Patent Application
20120219103
The present invention relates generally to a jet pump of a boiling water nuclear reactor and more specifically to a jet pump slip joint for vibration reduction.
Jet pumps are used to circulate a coolant fluid, such as water, through the fuel core of a boiling water nuclear reactor. The jet pumps are located in a downcomer annulus between a shroud surrounding the core and the interior of the pressure vessel where the coolant is forced into the inlet end or bottom of the core. A slip joint is used along the length of the jet pump typically to accommodate differential thermal expansion that may occur along the jet pump. The slip joint typically has a narrow gap between two nearly concentric cylinders through which coolant fluid may pass under differential pressure.
Boiling water reactor jet pumps experience flow induced vibrations. Flow induced vibration occurs in leakage flow situations under certain circumstances such as flow through a narrow passage with a differential pressure imposed, among which include the BWR slip joint.
U.S. Pat. No. 3,378,456 discloses a jet pump means for a nuclear reactor. The configuration disclosed is what is known to one of skill in the art. The jet pump includes a nozzle, an inlet section, a mixer section and a diffuser section.
U.S. Pat. No. 4,285,770 discloses a jet pump seal configuration to reduce leakage by modifying the cylinder design to incorporate a labyrinth seal. The labyrinth seal is in the form of a series of flow expansion chambers which increase flow resistance and therefore decrease leakage flow. The expansion chambers may be provided by a series of spaced annular grooves formed in the mixer slip joint surface or in the diffuser slip joint
U.S. Pat. No. 3,378,456 teaches an increase, from bottom to top, in the annular gap (flow passage) size between the mixer and the diffuser. This is in the direction of the leakage flow through the slip joint. Although this helps facilitate putting the top piece in the bottom piece, these leave the slip joint unstable under flow conditions with sufficiently high differential pressure. U.S. Pat. No. 4,285,770 teaches attempting to reduce flow induced vibrations by attempting to decrease the flow rate through the slip joint at a constant pressure differential.
An object of the present invention is to reduce the vibration of jet pumps associated with leakage flow in the slip joint and improve the stability at the slip joint.
A method for retrofitting a boiling water reactor is provided. The method includes removing a mixing chamber from a slip joint defined by a diffuser and the mixing chamber, the mixing chamber having an inner surface and a bottom edge directing flow to the diffuser such that a recirculation zone at an entrance to the slip joint creates a diverging effective path for the leakage flow entering the slip joint. The method also includes providing a new inner surface and new bottom edge, the new inner surface and the new bottom edge being reshaped to decrease the size of the recirculation zone.
A jet pump of a boiling water reactor is also provided. The jet pump includes a mixing chamber and a diffuser positioned below the mixing chamber and receiving the mixing chamber at a slip joint such that an outer diameter of the mixing chamber is received in an inner diameter of the diffuser in a longitudinally slidable manner. Water leaks upward through the slip joint. An inner diameter and a bottom edge of the mixing chamber are shaped to decrease the size of a recirculation zone formed at an entrance of the slip joint.
Another method for retrofitting a boiling water reactor is also provided. The method includes removing a mixing chamber from a slip joint defined by a diffuser and the mixing chamber, the mixing chamber having an inner surface directing flow to the diffuser and an outer surface defining part of the slip joint and having an insertion depth in the diffuser. The method also includes providing at least one of a new inner surface, a new outer surface and a new insertion depth to permit reduced vibration at the slip joint.
