Abstract:
Fuel supports have specially configured flow paths useable in reactor cores to achieve desired levels of flow at given positions. Any number of differently-configured inlet orifices, from three to hundreds, are useable in a given core. Inlet orifice configuration may include diameter sizing or presence of flow blockages such as filters, venturis, choke plates, and/or obstructions. Fuel supports may be positioned within a core plate in the nuclear reactor, with openings for a control blade and instrumentation tubes to pass through or between the fuel supports. Different fuel support configurations may be used at outer core periphery, inner core periphery, and central core portions. Example methods configure fuel support characteristics by examining the effect of modifying flow loss coefficients at particular bundle locations and configuring associated inlet orifices to achieve the modified flow loss coefficients, if the effect is a positive one.

Description:
BACKGROUND 
       [0001]    As shown in  FIG. 1 , a conventional nuclear reactor, such as a Boiling Water Reactor (BWR), may include a reactor pressure vessel (RPV)  12  with a generally cylindrical shape. RPV  12  may be closed at a lower end by a bottom head  28  and at a top end by a removable top head  29 . A cylindrically-shaped core shroud  34  may surround reactor core  36 , which includes several nuclear fuel elements or assemblies, called bundles herein, that generate power through fission. Shroud  34  may be supported at one end by a shroud support  38  and may include a removable shroud head  39  and separator tube assembly at the other end. One or more control blades  20  may extend upwards into core  36 , so as to control the fission chain reaction within fuel elements of core  36 . Additionally, one or more instrumentation tubes  50  may extend into reactor core  36  from outside RPV  12 , such as through bottom head  28 , permitting instrumentation, such as neutron monitors and thermocouples, to be inserted into and enclosed within the core  36  from an external position. 
         [0002]    Fuel bundles may be aligned and supported by fuel supports  48  located on a core plate  49  at the base of core  36 . Fuel supports  48  may receive individual fuel bundles or groups of bundles and permit coolant flow through the same. Fuel supports  48  may further permit instrumentation tubes  50 , control blades  20 , and/or other components to pass into core  36  through or between fuel supports  48 . A fluid, such as light or heavy water, is circulated up through core  36  and core plate  48 , and in a BWR, is at least partially converted to steam by the heat generated by fission in the fuel elements. The steam is separated and dried in separator tube assembly and steam dryer structures  15  and exits RPV  12  through a main steam nozzle  3  near a top of RPV  12 . Other fluid coolant/moderators may be used in other reactor designs, with or without phase change. 
         [0003]      FIGS. 2A and 2B  are detailed views of a related art fuel support  48  useable in the nuclear plant of  FIG. 1 , for example, that can receive and support up to four individual fuel bundles. As shown in  FIGS. 2A and 2B , fuel support  48  includes openings  90  shaped to receive a lower end of a fuel bundle so as to support and align fuel bundles seated in fuel support  48 . Openings  90  are open and permit coolant flow  80  through fuel support  48  into fuel bundles supported thereon. Openings  90  receive fluid flow through fuel support  48  from inlet or lower orifices  95 , permitting fluid coolant/moderator to flow through fuel support  48 . A cruciform or other opening  21  may permit a control blade  20  to pass between bundles supported by fuel support  48 . It is understood however, that control blades  20  may not be present in every possible core location, such that opening  21  may be unfilled or nonexistent. 
         [0004]      FIG. 3  is an illustration of a conventional core map, showing a quadrant of a conventional reactor core  36 . Each grid location in  FIG. 3  represents a fuel bundle location in the core  36 , with each fuel bundle seating into an associated opening in a fuel support. Each grid location in  FIG. 3  is identified with a number showing fuel support inlet orifice configuration in a conventional reactor core  36 . That is, orifices  95  ( FIGS. 2A &amp; 2B ) conventionally have two different sizes, or diameters, to achieve two different flow rates through the core. Grid locations marked with a “1” in core  36  of  FIG. 3  correspond to locations with orifices sized for central fuel bundles. Orifices for central fuel bundles at “1” locations are larger, permitting increased coolant and moderator flow through fuel supports and fuel bundles at the associated location. Orifices for peripheral fuel bundles at “2” locations are smaller, permitting less coolant and moderator flow into bundles at the periphery. In this way, as shown in  FIG. 3 , conventional cores  36  may have standard, larger fluid flow through central fuel bundles with a lower level of fluid flow occurring in the outermost ring or periphery of fuel bundles in the core. 
