Patent Publication Number: US-9887015-B2

Title: Suspended upper internals with tie rod couplings for compact nuclear reactor

Description:
This application claims the benefit of U.S. Provisional Application No. 61/625,764 filed Apr. 18, 2012 and titled “UPPER INTERNALS”. U.S. Provisional Application No. 61/625,764 filed Apr. 18, 2012 titled “UPPER INTERNALS” is hereby incorporated by reference in its entirety into the specification of this application. 
     This application claims the benefit of U.S. Provisional Application No. 61/625,280 filed Apr. 17, 2012 and titled “TIE ROD COUPLING”. U.S. Provisional Application No. 61/625,280 filed Apr. 17, 2012 titled “TIE ROD COUPLING” is hereby incorporated by reference in its entirety into the specification of this application. 
    
    
     BACKGROUND 
     The following relates to the nuclear reactor arts and related arts. 
     There is increasing interest in compact reactor designs. Benefits include: reduced likelihood and severity of abnormal events such as loss of a coolant accident (LOCA) event (both due to a reduction in vessel penetrations and the use of a smaller containment structure commensurate with the size of the compact reactor); a smaller and more readily secured nuclear reactor island (see Noel, “Nuclear Power Facility”, U.S. Pub. No. 2010/0207261 A1 published Aug. 16, 2012 which is incorporated herein by reference in its entirety); increased ability to employ nuclear power to supply smaller power grids, e.g. using a 300 MWe or smaller compact reactor, sometimes referred to as a small modular reactor (SMR); scalability as one or more SMR units can be deployed depending upon the requisite power level; and so forth. 
     Some compact reactor designs are disclosed, for example, in Thome et al., “Integral Helical-Coil Pressurized Water Nuclear Reactor”, U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which is incorporated by reference in its entirety; Malloy et al., “Compact Nuclear Reactor”, U.S. Pub. No. 2012/0076254 A1 published Mar. 29, 2012 which is incorporated by reference in its entirety. These compact reactors are of the pressurized water reactor (PWR) type in which a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel, and the primary coolant is suitably light water maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel and (together with the core basket or shroud) defines a primary coolant circuit in which coolant flows upward through the reactor core and central riser, discharges from the top of the central riser, and reverses direction to flow downward back to below the reactor core through a downcomer annulus defined between the pressure vessel and the central riser. The nuclear core is built up from multiple fuel assemblies each comprising a bundle of fuel rods containing fissile material (typically  235 U). The compact reactors disclosed in Thome et al. and Malloy et al. are integral PWR designs in which the steam generator(s) is disposed inside the pressure vessel, namely in the downcomer annulus in these designs. Integral PWR designs eliminate the external primary coolant loop carrying radioactive primary coolant. The designs disclosed in Thome et al. and Malloy et al. employ internal reactor coolant pumps (RCPs), but use of external RCPs (e.g. with a dry stator and wet rotor/impeller assembly, or with a dry stator and dry rotor coupled with a rotor via a suitable mechanical vessel penetration) is also contemplated (as is a natural circulation variant that does not employ RCPs). The designs disclosed in Thome et al. and Malloy et al. further employ internal pressurizers in which a steam bubble at the top of the pressure vessel is buffered from the remainder of the pressure vessel by a baffle plate or the like, and heaters, spargers, or so forth enable adjustment of the temperature (and hence pressure) of the steam bubble. The internal pressurizer avoids large diameter piping that would otherwise connect with an external pressurizer. 
     In a typical PWR design, upper internals located above the reactor core include control rod assemblies with neutron-absorbing control rods that are inserted into/raised out of the reactor core by control rod drive mechanisms (CRDMs). These upper internals include control rod assemblies (CRAs) comprising neutron-absorbing control rods yoked together by a spider. Conventionally, the CRDMs employ motors mounted on tubular pressure boundary extensions extending above the pressure vessel, which are connected with the CRAs via suitable connecting rods. In this design, the complex motor stator can be outside the pressure boundary and magnetically coupled with the motor rotor disposed inside the tubular pressure boundary extension. The upper internals also include guide frames constructed as plates held together by tie rods, with passages sized to cam against and guide the translating CRA&#39;s. 
     For compact reactor designs, it is contemplated to replace the external CRDM motors with wholly internal CRDM motors. See Stambaugh et al., “Control Rod Drive Mechanism for Nuclear Reactor”, U.S. Pub. No. 2010/0316177 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety; and DeSantis, “Control Rod Drive Mechanism for Nuclear Reactor”, U.S. Pub. No. 2011/0222640 A1 published Sep. 15, 2011 which is incorporated herein by reference in its entirety. Advantageously, only electrical vessel penetrations are needed to power the internal CRDM motors. In some embodiments, the scram latch is hydraulically driven, so that the internal CRDM also requires hydraulic vessel penetrations, but these are of small diameter and carry primary coolant water as the hydraulic working fluid. 
