Abstract:
In a nuclear reactor core, a lower tie plate assembly is provided with asymmetric features designed to control or vary a loss coefficient as a function of rotation of the associated fuel assembly. An associated method is provided to control the flow of coolant through the associated fuel assembly via rotation of the fuel assembly relative to the fuel support member. Control of the flow can be used to adjust assembly flow rate, assembly power and flow quality within the fuel assembly, among other assembly operational characteristics. Such flow control will impact the flow through other assemblies as well, since core flow remains generally fixed. On a core-wide basis, such flow control can be used to optimize core wide parameters. Optimization parameters of particular interest are the fuel cycle cost and moisture carryover.

Description:
TECHNICAL FIELD 
       [0001]    The technical field is generally nuclear reactors and, more specifically, systems and methods for selectively and variably controlling coolant flow through a fuel assembly 
       BACKGROUND 
       [0002]    A nuclear reactor core, and in particular a boiling water reactor, includes a plurality of individual fuel assemblies that have different characteristics that affect the strategy for operation of the core. The use of coolant in the core also affects the strategy for operation of the core. Coolant is introduced in the core to cool the core, to be transitioned into steam as a working fluid for energy generation, and to provide thermal neutron source aid in the nuclear reaction. 
         [0003]    Fuel support members support the lower end of the fuel assemblies and provide a flow path for coolant to enter the lower end of the fuel assemblies. For most fuel assemblies, the flow path proceeds vertically and upward as it approaches a fuel support member. The coolant then flows horizontally into a side entry inlet orifice for entry into the fuel support member, then vertically and upward again for the approach to the fuel assembly. The flow path in this final approach to the most of the fuel assemblies is not symmetric with respect to the fuel assembly flow path. In fact, only some peripheral fuel assemblies experience symmetric flow through the fuel support member and into the fuel assembly. The flow enters the fuel assembly through the lower tie plate, which supports fuel rods and is also shaped to facilitate seating of the fuel assembly into the fuel support member. 
         [0004]    One of the key design considerations for a fuel assembly is the minimum critical power ratio (MCPR), which is a limit selected to protect the fuel assembly from undergoing a boiling transition, which would expose the fuel assembly to excessive temperature. The MCPR is directly related to the flow quality, which is a positive linear function of the fuel assembly power divided by fuel assembly flow rate. High power fuel assemblies require more flow to maintain the same MCPR but, with current side entry inlet orifice layouts, generally receive less flow than the average fuel assembly. This is because the interior fuel support members have the same inlet orifice size, and thus the same loss coefficient. High power fuel assemblies produce more steam, which increases the fuel assembly pressure losses and reduces the flow relative to the average fuel assembly. Consequently, high power fuel assemblies produce especially high quality flow, while low power fuel assemblies produce especially low quality flow. 
         [0005]    The fuel support members include an inlet orifice to control coolant distribution between the fuel assemblies and to assist in thermal-hydraulic stability performance of the reactor core. Generally, the inlet orifices of fuel support members were designed at the time of reactor construction to have loss coefficients optimized for then-existing modes of core operation and fuel designs. There have since been changes in fuel designs and core operation, and the orifices are no longer optimized for current fuel assembly designs and/or core operation. Generally, there are two orifice loss coefficients: a high loss coefficient for the fuel assemblies around the periphery of the core and a lower one for all other fuel assemblies. As the fuel assemblies on the periphery of the core have significant neutron leakage, the power in these fuel assemblies is relatively low. The flow to these assemblies is reduced due to orificing, but not sufficiently reduced for current fuel assembly designs and core operation. 
         [0006]    The non-uniform exit quality distribution combined with lower values of average exit quality due to non-optimum orificing reduces the capability for reactor systems such as steam separators and dryers to operate effectively. This results in higher amounts of moisture carryover to the turbine, which causes increased erosion damage to turbine blades and steam cycle piping. Erosion damage is costly as it impacts operational life of the affected components and reduces efficiency of the plant&#39;s thermodynamic cycle. The moisture also carries small, irradiated particles that collect in various locations of the turbine system. These particles become the sources of higher radiation exposure in the balance of the plant. 
