Method and apparatus for a BWR inlet mixer clamp assembly

A method and apparatus for physically restraining an inlet mixer of a boiling water reactor (BWR) jet pump assembly. An inlet mixer clamp assembly is attached to the top of a transition piece of the BWR jet pump assembly, to provides a downward clamping force on the top of the inlet mixer. The clamping force restrains the inlet mixer from experiencing vibration and mitigates leakage between the inlet mixer and the transition piece sealing surfaces of the jet pump assembly. The clamping force also restrains the inlet mixer from undesirable rotation, especially during operation of the jet pump assembly. The inlet mixer clamp assembly may be used as an alternative to conventional jet pump beam assemblies. Alternatively, the inlet mixer clamp assembly may be used as a redundant physical restraint in addition to the conventional jet pump beam assemblies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Example embodiments relate generally to nuclear reactors, and more particularly to a method and apparatus for a Boiling Water Reactor (BWR) inlet mixer clamp assembly that physically restrains and applies a downward force on an inlet mixer of a boiling water reactor (BWR) jet pump assembly. The inlet mixer main clamp assembly may be used as an alternative to conventional jet pump beam assemblies. Alternatively, the inlet mixer clamp assembly may be used as a redundant physical restraint in addition to conventional jet pump beam assemblies.

2. Related Art

A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends (for example 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 between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.

In a BWR, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within jet pump restrainer brackets by a gravity actuated wedge.

Conventionally, the inlet mixer is held in place by jet pump beam assemblies at the top of the BWR jet pump assembly. The jet pump beam assemblies press down and provide a clamping force on the inlet mixer to mitigate movement, rotation and vibration of the inlet mixer and reduce potential leakage between the inlet mixer and the transition piece sealing surfaces of a jet pump assembly. Additionally, the jet pump beam assemblies mitigate the potential that the inlet mixer may be ejected from the diffusers of the jet pump assembly during operation of the jet pump assembly. Conventionally, jet pump beam assemblies are highly stressed, they are limited in their clamping force, and they are susceptible to stress corrosion cracking. Field failures of jet pump assemblies have previously caused significant plant downtime and expensive repair.

SUMMARY OF INVENTION

Example embodiments provide a method and an apparatus for providing vertical restraint of an inlet mixer of a BWR jet pump assembly to reduce leakage and vibration of the inlet mixer. Example embodiments provide a clamping force, preferably at locations off of the centerline of the transition seat, to resist inlet mixer rotation. The clamping force of example embodiments may exceed the force that may otherwise be applied by conventional jet pump beam assemblies to ensure that the inlet mixer stays seated against the transition piece. The clamping force of example embodiments may eliminate a single point of failure for stabilizing the inlet mixer, as the clamping force may be a redundant downward force on the inlet mixer that may be applied in addition to conventional jet pump beam assemblies. Alternatively, the clamping force of example embodiments may be used in lieu of conventional jet pump beam assemblies.

DETAILED DESCRIPTION

FIG. 1is a perspective view of a conventional boiling water nuclear reactor (BWR) jet pump assembly. The jet pump assembly includes a riser pipe1that brings water into inlet mixers2. The inlet mixers discharge water into diffusers4. The jet pump beam bolts6of the conventional jet pump beam assemblies (seen in better detail, inFIG. 2) can be seen toward the top of the inlet mixer2.

FIG. 2is a detailed view of the top of an inlet mixer2, with conventional jet pump beam assemblies8in place. The jet pump beam assemblies8are attached underneath a bridge10aportion of transition piece10of a conventional jet pump assembly. The transition piece10is welded to the top of the riser pipe1(seeFIG. 1) to ensure stability of the transition piece10. The jet pump beam assemblies provide a downward clamping force on the top of inlet mixer2, to resist the vertical hydraulic load the inlet mixers see during operation and to reduce vibration of the inlet mixer.

FIG. 3is a detailed view of a BWR inlet mixer clamp assembly20, in accordance with example embodiments. The inlet mixer clamp assembly20may include an inlet mixer clamp plate22. The inlet mixer clamp assembly may include inlet mixer jacking bolts24that provide a downward clamping force that is applied to the top of the inlet mixer2. The jacking bolts24may be threaded bolts that mate with eyelets of the clamp plate22as well as threaded connections that may be made in the top surface of the existing inlet mixer2(shown in detail inFIGS. 5 and 6). The jacking bolts may alternatively penetrate clamp plate22and contact and/or press on the top surface of the existing inlet mixer2without actually penetrating the top surface of the inlet mixer. The inlet jacking bolts24may be lined up in a row along the centerline of the clamp plate22, as shown inFIG. 3. Alternatively, jacking bolts24may be provided on other convenient locations on the clamp plate22. The number of jacking bolts24may vary, although four jacking bolts24are shown in example embodiments.

