TOP END SUPPORT FOR WATER DISPLACEMENT ROD GUIDES OF PRESSURIZED WATER NUCLEAR REACTOR

Abstract: A top end support (72, 80) for rod guides (32) of a pressurized water reactor includes an axially extending sleeve (72) affixed to the upper end of a rod guide (32) and a fixed cylindrical support (80) having an integral depending sidewall (81) of generally cylindrical configuration received in telescoping relationship within said rod guide sleeve (72) when in assembled relationship therewith.

Description: This invention relates to pressurized water reactors and, more particularly, to a top end support for the water displacement rod guides positioned within the inner barrel assembly of a pressurized water reactor. Certain advanced designs of nuclear reactors incorporate at successively higher, axially aligned eleva­ tions within the reactor vessel, a lower barrel assembly, an inner barrel assembly, and a calandria, each of general­ ly cylindrical configuration, and an upper closure dome. The lower barrel assembly generally has mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies which are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates. Within the inner barrel assembly there is provided a large number of rod guides disposed in closely spaced relationship, in an array extending substantially throughout the cross-sectional area of the inner barrel assembly. The rod guides are of first and second types, respectively, housing therewithin reactor control rod clusters (RCC) and water displacer rodlet clusters (WDRC); these clusters, as received within their respectively associated guides, generally are aligned with the fuel rod assemblies. The calandria includes a lower calandria plate and an upper calandria plate. The rod guides are secured in position at the lower and upper ends thereof, respectively, to the upper core plate and the lower calan­ dria plate. the inner barrel assembly thus is defined in axial height between a first plate of a lower elevation comprising the upper core plate and a second plate of a higher elevation comprising the lower calandria plate. Within the calandria and extending between the lower and upper plates thereof is mounted a plurality of calandria tubes in parallel axial relationship and respectively aligned with the rod guides. A number of flow holes are provided in remaining portions of the calandria plates, intermediate the calandria tubes, through which passes the reactor core outlet flow as it exits from its passage through the inner barrel assembly. In similar parallel axial and aligned relation­ ship, the calandria tubes are joined to corresponding flow shrouds which extend to a predetermined elevation within the dome, and which in turn are connected to corresponding head extensions which pass thorugh the structural wall of the dome and carry, on their free ends at the exterior of and vertically above the dome, corresponding adjustment mechanisms. The adjustment mechanisms have corresponding control lines which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated clusters of RCC rods and WDRC rods, and serve to adjust their elevational positions within the inner barrel assembly and, particular­ ly, the level to which same are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the activity within the core. A critical design criterion of such reactors is to minimize wear of the rodlets in the rod guides, and thus to reduce or eliminate the factors which produce wear, such as flow-induced vibration. Because of the relatively dense packing of the rod guides within the inner barrel assembly, it is critical to maintain substantially uniform distribu­ tion of the outlet flow from the reactor core, and an axial direction of that flow through the upper barrel assembly. Even if a uniform, axial flow of the core outlet is achieved, the effects of differential pressure and tempera­ ture across the array of rod guides or an individual rod guide can produce significant reaction loads at the support points, or support connections, for the rod guides. These reaction loads, coupled with the flow-induced vibration, create a high potential for wear at the support points for the rod guides as well as the rodlets. Additionally, the provision of the calandria, and particularly the lower plate thereof, presents an interface with the top end of the rod guides which does not exist in conventional pres­ surized water reactors. That interface must be capable of accommodating differential thermal expansions between the lower calandria plate and the inner barrel in order to prevent large thermal stresses from developing. Further­ more, the lower calandria plate and the upper core plate are essentially structurally independent; therefore, vibration of the internals can result in significant relative movement between the supporting connections of the rod guides at their lower and upper ends to the respective upper core plate and the bottom calandria plate. The wear potential