Abstract:
The present invention provides a top nozzle for use with PWR nuclear reactors and power plants, and in particular, VVER nuclear reactors. The top nozzle includes a plate portion having a peripheral portion; a hub portion spaced from the plate portion; a plurality of support portions extending from the plate portion to the hub portion; and at least one deflector portion extending inwardly from the peripheral portion at an acute angle with respect to the plate portion.

Description:
BACKGROUND 
       [0001]    Field 
         [0002]    The disclosed concept pertains generally to nuclear fuel assemblies and more particularly to top nozzles for pressurized water nuclear reactors. 
         [0003]    Background Information 
         [0004]    In nuclear reactors of the type designed in the former Soviet Union, the reactor core is comprised of a large number of elongated fuel assemblies, each having a plurality of fuel rods held in an organized hexagonal array by a plurality of grids spaced longitudinally along the fuel rods and secured to stainless steel control rod guide thimbles. The stainless steel control rod guide thimbles extend above and below the ends of the fuel rods and are attached to the top and bottom nozzles, respectively. The fuel assemblies are arranged in the reactor vessel with the bottom nozzles resting on a lower core plate. An upper core plate rests on the top nozzles. 
         [0005]    The top nozzles in the Soviet design are non-removably fixed to the stainless steel control rod guide thimbles of the fuel assembly. These complex nozzles perform several functions. First, they position the rod control cluster assembly (RCCA) relative to the guide tubes within the core so that the position of the RCCA relative to the upper core plate is fixed. The RCCA positions the control rods, which are inserted into the fuel assembly as a group or cluster. 
         [0006]    The Soviet nozzle also dampens the velocity of the control rods using springs to remove energy when the RCCA rods are dropped into the reactor core during an emergency shutdown, commonly known as a “scram.” The nozzle also supplies spring loads for supporting the internals. When the upper core plate is lowered onto the nozzles, it compresses the nozzle spring. In addition, the Soviet nozzle is designed to protect the control rods when the fuel assembly is removed from the reactor vessel. Under these conditions, the RCCA is at or below the top edge of the nozzle. Finally, the Soviet design of the top nozzle allows the fuel assembly to be handled when lifted out of the core by transferring the loads through the nozzle. 
         [0007]    In a VVER-1000 type Soviet designed reactor, the inventors have discovered problems with loading of fuel assemblies. More specifically, during refueling outages, the bottom nozzles of fuel assemblies being loaded into the reactor core make solid contact with the top nozzles of the fuel assemblies already loaded into the core. These interferences cause the fuel assemblies being loaded to be undesirably prevented from moving past the already loaded fuel assembly. This type of incident uses a significant amount of additional critical path time while loading the core. The result is extensions of outage time during fuel assembly loading that correspond to an increase in costs. 
         [0008]    Accordingly, a new top nozzle and pressurized water nuclear reactor including a top nozzle are desired that will assure that engagements between the top nozzle and the bottom nozzle of a fuel assembly being loaded into the reactor core do not result in an undesirable extension of time during a refueling outage. 
       SUMMARY 
       [0009]    These needs and others are met by the disclosed concept in which a number of deflector portions are included in a top nozzle of pressurized water nuclear reactor. 
         [0010]    In accordance with one aspect of the disclosed concept, a top nozzle for a pressurized water nuclear reactor is provided. The top nozzle comprises a plate portion having a peripheral portion; a hub portion spaced from the plate portion; a plurality of support portions extending from the plate portion to the hub portion; and at least one deflector portion extending inwardly from the peripheral portion at an acute angle with respect to the plate portion. 
         [0011]    As another aspect of the disclosed concept, a pressurized water nuclear reactor comprises a pressure vessel; a plurality of fuel assemblies housed by the pressure vessel, each fuel assembly comprising a bottom nozzle, and a top nozzle comprising a plate portion having a peripheral portion, a hub portion spaced from the plate portion, a plurality of support portions extending from the plate portion to the hub portion, and at least one deflector portion extending inwardly from the peripheral portion at an acute angle with respect to the plate portion; and a plurality of fuel rods disposed intermediate the top nozzle and the bottom nozzle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    A full understanding of the disclosed concept can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0013]      FIG. 1  is a vertical elevation view of a fuel assembly for use with a pressurized water nuclear reactor, in accordance with the disclosed concept; 
           [0014]      FIG. 2  is a simplified view of portions of the fuel assembly of  FIG. 1 , shown during loading; 
           [0015]      FIG. 3  is a front isometric view of a top nozzle for the fuel assembly of  FIG. 1 ; 
           [0016]      FIG. 4  is top plan view of the top nozzle of  FIG. 1 ; 
           [0017]      FIG. 5  is a section view of a portion of the top nozzle of  FIG. 4 , taken along line A-A of  FIG. 4 ; 
           [0018]      FIG. 6  is a front elevation view of a portion of the top nozzle of  FIG. 1 ; and 
           [0019]      FIG. 7  is a front elevation view of another portion of the top nozzle of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality). 