The clamp plate22may include beam bolt holes30, especially in the case where the clamp assembly20is to be used in conjunction with the conventional beam assemblies8. The beam bolt holes30allow for clearance of the beams bolts6(as shown inFIG. 5).

The clamp plate22may include mounting blocks28, that may be “C”-shaped components affixed to a bottom surface of the clamp plate22. Mounting blocks28may be securely attached to clamp plate22via mounting block bolts26. Alternative to mounting block bolts26, other reasonable means of securing mounting blocks28to clamp plate22may be used, including welding.

All clamp assembly components may be made by materials that are known to be acceptable for a nuclear environment. For instance, stainless steel (304, 316, XM-19, or equivalent) or nickel based alloys (Iconel, X-750, X-718, or equivalent) may be used.

FIG. 4is a detailed bottom view of a BWR inlet mixer clamp assembly20, in accordance with example embodiments. Note that mounting blocks28, formed in a “C”-shape, include mounting block apertures28athat cradle the transition piece bridge10a(shown inFIG. 2). The mounting blocks28grip the transition piece10to provide an anchor for the clamp plate22, thereby allowing jacking bolts24to then press forcefully down on the top of inlet mixer2.

FIG. 5is a detailed view of a BWR inlet mixer clamp assembly20in use in conjunction with conventional jet pump beam assemblies8, in accordance with example embodiments. Note that jacking bolts24may mate with threaded connections that are drilled into portions of the thick upper surface2cof the inlet mixer2. Mounting blocks28are shown cradling the bridge10a(seeFIG. 2) of the transition piece10, to stabilize the clamp plate22. Existing beam bolts6of the conventional jet pump beam assemblies8are shown protruding through the beam bolt holes30, as the beam bolt holes30may be sized to allow the beam bolts6to clear the clamp plate22when the clamp assembly20is attached to the inlet mixer2. In this example embodiment, conventional jet pump beam assemblies8are working in unison with the clamp assembly20to provide redundant downward force on the top of the inlet mixer.

The clamp plate22is sized to fit between the lifting eyelets12of the inlet mixers2, but may alternatively extend beyond the lifting eyelets. The clamp plate has also been sized to fit over and cover the upper surfaces of opposing portions of transition piece10(note that only one side of transition piece10is shown inFIG. 5, as the opposing side of transition piece10is obscured from the drawing and located on the other side of the inlet mixer2as shown inFIG. 2). Lobes22bare sized to extend from the clamping plate22at locations directly above the transition piece bridges10a(bridges10amay be seen inFIG. 2). The lobes22aare located at this position of the clamping plate22to support mounting blocks28underneath the clamp plate22and allow the mounting blocks28to cradle the transition piece bridges10a.

By positioning the jacking bolts24at locations on either side of “centerline”22bof plate22, the clamp assembly20restrains the inlet mixer2from rotation during jet pump assembly operation. Note that the “centerline”22bdivides the clamp plate22approximately in half, and runs through the center of each lobe22a(and, centerline22balso runs directly through the middle and above both transition piece bridges10a). For the jacking bolts24to be effective in mitigating rotation of the inlet mixer2, it is preferable to have at least one jacking bolt24on either side of centerline22b. Furthermore, the farther the jacking bolts24are located away from centerline22b, the more effective the clamp assembly20will be at mitigating clockwise/counterclockwise rotation of the inlet mixers2. For this reason, lobes22cmay be used to extend the location of jacking bolts24away from centerline22bto provide added leverage to allow clamp assembly20to mitigate inlet mixer2rotation that may occur especially during jet pump assembly operation. In one embodiment, the jacking bolts24may be located on the clamp plate22in a line that runs approximately perpendicular to the centerline22b. Alternatively, if jet pump beam bolts6are to be removed from operation upon the installation of clamp assembly, jacking bolts24may be located where jet beam bolts6would otherwise penetrate plate22(i.e., jacking bolts24may be located where beam bolt holes30may otherwise be located). Alternatively, only one jacking bolt24may be located on clamp plate22at a location that intersects centerline22b.

FIG. 6is a detailed view of a BWR inlet mixer clamp assembly20in use (without conventional jet pump beam assemblies8), in accordance with example embodiments. Note the vacant space8athat is left under the clamp plate22, where the conventional jet pump beam assemblies8would otherwise be located. Also note that the beam bolt holes30do not have beam bolts6of the jet pump beam assemblies8penetrating through the holes30. Therefore, the beam bolt holes30are optional in this configuration.

The configuration ofFIG. 6allows the clamp assembly20to be used in lieu of the conventional jet pump beam assemblies8, such that the jet pump beam assemblies8may be removed prior to installation of the clamp assembly20.