Patent Publication Number: US-2011069804-A1

Title: Jet Pump Slip Joint Modification for Vibration Reduction

Description:
Priority to U.S. Provisional Patent Application Ser. No. 61/276,973 filed Sep. 18, 2009, is claimed, the entire disclosure of which is hereby incorporated by reference. 
    
    
     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. 
     BACKGROUND 
     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. 
     SUMMARY OF THE INVENTION 
     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 slip joint of a jet pump to reduce vibrations is provided. The method includes removing a mixing chamber from an existing slip joint defined by a diffuser and the mixing chamber, the existing slip joint defining an existing annular gap, and providing a new slip joint defining a new annular gap, the new annular gap being reshaped to permit reduced vibration. 
     A jet pump of a boiling water nuclear 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 and at least one of the mixing chamber or the diffuser being shaped to provide an increased pressure profile to the water leaking upward. 
     A method of operating a jet pump is also provided. The method includes passing water downward through a mixing chamber into a diffuser and directing water leaking upward through a slip joint connecting the mixing chamber and the diffuser to reduce oscillations at the slip joint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is shown with respect to the drawings in which: 
         FIG. 1  schematically shows the lower portion of a boiling water nuclear reactor; 
         FIG. 2  shows an isometric view of a jet pump assembly; 
         FIG. 3  shows an embodiment of a conventional slip joint; 
         FIG. 4  shows a slip joint according to a first embodiment of the present invention; 
         FIG. 5  shows a slip joint according to a second embodiment of the present invention; 
         FIG. 6  shows a slip joint according to a third embodiment of the present invention; and 
         FIG. 7  shows a slip joint according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows the lower portion of a boiling water nuclear reactor  50 . Reactor  50  includes a pressure vessel  14  closed at a lower end by a dish shaped bottom head  10 . A shroud  26  is located radially inside of pressure vessel  14 . Between a wall of pressure vessel  14  and shroud  26  is a downcomer annulus  4 . A reactor core fuel assembly  28  is housed inside of shroud  26 , which comprises fuel assemblies  2 . Fuel assemblies  2  may be arranged in groups of four, with each group being attached to guide tubes  12  at lower ends fuel assemblies  2 . Upper ends of guide tubes  12  are sealed by a horizontal bottom grid plate  6  mounted across the bottom of shroud  26 . Multiple jet pumps  18 , one of which is shown schematically in  FIG. 1 , are mounted in downcomer annulus  4  circumferentially spaced about shroud  26 . 
       FIG. 2  shows an isometric view of a jet pump assembly  40 . Jet pump assembly  40  includes two jet pumps  18  that are coupled to a riser pipe  42  by a ram&#39;s head  22 . Water enters riser pipe  42 , passes through ram&#39;s head  22  and is then driven downward into a mixing chamber  30  by drive nozzles  20 . Mixing chamber  30  merges with a diffuser  32  at a slip joint  16 , with mixing chamber  30  being independently supported with respect to diffuser  32  so that mixing chamber  30  is longitudinally slidable with respect to diffuser  32 . 
       FIG. 3  schematically shows an embodiment of a conventional slip joint  116 , in which the bottom of a mixing chamber  130  is positioned to be longitudinally slidable within the top of a diffuser  132 . The bottom of mixing chamber  130  includes a gap forming portion  138  defined by an outer diameter of mixing chamber  130  that runs parallel to an inner diameter diffuser  132  so that radial distance of an annular gap  134 , formed between mixing chamber  130  and diffuser  132  at slip joint  116 , has constant width along the length of annular gap  134 . At slip joint  116 , annular gap  134 , which is for example sized to be 0.008 inches (0.020 cm) wide and has a height hl of at least 1.0 inch (2.54 cm) to limit leakage, is formed between the parallel portions of an outer diameter of mixing chamber  130  and the inner diameter of diffuser  132  to allow mixing chamber  130  to slide within diffuser  132 . Mixing chamber  130  has an inner diameter IDm of approximately 6 to 8 inches (15.2 cm to 20.3 cm) and diffuser  132 , at slip joint  116 , has an inner diameter IDd of approximately 7 to 9 inches (17.8 cm to 22.9 cm), such that the thickness of portion  138  is approximately 0.5 inches (1.27 cm). Below gap forming portion  138 , mixing chamber  130  includes a lead-in portion  136  to allow for ease of inserting mixing chamber  130  into diffuser  132 . Lead-in portion  136  has a height h 2  of between 0.25 and 0.5 inches (0.64 cm to 1.27 cm) and converges over a width of lead-in portion  136  towards an inner diameter IDd of diffuser  132  to define a bottom of annular gap  134 . As water is forced downward through mixing chamber  130  into diffuser  132 , leakage occurs upward through slip joint  116  causing mixing chamber  130  to oscillate laterally, which causes mixing chamber  130  and diffuser  132  to disadvantageously vibrate and potentially impact each other. The change in the width of lead-in portion  136  is too large with respect to the change in height of lead-in portion  136  (i.e., the angle of slope of lead-in portion  136  vertically upward towards diffuser  132 , which is for example  15  degrees, is too large) for the leakage to be able to force mixing chamber radially inward and prevent or limit the vibrations between mixing chamber  130  and diffuser  132 . 