         [0007]    Additionally, non-optimum orificing incurs a higher fuel cycle cost. As noted above, high power fuel assemblies are significantly more limiting than other fuel assemblies in terms of MCPR. This limitation translates into non-optimal fuel assembly design and reactor operating conditions in order to comply with the MCPR limits. 
         [0008]    Previous attempts to address these problems involve changing the design of the fuel support member. However, the fuel support members are typically installed when the reactors are built and are not easily replaced. Moreover, design changes such as replacing members are relatively permanent, and provide a static adjustment to loss coefficient. As such, permanent modifications do not facilitate customized and adjustable core control strategy planning and implementation. Accordingly, there is a need for a dynamic solution to the problem of control of coolant flow through fuel assemblies. 
       SUMMARY 
       [0009]    The various embodiments provide systems and methods for varying the loss coefficient by reconfiguring the cross-sectional flow geometry of the flow into a fuel assembly, thereby selectively controlling coolant flow through a lower tie plate. The systems and methods facilitate optimization of loss coefficients for different fuel assembly designs and/or core operation strategies. As such, the systems and methods can optimize or customize flow and power output, provide more uniform exit quality distribution, provide lower average exit quality, improve MCPR performance by forcing flow to high power fuel assemblies, reduce both fuel cycle costs and reactor system costs, combinations thereof, and the like. 
         [0010]    Generally, the exemplary environment is in the context of a boiling water reactor (BWR) core control strategy that involves optimizing the flow of coolant through fuel assemblies, based on actual or desired characteristics of the fuel assemblies, such as uranium enrichment, gadolinium concentration, power level, exit quality, exposure level, presence in a control cell, and position in the overall arrangement of fuel assemblies in the reactor core. Such characteristics may constitute critical parameters for optimizing the core strategy by controlling the critical parameters to a predefined acceptable range. 
         [0011]    The teachings of the present invention can be applied in concert with conventional or yet to be developed control cell core (CCC) strategy wherein, for example, low reactivity fuel assemblies are placed adjacent to control blades that are inserted and distributed in a purposeful arrangement within the overall layout of the core. 
         [0012]    According to one aspect, any core strategy can be further optimized using the systems and methods described herein, by selectively varying the coolant flow through specific fuel assemblies, thereby achieving an overall core strategy that is customized to the actual parameters that are applicable to the fuel assemblies that are present in the reactor core. 
         [0013]    To achieve such optimization, in certain embodiments, a lower tie plate of a fuel assembly is configured to provide different loss coefficients upon rotation of the fuel assembly relative to a side entry inlet orifice formed in a fuel support member. More specifically, a rotatable damper is disposed between the inlet orifice and an inlet nozzle of the fuel assembly. In the open position, the primary flow area through the lower tie plate is essentially unobstructed by the damper, as its flow geometry is maintained substantially at status quo. By rotating the fuel assembly, the damper can be positioned to partially obstruct the primary flow path. By doing so, the flow is diverted along a more obstructed path, yielding a higher associated loss coefficient. 
         [0014]    The damper may be configured and positioned to coordinate with the existing geometry of the fuel support member so as to create sufficient variability to fine tune the performance of individual fuel assemblies. In certain of these embodiments, for example, the damper encloses approximately one third of typical inlet orifice area, and the fuel assembly can be rotated to four positions (unrotated, rotated 90 degrees, 180 degrees or 270 degrees), each yielding a different flow geometry that produces a different loss coefficient. It is contemplated that the shape and size of the damper can be as needed to achieve four unique loss coefficients. 
         [0015]    According to an exemplary method, the flow is optimized by rotating at least one fuel assembly in a reactor core layout relative to the side entry inlet orifice of the fuel support member that supports the fuel assembly, to adjust the flow geometry as coolant enters the lower tie plate of the fuel assembly. 
         [0016]    The foregoing has broadly outlined some of the aspects and features of the various embodiments, which should be construed to be merely illustrative of various potential applications. Other beneficial results can be obtained by applying the disclosed information in a different manner or by combining various aspects of the disclosed embodiments. Other aspects and a more comprehensive understanding may be obtained by referring to the detailed description of the exemplary embodiments taken in conjunction with the accompanying drawings, in addition to the scope defined by the claims. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic cross-sectional view illustrating the structure of a reactor pressure vessel in accordance with an exemplary environment of the invention. 
