Induced nuclear reactions: processes – systems – and elements – Seal arrangements
Reexamination Certificate
1998-08-07
2001-02-13
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Seal arrangements
C376S204000, C376S260000, C376S263000, C376S294000, C228S184000, C029S428000
Reexamination Certificate
active
06188741
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to boiling water nuclear reactors and more particularly, to sealing elongate hollow members such as control rod drive housings to a bottom head dome of a nuclear reactor pressure vessel of such reactors.
BACKGROUND OF THE INVENTION
Boiling water nuclear reactors typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick. The cylindrical shell is closed at its top end by a removable top head. The top head is removable so that components, such a fuel bundles, located in the RPV can be accessed. The RPV cylindrical shell is closed at its bottom end by a dome shaped bottom head assembly welded to the shell.
A plurality of openings, sometimes referred to as penetrations, are formed in the bottom head dome so that components, such as control rod drive assemblies, can extend within the RPV. Typically, a substantially cylindrical stub tube having a bore extending therethrough is welded to the bottom head dome and the tube bore aligns with an opening in the bottom head dome. The cylindrical stub tube typically is fabricated from a corrosion resistant material such as stainless steel or Ni—Cr—Fe.
With a control rod drive assembly, for example, the control rod drive housing, e.g., a tube, is inserted through the bottom head dome opening and stub tube bore, and the housing extends into the RPV. The control rod drive (CRD) housing is welded to the stub tube to maintain the housing in the desired position. The stub tube thus serves as a transition piece between the bottom head dome, which typically is fabricated from low alloy steel (LAS), and the CRD housing, which typically is fabricated from stainless steel.
Stress corrosion cracking (SCC) is a known phenomenon occurring to adjacent stub tube welds connecting the bottom head dome to the stub tube and connecting the stub tube to the CRD housing. Particularly, the stub tube welds are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. Such stresses may, at times, cause cracks adjacent the stub tube welds or may even cause cracks in the weld materials.
To avoid SCC, it is desirable to eliminate welds. In addition, the welds which are required should be inspectable. It would be desirable to provide a stainless steel penetration to LAS head attachment that eliminates the stub tube to head weld, meets all design stress requirements, and simplifies fabrication.
SUMMARY OF THE INVENTION
These and other objects may be attained by an integral stub tube which simplifies the reactor pressure vessel fabrication process, provides a structural transition between the penetration and head, and facilitates future remote inspection of the attachment weld. In one embodiment, a stub tube is machined into the bottom head dome. Specifically, a penetration is formed in the bottom head dome by a bore having a stub tube portion. The stub tube portion has a cylindrical shape and a length of the stub tube portion is selected to provide a transition between a penetration housing and an adjacent portion of bottom head dome. The penetration housing extends through the penetration, in the dome. A weld attaches the stub tube portion to the penetration housing.
The above described integral stub tube provides a pressure boundary attachment at the junction between control and instrumentation housings and the bottom head of the pressure vessel. Such stub tube also provides a structural transition between the penetration housing (which is typically austenitic stainless steel) and the LAS bottom head. This structural transition provides a transition in stiffness and smooths out temperature gradients, and has the advantage of being an integral part of the bottom head.
In addition, such integral stub tube requires only one weld for each penetration, and the weld is axisymmetric. An axisymmetric weld is easier to make than a non-axisymmetric weld. Requiring just one weld for each penetration also reduces the number of welds that must be inspected during service by eliminating the stub tube to head weld. The weld also is readily accessible for inspection.
Also, all machining required to form the stub tube portion can be performed by conventional boring and tree-panning operations. Complex multi-axis machining is eliminated, which results in a potential fabrication cost savings as compared to known stub tubes. Reliability of the reactor is believed to be improved and the risk of future materials related problems is reduced because Ni—Cr—Fe material is not used in the stub tube.
REFERENCES:
patent: 3945431 (1976-03-01), Straub
patent: 4057163 (1977-11-01), Widart et al.
patent: 5006300 (1991-04-01), Jonsson et al.
patent: 5426675 (1995-06-01), Kumar et al.
patent: 5519744 (1996-05-01), Relf
patent: 5721758 (1998-02-01), Fife et al.
patent: 5796797 (1998-08-01), Fallas
patent: 5802125 (1998-09-01), Fife
patent: 5809098 (1998-09-01), Deaver
Ballas Gary J.
Fife Alex B.
Matsumoto Jack T.
Armstrong Teasdale LLP
General Electric Company
Jordan Charles T.
Keith Jack
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