The present invention is shown with respect to the drawings in which:
a to 9c show mixing chambers according to further embodiments of the present invention;
a shows partial cross-sections of a plurality of different embodiments of the present invention;
b shows two views of one of the embodiments of the mixing chambers shown in
a to 12c show plots of pressure power spectral density versus frequency for vibrations occurring at the slip joints of four samples;
a to 13c shows stability map of the four sample plotting thresholds of slip joint differential pressure versus flow rate; and
Above radially exterior portion 242, mixing chamber 230 converges inwardly toward diffuser 232, such that radially exterior portion 242 is formed by peaks of two opposing frusticonical portions coming substantially to a point to have approximately a V-shape. In other embodiments, radially exterior portion 242 may have approximately a U-shape or may include a portion that runs parallel to inner diameter IDd of diffuser 232. The radial width of annular gap 234 varies along the length of tapered portion 240, for example by approximately 1 to 5 degrees, most preferably by approximately 1 to 3 degrees, so tapered portion 240 directs water entering annular gap 234 to push against mixing chamber 230 and holds mixing chamber 230 radially away from diffuser 232 to prevent or limit mixing chamber 230 and diffuser 232 from contacting each other. The gradually varying width of annular gap 234, with respect to conventional annular gap 134, advantageously causes leakage to apply a radial force against mixing chamber 230 and helps hold mixing chamber 230 away from diffuser 232, preventing or reducing vibrations that could result if mixing chamber 230 and diffuser 232 contact one another.
a shows a mixing chamber 630 according to an embodiment of the present invention. A bottom of mixing chamber 630 is slidably positioned within a top of a diffuser 632 such that an outer surface 652 of mixing chamber 630 and an inner surface 654 of diffuser 632 form a slip joint 616 in which leakage flows upward. An inner surface 650 of mixing chamber 630 is tapered with respect to a vertical axis that runs parallel to a center axis CA of mixing chamber 630 such that an inner diameter of mixing chamber 630 decreases as mixing chamber 630 extends away upward from diffuser 632 and inner surface 650 has a frusticonical shape. A bottom edge or tip 656 of mixing chamber 630 comes to substantially a point, such that tip 656 forms a blade edge for guiding the path of the leakage flow. The tapering of inner surface 650 of mixing chamber 630 and the shape of tip 656 provides a more gradual entrance to the leakage flow path through slip joint 616 and may prevent or mitigate vibration that may be caused by the leakage flow. Outer surface 652 of mixing chamber 630 is straight (i.e., untapered) such that an outer diameter of mixing chamber 630 is parallel to center axis CA along the entire length of slip joint 616 and does not include a lead-in portion. In preferred embodiments, inner surface 650 of mixing chamber 630 is tapered such that inner surface 650 is angled toward center axis CA approximately 1 to 5 degrees with respect to vertical.
b shows another embodiment of mixing chamber 630 according to the present invention. The bottom of mixing chamber 630 is slidably positioned within the top of diffuser 632 to form slip joint 616. In this embodiment inner surface 650 of mixing chamber 630 is straight (i.e., untapered) such that an inner diameter of mixing chamber 630 is parallel to center axis CA. However, outer surface 652 is tapered outward with respect to a vertical axis that runs parallel to center axis CA of mixing chamber 630 such that an outer diameter of mixing chamber 630 increases as mixing chamber 630 extends upward and outer surface 652 has a frusticonical shape. Tip 656 of mixing chamber 630 comes to substantially a point, such that tip 656 forms a knife edge for guiding the path of the leakage flow. Outer surface 652 is tapered such that a radially exterior portion of outer surface 652 at slip joint 616 is positioned at the top of the inner surface of diffuser 632. In preferred embodiments, outer surface 652 of mixing chamber 630 is tapered such that outer surface 652 is angled away from center axis CA approximately 1 to 5 degrees with respect to vertical.
