Patent Number: 047298685
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to pressurized water reactors and, more particularly, to a vibration arrestor for rod guides positioned within the inner barrel assembly of a pressurized water reactor. 2. State of the Prior Art Certain advanced designs of nuclear reactors incorporate at successively higher, axially aligned elevations within the reactor vessel, a lower barrel assembly, an inner barrel assembly, and a calandria, each of generally cylindrical configuration, and an upper closure dome. The lower barrel assembly may be conventional, having 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 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 the reactor core outlet flow as it exits from its passage through the inner barrel assembly. In similar parallel axial and aligned relationship, 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 through 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, particularly, 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 at interfaces between the individual rodlets of a given cluster and known support plate structures within the rod guide through which the rodlets pass for support, and thus to reduce or eliminate the factors which produce wear, such as flow induced vibration and associated vibration of reactor internal structures. Because of the relatively dense packing of the rod guides within the inner barrel assembly, it is critical to maintain substantially uniform distribution 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 temperature 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 reactor loads, coupled with the flow induced vibrating create a high potential for wear of 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 pressurized 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. Furthermore, the bottom 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 respectively to the upper core plate and the bottom calandria plate. The wear potential under these circumstances is great. Thus, split pin connections of conventional types are inappropriate for use as the supporting connections for the top ends of the rod guides since 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 requirement that unimpeded flow holes must be provided in the calandria plates for the core outlet flow. While these foregoing factors severly 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 visible for inspection and replaceable without undue effort. Additionally, the assembly load of the calandria must be less than its dead weight and must be accomplished without access to the support region. This avoids having to apply force to the calandria before installing the vessel head. 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, a further critical requirement is that the wear potential of the support structure itself must be minimized. This is a critical requirement in view of the potential for intense vibration arising out of the core outlet flow and the development of high contact forces due to differential pressure and both steady state and transient temperature conditions across both the array of rod guides and the individual rod guides. Conventional reactor designs do not present the support problems attendant the dense packing of rod guides and associated rod clusters in advanced reactor designs of the type herein contemplated. Thus, there is no known solution to the problems of adequately supporting the rod guides, consistent with the requirements and taking into account the environmental factors which exist in operation of such reactors as hereinabove set forth. SUMMARY OF THE INVENTION A pressurized water nuclear reactor, of the type with which the vibration arrestors for rod guides of the inner barrel assembly in accordance with the present invention are intended for use, employs a large number of reactor control rods, or rodlets, typically arranged in what are termed reactor control rod clusters (RCC) and, additionally, a large number of water displacer rods, or rodlets, similarly arranged in water displacer rod clusters (WDRC). For example, in one such reactor, an array of 185 such clusters containing a total of 2800 rodlets (i.e., the total of reactor control rods and water displacer rods) are mounted in parallel axial relationship within the inner barrel assembly. Each of these clusters, moreover, is received within a corresponding rod guide structure. In operation, it is desired to maintain the core outlet flow in an axial flow condition and in a substantially uniform distribution throughout the cross-sectional area of the inner barrel assembly, as it passes through the inner barrel assembly, and thus to prevent cross-flow conditions (i.e., core flow in a direction transverse of the rod guides). This is a critical requirement in reactors of such advanced designs in which the inner barrel is densely loaded with rodlets, as before noted. The geometry of the reactor vessel itself introduces a structural anomaly which is contrary to maintaining the desired, substantially uniform axial flow condition. Particularly, the circular configuration of the reactor vessel, including the inner barrel assembly, is geometrically incompatible with the generally rectangular or square cross-sectional configuration of the individual rod guides, and correspondingly of an array thereof as stacked in closely adjacent relationship within the inner barrel assembly. Thus, in the peripheral regions between the inside diameter of the cylindrical inner barrel assembly and the outer periphery of the array of rod guides, no rodlets are present, resulting in a nonuniform flow distribution and presenting at least the potential of turbulence and