Induced nuclear reactions: processes – systems – and elements – Grids
Reexamination Certificate
1998-06-12
2001-05-29
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Grids
C376S438000, C376S260000, C411S075000, C411S077000, C411S080000
Reexamination Certificate
active
06240156
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to boiling water reactors and more particularly, to securing a top guide grid to a shroud of 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 formed by cross members, sometimes collectively referred to as a grid, and fuel bundles are inserted through the openings and are supported by the core plate. The top guide is supported vertically and horizontally by the shroud, and the top guide grid restrains horizontal movement of the upper ends of the fuel assemblies. Under normal operating conditions, there are no substantial vertical loads applied to the grid.
Irradiation is know to assist in the cracking of some BWR materials. The grid is located in a high fluence region and may be susceptible to irradiated assisted stress corrosion cracking (IASCC). The core support structure is exposed to the highest fluence. The core plate also is in a high fluence region and may be susceptible to IASCC.
The top guide grid must be removed if it becomes necessary to replace the top guide or the core plate. In some known BWRs, however, the grid is integral with the upper shroud assembly. Therefore, the grid cannot be replaced without removing the entire upper shroud assembly. Removing the entire upper shroud assembly requires removing approximately 90 stud
ut/keeper assemblies and disconnecting two to four core spray piping lines.
Other known BWRs have removable top guide grids. Specifically, the grid is secured to other components of the shroud assembly by bolted joints. Vertical loads are carried by the preload of the bolts, and horizontal loads are carried by shear pins and/or friction in the joint. Assembling such top guides is a tedious and time consuming task. Specifically, the grid must be accurately aligned so that the shear pins can be properly positioned, and each bolt must be torqued to the required pre-load.
It would be to desirable to provide a top guide grid which can be easily removed and replaced. It also would be desirable to provide such a grid which is easy to install.
SUMMARY OF THE INVENTION
These and other objects may be attained by a top guide assembly for securing a top guide to a shroud which enables the top guide to be remotely removed without removing any other parts of the shroud or the core support structure. More specifically, and in accordance with one embodiment, the top guide assembly includes a hanger for being secured in a groove formed in an inner surface of the shroud, and the hanger includes a ledge having a plurality of openings therein. The top guide rests on an upper surface of the hanger ledge. The assembly also includes wedges for being positioned in pockets formed in the top guide and tightly fit between the hanger and the top guide, and threaded studs that extend through respective aligned bores in the wedges and thread into the hanger ledge. A nut and a keeper are engaged to one end of each stud and retain the wedge in the pockets of the grid.
Vertical support for the top guide is provided by the groove machined on the inside surface of the shroud, and no flange or ledge in the shroud is required to vertically support the top guide. Therefore, the inside surface of the shroud can be a constant diameter at the top guide interface. In addition, no threaded holes are required in the shroud since all threaded components are separate from the shroud and are removable. Also, wedges allow for horizontal alignment/positioning of the top guide and provide the horizontal load path from the top guide to the shroud. No alignment, eccentric or shear pins are required. Further, bolt preloading, as compared to known bolting methods, is minimized by the wedges.
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patent: 208951 (1956-08-01), None
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Fife Alex B.
Matsumoto Jack T.
Armstrong Teasdale LLP
Carone Michael J.
General Electric Company
Keith Jack
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