Induced nuclear reactions: processes – systems – and elements – Handling of fission reactor component structure within...
Reexamination Certificate
1999-02-18
2001-05-22
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Handling of fission reactor component structure within...
C376S203000, C376S282000, C376S286000, C376S361000, C285S404000, C285S015000
Reexamination Certificate
active
06236700
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to nuclear reactors and more particularly, to apparatus and methods for coupling piping within reactor pressure vessels of such reactors.
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 core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure.
Boiling water reactors have numerous piping systems, and such piping systems are utilized, for example, to transport water throughout the RPV. For example, core spray piping is used to deliver water from outside the RPV to core spray spargers inside the RPV. The core spray piping and spargers deliver water flow to the reactor core.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high temperature water. The reactor components 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. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to SCC.
Reactor internal piping, such as T-boxes and core spray line downcommers, occasionally require replacement as a result of SCC. Replacing the core spray piping often requires removing and replacing the core spray line downcommer. The core spray line downcommer attachment to the shroud, however, typically is installed during original reactor construction and is difficult to access. In addition, replacing the core spray line downcommer is complicated by the limited available working space.
The core spray line downcommer includes a lower elbow secured to a shroud core spray sparger T-box. Installing a replacement downcommer pipe into the reactor requires that the replacement lower elbow be welded to the shroud. However, as explained above, it is difficult to access this area of the reactor.
It would be desirable to provide an apparatus which facilitates replacing a core spray line downcommer attached to the shroud core spray sparger T-box. It would also be desirable to provide such an apparatus which is installed without the necessity of field welding.
BRIEF SUMMARY OF THE INVENTION
Replacement of a lower elbow section of a downcommer pipe without field welding may be attained by a downcommer pipe coupling apparatus which, in one embodiment, includes a wedge, a wedge flange, and a wedge housing. The wedge is a split tapered sleeve having a plurality of flexible thinned segments extending lengthwise. The separate wedge flange is configured to be located over the wedge and includes two openings sized for wedge flange bolts with each opening having a square recess to mate with a bolt locking collar. The wedge housing is substantially frustro-conical shaped and has a wedge housing flange at a larger diameter end, a tapered bore matching the wedge, and two opposing threaded dowel bolt openings threaded to receive dowel bolts. Each dowel bolt opening has a square recess to mate to a bolt locking collar.
The downcommer pipe is coupled to the apparatus using two wedge flange bolts and two dowel bolts. Particularly, the wedge flange bolts extend through the wedge flange and into the wedge housing so that as the wedge flange bolts are secured, the wedge clamps the downcommer pipe tightly to the wedge housing. The dowel bolts extend through the dowel bolt openings and the downcommer pipe, forming a shear connection between the wedge housing and the downcommer pipe. The tightened wedge, wedge housing and downcommer pipe form a rigid connection which is aligned to the downcommer pipe and the dowel bolt openings.
The apparatus further includes a substantially cylindrical pipe connected to the smaller diameter end of the wedge housing, an elbow connected to the second end of the cylindrical pipe, and a substantially circular shaped lower flange connected to the second end of the elbow. The lower flange includes a T-box opening having an inner diameter sized to receive a portion of a T-box extending from the shroud, and a circular lip on the lower flange face is configured to engage the shroud spot face to transmit applied piping shear loads and the joint preload forces. The lower flange also includes eight lower flange bolt openings sized to receive lower flange bolts. Each lower flange bolt opening has a square recess to mate to a bolt locking collar. Lower flange bolts are installed in three of the eight possible locations, depending on the available access.
Additionally, the coupling apparatus includes a nutbar and a substantially L-shaped clamp having a bolt opening sized to receive one of the lower flange bolts. The nutbar is sized to receive two lower flange bolts using the connection of the two nuts to act as an anti-rotational mechanism. The clamp is sized to axially restrain the T-box to the shroud so that the T-box does not move inward toward the reactor core.
The two connecting ends of the coupling apparatus are provided with seals to minimize leakage. Both the pipe seal and T-box seal are double diaphragm type seals which are self-energizing so that the pressure difference tending to cause leakage also increases the seal surface contact force, tightening the seal. This flexible type of seal minimizes the added joint load required to accomplish sealing, so the sizing of the required bolts and bolt openings is minimized to fit in the available access. In particular, access on the inside of the shroud is limited by numerous existing core spray sparger nozzles.
The coupling apparatus is mechanically joined to the shroud and downcommer pipe with sufficient strength and stiffness to react the originally specified piping loads from seismic, weight, thermal expansion and hydraulic conditions without significantly changing the piping system loads. The wedge and wedge housing grip the downcommer pipe tightly over a sufficient length to develop the full bending strength and stiffness of a continuous pipe.
The dowel bolts are sized to react the axial twisting and vertical loads. The bolted lower flange connection to the shroud is a similarly stiff connection, with the bolt preload sufficient to prevent separation of the lower flange from the shroud. Placement of the lower flange bolts outside the downcommer pipe minimizes the preload change due to temperature transients in the fluid inside the downcommer pipe.
The coupling apparatus is constructed, typically, of materials matching the thermal properties of the attached downcommer pipe and shroud, for example, 316 stainless steel. Components of the coupling apparatus are fabricated to ease assembly, provide strength, and provide optimum corrosion resistance.
To install the coupling apparatus, a lower portion of the existing downcommer pipe is removed, for example, by cutting-off a portion of the downcommer pipe near the lower elbow and removing the lower elbow from the shroud. Two dowel bolt openings are then machined in the downcommer pipe, aligned with the shroud outside diameter and downcommer pipe end. A circular groove surrounding the T-box in the shroud outside diameter and the cut end of the T-box are then spot face machined to the correct depth. Three lower flange bolt openings are then machined through the shroud at accessible locations.
After inserting the pipe seals into the wedge housing and lower flange groove, the replacement coupling apparatus, wedge flange, wedge, and wedge housing are installed over the end of the downcommer pipe, until the downcommer pipe is seated against the seal inside the bottom of the wedge housing. The two dowel bolts are then engaged through the wedge housing into the downcommer pipe openings, aligning the coupling apparatus to the shroud outside diameter and applying a controlled
Erbes John G.
Rousar David L.
Schrag Michael R.
Armstrong Teasdale LLP
Carone Michael J.
General Electric Company
Keith Jack
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