain centering of the thermal sleeve lower end during transient conditions. 
     Flow in the crevice region  72  behind the thermal sleeve  26  is restricted by small clearances between the sleeve and the nozzle bore at both the upper end raised cladding surface and at the lower end. Thus, an improved thermal sleeve attachment to the interior surface of the nozzle is provided that can accommodate thermal growth without adding substantial stress to the nozzle. 
     While specific embodiments of the invention have been described in detail, it will be 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.
A thermal sleeve is mechanically attached to the bore of a surge nozzle of a pressurizer for the primary circuit of a pressurized water reactor steam generating system. The thermal sleeve is attached with a series of keys and slots which maintain the thermal sleeve centered in the nozzle while permitting thermal growth and restricting flow between the sleeve and the interior wall of the nozzle.