         [0021]    As employed herein, the statement that two or more parts are “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts. 
         [0022]    As employed herein, the statement that two or more parts or components “engage” one another shall mean that the parts touch and/or exert a force against one another either directly or through one or more intermediate parts or components. 
         [0023]      FIG. 1  shows a fuel assembly  2  for use with a pressurized water nuclear reactor. The fuel assembly  2  is surrounded by a pressure vessel  200  (shown in simplified form in  FIG. 1 ), and includes a bottom nozzle  4 , a plurality of fuel rods  8 , and a top nozzle assembly  9 . The top nozzle assembly  9  includes an upper barrel assembly  10  and a top nozzle  20 . The fuel rods  8  are located between the bottom nozzle  4  and the top nozzle  20 . The upper barrel assembly  10  includes a plate portion  12 , and in operation, an upper core plate (not shown) of the nuclear reactor presses down on the upper barrel assembly  10  and causes the plate portion  12  to compress a number of hold down springs (not shown) located internal the upper barrel assembly  10 . This allows the fuel rods  8  to be held in place in the fuel assembly  2 . 
         [0024]      FIG. 2  shows a simplified view of a possible configuration of the bottom nozzle  4  and the top nozzle assembly  9  at an instant in time when the bottom nozzle  4  is being loaded into the fuel assembly  2  ( FIG. 1 ) during a refueling outage. The bottom nozzle  4  has a lower portion  6  that is structured to move past (i.e., down with respect to the orientation of  FIG. 2 ) the top nozzle  20  even in situations where the lower portion  6  engages the top nozzle  20 . This is distinct from many known nuclear reactors (not shown) in which interferences, or engagements, between bottom nozzles of fuel assemblies being loaded into the core and top nozzles of fuel assemblies already loaded in the core require additional extensions of outage time to allow the bottom nozzle to move past. As a result, undesirable additional extensions of outage time during refueling outages are avoided, thereby saving time/labor and reducing costs. Additionally, although  FIG. 2  shows the lower portion  6  being partially located above a portion of the top nozzle  20  during the illustrated instance in time, it will be appreciated that loading of the bottom nozzle  4  into the fuel assembly  2  ( FIG. 1 ) often results in the bottom nozzle  4  not overlaying the top nozzle  20  during passing, a situation in which there would be no engagement at all between the bottom nozzle  4  and the top nozzle  20 . 
         [0025]    Referring to  FIGS. 3 and 4 , the top nozzle  20  includes a hexagonal-shaped plate portion  22 , a hub portion  24  spaced from the plate portion  22 , and a plurality of support portions  26 , 28 , 30 , 32 , 34 , 36  extending from the plate portion  22  to the hub portion  24 . The plate portion  22  has a peripheral portion  38  and the top nozzle  20  further includes a number of deflector portions (six deflector portions  100 , 110 , 120 , 130 , 140 , 150  are shown) that each extend radially inwardly from the peripheral portion  38  at an acute angle with respect to an interior of the plate portion  22 . During loading of the bottom nozzle  4  ( FIGS. 1 and 2 ) into the fuel assembly  2  ( FIG. 1 ), a potential engagement between the lower portion  6  and the top nozzle  20  would result in the lower portion  6  engaging and exerting a force on one of the deflector portions  100 ,  110 , 120 , 130 , 140 , 150 . The corresponding opposing normal force on the lower portion  6  by the respective deflector portion  100 , 110 , 120 ,  130 , 140 , 150  would cause the bottom nozzle  4  to be deflected left, with respect to the orientation of  FIG. 2 . Because the peripheral portion  38  is the outermost circumferential portion of the plate portion  38 , deflection of the bottom nozzle  4  by the deflector portions  100 , 110 , 120 , 130 ,  140 , 150  would allow the bottom nozzle  4  to continue to be loaded during the refueling outage without requiring an undesirable extension of outage time. In the example of  FIG. 2 , it will be appreciated that the deflector portion  140  (shown in  FIGS. 3 and 4 , but hidden in  FIG. 2  by the support portion  26 ) would engage the lower portion  6  during loading of the fuel assembly  2  ( FIG. 1 ) and cause the bottom nozzle  4  to be deflected left. 
