Patent Number: 059206039
Section: summary

FIELD OF THE INVENTION This invention relates generally to boiling water reactors and more particularly, to a forged core plate for such a reactor. BACKGROUND OF THE INVENTION A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The top guide includes several openings, and fuel bundles are inserted through the openings and are supported by the core plate. Known core plates are fabricated from several stainless steel plates and formed ring segments joined together by welding. A typical core plate has over 200 welds. Fabricating such a core plate is very labor intensive. In addition, maintaining the required core plate geometry is difficult because of weld distortion and tolerance stack-ups between the several plates and ring segments. The core plate welds also, increase the susceptibility of the core plate material to a detrimental effect known as inter-granular stress corrosion cracking (IGSCC). Although no cracking currently is known to be occurring in a core plate, cracking may occur in the heat affected zone of the welds. Eliminating welds in the core plate therefore would eliminate the potential of cracks initiating in welds. SUMMARY OF THE INVENTION These and other objects may be attained by a forged core plate fabricated from a single piece stainless steel forging. The plate has a substantially circular shape and penetrations are provided in the core plate to allow passage of control rod blades into the core and direct recirculation flow past the fuel assemblies. The penetrations are formed from machining "5 holes" per penetration. Particularly, each penetration includes a center through hole to allow passage of the control rod blade and control rod blade guide tube. The through hole has a smaller diameter at the plate top surface to provide a tighter fit to the outside diameter of the guide tube to maintain alignment and minimize flow leakage. The through hole is enlarged at the core plate bottom surface to allow clearance in order to improve the installation and removal of the guide tube. Each penetration also includes four peripheral blind holes to direct the recirculation flow into the flow holes located on the sides of the guide tube. The blind holes do not extend completely through the plate. When the guide tube is installed in the core plate, the flow holes in the side of the guide tube are located approximately 1/3 of the way down from the plate top surface. The above described core plate enables the elimination of welds, minimizes the size of the forging required for fabrication, provides all the same flow paths as provided with known core plates, and has improved the producibility. Specifically, all welds in the core plate are eliminated, which eliminates the possibility of cracking in the heat affected zone of welds. Further, no welds need to be inspected during manufacturing and during service life. Eliminating welding also eliminates weld distortion and thus improves producibility. The single piece construction also eliminates fit-up requirements and thus improves producibility. In addition, the five hole pattern is the same for all central fuel support locations. This pattern provides structural webs on four sides of the fuel support location, which minimizes the required depth of the core plate. Core plates fabricated by welding together plates have structural webs on only one or two sides of each fuel support location. Also, all flow paths through the core plate to the fuel are the same, which standardizes the fuel orifice design. Core plates fabricated by welding together plates have two or three different flow paths at each fuel support location. Also, the 5 hole configuration can be machined by drilling rather than my milling, which improves producibility. The 5 hole configuration also improves producibility by eliminating small fillet radii.