Suspended upper internals with tie rod couplings for compact nuclear reactor

A suspended basket includes a plurality of plates, tie rods, and adjustable length threaded tie rod couplings connecting threaded ends of the tie rods with threaded features of the plates. Control rod drive mechanisms (CRDMs) with CRDM motors are mounted in the suspended basket, which is suspended in a pressure vessel above a nuclear reactor core to control insertion of control rods into the reactor core. In one embodiment each adjustable length threaded tie rod coupling is a turnbuckle coupling that includes a sleeve threaded onto the threaded end of the tie rod and onto the threaded feature of the plate, and the sleeve is rotatable to adjust the position of the tie rod respective to the plate. Guide frames may be mounted in the suspended basket between the CRDMs and the nuclear reactor core to guide portions of the control rods withdrawn from the nuclear reactor core.

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 (typically235U). 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'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'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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIG. 1, a small modular reactor (SMR)1of the of the integral pressurized water reactor (PWR) variety is shown in partial cutaway to reveal selected internal components. The illustrative PWR1includes a nuclear reactor core2disposed in a pressure vessel comprising a lower vessel portion3and an upper vessel portion4. The lower and upper vessel portions3,4are connected by a mid-flange5. Specifically, a lower flange5L at the open top of the lower vessel portion3connects with the bottom of the mid-flange5, and an upper flange5U at the open bottom of the upper vessel portion4connects with a top of the mid-flange5.

The reactor core2is disposed inside and at or near the bottom of the lower vessel portion3, and comprises a fissile material (e.g.,235U) immersed in primary coolant water. A cylindrical central riser6is disposed coaxially inside the cylindrical pressure vessel and a downcomer annulus7is defined between the central riser6and the pressure vessel. The illustrative PWR1includes internal control rod drive mechanisms (internal CRDMs)8with internal motors8mimmersed in primary coolant that control insertion of control rods to control reactivity. Guide frames9guide 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 PWR1employs one or more internal steam generators10located inside the pressure vessel and secured to the upper vessel portion4, 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 generator10is of the once-through straight-tube type with internal economizer, and is fed by a feedwater inlet11and deliver steam to a steam outlet12. 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 PWR1includes an integral pressurizer14at the top of the upper vessel section4which defines an integral pressurizer volume15; however an external pressurizer connected with the pressure vessel via suitable piping is also contemplated. The primary coolant in the illustrative PWR1is circulated by reactor coolant pumps (RCPs) comprising in the illustrative example external RCP motors16driving an impeller located in a RCP plenum17disposed inside the pressure vessel.

With reference toFIGS. 2 and 3, a variant PWR design1′ is shown, which differs from the PWR1ofFIG. 1by having a differently shaped upper vessel section4′ and internal RCPs16′ in place of the external pumps16,17of the PWR1.FIG. 2shows the pressure vessel with the upper vessel section4′ lifted off, as is done during refueling. The mid-flange5remains disposed on the lower flange5L of the lower vessel3.FIG. 3shows an exploded view of the lower vessel section3and principle components contained therein, including: the nuclear reactor core2comprising fuel assemblies2′ contained in a core former20disposed in a core basket22.

With continuing reference toFIGS. 1 and 3and with further reference toFIGS. 4 and 5, above the reactor core assembly2,20,22are the upper internals which include a suspended support assembly24comprising an upper hanger plate30, a mid-hanger plate32, and a lower hanger plate34suspended by tie rods36from the mid-flange5. More particularly, in the illustrative embodiment the upper ends of the tie rods36are secured to a riser transition section38that is in turn secured with the mid-flange5. The central riser6disposed in the upper vessel section4,4′ (shown only inFIG. 1) is connected with the core basket22in the lower vessel section3by the riser cone (not shown) riser transition section38to 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 assembly24comprising hanger plates30,32,34interconnected by tie rods36provides the structural support for the CRDMs8and the guide frames9(note the CRDMs8and guide frames9are omitted inFIG. 3). The CRDMs8are disposed between the upper hanger plate30and the mid-hanger plate32, and are either (1) top-supported in a hanging fashion from the upper hanger plate or (2) bottom-supported on the mid-hanger plate32(as in the illustrative embodiments described herein). Lateral support for the CRDMs8is provided by both plates30,32. (Note that in the illustrative embodiment, the CRDMs8actually pass through openings of the upper hanger plate30so that the tops of the CRDMs8actually extend above the upper hanger plate30, as best seen inFIG. 1). The guide frames9are disposed between the mid-hanger plate32and the lower hanger plate34, and are likewise either (1) top-supported in a hanging fashion from the mid-hanger plate32(as in the illustrative embodiments described herein) or (2) bottom-supported on the lower hanger plate. Lateral support for the guide frames9is provided by both plates32,34.

