Patent Publication Number: US-11031146-B2

Title: Method for heating a primary coolant in a nuclear steam supply system

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 14/620,390 filed Feb. 12, 2015, which is a national stage entry of International Patent Application No. PCT/US2013/054961 filed Aug. 14, 2013, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/683,021, filed Aug. 14, 2012; the entireties of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a nuclear steam supply system, and more particularly to a start-up sub-system for heating a primary coolant in a nuclear steam supply system. 
     BACKGROUND OF THE INVENTION 
     For starting up a nuclear steam supply system in a typical pressurized water reactor, it is necessary to heat the reactor coolant water to an operating temperature, which is known in the art as the no-load operating temperature of the reactor coolant water. Furthermore, in conventional nuclear steam supply systems it is necessary to ensure full flow through the coolant loop and the core. This is necessary to ensure that a completely turbulated flow across the fuel core exists as the control rods are being withdrawn in order to avoid localized heating and boiling, and to ensure that the reactivity of water is in the optimal range during start-up and during normal operation. 
     In the present state of the art, the desired start-up condition is achieved by the use of the reactor coolant pump whose primary function is to circulate coolant through the reactor core during normal operating conditions. In normal operation, the substantial frictional heat produced by the reactor coolant pumps is removed by external cooling equipment (heat exchangers) to maintain safe operating temperature. However, during start-up external cooling is disabled so that the frictional heat can be directly transferred to the reactor coolant water, enabling it to reach no-load operating temperature. As the reactor coolant water is being heated, the pressure in the reactor coolant loop is increased using a bank of internal heaters which evaporates some reactor coolant water and increases the pressure in the reactor coolant system by maintaining a two-phase equilibrium. 
     The above process for heating the reactor water inventory during start-up is not available in a passively safe nuclear steam supply system. This is because such a passively safe nuclear steam supply system does not include or require any pumps, and thus the use of the frictional heat is unavailable for heating the reactor water inventory. Thus, a need exists for a start-up system for heating the reactor water inventory in a passively safe nuclear steam supply system. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved nuclear steam supply system and start-up sub-system therefor that overcomes the deficiencies of the foregoing existing arrangements. The present invention also provides an improved method of heating a primary coolant in a nuclear steam supply system to a no load operating temperature. 
     In one aspect, the invention can be a nuclear steam supply system comprising: a reactor vessel having an internal cavity, a reactor core comprising nuclear fuel disposed within the internal cavity; a steam generating vessel fluidly coupled to the reactor vessel; a riser pipe positioned within the steam generating vessel and fluidly coupled to the reactor vessel; a primary coolant at least partially filling a primary coolant loop formed within the reactor vessel and the steam generating vessel; and a start-up sub-system comprising: an intake conduit having an inlet located in the primary coolant loop; a pump fluidly coupled to the intake conduit for pumping a portion of the primary coolant from the primary coolant loop through the intake conduit and into an injection conduit; at least one heating element for heating the portion of the primary coolant to form a heated portion of the primary coolant; and an injection nozzle fluidly coupled to the injection conduit and positioned within the riser pipe for injecting the heated portion of the primary coolant into the riser pipe. 
     In another aspect, the invention can be a nuclear steam supply system comprising: a reactor vessel having an internal cavity, a reactor core comprising nuclear fuel disposed within the internal cavity; a steam generating vessel fluidly coupled to the reactor vessel; a primary coolant loop formed within the reactor vessel and the steam generating vessel, a primary coolant in the primary coolant loop; and a start-up sub-system fluidly coupled to the primary coolant loop, the start-up sub-system configured to: (1) receive a portion of the primary coolant from the primary coolant loop; (2) heat the portion of the primary coolant to form a heated portion of the primary coolant; and (3) inject the heated portion of the primary coolant into the primary coolant loop. 
     In yet another aspect, the invention can be a method of heating a primary coolant to a no-load operating temperature in a nuclear steam supply system, the method comprising: a) filling a primary coolant loop within a reactor vessel and a steam generating vessel that are fluidly coupled together with a primary coolant; b) drawing a portion of the primary coolant from the primary coolant loop and into a start-up sub-system; c) heating the portion of the primary coolant within the start-up sub-system to form a heated portion of the primary coolant; and d) injecting the heated portion of the primary coolant into the primary coolant loop. 
     In a further aspect, the invention can be a method of starting up a nuclear steam supply system, the method comprising: a) at least partially filling a primary coolant loop within a reactor vessel and a steam generating vessel that are fluidly coupled together with a primary coolant, wherein the primary coolant loop comprises a riser pipe in the steam generating vessel; b) drawing a portion of the primary coolant from the primary coolant loop and into a start-up sub-system; c) heating the portion of the primary coolant within the start-up sub-system to form a heated portion of the primary coolant; and d) introducing the heated portion of the primary coolant into the riser pipe of the steam generating vessel. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the exemplary embodiments of the present invention will be described with reference to the following drawings, where like elements are labeled similarly, and in which: 
         FIG. 1  is front view of a nuclear steam supply system including a reactor vessel, a steam generating vessel and a start-up sub-system in accordance with an embodiment of the present invention; 
         FIG. 2  is an elevation cross-sectional view of the reactor vessel of  FIG. 1 ; 
         FIG. 3  is an elevation cross-sectional view of the bottom portion of the steam generating vessel of  FIG. 1 ; 
         FIG. 4  is an elevation cross-sectional view of the top portion of the steam generating vessel of  FIG. 1 ; 
         FIG. 5A  is a close-up view of the reactor vessel and a portion of the steam generating vessel of  FIG. 1  illustrating the location of an intake conduit of the start-up sub-system in accordance with a first embodiment of the present invention; 
         FIG. 5B  is the close-up view of  FIG. 5A  illustrating the location of the intake conduit of the start-up sub-system in accordance with a second embodiment of the present invention; 
         FIG. 5C  is the close-up view of  FIG. 5A  illustrating the location of the intake conduit of the start-up sub-system in accordance with a third embodiment of the present invention; 
         FIG. 6  is a close-up view of area VI of  FIG. 1 ; 
         FIG. 7  is a schematic illustrating the connection between the start-up sub-system and the reactor vessel; and 
         FIG. 8  is a graph illustrating the primary coolant pressure vs. the primary coolant temperature. 
