Patent Number: 047524413
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to pressurized water reactors and, more particularly, to a modular former for use in the inner barrel assembly of such reactors. 2. State of the Prior Art Baffle type structures have been known for use in the portion of a reactor which surrounds the reactor core, usually termed the lower barrel assembly. Such baffle structures typically are comprised of plates which are bolted together to form a unitary baffle structure. Since not positioned in the core outlet flow, such known baffle structures are not subjected to the same type of thermal transients and flow loads as can exist within the inner barrel assembly of a reactor, and particularly a reactor of the advanced design to which the present invention primarily is directed. Specifically, in a reactor of the type with which the modular former of the present invention is employed, the reactor internals of the inner barrel assembly includes several hundred rods, or rodlets, which are selectively movable in an axial, vertically oriented direction, into and out of the lower barrel assembly, to control the activity of the reactor core. The rodlets thus are positioned directly in the core outlet flow, rendering it critical to maintain substantially uniform distribution of the outlet flow from the reactor core in an axial direction as the flow passes along the rodlets and through the upper barrel assembly. The core outlet flow, moreover, at least potentially presents significant thermal stresses due to the core outlet flow transients and induces vibrations. Since these conditions and corresponding flow control requirements do not exist in conventional reactors, no corresponding structures are known in the prior art for performing the functions of the modular formers of the present invention. SUMMARY OF THE INVENTION A pressurized water nuclear reactor of the type with which the modular formers 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 clusters (RCC) and, additionally, a large number of water displacer rods, or rodlets, similarly arranged in water displacer rodlet 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-section of the inner barrel assembly, as it passes through the inner barrel assembly, and thus prevents 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 of an array thereof as stacked in closely adjacent relationship within the inner barrel. 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. The modular formers of the present invention thus are configured to be received and rigidly supported 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. The formers thus are directly exposed to the core outlet flow and are potentially subjected to flow induced vibrations and significant thermal stresses due to core outlet flow transients. To accommodate these stringent operating and environmental conditions, the formers are of a modular configuration, each including upper and lower, horizontally and radially inwardly extending former plates interconnected by vertically (i.e., axially) extending corrugated columns which are welded at their opposite ends to the respective upper and lower former plates. In a specific embodiment herein disclosed, the modular formers are of two different configurations, respectively corresponding to the two different spacings, or shapes, of the peripheral regions, each extending for only a limited arcuate segment of the circumferential distance about the inside diameter of the inner barrel assembly. In the disclosed design, eight such modules, four of each of the two types, are disposed in a common horizontal rank, and three such ranks are disposed in vertically displaced positions, or elevations, within the inner barrel, thereby to obtain the proper pressure drop for assuring that the axial flow and uniform distribution conditions in the rod guide region are achieved. Each module is fabricated by welding so as to form a unitary structure prior to being mounted within the upper barrel assembly. Cantilever attachment elements are welded, in advance, onto the remote surfaces of the upper and lower former plates and include parallel-extending shanks. The shanks are inserted through corresponding holes provided therefore in the sidewall of the inner barrel, and then welded to the sidewall from the exterior of the inner barrel sidewall. The structural configuration and assembly of the modules, including the mounting of the cantilever attachment means thereon, readily adapts same to efficient, automated production, permitting complete assembly of the modules in advance of positioning same within the inner barrel; as well, installation of the modules can be performed quickly and easily, in view of the capability of the exterior welding of the cantilever attachment elements to the sidewall of the inner barrel. The welded, unitary construction of each modular former and the welded attachment to the inner barrel sidewall furthermore eliminates the use of bolts, such as are employed in prior art core baffle structures and the problem of maintaining preloads on such bolts, which problem is far more severe in the core outlet flow environment which exists within the inner barrel than the environment which exists within the region of the core itself. These and other objects and advantages of the modular formers for the inner barrel assembly of a pressurized water reactor in accordance with the invention will be more apparent from the following detailed description of the invention and the accompanying drawings.