       SUMMARY 
       [0005]    Example embodiments are directed to fuel supports and reactor cores including the same. Example embodiment fuel supports include an inlet orifice that permits a coolant/moderator to flow through the support into an associated fuel bundle seated into the support, and the inlet orifice is specially designed to achieve a desired fluid flow characteristic, such as coolant/moderator flow rate through the associated fuel bundle. The desired fluid flow characteristic may be determined based on a position of a bundle associated with the inlet orifice within a core of the nuclear reactor. Any number of differently-configured inlet orifices, having different associated fluid flow characteristics, may be used throughout the core and in individual supports. Example embodiment fuel support configurations may include different inlet orifice diameters or use of flow blockages such as filters, venturis, choke plates, etc., to achieve a desired flow loss coefficient or flow rate under known conditions, for example. Example embodiment fuel support may be positioned within a core plate in the nuclear reactor, permitting coolant/moderator flow and potentially a control blade and instrumentation tubes to pass through or between the fuel supports. Several example embodiment fuel supports may be placed at the base of the reactor core, each support having physical configuration to achieve a desired flow characteristic at the associated fuel bundle position. For example, three different configurations may be used at outer core periphery, inner core periphery, and central portions of the core. The configuration at the outer periphery—those positions at the edge of the core and not surrounded by fuel bundles on each side—may have a highest flow loss coefficient so as to limit coolant/moderator flow to periphery bundles requiring less moderation and heat transfer. The configuration at an inner periphery, defined herein as the two or three bundle positions immediately inside the outer periphery, may have intermediate flow loss coefficients, and the configuration in the central portion may have the lowest flow loss coefficients, providing the highest levels of coolant/moderator flow to central bundles at higher power levels. 
         [0006]    Example methods configure flow path characteristics of fuel supports in a nuclear core. Example methods may include modifying flow loss coefficients at particular bundle locations, simulating core performance with the modified flow loss coefficients, analyzing the simulated core performance, and/or configuring at least one fuel support to achieve the modified flow loss coefficients. Analyzing may be performed by comparing simulated core performance against desired performance characteristics or comparing simulated core performance against a previously simulated core performance with different flow loss coefficients, in an iterative manner. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is an illustration of a conventional nuclear fuel reactor. 
           [0008]      FIGS. 2A and 2B  are two different views of a conventional fuel support. 
           [0009]      FIG. 3  is an illustration of a core map of a conventional reactor core. 
           [0010]      FIG. 4  is an illustration of an example embodiment fuel support. 
           [0011]      FIG. 5  is an illustration of a core map of an example embodiment reactor core. 
           [0012]      FIG. 6  is a graph of experimental results using example embodiment versus conventional fuel supports. 
           [0013]      FIG. 7  is a flow chart illustrating example methods. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Hereinafter, example embodiments will be described in detail with reference to the attached drawings. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. For example, although example embodiments and methods are described in connection with a Boiling Water Reactor (BWR), it is understood that example embodiments and methods are useable with several other reactor types, including PWRs, ESBWRs, heavy-water reactors, breeder reactors, etc. all using a fluid coolant and/or moderator. The example embodiments may be embodied in many alternate forms and should not be construed as limited to only example embodiments set forth herein. 
         [0015]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0016]    It will be understood that when an element is referred to as being “connected,” “coupled,” “mated,” “attached,” or “fixed” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