     
    
    
     All drawings are schematic and not necessarily to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The features and benefits of the invention are illustrated and described herein by reference to exemplary embodiments. This description of exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description of embodiments disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. Accordingly, the disclosure expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features. 
     Referring first to  FIG. 1 , a nuclear steam supply system  100  is illustrated in accordance with an embodiment of the present invention. Although described herein as being a nuclear steam supply system, in certain embodiments the system may be generally referred to herein as a steam supply system. The inventive nuclear steam supply system  100  is typically used in a nuclear pressurized water reactor and generally comprises a reactor vessel  200 , a steam generating vessel  300  and a start-up sub-system  500 . Of course, the nuclear steam supply system  100  can have uses other than for nuclear pressurized water reactors as can be appreciated. 
     During normal operation of the nuclear steam supply system  100 , a primary coolant flows through a primary coolant loop  190  within the reactor vessel  200  and the steam generating vessel  300 . This primary coolant loop  190  is depicted with arrows in  FIG. 1 . Specifically, the primary coolant flows upwardly through a riser column  224  in the reactor vessel  200 , from the reactor vessel  200  to the steam generating vessel  300  through a fluid coupling  270 , upwardly through a riser pipe  337  in the steam generating vessel  300  to a top of the steam generating vessel  300  (i.e., to a pressurizer  380 ), and then downwardly through tubes  332  (see  FIGS. 3 and 4 ) in a tube side  304  of the steam generating vessel  300 , from the steam generating vessel  300  to the reactor vessel  200  through the fluid coupling  270 , downwardly through a downcomer  222  of the reactor vessel  200 , and then back from the downcomer  222  of the reactor vessel  200  to the riser column  224  of the reactor vessel  200 . The primary coolant continues to flow along this primary coolant loop  190  as desired without the use of any pumps during normal operation of the nuclear steam supply system  100 . 
     It should be appreciated that in certain embodiments the primary coolant loop  190  is filled or partially filled with the primary coolant when the nuclear steam supply system  100  is shut down and not operating. By filled it may mean that the entire primary coolant loop  190  is completely filled with the primary coolant, or that the primary coolant loop  190  is almost entirely filled with the primary coolant with some room for air which leaves space for the addition of more primary coolant if desired or the expansion of the primary coolant as it heats up during the start-up procedures discussed below. In certain embodiments, before start-up the primary coolant is static in the primary coolant loop  190  in that there is no flow of the primary coolant along the primary coolant loop. However, during a start-up procedure utilizing the start-up sub-system  500  discussed in detail below, the primary coolant is heated and caused to flow through the primary coolant loop  190  and eventually is able to flow through the primary coolant loop  190  passively and unaided by any pumps due to the physics concept of thermosiphon flow. 
     Before nuclear fuel within the reactor core engages in a fission chain reaction to produce heat, a start-up process using the start-up sub-system  500  takes place to heat the primary coolant to a no-load operating temperature, as discussed in more detail below. During normal operation of the nuclear steam supply system  100 , the primary coolant has an extremely high temperature due to its flowing through the reactor core. Specifically, nuclear fuel in the reactor vessel  200  engages in the fission chain reaction, which produces heat and heats the primary coolant as the primary coolant flows through the reactor core of the reactor vessel  200 . This heated primary coolant is used to phase-change a secondary coolant from a liquid into steam as discussed below. 
     While the primary coolant is flowing through the primary coolant loop  190  during normal operation, the secondary coolant is flowing through a second coolant loop. Specifically, the secondary coolant is introduced into the shell side  305  ( FIGS. 3 and 4 ) of the steam generating vessel  300  at the secondary coolant in location indicated in  FIG. 1 . The secondary coolant then flows through the shell side  305  ( FIGS. 3 and 4 ) of the steam generating vessel  300  where it is heated by heat transfer from the primary coolant. The secondary coolant is converted into steam due to the heat transfer, and the steam flows from the steam generating vessel  300  to a turbine  900  as indicated in  FIG. 1 . The turbine  900  drives an electric generator which is connected to the electrical grid for power distribution. The steam then travels from the turbine  900  to a condenser (not illustrated) whereby the steam is cooled down and condensed. Thus, the condenser converts the steam back to a liquid (i.e., the secondary coolant) so that it can be pumped back into the steam generator  300  at the secondary coolant in location and repeat its flow through the flow path discussed above. 
     In certain embodiments both the primary coolant and the secondary coolant may be water, such as demineralized water. However, the invention is not to be so limited and other liquids or fluids can be used in certain other embodiments, the invention not being limited to the material of the primary and secondary coolants unless so claimed. 
     The primary coolant continues to flow through the primary coolant loop and the secondary coolant continues to flow in the second coolant loop during normal operation of the nuclear steam supply system  100 . The general operation of the nuclear steam supply system  100  and details of the components is described in detail in International Application No. PCT/US13/38289, filed on Apr. 25, 2013, the entirety of which is incorporated herein by reference. 
     Referring to  FIGS. 1-4 , the general details of the components and the operation of the nuclear steam supply system  100 , and specifically of the reactor vessel  200  and the steam generating vessel  300 , will be described. In the exemplified embodiment, the reactor vessel  200  and the steam generating vessel  300  are vertically elongated and separate components which hydraulically are closely coupled, but are discrete vessels in themselves that are thermally isolated except for the exchange of primary coolant (i.e. reactor coolant) flowing between the vessels in the fluid coupling  270  of the primary coolant loop  190  as discussed above. In one non-limiting embodiment, each of the reactor vessel  200  and the steam generating vessel  300  may be made of a corrosion resistant metal such as stainless steel, although other materials of construction are possible. 
     Referring to  FIGS. 1 and 2  concurrently, the reactor vessel  200  will be further described. The reactor vessel  200  in one non-limiting embodiment is an ASME code Section III, Class  1  thick-walled cylindrical pressure vessel comprised of a cylindrical sidewall shell  201  with an integrally welded hemispherical bottom head  203  and a removable hemispherical top head  202 . The shell  201  defines an internal cavity  208  configured for holding the reactor core which comprises the nuclear fuel. Specifically, the reactor vessel  200  includes a cylindrical reactor shroud  220  which contains the reactor core defined by a fuel cartridge  230  (i.e., nuclear fuel). The reactor shroud  220  transversely divides the shell portion of the reactor vessel into two concentrically arranged spaces: (1) an outer annulus  221  defining the annular downcomer  222  for primary coolant entering the reactor vessel which is formed between the outer surface of the reactor shroud  220  and an inner surface  206  of the shell  201 ; and (2) a passageway  223  defining the riser column  224  for the primary coolant leaving the reactor vessel  200  heated by fission in the reactor core. 