       FIG. 4  shows a slip joint  236  according to one embodiment of the present invention, in which the bottom of a mixing chamber  230  is slidably positioned within the top of a diffuser  232 . The bottom of mixing chamber  230  includes a continuously tapered portion  240  forming an annular gap  234  that decreases in size between a bottom and a top of slip joint  216  to stabilize slip joint  216  under flow conditions. As a result, slip joint  216  may converge from bottom to top along substantially the entire length of annular gap  234  so portions of annular gap  234  are wider than the conventional annular gap  134  shown in  FIG. 3 . Mixing chamber  230  has an inner diameter IDm of approximately 6 to 8 inches (15.2 cm to 20.3 cm) and diffuser  232 , at slip joint  216 , has an inner diameter IDd of approximately 7 to 9 inches (17.8 cm to 22.9 cm), such that the thickness of portion  240  is approximately 0.5 inches (1.27 cm) at a radially exterior portion  242 , or peak, of each continuously tapered portion  240 . At slip joint  216 , annular gap  234 , which is for example sized to be 0.008 inches (0.020 cm) wide at radially exterior portion  242  and has a height h 3  of for example of approximately at least 1.0 inch (2.54 cm), is formed between tapered portion  240  and inner diameter IDd of diffuser  232 . Below tapered portion  240 , mixing chamber  230  may include a lead-in portion  236  to allow for ease of inserting mixing chamber  230  into diffuser  232 . Lead-in portion  236  may for example have a height h 4  of between 0.15 and 0.4 inches (0.38 cm to 1.02 cm) and may converge over a width of lead-in portion  236  towards an inner diameter IDd of diffuser  232  at slip joint  216 . 
     Above radially exterior portion  240 , mixing chamber  230  converges inwardly toward diffuser  232 , such that radially exterior portion  240  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  240  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. 
       FIG. 5  shows a slip joint  316  according to another embodiment of the present invention, in which the bottom of a mixing chamber  330  is slidably positioned within the top of a diffuser  332 . The bottom of mixing chamber  330  includes a continuously tapered portion  340  forming an annular gap  334  that decreases in size from the top of a lead-in portion  336  to a radially exterior portion  342  of mixing chamber  330  to stabilize slip joint  316  under flow conditions. Tapered portion  340  is formed similar to taper portion  240 , converging approximately 1 to 5 degrees, most preferably 1 to 3 degrees, with the addition that tapered portion  340  is formed with a plurality of annular grooves  338  on the surface of tapered portion  340  so that tapered portion  340  includes a labyrinth-seal type feature. Grooves  338  may help further stabilize mixing chamber  330  by providing pockets in tapered portion  340  to receive additional force from water passing through annular gap  334 . 
       FIG. 6  shows a slip joint  416  according to one embodiment of the present invention, in which the bottom of a mixing chamber  430  is slidably positioned within the top of a diffuser  432 . The bottom of mixing chamber  430  includes a stepped portion  440  forming an annular gap  434  that decreases in size from the top of a lead-in portion  436  to a radially exterior portion  442  of mixing chamber  430  to stabilize slip joint  416  under flow conditions. Stepped portion  440  is formed similar to taper portion  240 , converging approximately 1 to 5 degrees, most preferably approximately 1 to 3 degrees. 
       FIG. 7  shows a slip joint  516  according to one embodiment of the present invention, in which the bottom of a mixing chamber  530  is slidably positioned within the top of a diffuser  532 . The bottom of mixing chamber  530  is formed with a constant outer diameter at an annular gap  534 . However, annular gap  534  decreases in size because diffuser  532  includes a continuously tapered portion  546  that increases in width from top to bottom by approximately 1 to 5 degrees, most preferably 1 to 3 degrees, which may allow a sufficient volume of water to enter annular gap  534  to push mixing chamber  530  radially away from diffuser  532 . Annular gap  534  advantageously may prevent or minimize vibrations between mixing chamber  530  and diffuser  532 . In other embodiments, both the mixing chamber  530  and diffuser  532  may be continuously tapered from top to bottom. Also, tapered portion  546  of diffuser  532  may include grooves similar to grooves  338  ( FIG. 5 ) so that tapered portion  546  includes a labyrinth-seal type feature. In a preferred embodiment, slip joint  516  only decreases in width between the bottom of slip joint  516  and the top of annular gap  534  and does not including any portion that increases in width. 
       FIG. 8  shows a graph illustrating the pressure profile in a slip joint, comparing a tapered annular gap converging at 1 degree in accordance with an embodiment of the present invention with an annular gap following a parallel path in accordance with a conventional slip joint. The graph plots pressure versus distance from the bottom of the annular gap for both the tapered annular gap and the parallel annular gap. As shown in  FIG. 8 , the tapered annular gap generates an increased pressure profile along the length of the slip joint than the parallel annular gap of the conventional slip joint. 
     The embodiments of the present invention described herein vary from conventional approaches in that, instead of attempting to reduce the amount of leakage through slip joint to reduce flow induced vibrations, the embodiments involve shaping slip joints  216 ,  316 ,  416 ,  516  to use the leakage itself to create force between the respective mixing chambers  230 ,  330 ,  430 ,  530  and respective diffusers  232 ,  332 ,  432 ,  532  to prevent or minimize vibrations. 
     In preferred embodiments, jet pumps  18  may be retrofitted to prevent or minimize vibrations. Retrofitting of jet pumps  18  may be achieved by retrofitting conventional mixing chamber  130  to form mixing chambers  230 ,  330 ,  430  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 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  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  and stepped portion  440  may be formed in respective mixing chambers  230 ,  330 ,  430  during fabrication of mixing chambers  230 ,  330 ,  430  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.