         [0017]    As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the language explicitly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0018]    It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures or described in the specification. For example, two figures or steps shown in succession may in fact be executed in parallel and concurrently or may sometimes be executed in the reverse order or repetitively, depending upon the functionality/acts involved. 
       Example Embodiments 
       [0019]      FIG. 4  is an illustration of example embodiment fuel support  148  that includes inlet orifices  195  with optimized fluid flow properties. Example embodiment fuel supports are shown as similar to, and may be used in place of, conventional fuel supports  48  shown in  FIGS. 2A and 2B  to provide alignment, support, and coolant/moderator flow to fuel bundles seated in the supports; for example, an optional control blade opening  121  and/or four fuel bundle openings  190  may be present in example embodiment fuel support  148  to match conventional fuel support characteristics. It is understood, however, that example embodiment fuel supports may have several different features and configurations, including physical shape, opening number, control blade accommodation, etc., from conventional fuel supports  48  ( FIGS. 2A &amp; 2B ) and example embodiment fuel support  148  shown in  FIG. 4 . Example embodiment fuel supports  148  include one or more inlet orifices  195  that are specially sized or configured to optimize fluid flow rates through associated fuel bundles, based on position within a nuclear core. Individual inlet orifices  195   a, b  may each be uniquely configured and differ from each other, or may be substantially the same. Inlet orifices  195  of different example embodiment fuel supports  148  within a core may be configured the same as some other inlet orifices in other fuel supports or be entirely unique, all based on the desired flow characteristics at the position of the inlet orifices  195 . That is, although example embodiment fuel support  148  is shown with two inlet orifices  195   a  and  195   b  having different respective diameters da and db, it is understood that all inlet orifices  195  in example embodiment fuel support  148  could have a same diameter and other aspects, while differing from other inlet orifices in other example embodiment fuel supports positioned elsewhere in a core and not shown in  FIG. 4 . Example methods for determining orifice configuration of example embodiment fuel supports and associated fluid flow characteristics are discussed following example embodiments below. 
         [0020]    Inlet orifices  195  of example embodiment fuel supports  148  are physically configured to provide a desired flow level of fluid coolant/moderator through an associated bundle during plant operation. The configuration may be achieved in several ways. For example, diameter d a  of inlet orifice  195   a  may be set during fabrication of fuel support  148  to permit a desired level of coolant flow  180   a  therethrough. Similarly, diameter d a  may be achieved or adjusted following fabrication through machining or remolding, for example. Or, for example, diameter d b  of inlet orifice  195   b  may be achieved or adjusted through addition of an insert, such as an annular choke plate, that reduces diameter d b  and achieves a desired lower flow rate  180   b  therethrough. Additionally, inserts, baffles, filters and/or any other structure may be used in example embodiment fuel support  148 , on either side of inlet orifices  195 , to affect fluid flow loss coefficients of, and a resulting amount of fluid flow through, a given inlet orifice  195  to a desired level during plant operation. For example, a flow restrictor or blockage may be placed in a flow path prior to or in opening  190  to adjust an amount of coolant/moderator flowing into an associated inlet orifice  195  and ultimately through the fuel bundle seated into the associated opening  190 . 
         [0021]    The levels of fluid coolant/moderator flow permitted by various diameters and/or other configurations of inlet orifices  195  in example embodiment fuel supports  148  may be set at any desired level. Local flow loss coefficients caused by these configurations on a given fluid may provide a universal metric to compare individual inlet orifice  195  functionality in an operating nuclear plant. For a universal inlet pressure and fluid, a higher loss coefficient correlates with less fluid moderator/coolant flow through an orifice and associated fuel bundle, resulting in less moderation and fuel usage while directing more flow to other bundles. Higher loss coefficients may be achieved in example embodiment fuel supports  148  by decreasing inlet orifice  195  diameter and/or providing other flow-interrupting structures within inlet orifice  