     The use of internal CRDM motors shortens the connecting rods, which reduces the overall weight, which in turn reduces the gravitational impetus for scram. To counteract this effect, some designs employ a yoke that is weighted as compared with a conventional spider, and/or may employ a weighted connecting rod. See Shargots et al., “Terminal Elements for Coupling Connecting Rods and Control Rod Assemblies for a Nuclear Reactor”, U.S. Pub. No. 2012/0051482 A1 published Mar. 1, 2012 which is incorporated herein by reference in its entirety. Another design improvement is to replace the conventional guide frames which employ spaced apart guide plates held together by tie rods with a continuous columnar guide frame that provides continuous guidance to the translating CRA&#39;s. See Shargots et al, “Support Structure for a Control Rod Assembly of a Nuclear Reactor”, U.S. Pub. No. 2012/0099691 A1 published Apr. 26, 2012 which is incorporated herein by reference in its entirety. 
     The use of internal CRDMs and/or continuous guide frames and/or internal RCPs introduces substantial volume, weight, and complexity to the upper internals. These internals are “upper” internals in that they are located above the reactor core, and they must be removed prior to reactor refueling in order to provide access to the reactor core. In principle, some components (especially the internal RCPs) can be located below the reactor core, but this would introduce vessel penetrations below the reactor core which is undesirable since a LOCA at such low vessel penetrations can drain the primary coolant to a level below the top of the reactor core, thus exposing the fuel rods. Another option is to employ external RCPs, but this still leaves the complex internal CRDMs and guide frames. 
     Disclosed herein are improvements that provide various benefits that will become apparent to the skilled artisan upon reading the following. 
     BRIEF SUMMARY 
     In one disclosed aspect, an apparatus comprises a suspended basket including plurality of plates connected together by tie rods. The suspended basket is configured to support upper internals (such as control rod drive mechanisms and/or guide frames) of a nuclear reactor. The suspended basket includes adjustable length threaded connections between ends of tie rods and at least one of the plates. In some embodiments each adjustable length threaded connection includes a plate thread feature extending from the plate, a threaded end of the tie rod, and a tie rod coupling portion having threading engaging both the plate thread feature and the threaded end of the tie rod, and rotating the tie rod coupling portion adjusts the position of the tie rod respective to the plate. In some such embodiments the plate thread feature has outside threading, the threaded end of the tie rod has outside threading, and the tie rod coupling portion comprises a sleeve with inside threading engaging both the outside threading of the plate thread feature and the outside threading of the threaded end of the tie rod. 
     In another disclosed aspect, an apparatus comprises a suspended basket including plurality of plates connected together by tie rods, and control rod drive mechanisms (CRDMs) with CRDM motors mounted in the suspended basket. The tie rod couplings to the plates comprise threaded turnbuckle connections each including a rod coupling portion that is rotatable to adjust the position of the tie rod respective to the plate. In some embodiments each threaded turnbuckle connection further includes a plate thread feature threadedly engaged with the rod coupling portion and a threaded end of the tie rod threadedly engaged with the rod coupling portion. The apparatus may further include guide frames mounted in the suspended basket. In such embodiments, the plurality of plates of the suspended basket may suitably include an upper hanger plate, a mid-hanger plate, and a lower hanger plate, with the CRDMs mounted in the suspended basket via the upper hanger plate and the mid hanger plate, and the guide frames mounted in the suspended basket via the mid-hanger plate and the lower hanger plate. 
     In another disclosed aspect, an apparatus as set forth in any of the two immediately preceding paragraphs further includes a pressure vessel and a nuclear reactor core comprising fissile material disposed in the pressure vessel, and the suspended basket is suspended inside the pressure vessel above the nuclear reactor core, e.g. with the mounted CRDMs arranged to control insertion of control rods into the nuclear reactor core. 
     In another disclosed aspect, an apparatus comprises: a plurality of plates; tie rods; adjustable length threaded tie rod couplings connecting threaded ends of the tie rods with threaded features of the plates to form a suspended basket; control rod drive mechanisms (CRDMs) with CRDM motors mounted in the suspended basket; a pressure vessel; and a nuclear reactor core comprising fissile material disposed in the pressure vessel. The suspended basket is suspended inside the pressure vessel above the nuclear reactor core with the mounted CRDMs arranged to control insertion of control rods into the nuclear reactor core. In some embodiments each adjustable length threaded tie rod coupling includes a threaded sleeve that is threaded onto the threaded end of the tie rod and is threaded onto the threaded feature of the plate, the sleeve being rotatable to adjust the position of the tie rod respective to the plate. In some embodiments each adjustable length threaded tie rod coupling comprises a turnbuckle coupling. In some embodiments the apparatus further comprises guide frames mounted in the suspended basket between the CRDMs and the nuclear reactor core to guide portions of the control rods withdrawn from the nuclear reactor core. In a suitable arrangement, the plurality of plates include an upper hanger plate, a mid-hanger plate, and a lower hanger plate, the upper hanger plate being furthest from the nuclear reactor core and the lower hanger plate being closest to the nuclear reactor core, the CRDMs are mounted in the suspended basket via the upper hanger plate and the mid hanger plate, and the guide frames are mounted in the suspended basket via the mid-hanger plate and the lower hanger plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
         FIG. 1  illustrates a compact nuclear reactor in partial cutaway perspective view revealing selected internal components. 