     The reactor shroud  220  is elongated and extends in an axial direction along a vertical axis A-A of the reactor vessel  200 . The reactor shroud  220  includes an open bottom end  225  and a closed top end  226 . In one embodiment, the open bottom end  225  of the reactor shroud  220  is vertically spaced apart by a distance from the bottom head  203  of the reactor vessel  200  thereby forming a bottom flow plenum  228  between the bottom end  225  of the reactor shroud  220  and the bottom head  203  of the reactor vessel  200 . As will be discussed in more detail below, during flow of the primary coolant through the primary coolant loop  190 , the bottom flow plenum  228  collects the primary coolant from the annular downcomer  222  and directs the primary coolant flow into the inlet of the riser column  224  formed by the open bottom end  225  of the reactor shroud  220 . 
     In certain embodiments, the reactor shroud  220  is a double-walled cylinder which may be made of a corrosion resistant material, such as without limitation stainless steel. This double-wall construction of the reactor shroud  220  forms an insulated structure designed to retard the flow of heat across it and forms a smooth vertical riser column  224  for upward flow of the primary coolant heated by the fission in the fuel cartridge  230  (“core”), which is preferably located at the bottom extremity of the shroud  220  in one embodiment as shown in  FIG. 2 . In certain embodiments, shroud  220  may be a multi-walled cylinder having more than two walls to further decrease the heat transfer across the shroud from the hot riser to the cold downcomer. The vertical space above the fuel cartridge  230  in the reactor shroud  220  may also contain interconnected control rod segments along with a set of “non-segmental baffles” that serve to protect them from flow induced vibration during reactor operations. The reactor shroud  220  is laterally supported by the reactor vessel by support members  250  to prevent damage from mechanical vibrations that may induce failure from metal fatigue. 
     In certain embodiments, the fuel cartridge  230  is a unitary autonomous structure containing upright fuel assemblies, and is situated in a region of the reactor vessel  200  that is spaced above the bottom head  203  so that a relatively deep plenum of water lies underneath the fuel cartridge  230 . The fuel cartridge  230  is insulated by the reactor shroud  220  so that a majority of the heat generated by the fission reaction in the nuclear fuel core is used in heating the primary coolant flowing through the fuel cartridge  230  and adjoining upper portions of the riser column  224 . In certain embodiments, the fuel cartridge  230  is an open cylindrical structure including cylindrically shaped sidewalls, an open top, and an open bottom to allow the primary coolant to flow upward completely through the cartridge (see directional flow arrows, described in detail above with specific reference to  FIG. 1 ). In one embodiment, the sidewalls of the fuel cartridge  230  may be formed by multiple arcuate segments of reflectors which are joined together by suitable means. The open interior of the fuel cartridge  230  may be filled with a support grid for holding the nuclear fuel rods and for insertion of control rods into the core to control the fission reaction as needed. 
     In the interconnecting space between the reactor vessel  200  and the steam generating vessel  300  there is a fluid coupling  270  that comprises an inner flow path  271  and an outer flow path  272  that concentrically surrounds the inner flow path  271 . As will be discussed in more detail below, during flow of the primary coolant the primary coolant flows upwardly within the riser column  224  and through the inner flow path  271  of the fluid coupling  270  to flow from the reactor vessel  200  to the steam generating vessel  300 . After the primary coolant gets to the top of the steam generating vessel  300 , the primary coolant begins a downward flow through the steam generating vessel  300  and then flows through the outer flow path  272  from the steam generating vessel  300  and into the downcomer  222  of the reactor vessel  200 . Again, this flow path will be described in more detail below. 
     Turning now to  FIGS. 1, 3 and 4  concurrently, the details of the steam generating vessel  300  will be described in more detail. In certain embodiments, the steam generating vessel  300  includes a preheater section  320 , a steam generator section  330 , a superheater section  340  and a pressurizer  380 . However, the invention is not to be so limited and one or more of the sections of the steam generating vessel  300  may be omitted in certain other embodiments. Specifically, in certain embodiments the preheater section  320  may be omitted, or may itself be considered a part of the steam generator section  330 . As discussed above, it is within the steam generator vessel  300  that the secondary coolant that is flowing through the shell side  305  of the steam generator vessel  300  is converted from a liquid (i.e., secondary coolant inlet illustrated in  FIG. 1 ) to a superheated steam that is sent to the turbine  900  ( FIG. 1 ) for electricity generation. The secondary coolant flows in the second coolant loop through the shell side of the steam generating vessel  300 , out to the turbine  900 , from the turbine  900  to a condenser, and then back into the shell side of the steam generating vessel  300 . 
     In the exemplified embodiment, each of the preheater  320 , the steam generator  330 , and the superheater  350  are tubular heat exchangers having a tube side  304  and a shell side  305 . The tube side  304  of the tubular heat exchangers include a tube bundle comprising a plurality of parallel straight tubes  332  and tubesheets  333  disposed at the extremities or ends of each tube bundle that support the tubes. In the exemplified embodiment, only two tubes  332  are illustrated to avoid clutter. However, in actual use tens, hundreds or thousands of tubes  332  can be positioned within each of the sections of the steam generating vessel  300 . In certain embodiments, a bottom-most one of the tubesheets  333 A is located in the preheater section  320  or in the steam generator section  330 . This bottom-most tubesheet  333 A will be discussed in more detail below with regard to a location of injection from the start-up sub-system  500  in one exemplified embodiment. 