195  or fuel support  148 , as discussed above. Under the same universal inlet pressure and coolant/moderator fluid, a lower loss coefficient correlates with increased fluid moderator/coolant flow through an orifice and associated fuel bundle, resulting in greater moderation and fission energy generation while decreasing flow available to other bundles. Lower loss coefficients may be achieved in example embodiment fuel supports  148  by increasing inlet orifice  195  diameter and/or removing flow-interrupting structures in example embodiment fuel supports  148 . Several different types of orifice configurations may be used together on a same fuel support  148  or even on a same inlet orifice  195 , based on the flow characteristics desired of that orifice. 
         [0022]      FIG. 5  is an illustration of a core map showing how a quadrant of an example embodiment core  236  may be populated with example embodiment fuel supports  148  ( FIG. 4 ). Each grid position in  FIG. 5  corresponds to a fuel bundle location and associated fuel support inlet orifice  195  ( FIG. 4 ) providing a coolant/moderator flow  180  into the bundle. As such, it is understood that example embodiment fuel supports  148  ( FIG. 4 ) may span one or more grid positions of  FIG. 5 , depending on the number of orifices and shape of example embodiment fuel supports. Although example embodiment core  236  is shown with 19×19 radial fuel bundles in a quadrant, it is understood that other numbers of fuel bundles and core shapes are useable with example embodiments and methods, including BWR, ESBWR, ABWR, PWR, non-LWR designs, and/or any other type of reactor design where fuel supports with orifices are useable. 
         [0023]    As shown in  FIG. 5 , each grid location is associated with a unique orifice configuration denoted by a numeral “1,” “2,” “3,” or “4” in the grid location. As shown in the legend of  FIG. 5 , inlet orifices at “1” locations have a central configuration, with the lowest loss coefficients, achieved with largest orifice diameters and/or fewest flow obstructions, for example. Inlet orifices at “2” locations have an outer peripheral configuration, with the highest loss coefficients and smallest orifice diameters/most flow obstructions. As a specific example, the loss coefficient for orifices in example embodiment fuel supports at “1” central locations may be approximately 20-25%, such as 21%, of the loss coefficient for orifices at “2” peripheral locations; i.e., greater flow losses and less flow occur at “2” positions. Orifices at “3” and “4” inner periphery positions, defined herein as the two or three bundle positions immediately inside the outer periphery, have intermediate loss coefficients, between those of orifices at “1” and “2” central and peripheral positions. For example, orifices at “3” positions may have approximately 77-83%, such as 80%, the loss coefficient of orifices at “2” peripheral positions, and orifices at “4” positions may have approximately 37-43%, such as 40%, the loss coefficient of orifices at “2” peripheral positions. These unique loss coefficients of different orifices in fuel supports may be achieved through the different configuring techniques discussed above for changing the flow parameters discussed above, including varying orifice diameter, adding/removing flow obstructions, etc. 
         [0024]    In this way, example embodiment core  236  includes example embodiment fuel supports having several types of orifices with intermediate variations of loss coefficients, from outer periphery orifices with the highest loss coefficients to inner central orifices with the lowest loss coefficients. The quadrant shown in  FIG. 5  may be mirrored about three axes to produce a full example embodiment core  236  that is symmetrical in orifice layout about these axes. 
         [0025]    Bundles at peripheral and intermediate positions “2,” “3,” and “4” in example embodiment core  236  may possess lower fuel enrichment (through age or initial enrichment) and suffer from increased neutron loss at core boundaries, resulting in lower fission energy production. Due to the lower power levels at peripheral and inner peripheral positions, less moderator/coolant flow may be required to maintain bundles at these positions at operating temperature and maximum power production. Example embodiment core  236  provides higher loss coefficients, and thus less flow, for fluid moderator/coolant through intermediate bundles with orifices at “3” and “4” locations, compared to conventional cores, such as the core shown in FIG.  3 , which provide full, central orifices for the same bundles at intermediate, inner periphery locations. In this way, example embodiment core  236  may direct more moderator/coolant to bundles at central locations “1,” while directing less moderator/coolant to bundles at intermediate peripheral locations “3” or “4,” with the same whole-core flow rates, compared to conventional cores such as those shown in  FIG. 3 . Bundles at central “1” locations may have higher enrichment and power rates compared to intermediate or peripheral locations “2,” “3,” or “4”. Bundles at central “1” locations may thus benefit from the increased neutron moderation and fluid energy transfer from the fluid coolant/moderator in example embodiment core  236 . 