c shows another embodiment of mixing chamber 630 according to the present invention. The bottom of mixing chamber 630 is slidably positioned within the top of diffuser 632 to form slip joint 616. In this embodiment inner surface 650 of mixing chamber 630 is tapered with respect to a vertical axis that runs parallel to center axis CA of mixing chamber 630 such that an inner diameter of mixing chamber 630 decreases as mixing chamber 630 extends away upward from diffuser 632 and inner surface 650 has a frusticonical shape. Also, outer surface 652 is tapered outward with respect to a vertical axis that runs parallel to center axis CA of mixing chamber 630 such that an outer diameter of mixing chamber 630 increases as mixing chamber extends upward and outer surface 652 has a frusticonical shape. Tip 656 of mixing chamber 630 comes to substantially a point, such that tip 656 forms a knife edge for guiding the path of the leakage flow. Outer surface 652 is tapered such that a radially exterior portion of outer surface 652 at slip joint 616 is positioned at the top of the inner surface of diffuser 632. In preferred embodiments, outer surface 652 of mixing chamber 630 is tapered such that outer surface 652 is angled away from center axis CA approximately 1 to 3 degrees with respect to vertical and inner surface 650 of mixing chamber 630 is tapered such that inner surface 650 is angled toward center axis CA approximately 1 to 3 degrees with respect to vertical.
a shows partial cross-sections of a plurality of different embodiments for mixing chamber 630, most of which include tapering both inner surface 650 and outer surface 652 of mixing chamber 630. In all of details 10a-1 to 10a-5, inner surface 650 of mixing chamber 630 is tapered and forms and angle of approximately 3 degrees with respect to vertical over the bottom of mixing chamber 630. The tapered portion of inner surface 650 extends a distance d1 from the bottom of mixing chamber 630, with the remaining inside surface of mixing chamber extending parallel to center axis CA (
b shows two views of the embodiment of mixing chamber 630 shown in detail 10a-5. A detail 10b-1 is cross-sectional view of mixing chamber 630, with the bottom 2.0 of mixing chamber 630 having an inner diameter that tapers by 3.0 degrees. A detail 10b-2 is a side view of mixing chamber 630, showing the bottom of mixing chamber 630 having an outer diameter that tapers by 2.0 degrees.
In accordance with further embodiments of the present invention, the embodiments described above may be combined to effectively reduce vibrations caused by leakage flow through a slip joint. For example, in one embodiment, the three main vibration reduction techniques may be employed together—the inner surface of a mixing chamber may be tapered outward at the bottom of the mixing chamber, the outer surface of the mixing chamber may be tapered inward at the bottom of the mixing chamber and the mixing chamber may be inserted deeper into the diffuser than is conventional. Deeper insertion of the mixing chamber into the diffuser may be helpful in situations where the outer diameter of the mixing chamber has been tapered too much, resulting in too large of a gap between the mixing chamber and the diffuser at the bottom of the slip joint. In such a situation, the insertion depth of the mixing chamber in the diffuser may be increased until the vibrations are minimized to an acceptable or stable level. In other embodiments, only the inner surface of the mixing chamber or the outer surface of the mixing chamber may be tapered and the mixing chamber may be inserted into the diffuser deeper than is conventional. Also, in even further embodiments, the inner surface of the mixing chamber may be tapered and the outer surface of the mixing chamber may be tapered, but the mixing chamber may be inserted into diffuser at a conventional insertion depth.
The vibrations at the slip joint have been determined to be caused by three main interrelated parameters: (1) slip joint differential pressure, (2) water temperature and (3) drive flow. An increase in one of these parameters, with all other variables remaining the same, increases the likelihood that vibrations will be induced. The tapering of the inner surface of a mixing chamber outward at the bottom of the mixing chamber, the tapering of the outer surface of the mixing chamber inward at the bottom of the mixing chamber and increasing the insertion depth of the mixing chamber in the diffuser may be used to increase the thresholds at which these three parameters cause unstable vibrations. Accordingly, altering the slip joint and increasing the thresholds eliminates or minimizes the likelihood of flow induced unstable vibrations. In particular, altering the mixing chamber or diffuser as described herein may then allow a nuclear reactor to be operated at a higher slip joint differential pressure and/or drive flow, advantageously giving operators of the nuclear reactor more operating flexibility.