under these circumstances is great. Thus, split, or two-pin connections of conven­ tional types are inappropriate for use as the supporting connections for the top ends of the rod guides since they have too much torsional stiffness. If used, they would wear rapidly, with the result that the top ends of the rod guides would become loose. Rod guides having such loose top end connections are unacceptable because of the rapid rate of wear of the rodlets which would result. Other known mounting devices as well are inappropriate. For example, leaf springs cannot be used to support all of the rod guides because sufficient space is not available within the inner barrel assembly to provide leaf springs of the proper size for the large number of rod guides which are present, even if high strength material is used for the leaf springs. Beyond the unsuitability of existing, known structural support arrangements, further factors must be taken into account in the consideration of possible designs for the support of the top end of the rod guides within the inner barrel assembly. For example, both the RCC and the WDRC rod clusters should be removable without requiring that the guides be disassembled. This requirement imposes a severe space limitation in view of the dense packing of the guides and their associated rod clusters within the inner barrel assembly. For example, in one such reactor design, over 2,800 rods are mounted in 185 clusters, the latter being received within a corresponding 185 guides. The space limitation is further compounded by the require­ ment that unimpeded flow holes must be provided in the calandria plates for the core outlet flow. While these foregoing factors severely restrict the available space envelope in the horizontal cross-sectional dimension of the inner barrel assembly, axial or vertical limitations on this space envelope must also be considered. For example, the presence of the support members should not require any increase in the height of the vessel. From a maintenance standpoint, the support members should be replaceable without undue effort. Additionally, the assembly of the calandria must be accomplished without access to the support region. While the supports for the rod guides must therefore satisfy a wide range of structural and functional requirements relating to, or imposed by, the inner barrel assembly itself, it is the principal object of the present invention to provide a rod guide structure with minimum wear potential. With this object in view, the present invention resides in a top end support for rod guides disposed in closely spaced, parallel axial relationship in an inner barrel assembly of a pressurized water reactor vessel, the inner barrel assembly occupying a central portion of the vessel and being of a vertical height extending from a first plate of lower elevation to a second plate of higher elevation within the vessel, each of said rod guides having sidewalls extending between said first and second plates and being of generally square cross-section, each rod guide accommodating in axial, sliding relationship therewithin a respectively associated cluster of elongated rods intercon­ nected to extend in parallel axial relationship, charac­ terized in that each said rod guide includes an axially extending sleeve affixed at a lower end thereof to the upper end of a respective said sidewall of said rod guide and of an axial length such that said upper end thereof is closely adjacent to, but displaced from, the lower surface of the second plate when in assembled relationship there­ with, said sleeve having an outer surface generally corre­ sponding in cross-section to that of said side wall of said rod guide and an inner circumferential surface of generally cylindrical cross-section; a fixed cylindrical support having an upper end closure adapted for being secured to the lower surface of the second plate and having an inte­ gral depending sidewall of generally cylindrical configu­ ration received in telescoping relationship within said rod guide sleeve when in assembled relationship therewith; and means for securing said fixed cylindrical support to the second plate. The top end support of the present invention thus provides overall support and alignment between the calan­ dria and the associated rod guides, loads exerted on the associated rod guides being reacted into the calandria. The telescoping interconnection between the fixed cylindri­ cal supports and the associated rod guide sleeves, on the other hand, provides compensation for thermal stresses and accommodates tolerances in the axial spacing between the lower calandria plate and the upper core plate and the vibrations encountered during operation, while facilitating removal and reassembly of the calandria with the rod guides for routine maintenance. Moreover, the configuration of the top end support maximizes the area of the associated wear surfaces, affording a long-life device. Significant­ ly, the top end support is configured to provide the requisite flow paths through the lower calandria plate for the core outlet flow. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown by way of example only, in the accompanying drawings, wherein: Figure 1 is an elevational view, partially in cross-section, of a pressurized water reactor of the type with which the top end supports of the present invention are intended to be employed; Figure 2 is a plan view of a top end support in accordance with the present invention, taken along the line 2-2 in Fig. 1, and thus showing in cross-section the sidewalls of the WDRC rod guide associated with the illus­ trated top end support; Figure 3 is a cross-sectional, elevational view taken along the line 3-3 in Fig. 2; Figure 4 is a cross-sectional, elevational view taken along the line 4-4 in Fig. 2; and Figure 5 is a schematic, plan view illustrating a WDRC rod cluster in a configuration for being received within the rod guide and associated top end support in accordance with the present invention as illustrated in the foregoing figures. As shown in Fig. 1, pressurized water reactor 10 comprises a vessel 12 including an upper dome 12a, cylin­ drical sidewalls 12b, and a bottom closure 12c comprising the base of the reactor 10. Within the bottom closure 12c, as schematically indicated, is so-called base-mounted instrumentation 14. The lower barrel assembly 16 comprises a generally cylindrical sidewall 17 affixed at its lower and upper ends to respective lower and upper core plates 18 and 19. Fuel rods assemblies 20 are positioned in generally vertically oriented, parallel axial relationship within the lower barrel assembly 16. A radiation reflec­ tion shield 21 is mounted interiorly of the cylindrical sidewalls 17, in conventional fashion. The inner barrel assembly 24 includes a cylindri­ cal sidewall 26 within which are positioned a plurality of rod guides in closely spaced, parallel axial relationship; for simplicity of illustration, only two such rod guides are shown in Fig. 1, namely rod guide 28 housing a cluster of radiation control rods 30 (RCC) and a rod guide 32 housing a cluster of water displacement rods 33 (WDRC). Mounting means 36 and 37 are provided at the respective upper and lower ends of the rod guide 28 and, correspond­ ingly, mounting means 38 and 39 are provided at the respec­ tive upper and lower ends of the rod guide 32, the lower end mounting means 37 and 39 mounting the respective rod guides 28 and 32 to the upper core plate 19, and the upper mounting means 36 and 38 mounting the respective rod guides 28 and 32 to a calandria assembly 50, and more particularly to the lower calandria plate 52. The inner barrel assembly 24 thus is defined to extend, in axial height, from a first plate of lower elevation comprising the upper core plate 19 to a second plate of higher elevation comprising the lower calandria plate 52. The calandria assembly 50 includes a lower calandria plate 52, an upper calandria plate 54, and a plurality of parallel axial calandria tubes 56 which are positioned in alignment with corresponding apertures in the lower and upper calandria plates 52 and 54 and to which the calandria tubes 56 are mounted at their respective, oppo­ site ends. Extending upwardly beyond the upper calandria plate 54 and, more particularly, within the dome 12a of the vessel 12, there is provided a plurality of flow shrouds 60 respectively aligned with the calandria tubes 56. A corresponding plurality of head extensions 62 is aligned with the plurality of flow shrouds 60, with respective adjacent ends thereof in generally overlapping relationship. Control rod cluster (RCC) displacement mechanisms 64 and water displacement rod cluster (WDRC) displacement mechanisms 66 are associated with the respec­ tive head extensions 62, flow shrouds 60 and calandria tubes 56 which, in turn, are respectively associated with the respective clusters of radiation control rods 30 and water displacement rods 34. Particularly, the RCC and WDRC displacement mechanisms 64 and 66 connect through corre­ sponding lines to the respective clusters of radiation control rods and water displacement rods 30 and 34, to control the respective vertical positions thereof and, particularly, to selectively lower same through correspond­ ing openings (not shown) provided therefore in the upper core plate 19 into surrounding relationship with respec­ tively associated fuel rod assemblies 20. In this regard, the clusters 30 and 34 have an extent of travel represented by the distance D designated in Fig. 1. While the particu­ lar control function is not relevant to the present inven­ tion, insofar as the control over the reaction within the core is effected by the selective positions of the respec­ tive rod clusters 30 and 34, it is believed that those skilled in the art will appreciate that moderation or control of the reaction is accomplished in accordance with the extent to which the