Patent Number: 048572640
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the pressure vessel of a pressurized water reactor system of an advanced design in which plural rod guides are cantilever-mounted at their lower ends and extend in parallel, vertical relationship to dispose the upper ends thereof adjacent a calandria assembly or other removable support and, more particularly, to improved, frictionally loaded top end supports for such rod guides. 2. State of the Relevant Art Conventional pressurized water reactors employ a number of control rods which are mounted within the reactor vessel, generally in parallel axial relationship, for axial translational movement in telescoping relationship with the fuel rod assemblies. The control rods contain materials which absorb neutrons and thereby lower the neutron flux level within the core. Adjusting the positions of the control rods relatively to the respectively associated fuel rod assemblies thereby controls and regulates the reactivity and correspondingly the power output level of the reactor. Typically, the control rods, or rodlets, are arranged in clusters, and the rods of each cluster are mounted at their upper ends to a common, respectively associated spider. Each spider, in turn, is connected through a drive rod to a respectively associated adjustment mechanism for raising or lowering the associated rod cluster. In certain advanced designs of such pressurized water reactors, there are employed both control rod clusters (RCC's) and water displacement rod clusters (WDRC's), and also so-called gray rod clusters which, to the extent here relevant, are structurally identical to the RCC's and therefore both are referred to collectively hereinafter as RCC's. In an exemplary such reactor design, a total of over 2800 reactor control rods and water displacer rods are arranged in 185 clusters; typically, the rods of each cluster are individually mounted to a respectively corresponding spider. Further, there are provided, at successively higher, axially aligned elevations within the reactor vessel, a lower barrel assembly, an inner barrel assembly and a calandria assembly, each of generally cylindrical configuration; a removable, upper closure dome seals the top of the vessel and is removable to gain access to the vessel interior. The lower barrel assembly has mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies comprising the reactor core. The fuel rod assemblies are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates. The inner barrel assembly comprises a cylindrical sidewall which is welded at its bottom edge to the upper core plate. Within the inner barrel assembly there are mounted 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 the reactor-control rod clusters (RCC's) and the water displacer rod clusters (WDRC's); these clusters, as received in telescoping relationship within their respectively associated guides, generally are aligned with respectively associated fuel rod assemblies. One of the main objectives of the advanced design, pressurized water reactors to which the present invention is directed, is to achieve a significant improvement in the fuel utilization efficiency, resulting in lower overall fuel costs. Consistent with this objective, the water displacement rodlet clusters (WDRC's) function as a mechanical moderator and provide spectral shift control of the reactor. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. When initiating a new fuel cycle, all of the WDRC's are fully inserted into association with the fuel rod assemblies, and thus into the reactor core. As the excess reactivity level of the fuel diminishes over the cycle, the WDRC's, in groups, are withdrawn progressively from the core so as to enable the reactor to maintain the same reactivity level even though the reactivity level of the fuel rod assemblies is reducing due to dissipation over time. Conversely, the control rod clusters are moved, again in axial translation and thus telescoping relationship relatively to the respectively associated fuel rod assemblies but on a continuing basis, for control of the reactivity and correspondingly the power output level of the reactor, for example in response to load demands, in a manner analogous to conventional reactor control operations. A reactor incorporating WDRC's is disclosed in application Ser. No. 946,111, filed Dec. 24, 1986, a continuation of Ser. No. 217,053, filed Dec. 16, 1980 and entitled "MECHANICAL-SPECTRAL SHIFT REACTOR" and in further applications cited therein. A system and method for achieving the adjustment of both the RCC's and WDRC's are disclosed in the co-pending application Ser. No. 806,719, filed Dec. 9, 1985 of Altman et al. and entitled "VENT SYSTEM FOR DISPLACER ROD DRIVE MECHANISM OF PRESSURIZED WATER REACTOR AND METHOD OF OPERATION." Each of the foregoing applications is assigned to the common assignee hereof and is incorporated herein by reference. A critical design criterion of such advanced design reactors is to minimize vibration of the reactor internal structures, as may be induced by the core outlet flow as it passes therethrough. A significant factor for achieving that criterion is to maintain the core outlet flow in an axial direction throughout the inner barrel assembly of the pressure vessel and thus in parallel axial relationship relative to the rod clusters and associated rod guides. The significance of maintaining the axial flow condition is to minimize the exposure of the rod clusters to cross-flow, a particularly important objective due both to the large number of rods and also to the type of material required for the WDRC's, which creates a significant wear potential. This is accomplished by increasing the vertical length, or height, of the vessel