Patent Publication Number: US-2023162879-A1

Title: Stress relieving attachment of tube to tubesheet, such as in a pressure vessel shell of a nuclear reactor power system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/282,053, filed Nov. 22, 2021, and titled “STRESS RELIEVING ATTACHMENT OF TUBE TO TUBESHEET IN A PRESSURE VESSEL SHELL OF A NUCLEAR POWER SYSTEM,” which is incorporated herein by reference in its entirety. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under Contract #DE-NE-000-8928 awarded by the Department of Energy. The government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present technology is related to steam generator systems including tubesheet assemblies for use in, for example, nuclear reactor power systems. More particularly, the present technology is related to stress-relieving attachments for attaching a tubesheet to a reactor vessel. 
     BACKGROUND 
     Nuclear reactor systems often include one or more steam generators positioned within a nuclear reactor vessel. The reactor vessel houses a reactor core and a primary coolant that absorbs heat produced from a nuclear reaction (e.g., a fission reaction) within the reactor core. Such a steam generator can include multiple tubes (e.g., helical tubes) within the reactor vessel that extend between a feedwater header and a steam header. Secondary coolant (e.g., water) enters the tubes at the feedwater header, rises through the tubes and converts to vapor (e.g., steam) as the secondary coolant absorbs heat from the primary coolant, and exits the tubes at the steam header for use in a power conversion system. The tubes can be connected to a tubesheet, such as a perforated plate, at and/or proximate to the feedwater header and/or the steam header (e.g., via tube-to-tubesheet (TTS) welds). The tubesheets can be integral with or attached to the reactor vessel. 
     Large stresses can develop locally in the tubesheets and/or in the tube-to-tubesheet (TTS) welds due to incompatible motion under pressure and thermal loading of the tubesheets and the reactor vessel caused by the differing geometries thereof. As the nuclear reactor system undergoes transients, including startups and shutdowns, the stresses in the tubesheets can be cyclic, which can lead to fatigue and premature decommissioning of the steam generator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present technology can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Instead, emphasis is placed on clearly illustrating the principles of the present technology. 
         FIG.  1    is a partially schematic, partially cross-sectional view of a nuclear reactor system configured in accordance with embodiments of the present technology. 
         FIG.  2 A  is a cross-sectional side view of a nuclear reactor system including a steam generator system configured in accordance with embodiments of the present technology. 
         FIG.  2 B  is an enlarged side cross-sectional view of an upper tubesheet assembly of the steam generator system of  FIG.  2 A  in accordance with embodiments of the present technology. 
         FIG.  3 A  is an enlarged cross-sectional side view of the nuclear reactor system and the steam generator system of  FIG.  2 A  configured in accordance with additional embodiments of the present technology. 
         FIG.  3 B  is an enlarged isometric view of the steam generator system of  FIG.  3 A  showing a lower tubesheet configured in accordance with embodiments of the present technology. 
         FIG.  4 A  is an enlarged cross-sectional side view of the nuclear reactor system and the steam generator system of  FIG.  2 A  configured in accordance with additional embodiments of the present technology. 
         FIG.  4 B  is an enlarged cross-sectional side view of the steam generator system of  FIG.  4 A  showing a lower tubesheet assembly in accordance with embodiments of the present technology. 
         FIGS.  5 A- 5 D  are an isometric front view, a rear view, a cross-sectional side view, and a cross-sectional isometric view, respectively, of a portion of a nuclear reactor system configured in accordance with embodiments of the present technology. 
         FIGS.  6 A- 6 C  are cross-sectional side views of a connection portion of a steam generator system of the nuclear reactor system of  FIGS.  5 A- 5 D  illustrating different profiles for an outer surface of the connection portion in accordance with embodiments of the present technology. 
         FIGS.  7 A- 7 C  are a cross-sectional front view, a cross-sectional rear view, and a cross-sectional side view, respectively, of the nuclear reactor system of  FIGS.  5 A- 5 D  including a connection portion of the steam generator system configured in accordance with additional embodiments of the present technology. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure are directed generally toward steam generator systems including tubesheet assemblies, such as for use in nuclear reactor power systems, and associated devices and methods. More particularly, some aspects of the present disclosure are directed toward stress-relieving attachments for attaching a tubesheet to a reactor vessel within a nuclear reactor power system. In several of the embodiments described below, for example, a representative steam generator system can be installed in a nuclear reactor vessel (e.g., a reactor pressure vessel shell) positioned to house a primary coolant. The steam generator system can include a tubesheet assembly defining a plenum and comprising a tubesheet and a flexible connection portion coupling the tubesheet to the reactor vessel. The tubesheet can include a plurality of perforations fluidly coupled to the plenum. The steam generator system can further comprise a plurality of heat transfer tubes fluidly coupled to the perforations and configured to receive a flow of a secondary coolant. The connection portion can be more flexible than the tubesheet and the reactor vessel to reduce stresses on the tubesheet and the connections (e.g., tube-to-tubesheet (TTS) welds) between the heat transfer tubes and the tubesheet during operation of the nuclear reactor system. For example, the connection portion can be thinner than both the tubesheet and the adjoining reactor vessel. 
     Accordingly, in some aspects of the present technology, the connection portion can mitigate or reduce stresses (e.g., discontinuity stresses and/or fatigue) in the tubesheet and/or in the associated connections (e.g., tube-to-tubesheet (TTS) welds) between the tubesheet and the corresponding heat transfer tubes by functioning as a flexible connection between the reactor vessel and the tubesheet. Such a flexible connection can decouple the incompatible deformation between the differing geometries of the tubesheet (e.g., a perforated flat plate) and the reactor vessel (e.g., a cylindrical vessel) during cyclic loads. In some embodiments, the cyclic fatigue life of the tubesheet and associated TTS welds can be increased by one order of magnitude, two orders of magnitude, or more—increasing the lifespan of the steam generator system. 