One of the hanger plates, namely the mid-hanger plate32in the illustrative embodiments, also includes or supports a distribution plate that includes mineral insulated cabling (MI cables) for delivering electrical power to the CRDM motors8M and, in some embodiments, hydraulic lines for delivering hydraulic power to scram latches of the CRDMs8. In the embodiment ofFIGS. 2 and 3(and as seen inFIG. 3), the internal RCPs16′ are also integrated into the upper internals assembly24, for example on an annular pump plate providing both separation between the suction (above) and discharge (below) sides of the RCPs16′ and also mounting supports for the RCPs16′.

The disclosed upper internals have numerous advantages. The suspension frame24hanging from the mid-flange5is a self-contained structure that can be lifted out of the lower vessel section3as a unit during refueling. Therefore, the complex assembly of CRDMs8, guide frames9, and ancillary MI cabling (and optional hydraulic lines) does not need to be disassembled during reactor refueling. Moreover, by lifting the assembly5,24,8,9out of the lower vessel3as a unit (e.g. using a crane) and moving it to a suitable work stand, maintenance can be performed on the components5,24,8,9simultaneously with the refueling, thus enhancing efficiency and speed of the refueling. The tensile forces in the tie rods36naturally tend to laterally align the hanger plates30,32,34and thus the mounted CRDMs8and guide frames9.

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 frame24) 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 plates30,32,34are positioned at varying elevations and are connected to each other, and the remainder of the upper internals, via the tie rods36. The design of the illustrative upper internals basket24is such that the control rod guide frames9are hung from the mid-hanger plate32(although in an alternative embodiment the guide frames are bottom-supported by the lower hanger plate). In the top-supported hanging arrangement, the guide frames9are laterally supported at the bottom by the lower hanger plate34. The upper internals are aligned with the core former20and/or core basket22to ensure proper fit-up of the fuel to guide frame interface. This alignment is achieved by keying features of the lower hanger plate34.

With reference toFIGS. 6 and 7, alternative perspective views are shown of the hanger plates30,32,34connected by tie rods36and with the guide frames9installed, but omitting the CRDMs8so as to reveal the top surface of the mid-hanger plate32. In the illustrative embodiment, a distribution plate40is disposed on top of the mid-hanger plate32, as best seen inFIG. 6. The distribution plate40is a load-transferring element that transfers (but does not itself support) the weight of the bottom-supported CRDMs8to the mid-hanger plate32. 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 CRDMs8).

With reference toFIG. 8, which shows a corner of the upper hanger plate30as an illustrative example, the tie rods36are coupled to each plate by tie rod couplings42, 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 rods36extend between the upper and mid-hanger plates30,32with their upper ends terminating at tie rod couplings42at the upper hanger plate30and their lower ends terminating at tie rod couplings42at the mid-hanger plate32; and similarly, the lower tie rods36extend between the mid-hanger plate32and the lower hanger plate34with their upper ends terminating at tie rod couplings42at the mid-hanger plate32and their lower ends terminating at tie rod couplings42at the lower hanger plate34.

With reference toFIGS. 9 and 10, the lower hanger plate34in the illustrative embodiment provides only lateral support for the guide frames9which are top-supported in hanging fashion from the mid-hanger plate32. Consequentially, the lower hanger plate34is suitably a single plate with openings50that mate with the bottom ends of the guide frames (seeFIG. 10). To simplify the alignment, in some embodiments guide frame bottom cards52(seeFIG. 9) are inserted into the openings50and are connected with the bottom ends of the guide frames9by fasteners, welding, or another technique. (Alternatively, the ends of the guide frames may directly engage the openings50of the lower hanger plate34).

In addition to providing lateral support for each control rod guide frame9, locking each in laterally with a honeycomb-type structure (seeFIG. 10), the lower hanger plate34also includes alignment features54(seeFIG. 10) that align the upper internals with the core former20or with the core basket22. The illustrative alignment features are peripheral notches54that engage protrusions (not shown) on the core former20; 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 inFIG. 10are peripheral openings56in the lower hanger plate34into which the tie rod couples42of the lower hanger plate fit. The lower hanger plate34is suitably machined out of plate material or forging material. For example, in one contemplated embodiment the lower hanger plate34is machined from 304L steel plate stock.