           [0018]      FIGS. 2 and 3  are partial cross-sectional elevation views illustrating a fuel assembly and a fuel support member of the reactor of  FIG. 1 . 
           [0019]      FIG. 4  is a partial exploded perspective view of the fuel assembly and fuel support member of  FIGS. 2 and 3 . 
           [0020]      FIG. 5  is a schematic plan view of the fuel assembly and fuel support member of  FIGS. 2 and 3 . 
           [0021]      FIG. 6  is a schematic plan view of an arrangement, in a reactor pressure vessel, of fuel assemblies and fuel support members that lack variable orificing in accordance with the prior art. 
           [0022]      FIG. 7  is a schematic plan view of an arrangement of fuel assemblies and fuel support members of the reactor pressure vessel of  FIG. 1 , incorporating exemplary dampers for achieving variable orificing by rotating the fuel assemblies in accordance with one embodiment of the invention. 
           [0023]      FIGS. 8 and 9  are partial cross-sectional elevation views of configurations of the fuel assembly and the fuel support of  FIGS. 2 and 3 , incorporating an embodiment of the invention. 
           [0024]      FIG. 10  is a graphical illustration of effect of damper position on loss coefficients associated with configurations of  FIG. 7 . 
           [0025]      FIG. 11  shows a schematic plan view of another arrangement of fuel assemblies and fuel support members of the reactor pressure vessel of  FIG. 1 , incorporating exemplary dampers for achieving variable orificing by rotating the dampers without rotating the fuel assemblies, according to an alternative embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    As required, detailed embodiments are disclosed herein. It must be understood that the disclosed embodiments are merely exemplary of and may be embodied in various and alternative forms, and combinations thereof. As used herein, the word “exemplary” is used expansively to refer to embodiments that serve as illustrations, specimens, models, or patterns. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. In other instances, well-known components, systems, materials, or methods that are known to those having ordinary skill in the art have not been described in detail in order to avoid obscuring the present disclosure. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art. 
         [0027]    Generally described, the disclosure teaches systems and methods for controlling loss coefficients related to a fuel support member and a fuel assembly. The loss coefficient is controlled as a function of a rotation angle of the fuel assembly relative to the fuel support member (for example, relative to the entry orifice of the fuel support member). The systems and methods can be used to control loss coefficients throughout the boiling water reactor to optimize fuel cycle cost, flow distribution, flow rates, fuel assembly power, fuel assembly axial power distribution, plutonium utilization, exit quality distribution, average exit quality, minimum critical power ratio performance, reactor system costs, combinations thereof, and the like. Although, the embodiments are generally described herein with respect to loss coefficients, it should be understood that such systems and methods can similarly be applied to control flow rate, pressure loss, minimum critical power ratio, and the like. 
         [0028]    Referring to  FIG. 1 , the general structure of a boiling water reactor (BWR)  10  is illustrated. The BWR  10  includes a reactor pressure vessel  20 , a steam dryer  22 , a steam separator  24 , a top guide  26 , a core shroud  28 , a core support plate  30 , fuel assemblies  32 , control rods  34 , fuel support members  36 , control rod guide tubes  38 , a lower plenum  40 , reinforcing beams  42 , recirculation pumps  44 , and main steam lines  46 . 
         [0029]    Pressure is generated in the lower plenum  40  by the recirculation pumps  44  such that coolant (e.g., water) flows from the lower plenum  40  through the fuel support members  36  into the fuel assemblies  32 . In the fuel assemblies  32 , the coolant is heated to produce a two-phase flow including vapor and liquid components. The vapor and liquid components are separated by reactor systems including steam separators  24  and the steam dryer  22 . For example, liquid is separated from vapor by the steam separator  24 , with the liquid returned to an annulus (downcomer)  48  and then to the recirculation pumps  44 , and the vapor (with a small amount of residual liquid) directed into the steam dryer  22 . The remaining liquid is separated from the vapor by the steam dryer  22 , again with the liquid returned to the downcomer  48 , and the vapor directed into a turbine (not shown) through main steam lines  46 . At this point the steam contains very little liquid, on the order of 0.1% by weight. 