         [0026]    Further, near end of operating cycles, bundles in a given core are more depleted in fissionable material, and bundles at peripheral and intermediate positions “2,” “3,” and “4” in example embodiment core  236  may possess especially low fuel enrichment due to age and lower initial enrichment. Operators in end of cycle conditions may increase total core flow so as to provide additional moderator to the depleted bundles, sustaining a fission chain reaction for several more days beyond what typical or rated core flow would be able to sustain. However, due to the lower power levels from low enrichment and neutron loss at peripheral and inner peripheral positions, the increased moderator/coolant flow in end of cycle conditions may be wasted on peripheral and inner peripheral positions and result in wet moderator with high moisture carryover to pass through the core through these positions. Example embodiment core  236  provides higher loss coefficients for increased core flow, and thus even less flow at end of cycle conditions using increased core flow, for fluid moderator/coolant through intermediate bundles with orifices at “3” and “4” locations, compared to conventional cores. In this way, example embodiment core  236  may further decrease moisture carry-over and increase steam quality and plant efficiency for plants operating with increased core flow to extend cycle life. 
         [0027]    Other example embodiment core configurations are achievable with example embodiment fuel supports and individualized orifices therein. For example, as shown in scenarios 1-3 below in  FIG. 6 , only a single type of intermediate orifice may be used at both “3” and “4” inner periphery positions of example embodiment core  236  in order to increase fuel support standardization or achieve other core flow characteristics. Or, for example, inlet orifices with higher loss coefficients may be used at controlled locations, which are bundle positions directly adjacent to a control blade that typically require less moderation and coolant. Using example embodiment fuel supports to restrict coolant/moderator flow at controlled positions may further decrease moisture carryover and/or provide additional flow to higher-energy fuel bundles to increase plant efficiency. Because of the flexibility offered by example embodiment fuel supports, almost any desired flow characteristics can be achieved with proper fuel bundle configuration, resulting in an example embodiment core configuration having desired coolant flow, and thus energy generating or safety-margin complying, properties. 
         [0028]    The inventors compared example embodiment fuel supports and cores, with more than two different inlet orifice and thus bundle flow characteristics, with conventional cores having only two, central and peripheral, inlet orifice flow characteristics.  FIG. 6  is a graph of the results showing the percent difference of fluid coolant/moderator flow through example embodiment fuel supports in central locations of example embodiment cores versus conventional fuel support orifices in central locations of conventional cores (bars).  FIG. 6  further shows the change in Minimum Critical Power Ratio (MCPR, a ratio between power levels producing critical boiling transition in a single fuel bundle versus operating power levels) value between the same scenarios (line and points). 
         [0029]    To generate the results of  FIG. 6 , five ESBWR cores having a same whole-core flow rate of 77 Mlb/hr and energy density of 54 kw/l for 100% rated power were simulated using a known PANACEA (TRACG04/PANAC11) core thermodynamic code. Each core contained  1132  bundles at associated positions, such as the layout of example embodiment core  236  shown in  FIG. 5 . The only parameter varied in the simulations was the fuel support configuration at specific locations to achieve different loss coefficients and flow rates at different core locations, as done with example embodiment cores. The exception is scenario 5, which used the same parameters of scenario 4, but with different operation cycle length and reduced reload fuel requirements in combination with an optimized fuel design. Table 1 summarizes the varied parameters of scenarios 1-5, shown in  FIG. 6 : 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                   
                 Intermediate (3, 4)-to-Peripheral 
                 Central (1)-to-Peripheral (2) 
               