control rod clusters 30 are insert­ ed into the core and with the effective water displacement adjustment which is achieved by selective positioning of the water displacement rods 34. A first matrix of calandria extensions 58 project downwardly from the calandria tubes 56 and connect to corresponding mounting means 36 for the upper ends, or tops, of the RCC rod guides 28. As before noted, the top end support for the RCC rod guides 28 may be in accordance with the structure dislosed in a concurrently filed appli­ cation of one of the named co-inventors herein, entitled "Flexible Rod Guide Support Structure for Inner Barrel Assembly of Pressurized Water Reactor," assigned to the common assignee herewith. A second matrix of calandria extensions 59, in interleaved relationship with the matrix of extensions 58, projects downwardly from the respectively corresponding calandria tubes 56, each extension 59 connecting to a corresponding mounting means 38 for a WDRC rod guide 32, in accordance with the present invention. As before briefly noted, each of the mounting means 38 for the WDRC rod guides provides a telescoping interconnection between the lower calandria plate 52 and the respectively associated WDRC rod guide 32, thereby affording axial alignment and lateral support of the associated, individual WDRC rod guide 32. The calandria extensions 59 function to react seismic forces from the rod guides 32 into the calandria, while accommodating axial height tolerances and thermal stresses at the interface of the upper ends of the rod guides 32 and the lower calandria plate 52. Figure 2 is a plan view of the top end support in accordance with the present invention, taken along the line 2-2 in Fig. 1 and thus showing an upper end portion of a WDRC rod guide 32 in cross-section, and a bottom plan view of the associated top end support 38. The top end support 38 furthermore will be described with concurrent reference to Figs. 3 and 4, which respectively comprise cross-­ sectional elevational views taken along the lines 3-3 and 4-4 in Fig. 2. The WDRC rod guide, throughout substantially its entire axial length, comprises a relatively thin metal sidewall 70 of generally square cross-sectional configura­ tion which carries, at its upper extremity, a reinforced, generally coaxial sleeve 72 having a generally square cross-sectional configuration corresponding to the outer perimeter of the thin sidewall 70 and which is permanently joined at its bottom end to the top end of the latter at their common outer perimeters, as illustrated by weld bead 74. The sleeve 72 is one component of a pair of wear couple components which axially align and laterally support the rod guide with respect to the lower calandria plate 52. The second component of the top end support 38 comprises a fixed cylindrical support 80 having a generally cylindrical sidewall 81 and an end closure 82. The end closure 82 includes a central aperture 82' for receiving therethrough the calandria extension 59 in closely engaged, coaxial relationship. Flow holes 100 are disposed in a symmetrical array about the axis of the calandria extension 59 associ­ ated with the WDRC rod guide 32 and its associated top end support 38, which must be unobstructed so as to permit unimpeded passage of the core output flow axially there­ through and into the calandria assembly 50. Accordingly, the cylindrical sidewall 81 is discontinuous, or termi­ nates, at the corresponding perimeters of the flow holes 100 and thus comprises a plurality of arcuate segments 81' (best seen in Fig. 2), each bounded at its opposite ends by the corresponding perimeters of the adjacent flow holes 100. The end closure 82 correspondingly is configured to accommodate the flow holes 100 and thus includes a number of arcuate indentations, or recesses, 82' ' corresponding to the inner perimeter portions of the flow holes 100 adjacent the calandria extension 59. As best see in Fig. 4, the calandria extension 59 is rigidly secured to the lower calandria plate 52 by a weld bead 59' which extends about the entire circumference of the extension 59, the calandria extension 59 being of sufficient axial length such that its lower end 59' ' is flush with the lower surface of the end closure 82. The end closure 82 furthermore includes a number of countersunk bores 85 through which bolts 86 are received and secured in tightly threaded engagement with the corresponding threaded bores 87 in the calandria lower plate 52. The interior surfaces of the arcuate segments 81' include axially extending grooves 88, for a purpose to be explained. The rod guide sleeve 72 is machined so as to include an inner surface having a configuration which accommodates the flow holes 100, and defines a mating, wear surface with the outer circumferences of the arcuate segments 81' of the cylindrical sidewall 81. More specifi­ cally, as best seen in Fig. 2, the rod guide sleeve 72 includes plural interior arcuate surfaces 72' in precise