         [0030]    The upper ends of the fuel assemblies  32  are supported by the top guide  26  and the lower ends of the fuel assemblies  32  are supported by the fuel support members  36 . Referring to  FIGS. 2-4 , generally described, the fuel support member  36  is configured to support four fuel assemblies  32  in a lattice arrangement and to direct flow from the lower plenum  40  into the fuel assemblies  32 . The fuel support member  36  is inserted into the upper end of the control rod guide tube  38  so as to be positioned at the upper end of the control rod guide tube  38 . 
         [0031]      FIGS. 2-5  illustrate one of the fuel assemblies  32  and the associated fuel support member  36 , which are now described in further detail. Referring to  FIG. 2 , the fuel assembly  32  includes an assembly channel  50 , fuel rods  52 , fuel spacers  54 , an upper tie plate  56 , and a lower tie plate  58 . The assembly channel  50  has an elongated shape with a square cross-section and upper and lower open ends in which the upper tie plate  56  and lower tie plate  58  are received. The fuel rods  52  are arranged in parallel and contain fissionable material. The fuel spacers  54  support the fuel rods  52  at several positions along the length of the assembly channel  50 . The upper and lower tie plates  56 ,  58  secure upper and lower ends of the fuel rods  52  so that the coolant can pass therethrough. 
         [0032]    Continuing with  FIG. 2 , the lower tie plate  58  includes an inlet nozzle  60  that leads to an enlarged volume  62  within the lower tie plate  58 . The lower tie plate  58  further includes a rod supporting grid  64  that is located at an upper end of the enlarged volume  62  and houses ends of the fuel rods  52 . The rod supporting grid  64  directs the flow of coolant from the enlarged volume  62  into the fuel assemblies  32  between the fuel rods  52 . 
         [0033]    The lower tie plate  58  further includes a coupling that includes a bail  66 , which extends out from the inlet nozzle  60 . Generally, the bail  66  is used as a handle that is configured to facilitate directing the lower end of the fuel assembly  32  so as to be received by the fuel support members  36 . Here, the bail  66  includes three curved bars  68  spaced approximately 120 degrees apart and that converge at a point P. The pointed or conical shape of the bail  66  facilitates receiving and maintaining the bail  66  into a fuel support member  36 . Further, referring to  FIGS. 8 and 9 , the bail  66  is configured to modulate flow F. Referring to  FIGS. 3 and 5 , to modulate flow F, the bail  66  includes a damper  70  and a port  72  the port  72  essentially being defined as an opening formed in the bail  66 . The exemplary damper  70  is formed by closing in the space between two of the curved bars  68  that define the bail  66 . The port  72  is defined by the other two spaces between the curved bars  68 , so only about a third of the exemplary bail  66  is enclosed by the damper  70 , although other damper/port ratios are foreseeable, as discussed below. Referring momentarily to  FIGS. 5-9 , as will be described in further detail below, the damper  70  and port  72  are configured so as to be alternatively positioned as a function of a rotational position of the fuel assembly  32  relative to the fuel support members  36  (for example, relative to a lower side entry orifice  82  described in further detail below). The port  72  is offset or asymmetrical with respect to a longitudinal axis  74  of the fuel assembly. 
         [0034]    In alternative embodiments, the damper  70  spans two of the spaces between curved bars  68  and the port  72  is the other space between two of the curved bars  68 . In alternative embodiments the port and damper can be alternatively configured. For example, a bail can have different numbers of bars with different spaces between bars selected to define the port and the damper, or the bail may be a structure without bars. Further, a structure separate from the bail can define the port and the damper. 
         [0035]    Continuing with  FIGS. 2-4 , the fuel support member  36  includes four support channels  80  that are configured to direct flow F from the lower plenum  40  into the fuel assemblies  32 . The fuel support member  36  also includes a control rod insertion slot  86  that is centrally positioned between the support channels  80 . The control rod insertion slot  86  is configured to receive the control rod  34 , which is positioned in between the lattice arrangement of fuel assemblies  32 . 