         FIG. 2  illustrates a variant compact nuclear reactor in perspective view with the upper vessel lifted off. 
         FIG. 3  shows an exploded perspective view of the variant compact nuclear reactor of  FIG. 2  showing principle internal components. 
         FIGS. 4 and 5  shows perspective and top views, respectively, of an illustrative embodiment of the suspended upper internals. 
         FIGS. 6 and 7  show alternative perspective views of the upper internals of  FIGS. 4 and 5  with the control rod drive mechanisms (CRDMs) removed. 
         FIG. 8  shows an enlarged perspective view of two tie rod couplings of the upper internals of  FIGS. 4 and 5 . 
         FIG. 9  shows an enlarged perspective view from below of the lower hanger plate showing guide frame bottom cards. 
         FIG. 10  shows a perspective view of the lower hanger plate of the upper internals of  FIGS. 4 and 5 . 
         FIG. 11  shows a perspective view from below of the mid-hanger plate of the upper internals of  FIGS. 4 and 5 . 
         FIGS. 12 and 13  illustrate alternative embodiments for manufacturing the mid-hanger plate of  FIG. 11 . 
         FIG. 14  illustrates an alternative embodiment in which the guide frames are bottom-supported by the lower hanger plate. 
         FIG. 15  shows a perspective view of the upper hanger plate of the upper internals of  FIGS. 4 and 5 . 
         FIG. 16  shows a side view of the suspended support assembly of the upper internals of  FIGS. 4 and 5 . 
         FIGS. 17 and 18  shows enlarged perspective and enlarged perspective cutaway views, respectively, of one of the tie rod couplings of  FIG. 16 . 
         FIG. 19  shows a perspective view of the riser transition section from which the upper internals are suspended. 
         FIG. 20  shows a diagrammatic side view of one of the gussets of the riser transition section shown in  FIG. 19 . 
         FIG. 21  shows a perspective view of an upper portion of a CRDM including straps retaining a hydraulic line in which one of the straps is modified to include compliance features. 
         FIG. 22  shows a perspective view of the upper portion of the CRDM of  FIG. 21  installed with the compliance features seated in the upper hanger plate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , a small modular reactor (SMR)  1  of the of the integral pressurized water reactor (PWR) variety is shown in partial cutaway to reveal selected internal components. The illustrative PWR  1  includes a nuclear reactor core  2  disposed in a pressure vessel comprising a lower vessel portion  3  and an upper vessel portion  4 . The lower and upper vessel portions  3 ,  4  are connected by a mid-flange  5 . Specifically, a lower flange  5 L at the open top of the lower vessel portion  3  connects with the bottom of the mid-flange  5 , and an upper flange  5 U at the open bottom of the upper vessel portion  4  connects with a top of the mid-flange  5 . 
     The reactor core  2  is disposed inside and at or near the bottom of the lower vessel portion  3 , and comprises a fissile material (e.g.,  235 U) immersed in primary coolant water. A cylindrical central riser  6  is disposed coaxially inside the cylindrical pressure vessel and a downcomer annulus  7  is defined between the central riser  6  and the pressure vessel. The illustrative PWR  1  includes internal control rod drive mechanisms (internal CRDMs)  8  with internal motors  8   m  immersed in primary coolant that control insertion of control rods to control reactivity. Guide frames  9  guide the translating control rod assembly (e.g., each including a set of control rods comprising neutron absorbing material yoked together by a spider and connected via a connecting rod with the CRDM). The illustrative PWR  1  employs one or more internal steam generators  10  located inside the pressure vessel and secured to the upper vessel portion  4 , but embodiments with the steam generators located outside the pressure vessel (i.e., a PWR with external steam generators) are also contemplated. The illustrative steam generator  10  is of the once-through straight-tube type with internal economizer, and is fed by a feedwater inlet  11  and deliver steam to a steam outlet  12 . See Malloy et al., U.S. Pub. No. 2012/0076254 A1 published Mar. 29, 2012 which is incorporated by reference in its entirety. The illustrative PWR  1  includes an integral pressurizer  14  at the top of the upper vessel section  4  which defines an integral pressurizer volume  15 ; however an external pressurizer connected with the pressure vessel via suitable piping is also contemplated. The primary coolant in the illustrative PWR  1  is circulated by reactor coolant pumps (RCPs) comprising in the illustrative example external RCP motors  16  driving an impeller located in a RCP plenum  17  disposed inside the pressure vessel. 
     With reference to  FIGS. 2 and 3 , a variant PWR design  1 ′ is shown, which differs from the PWR  1  of  FIG. 1  by having a differently shaped upper vessel section  4 ′ and internal RCPs  16 ′ in place of the external pumps  16 ,  17  of the PWR  1 .  FIG. 2  shows the pressure vessel with the upper vessel section  4 ′ lifted off, as is done during refueling. The mid-flange  5  remains disposed on the lower flange  5 L of the lower vessel  3 .  FIG. 3  shows an exploded view of the lower vessel section  3  and principle components contained therein, including: the nuclear reactor core  2  comprising fuel assemblies  2 ′ contained in a core former  20  disposed in a core basket  22 . 
     With continuing reference to  FIGS. 1 and 3  and with further reference to  FIGS. 4 and 5 , above the reactor core assembly  2 ,  20 ,  22  are the upper internals which include a suspended support assembly  24  comprising an upper hanger plate  30 , a mid-hanger plate  32 , and a lower hanger plate  34  suspended by tie rods  36  from the mid-flange  5 . More particularly, in the illustrative embodiment the upper ends of the tie rods  36  are secured to a riser transition section  38  that is in turn secured with the mid-flange  5 . The central riser  6  disposed in the upper vessel section  4 ,  4 ′ (shown only in  FIG. 1 ) is connected with the core basket  22  in the lower vessel section  3  by the riser cone (not shown) riser transition section  38  to form a continuous hollow cylindrical flow separator between the columnar hot leg of the primary coolant path flowing upward and the cold leg that flows through the downcomer annulus surrounding the hot leg. The suspended support assembly  24  comprising hanger plates  30 ,  32 ,  34  interconnected by tie rods  36  provides the structural support for the CRDMs  8  and the guide frames  9  (note the CRDMs  8  and guide frames  9  are omitted in  FIG. 3 ). The CRDMs  8  are disposed between the upper hanger plate  30  and the mid-hanger plate  32 , and are either (1) top-supported in a hanging fashion from the upper hanger plate or (2) bottom-supported on the mid-hanger plate  32  (as in the illustrative embodiments described herein). Lateral support for the CRDMs  8  is provided by both plates  30 ,  32 . (Note that in the illustrative embodiment, the CRDMs  8  actually pass through openings of the upper hanger plate  30  so that the tops of the CRDMs  8  actually extend above the upper hanger plate  30 , as best seen in  FIG. 1 ). The guide frames  9  are disposed between the mid-hanger plate  32  and the lower hanger plate  34 , and are likewise either (1) top-supported in a hanging fashion from the mid-hanger plate  32  (as in the illustrative embodiments described herein) or (2) bottom-supported on the lower hanger plate. Lateral support for the guide frames  9  is provided by both plates  32 ,  34 . 
     One of the hanger plates, namely the mid-hanger plate  32  in the illustrative embodiments, also includes or supports a distribution plate that includes mineral insulated cabling (MI cables) for delivering electrical power to the CRDM motors  8 M and, in some embodiments, hydraulic lines for delivering hydraulic power to scram latches of the CRDMs  8 . In the embodiment of  FIGS. 2 and 3  (and as seen in  FIG. 3 ), the internal RCPs  16 ′ are also integrated into the upper internals assembly  24 , for example on an annular pump plate providing both separation between the suction (above) and discharge (below) sides of the RCPs  16 ′ and also mounting supports for the RCPs  16 ′. 
     The disclosed upper internals have numerous advantages. The suspension frame  24  hanging from the mid-flange  5  is a self-contained structure that can be lifted out of the lower vessel section  3  as a unit during refueling. Therefore, the complex assembly of CRDMs  8 , guide frames  9 , and ancillary MI cabling (and optional hydraulic lines) does not need to be disassembled during reactor refueling. Moreover, by lifting the assembly  5 ,  24 ,  8 ,  9  out of the lower vessel  3  as a unit (e.g. using a crane) and moving it to a suitable work stand, maintenance can be performed on the components  5 ,  24 ,  8 ,  9  simultaneously with the refueling, thus enhancing efficiency and speed of the refueling. The tensile forces in the tie rods  36  naturally tend to laterally align the hanger plates  30 ,  32 ,  34  and thus the mounted CRDMs  8  and guide frames  9 . 
     The upper internals are thus a removable internal structure that is removed as a unit for reactor refueling. The upper internals basket (i.e., the suspension frame  24 ) is advantageously flexible to allow for movement during fit-up when lowering the upper internals into position within the reactor. Toward this end, the horizontal plates  30 ,  32 ,  34  are positioned at varying elevations and are connected to each other, and the remainder of the upper internals, via the tie rods  36 . The design of the illustrative upper internals basket  24  is such that the control rod guide frames  9  are hung from the mid-hanger plate  32  (although in an alternative embodiment the guide frames are bottom-supported by the lower hanger plate). In the top-supported hanging arrangement, the guide frames  9  are laterally supported at the bottom by the lower hanger plate  34 . The upper internals are aligned with the core former  20  and/or core basket  22  to ensure proper fit-up of the fuel to guide frame interface. This alignment is achieved by keying features of the lower hanger plate  34 . 
     With reference to  FIGS. 6 and 7 , alternative perspective views are shown of the hanger plates  30 ,  32 ,  34  connected by tie rods  36  and with the guide frames  9  installed, but omitting the CRDMs  8  so as to reveal the top surface of the mid-hanger plate  32 . In the illustrative embodiment, a distribution plate  40  is disposed on top of the mid-hanger plate  32 , as best seen in  FIG. 6 . The distribution plate  40  is a load-transferring element that transfers (but does not itself support) the weight of the bottom-supported CRDMs  8  to the mid-hanger plate  32 . This is merely an illustrative example, and the distribution plate can alternatively be integral with the mid-hanger plate (e.g., comprising MI cables embedded in the mid-hanger plate) or located on or in the upper hanger plate. (Placement of the distribution plate in the lower hanger plate is also contemplated, but in that case MI cables would need to run from the distribution plate along the outsides of the guide frames to the CRDMs. As yet another option, the distribution plate can be omitted entirely in favor of discrete MI cables run individually to the CRDMs  8 ). 
     With reference to  FIG. 8 , which shows a corner of the upper hanger plate  30  as an illustrative example, the tie rods  36  are coupled to each plate by tie rod couplings  42 , which optionally incorporate a turnbuckle (i.e. length adjusting) arrangement as described elsewhere herein. Note that the ends of the tie rods connect with a hanger plate, with no hanger plate connecting at a middle of a tie rod. Thus, the upper tie rods  36  extend between the upper and mid-hanger plates  30 ,  32  with their upper ends terminating at tie rod couplings  42  at the upper hanger plate  30  and their lower ends terminating at tie rod couplings  42  at the mid-hanger plate  32 ; and similarly, the lower tie rods  36  extend between the mid-hanger plate  32  and the lower hanger plate  34  with their upper ends terminating at tie rod couplings  42  at the mid-hanger plate  32  and their lower ends terminating at tie rod couplings  42  at the lower hanger plate  34 . 
     With reference to  FIGS. 9 and 10 , the lower hanger plate  34  in the illustrative embodiment provides only lateral support for the guide frames  9  which are top-supported in hanging fashion from the mid-hanger plate  32 . Consequentially, the lower hanger plate  34  is suitably a single plate with openings  50  that mate with the bottom ends of the guide frames (see  FIG. 10 ). To simplify the alignment, in some embodiments guide frame bottom cards  52  (see  FIG. 9 ) are inserted into the openings  50  and are connected with the bottom ends of the guide frames  9  by fasteners, welding, or another technique. (Alternatively, the ends of the guide frames may directly engage the openings  50  of the lower hanger plate  34 ). 
     In addition to providing lateral support for each control rod guide frame  9 , locking each in laterally with a honeycomb-type structure (see  FIG. 10 ), the lower hanger plate  34  also includes alignment features  54  (see  FIG. 10 ) that align the upper internals with the core former  20  or with the core basket  22 . The illustrative alignment features are peripheral notches  54  that engage protrusions (not shown) on the core former  20 ; however, other alignment features can be employed (e.g., the lower hanger plate can include protrusions that mate with notches of the core former). Also seen in  FIG. 10  are peripheral openings  56  in the lower hanger plate  34  into which the tie rod couples  42  of the lower hanger plate fit. The lower hanger plate  34  is suitably machined out of plate material or forging material. For example, in one contemplated embodiment the lower hanger plate  34  is machined from 304L steel plate stock. 
     With continuing reference to  FIGS. 6 and 7  and with further reference to  FIG. 11 , the mid-hanger plate  32  provides top support for the guide frames  9  and bottom support for the CRDMs  8 . The mid-hanger plate  32  acts as a load distributing plate taking the combined weight of the CRDMs  8  and the guide frames  9  and transferring that weight out to the tie rods  36  on the periphery of the upper internals basket  24 . In the illustrative embodiment, the power distribution plate  40  is also bottom supported. Like the lower hanger plate  34 , the mid-hanger plate  32  includes openings  60 . The purpose of the openings  60  is to enable the connecting rod, translating screw, or other coupling mechanism to connect each CRDM  8  with the control rod assembly driven by the CRDM. To facilitate hanging the guide frames  9  off the bottom of the mid-hanger plate  32 , an egg crate-type structure made of orthogonally intersecting elements  61  is provided for increased strength and reduced deflection due to large loads. 