sufficiently such that the rods, even in the fully withdrawn (i.e., raised) positions within their inner barrel assembly, remain located below the vessel outlet nozzles, whereby the rods are subjected only to axial flow, and through the provision of a calandria assembly, which is disposed above the inner barrel assembly and thus above the level of the rods and which is constructed to withstand the cross-flow conditions. In general, the calandria assembly comprises a lower calandria plate and an upper calandria plate which are joined by a cylindrical side wall, and an annularly flanged cylinder which is joined at its lower cylindrical end to the upper calandria plate and is mounted by its upper, annularly flanged end on an annular supporting ledge of the pressure vessel. The rod guides are semipermanently, cantilever-mounted at their lower ends to the upper core plate and releasably affixed at their upper ends to the lower calandria plate. Within the calandria assembly and extending between aligned apertures in the lower and upper calandria plates is mounted a plurality of calandria tubes, positioned in parallel axial relationship and respectively aligned with the rod guides. A number of flow holes are provided in the lower calandria plates, at positions displaced from the apertures associated with the calandria tubes, through which the reactor core outlet flow passes as it exits from its upward passage through the inner barrel assembly. The calandria assembly receives the axial core outlet flow, and turns the flow from the axial direction through 90.degree. to a radially outward direction for passage through the radially oriented outlet nozzles of the vessel. The calandria thus withstands the cross-flow generated as the coolant turns from the axial and upward to the radial and outward directions, and provides for shielding the flow distribution in the upper internals of the vessel. Advanced design pressurized water reactors of the type here considered incorporating such a calandria assembly are disclosed in the co-pending applications: Ser. No. 490,101 to James E. Kimbrell et al., for "NUCLEAR REACTOR"; application Ser. No. 490,059 to Luciano Veronesi for "CALANDRIA"; and application Ser. No. 490,099, "NUCLEAR REACTOR", all thereof concurrently filed on Apr. 29, 1983 and incorporated herein by reference. As before noted, the rod guides for each of the RCC and WDRC rod clusters are mounted securely and semi-permanently at their bottom ends to the upper core plate, preferably by being bolted thereto, and extend in parallel axial relationship to dispose the upper, free ends thereof adjacent the lower calandria plate. This cantilever-type mounting is necessitated to accommodate both axial (i.e., vertical) movement of the free ends of the rod guides, which occurs due to thermal expansion and thus axial elongation of the rod guides, and also fixed end motion, which is caused by vibration and/or flexing of the upper core plate to which the bottom, fixed ends of the rod guides are mounted. Because of these factors, it is not possible to rigidly and permanently secure the free, upper ends of the rod guides to the lower calandria plate. For example, routine refueling and maintenance operations performed on such reactors require disassembly of major components including removal of the head assembly, the calandria assembly and the inner barrel assembly to gain access to the core for replacing or relocating fuel rod assemblies, as required. Inspection and replacement, as required, of other components usually is performed in conjunction with refueling; accordingly, the calandria assembly typically is removed from within the inner barrel assembly, necessitating separation of the rod guides from the lower calandria plate. This most readily is accomplished by providing support structures or mounting means for the upper ends of the rod guides, which means are secured to the lower calandria plate and releasably engage and support the top ends of the rod guides, preferably without the use of special tools. Despite being releasable, the mounting means for the upper, free ends of the rod guides not only must constrain the same against lateral motion, caused by flow-induced vibration and flow and thermal forces imposed thereon while nevertheless accommodating the aforedescribed axial movement of the free ends of the rod guides, but also must avoid excessive wear of the reactor internals. In some existing designs and as in conventional reactors, split pins are employed at the free ends of the rod guides for restricting lateral motion while permitting a limited extent of axial motion; such designs, however, present wear concerns. In fact, due to the high loads and large axial motion of the free ends in the advanced design pressure vessels, the use of split pins for the free end supports is deemed not practical. There thus exists a substantial need for a top end support structure for the top, free ends of the rod guides in such advanced design reactors, which satisfies these complex structural and operational requirements but which is of simple design and small physical size and employs a minimum number of parts, thereby to achieve cost economies, both in the cost of components and in the size of the reactor vessel and also in simplifying and thereby expediting the performance of maintenance operations on such reactors and correspondingly reducing down-time. Moreover, in view of the different configurations of the rod guides which accommodate the respective, different rod cluster types, respectively corresponding such top support structures of different configurations are required which are mutually compatible. CROSS-REFERENCE TO RELATED APPLICATIONS The co-pending application Ser. No. 798,194, filed Nov. 14, 1985, of D. G. Sherwood et al., entitled "TOP END SUPPORT FOR WATER DISPLACEMENT ROD GUIDES OF PRESSURIZED WATER REACTOR", assigned to the common assignee hereof and incorporated herein by reference, discloses a telescoping interconnection between a cylindrical support element which is affixed to and extends downwardly from the lower calandria plate and an apertured sleeve affixed to the top end of each rod guide. The configuration of the telescoping elements maximizes the area of the wear surface, thereby to resist wear during normal operation, while affording ease of removal of the calandria to gain access to the rod clusters and of reassembly of same. An alternative top end support arrangement is disclosed in the co-pending application of Ekeroth and Veronesi, the latter a common inventor herein, entitled "RESILIENTLY LOADED LATERAL SUPPORTS FOR CANTILEVER-MOUNTED ROD GUIDES OF A PRESSURIZED WATER REACTOR" filed Nov. 3, 1986, Ser. No. 926,301. As disclosed therein, a mount, which may be of cylindrical configuration, is secured to the lower surface of a lower calandria plate and a sleeve is secured to the upper end of a cantilever-mounted rod guide; means are provided for releasably interconnecting the sleeve and the support and for producing resilient, generally lateral (i.e., radially-oriented) loading therebetween. A variety of embodiments are disclosed in which the releasably interconnecting means may comprise links which are either inherently flexible or pivotally connected to the cylindrical support, generally extending downwardly therefrom such that the same are engaged on receiving sockets at the upper end of the sleeve as the calandria assembly is lowered into position. As an alternative to resiliency of the links, in one disclosed embodiment, a rigid link pivotally joined to the cylindrical socket engages a flexibly mounted receiving socket in the sleeve, to achieve the resilient loading effect. In certain disclosed embodiments, moreover, the support and sleeve may have mating surfaces which are nominally spaced by the resilient loading of the releasably interconnecting means and which serve as an abutment stop, or load pick-up surface, when lateral forces imposed on the guide exceed the resilient, lateral loading of the releasable interconnecting means, thereby to translate excessive lateral forces directly to the calandria. While the lateral loading and nominal spacing of contiguous parts, thus afforded, offers the advantage of reduced wear under normal loading conditions, the pivotal interconnection between the fixed support and the sleeve affords little, if any, axially oriented frictional force for restraining axially directed vibrational or translational movement of the rod guide, or of the lower and upper support plates with which it is associated (i.e., the upper core plate and the lower calandria plate). Moreover, the receiving socket structures required in the upper end of the sleeves encumber the requisite openings therein through which the respective rod clusters must pass during disassembly and assembly procedures. Yet another alternative top end support assembly is disclosed in U.S. Pat. No. 4,687,628 issued Aug. 18 1987 from the previously co-pending application Ser. No. 923,059, filed Oct. 24, 1986, a continuation of parent application Ser. No. 798,220, filed Nov. 14, 1985, of Gillett et al. entitled "FLEXIBLE ROD GUIDE SUPPORT STRUCTURE FOR INNER BARREL ASSEMBLY OF PRESSURIZED WATER REACTOR", assigned to the common assignee hereof and incorporated herein by reference. Respective, differently configured top support plates are mounted on the free ends of the RCC and the WDRC rod guides, respectively, and have mating, respective exterior and interior vertices to permit assemblage of same in an interdigitized array. Flexible linkages connect the top plates in a concatenated relationship, and serve to restrain relative, lateral movement while permitting relative axial movement therebetween. Stop pins are received in aligned bores of the contiguous interdigitated top plates and serve to limit the extent of load which can be applied to the linkages and thus the ultimate extent of relative movement between the concatenated top plates. The RCC top plates include openings, preferably of cylindrical configuration, which receive corresponding cylindrical extensions which are secured to and extend downwardly from the lower calandria plate, thereby establishing basic alignment of the concatenated and interleaved matrices of the plates. Leaf springs secured to the calandria bottom plate engage and exert a downward force on the top surfaces of the RCC top plates, thereby establishing a frictional force which further opposes lateral movement of the RCC top plates and, through the concatenated and interleaved arrangement, correspondingly opposes any lateral movement of the WDRC top plates, as well, while permitting restrained axial displacement or movement of the individual RCC and WDRC rod guides. While the flexible support structure of the referenced Gillett et al. application satisfies many of the requirements of the rod guide top end supports, the structure is of complex design and requires the use of numerous elements, contributing to increased costs of construction and maintenance of the reactor. Accordingly, there remains a need for a lateral support for the top, free ends of the cantilever-mounted rod guides of the pressurized water nuclear reactors of the advanced designs herein contemplated, which is of simplified design and reduced cost, yet which affords the requisite support functions while reducing and/or substantially eliminating wear concerns. SUMMARY OF THE INVENTION In accordance with the present invention, frictionally loaded top end supports are provided at the