Patent Number: 048760617
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 and, more particularly, to an improved, resiliently loaded lateral support between the top, free ends of the cantilever-mounted rod guides and the calandria assembly of an advanced design, pressurized water reactor. 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 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) and water displacement rod clusters (WDRC), 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 displacement rods are arranged in 185 clusters, each of the rod clusters having a respectively corresponding spider to which the rods of the cluster are individually mounted. 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) and the water displacement rod clusters (WDRC); 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 are progressively, in groups, withdrawn 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, for control of the reactivity and correspondingly the power output level of the reactor on a continuing basis, 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. 217,503, 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 of Altman et al., entitled "DISPLACER ROD DRIVE MECHANISM VENT SYSTEM." 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 internals 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 their fully withdrawn (i.e., raised) positions within the 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 thus above the level of the rods and which is designed 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 cantilever-mounted at their lower ends to the upper core plate and 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 plate, 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 to the radial 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 C. Veronesi for "CALANDRIA"; and application Ser. No. 490,099, "NUCLEAR REACTOR", all thereof concurrently filed on Apr. 29, 1983 and incorporated herein by reference. Maintenance of such reactors, for example, requires that the upper closure dome be removed to provide access to the calandria assembly which in turn is removed to afford access to the WDRC and RCC rod clusters for repair or replacement, and as well to the core for rearrangement or replacement of the fuel rod assemblies. To accomplish this, the calandria assembly typically is removable from the inner barrel assembly, withdrawing thereby the WDRC and RCC rods from within the corresponding rod guides. As before noted, the rod guides for each of the RCC and WDRC rod clusters are mounted rigidly 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 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 fixed end motion 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 ends of the rod guides to the lower calandria plate. Preferably, the design of the pressure vessel and particularly of the support structures which mount the free ends of the rod guides to the lower calandria plate permit both the assembly and removal of the calandria, relatively thereto, without special tools. Nevertheless, the mounting means for the free ends of the rod guides not only must constrain the same against lateral motion due to vibration, flow and thermal forces while accommodating the aforesaid axial movement of the free ends of the rod guides, but also must avoid wear of the reactor internals arising out of loads imposed on the guides and the previously discussed axial motion of the free ends of the guides. In some existing designs and as used with 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 for the reasons above-noted. 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 yet which is of simple design and employs a minimum number of parts, thereby to achieve cost economies both in the construction and also in the maintenance of such reactors. CROSS-REFERENCE TO RELATED APPLICATIONS The co-pending application of J. E. Gillett 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, for the reasons aforenoted. An alternative top end support assembly is disclosed in the co-pending application 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 therebetween while permitting independent axial movement. 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 correspondingly, through the concatenated and interleaved arrangement, 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, an improved lateral support is provided at the interface between the upper, free ends of cantilever-mounted rod guides and the lower calandria plate of a calandria assembly, as employed in a pressurized water reactor of the advanced design type herein contemplated. While the improved lateral support of the invention is directed to the particular problems presented by such advanced design pressurized water reactors, it will be appreciated that the lateral support of the invention may be employed in other reactors with the alignment and lateral top end support requirements are imposed even though the further concerns of vibration and axial movement of the rod guides are not as severe a concern. More particularly, an extension element having a central aperture therethrough, corresponding to an aperture in the lower calandria plate which accommodates a drive rod for an associated rod cluster, is aligned axially with a respectively corresponding aperture in the lower calandria plate and secured to the lower calandria plate so as to depend downwardly therefrom. Plural pivotal mounting sockets defining horizontal axes of rotation are formed in the extension element, each socket receiving and pivotally mounting therein a first end of a corresponding link, the plurality of links extending generally radially and in angularly displaced relationship relatively to the common axis of the aligned apertures. The top end of each rod guide includes a plurality of corresponding receiving sockets, each receiving socket having a mating configuration with respect to the free end of a corresponding link for releasably receiving and engaging same. The number of links and associated mounting and receiving sockets, and the corresponding angular relationships thereof are determined in accordance with the configuration of the top end of the associated rod guide. Illustratively, for a rod guide top end of generally square cross-sectional configuration, four such links and respectively associated mating and receiving sockets are provided, the links being relatively angularly displaced at right angles and extending radially outwardly from the extension element so as to engage and be received in correspondingly disposed receiving sockets in the top end of the associated rod guide. In use, as the calandria assembly is lowered into the pressure vessel, the links are normally pivoted downwardly, through force of gravity, and thus disposed at radially inwardly, retracted or disengaged positions. The corresponding receiving sockets formed at the upper and inner portions of the corresponding rod guide top end define an engagement ledge which is aligned with the free end of the link in its inwardly retracted position. As the downward movement of the calandria assembly continues, the free end of each link contacts the engagement ledge and is pivoted thereby upwardly and thus moved radially outwardly, to a releasably engaged position in its corresponding receiving socket in the assembled position of the calandria. Removal of the calandria is performed simply by lifting the calandria this causes the links to pivot downwardly and the free ends to move inwardly, thereby being withdrawn from the receiving sockets to the normal, retracted positions. The links may assume any of various configurations, the principle requirement being that a limited degree of lateral, i.e., radially oriented, flexibility be afforded through the resulting connection between the extension element and the rod guide top end. This is afforded in different embodiments of the lateral support of the present invention, alternatively by use of link configurations which themselves afford a required degree of flexibility in the lateral, or radial direction, or by the use of links of more rigid configuration but wherein the receiving socket is flexibly mounted in the top end of the associated rod guide. In all of these embodiments of the invention, the links are loaded laterally into the top end of the rod guide, and serve to center same, both maintaining the intended alignment and preventing lateral motion of the rod guide; further, since the links remain capable of pivotal movement even in the engaged position, they accommodate, through slight pivotal movement, axial movement of the free ends of the rod guides as may be produced by axial thermal growth and vibrations. The resiliently loaded lateral supports of the present invention thus provide for substantially rigid lateral restraint of the rod guide top, free ends, translating lateral forces from the rod guides directly to the calandria bottom plate and thus maintaining alignment and eliminating lateral movement of the rod guide free ends, while allowing axial, vertical motion for accommodating axial thermal expansion growth and base plate and related rod guide axial vibrations. The support, moreover, facilitates both the installation and the removal of the calandria, as required for routine maintenance and inspection operations. 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.