conformity and mating relationship with the corresponding exterior surfaces of the arcuate segments 81' of the cylindrical sidewall 81, and generally arcuate recesses 72' ' conforming to the corresponding outer perimeter portions of the flow holes 100. Within each of the arcuate recesses 72' ' there additionally are formed axially extend­ ing grooves 75, for a purpose to be explained. As best seen in Figs. 3 and 4, the fixed cylin­ drical support 80 and the rod guide sleeve 72 are assembled in coaxial, concentric and telescoping relationship, permitting axial movement therebetween while restraining any lateral movement such that forces acting on the rod guide 32 are reacted through the top end support 38 and into the lower calandria plate 52. To facilitate the telescoping assemblage, the sleeve 72 includes an outward bevel 78, and the cylindrical support 80 includes an inward bevel 89. Figure 5 comprises a simplified, schematic plan view of the WDRC rod clusters 34, more particularly compris­ ing a spider 90 having a plurality of radially extending arms 92 connected to a central hub 93; further, alternate ones of the arms 92 include transverse cross-arms 92a. A plurality of WDRC rods 94 are appropriately connected to the arms 92 and the cross-arms 92a and depend therefrom in parallel axial relationship. With concurrent reference to Figs. 2 to 5, the grooves 75 and 88 are designed to accommodate the outer pair of rods 94 on the outermost cross-bars 92, and the rods 94 at the extremities of the single radial arms 92, respectively, so that a given cluster 34 may be raised to the extent necessary within the inner barrel assembly and particularly within its associated rod guide 32 for the aforementioned control purposes. The provision of the grooves 75 and 88 more particularly avoids the necessity of increasing the height of the inner barrel assembly 24 in the event that adequate axial height is not available for permitting the cluster 34 to be raised to the required height. Where adequate vertical space is available, the groove 75 and 88 are not required,and accordingly are optional. In accordance with the foregoing, it will be understood that secure mounting of the top ends of the WDRC rod guides is afforded, while permitting the required vertical displacement of the associated WDRC rod clusters. Further, the telescoping interconnections of the sleeves 72 and the corresponding fixed cylindrical supports 80 permit the calandria 50 to be raised and withdrawn, affording access to the rod clusters 34 within the respective guides 32 for normal maintenance purposes. In an actual pressurized water reactor of the advanced design herein contemplated and incorporating the present invention, the thin wall section 70 of the rod guide 32 is formed of sheet metal of approximately 1/8 inch thickness. The rod guide 32 is approximately 12 inches wide, in both dimensions of its generally square cross-section, and approximately 174" (14½ feet) in height. The rod guide sleeve 72 has an outer periphery, in cross-section, corresponding to that of the rod guide 70, as aforenoted, and defines an inner circumferential support/wear surface of approximately 11.20 inches in diameter. The fixed cylindrical support 80 has a corre­ sponding outer diameter of approximately 11.14 inches thereby to afford a nominal 0.030 inch radial clearance to permit the telescoping interconnection of the wear surfaces of the cylindrical support 80 and the sleeve 72. The end closure 82 of the cylindrical support 80 has a thickness as measured in the vertical axial direction of approximately 1.10 inches. The axial height of the sleeve 72 is approxi­ mately 4.0 inches whereas the axial length of the cylindri­ cal support 80 is approximately 4.60 inches in axial height, resulting in a clearance between the upper end of the sleeve 72 and the lower surface of the lower calandria plate 52 of approximately 0.60 inches. The bevels 80 and 89 are each of approximately 0.625 inches, resulting in an axial length of the engaged, wear surfaces of the inner circumference of the sleeve 72 and the outer circumference of the cylindrical support 80 of approximately 2.75 inches. The substantial wear surface area thus afforded provides for long life of the top end support of the present inven­ tion, as before noted. Further enhancement of the effective life of the top end support of the invention may be achieved by appro­ priate selection of the materials used in fabricating the cylindrical support 80 and the sleeve 72, on at least the engaged, wear surfaces thereof. Various weld overlay materials are suitable for this purpose. An electro-spark deposited coating having particularly desirable character­ istics for this purpose, in view of the reduced amount of cobalt contained therein, is disclosed in a concurrently filed application entitled "Wear Resistant Zirconium Base Alloy Article for Water Reactors," Westinghouse 52,689, assigned to the common assignee herewith.