         [0026]    The peripheral portion  38  has six outer edge portions  40 , 42 , 44 , 46 , 48 , 50  that each have a respective midpoint  41 , 43 , 45 , 47 , 49 , 51 . Each of the deflector portions  100 , 110 , 120 , 130 ,  140 , 150  extends radially inwardly from a corresponding one of the midpoints  41 , 43 , 45 , 47 , 49 , 51 . The inventors have discovered that midpoints of edge portions of similar top nozzles that do not have deflector portions (not shown) are the locations where interference with bottom nozzles during refueling outages occurs most often. As such, by having the deflector portions  100 , 110 , 120 , 130 , 140 , 150  extend from the corresponding midpoints  41 , 43 , 45 , 47 , 49 , 51 , in the event of an engagement with the top nozzle  20 , the bottom nozzle  4  has a relatively large probability of engaging one of the deflector portions  100 , 110 , 120 , 130 , 140 , 150  and being deflected. 
         [0027]    Additionally, each of the deflector portions  100 , 110 , 120 , 130 , 140 , 150  is advantageously relatively small. More specifically, as shown in the top plan view of  FIG. 4 , each of the deflector portions  100 , 110 , 120 , 130 , 140 , 150  includes a distal portion  101 , 111 , 121 ,  131 , 141 , 151  located opposite and distal the peripheral portion  38 , and located between the peripheral portion  38  and the hub portion  24 . As seen, each deflector portion  100 , 110 , 120 , 130 ,  140 , 150  narrows from the peripheral portion  38  to the respective distal portion  101 , 111 , 121 , 131 ,  141 , 151 . In this manner, pressure drops associated with fluid flow past the deflector portions  100 , 110 , 120 , 130 , 140 , 150  during operation of the nuclear reactor are advantageously relatively small. Furthermore, the edge portion  38  further includes a plurality of corner portions  52 , 54 , 56 ,  58 , 60 , 62  from which each of the corresponding support portions  26 , 28 , 30 , 32 , 34 , 36  extends. The edge portions  40 , 42 , 44 , 46 , 48 , 50  extend between the corner portions  52 , 54 , 56 , 58 , 60 , 62 , and as shown, the deflector portions  100 , 110 , 120 , 130 , 140 , 150  are each spaced from the corner portions  52 , 54 , 56 , 58 , 60 , 62 . Thus, by being centrally located on each respective edge portion  40 , 42 , 44 ,  46 , 48 , 50  and spaced from each of the corner portions  52 , 54 , 56 , 58 , 60 , 62 , the relatively small deflector portions  100 , 110 , 120 , 130 , 140 , 150  are not structured to cause significant pressure drops to fluid flowing past the deflector portions  100 , 110 , 120 , 130 , 140 , 150  during operation of the nuclear reactor. It will, however, be appreciated that a similar suitable alternative top nozzle (not shown) could have deflector portions extending from one corner portion entirely to an adjacent corner portion, without departing from the scope of the disclosed concept. 
         [0028]      FIG. 5  shows a section view of a portion of the top nozzle  20 . As shown, the deflector portion  100  includes a base portion  102  and an extension portion  103 . The base portion  102  extends inwardly from the peripheral portion  38  at an acute angle  104  of between 55 degrees and 75 degrees with respect to the plate portion  22 . The extension portion  103  extends inwardly from the base portion  102  at an angle  105  of between 150 degrees and 170 degrees with respect to the base portion  102 . By employing the base portion  102  and the extension portion  103 , the deflector portion  100  is advantageously able to maintain structural integrity while also extending a relatively large distance over the interior of the plate portion  22 , thereby having greater deflection capabilities. More specifically, the extension portion  103  is advantageously able to “catch” the bottom nozzle  4  over a greater range of distances (i.e., distances inward from the peripheral portion  38 ), as compared to a similar suitable alternative deflector portion (not shown) of equal length as the deflector portion  100 , but with a constant inwardly extending angle. It will be appreciated that each of the other deflector portions  110 , 120 , 130 , 140 , 150  similarly includes a base portion and an extension portion that are angled with respect to each other and the plate portion  22  substantially the same as the base portion  102  and the extension portion  103 . However, it is within the scope of the disclosed concept for a top nozzle (not shown) to have deflector portions having relatively constant acute angles (i.e., with a single base portion and no extension portion), without departing from the scope of the disclosed concept. 