cross-flow conditions with attendant problems of vibration. A related, copending application of a common one of the co-inventors herein, entitled "Modular Former For Inner Barrel Assembly Of Pressurized Water Reactoring", and assigned to the common assignee hereof, discloses an invention relating to modular formers which are configured to be mounted in these peripheral regions, to provide hydraulic resistance and thereby to maintain a primarily axial direction, and substantially uniform distribution, of the core outlet flow, throughout the length of the rod guides within the inner barrel assembly. Thus while the state of the art, in the design of the inner barrel assembly of such advanced types of pressurized water reactors, has addressed the problem of attempting to maintain relatively stable conditions by minimizing cross-flow, e.g. by maintaining substantially uniform distribution and axial direction of the core output flow throughout the inner barrel assembly, there remains the critical problem of properly supporting the rod guides within the inner barrel because of remaining excitation forces from internal vibration and axial flow turbulence, consistent with the objectives and the structural and operating conditions and parameters as hereinabove set forth. The vibration arrestors in accordance with the present invention, for use with rod guides of the inner barrel assembly of a pressurized water reactor, afford a highly efficient and effective structure for satisfying the critical design criteria relating to flow induced vibrations of structural components and lateral force effects, as particularly relate to the rod guides within the inner barrel assembly. In one preferred use or application of the vibration arrestors in accordance with the present invention, they are employed in combination with a flexible rod guide support structure which is the subject of a copending application of a common inventor hereof, entitled "Flexible Rod Guide Support Structure for Inner Barrel Assembly of Pressurized Water Reactor," assigned to the common assignee hereof. Particularly, the flexible rod guide support structures as disclosed in the referenced, copending application, comprise, as major components, interdigitized matrices of top plates for the rod guides, flexible linkages which interconnect the top plates in a concatenated arrangement, pin stops between the continuous top plates of the two matrices, mounting extensions from the calandria which engage the top plates of one matrix, and rod guide leaf springs which are mounted on the calandria and which exert a force against the top plates of the one matrix to restrain lateral movement. These components are configured in a pattern that is repeated across the interface between the tops of all the rod guides in the array and the bottom plate of the calandria. Each such flexible linkage is attached to a respective WDRC rod guide top plate and to each of the RCC guide top plates which contiguously surround the respective, given WDRC rod guide top plate. Thus, each WDRC guide is attached, or concatenated, laterally to its surrounding RCC rod guides via the flexible linkage. This concatenated assembly of linkages creates a stiff structure between the guides in a plane perpendicular to the axis of the rod guides. Thus, the guides are essentially bound together laterally; however, the linkages in the out-of-plane direction, i.e., axially, are flexible and thus accommodate relative axial motion between guides to permit bowing of adjacent guides. This capability of flexibility in one plane compensates for local differences in height of adjacent guides due to differential thermal expansion and bowing due to pressure differential across the guide. Thus, the flexible linkages are flexible in a direction parallel to the axis of the rod guide, but rigid in a plane perpendicular to the axis of the rod guide. Lateral loads exerted on the rod guides are reacted into the calandria either by the calandria extensions or by the leaf springs, at each of the RCC plates. The rod guide leaf springs, as mounted on the calandria plate and pressed against the RCC top plates, generate sufficient lateral frictional force such that fluctuating steady state loads exerted on the guides do not cause slippage. Moreover, the mounting extensions from the calandria provide overall lateral support during events such as seismic, which can exceed the lateral frictional force of the leaf springs, and provide alignment between the rod guides and the calandria, there being one extension for each of the RCC guides. Collectively, the calandria extensions react the seismic loads from the rod guides. Alignment of the RCC clusters in the rod guide top plates further is controlled by the calandria extensions. The vibration arrestors of the present invention may be of differing embodiments, two specific embodiments thereof being disclosed herein, and in either embodiment may be employed as an improvement, in the alternative to the leaf springs disclosed in the referenced, copending application. Whereas leaf springs of the type disclosed therein are appropriate choices for the function required thereof, as above described, in view of the prior experience in the use thereof in connection with fuel rod assemblies, the leaf springs present certain obstacles or disadvantages which are overcome by the vibration arrestors of the present invention. For example, the leaf springs introduce numerous individual parts (in the referenced, exemplary reactor vessel design, in excess of 2,000 parts), adding considerably to the time and cost of initial assembly and continuing maintenance expense, for a given reactor vessel installation. The vibration arrestors in