Claims: 1. A top end support (72, 80) for rod guides (32) disposed in closely spaced, parallel axial relation­ ship in an inner barrel assembly (24) of a pressurized water reactor vessel (12), the inner barrel assembly (24) occupying a central portion of the vessel and being of a vertical height extending from a first plate (19) of lower elevation to a second plate (52) of higher elevation within the vessel, each of said rod guides (32) having sidewalls extending between said first and second plates (19, 52) and being of generally square cross-section, each rod guide (32) accommodating in axial, sliding relationship there­ within a respectively associated cluster (34) of elongated rods interconnected to extend in parallel axial relation­ ship, characterized in that each said rod guide includes: an axially extending sleeve (72) affixed at a lower end thereof to the upper end of a respective said sidewall (70) of said rod guide (32) and of an axial length such that said upper end thereof is closely adjacent to, but displaced from, the lower surface of the second plate (52) when in assembled relationship therewith, said sleeve (72) having an outer surface generally corresponding in cross-section to that of said side wall (70) of said rod guide (32) and an inner circumferential surface (72') of generally cylindrical cross-section; a fixed cylindrical support (80) having an upper end closure (82) adapted for being secured to the lower surface of the second plate (52) and having an integral depending sidewall (81) of generally cylindrical configu­ ration received in telescoping relationship within said rod guide sleeve (72) when in assembled relationship therewith; and means (86, 87) for securing said fixed cylindri­ cal support (80) to the second plate (52). 2. A top end support (72, 80) as recited in claim 1, characterized in that an array of depending extensions (59) respectively corresponding to said plurali­ ty of rod guides (32) is rigidly secured to the second plate (52) and extends axially downwardly therefrom; and each said end closure (82) has a central aperture (82') extending axially therethrough for receiving a respectively corresponding downward extension (59) therein thereby to establish a predetermined position of each corresponding fixed cylindrical support (80) relative to the second plate (52) and thus within said inner barrel assembly (12) and, correspondingly, a predetermined, aligned position of the associated rod guide (32) when said sleeve (72) thereof is assembled in telescoping relation­ ship with said corresponding fixed cylindrical support (80). 3. A top end support (72, 80) as recited in claim 2, characterized in that a plurality of bores (85) are formed in each of said securing means (86, 87) so as to be disposed symmetrically about said central aperture (82') and extend in parallel axial relationship through said end closure (82) in alignment with respectively corresponding, threaded bores (87) in the second plate (52); and bolts (86) are disposed through said bores (85) in said end closure (82) so as to engage the respective aligned threaded bores (87) in the second plate (52) for securing said cylindrical support (80) thereto. 4. A top end support (72, 80) as recited in claim 1, 2 or 3, characterized in that the upper end of said sleeve (72) on said interi­ or circumferential surface thereof includes a radially outwardly projecting bevel (78); and said lower end of said cylindrical sidewall (81) of said fixed cylindrical support (80) on said outer circumferential surface thereof includes a radially inward­ ly projecting bevel (89) for facilitating assemblage of said fixed cylindrical support (80) and said sleeve (72) in telescoping relationship. 5. A top end support (72, 80) as recited in any one of claims 1 to 4, characterized in that the second plate (52) includes an array of flow holes (100) for passage therethrough of the core output flow within the vessel (12), characterized in that an array of depending extensions (59) is affixed to and extends through said second plate (52) in parallel axial relationship, each said depending extension (59) being disposed centrally of a predetermined number of respectively associated said flow holes (100) with the latter at a common radial distance therefrom; that each said fixed cylindrical support (80) is respectively associated with a corresponding one of said depending extensions (59), that each of said end closures (82) includes a central aperture (82') extending axially therethrough and receives the corresponding depending extension (59); that said rod guide (32) and associated sleeve (72) are of cross-sectional dimension so as to encompass therewithin, throughout the height of the inner barrel assembly (24), the spatial area of the associated flow holes (100); each said sleeve (72) including a recess (72' ') therein corresponding to an outer portion of the periphery of each said flow hole (100); and said cylindrical sidewall (81) of each said fixed cylindrical support (80) being discontinuous at the corre­ sponding peripheries of each of said flow holes so as to comprise a plurality of arcuate segments (81) extending intermediate successive said respectively associated flow holes (100), and said end closure (82) of said cylindrical support (80) including an arcuate recess (82' ') aligned with and corresponding to the remaining peripheral portion of each of said respectively associated flow holes (100).

Inventors: EKEROTH, DOUGLAS ERNEST, GILLETT, JAMES EDWIN, SHERWOOD, DONALD GUYER
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: G21C7
IPC codes: G21C5, G21C7