               
                 Scenario 
                 (2) Loss Coefficient Ratio 
                 Loss Coefficient Ratio 
               
               
                   
               
             
             
               
                 Reference 
                 0.23 
                 0.23 
               
               
                 1 
                 0.40 
                 0.20 
               
               
                 2 
                 0.60 
                 0.19 
               
               
                 3 
                 0.80 
                 0.19 
               
               
                 4 
                 0.40 (4), 0.80 (3) 
                 0.19 
               
               
                  5* 
                 0.40 (4), 0.80 (3) 
                 0.19 
               
               
                   
               
             
          
         
       
     
         [0030]    The simulated channel flow and MCPR values for scenarios 1-5 were compared against the results of the simulated channel flow and MCPR values for the Reference Scenario, and the percentage change or value difference was graphed in  FIG. 6 . As shown in  FIG. 6 , each example embodiment core using more than two different types of orifices showed significant improvement in channel flow (at least 3% increase) and MCPR (at least 0.02 improvement). At current uranium costs, every 0.01 MCPR improvement in an operating commercial light water reactor translates to approximately $400,000.00 in reduced fuel costs. As such, example embodiment fuel supports with variable orifice characteristics and flow rates based on core position of the orifice and associated bundle may be used in example embodiment reactor cores to increase reactor efficiency. 
       Example Methods 
       [0031]    Example methods generate nuclear core configurations having customized fuel supports to achieve several different desired levels of coolant/moderator flow within the core. As shown in  FIG. 7 , in S 100  a known core configuration, including fuel characteristics, bundle location, core operating parameters, etc. is identified for optimization. For example, a program may receive input of several reactor core operational characteristics in S 100 . In S 110 , one or more loss coefficients are proposed or modified for one or more associated bundle positions in the known core configuration. For example, a user may input or alter bundle location flow loss coefficients, or a computer processor may iteratively cycle through all potential flow loss coefficients within an acceptable range, in S 110 . The resulting core with modified core flow coefficients for each bundle position is then simulated with a thermodynamic reactor modeling code in S 120 . The results, including variables such as central bundle flow rate and MCPR, are determined by the simulator and output for analysis or comparison in S 130 . For example, a user or computer program may determine if the resulting core operational parameters exceed a minimum performance threshold or compare the operational results against previous results from previous iterations with different flow loss coefficients in S 130 . Actions S 100 , S 110 , S 120 , and/or S 130  may then optionally be repeated for any number of iterations until an acceptable or best flow loss coefficient map is determined for a particular core. In S 140 , the accepted core flow loss coefficient map is achieved by identifying fuel support configurations that possess the accepted flow loss coefficients for each core position. Example embodiment fuel supports may then be fabricated or otherwise configured to achieve the identified flow loss coefficients for each core location in S 140 . 
         [0032]    Example methods including S 100 -S 140  may be executed for each bundle location within a core or only a subset of bundle locations of interest. Alternatively, example methods may be executed only with respect to a particular bundle in order to, for example, optimize core operating characteristics or fix a limiting problem with respect to the particular bundle location. Similarly, example methods may be used as an integral part of core design or as a separate step performed alternatively and/or iteratively with other known methods of core design. For example, a known core design program may output a core map using fuel bundle characteristics and core parameters using uniform orifice configuration and associated flow loss coefficients. Example methods including S 100 -S 140  may then be performed on some or all fuel bundle locations involved in the map, changing their operational characteristics including flow loss coefficient. The core design program may then be re-executed with the modified characteristics, and this core configuring involving example and other core optimization methods may continue until no further optimization is possible or desired. Or, example methods may be used as an integral part of otherwise known core design methods, treating flow loss coefficient parameters affected by orifice configuration as additional variables in the core design process. It is also recognized that one or more actions S 100 -S 140  may be executed by different programs or parties in the fuel services and licensee context. 
         [0033]    Example embodiments and methods thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied through routine experimentation and without further inventive activity. For example, it is readily appreciated upon reading the above disclosure that other core configurations and fuel support shapes and capacities from the specific example embodiments described may be achieved. Variations are not to be regarded as departure from the spirit and scope of the example embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.