     Certain details are set forth in the following description and in  FIGS.  1 - 7 C  to provide a thorough understanding of various embodiments of the present technology. In other instances, well-known structures, materials, operations, and/or systems often associated with tubesheet assemblies, tubesheets, nuclear reactor power conversion systems, heat transfer tubes, steam generators, and the like, are not shown or described in detail in the following disclosure to avoid unnecessarily obscuring the description of the various embodiments of the technology. Those of ordinary skill in the art will recognize, however, that the present technology can be practiced without one or more of the details set forth herein, and/or with other structures, methods, components, and so forth. 
     The terminology used below is to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain examples of embodiments of the technology. Indeed, certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section. 
     The accompanying Figures depict embodiments of the present technology and are not intended to limit its scope unless expressly indicated. The sizes of various depicted elements are not necessarily drawn to scale, and these various elements may be enlarged to improve legibility. Component details may be abstracted in the Figures to exclude details such as position of components and certain precise connections between such components when such details are unnecessary for a complete understanding of how to make and use the present technology. Many of the details, dimensions, angles and other features shown in the Figures are merely illustrative of particular embodiments of the disclosure. Accordingly, other embodiments can have other details, dimensions, angles and features without departing from the present technology. In addition, those of ordinary skill in the art will appreciate that further embodiments of the present technology can be practiced without several of the details described below. 
     To the extent any materials incorporated herein by reference conflict with the present disclosure, the present disclosure controls. The headings provided herein are for convenience only and should not be construed as limiting the subject matter disclosed. 
     I. Select Embodiments of Nuclear Reactor Power Conversion Systems 
       FIG.  1    is a partially schematic, partially cross-sectional view of a nuclear reactor system  100  configured in accordance with embodiments of the present technology. The system  100  can include a power module  102  having a reactor core  104  in which a controlled nuclear reaction takes place. Accordingly, the reactor core  104  can include one or more fuel assemblies  101 . The fuel assemblies  101  can include fissile and/or other suitable materials. Heat from the reaction generates steam at one or more steam generator systems  130 , which direct the steam to a power conversion system  140 . The power conversion system  140  generates electrical power, and/or provides other useful outputs. A sensor system  150  is used to monitor the operation of the power module  102  and/or other system components. The data obtained from the sensor system  150  can be used in real time to control the power module  102 , and/or can be used to update the design of the power module  102  and/or other system components. 
     The power module  102  includes a containment vessel  110  (e.g., a radiation shield vessel, a radiation shield container, and/or the like) that houses/encloses a reactor vessel  120  (e.g., a reactor pressure vessel, a reactor pressure shell, a reactor pressure container and/or the like), which in turn houses the reactor core  104 . The containment vessel  110  can be housed in a power module bay  156 . The power module bay  156  can contain a cooling pool  103  filled with water and/or another suitable cooling liquid. The bulk of the power module  102  can be positioned below a surface  105  of the cooling pool  103 . Accordingly, the cooling pool  103  can operate as a thermal sink, for example, in the event of a system malfunction. 
     A volume between the reactor vessel  120  and the containment vessel  110  can be partially or completely evacuated to reduce heat transfer from the reactor vessel  120  to the surrounding environment (e.g., to the cooling pool  103 ). However, in other embodiments the volume between the reactor vessel  120  and the containment vessel  110  can be at least partially filled with a gas and/or a liquid that increases heat transfer between the reactor vessel  120  and the containment vessel  110 . 
     Within the reactor vessel  120 , a primary coolant  107  conveys heat from the reactor core  104  to the steam generator system  130 . For example, as illustrated by arrows located within the reactor vessel  120 , the primary coolant  107  is heated at the reactor core  104  toward the bottom of the reactor vessel  120 . The heated primary coolant  107  (e.g., water with or without additives) rises from the reactor core  104  through a core shroud  106  and to a riser tube  108 . The hot, buoyant primary coolant  107  continues to rise through the riser tube  108 , then exits the riser tube  108  and passes downwardly through the steam generator system  130 . The steam generator system  130  includes a multitude of conduits  132  (e.g., tubes, heat transfer tubes) that are arranged circumferentially around the riser tube  108 , for example, in a helical pattern, as is shown schematically in  FIG.  1   . The descending primary coolant  107  transfers heat to a secondary coolant (e.g., water) within the conduits  132 , and descends to the bottom of the reactor vessel  120  where the cycle begins again. The cycle can be driven by the changes in the buoyancy of the primary coolant  107 , thus reducing or eliminating the need for pumps to move the primary coolant  107 . 
     The steam generator system  130  can include a lower header assembly  131  (e.g., a lower plena assembly, a lower tubesheet assembly, a feedwater header assembly, a first header assembly, a first tubesheet assembly, and/or the like) at which the incoming secondary coolant enters the steam generator conduits  132 . The secondary coolant rises through the conduits  132 , converts to vapor (e.g., steam), and is collected at an upper header assembly  133  (e.g., an upper plena assembly, an upper tubesheet assembly, a steam header assembly, a second header assembly, a second tubesheet assembly and/or the like). The vapor exits the upper header assembly  133  and is directed to the power conversion system  140 . 
     The power conversion system  140  can include one or more steam valves  142  that regulate the passage of high pressure, high temperature steam from the steam generator system  130  to a steam turbine  143 . The steam turbine  143  converts the thermal energy of the steam to electricity via a generator  144 . The low-pressure steam exiting the steam turbine  143  is condensed at a condenser  145 , and then directed (e.g., via a pump  146 ) to one or more feedwater valves  141 . The feedwater valves  141  control the rate at which the feedwater re-enters the steam generator system  130  via the lower header assembly  131 . 
     The power module  102  includes multiple control systems and associated sensors. For example, the power module  102  can include a hollow cylindrical reflector  109  that directs neutrons back into the reactor core  104  to further the nuclear reaction taking place therein. Control rods  113  are used to modulate the nuclear reaction, and are driven via fuel rod drivers  115 . The pressure within the reactor vessel  120  can be controlled via a pressurizer plate  117  (which can also serve to direct the primary coolant  107  downwardly through the steam generator system  130 ) by controlling the pressure in a pressurizing volume  119  positioned above the pressurizer plate  117 . In some embodiments, the upper header assembly  133  can be at least partially integrated into the pressurizer plate  117 . 
     The sensor system  150  can include one or more sensors  151  positioned at a variety of locations within the power module  102  and/or elsewhere, for example, to identify operating parameter values and/or changes in parameter values. The data collected by the sensor system  150  can then be used to control the operation of the system  100 , and/or to generate design changes for the system  100 . For sensors positioned within the containment vessel  110 , a sensor link  152  directs data from the sensors to a flange  153  (at which the sensor link  152  exits the containment vessel  110 ) and directs data to a sensor junction box  154 . From there, the sensor data can be routed to one or more controllers and/or other data systems via a data bus  155 . 
     II. Select Embodiments of Steam Generator Systems and Tubesheet Assemblies 
       FIGS.  2 A- 5    illustrate various steam generator systems or portions thereof configured in accordance with embodiments of the present technology that can be used within the nuclear reactor system  100  and/or other nuclear reactor systems. For example, the various steam generator systems can be used in addition or alternatively to the steam generator system  130  described in detail above with reference to  FIG.  1   , and can function in a similar or identical manner. In some embodiments, the steam generator systems can include some features that are at least generally similar in structure and function, or identical in structure and function, to those of the steam generator systems disclosed in (i) U.S. Pat. No. 9,997,262, titled “INTEGRAL REACTOR PRESSURE VESSEL TUBE SHEET,” and filed Apr. 24, 2014 and/or (ii) U.S. Pat. No. 10,685,752, titled “STEAM GENERATOR WITH INCLINED TUBE SHEET,” and filed Feb. 10, 2015, each of is incorporated herein by reference in its entirety. 
       FIG.  2 A  is a cross-sectional side view of a nuclear reactor system  200  (e.g., a nuclear power system, a nuclear power conversion system, a nuclear reactor steam generation system, and/or the like) including a steam generator system  230  configured in accordance with embodiments of the present technology. The nuclear reactor system  200  can include a reactor vessel  220  configured to house (i) a reactor core (not shown in  FIG.  2 A ) that generates heat, (ii) a primary coolant that absorbs the heat from the nuclear reactor, and (iii) a riser tube  208 . The reactor vessel  220  can have a cylindrical shape. In the illustrated embodiment, the steam generator system  230  includes a plurality of heat transfer tubes  232  (e.g., conduits) positioned within the reactor vessel  220  and arranged circumferentially around the riser tube  208 . The tubes  232  can extend helically about the riser tube  208 . Individual ones of the tubes  232  have a lower portion fluidly coupled to a lower tubesheet assembly  231  (e.g., a lower header assembly) and an upper portion fluidly coupled to an upper tubesheet assembly  233  (e.g., an upper header assembly). The lower tubesheet assembly  231  and the upper tubesheet assembly  233  can be similar or identical in structure and/or function to the lower header assembly  131  and the upper header assembly, respectively, described in detail above with reference to  FIG.  1   . 
     In some embodiments, the steam generator system  230  includes multiple ones (e.g., four) of the lower tubesheet assemblies  231  and/or multiple ones (e.g., four) of the upper tubesheet assemblies  233  positioned circumferentially about the reactor vessel  220 . Pairs of the lower and upper tubesheet assemblies  231 ,  233  can be fluidly coupled to a set of the tubes  232  to define an individual steam generator circuit. The lower and upper tubesheet assemblies  231 ,  233  can be coupled to or integral with the reactor vessel  220  and are positioned to provide a fluid flow path from the tubes  232  to/from the reactor vessel  220  to/from an external power conversion system (e.g., the power conversion system  140  of  FIG.  1   ). In some embodiments, the lower and upper tubesheet assemblies  231 ,  233  are entirely contained within a containment vessel (e.g., the containment vessel  110  of  FIG.  1   ) that surrounds the reactor vessel  220 . 
     In operation, the primary coolant within the reactor vessel  220  is heated and rises through the riser tube  208  past the tubes  232  before descending past the tubes  232  outside the riser tube  208 . The tubes  232  receive a secondary coolant (e.g., water) via the lower tubesheet assemblies  231 . The secondary coolant rises through the tubes  232  and heat is thermally transferred to the secondary coolant from the primary coolant such that the secondary coolant become super-heated vapor (e.g., steam). The secondary coolant in the steam generator system  230  can be isolated from the primary coolant in reactor vessel  220  such that they are not allowed to mix or come into direct contact with one another. The vaporized secondary coolant exits the tubes  232  into the upper tubesheet assemblies  233  for transfer to a power conversion system. After the heat from the secondary coolant is utilized by the power conversion system, the secondary coolant can be returned to the steam generator system  230  via the lower tubesheet assemblies  231 . 
     The lower tubesheet assemblies  231  can be identical and can each include a body  234  (e.g., a wall, a body portion, a wall portion, and/or the like) integrally formed with or attached to the reactor vessel  220  and defining a plenum  235 . The body  234  can include a tubesheet  236  (e.g., a perforated plate) and/or the tubesheet  236  can be a separate component attached to the body  234 . The lower portions of the tubes  232  can be coupled (e.g., welded, affixed) to the tubesheet  236  which is positioned to route the secondary coolant from the plenum  235  to the tubes  232 . In the illustrated embodiment, the tubesheet  236  is positioned within the annular region between the reactor vessel  220  and the riser tube  208  and is oriented in a horizontal or radial position. That is, the tubesheet  236  can extend along an axis X that extends orthogonal to a longitudinal axis Y of the reactor vessel  220 . The body  234  can further include/define an inlet port  237  (e.g., a feed nozzle) that can be connected to a feed pipe for receiving the secondary coolant and that is positioned to direct the secondary coolant from the feed pipe into the plenum  235 . In some embodiments, the lower tubesheet assembly  231  further includes a removable cover plate  238  that can be coupled (e.g., bolted) to the body  234  to enclose the plenum  235 . The removable cover plate  238  can be removed from and/or installed on the body  234  during one or more operations such as maintenance, inspection, and/or installation. 
       FIG.  2 B  is an enlarged side cross-sectional view of one of the upper tubesheet assemblies  233  configured in accordance with embodiments of the present technology. Referring to  FIGS.  2 A and  2 B , the upper tubesheet assemblies  233  can be identical and can include some features generally similar or identical to those of the lower tubesheet assemblies  231 . For example, the upper tubesheet assemblies  233  can each include a body  244  integrally formed with or attached to the reactor vessel  220  and defining or bounding a plenum  245 . The body  244  can define or include a tubesheet  246  (e.g., a perforated plate) and/or the tubesheet  246  can be a separate component attached to the body  244 . In some embodiments, the tubesheet  246  comprises a portion of a pressurizer plate  217  of the nuclear reactor system  200 . The upper portions of the tubes  232  can be coupled (e.g., welded, affixed) to the tubesheet  246 , which is positioned to route the secondary coolant from the tubes  232  to the plenum  245 . In the illustrated embodiment, the tubesheet  246  is oriented in a horizontal or radial position. That is, the tubesheet  246  can extend along the X axis orthogonal to the longitudinal axis Y of the reactor vessel  220 . The body  244  can further include/define an outlet port  247  (e.g., a steam nozzle) that can be connected to a steam pipe  241  ( FIG.  2 B ) for receiving the secondary coolant and that is positioned to direct the secondary coolant from the plenum  245  to the steam pipe. In some embodiments, the upper tubesheet assembly  233  further includes a removable cover plate  248  that can be coupled (e.g., bolted) to the body  244  to enclose the plenum  245 . The removable cover plate  248  can be removed from and/or installed on the body  244  during one or more operations such as maintenance, inspection, and/or installation. Referring to  FIG.  2 B , the tubesheet  236  can be a generally flat plate including a plurality of perforations  239  (e.g., through-holes) arranged in rows. The perforations  239  extend through the tubesheet  236  and can be oriented parallel to the axis Y ( FIG.  2 A ). The perforations  239  can be coupled to corresponding ones of the tubes  232  (e.g., via welding). 
     In some embodiments, the lower tubesheet assemblies  231  and/or the upper tubesheet assemblies  233  can have different configurations. For example,  FIG.  3 A  is an enlarged cross-sectional side view of the nuclear reactor system  200  and the steam generator system  230  configured in accordance with additional embodiments of the present technology. In the illustrated embodiment, the body  234  is attached to or integrally formed with the reactor vessel  220  and is positioned generally radially outside the reactor vessel  220 . The tubesheet  236  is positioned at and/or proximate to the wall of the reactor vessel  220  (e.g., substantially outside the annular region between the reactor vessel  220  and the riser tube  208 ) and is oriented in a vertical position. That is, the tubesheet  236  can extend generally parallel to the longitudinal axis Y of the reactor vessel  220 . 
       FIG.  3 B  is an enlarged isometric view of the nuclear reactor system  200  showing one of the lower tubesheet assemblies  231  of  FIG.  3 A  in accordance with embodiments of the present technology. In the illustrated embodiment, the tubesheet  236  is a generally circular flat plate including a plurality of perforations  339  (e.g., through-holes) arranged in rows. The perforations  439  extend through the tubesheet  236  and can be oriented parallel to the axis X. The perforations  339  in individual ones of the rows can be coupled to corresponding ones of the tubes  232  in a vertical group of the tubes  232 . Only some of the tubes  232  are shown in  FIG.  3 B  for clarity. 
       FIG.  4 A  is an enlarged cross-sectional side view of the nuclear reactor system  200  and the steam generator system  230  configured in accordance with additional embodiments of the present technology.  FIG.  4 B  is an enlarged cross-sectional side view of the nuclear reactor system  200  showing one of the lower tubesheet assemblies  231  of  FIG.  4 A  in accordance with embodiments of the present technology. Referring to  FIGS.  4 A and  4 B , in the illustrated embodiment the body  234  is attached to or integrally formed with the reactor vessel  220  and is positioned partially within and partially outside the reactor vessel  220 . The tubesheet  236  similarly extends from within the reactor vessel  220  to outside the reactor vessel  220  and is angled (e.g., inclined) relative to the wall of the reactor vessel  220 . That is, the tubesheet  236  can extend at a non-zero angle relative to the longitudinal axis Y of the reactor vessel  220  and the orthogonal axis X. In some embodiments, the tubesheet  236  can be angled less than about 60°, between about 10°-50°, between about 15°-45°, between about 20°-40°, between about 25°-35°, and/or about 30° relative to the longitudinal axis Y of the wall of the reactor vessel  220 . As best seen in  FIG.  3 B , the tubesheet  236  can be a generally circular flat plate including a plurality of perforations  439  (e.g., through-holes). The perforations  439  extend through the tubesheet  236  and are angled (e.g., inclined) relative to the axes X, Y. The perforations  339  can be coupled to corresponding ones of the tubes  232 . 
     Referring to  FIGS.  2 A- 4 B , the tubesheets  236  of the lower tubesheet assemblies  231  and the tubesheets  246  of the upper tubesheet assemblies  233  can be perforated flat plates that are integrally attached or directly affixed to the reactor vessel  220 , which can have a cylindrical shape. Large stresses can develop locally in the tubesheets  236 ,  246  and/or in the connections (e.g., tube-to-tubesheet (TTS) welds) between the tubes  232  and the tubesheets  236 ,  246  due to incompatible motion under pressure and thermal loading of the steam generator system  230  and the reactor vessel  220  caused by the different geometries thereof. That is, the reactor vessel  220  can expand/contract at a different rate under thermal and pressure loads than the tubesheets  236 ,  246  because of the differing geometries of these components. This can lead to incompatible deformation at the interface between the tubesheets  236 ,  246  and the reactor vessel  220 —which can cause discontinuity stresses and fatigue at the tubesheets  236 ,  246  and/or in the connections (e.g., tube-to-tubesheet (TTS) welds) between the tubes  232  and the tubesheets  236 ,  246  during cyclic loads. For example, as the nuclear reactor system  200  undergoes transients, including startups and shutdowns, the stresses in the tubesheets  236 ,  246  can be cyclic, which can lead to fatigue and premature decommissioning of the steam generator system  230 . 
     In some aspects of the present technology, a tubesheet assembly (e.g., a header assembly) configured in accordance with the present technology can mitigate or reduce the stresses in a tubesheet and associated TTS welds by introducing a more flexible connection between the reactor vessel and the tubesheet. Such a flexible connection can decouple the incompatible deformation between the differing geometries of the tubesheet and the reactor vessel. For example, in some embodiments a tubesheet assembly can include a flexible section or portion between the reactor vessel and the tubesheet that is thinner than both the reactor vessel and the tubesheet. This can provide stress relief on the tubesheet by being more flexible than the parts it connects (e.g., the tubesheet and the reactor vessel). 
     For example,  FIGS.  5 A- 5 D  are an isometric front view, a rear view, a cross-sectional side view, and a cross-sectional isometric view, respectively, of a portion of a nuclear reactor system  500  in accordance with embodiments of the present technology. The nuclear reactor system  500  can include some features generally similar or identical to those of the nuclear reactor systems  100  and/or  200  described in detail above with reference to  FIGS.  1 - 4 B . For example, referring to  FIGS.  5 A- 5 D , the nuclear reactor system  500  includes a steam generator system having a tubesheet assembly  550  integrally formed with a reactor vessel  520 . More specially,  FIG.  5 A  is a front view of the tubesheet assembly  550  from within the reactor vessel  520  (e.g., generally facing in a direction toward an exterior of the reactor vessel  520 ) and  FIG.  5 B  is a rear view of the tubesheet assembly  550  from outside the reactor vessel  520  (e.g., generally facing in a direction toward an interior of the reactor vessel  520 ). The reactor vessel  520  can have a cylindrical shape. In other embodiments, the tubesheet assembly  550  can be a separate component coupled to the reactor vessel (e.g., via welding, bolts, fasteners, etc.). 
     In the illustrated embodiment, the tubesheet assembly  550  includes a body  534  defining or bounding (at least in part) a plenum  535  (obscured in  FIGS.  5 A and  5 B ). The body  534  can further define a tubesheet  536  (e.g., a perforated plate) and/or the tubesheet  536  can be a separate component attached to the body  534  and the reactor vessel  520 . In the illustrated embodiment, the tubesheet  536  is angled (e.g., inclined) by an angle A ( FIG.  5 C ) relative to a longitudinal axis Y ( FIG.  5 C ) of the reactor vessel  520  (and relative to an axis X shown in  FIG.  5 C  that is orthogonal to the longitudinal axis Y). The angle A can be less than about 60°, between about 10°-50°, between about 15°-45°, between about 20°-40°, between about 25°-35°, and/or about 30°. As best seen in  FIGS.  5 A and  5 B , the tubesheet  536  can be a generally circular flat plate including a plurality of perforations  539  (e.g., through-holes). The perforations  539  extend through the tubesheet  236  and are angled (e.g., inclined) relative to the axes X, Y. In some embodiments, the perforations  539  are arranged in a plurality of rows that can decrease in number in a downward direction along the longitudinal axis Y. The perforations  539  can be coupled to (e.g., welded to) corresponding ones of a plurality of heat transfer tubes (e.g., the heat transfer tubes  132  and/or  232  of  FIG.  1 A,  2 A , and/or  3 B) of the steam generator system. 
     The body  534  can further include/define a port  537  (e.g., a nozzle) fluidly coupled to the plenum  535 . In some embodiments, the tubesheet assembly  550  is a lower tubesheet assembly (e.g., a feed assembly) configured to (i) receive a secondary coolant through the port  537  and (ii) direct the secondary coolant through the plenum  535  and out of the perforations  539  into the corresponding heat transfer tubes (e.g., from lower portions of the heat transfer tubes) coupled to the tubesheet  536 . In other embodiments, the tubesheet assembly  550  is an upper tubesheet assembly (e.g., a vapor assembly) configured to (i) receive the secondary coolant in vapor form through the perforations  539  from the heat transfer tubes (e.g., from upper portions of the heat transfer tubes) coupled to the tubesheet  536  and (ii) direct the secondary coolant in vapor form through the plenum  535  to the port  537  for outlet to a power conversion system. 
     Referring to  FIG.  5 C , the tubesheet assembly  550  can further include a removable cover plate  538  that can be coupled to the body  534  via, for example, bolts  559 . When attached to body  534 , the cover plate  538  can enclose the plenum  535 . The cover plate  538  can be removed from and/or installed on the body  534  during one or more operations such as maintenance, inspection, and/or installation. The cover plate  538  is shown as removed in  FIGS.  5 A,  5 B, and  5 D  for clarity. 
     Referring to  FIGS.  5 A- 5 D , the tubesheet  536  includes an inner surface  551  (obscured in  FIG.  5 B ) positioned to face the interior of the reactor vessel  520  and an outer surface  552  (obscured in  FIG.  5 A ), opposite the inner surface  551 , and positioned to face the plenum  535  and the cover plate  538 . The perforations  539  can extend entirely through the tubesheet  536  from the inner surface  551  to the outer surface  552 . Referring to  FIG.  5 B- 5 D , the tubesheet assembly  550  further includes a groove  553  extending circumferentially about the tubesheet  536  and defining a connection portion  554  (which can also be referred to as a connection region, a flexible portion, a stress-relieving portion, a thinned portion, a weakened portion, and/or the like) between the tubesheet  536  and the adjoining portions of the reactor vessel  520  and/or the body  534 . Accordingly, the connection portion  554  can have a thickness T 1  ( FIG.  5 C ) that is less than a thickness T 2  ( FIG.  5 C ) of the tubesheet  536 . In some embodiments, the thickness T 2  can be between about 2-6 times larger, about two times larger, about three times larger, and/or about four times larger than the thickness T 1 . The tubesheet  536  can be integral with the connection portion  554 , the body  534 , and/or the reactor vessel  520 . 
     In the illustrated embodiment, the groove  553  and the connection portion  554  extend entirely circumferentially about the tubesheet  536  and each have a circular shape with a constant width W ( FIG.  5 C ). That is, the connection portion  554  can be a thinned annular ring around the tubesheet  536 . Moreover, the groove  553  extends from the outer surface  552  of the tubesheet  536  toward the inner surface  551  of the tubesheet  536  such that the connection portion  554  is positioned adjacent to the inner surface  551  of the tubesheet  536 . More specifically, referring to  FIGS.  5 C and  5 D , the connection portion  554  can include an inner surface  555  positioned to face the interior of the reactor vessel  520  and an outer surface  556 , opposite the inner surface  555 , positioned within the groove  553  and to face the plenum  535 . The inner surface  555  of the connection portion  554  can be coplanar with the inner surface  551  of the tubesheet  536 , while the outer surface  556  of the connection portion  554  can be offset from the outer surface  552  of the tubesheet  536 . In other embodiments, the groove  553  can extend only partially about the tubesheet  536 , can have other shapes (e.g., as shown in and described in detail with reference to  FIGS.  7 A- 7 C ), and/or can extend alternatively or additionally from the inner surface  551  (e.g., as shown in and described in detail with reference to  FIGS.  7 A- 7 C ). 
     In the illustrated embodiment, the inner and outer surfaces  555 ,  556  of the connection portion  554  are each planar/flat. In other embodiments, the inner and/or outer surfaces  555 ,  556  can have different profiles. For example,  FIGS.  6 A- 6 C  are cross-sectional side views of the connection portion  554  illustrating different profiles for the outer surface  556  in accordance with embodiments of the present technology. As shown in  FIGS.  6 A- 6 C , the outer surface  556  can have a curved spherical shape, a curved cylindrical shape, and/or an omega shape, respectively. In other embodiments, the inner surface  555  can have a similar shape and/or the inner surface  555  and/or the outer surface  556  can have other shapes (e.g., polygonal, irregular, etc.). 
     Referring to  FIGS.  5 A- 5 D , the connection portion  554  can be more flexible (e.g., less stiff) than the adjoining components it connects—e.g., the tubesheet  536 , the body  534 , and the reactor vessel  520 —because it is thinner than the adjoining components. In some embodiments, the connection portion  554  can alternatively or additionally be formed from a more flexible material than the reactor vessel  520  and the tubesheet  536 . Accordingly, in some aspects of the present technology, the connection portion  554  can mitigate or reduce the stresses (e.g., discontinuity stresses and/or fatigue) in the tubesheet  536  and/or in the associated connections (e.g., tube-to-tubesheet (TTS) welds) between the tubesheet  536  and the corresponding heat transfer tubes by functioning as a flexible connection between the reactor vessel  520  and the tubesheet  536 . Such a flexible connection can decouple the incompatible deformation between the differing geometries of the tubesheet  536  (e.g., a perforated flat plate) and the reactor vessel  520  (e.g., a cylindrical vessel) during cyclic loads. In some embodiments, the cyclic fatigue life of the tubesheet  536  and associated TTS welds can be increased by one order of magnitude, two orders of magnitude, or more—increasing the lifespan of the steam generator system  530 . 
     In some embodiments, individual ones of the perforations  539  can receive a corresponding one of the heat transfer tubes therein, and the heat transfer tube can be welded (e.g., via a TTS weld) or otherwise connected to the tubesheet  536  at and/or proximate to the outer surface  552  of the tubesheet  536 . Accordingly, the connections between the heat transfer tubes and the tubesheet  536  can be positioned adjacent the groove  553  opposite the connection portion  554 , which extends from proximate the inner surface  551  of the tubesheet  536 . Because the connection portion  554  does not extend at or proximate to the outer surface  556  of the tubesheet  536 , the tubesheet  536  can flex more readily near the outer surface  552 . Accordingly, in some aspects of the present technology, spacing the connections between the heat transfer tubes and the tubesheet  536  away from the connection portion  554  in this manner can further decrease the stresses in the connections during operation of the nuclear reactor system  500 . 
     In some embodiments, the connection portion  554  can be manufactured by milling the groove  553  via a tool inserted into the plenum  535  with the cover plate  538  removed. In some aspects of the present technology, forming the groove  553  to be circular and to have a constant width and depth can reduce the complexity of the manufacturing process used to form the groove  553 . 
     As noted above, tubesheet assemblies in accordance with the present technology can have other configurations of groove(s) extending around a tubesheet that provide a flexible coupling between the tubesheet and an adjoining reactor vessel.  FIGS.  7 A- 7 C , for example, are a cross-sectional front view, a cross-sectional rear view, and a cross-sectional side view, respectively, of the nuclear reactor system  500  including a connection portion  754  in accordance with additional embodiments of the present technology. Referring to  FIGS.  7 A- 7 C , other than the connection portion  754 , the components of the nuclear reactor system  500  can be similar or identical to those shown in and described in detail with reference to  FIGS.  5 A- 5 D , and are referenced with the same reference numbers. 
     In the illustrated embodiment, the tubesheet assembly  550  includes an inner groove  757  and an outer groove  758  extending circumferentially about the tubesheet  536  and defining the connection portion  754  between the tubesheet  536  and the adjoining portions of the reactor vessel  520  and/or the body  534 . Accordingly, the connection portion  754  can have a thickness T 1  ( FIG.  7 C ) that is less than a thickness T 2  ( FIG.  7 C ) of the tubesheet  536 . In some embodiments, the thickness T 2  can be between about 2-6 times larger, about two times larger, about three times larger, and/or about four times larger than the thickness T 1 . The tubesheet  536  can be integral with the connection portion  754 , the body  534 , and/or the reactor vessel  520 . 
     In the illustrated embodiment, the inner and outer grooves  757 ,  758  and the connection portion  754  extend entirely circumferentially about the tubesheet  536  and each have a lozenge-like or shield-like shape with a variable width W ( FIG.  7 C ) about the tubesheet  536 . In some embodiments, the width W can be selected to maintain a minimum clearance between the connection portion  754  and the perforations  539 . Moreover, the inner groove  757  extends from the inner surface  551  of the tubesheet  536  toward the outer surface  552  of the tubesheet, and the outer groove  758  extends from the outer surface  552  of the tubesheet  536  toward the inner surface  551  of the tubesheet  536  such that the connection portion  754  is positioned adjacent to a middle portion of the tubesheet  536 . More specifically, the connection portion  754  can include (i) an inner surface  755  positioned within the inner groove  757  to face the interior of the reactor vessel  520  and (ii) an outer surface  756 , opposite the inner surface  755 , positioned within the outer groove  758  and to face the plenum  535 . The inner surface  755  of the connection portion  754  can be offset from (e.g., spaced apart from) the outer surface  552  of the tubesheet  536 , and the outer surface  756  of the connection portion  754  can similarly be offset from the outer surface  552  of the tubesheet  536 . 
     Accordingly, in some aspects of the present technology the connection portion  754  can be more flexible (e.g., less stiff) than the adjoining components it connects—e.g., the tubesheet  536 , the body  534 , and the reactor vessel  520 —because it is thinner than the adjoining components. Accordingly, in some aspects of the present technology, the connection portion  754  can mitigate or reduce the stresses (e.g., discontinuity stresses and/or fatigue) in the tubesheet  536  and/or in the associated connections (e.g., tube-to-tubesheet (TTS) welds) between the tubesheet  536  and the corresponding heat transfer tubes by functioning as a flexible connection between the reactor vessel  520  and the tubesheet  536 . 
     Although  FIGS.  5 A- 7 C  illustrate tubesheet assemblies having a tubesheet  536  angled (e.g., inclined) relative to the longitudinal axis L of the reactor vessel  520 , the flexible connection portions (e.g., the connection portion  554  and/or the connection portion  754 ) of the present technology can be used with a tubesheet having any orientation/configuration described herein, and/or in any configuration in which a tubesheet is integrally attached to a reactor vessel. For example, referring to  FIG.  2 A , a flexible connection portion can be formed between any or all of the horizontally-oriented tubesheets  236  of the lower tubesheet assemblies  231  and the reactor vessel  220  and/or between the horizontally-oriented tubesheets  236  and the body  234  of the tubesheets  236 . Similarly, referring to  FIGS.  2 A and  2 B , a flexible connection portion can be formed in the pressurizer plate  217  between any or all of the tubesheets  246  of the upper tubesheet assemblies  233  between the tubesheets  246  and the reactor vessel  220  and/or between the tubesheets  246  and the body  244  of the tubesheets  246 . Likewise, referring to  FIGS.  3 A and  3 B , a flexible connection portion can be formed between any or all of the vertically-oriented tubesheets  236  of the lower tubesheet assemblies  231  and the reactor vessel  220  and/or between the vertically-oriented tubesheets  236  and the body  234  of the tubesheets  236 . 
     III. ADDITIONAL EXAMPLES 
     The following examples are illustrative of several embodiments of the present technology: 
     1. A steam generator system for use in a nuclear reactor system including a reactor vessel positioned to house a primary coolant, the steam generator system comprising:
         a tubesheet assembly coupled to the reactor vessel, forming at least a portion of a plenum, and comprising—
           a tubesheet including a plurality of perforations fluidly coupled to the plenum; and   a connection portion at least partially between the tubesheet and the reactor vessel, wherein the connection portion is more flexible than the tubesheet and the reactor vessel; and   
           a plurality of heat transfer tubes configured to receive a flow of a secondary coolant, wherein individual ones of the heat transfer tubes are fluidly coupled to corresponding ones of the perforations.       

     2. The steam generator system of example 1 wherein the tubesheet is a flat plate, and wherein the tubesheet assembly includes a groove extending circumferentially about the tubesheet and defining the connection portion. 
     3. The steam generator system of example 2 wherein the tubesheet includes an inner surface positioned to face an interior of the reactor vessel and an outer surface positioned to face the plenum, and wherein the groove extends partially from the outer surface toward the inner surface. 
     4. The steam generator system of example 2 or example 3 wherein the flat plate has a circular shape, and wherein the groove has a circular shape with a generally constant width and depth. 
     5. The steam generator system of example 2 or example 3 wherein the groove has a width that varies in a circumferential direction. 
     6. The steam generator system of any one of examples 1-5 wherein the tubesheet assembly is integrally formed with the reactor vessel. 
     7. The steam generator system of any one of examples 1-6 wherein the reactor vessel extends along a longitudinal axis, and wherein the tubesheet is positioned generally parallel to the longitudinal axis. 
     8. The steam generator system of any one of examples 1-6 wherein the reactor vessel extends along a longitudinal axis, and wherein the tubesheet is inclined relative to the longitudinal axis by an angle of between about 15°-45°. 
     9. The steam generator of any one of examples 1-6 wherein the reactor vessel extends along a longitudinal axis, and wherein the tubesheet is positioned generally perpendicular to the longitudinal axis. 