With continuing reference toFIGS. 6 and 7and with further reference toFIG. 11, the mid-hanger plate32provides top support for the guide frames9and bottom support for the CRDMs8. The mid-hanger plate32acts as a load distributing plate taking the combined weight of the CRDMs8and the guide frames9and transferring that weight out to the tie rods36on the periphery of the upper internals basket24. In the illustrative embodiment, the power distribution plate40is also bottom supported. Like the lower hanger plate34, the mid-hanger plate32includes openings60. The purpose of the openings60is to enable the connecting rod, translating screw, or other coupling mechanism to connect each CRDM8with the control rod assembly driven by the CRDM. To facilitate hanging the guide frames9off the bottom of the mid-hanger plate32, an egg crate-type structure made of orthogonally intersecting elements61is provided for increased strength and reduced deflection due to large loads.

With reference toFIGS. 12 and 13, the mid-hanger plate32can be manufactured in various ways. In one approach (FIG. 12), a forging machining process is employed to machine the mid-hanger plate32out of a 304L steel forged plate62. The machining forms the openings60and the intersecting elements61. In another approach (FIG. 13), a machined plate64and the intersecting elements61are manufactured as separate components, and the intersecting elements61are interlocked using mating slits formed into the intersecting elements61and welded to each other and to the machined plate64to form the mid-hanger plate32. As previously noted, the illustrative bottom-supported distribution plate40can alternatively be integrally formed into the mid-hanger plate.

With reference toFIG. 14, in an alternative embodiment the guide frames9are bottom supported by an alternative lower hanger plate34′, and are laterally aligned at top by an alternative mid-hanger plate32′. In this case the alternative lower hanger plate34′ may have the same form and construction as the main embodiment mid-hanger plate32ofFIGS. 11-13(but with suitable alignment features to align with the core former and/or core basket, not shown inFIG. 14), and the alternative mid-hanger plate32′ can have the same form and construction as the main embodiment lower hanger plate34ofFIG. 10(but without said alignment features). If the CRDMs remain bottom supported, then the alternative mid-hanger plate32′ should be made sufficiently thick (or otherwise sufficiently strong) to support the weight of the CRDMs. As another variant, the alternative mid-hanger plate32′ 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 frames9are continuous columnar guide frames9that 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 frame24operates 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 plate32is particularly well-suited to supporting conventional guide frames, as the hanging arrangement tends to self-align the guide frame plates.

With reference toFIG. 15, an illustrative embodiment of the upper hanger plate30is shown. Like the other hanger plates32,34, the upper hanger plate30includes openings70, in this case serving as passages through which the upper ends of the CRDMs8pass. The inner periphery of each opening70serves as a cam to laterally support and align the upper end of the CRDM8. The upper hanger plate30can also suitably be made by machining from either plate material or forging material, e.g. a 304L steel plate stock or forging.

With reference toFIGS. 16-18, the tie bar (alternatively “tie rod”) couplings42are further described.FIG. 16shows the suspended frame24including the upper, mid-, and lower hanger plates30,32,34held together by tie rods36. For clarity, the tie bars are denoted inFIG. 16as upper tie bars361and lower tie bars362, and the various levels of tie bar couples are denoted as upper tie bar couples421, middle tie bar couples422, and lower tie bar couples423. At the upper end, short tie rods (i.e. tie rod bosses)36B have upper ends welded to the riser transition38and have lower ends threaded into the tops of upper tie bar couplings421. The upper tie bars361have their upper ends threaded into the bottoms of upper tie bar couplings421and have their lower ends threaded into the tops of middle tie bar couplings422. The lower tie bars362have their upper ends threaded into the bottoms of middle tie bar couplings422and have their lower ends threaded into the tops of lower tie bar couplings423.

FIGS. 17 and 18show perspective and sectional perspective views, respectively, of the middle tie bar coupling422. As best seen inFIG. 18, the tie rod coupling422has a turnbuckle (i.e. length adjusting) configuration including outer sleeves81,82having threaded inner diameters that engage (1) the threaded outsides of the ends of the respective mating tie rods361,362, and (2) the threaded outsides of a plate thread feature84. Thus, by rotating the outer sleeve81the position of tie rod361respective to the mid-hanger plate32can be adjusted; and similarly, by rotating the outer sleeve82the position of tie rod362respective to the mid-hanger plate32can be adjusted. (Note that the plate thread feature84can be a single element passing through the mid-hanger plate32, or alternatively can be upper and lower elements extending above and below the mid-hanger plate32, respectively). The tie bar coupling421is the same as tie bar coupling422except that the upper outer sleeve81suitably engages the tie rod boss36B; while, the tie bar coupling42is the same as tie bar coupling422but omits the lower half (i.e. lower outer sleeve82and the corresponding portion of the plate thread feature84), since there is no tie rod “below” for the tie bar coupling423to engage.