         [0029]    The top nozzle  20  further includes a plurality of other support portions (one support portion  160  is shown in  FIG. 6  and two support portions  170 , 172  are shown in  FIG. 7 ) that provide structural support to each of the respective deflector portions  100 , 110 , 120 , 130 ,  140 , 150  and resist fracture in the event of an engagement with the bottom nozzle  4 . The plate portion  22  includes a plurality of ribs  70  extending across the plate portion  22  internal the peripheral portion  38 , and each of the support portions  160 , 170 , 172  extends from one of the ribs  70  to one of the deflector portions  100 , 110 , 120 , 130 , 140 , 150 . The support portions  160 , 170 , 172  are each perpendicular to a respective one of the ribs  70  from which they extend in order to provide maximum support to the respective deflector portions  100 , 110 , 120 , 130 , 140 , 150 . In the exemplary embodiment, each of the deflector portions  100 , 110 , 120 , 130 , 140 , 150  generally has one of two configurations, depending on the position of the respective deflector portion  100 , 110 , 120 , 130 , 140 , 150  with respect to the ribs  70 . The two configurations are depicted in  FIGS. 6 and 7 , respectively. 
         [0030]    As shown in  FIG. 6 , the deflector portion  130  has a center line  136  coinciding with the support portion  160 . That is, the center line  136  and the support portion  160  are each located in a common plane such that the support portion  160  extends from a center of the deflector portion  130 . As shown in  FIG. 7 , the deflector portion  140  has first and second half portions  146 , 147  and a center line  148  separating the first and second half portions  146 , 147 . The support portions  170 , 172  extend from a respective one of the first and second half portions  146 , 147 . That is, the support portion  170  substantially extends from the first half portion  146  and does not extend from the second half portion  147 , while the support portion  172  substantially extends from the second half portion  147  and does not extend from the first half portion  146 . 
         [0031]    Referring again to  FIG. 4 , the deflector portion  120  is situated substantially the same as the opposing deflector portion  130 , with respect to the configuration of the ribs  70 . Accordingly, it will be appreciated that the top nozzle  20  has another single support portion (not shown) extending from the deflector portion  120  to one of the ribs  70 , and coinciding with a corresponding center line of the deflector portion  120 . Additionally, the deflector portion  150  is situated substantially the same as the opposing deflector portion  140 , and the opposing deflector portions  100 , 110  are likewise situated substantially the same as the deflector portion  140 , with respect to the orientation of the ribs  70 . Accordingly, it will be appreciated that the top nozzle  20  further has two additional support portions (partially shown, but not indicated, in  FIG. 3 ) for each of the deflector portions  100 , 110 , 150  that extend from respective first and second half portions of the deflector portions  100 , 110 , 150  to a respective one of the ribs  70 . 
         [0032]    In operation, by having the top nozzle  20  include the deflector portions  100 , 110 ,  120 , 130 , 140 , 150 , undesirable extensions of outage time during refueling outages as a result of engagements between the bottom nozzle  4  and the top nozzle  20  are significantly minimized and/or avoided. The deflector portions  100 , 110 , 120 , 130 , 140 , 150  ensure that the lower portion  6  of the bottom nozzle  4  is able to move past (i.e., down with respect to the orientation of  FIG. 2 ) the top nozzle  20  during loading of the fuel assembly  2  ( FIG. 1 ). More specifically, any potential engagement between the lower portion  6  and the top nozzle  20  during loading of the fuel assembly  2  ( FIG. 1 ) will result in the lower portion  6  engaging a respective one of the deflector portions  100 , 110 , 120 , 130 , 140 , 150 . Because of the acute angle (see the angle  104  in  FIG. 5 ) of the deflector portions  100 , 110 , 120 , 130 , 140 , 150  with respect to the interior of the plate portion  22 , an engagement, though undesired, would result in a corresponding opposing normal force being exerted on the bottom nozzle  4  to the left. As a result, the bottom nozzle  4  would be deflected (i.e., bumped or moved) left and would thus be able to continue to move down and past the top nozzle  20  and be loaded into the fuel assembly  2  ( FIG. 1 ). This will significantly minimize and/or eliminate additional extensions of outage time during loading of the fuel assembly  2  ( FIG. 1 ) associated with interferences between the bottom nozzle  4  and the top nozzle  20 , thereby saving time/labor and reducing costs in connection with refueling outages. Additionally, the top nozzle  20  is preferably made of a single piece of material and can be manufactured by any suitable process known in the art such as, for example, casting. As such, the deflector portions  100 , 110 , 120 , 130 , 140 , 150  can be cast with the top nozzle  20  without requiring any additional assembly. 
         [0033]    While specific embodiments of the disclosed concept have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof.