Patent Number: 047599049
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 and, more particularly, to an improved calandria assembly within the pressure vessel which provides requisite mechanical support functions, taking into account acceptable stress conditions and vibration problems to which the calandria assembly is subjected, while affording enhanced flow conditions. 2. State of the Relevant Art As is well known in the 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 relative 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 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 displacer 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 one such reactor design, a total of over 2800 reactor control rods and water displacer 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. In the exemplary such advanced design pressurized water reactor, there are provided, at successsively 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 has mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies comprising the reactor core, and which are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates, the latter being welded to the bottom edges of the cylindrical sidewall of the inner barrel assembly. 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 reactor control rod clusters (RCC) and water displacer 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 control, all of the WDRC's being fully inserted into association with the fuel rod assemblies, and thus into the reactor core, when initiating a new fuel cycle. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. As the excess reactivity level 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. The WDRC's provide a mechanical means for spectral shift control of a reactor and a reactor incorporating same is disclosed in the copending application Ser. No. 946,112, filed Dec. 24, 1986 a continuation of application Ser. No. 217,503, filed Dec. 16, 1980 and entitled MECHANICAL SPECTERAL SHIFT REACTOR and applications cited therein; a system and method for achieving the adjustment of both the RCC's and WDRC's are disclosed in the copending application of Altman et al., Ser. No. 806,719, filed Sept. 12, 1985, and 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 crossflow, a particularly important objective due to the large number of rods and the type of material required for the WDRC's which creates a significant wear potential. This is accomplished by increasing the vessel length sufficiently so as to locate the rods below the vessel outlet nozzles, whereby the rods are subjected only to axial flow. The calandria then is provided as an additional structure, disposed above the inner barrel assembly and thus above the level of the rods. The calandria receives the axial core outlet flow, and turns the flow through 90.degree. to a radial direction for exiting from the radially oriented outlet nozzles of the vessel. The calandria thus must withstand the crossflow generated as the coolant turns from the axial to the radial directions, and must provide for shielding and flow distribution in the upper internals of the vessel. Advanced design pressurized water reactors of the type here considered incorporating such calandria structures are disclosed in the copending application 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. Additionally, structural elements known as formers are included within the vessel to assist in maintaining the desired axial flow condition within the inner barrel, assembly; modular such formers are disclosed in the copending application Ser. No. 798,195, filed Nov. 14, 1985, and entitled "MODULAR FORMER FOR INNER BARREL ASSEMBLY OF PRESSURIZED WATER REACTOR," having a common coinventor herewith and assigned to the common assignee hereof. In general, 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 aligned apertures in the lower and upper plates thereof is mounted a plurality of calandria tubes in parallel axial relationship, 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 core outlet flow or a major portion thereof, as received in the calandria, turns from the axial flow direction to a radial direction for passage through radially outwardly oriented outlet nozzles which are in fluid communication with the calandria. 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 in alignment with and in close proximity 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, as above noted. The adjustment mechanisms have corresponding drive rods which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated spiders to which the clusters of RCC rods and WDRC rods are mounted, and serve to adjust their elevational positions within the inner barrel assembly and, correspondingly, the level to which the rods are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the reactivity within the core. The calandria, as before noted, performs the important function of shielding the drive rods and performing flow distribution in the upper internals. Since the radial flow, or crossflow, velocities are the range of 40 feet per second, it must be robust and able to withstand the vibrational loading imposed thereon by such crossflow. Further, the vessel provides a flow path for the coolant to enter the head region directly, for cooling the adjustment mechanisms mounted on the head assembly and vessel dome, and a downcomer flow path through which the head coolant normally passes from the head region to mix with the core outlet flow and exit from the vessel through the outlet nozzles. The head region also serves as a reservoir of low temperature coolant which passes through the downcomer flow path and ultimately into the lower internals, to cool the core in the event of a LOCA (loss of coolant accident). The calandria thus is an interface between the high temperature core outlet flow and the low temperature coolant of the head region, and accordingly is subjected to the significant temperature differential which exists therebetween, and must be flexible in order to limit the magnitude of the resulting thermal stresses. Conventional reactor internals have no structural analogy to the calandria assembly of such advanced design reactors, and thus there are no known solutions for satisfying the requirements of such a calandria assembly as above set forth and to which the present invention relates. SUMMARY OF THE INVENTION As before noted, a pressurized water nuclear reactor incorporating a calandria assembly, and particularly the improved calandria assembly of the present invention, employs a large number of 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), an array of 185 such clusters containing a total of 2800 rodlets (i.e., the total of reactor control rods and water displacer rods) being mounted in respective, corresponding rod guides and in parallel axial relationship within the inner barrel assembly of the reactor vessel. More specifically, the rods of each cluster are mounted at their upper ends to a corresponding spider, and the spider-mounted cluster is received in telescoping relationship within the corresponding rod guide. The spider is connected through a drive rod to a corresponding adjustment mechanism disposed on the exterior of the head assembly of the vessel, which provides for selectively raising or lowering the rod cluster relatively to an associated group of fuel rod assemblies, to control the reactivity, and thus the power output level of the reactor, as before described. The basic calandria structure comprises an annular, flanged cylinder, the flange of which is received on a supporting ledge of the vessel and the lower end of the cylinder being connected to the periphery of a main structural support plate, termed the upper calandria plate, of corresponding, generally circular configuration. A connecting cylinder is connected at its upper end to the periphery of, and depends from, the main structural support plate and is connected at its lower end to the perimeter of a generally circular, lower calandria plate which is much thinner than the upper calandria plate. Hollow tubes of generally circular cross-section, termed calandria tubes, extend in a parallel axial direction between the upper and lower calandria plates and are aligned with corresponding holes provided therefor in those plates. As before noted, the drive rods for the rod clusters are received through the calandria tubes and are shielded thereby from the crossflow within the calandria. The present invention provides for welded connections between the calandria tubes and the upper and lower calandria plates, which eliminate the potential of loosening, due to flow induced vibration, of mechanical connections which potentially could be employed for this purpose, and afford the further advantage of requiring less space than a mechanical connection requires. The resulting construction is quite stiff, consistent with the support requirements of the calandria, but introduces the potential of being susceptible to developing significant thermal stresses due to the differences in the material structure and geometry, and particularly the redundant structure between the lower and upper calandria plates, as presented by the calandria tubes and the connecting cylinder, or skirt, in view of the temperatures to which they are subjected. Further, whereas the upper calandria plate is relatively massive and stable, temperature differentials or gradients to which it is subjected may cause it to bend; the connecting cylinder, or skirt, is likewise very stiff, but is much thinner than the upper calandria plate and therefor exhibits a different thermal response. Thus, there is a critical requirement to relieve or limit the levels of thermal stress which can develop in the calandria assembly. In accordance with the present invention, the potentially significant thermal stresses are limited and relieved by controlling the stiffness of the lower calandria plate, in the axial direction, achieved in accordance with the proper selection of plate thickness and flow hole pattern therein, and the provision of flexible weld joints between the plate and the calandria tubes. Specifically, the lower calandria plate is selected to be of a thickness of approximately 1.5 inches and the flow hole pattern comprises a substantially symmetrical distribution of flow holes about each of the mounting holes associated with the calandria tubes; further, flexible welds are formed between the calandria tubes and the lower calandria plate, achieved in the embodiment disclosed herein by counterbored annular weld interfaces of a "J-shaped" configuration. These combined features afford the requisite stiffness for affording the requisite structural support and withstanding vibration, while relieving thermal stresses. As before noted, flow shrouds are provided in the head assembly to protect the drive lines, or drive rods, from direct exposure to the head coolant flow which, if it contacted the drive rods directly, could cause unacceptable levels of drive rod vibration due to the long, unsupported lengths of the drive rods. The flow shrouds, however, if implemented as simple cylinders surrounding the drive rods, would preclude the blowdown flow of a large portion of the coolant in the head assembly, as is relied upon for cooling the core in the event of a LOCA. Particularly, the coolant flow path from the head region to the core during blowdown is through the inside annuli intermediate the outer diameter of the drive rods and the inner diameter of the corresponding calandria tubes. Thus, once the head region drains to the tops of the flow shrouds, the remaining coolant is trapped within the head above the upper calandria plate and can no longer pass through the flow shrouds/calandria tube annuli and drain into the core. To solve this problem, the present invention introduces flow holes at the base of the flow shrouds and above the top surface of the upper calandria plate, and a flow diverter which is disposed coaxially within each flow shroud and in surrounding, shielding relationship with respect to the drive rod at the vicinity of the flow holes. The flow holes thus permit drainage of the complete head cooling region during blowdown, while the flow diverter protects the drive rod from exposure to jetting of the head coolant flow in its passage through the flow holes, the latter presenting an undesirable condition which can result in increased drive rod lateral motion and corresponding wear. The flow diverter, moreover, incorporates a flow restrictor on its interior portion contiguous the drive rod therein and disposed above the flow holes, to prevent flashing of steam from blocking the flow of coolant through the flow holes during blowdown. Such blockage potentially can occur when the liquid level within the head reaches the top of the flow shroud during blowdown, if the flow path from the top of the flow shrouds to the flow holes is not restricted. The flow restrictor more particularly provides sufficient flow resistance, such that the head coolant will continue to enter the flow holes at the base of the flow shroud without being choked by steam entering the top of the flow shroud. Accordingly, the calandria and the associated shrouds and flowhole/diverter structures afford complete shielding of the drive rods from the reactor cooant crossflow throughout the entire extent of the drive rods from the head region to the top of the rod guides, without impairment of and, indeed, while assuring the requisite head coolant flow to the core during blowdown. 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.