     With reference to  FIGS. 12 and 13 , the mid-hanger plate  32  can be manufactured in various ways. In one approach ( FIG. 12 ), a forging machining process is employed to machine the mid-hanger plate  32  out of a 304L steel forged plate  62 . The machining forms the openings  60  and the intersecting elements  61 . In another approach ( FIG. 13 ), a machined plate  64  and the intersecting elements  61  are manufactured as separate components, and the intersecting elements  61  are interlocked using mating slits formed into the intersecting elements  61  and welded to each other and to the machined plate  64  to form the mid-hanger plate  32 . As previously noted, the illustrative bottom-supported distribution plate  40  can alternatively be integrally formed into the mid-hanger plate. 
     With reference to  FIG. 14 , in an alternative embodiment the guide frames  9  are bottom supported by an alternative lower hanger plate  34 ′, and are laterally aligned at top by an alternative mid-hanger plate  32 ′. In this case the alternative lower hanger plate  34 ′ may have the same form and construction as the main embodiment mid-hanger plate  32  of  FIGS. 11-13  (but with suitable alignment features to align with the core former and/or core basket, not shown in  FIG. 14 ), and the alternative mid-hanger plate  32 ′ can have the same form and construction as the main embodiment lower hanger plate  34  of  FIG. 10  (but without said alignment features). If the CRDMs remain bottom supported, then the alternative mid-hanger plate  32 ′ should be made sufficiently thick (or otherwise sufficiently strong) to support the weight of the CRDMs. As another variant, the alternative mid-hanger plate  32 ′ can be made too thin to directly support the CRDMs, and an additional thicker upper plate added to support the weight of the CRDMs. In this case the thicker plate would be the one connected with the tie rods to support the CRDMs. 
     In the illustrative embodiments, the guide frames  9  are continuous columnar guide frames  9  that provide continuous guidance to the translating control rod assemblies. See Shargots et al, “Support Structure for a Control Rod Assembly of a Nuclear Reactor”, U.S. Pub. No. 2012/0099691 A1 published Apr. 26, 2012 which is incorporated herein by reference in its entirety. However, the described suspended frame  24  operates equally well to support more conventional guide frames comprising discrete plates held together by tie rods. Indeed, the main illustrative approach in which the guide frames are top-supported in hanging fashion from the mid-hanger plate  32  is particularly well-suited to supporting conventional guide frames, as the hanging arrangement tends to self-align the guide frame plates. 
     With reference to  FIG. 15 , an illustrative embodiment of the upper hanger plate  30  is shown. Like the other hanger plates  32 ,  34 , the upper hanger plate  30  includes openings  70 , in this case serving as passages through which the upper ends of the CRDMs  8  pass. The inner periphery of each opening  70  serves as a cam to laterally support and align the upper end of the CRDM  8 . The upper hanger plate  30  can also suitably be made by machining from either plate material or forging material, e.g. a 304L steel plate stock or forging. 
     With reference to  FIGS. 16-18 , the tie bar (alternatively “tie rod”) couplings  42  are further described.  FIG. 16  shows the suspended frame  24  including the upper, mid-, and lower hanger plates  30 ,  32 ,  34  held together by tie rods  36 . For clarity, the tie bars are denoted in  FIG. 16  as upper tie bars  36   1  and lower tie bars  36   2 , and the various levels of tie bar couples are denoted as upper tie bar couples  42   1 , middle tie bar couples  42   2 , and lower tie bar couples  42   3 . At the upper end, short tie rods (i.e. tie rod bosses)  36 B have upper ends welded to the riser transition  38  and have lower ends threaded into the tops of upper tie bar couplings  42   1 . The upper tie bars  36   1  have their upper ends threaded into the bottoms of upper tie bar couplings  42   1  and have their lower ends threaded into the tops of middle tie bar couplings  42   2 . The lower tie bars  36   2  have their upper ends threaded into the bottoms of middle tie bar couplings  42   2  and have their lower ends threaded into the tops of lower tie bar couplings  42   3 . 
