Induced nuclear reactions: processes – systems – and elements – Reactor protection or damage prevention – Core restraint means
Reexamination Certificate
1998-09-25
2001-05-15
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Reactor protection or damage prevention
Core restraint means
C376S260000, C376S372000, C376S461000, C219S069110, C219S069150, C219S070000
Reexamination Certificate
active
06233301
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to nuclear reactors and more particularly, to methods and apparatus for securing pressure lines to jet pumps in a nuclear 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 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. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
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 routed over and through the shroud to deliver water to the reactor core.
Jet pump diffusers in a nuclear reactor, such as in a boiling water nuclear reactor, typically form part of a jet pump and are utilized to maintain the floodability of the reactor core at a safe level. The jet pump diffuser also typically discharges water from an inlet mixer into the lower core plenum to maintain adequate recirculation flow.
One known jet pump diffuser has a generally conical shape, and the larger diameter end of the diffuser is welded to an adapter assembly engaged to an INCONEL nickel-chromium alloy shroud support plate. The smaller diameter end of the diffuser forms a slip fit connection to the inlet mixer section of the jet pump.
Water, generally under high pressure, flows through the jet pump diffuser from the smaller diameter end to the larger diameter end. The water exits the diffuser at the larger diameter end and is discharged through the adapter assembly into the lower core plenum. The discharged water mixes with the water in the lower core plenum and may cycle through the reactor.
As the water flows through the jet pump diffuser, the speed of the water decreases due to the increasing diameter of the diffuser. Decreasing the water speed and pressure just prior to discharging the water into the lower core plenum improves the mixing and flow characteristics of the discharged water.
For safe operation of a nuclear plant, the pressure drop across the diffuser is continually monitored. For this purpose, small sensing lines are mounted to the exterior of the diffuser, and communicate through the walls of the diffuser so that the pressure differential may be sensed.
Under certain conditions, the pressure sensing lines may vibrate. This vibration subjects the stainless steel stand off blocks mounted between the diffuser and line to sever between the block and line. This severance of the line from its mounted stand off block is an undesirable condition.
T-Bolt clamps have been developed that clamp the diffuser sensing lines to the diffuser providing added support and minimizing vibration. To secure the T-bolt clamp to the diffuser, slots need to be machined into the diffuser so that the T-bolt can be inserted into the slot. The T-bolt clamp is then secured around the sensing line to prevent excessive vibration of the sensing line. The slot must be positioned behind the sensing line. Typically such slots are burned or machined into the diffuser by using an electrode discharge machining (EDM) process. A major problem to overcome is burning the slot into the diffuser behind the sensing line without burning and damaging the sensing line at the same time.
It would be desirable to provide a EDM tool for burning slots or holes in the diffuser behind the sensing line that will not damage the sensing line during the EDM process.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained by an electrode discharge machining apparatus capable of machining precisely located and sized slots in a jet pump diffuser in a nuclear reactor. Particularly, the EDM apparatus, or tool, is capable of machining two slots in the jet pump diffuser at a location behind the diffuser sensing line without damaging the sensing line. Behind the sensing line may be defined as within the plane formed by the center of the diffuser and the sensing line.
The EDM tool includes a housing, two EDM electrode assemblies, two clamp assemblies, and a hook assembly. Each EDM electrode assembly includes an electrode holder and an electrode configured to be coupled to the holder and to form the desired slot in the diffuser. Each slot extends through the wall of the jet pump diffuser and is configured to accept a T-bolt vibration clamp that is configured to secure the sensing line to the sensing line standoff to prevent vibration of the sensing line during operation of the reactor. In one embodiment of the present invention, each slot machined in the diffuser has the dimension of about 1.06 by 0.4 inches.
The EDM apparatus also includes a stepper drive motor coupled to the electrode assemblies. The stepper drive motor moves the electrode assemblies so that the electrodes maintain a predetermined distance from the diffuser during the electrode discharge machining process. Particularly, as the electrode machines a hole through the diffuser the position of the electrode relative to the diffuser is adjusted by the stepper drive motor so that a constant distance is maintained between the electrode and the bottom of the hole being machined in the diffuser.
Each clamp assembly includes a clamp arm coupled to a movable shaft. The shaft is movably coupled to an actuation cylinder. The actuation cylinder may be a fluid or an air actuation cylinder. The clamp arm is configured to engage the sensing line to clamp the EDM apparatus to the sensing line. Particularly, the clamp arm exerts a clamping force on the sensing line to clamp the line between the arm and the housing.
The hook assembly includes a flat hook shaped member configured to engage the sensing line standoff block. The hook assembly also includes a cylindrically shaped member depending horizontally from the flat hooked shaped member. The hook assembly is coupled to the housing at an angle and is configured so that when the flat hooked shaped member engages the standoff, the EDM apparatus is positioned at an angle from a plane formed by the center of the jet pump diffuser and the sensing line. In one embodiment, the hook assembly is configured so that the EDM apparatus is positioned at about a 21° angle from the plane formed by the center of the jet pump diffuser and the sensing line. This configuration is used so that the holes can be machined behind the sensing line without damaging the sensing line.
To machine slots in the jet pump diffuser configured for the installation of T-bolt clamps, the EDM apparatus is lowered into the annulus of the reactor pressure vessel with a handling pole. The EDM apparatus is positioned adjacent the diffuser with the hook assembly engaging the sensing line standoff. Because of restricted spacing around the jet pump diffusers in the annulus of the reactor, the EDM apparatus may be configured to be installed clockwise around the sensing line or may be configured to be installed counter-clockwise around the sensing line.
When properly positioned, the hook assembly engages the sensing line standoff and positions the EDM apparatus at an angle of 21° from the plane formed by the center of the diffuser and the sensing line. The clamp assemblies are then actuated causing the clamping arms to clamp the EDM apparatus to the sensing line. In this configuration the EDM apparatus is properly positioned so that each EDM electrode is adjacent the diffuser to burn a slot or hole in the diffuser behind the sensing line. The EDM process is then actuated and each electrode machines a 1.06 by 0.4 inch hole in the diffuser behind the sensing line. Typically, the holes are machined simultaneously. Each hole is configured to accept a T-bolt vibration clamp that is configured to secure the sensing line to the sensing line sta
Armstrong Teasdale LLP
Carone Michael J.
General Electric Company
Keith Jack
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