interface between the upper, free ends of cantilever-mounted rod guides, respectively housing control rod clusters (RCC's) and water displacement rod clusters (WDRC's), and a support plate disposed thereabove. In the specific embodiment herein disclosed relating to a pressurized water reactor of the advanced design type, the support plate comprises the lower calandria plate of a calandria assembly. While the frictionally loaded top end supports of the invention are directed to overcoming the particular problems presented by such advanced design pressurized water reactors, it will be appreciated that they may be employed in other reactors for satisfying alignment and lateral support requirements for the top ends of rod guides, even though the further concerns of vibration and axial movement of the rod guides are not as severe, or not present in the first instance. More particularly in accordance with the present invention, the frictionally loaded top end supports or mounting means, for each of the RCC and WDRC rod guides, comprise corresponding, generally cylindrical, fixed supports which are secured to the lower calandria plate and extend axially downwardly therefrom, and respectively mating, reinforced sleeves which are affixed to the upper ends of the associated RCC and WDRC rod guides and which define generally cylindrical central openings for receiving the respective, cylindrical fixed supports in telescoping, sliding relationship therein. Each of the top end supports includes an upper, continuous annular collar portion and a lower, continuous annular base portion integrally interconnected by a generally cylindrical sidewall portion. Plural leaf springs are incorporated in each reinforced sleeve, positioned at angularly displaced locations about the common alignment axis of the cylindrical support and associated rod guide sleeve, e.g., four leaf springs at 90.degree. displaced locations. Each leaf spring has a flexible shank portion extending from a base portion at the lower end of the sleeve to an arcuate segment lip portion adjacent the upper end of the sleeve, the arcuate segments at the upper ends being of a common radius, substantially that of the exterior surface of the fixed cylindrical support, and which are resiliently biased by the shank and base portions in a radially inward direction to bear against same. The leaf springs present a relatively low preload, thereby inducing only a low frictional load during installation as the cylindrical supports are telescopingly inserted and received into the corresponding cylindrical openings of the sleeves, yet exerts sufficient lateral, resilient force to oppose lateral loads within the typical range experienced in normal operational flow conditions and sufficient axially directed frictional forces to oppose axial, or vertical, movement of the free ends of the rod guides. The continuous, annular collar portion of each sleeve, moreover, presents a non-yielding, load pick-up surface which backs the arcuate segment lip portions of the preload springs and is capable of carrying high lateral loads with low deflections. Thus, high lateral loads, including both those exceeding the typical range in normal operation and those which are produced under accident (e.g., seismic and LOCA) conditions, are transferred from the load pick-up surface of the sleeve directly to the cylindrical support and thus into the calandria. Accordingly, the top end supports of the invention are both compliant so as to afford easy installation and also of sufficient resilient strength to maintain alignment and stability during most normal operating conditions, and yet also are strong and rugged to resist abnormally high lateral forces which can occur during accident conditions. Because of the dense packing of the RCC and WDRC control rod clusters in the internals of the advanced design reactor vessels, the top end supports correspondingly are densely packed and thus must be of an efficient design and configuration so as to occupy a minimum of space, individually, while satisfying the above-described support functions. In general, each of the RCC clusters has an "X"shaped configuration in cross-section, i.e., four arms extending radially at mutually displaced right angles from a common, central axis, as viewed in a cross-section in a plane perpendicular to the axis, whereas each of the WDRC rod clusters has a periphery of generally square cross-sectional configuration, likewise symmetrically disposed about a central axis. In assembled relationship, the four exterior vertices of each WDRC support are received in mating relationship within corresponding, interior vertices of four surrounding RCC top end supports; likewise, each RCC top end support receives within each of its four interior vertices corresponding exterior vertices of four WDRC top end supports which are disposed thereabout in surrounding relationship. While thus in an interspersed array, the individual RCC and WDRC top end supports are independent of each other, each providing the required support function for its associated rod guide. Because of the greater lateral forces to which the WDRC rod guides are subjected, a more massive fixed cylindrical support is provided for the WDRC top end supports, as compared to the RCC top end supports. Conversely, while the RCC top end supports need only accommodate a lower level of lateral forces and correspondingly have less massive fixed cylindrical supports, a smaller spatial envelope is available in these structures to accommodate the leaf springs. Accordingly, the RCC supports incorporate a retainer pin which is received in a clearance hole in the top, free end of each leaf spring, as an added safety precaution, for capturing the spring in the unlikely event of breakage. More