Said another way, the tie rod coupling portions81,82can be threaded on their inner diameter with threads matching that of the outer diameter of the tie rods36and of the threading feature84of any of the plates30,32,34or riser transition38. This allows the coupling42to be threaded onto the tie rod36and onto the threading feature84of 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 components30,32,34,38within 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 couplings42is 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 portions81,82with inner diameter threading by threaded studs with outside diameter threading, and correspondingly replacing the outside threading on the tie rod36and the plate threading feature84with 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 couplings42would 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 ofFIG. 16it is contemplated to replace the threaded tie rod couplings423with welded couplings while retaining threaded tie rod couplings422to provide length adjustment between the plates32,34.

With reference toFIG. 19, the riser transition38is shown in perspective view. The riser transition assembly38performs several functions. The riser transition38provides load transfer from the tie rods36of the upper internals basket24to the mid-flange5of the reactor pressure vessel. Toward this end, the riser transition38includes gussets90by which the riser transition38is welded to the mid-flange5. (See alsoFIGS. 4 and 5showing the riser transition38with gussets90welded to the mid-flange5). One or more of these gussets90may include a shop lifting lug91or other fastening point to facilitate transport, for example when the upper internals are lifted out during refueling. The load transfer from the tie rods36to the mid-flange5is 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 gussets90and the mid-flange5, and the riser transition38has to also absorb these thermal loads. As already mentioned, the riser cone and riser transition38also acts (in conjunction with the central riser6and core basket22) as the flow divider between the hot leg and cold leg of the primary coolant loop. Still further, the riser transition38also houses or includes an annular hydraulic collection header92for supplying hydraulic power via vertical hydraulic lines94to the CRDMs (in the case of embodiments employing hydraulically driven scram mechanisms). The riser transition38also has an annular interface feature96for fit-up with the riser cone or other connection with the central riser6, and feature cuts98to allow the passing of the CRDM electrical MI cable.

With brief returning reference toFIGS. 4 and 5, the gussets90are suitably welded to the mid-flange5at one end and welded to the main body portion of the riser transition assembly38at the other end. The riser transition38is suitably made of 304L steel, in some embodiments, e.g. by machining from a ring forging.

With reference toFIG. 20, an illustrative gusset90is shown, having a first end100that is welded to the mid-flange5and a second end102that is welded to the riser transition38as already described. The gusset90includes horizontal cantilevered portion104, and a tensile-strained portion106that angles generally downward, but optionally with an angle A indicated inFIG. 20. The horizontal cantilevered portion104has a thickness dcantthat is relatively greater than a thickness dGof the tensile-strained portion106. The thicker cantilevered portion104handles the vertical loading component, while the tensile-strained portion106allows the gusset90to deflect in the lateral direction to absorb lateral loading due to thermal expansion. The angle A of the tensile-strained portion106provides for riser cone lead-in. The end102of the gusset90that is welded to the riser transition38includes an upper ledge108that serves as a riser cone interface.

In the illustrative embodiments, the CRDMs8are bottom supported from the mid-hanger plate32, and the tops of the CRDMs8are supported by the upper hanger plate30, which serves as the lateral support for each CRDM, locking each in laterally with a honeycomb type structure (seeFIG. 15). Even with this support structure, however, the CRDM8should 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 CRDM8satisfies both of these requirements, as well as providing compliance that accommodates any differential thermal expansion.

Integrating compliance features into support straps of the CRDM8allows the CRDM'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 plate30. 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 plate30allows 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 toFIGS. 21 and 22, an upper end of a CRDM8includes a hydraulic line110delivering hydraulic power to a scram mechanism. Straps112,114secure the hydraulic line110to the CRDM8. The strap114is modified to include compliance features116. As seen inFIG. 22, the compliance features116comprise angled spring blocks that wedges into the opening70of the upper hanger plate30when the CRDM8is fully inserted. It will be appreciated that such compliance features116can be incorporated into straps retaining other elements, such as electrical cables (e.g. MI cables). The illustrative compliance features116can be constructed as angled leaf springs cut into the (modified) strap114. Alternatively, such leaf springs can be additional elements welded onto angled ends of the strap114. By including such springs on straps114on opposite sides of the CRDM8, four contact points are provided to secure the CRDM against lateral motion in any direction. The wedged support provided by the straps114also leave substantial room for coolant flow through the opening70in the upper hanger plate30.

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 plate32could 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 assembly24is suspended from the mid-flange5via the riser transition38. 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-flange5could 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-flange5omitted, the upper and lower flanges5U,5L 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-flange5, the upper internals would be lifted out by the support ring.

In the embodiment ofFIGS. 2 and 3, the internal RCPs16′ 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.