For example,
a shows a stability map of the first sample plotting thresholds of slip joint differential pressure versus flow rate. A line 901 represents a curve of maximum thresholds, with slip joint differential pressures exceeding the thresholds causing unstable vibrations at the slip joint. A line 902 represents a curve of minimum thresholds. If the slip joint differential pressure for a particular flow rate exceeds the maximum threshold of line 901 and unstable vibrations begin, the slip joint differential pressure will have to reduced to below the minimum threshold of line 902 to make the vibrations stable again.
b shows a stability map of the second sample plotting thresholds of slip joint differential pressure versus flow rate. Lines 903, 904 form essentially an island of instability. Instability at the slip joint only results if the slip joint differential pressure is greater than line 903, but less than line 904, with line 903 also defining the maximum flow rate at which the unstable vibrations occur. Unstable vibrations did not occur for pressures and flow rates outside of the island formed by lines 903, 904 for the second sample.
c shows a stability map of the third and fourth samples plotting thresholds of slip joint differential pressure versus flow rate. As shown in
One embodiment of the present invention is a method for determining optimal shape and insertion depth of a mixing chamber into a diffuser. The method includes operating a boiling water reactor to determine unstable vibration thresholds for a jet pump of the boiling water reactor by varying the drive flow produced by drive nozzles in the jet pump and/or the slip joint differential pressure of the jet pump. The method then includes varying the shape of the bottom of the mixing chamber or the insertion depth of the bottom of the mixing chamber into the diffuser to increase the unstable vibration thresholds for the jet pump so that the jet pump may be operated at higher drive flows and/or higher slip joint differential pressures without inducing unstable vibrations.
One embodiment of the present invention is a method for determining the optimal shape of a mixing chamber in a jet pump. The method involves varying the inner surface of the mixing chamber and a bottom edge of the mixing chamber to decrease the size of a recirculation zone formed at an entrance to a slip joint formed by the mixing chamber and a diffuser. When the bottom edge of the mixing chamber has a wide surface and the inner surface of the mixing chamber is straight, the recirculation zone at the entrance of a slip joint may be large, causing the leakage flow to enter the slip joint through a small path that immediately diverges, resulting in instability. The wider of the bottom edge of the mixing chamber, the greater the recirculation zone and the instability. Decreasing the width of the bottom edge of the mixing chamber by machining the mixing chamber decreases the size of the recirculation zone, minimizing the divergence of the effective path of the leakage flow, and increases the stability of the slip joint.
In preferred embodiments, jet pumps 18 may be retrofitted to prevent or minimize unstable vibrations. Retrofitting of jet pumps 18 may be achieved by retrofitting conventional mixing chamber 130 to form mixing chambers 230, 330, 430, 630 or by retrofitting conventional diffuser 132 to form diffuser 532. This may be accomplished by removing mixing chamber 130 from conventional slip joint 116 defined by diffuser 132 and mixing chamber 130 and then removing material from mixing chamber 130 (i.e., portions of gap forming portion 138 and lead-in portion 136 or the inner surface of mixing chamber 130) or diffuser 132, for example by electrical discharge machining By machining existing slip joint 116 having existing annular gap 134, new slip joints 216, 316, 416, 516 defining new annular gaps 234, 334, 434, 534 are provided. Jet pump 18 may also be retrofitted by removing conventional mixing chamber 130 or conventional diffuser 132 from jet pump assembly 40, and then placing mixing chambers 230, 330, 430, 630 or diffuser 532, or a portion thereof, in jet jump assembly 40. In embodiments where mixing chamber 130 or diffuser 532 are removed and replaced, tapered portions 240, 340, stepped portion 440 and inner surface 650 and tip 656 may be formed in respective mixing chambers 230, 330, 430, 630 during fabrication of mixing chambers 230, 330, 430, 630 or may be machined therein after fabrication and tapered portions 546 may be formed in diffuser 532 during fabrication of diffuser 532 or may be machined therein after fabrication.
In the preceding specification, the invention has been described with reference to specific exemplary embodiments and examples thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.
Priority to U.S. Provisional Patent Application Ser. No. 61/446,630 filed Feb. 25, 2011, is claimed, the entire disclosure of which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61446630 | Feb 2011 | US |