specifically, with respect to each WDRC top end support, the associated, fixed cylindrical support comprises a cylindrical sidewall and an integral, end closure having a central aperture therein which is aligned with a corresponding aperture in the lower calandria plate for receiving therethrough a drive rod for the WDRC rod cluster associated with the respective WDRC rod guide. Preferably, the upper surface of the end closure includes an annular projection which is received in a corresponding annular recess in the lower calandria plate, coaxial with the drive rod aperture; the calandria extension then is received through the aligned apertures in the lower calandria plate and the end closure, the extension being welded to the plate. The assemblage thus provides directly interlocking interfaces, preventing any lateral displacement. Bolts then are received through corresponding apertures in the end closure and into threaded engagement in the calandria plate, to axially secure the fixed cylindrical support to the calandria plate. Arcuate recesses extend through the end closure of the fixed cylindrical support and complementary recesses extend axially through the cylindrical sidewall of the support and the reinforced sleeve, corresponding to respective flow holes in the lower calandria plate, to assure unimpeded vertical flow from the core and through the inner barrel assembly. Because of the square cross-sectional configuration and larger dimensions of the WDRC support, it is convenient to form the leaf springs by machining the corresponding, major faces of the sleeve, such that the base portion of each spring is integral with the lower, continuous annular base portion of the sleeve and the arcuate segment lip portion is positioned slightly, axially below the annular collar portion. The interior, engaging surface of the arcuate segment lip portion of each spring is biased by the shank portion so as to be positioned nominally radially inwardly of the interior, load pick-up surface of the annular collar portion of the sleeve. With respect to the RCC top end supports, and recalling the reduced lateral force which the same must withstand but also the smaller spatial envelope of the structure, the fixed cylindrical support comprises solely a calandria extension, similar in configuration to the calandria extension of the WDRC support and likewise extending through and secured to the lower calandria plate. The reinforced sleeve, as before-noted, is of X-shaped configuration, a central portion thereof from which the four, 90.degree.-displaced radially extending arms project, having a generally cylindrical interior of a greater diameter than the external diameter of the calandria extension and thus defining an annular gap therebetween. The leaf springs preferably are formed as separate elements. The base portion of each spring has a flat or planar inner surface, and an arcuate outer surface which matches the interior circumference of the continuous annular base portion of the sleeve, and is secured thereto by bolts which pass through the sleeve sidewall. The shank integrally connects the base portion with the arcuate segment lip portion and positions the latter in the annular gap between the calandria extension and the upper, continuous collar portion of the sleeve; the collar, due to its concentric and surrounding relationship, therefore comprises the non-yielding, load pick-up surface. Retainer pins extend through the collar portion of the sleeve adjacent its upper end and into clearance holes extending radially and partially into the respective, arcuate segment lip portions. The retainer pins thus capture and retain the RCC leaf springs, in the unexpected event of breakage. While structurally different, the RCC top end supports function in substantially the same manner as the WDRC top end supports, the leaf springs presenting relatively low preloads which are readily overcome during assembly and which nevertheless are adequate to resist excessive flow-induced vibration and axial and lateral loads in normal operation. High lateral loads, which exceed the typical range in normal operation and as are experienced during accident conditions, are transferred from the load pick-up surface of the continuous collar portion of the sleeve and through the intervening arcuate lip portion of the correspondingly positioned spring to the calandria extension. Accordingly, the RCC top end support likewise affords a load pick-up surface which is stiff and strong, for transfer of lateral loads of excessive levels and in accident conditions directly to the calandria. The reinforced sleeves of both the WDRC and RCC top end supports furthermore are machined to define interior, axially extending passageways for the respective RCC and WDRC rod clusters, thereby to enable their removal from within the corresponding rod guides in conjunction with removal of the calandria assembly from the upper internals, without the necessity of structural modification or disassembly of the rod guide top end support structures. Accordingly, the frictionally loaded, WDRC and RCC top end supports of the present invention function to prevent both lateral and axial displacement and vibrational movement of the associated rod guides as well as axial vibrational movement of the upper core plate, while affording ease of installation and of removal of the calandria assembly and providing the requisite flow paths to the calandria assembly, yet are of a simplified design and employ a minimum number of parts. These and other advantages of the present invention will become more apparent from the following detailed description, taken with reference to the enclosed figures, in which like reference numerals and letters refer to like parts throughout.