         [0036]    Each support channel  80  leads from a lower side entry orifice  82  that is configured to be positioned in the lower plenum  40  to a lower tie plate (LTP) seating orifice  84  that is configured to receive, support, and interface with the bail  66  and inlet nozzle  60  of the fuel assembly  32 . The support channel  80  includes a substantially vertical portion  96  and a conical portion  92  adjacent the seating orifice  84 . The conical portion  92  tapers outwardly and has a bowl-shaped, concave geometry that corresponds to conical shape of the bail  66 . The conical portion  92  expands asymmetrically with respect to a center axis  94  of the substantially vertical portion  96  of the support channel  80  and includes an axially aligned area  98  and an offset area  100 , which in the exemplary embodiment is curved to approximately the same extent as the bail  66 . The aligned area  98  is substantially aligned with the vertical portion  96  and the offset area  100  is completely offset from the vertical portion  96 . An edge  102  represents a division between the aligned area  98  and the offset area  100 . A center axis  90  of the conical portion  92  is offset from the center axis  94  of the substantially vertical portion  96 . 
         [0037]    When the lower tie plate  58  is received in the support channel  80 , the bail  66  extends into the conical portion  92  of the support channel  80  through the seating orifice  84  and is adjacent to the wall of the support channel  80  in the offset area  100  of the conical portion  92 . The outer surface of the lower tie plate  58  seals against a lip of the seating orifice  84 . As such, the support channel  80 , the lower tie plate  58 , and the assembly channel  50  provide a substantially continuous flow channel. 
         [0038]    When the fuel assembly  32  is supported by the fuel support member  36 , the longitudinal axis  74  of the fuel assembly  32  is aligned with the center axis  90  of the conical portion  92 . The axes  74 ,  90  are offset from the center axis  94  of the substantially vertical portion  96  of the support channels  80 . 
         [0039]    Referring to  FIGS. 4 ,  5 , and  7 , each of the fuel assemblies  32  has a substantially square cross-section represented by four walls  110  of the assembly channel  50 . The fuel support member  36  and the control rod  34  position the fuel assemblies  32  adjacent one another in a lattice arrangement. Each fuel assembly  32  is oriented in one of four rotational positions θ 1 , θ 2 , θ 3 , θ 4  (various rotational positions shown in  FIG. 7 ) relative to the control rod  34 . Each rotational position places a different pair of walls  110  adjacent to the control rod  34 . Each rotational position also positions the port  72  differently with respect to the lower side entry inlet orifice  82  and provides different flow geometry  120 . Referring to  FIGS. 5 and 7 , for purposes of teaching, flow geometry  120  is represented by the overlap between the port  72  and the substantially vertical portion  96  of the support channel  80  (and the substantially aligned area  98  of the conical portion  92 ). The overlap area is shown with a stipple pattern to represent flow geometry  120 . Flow geometry  120  includes the geometry of the support channel  80 , the geometry of the bail  66 , and the position of the port  72 , among other things. 
         [0040]    Referring to  FIG. 7 , for purposes of teaching, rotational positions θ 1 , θ 2  θ 3 , θ 4  are described with respect to the structure of an associated support channel  80  (specifically, side entry inlet orifice  82  which is positioned opposite the intersection of the control rod  34 ). Each of the fuel assemblies  32  can have the same rotational position with respect to an associated side entry inlet orifice  82  of a support channel  80 . For example, center fuel assemblies  32  have a rotational position θ 1  such that the fuel assemblies  32  have the same flow geometry  120 . As such, flow F is directed through each port  72  in substantially the same way and each associated fuel assembly  32  and fuel support member  36  has substantially the same loss coefficient. 
         [0041]    Referring to  FIG. 7-10 , certain fuel assemblies  32  have different rotational positions θ 1 , θ 2 , θ 3 , θ 4  with respect to an associated side entry orifice  82  of a respective support channel  80 . Here, various ports  72  overlap to different degrees with the aligned area  98  and the offset area  100 , providing different flow geometries  120  that direct the flow F differently. For example, referring to  FIGS. 8 and 9 , according to one rotational position θ 1  a port  72  directs flow through the aligned area  98  and according to another rotational position θ 2 , θ 3 , θ 4  the port  72  alternatively diverts greater flow F through the offset area  100 . 