     As noted above, in one embodiment the preheater section  320  can be considered as a part of the steam generator section  330 . In such embodiments the steam generator section  330  and the superheater section  350  can be considered as stacked heat exchangers such that the superheater section  350  is disposed above the steam generator section  330 . In certain embodiments, the preheater section  320 , steam generator section  330 , and superheater section  350  are positioned to form a parallel counter-flow type heat exchanger arrangement in which the secondary coolant (Rankine cycle) flows in an opposite, but parallel direction to the primary coolant (see  FIGS. 3 and 4 ). Specifically, the arrows labeled A indicate the flow direction of the primary coolant through the riser pipe  337  that is positioned within the steam generating vessel  300 , the arrows labeled B indicate the flow direction of the primary coolant through the tubes  332  of the steam generating vessel  300 , and the arrows labeled C indicate the flow direction of the secondary coolant through the shell side  305  of the steam generating vessel  300 . The trio of the foregoing tubular heat exchangers (i.e. preheater, steam generator, and superheater) are hydraulically connected in series on both the tube side  304  (primary coolant) and the shell side  305  (the secondary coolant forming the working fluid of the Rankine Cycle which changes phase from liquid to superheated gas). 
     In the exemplified embodiment, the steam generating vessel  300  includes a top  310 , a bottom  311 , an axially extending cylindrical shell  312 , and the internal riser pipe  337  which is concentrically aligned with the shell  312  and in the exemplified embodiment lies on a centerline C-C of the steam generating vessel  300 . The tubes  332  are circumferentially arranged around the outside of the riser pipe  337  between the riser pipe  337  and the shell  312  in sections of the steam generating vessel  300  which include the preheater  320 , the steam generator  330 , and the superheater  350 . In one embodiment, the riser pipe  337  extends completely through all of the tubesheets  333  associated with the preheater  320 , the steam generator  330 , and the superheater  350  from the top of the steam generating vessel  300  to the bottom to form a part of the continuous primary coolant loop  190  between the reactor vessel  200  and the steam generating vessel  300  all the way to the pressurizer  380 . 
     The fluid coupling  270  includes an inner flowpath  371  and an outer flowpath  372  on the steam generating vessel  300  side of the fluid coupling  270 . The inner flowpath  371  is fluidly coupled to the inner flow path  271  and the outer flowpath  372  is fluidly coupled to the outer flowpath  272 . Thus, via these operable couplings the steam generating vessel  300  is fluidly coupled to the reactor vessel  200  to complete the primary coolant loop  190  for flow of the primary coolant through both the reactor vessel  200  and the steam generating vessel  300 . An annular space is formed between the riser pipe  337  and the shell  312 , which forms a bottom plenum  338 . The bottom plenum  338  collects and channels the primary coolant from the steam generating vessel  300  back to the reactor vessel  200  via the outer flow paths  272 ,  372 . Thus, in the exemplified embodiment the primary coolant flows from the reactor vessel  200  to the steam generating vessel  300  through the inner flow paths  271 ,  371  and the primary coolant flows from the steam generating vessel  300  to the reactor vessel  200  through the outer flow paths  272 ,  372 . However, the invention is not to be so limited and in other embodiments the use of the flow paths  271 ,  272 ,  371 ,  372  can be reversed 
     The superheater  350  is topped by a pressurizer  380  as shown in  FIGS. 1 and 4 , which is in fluid communication with both the top or outlet of the riser pipe  337  and the inlet to the tubes  332  of the superheater  350 . In one embodiment, the pressurizer  380  is mounted directly to the shell  312  of the steam generating vessel  300  and forms a top head  336   a  on the shell. In one embodiment, the pressurizer has a domed or hemispherical head and may be welded to the shell  312 , or alternatively bolted in other possible embodiments. The pressurizer  380  forms an upper plenum which collects reactor primary coolant rising through riser pipe  337  and distributes the primary coolant from the riser pipe  337  to the superheater tubes  332 . In certain embodiments, the pressurizer  380  includes a heating/quenching element  38 . (i.e. water/steam) for pressure control of the reactor primary coolant. 
     Shown schematically in  FIG. 4 , the heating/quenching element  383  is comprised of a bank of electric heaters which are installed in the pressurizer section that serve to increase the pressure by boiling some of the primary coolant and creating a steam bubble that resides at the top of the pressurizer near the head (above the liquid/gas interface  340  represented by the dashed line). A water spray column  384  is located near the top head  336   a  of the pressurizer  380  which sprays water into the steam bubble thereby condensing the steam and reducing the size of the steam bubble. The increase/decrease in size of the steam bubble serves to increase/decrease the pressure of the primary coolant inside the reactor coolant system. In one exemplary embodiment, a representative primary coolant pressure maintained by the pressurizer  380  and the heating/quenching element  383  may be without limitation about 2,250 psi. In alternative embodiments, as noted above, the liquid/gas interface  340  is formed between an inert gas, such as nitrogen (N2) supplied by supply tanks (not shown) connected to the pressurizer  380 , and the liquid primary coolant. 
     In one embodiment, the external surfaces of the tubes  332  may include integral fins to compensate for the reduced heat transfer rates in the gaseous superheated steam media. The superheater tube bundle is protected from erosion (i.e. by tiny water droplets that may remain entrained in the up-flowing steam) by ensuring that the steam flow is counter-flow being parallel along, rather than across, the tubes  332  in the tube bundle. 
     Referring now to  FIGS. 1 and 5A , the start-up sub-system  500  of the nuclear steam supply system  100  will be described in accordance with one embodiment of the present invention. In addition to discussing the components of the start-up sub-system  500  below, the operation of the start-up sub-system  500  in conjunction with the operation of the nuclear steam supply system  100  as a whole will be discussed below. Prior to the start-up processes taking place as will be discussed in more detail below, the primary coolant loop  190  is filled with the primary coolant, but the primary coolant is at ambient temperature and is not flowing through the primary coolant loop  190 . Utilizing the start-up sub-system  500  of the present invention, the primary coolant is heated, made to flow through the primary coolant loop  190 , and then able to continue passively flowing through the primary coolant loop  190  without the use of any pumps after disconnecting the start-up sub-system  500  from the primary coolant loop  190 . It will be appreciated that in certain embodiments, nuclear heat from the reactor may be used to heat the primary coolant and the start-up system to provide circulation up to a certain fraction of full natural circulation flow. 
