Induced nuclear reactions: processes – systems – and elements – Reactor protection or damage prevention – Expansion means
Reexamination Certificate
1998-12-16
2001-05-15
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Reactor protection or damage prevention
Expansion means
C376S287000, C376S289000, C376S294000
Reexamination Certificate
active
06233300
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to nuclear reactors and more particularly, to guide pins located between the shroud head flanges and the top guide in a boiling water reactor.
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 core center axis is substantially coaxial with the center axis of the shroud, and the shroud is open at both ends so that water can flow up through the lower end of the shroud and out through the upper end of the shroud. The shroud, top guide, and core plate limit lateral movement of the core fuel bundles.
The upper portion of the shroud, commonly referred to as the shroud head, is coupled to the top guide. Particularly, the shroud head flange mates with the top guide flange. The top guide flange typically includes cylindrically shaped guide/shear pins, and the shroud head flange includes corresponding cylindrically shaped mating holes. The purpose of the guide pins is to align the shroud head in the proper position during assembly of the reactor. Additionally the pins provide support and rigidity to the structure during a seismic event. Typically small clearances between the pins and their corresponding mating holes minimize the impact loading on the pins and flange holes caused by horizontal seismic accelerations. The small clearances make routine refueling assembly and disassembly operations difficult.
During assembly, the shroud head is typically suspended from an overhead crane and lowered into mating position with the top guide flange. Because of the large size and weight of the shroud head, the shroud head flange may flex between the lifting points. The flexing of the shroud head flange may cause the cylindrical holes to become misaligned with the guide pins on the top guide flange. The misalignment causes an interference condition which may not allow the top guide flange and the shroud head flange to mate properly.
It would be desirable to provide a shroud head and a top guide that include corresponding mating guide pins and flange holes that have a suitable clearance to accommodate any flexing of the shroud head flange during assembly. It would also be desirable to provide a shroud head and a top guide that include corresponding mating guide pins and flange holes that have small clearances after assembly to minimize the impact loading on the alignment pins and flange holes caused by horizontal seismic accelerations during a seismic event.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained by a top guide to shroud head interface that in one embodiment includes a top guide having a plurality of frusto-conical shaped alignment/guide pins and a shroud head having a plurality of corresponding guide pin openings. Each guide pin opening is configured to receive a frusto-conical guide pin.
The top guide includes a flange that is configured to engage a corresponding flange of the shroud head. The top guide flange includes a plurality of frusto-conical shaped guide pins extending from the top surface of the flange. The guide pins are located around the circumference of the top guide flange.
The shroud head flange includes a plurality of guide pin openings configured to align with the guide pins located on the top guide flange. Each guide pin opening includes a frusto-conical portion that extends through the flange from the bottom surface of the shroud head flange and has a slope equal to the slope of the frusto-conical pins. Each guide pin opening also includes a cylindrical portion that extends from the small base of the frusto-conical portion of the guide pin opening to the top surface of the shroud head flange. The diameter of the frusto-conical guide pin opening at the bottom surface of the shroud head flange is configured to be larger than the diameter of the base of the frusto-conical guide pin immediately adjacent the top surface of the top guide flange.
During assembly, the shroud head is suspended from an overhead crane and lowered into engagement with the top guide flange. Particularly, the shroud head is lowered so that the guide pin openings in the shroud head flange align with the frusto-conical guide pins extending from the top guide flange. The shroud head is lowered until the bottom surface of the shroud head flange is in surface to surface contact with the top surface of the top guide flange. The conical shape of the guide pins and the conical shape of the guide pin openings provide greater clearance between the pin and the side wall of the opening as the two flanges approach engagement than the clearance when the guide pins are cylindrically shaped. In the assembled condition, there is less than about 1.0 millimeters of clearance between the pin and the opening side wall. At a position where the distance between the two flanges is equal to the height of the guide pins, the clearance between the guide pins and the opening is about 4.0 millimeters when the cone angle of the guide pins is 60 degrees.
The above described top guide to shroud head interface includes frusto-conical guide pins and guide pin openings that provide for suitable clearances between the guide pins and the guide pin openings to accommodate flexing of the shroud head flange during installation. Additionally, the frusto-conical guide pins and guide pin openings provide less than 1.0 millimeter of clearance in the installed position to minimize the impact loading on the guide pins and openings caused by horizontal seismic accelerations during a seismic event.
REFERENCES:
patent: 4096034 (1978-06-01), Anthony
patent: 4100021 (1978-07-01), Silverblatt
patent: 4192716 (1980-03-01), Anthony
patent: 4818476 (1989-04-01), Gasparro
patent: 5325407 (1994-06-01), Forsyth et al.
patent: 5675619 (1997-10-01), Erbes et al.
patent: 5797228 (1998-08-01), Kemeny
Armstrong Teasdale LLP
General Electric Company
Jordan Charles T.
Mun Kyongtack K.
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