accordance with the present invention have an optimum design for use with rod guide top plates of the general configuration and mounting provisions of the RCC top plates hereinabove described, and achieve a substantial reduction in the number of parts--at the level of an order of magnitude smaller--while yielding superior structural performance at reduced stress levels. More particularly, the vibration arrestors of the present invention comprises a central hub of generally annular configuration, functioning as a mounting base, and integral spring arms which extend therefrom in a pattern which is symmetrical about the center of the hub. In one specifically disclosed embodiment of the arrestors, a single pair of two such spring arms extend in aligned and oppositely oriented directions from the hub (and thus angularly displaced by 180.degree. about the hub). In a second embodiment, two such pairs of spring arms are formed integrally with the hub and extend therefrom in quadrature relationship. The arrestors are formed of metal of constant thickness, and each of the spring arms has a simple taper in the width dimension along the generally radial length thereof. The hub includes a central aperture by which it is received over a corresponding calandria extension. A clamping, or stiffening, ring having an outer periphery corresponding to the hub portion of the arrestor is received over the calandria extension in superposed relationship with the hub, the ring and the hub having holes extending therethrough which are positioned in alignment with corresponding threaded bores in the lower calandria plate for receiving attachment bolts thereby to secure the arrestor to the calandria. The calandria extensions both position and laterally support the respective vibration arrestors, and thus prevent the attachment bolts from reacting lateral loads which may be imparted on the spring arms and transmitted thereby through the hub to the calandria. Moreover, the symmetrical configuration of the vibration arrestors prevents bending torques from being applied to the attachment bolts, because the compression loads applied to the ends of the symmetrically oriented spring arms are substantially the same and thus exert no net external moment. Due to the symmetrical configuration of the vibration arrestors, forces applied to the spring arms generate primarily an internal moment in the hub of the arrestor, which is made of sufficiently high strength material to withstand the stress. The clamping ring moreover reinforces the arrestor hub and prevents localized stresses therein due to attachment bolt preload effects. There results primarily only tensile loads on the bolts, with minimal, if any, shear and moment forces which is a highly desirable and acceptable condition. The vibration arrestors of the present invention thus achieve not only a reduction in the number of parts and corresponding time and cost of assembly, relative to the leaf spring implementation, but additionally the clamping ring attachment structure and the balanced reaction to compression loads afforded by the symmetrical configuration thereof provide improved operational characteristics and enhanced reliability by substantially eliminating the bending moment on the attachment bolts. While an exemplary application of the vibration arrestors of the present invention may be in assemblage with a flexible rod guide support structure of the type disclosed in the above identified and similarly entitled copending application, the vibration arrestors as well may be employed independently with rod guides of any desired type, and are not restricted in use to the specific interleaved matrices of first and second different types of rod guides, as disclosed in that copending application. Thus, for example, where employed independently with a given type of rod guides, such as the RCC rod guides, alternative mounting means may be employed for other types of rod guides, such as the WDRC rod guides; in such an installation, the WDRC rod guides may be supported independently, for example, by the top end support structure disclosed in the copending application entitled: "Top End Support for Water Displacement Rod Guides of Pressurized Water Reactor," having a common coinventor herewith and assigned to the common assignee hereof. The vibration arrestors of the present invention thus afford greatly enhanced beneficial effects, corresponding and relative to the leaf springs as described in the earlier-referenced copending application; thus, the arrestors as well react lateral force components on the associated rod guides, even in the event of wear of the calandria extensions, and thus suppress top end lateral motion and correspondingly prevent any increase in the excitation of the associated rodlets. This assures that rodlet wear does not increase, despite the potential of slippage due to inadvertent wear of the rod guide support, the need for significant gaps to permit assembly, and resultant increased tolerances between adjoining parts. Likewise, the vibration arrestors of the present invention increase, by more than an order of magnitude, the allowable wear depth on the calandria extension before alignment between the rodlet clusters and respective rod guides is compromised. Further, regardless of the gap size between the calandria extension and the respective rod guide top plate, the lateral excitation of rodlets within the respective rod guides is not affected. These and other advantages of the vibration arrestors for the rod guide supports in the inner barrel assembly of a pressurized water reactor, in accordance with the present invention, will become more apparent from the following detailed description and drawings.