         [0042]    Referring to  FIG. 10 , a loss coefficient K through the fuel support member  36  and lower tie plate  58  is a function of the flow geometry  120  of the support channel  80 , the bail  66 , and the position of the port  72 , among other things. In general, the less obstructed and more linear the flow, the lower the loss coefficient K. The interaction of the flow geometry  120  of the support channel  80  and the structure of the bail  66  impacts the loss coefficient K. The loss coefficient K is a function of the rotational position of the fuel assembly  32  because rotation of the fuel assembly  32  changes the position of the damper  70  and the port  72 . Referring to  FIG. 10 , inlet loss coefficients K associated with four rotational positions θ 1 , θ 2 , θ 3 , θ 4  are illustrated. An inlet loss coefficient K can be determined according to the following relationship: 
         [0000]    
       
         
           
             K 
             ∝ 
             
               Δ 
               
                 Q 
                 2 
               
             
           
         
       
     
         [0000]    where Δ is the pressure loss due to local losses from the lower plenum  40  through the rod supporting grid  64  and Q is the flow rate entering the lower tie plate  58 . 
         [0043]    In general, the different positions of the port  72  result in different flow F paths through the fuel support member  36  and lower tie plate  58 . In the illustrated arrangement, a primary flow path Y is offset from the center axis  94  of the substantially vertical portion  96  and along the inside of the support channel  80 , opposite the side entry inlet orifice  82 . Flow geometry  120  that allows the flow Y to follow the primary flow path Y (e.g., port  72  aligned with the aligned area  98  in  FIG. 8 ) generally results in lower loss and flow geometry  120  than directing the flow Y along an alternate flow path (e.g., damper  70  aligned with the aligned area  98  in  FIG. 9 ) generally results in higher loss. 
         [0044]    For a rotational position where the port  72  is substantially aligned with the primary flow path (referred to as an “open” position), the flow F is relatively unobstructed and can have a loss coefficient that is comparable to a bail  66  without a closed portion  70  (see, e.g.,  FIG. 6  illustrating bails without closed portions and see  FIG. 7 , at center, illustrating bails in an open position). Here, the flow F can substantially follow the primary flow path with less loss and the port  72  is positioned in the aligned area  98 . For a rotational position where the damper  70  is at least partially aligned with the primary flow path Y, the flow F is obstructed by the damper  70  and diverted into the offset area  100 . Here, the flow F has a less direct path with greater loss coefficient. Accordingly, the combination of the selective rotational positioning of the bail  66  and the intentional misalignment of the bail  66  with vertical portion  96  of the support channel  80  adjustably restricts flow. 
         [0045]    Referring to  FIGS. 6 and 7 , an exemplary method of rotating fuel assemblies  32  to customize inlet loss coefficients K is described.  FIGS. 6 and 7  schematically illustrate an exaggerated cross-sectional view of a BWR  10  core (with an unrealistically small number of fuel assemblies  32 ) showing the support channels  80 , lower tie plates  58  of fuel assemblies  32 , and control rods  34 . The stippled areas represent the flow geometry  120  through the lower tie plate  58 . Relative bundle exit quality levels  130  are shown as a schematic (not to scale) on the right side of each fuel assembly  32 . The relationship between loss coefficient and both MCPR and exit quality is described in further detail below. 
         [0046]    Typically, there are two types of fuel assemblies  32  that are at very low power throughout the cycle: those that are on the periphery of the reactor pressure vessel  20  and those that are in a control cell in accordance with a known or yet to be developed BWR core design techniques, such as a control cell core (CCC) strategy. The periphery of the reactor pressure vessel  20  has significant neutron leakage, and as such, fuel assemblies  32  located here have lower power than those at the center of the reactor pressure vessel  20 . A control cell embodies a CCC operational strategy in which high exposure fuel assemblies  32  (those that have resided in the reactor for a long time) are placed adjacent to a control rod  34  that is in the inserted position for most of the cycle. By design, these fuel assemblies  32  have low power throughout the cycle. 