     In order to start up the nuclear steam supply system  100  and begin withdrawing the control rods to initiate a fission chain reaction by the nuclear fuel in the reactor vessel  200 , the primary coolant should be heated to a no load operating temperature, which in certain embodiments can be between 500° F. and 700° F., more specifically between 550° F. and 650° F., and more specifically approximately 600° F. Ensuring that the primary coolant is at the no load operating temperature before normal operation (i.e., before flowing the steam to the turbine and before withdrawing the control rods) is beneficial for several reasons. First, it ensures that the primary coolant has a completely turbulated flow across the fuel core while the control rods are being withdrawn, which avoids localized heating and boiling. Second, it ensures that the reactivity of the water is in the optimal range during start-up and normal operation. Because the nuclear steam supply system  100  does not utilize any pumps to flow the primary fluid through the primary coolant loop  190  during normal operation but rather relies on thermosiphon flow as discussed above, conventional means of using frictional heat from the pumps to heat up the primary coolant is unavailable. Thus, the inventive nuclear steam supply system  100  uses the start-up sub-system  500  to heat the primary coolant up to the no load operating temperature during start up procedures. 
     The start-up sub-system  500  is designed to have a high margin of safety. The start-up sub-system  500  also ensures a fully turbulent flow across the fuel core in the reactor vessel  200  and heats the water to no-load operating temperature prior to any withdrawal of the control rods. As discussed in detail above, during start-up of the nuclear steam supply system  100 , the primary coolant is located within the primary coolant loop  190  in the reactor vessel  200  and in the steam generating vessel  300 , but it does not flow through the primary coolant loop  190  initially. While the primary fluid is positioned in the primary coolant loop  190 , the start-up sub-system  500  draws or receives a portion of the primary coolant from the primary coolant loop  190 , heats up the portion of the primary coolant to form a heated portion of the primary coolant, and injects the heated portion of the primary coolant back into the primary coolant loop  190 . Thus, the start-up sub-system  500  forms a fluid flow circuit that withdraws some of the primary coolant from the primary coolant loop  190  and heats the primary coolant prior to re-injecting that portion of the primary coolant into the primary coolant loop  190 . 
     When the start-up sub-system  500  injects the heated portion of the primary coolant into the primary coolant loop  190 , this initiates a venturi effect that creates fluid flow of the entire body of the primary coolant within the primary coolant loop  190 . Specifically, the injected heated portion of the primary coolant flows within the primary coolant loop and pulls the initially static primary coolant within the primary coolant loop  190  with it as it flows, thereby creating an entire turbulent flow of the primary coolant (including the original static primary coolant and the heated portion of the primary coolant) through the primary coolant loop  190 . Furthermore, because the primary coolant injected from the start-up sub-system is heated relative to the temperature of the primary coolant within the primary coolant loop  190 , this injection begins to heat up the primary coolant inventory within the primary coolant loop  190 . When the primary coolant within the primary coolant loop  190  reaches the no-load operating temperature, the start-up sub-system  500  can be fluidly disconnected from the reactor vessel  200  and the steam generating vessel  300  and flow of the primary coolant through the primary coolant loop  190  will continue due to thermosiphon properties. 
     In the exemplified embodiment, the start-up sub-system  500  comprises an intake conduit  501 , a pump  502 , an injection conduit  503 , a heating element  504  and an injection nozzle  505 . The intake conduit  501 , the pump  502 , the injection conduit  503  and the injection nozzle  505  are all fluidly coupled together so that a portion of the primary coolant that is received by the start-up sub-system  500  will flow through each of the intake conduit  501 , the pump  502 , the injection conduit  503  and the injection nozzle  505 . 
     In the exemplified embodiment, the entire nuclear steam supply system  100  including the reactor vessel  200 , the steam generating vessel  300  and the start-up sub-system  500  are housed within a containment vessel  400 . This ensures that in the event of a loss-of-coolant accident during start-up, all of the high energy fluids are contained within the containment boundary of the containment vessel  400 . The details of the containment vessel  400  can be found in PCT/US13/42070, filed on May 21, 2013, the entirety of which is incorporated herein by reference. Furthermore, the start-up sub-system  500  is at least partially positioned external to the reactor vessel  200  and to the steam generating vessel  300 . Specifically, in the exemplified embodiment while the intake conduit  501  is at least partially positioned within one of the reactor vessel  200  or the steam generating vessel  300  to draw a portion of the primary coolant into the start-up sub-system  500  and the injection nozzle  505  is at least partially positioned within one of the reactor vessel  200  or the steam generating vessel  300  to inject the heated portion of the primary coolant back into one of the reactor vessel  200  or the steam generating vessel  300 , the pump  502  and the heating element  504  are positioned entirety external to the reactor vessel  200  and to the steam generating vessel  300 . 
     The portion of the primary coolant that is introduced into the start-up sub-system  500  flows in a single direction through the start-up sub-system  500  from the intake conduit  501  to the injection nozzle  505 . The intake conduit  501  and the injection conduit  503  can be a single pipe or conduit or can be multiple pipes or conduits that are fluidly coupled together. In some embodiments, the intake conduit  501  and the injection conduit  504  comprise heavy wall pipes that are sized to be between five and seven inches in diameter, and more specifically approximately six inches in diameter. Furthermore, the injection nozzle  505  has a smaller diameter than the diameter of the intake conduit  501  and the injection conduit  504 , and can be between two and four inches, or approximately three inches. However, the invention is not to be so limited and the sizing of the intake conduit  501 , the injection conduit  504  and the injection nozzle  505  can be greater than or less than the noted ranges in other embodiments. 
     In the exemplified embodiment, the pump  502  is a centrifugal pump designed to pump a sufficiently large flow of the primary coolant to develop turbulent conditions in the reactor core. Specifically, in certain embodiments the pump  502  can pump approximately 10% of the normal flow through the primary coolant loop  190  and is able to overcome any pressure differential through the riser pipe  337 . Of course, the invention is not to be so limited and the pump  502  can be any type of pump and can pump any amount of the primary coolant through the start-up sub-system  500  as desired or needed for start-up procedures to be successful. 
     The heating element  504  can be any mechanism that is capable of transferring heat into the portion of the primary coolant that is flowing through the start-up sub-system  500 . The heating element  504  can be a single heater or a bank of heaters. The heating element can take on any form, including being a resistance wire, molybdenum disilicide, etched foil, a heat lamp, PTC ceramic, a heat exchanger or any other element that can provide heat to a liquid that is flowing through a conduit. In certain embodiments the heating element  504  can be powered by electrically powered resistance rods. In other embodiments, the heating element  504  can be powered by tubular heat exchangers supplied with steam by an auxiliary steam boiler. Any mechanism can be used as the heating element  504  so long as the heating element  504  can transfer heat into the primary coolant in order to heat up the portion of the primary coolant that is flowing through the start-up sub-system  500 . 