     10. The steam generator system of any one of examples 1-9 wherein the tubesheet is a flat plate having an inner surface positioned to face an interior of the reactor vessel and an outer surface positioned to face the plenum, wherein the tubesheet assembly includes a first groove extending circumferentially about the tubesheet from the inner surface partially toward the outer surface and a second groove extending circumferentially about the tubesheet from the outer surface partially toward the inner surface, and wherein the first groove and the second groove define the connection portion. 
     11. The steam generator system of any one of examples 1-10 wherein the tubesheet has a first thickness, and wherein the connection portion has a second thickness less than the first thickness. 
     12. The steam generator system of example 11 wherein the first thickness is less than half the second thickness. 
     13. The steam generator system of any one of examples 1-12 wherein the tubesheet assembly further comprises an inlet port fluidly coupled to the plenum, and wherein the tubesheet assembly is positioned to receive the secondary coolant in liquid form via the inlet port and route the secondary coolant in liquid form through the plenum into the perforations and into the heat transfer tubes. 
     14. The steam generator system of any one of examples 1-12 wherein the tubesheet assembly further comprises an outlet port fluidly coupled to the plenum, and wherein the tubesheet assembly is positioned to receive the secondary coolant in vapor form from the heat transfer tubes and route the secondary coolant in vapor form through the perforations into the plenum and into the outlet port. 
     15. A tubesheet assembly for use in a nuclear reactor system including a reactor vessel, the tubesheet assembly comprising:
         a body bounding at least a portion of a plenum;   a tubesheet including a plurality of perforations fluidly coupled to the plenum, wherein the tubesheet assembly is coupled to the body and the reactor vessel; and   a connection portion at least partially between the tubesheet and the reactor vessel, wherein the connection portion is more flexible than the tubesheet and the reactor vessel.       

     16. The tubesheet assembly of example 15 wherein the tubesheet is a circular flat plate having an inner surface positioned to face an interior of the reactor vessel and an outer surface positioned to face the plenum, wherein the tubesheet assembly includes a circular groove extending circumferentially about the tubesheet and defining the connection portion, and wherein the circular groove extends from the outer surface partially toward the inner surface. 
     17. The tubesheet assembly of example 15 or example 16 wherein the tubesheet is a circular flat plate having an inner surface positioned to face an interior of the reactor vessel and an outer surface positioned to face the plenum, wherein the tubesheet has a first thickness in a direction between the inner surface and the outer surface, and wherein the connection portion has a second thickness, less than the first thickness, in the direction between the inner surface and the outer surface. 
     18. The tubesheet assembly of any one of examples 15-17 wherein the reactor vessel extends along a longitudinal axis, and wherein the tubesheet is inclined relative to the longitudinal axis by an angle of about 30°. 
     19. A nuclear reactor system, comprising:
         a reactor vessel positioned to house a reactor core and a primary coolant, wherein the primary coolant is positioned to absorb heat from a nuclear reaction within the reactor core; and   a steam generator assembly, comprising:
           a first tubesheet assembly including a first tubesheet, wherein the first tubesheet is coupled to the reactor vessel and includes a plurality of first perforations, wherein the first tubesheet assembly includes a flexible connection portion positioned between the first tubesheet and the reactor vessel, wherein the flexible connection portion comprises an annular ring around the first tubesheet having a thickness less than a thickness of the first tubesheet;   a second tubesheet assembly including a second tubesheet, wherein the second tubesheet is coupled to the reactor vessel and includes a plurality of second perforations; and   a plurality of heat transfer tubes configured to receive a secondary coolant, wherein the secondary coolant is configured to absorb heat from the primary coolant through the heat transfer tubes, and wherein individual ones of the heat transfer tubes have a first portion fluidly coupled to a corresponding one of the first perforations of the first tubesheet and a second portion fluidly coupled to a corresponding one of the second perforations of the second tubesheet.   
               

     20. The nuclear reactor system of example 19 wherein the first tubesheet assembly is positioned to receive the secondary coolant in liquid form and route the secondary coolant in liquid form to the heat transfer tubes via the first perforations, wherein the reactor vessel extends along a longitudinal axis, and wherein the first tubesheet is inclined relative to the longitudinal axis by an angle of between about 15°-45°. 
     IV. CONCLUSION 
     All numeric values are herein assumed to be modified by the term about whether or not explicitly indicated. The term about, in the context of numeric values, generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (e.g., having the same function and/or result). For example, the term about can refer to the stated value plus or minus ten percent. For example, the use of the term about 100 can refer to a range of from 90 to 110, inclusive. In instances in which the context requires otherwise and/or relative terminology is used in reference to something that does not include, or is not related to, a numerical value, the terms are given their ordinary meaning to one skilled in the art. 
     The above detailed description of embodiments of the present technology are not intended to be exhaustive or to limit the technology to the precise forms disclosed above. Although specific embodiments of, and examples for, the technology are described above for illustrative purposes, various equivalent modifications are possible within the scope of the technology, as those skilled in the relevant art will recognize. For example, although steps may be presented in a given order, in other embodiments, the steps may be performed in a different order. The various embodiments described herein may also be combined to provide further embodiments. 
     From the foregoing, it will be appreciated that specific embodiments of the technology have been described herein for purposes of illustration, but well-known structures and functions have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments of the technology. Where the context permits, singular or plural terms may also include the plural or singular term, respectively. 
     As used herein, the phrase and/or as in A and/or B refers to A alone, B alone, and A and B. Additionally, the term comprising is used throughout to mean including at least the recited feature(s) such that any greater number of the same feature and/or additional types of other features are not precluded. It will also be appreciated that specific embodiments have been described herein for purposes of illustration, but that various modifications may be made without deviating from the technology. Further, while advantages associated with some embodiments of the technology have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the technology. Accordingly, the disclosure and associated technology can encompass other embodiments not expressly shown or described herein.