         [0047]    Additionally, fuel assembly  32  power has a strong dependence on exposure (defined as fuel assembly power integrated with residence time). High exposure fuel assemblies  32  generally have lower power than “fresh,” or low exposure, fuel assemblies  32  due to the reduction of the uranium-233 and uranium-235 concentrations present in the fuel assembly  32  from fission reactions. 
         [0048]    There are many advantages to using customized inlet loss coefficients K in the design and operation of boiling water reactor  10 . A first is an increase in the exit quality of low power fuel assemblies, with a corresponding decrease in the exit quality of high power fuel assemblies. A second is an increase in the MCPR for high power fuel assemblies with a corresponding decrease in the MCPR for low power fuel assemblies. These two goals are interrelated and complementary, that is, a decrease in exit quality will result in an increase in MCPR. 
         [0049]    There are other secondary effects to the implementation of customized inlet loss coefficients K. Some of the more important secondary effects may include the following. The flow quality in the fuel assembly impacts the neutronics of the fission reaction and thus axial power generation. As such, the nuclear operation of any fuel assembly  32  may be improved or optimized through flow quality customization via the inlet loss coefficient K. A change in flow Y can change the axial power distribution in the fuel assembly  32  as well as the total bundle power. A change in the flow Y can thus be used to impact fuel assembly  32  parameters of interest, such as linear heat generation rate, as well as core-wide parameters of interest, such as core-wide axial and radial power profiles. A change in flow Y can also be used to impact the neutron spectrum and thus the generation and fissioning of plutonium. 
         [0050]    As mentioned above, for purposes of comparison,  FIG. 6  illustrates a boiling water reactor  10  where the lower tie plates  58  are without flow control structure (e.g., dampers  70 ) such that the flow geometry  120  and flow path through the peripheral fuel bundles  32  is substantially the same as the flow geometry  120  and flow path through the central fuel bundles  32 . Here, the peripheral bundles  32  have very low exit qualities  130  while the central bundles  32  have very high exit qualities  130 . 
         [0051]      FIG. 7  illustrates a boiling water reactor  10  where lower tie plates  58  include dampers  70  and the fuel bundles  32  are configured to customize inlet loss coefficients K according to an exemplary method. Here, the peripheral bundles  32  are rotationally positioned θ 3  to have flow geometry  120  that is least aligned with the primary flow path Y and the central bundles  32  are rotationally positioned θ 1  to have flow geometry  120  that is most aligned with the primary flow path Y. As a result, the exit quality  130  of the boiling water reactor  10  is more evenly distributed. The peripheral bundles  32  have exit qualities  130  that are similar to those of the central bundles  32 . 
         [0052]    Rotating peripheral fuel bundles  32  such that the flow geometry  120  creates a flow path that is different from the primary flow path Y increases the inlet loss coefficient K of low power (e.g., peripheral) fuel assemblies  32 . Rotating central fuel bundles  32  such that the flow geometry  120  creates a flow path that is substantially that of the primary flow path Y and leaves the inlet loss coefficient K of high power (e.g., central) fuel assemblies  32  essentially unchanged. This results in throttled flow for low power fuel assemblies  32 , increased system pressure drop across all fuel assemblies  32 , and reduced core flow rate Q to some degree. The increase in the system pressure drop across the fuel assemblies will increase flow to all non-throttled (i.e., high power) fuel assemblies  32 . 
         [0053]    In certain embodiments, this method of rotating fuel assemblies  32  is applied once per fuel cycle. During the process of shuffling fuel assemblies  32  to different locations or loading new fuel assemblies, the rotational position A of each fuel assembly  32  is selected without the need to spend additional time to insert the fuel assembly  32 . As such, fuel assemblies  32  that are selected to be throttled (e.g., reduced flow) will generally be operating at a low power throughout the cycle. 
         [0054]    Although damper actuation has been described in the context of rotating the fuel assemblies, it is contemplated that damper actuation can occur by rotating the dampers independently of the fuel assemblies. Additionally, it is contemplated that different fuel assemblies may have dampers of varying sizes, shapes, orientations, and rotation positions. Either of these scenarios can yield even more custom core flow patterns, as shown in  FIG. 11 . 
         [0055]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.