     In the exemplified embodiment, the intake conduit  501  comprises an inlet  506  that is located within the primary coolant loop  190 . More specifically, in the embodiment of  FIG. 1  the inlet  506  of the intake conduit  501  is positioned at a bottom of the reactor vessel  200 . This may include positioning the inlet  506  of the intake conduit  501  within the bottom flow plenum  228  of the reactor vessel  200 . However, the invention is not to be so limited and the bottom of the reactor vessel  200  may include positioning the inlet  506  of the intake conduit  501  adjacent to the bottom end  225  of the shroud  220 . Furthermore, in other embodiments the inlet  506  of the intake conduit  501  can be located in a central vertical region of the reactor vessel  200  or in a top vertical region of the reactor vessel  200  or within the steam generating vessel  300  as discussed in more detail below with reference to  FIGS. 5A-5C . Positioning the inlet  506  of the intake conduit  501  at the bottom of the reactor vessel  200  ensures that the portion of the primary coolant that is removed from the primary coolant loop and received by the start-up sub-system  500  is the coolest or coldest primary coolant available in the primary coolant loop. Such positioning of the inlet  506  of the intake conduit  501  can reduce start-up time. However, the invention is not to be limited by positioning the inlet  506  of the intake conduit  501  at the bottom of the reactor vessel  200 , and other positions are possible as discussed above and again below with regard to  FIGS. 5A-5C . 
     Specifically,  FIGS. 5A-5C  show different places that the inlet  506  of the intake conduit  501  can be positioned in different embodiments. The positioning of the inlet  506  of the intake conduit  501  illustrated in  FIGS. 5A-5C  are merely exemplary and are not intended to be limiting of the present invention. Therefore, it should be understood that the inlet  506  of the intake conduit  501  can be located at any other desired location along the primary coolant loop. In  FIG. 5A , the inlet  506  of the intake conduit  501  is positioned at the bottom of the reactor vessel  200 . In  FIG. 5B , the inlet  506  of the intake conduit  501  is positioned at the bottom of the steam generating vessel  300  or within the outer flow path  272 ,  372  of the fluid coupling  270  between the steam generating vessel  300  and the reactor vessel  200 . In  FIG. 5C , the inlet  506  of the intake conduit  501  is positioned within the riser pipe  337  or within the inner flow path  271 ,  371  of the fluid coupling  270  between the steam generating vessel  300  and the reactor vessel  200 . The inlet  506  of the intake conduit  501  can also be positioned within the riser pipe  337  upstream of the fluid coupling  270  or at any other desired location within the primary coolant loop  190 . Regardless of its exact positioning, the location of the inlet  506  of the intake conduit  501  is the location from which the portion of the primary coolant is withdrawn for introduction into the start-up sub-system  500 . 
     In certain embodiments, the pump  502  may be fluidly coupled to more than one intake conduit or more than one inlet so that the primary coolant can be drawn from the primary coolant loop  190  and introduced into the start-up sub-system  500  from more than one location simultaneously, or so that an operator can determine the location from which the primary coolant can be taken based on desired applications and start-up time requirements. Specifically, there may be multiple intake conduits that are connected to the injection conduit such that there are valves associated within each intake conduit. One of the intake conduits can have an inlet located at a bottom of the reactor vessel  200  and another one of the intake conduits can have an inlet located at a bottom of the steam generating vessel  300 . Thus, an operator can open one or more of the valves while leaving the other valves closed to determine the location(s) within the primary coolant loop  190  from which the primary coolant will be drawn for introduction into the start-up sub-system  500 . 
     Referring back to  FIG. 1 , regardless of the exact positioning of the inlet  506  of the intake conduit  501 , a portion of the primary coolant is drawn from the primary coolant loop  190  into the intake conduit  501  of the start-up sub-system  500  when it is desired to start the nuclear steam supply system  100 . More specifically, in the exemplified embodiment the primary coolant is drawn from the primary coolant loop  190  by the operation of the pump  502 . Specifically, in the exemplified embodiment when the pump  502  is turned on, the portion of the primary coolant is drawn from the primary coolant loop  190  and into the start-up sub-system  500 . When the pump is turned off, none of the primary coolant is drawn from the primary coolant loop  190  and into the start-up sub-system  500 . 
     Although the use of the pump  502  for drawing the portion of the primary coolant into the start-up sub-system  500  is described above, the invention is not to be so limited. In certain other embodiments, the start-up sub-system  500  may include a valve positioned at some point along the intake conduit  501 . In some embodiments, the start-up sub-system  500  may also or alternatively include another valve positioned at some point along the injection conduit  503 . The use of valves enables the start-up sub-system to be cut off from the reactor vessel  200  and the steam generating vessel  300  from a fluid flow standpoint. Specifically, by closing the valves the primary coolant will be unable to enter into the start-up sub-system  500 , and the primary coolant loop will form a closed-loop path. One embodiment of the use of valves in the start-up sub system  500  and the connection/placement of those valves will be described in more detail below with reference to  FIG. 7 . 
     Where valves are used, the valves can be alterable between an open state whereby a portion of the primary coolant flows from the primary coolant loop and into the start-up sub-system  500  and a closed state whereby the primary coolant is prevented from flowing into the start-up sub-system  500 . In some embodiments, both the pump  502  and one or more valves may be used in conjunction with one another to facilitate the flow of the portion of the primary coolant into the start-up sub-system  500 . 
     Still referring to  FIG. 1 , when the pump  502  is operating (and any valves positioned between the reactor vessel  200  and the start-up sub-system  500  and between the steam generating vessel  300  and the start-up sub-system  500  are open), the portion of the primary coolant flows from the primary coolant loop  190  and into the intake conduit  501  through the inlet  506 . In  FIG. 1 , this portion of the primary coolant is taken from the bottom of the reactor vessel  200  where the primary coolant is at its coldest. However, as discussed above the primary coolant can be taken from any location along the primary coolant loop  190 , including from within the steam generating vessel  300  and within the riser pipe  337 . The portion of the primary coolant flows through the intake conduit  501 , passes through the pump  502  and flows into the injection conduit  503  whereby the portion of the primary coolant passes through the heating element  504 . As the portion of the primary coolant passes through or by the heating element  504 , the portion of the primary coolant is heated and becomes a heated portion of the primary coolant. The heated portion of the primary coolant then continues to flow along the injection conduit  503  and into the injection nozzle  505  where the heated portion of the primary coolant is injected back into the primary coolant loop  190 . 
     Referring to  FIGS. 1 and 6  concurrently, the injection of the heated portion of the primary coolant into the primary coolant loop  190  will be discussed in more detail. In the exemplified embodiment, the injection nozzle  505  is positioned within the riser pipe  337  of the steam generating vessel  300 . Of course, the invention is not to be so limited and the injection nozzle  505  can be positioned at other locations within either the reactor vessel  200  or the steam generating vessel  300  as desired. Specifically, the injection conduit  505  can be located within the riser column  224  of the reactor vessel  200 , within the downcomer  222  of the reactor vessel  200 , within the pressurizer  380  of the steam generating vessel  300  or at any other desired location. 
     In the exemplified embodiment the injection nozzle  505  is centrally located within the riser pipe  337  so as to be circumferentially equidistant from the inner surface of the riser pipe  337 . Furthermore, the injection nozzle  505  faces in an upwards direction so that the heated portion of the primary coolant injected from the injection nozzle  505  is made to flow in a vertical upward direction. In the exemplified embodiment, the injection conduit  503  enters into the steam generating vessel  300  at the bottom-most tubesheet  333 A elevation, and the injection nozzle  505  is positioned near or at the elevation of the bottom-most tubesheet  333 A. More specifically, the injection conduit  503  extends horizontally into the riser  337  just below the bottom-most tubesheet  333 A, an elbow connects the injection conduit  503  to the injection nozzle  505 , and the injection nozzle  505  extends vertically from the elbow within the riser pipe  337 . Specifically, the injection nozzle  505  in one embodiment is located so as to inject the heated portion of the primary coolant just above the bottom-most tubesheet  333 A. Thus, in the exemplified embodiment the injection nozzle  505  is located at and injects the heated portion of the primary coolant to a location above the bottom plenum  338  of the steam generating vessel  300 . Of course, the invention is not to be so limited in all embodiments and as discussed above the location at which the heated portion of the primary coolant is injected can be modified as desired. 
     In the exemplified embodiment, the injection nozzle  505  of the start-up sub-system  500  injects a heated portion of the primary coolant (indicated with arrows as  511 ) into the riser pipe  337  in a first vertical direction. At the time of the initial injection of the heated portion of the primary coolant  511  into the riser pipe  337 , the primary coolant (indicated with arrows as  512 ) is positioned in the primary coolant loop  190  including within the riser pipe  337  but is static or non-moving. After the start-up sub-system  500  begins injecting the heated portion of the primary coolant  511  into the riser pipe  337  in the first vertical direction, the entire body of the primary coolant  512  within the primary coolant loop  190  begins to flow in the first vertical direction due to the venturi effect, as discussed below. In certain embodiments, once the primary coolant  512  within the primary coolant loop  190  begins to flow, it flows at a first flow rate. Furthermore, the heated portion of the primary coolant  511  is injected at a second flow rate, the second flow rate being greater than the first flow rate. 
     In the exemplified embodiment, the injection of the heated portion of the primary coolant  511  creates a venturi effect in the closed loop path  190 , and more specifically in the riser pipe  337 . Specifically, introducing a jet of high velocity heated primary coolant  511  into the riser pipe  337  creates a venturi effect in the riser pipe  337  that creates a low pressure in the vicinity of the injection nozzle  505 . This low pressure pulls the primary coolant  512  from the bottom of the riser pipe  337  upwardly in the direction of the flow of the heated portion of the primary coolant  511  to the top of the steam generating vessel  300  and facilitates the flow of the primary coolant through the primary coolant loop  190 . Thus, the injection of the heated portion of the primary coolant  511  from the start-up sub-system  500  initiates start-up of the nuclear steam supply system  100  by facilitating the flow of the primary coolant  512  through the primary coolant loop  190 . Specifically, due to the venturi effect the mixture of the heated portion of the primary coolant  511  and the primary coolant  512  flows upwardly within the riser pipe  337 , and due to gravity the mixed primary coolant  511 / 512  flows downwardly through the tubes  332  in the steam generating vessel  300  and downwardly through the downcomer  222  in the reactor vessel  200  due to thermosiphon flow. When the heated portion of the primary coolant  511  mixes with the primary coolant  512  in the riser pipe  337 , this heated mixture expands and becomes less dense and more buoyant than the cooler primary coolant below it in the primary coolant loop. Convection moves this heated liquid upwards in the primary coolant loop as it is simultaneously replaced by cooler liquid returning by gravity. 
     Once the primary coolant gets heated up to the no-load operating temperature, the flow of the primary coolant in the primary coolant loop  190  is continuous without the use of an external pump. The start-up sub-system  500  and the pump  502  associated therewith merely operate to heat up the temperature of the primary coolant and to begin the flow of the primary coolant in the primary coolant loop  190  and to heat up the primary coolant in the primary coolant loop  190 . However, the start-up sub-system  500  can be disconnected from the primary coolant loop  190  once no-load operating temperature of the primary coolant is reached and thermosiphon flow of the primary coolant in the primary coolant loop is achieved. 
     As discussed above, as the primary coolant in the primary coolant loop  190  heats up, the primary coolant expands. Thus, in certain embodiments the system  100  may be fluidly coupled to a chemical and volume control system which can remove the additional volume of the primary coolant as needed. Furthermore, such a chemical and volume control system can also remove dissolved gases in the primary coolant. Thus, the chemical and volume control system can be used to control the liquid level by draining and adding additional primary coolant into the primary coolant loop  190  as needed. In certain embodiments, the chemical and volume control system may be capable of adding and/or removing the primary coolant at a desired rate, such as at a rate of sixty gallons per minute in some embodiments. When used, the chemical and volume control system can be fluidly coupled to the nuclear steam supply system  100  at any desired location along the primary coolant loop  190 . 
     During start-up of the nuclear steam supply system  100 , the start-up sub-system  500  continues to take a portion of the primary coolant from the primary coolant loop  190 , heat the portion of the primary coolant to form a heated portion of the primary coolant, and inject the heated portion of the primary coolant into the primary coolant loop  190 . The flow of the heated portion of the primary coolant into the primary coolant loop  190  serves to heat up the primary coolant (which is actually a mixture of original primary coolant and the heated portion of the primary coolant) during the start-up process. Once the primary coolant in the primary coolant loop  190  reaches the no load operating temperature, the pump  502  is turned off or the start-up sub-system  500  is otherwise isolated/disconnected/valved off from the primary coolant loop  190 . In certain embodiments, only after the primary coolant reaches the no load operating temperature do the control rods begin to be withdrawn. 
     During the start-up procedures discussed above, the secondary coolant (i.e., feedwater) continues to be circulated on the shellside  305  of the steam generating vessel  300 . Thus, as the primary coolant heats up due to the start-up procedures and begins to flow through the primary coolant loop  190  including through the tubes  332  of the steam generating vessel, the secondary coolant flowing through the shellside  305  of the steam generating vessel  300  boils to produce steam. This steam is held inside of the steam generating vessel  300  until a desired pressure is reached. Once the desired pressure is reached, a steam isolation valve (i.e., a valve between the steam generating vessel  300  and the turbine  900 ) is opened and a portion of the steam is sent to the turbine  900  for turbine heat-up and the remainder of the steam is sent to the condenser in a bypass operation. 
     In certain embodiments, the steam is sent to the turbine  900  for power production only when all of the control rods are fully withdrawn and the nuclear steam supply system  100  is at full power. Furthermore, as noted above the control rods are only fully withdrawn in some embodiments after the primary coolant reaches the no-load operating temperature. Thus, in those embodiments, during the start-up process no steam is sent to the turbine  900  for power production (although it may be sent to the turbine  900  for turbine heat-up). Power production begins in such embodiments only when the start-up process is complete and the primary coolant flows through the primary coolant loop  190  passively without the operation of a pump. 
     In addition to heating the primary coolant within the primary coolant loop  190 , the start-up sub-system  500  can also be used for draining the primary coolant from the primary coolant loop  190  if the need arises. In certain embodiments, such as the embodiment depicted in  FIGS. 1 and 5A  whereby the inlet  506  of the intake conduit  501  is positioned at a bottom of the reactor vessel  300 , this can include draining primary coolant from the reactor vessel  200 . Furthermore, the start-up supply system  500  can be used to remove debris that may accumulate at the bottom of the reactor vessel  200  or at the bottom of the steam generating vessel  300 , depending on the location of the inlet  506  of the intake conduit  501 . 
     In certain embodiments, as the primary coolant is being heated by injecting the heated portion of the primary coolant into the primary coolant loop  190  using the start-up sub-system  500 , pressure in the primary coolant loop  190  is increased in stages by introducing high pressure inert gas into the pressurizer  380  volume. The two-phase (inert gas—water vapor with liquid water) equilibrium maintains the liquid level in the pressurizer  380  volume. The staged increase in pressure follows the typical heat-up curve as shown in  FIG. 8 , which is based on a brittle toughness curve specific to the primary coolant loop  190 , reactor vessel  200  and steam generating vessel  300  material of construction. 
     Referring now to  FIG. 7 , the interconnection between the start-up sub-system  500  and the reactor vessel  200  will be described. Although  FIG. 7  only depicts the connection between the start-up sub-system  500  and the reactor vessel  200 , it should be appreciated that an identical connection can be used for connecting the start-up sub-system  500  to the steam generating vessel  300 . Stated another way,  FIG. 7  illustrates the manner in which the intake conduit  501  is connected to the reactor vessel  200  in a manner that prevents or eliminates or substantially reduces the likelihood of a loss-of-coolant accident. Of course, certain embodiments may omit the valves discussed below, and in certain embodiments the connection between the start-up sub-system  500  and the reactor vessel  200  and the steam generating vessel  300  may be achieved in other manners than that discussed directly below. 
     As illustrated in  FIG. 7 , the intake conduit  501  comprises a concentric pipe construction including an inner pipe  508  that carries the portion of the primary fluid from the primary coolant loop  190  and an outer pipe  509  that concentrically surrounds the inner pipe  508 . The outer pipe serves as a redundant pressure boundary to contain the portion of the primary coolant within the piping in case the inner pipe  508  were to develop a leak. Two independent pressure enclosures (i.e., the inner pipe  508  and the outer pipe  509 ) serve to render the potential of a pipe break loss-of-coolant accident non-credible. 
     The inner pipe  508  is directly connected to a valve  600 . Furthermore, the valve  600  is enclosed in a pressure vessel  602  which encloses the entirety of the valve  600  except for the valve stem  601 . Thus, the valve stem  601  extends from the pressure vessel  602  so that manual opening and closing of the valve  600  is still possible while the pressure vessel  602  remains enclosing the valve  600 . The inner pipe  509  connects to the valve  600  within the pressure vessel  602 . Thus, the pressure vessel  602  prevents any loss-of-coolant accident event initiating at the weldment between the valve  600  and the inner/outer pipe  508 ,  509  arrangement. Specifically, if there was a breakage at the weldment between the valve  600  and the inner pipe  508 , any coolant leakage would occur within the pressure vessel  602  and would not escape into the environment or elsewhere where it could cause harm. 
     Furthermore, the reactor vessel  200  comprises a forging  290  extending from the sidewall thereof. The valve  600  is directly welded to the forging  290 . This eliminates the possibility of pipe breakage between the reactor vessel  200  and the valve  600 . Furthermore, the connection between the forging  290  and the valve  600  occurs within the pressure vessel  602  so that a break at the weldment between the forging  290  and the valve  600  would result in coolant leakage occurring within the pressure vessel  602 . 
     Unless otherwise specified, the components described herein may generally be formed of a suitable material appropriate for the intended application and service conditions. All conduits and piping are generally formed from nuclear industry standard piping. Components exposed to a corrosive or wetted environment may be made of a corrosion resistant metal (e.g. stainless steel, galvanized steel, aluminum, etc.) or coated for corrosion protection. 
     While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques. It is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention. Thus, the spirit and scope of the invention should be construed broadly as set forth in the appended claims.