       FIGS. 17 and 18  show perspective and sectional perspective views, respectively, of the middle tie bar coupling  42   2 . As best seen in  FIG. 18 , the tie rod coupling  42   2  has a turnbuckle (i.e. length adjusting) configuration including outer sleeves  81 ,  82  having threaded inner diameters that engage (1) the threaded outsides of the ends of the respective mating tie rods  36   1 ,  36   2 , and (2) the threaded outsides of a plate thread feature  84 . Thus, by rotating the outer sleeve  81  the position of tie rod  36   1  respective to the mid-hanger plate  32  can be adjusted; and similarly, by rotating the outer sleeve  82  the position of tie rod  36   2  respective to the mid-hanger plate  32  can be adjusted. (Note that the plate thread feature  84  can be a single element passing through the mid-hanger plate  32 , or alternatively can be upper and lower elements extending above and below the mid-hanger plate  32 , respectively). The tie bar coupling  42   1  is the same as tie bar coupling  42   2  except that the upper outer sleeve  81  suitably engages the tie rod boss  36 B; while, the tie bar coupling  42  is the same as tie bar coupling  42   2  but omits the lower half (i.e. lower outer sleeve  82  and the corresponding portion of the plate thread feature  84 ), since there is no tie rod “below” for the tie bar coupling  42   3  to engage. 
     Said another way, the tie rod coupling portions  81 ,  82  can be threaded on their inner diameter with threads matching that of the outer diameter of the tie rods  36  and of the threading feature  84  of any of the plates  30 ,  32 ,  34  or riser transition  38 . This allows the coupling  42  to be threaded onto the tie rod  36  and onto the threading feature  84  of any other component. The advantages to a coupling such as this is that a very accurate elevation can be held with each of the above mentioned components  30 ,  32 ,  34 ,  38  within the upper internals, and that each of the above components can hold a very accurate parallelism with one another. Essentially, the couplings allow for very fine adjustments during the final assembly process. They also allow for a quick and easy assembly process. Another advantage to the couplings  42  is that they allow for the upper internals to be separated at the coupling joints fairly easily for field servicing or decommissioning of the nuclear power plant. It will be appreciated that the turnbuckle (i.e. length-adjusting) tie rod couplings can have alternative configurations, such as for example replacing the illustrative hollow tie rod coupling portions  81 ,  82  with inner diameter threading by threaded studs with outside diameter threading, and correspondingly replacing the outside threading on the tie rod  36  and the plate threading feature  84  with hollowed ends or bores having mating inside diameter threading (alternative not illustrated). 
     In an alternative tie rod coupling approach, it is contemplated for the tie rods to be directly welded to any of the plates or riser transition, in which case the tie rod couplings  42  would be suitably omitted. However, this approach makes it difficult to keep the tie rod perpendicular to the plates making assembly of the upper internals more difficult. It also makes breaking the upper internals down in the field more difficult. In contemplated hybrid embodiments, some threaded tie rod couplings are replaced by welded connections. For example, in the structure of  FIG. 16  it is contemplated to replace the threaded tie rod couplings  42   3  with welded couplings while retaining threaded tie rod couplings  42   2  to provide length adjustment between the plates  32 ,  34 . 
     With reference to  FIG. 19 , the riser transition  38  is shown in perspective view. The riser transition assembly  38  performs several functions. The riser transition  38  provides load transfer from the tie rods  36  of the upper internals basket  24  to the mid-flange  5  of the reactor pressure vessel. Toward this end, the riser transition  38  includes gussets  90  by which the riser transition  38  is welded to the mid-flange  5 . (See also  FIGS. 4 and 5  showing the riser transition  38  with gussets  90  welded to the mid-flange  5 ). One or more of these gussets  90  may include a shop lifting lug  91  or other fastening point to facilitate transport, for example when the upper internals are lifted out during refueling. The load transfer from the tie rods  36  to the mid-flange  5  is mostly vertical loading due to the overall weight of the upper internals. However, there is also some radial differential of thermal expansion between the riser transition gussets  90  and the mid-flange  5 , and the riser transition  38  has to also absorb these thermal loads. As already mentioned, the riser cone and riser transition  38  also acts (in conjunction with the central riser  6  and core basket  22 ) as the flow divider between the hot leg and cold leg of the primary coolant loop. Still further, the riser transition  38  also houses or includes an annular hydraulic collection header  92  for supplying hydraulic power via vertical hydraulic lines  94  to the CRDMs (in the case of embodiments employing hydraulically driven scram mechanisms). The riser transition  38  also has an annular interface feature  96  for fit-up with the riser cone or other connection with the central riser  6 , and feature cuts  98  to allow the passing of the CRDM electrical MI cable. 
     With brief returning reference to  FIGS. 4 and 5 , the gussets  90  are suitably welded to the mid-flange  5  at one end and welded to the main body portion of the riser transition assembly  38  at the other end. The riser transition  38  is suitably made of 304L steel, in some embodiments, e.g. by machining from a ring forging. 
