Patent Number: 046876286
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to pressurized water reactors and, more particularly, to a flexible support for the 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 predtermined 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 unipeded 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 flexible rod guide support structures of 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 application of a common one of the co-inventors herein, entitled "Modular Former For Inner Barrel Assembly Of Pressurized Water Reactoring", filed concurrently herewith 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 present invention solves these mounting requirements for the top end supports of the rod guides and achieves the aforestated objectives through the provision of flexible rod guide support structures. Particularly, the flexible support structures for the top ends of the rod guides, in accordance with the present invention, 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. In a preferred embodiment, the flexible linkages are generally of square configuration, each being attached at four places to its respective WDRC guide top plate. Further, each such flexible linkage is attached to each of the four contiguous, surrounding, RCC guide top plates, at four corresponding, individual connections or attachment points. Thus, each WDRC guide is attached, or concatenated, laterally to the four 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 must be provided to compensate 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 in accordance with the invention will be understood to be 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. It will also be understood that the assemblage of linkages in the concatenated arrangement as well are rigid in the plane perpendicular to the parallel axes of the array of rod guides. Absent this flexibility of the linkages, the linkages would be overstressed and would fail. Further, the flexible linkages are recessed into the top plates to prevent damage during assembly. 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 pin stops are located on the top of the WDRC guides and mate with openings provided therefor in the associated four, adjacent RCC guides. The pin stops serve three purposes. First, during assembly of the guides, the pin stops provide rough positioning of adjacent rod guides prior to attachment of the square flexes. Secondly, the pin stops limit the amount of deflection possible along the axis of the flexible linkage, and thereby prevent excessive stress of any individual element due to an unanticipated and transient conditions. Finally, the pin stops provide an ultimate load capacity for very large loads. Essentially, the pin stops have a greater load capability than that of the WDRC guide itself. Proper selection of the material for the pin stop enables the attachment between adjacent guides to have the same or greater lateral load capacity than the rod guide enclosures. This provides a balanced design where the method of attachment of the rod guide is not structurally limiting. By this configuration and arrangement, the rod guide flex support of the invention virtually totally assures that loads on the WDRC rod guides can be transmitted through the RCC rod guide top plates and into the calandria. Thus, normal operational fluctuating loads are reacted laterally by the frictional forces generated by the leaf springs attached to the calandria and which press onto the tops of the RCC guides. The flexible linkages, moreover, transmit loads from the WDRC guide top plates to the adjacent RCC guide top plates; as a result of this design, impact between the rod guide top plates and the calandria extensions are essentially precluded. A further major advantage of the combined flexible linkage support and leaf spring frictional support is that wear or gap size between the calandria and the rod guides can be virtually ignored due to the continuing ability of the leaf springs to react lateral force components on the rod guides, even in the event of wear of the calandria extensions, and thus to suppress top end lateral motion and correspondingly prevent any increase in the excitation of the 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. Further, the concatenated interconnections of the flexible linkages distribute loads between adjacent extensions. Additionally, because of the coupled effect of the flexible linkages and the leaf springs, axial core plate vibration and resulting wear are of reduced concern since the leaf springs 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 flexible rod guide support for 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.