     With reference to  FIG. 20 , an illustrative gusset  90  is shown, having a first end  100  that is welded to the mid-flange  5  and a second end  102  that is welded to the riser transition  38  as already described. The gusset  90  includes horizontal cantilevered portion  104 , and a tensile-strained portion  106  that angles generally downward, but optionally with an angle A indicated in  FIG. 20 . The horizontal cantilevered portion  104  has a thickness d cant  that is relatively greater than a thickness d G  of the tensile-strained portion  106 . The thicker cantilevered portion  104  handles the vertical loading component, while the tensile-strained portion  106  allows the gusset  90  to deflect in the lateral direction to absorb lateral loading due to thermal expansion. The angle A of the tensile-strained portion  106  provides for riser cone lead-in. The end  102  of the gusset  90  that is welded to the riser transition  38  includes an upper ledge  108  that serves as a riser cone interface. 
     In the illustrative embodiments, the CRDMs  8  are bottom supported from the mid-hanger plate  32 , and the tops of the CRDMs  8  are supported by the upper hanger plate  30 , which serves as the lateral support for each CRDM, locking each in laterally with a honeycomb type structure (see  FIG. 15 ). Even with this support structure, however, the CRDM  8  should be protected during an Operating Basis Earthquake (OBE) or other event that may cause mechanical agitation. To achieve this, it is desired to support the upper end of the CRDM to prevent excessive lateral motion and consequently excessive loads during an OBE. It is disclosed to employ a restraining device which still allows for ease of maintenance during an outage. Using spring blocks integrated into the CRDM  8  satisfies both of these requirements, as well as providing compliance that accommodates any differential thermal expansion. 
     Integrating compliance features into support straps of the CRDM  8  allows the CRDM&#39;s to be removed while still maintaining lateral support. As the CRDM is lowered into its mounting location the compliant features come into contact with the upper hanger plate  30 . The compliance allows them to maintain contact with the upper hanger plate yet allow for misalignment between the CRDM standoff mounting point and the upper hanger plate. Their engagement into the upper hanger plate  30  allows them to be of sufficient height vertically from the mounting base of the CRDMs to minimize the loads experienced at the base in an OBE event. Having no feature that extends below the upper hanger plate allows the CRDM to be removed from the top for service. 
     With reference to  FIGS. 21 and 22 , an upper end of a CRDM  8  includes a hydraulic line  110  delivering hydraulic power to a scram mechanism. Straps  112 ,  114  secure the hydraulic line  110  to the CRDM  8 . The strap  114  is modified to include compliance features  116 . As seen in  FIG. 22 , the compliance features  116  comprise angled spring blocks that wedges into the opening  70  of the upper hanger plate  30  when the CRDM  8  is fully inserted. It will be appreciated that such compliance features  116  can be incorporated into straps retaining other elements, such as electrical cables (e.g. MI cables). The illustrative compliance features  116  can be constructed as angled leaf springs cut into the (modified) strap  114 . Alternatively, such leaf springs can be additional elements welded onto angled ends of the strap  114 . By including such springs on straps  114  on opposite sides of the CRDM  8 , four contact points are provided to secure the CRDM against lateral motion in any direction. The wedged support provided by the straps  114  also leave substantial room for coolant flow through the opening  70  in the upper hanger plate  30 . 
     The disclosed embodiments are merely illustrative examples, and numerous variants are contemplated. For example, the suspended frame of the upper internals can include more than three plates, e.g. the power distribution plate could be a separate fourth plate. In another variant, the mid-hanger plate  32  could be separated into two separate hanger plates—an upper mid-hanger plate bottom-supporting the CRDMs, and a lower mid-hanger plate from which the guide frames are suspended. In such a case, the two mid-hanger plates would need to be aligned by suitable alignment features to ensure relative alignment between the CRDMs and the guide frames. 
     The use of at least three hanger plates is advantageous because it provides both top and bottom lateral support for both the CRDMs and the guide frames. However, it is contemplated to employ only two hanger plates if, for example, the bottom support of the CRDMs is sufficient to prevent lateral movement of the CRDMs. 
     In the illustrative embodiments, the suspended support assembly  24  is suspended from the mid-flange  5  via the riser transition  38 . However, other anchor arrangements are contemplated. For example, the suspended support assembly could be suspended directly from the mid-flange, with the riser transition being an insert secured to the gussets. The mid-flange  5  could also be omitted. One way to implement such a variant is to include a ledge in the lower vessel on which a support ring sits, and the suspended support assembly is then suspended from the support ring. With the mid-flange  5  omitted, the upper and lower flanges  5 U,  5 L of the upper and lower vessel sections can suitably connect directly (i.e., without an intervening mid-flange). Instead of lifting the upper internals out by the mid-flange  5 , the upper internals would be lifted out by the support ring. 
     In the embodiment of  FIGS. 2 and 3 , the internal RCPs  16 ′ are incorporated into the upper internals and are lifted out with the upper internals. Other configurations are also contemplated—for example, internal RCPs could be mounted in the upper vessel and removed with the upper vessel. 
     The preferred embodiments have been illustrated and described. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.