Abstract:
A control strategy for a pressurized water nuclear reactor that employs separate, independent control rod banks for respectively controlling T avg  and axial offset within corresponding deadbands. The strategy does not permit the control banks controlling reactor core power and the control banks controlling axial offset to move together, but normally gives preference to the control banks controlling the T avg  except when a demand signal is received simultaneously by both independent control rod banks to move in a same direction, in which case, the control bank compensating for the axial offset is given preference.

Description:
BACKGROUND 
     1. Field 
     This invention pertains generally to a method for operating a pressurized water nuclear reactor and more particularly to a method for automatically controlling the average coolant temperature and the axial power distribution of such a reactor. 
     2. Description of the Related Art 
     The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated and in heat exchange relationship with a secondary circuit for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material. The primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump, and the system of pipes which are connected to the vessel form a loop of the primary side. 
     For the purpose of illustration,  FIG. 1  shows a simplified nuclear reactor primary system, including a generally cylindrical pressure vessel  10  having a closure head  12  enclosing a nuclear core  14 . A liquid reactor coolant, such as water or borated water, is pumped into the vessel  10  by pump  16  through the core  14  where heat energy is absorbed and is discharged to a heat exchanger  18 , typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump  16 , completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel  10  by reactor coolant piping  20 . 
     An exemplary reactor design is shown in more detail in  FIG. 2 . In addition to the core  14  comprised of a plurality of parallel, vertical, co-extending fuel assemblies  22 , for purpose of this description, the other vessel internal structures can be divided into the lower internals  24  and the upper internals  26 . In conventional designs, the lower internals&#39; function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies  22  (only two of which are shown for simplicity in  FIG. 2 ), and support and guide instrumentation and components, such as control rods  28 . In the exemplary reactor shown in  FIG. 2 , coolant enters the reactor vessel through one or more inlet nozzles  30 , flows down through an annulus between the reactor vessel and the core barrel  32 , is turned 180° in a lower plenum  34 , passes upwardly through a lower support plate  37  and a lower core plate  36  upon which the fuel assemblies are seated and through and about the assemblies. In some designs, the lower support plate  37  and the lower core plate  36  are replaced by a single structure, a lower core support plate having the same elevation as  37 . The coolant flow through the core and surrounding areas  38  is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and friction of forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate  40 . Coolant exiting the core  14  flows along the underside of the upper core plate  40  and upwardly through a plurality of perforations  42 . The coolant then flows upwardly and radially to one or more coolant nozzles  44 . 
     The upper internals  26  can be supported from the vessel or the vessel head and include an upper support assembly  46 . Loads are transmitted between the upper support assembly  46  and the upper core plate, primarily by a plurality of support columns  48 . A support column is aligned above a selected fuel assembly  22  and perforations  42  in the upper core plate  40 . 
     Rectilinearly moveable control rods  28 , which typically include a drive shaft  50  and spider assembly  52  of neutron poison rods, are guided through the upper internals  26  and into aligned fuel assemblies  22  by control rod guide tubes  54 . The guide tubes are fixedly joined to the upper support assembly  46  and the top of the upper core plate  40 . The support column  48  arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability. 
       FIG. 3  is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character  22 . The fuel assembly  22  is of the type used in a pressured water reactor and has a structural skeleton which, at its lower end, includes a bottom nozzle  58 . The bottom nozzle  58  supports the fuel assembly  22  on the lower core plate  36  in the core region of the nuclear reactor. In addition to the bottom nozzle  58 , the structural skeleton of the fuel assembly  22  also includes a top nozzle  62  at its upper end and a number of guide tubes or thimbles  84  which align with the guide tubes  54  in the upper internals. The guide tubes or thimbles  84  extend longitudinally between the bottom and top nozzles  58  and  62  and at opposite ends are rigidly attached thereto. 
     The fuel assembly  22  further includes a plurality of transverse grids  64  axially spaced along and mounted to the guide thimbles  84  and an organized array of elongated fuel rods  66  transversely spaced and supported by the grids  64 . Also, the fuel assembly  22 , as shown in  FIG. 3 , has an instrumentation tube  68  located in the center thereof that extends between and is captured by the bottom and top nozzles  58  and  62 . With such an arrangement of parts, fuel assembly  22  forms an integral unit capable of being conveniently handled without damaging the assembly of parts. 
     As mentioned above, the fuel rods  66  in the array thereof in the assembly  22  are held in spaced relationship with one another by the grids  64  spaced along the fuel assembly length. Each fuel rod  66  includes a plurality of nuclear fuel pellets  70  and is closed at its opposite ends by upper and lower end plugs  72  and  74 . The pellets  70  are maintained in a stack by a plenum spring  76  disposed between the upper end plug  72  and the top of the pellet stack. The fuel pellets  70  composed of fissile material, are responsible for creating the reactive power of the reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission byproducts from entering the coolant and further contaminating the reactor system. 
     To control the fission process, a number of control rods  78  are reciprocally moveable in the guide thimbles  84  located at predetermined positions in the fuel assemblies  22 . A rod cluster control mechanism  80 , positioned above the top nozzle  62 , supports a plurality of the controls  78 . The control mechanism has an internally threaded cylindrical hub member  82  with a plurality of radially extending flukes or arms  52  that form the spider previously noted with regard to  FIG. 2 . Each arm  52  is interconnected to a control rod  78 , such that the control rod mechanism  80  is operable to move the control rods vertically in the guide thimbles  84  to thereby control the fission process in the fuel assembly  22  under the motive power of a control rod drive shaft  50  which is coupled to the control rod hub  80 , all in a well known manner. 
     The newer reactors, such as the AP1000 nuclear plant design offered by Westinghouse Electric Company LLC, Cranberry Township, Pennsylvania, employ two different types of control rods, i.e., the traditional control rods (black control rods) and gray control rods, the latter having a reduced reactivity worth, i.e., control rods that absorb fewer neutrons per unit area than the traditional control rods. The gray control rods are employed to implement a MSHIM operation and control strategy which has as an objective constant axial offset control. The term MSHIM is derived from the fact that reactivity control uses the gray control rod banks as a “mechanical shim” rather than the chemical shim, i.e., changes in soluble boron concentration, employed in a number of operating commercial reactors today, in order to provide fine reactivity control. the MSHIM strategy employs two independently controlled control rod groups to provide fine control of both the core reactivity and axial power distribution during a wide range of operational scenarios. 
     In the AP1000 reactor design, the MSHIM operation and control strategy is implemented by a digital rod control system that automatically controls the core reactivity (reactor coolant system temperature) using four banks of gray control rods and two banks of traditional control rods, all moving in a defined overlap. Furthermore, automatic axial power distribution (i.e., the axial offset, also known as the core axial flux difference) control is provided using a single, heavy bank of traditional control rods which move independently of the reactivity control banks. Changes in the concentration of the chemical shim within the reactor coolant is generally limited to only that required to directly compensate for fuel and/or burnable absorber depletion during a given fuel cycle. 
     The digital rod control system that is responsible for implementing the MSHIM operation and control strategy is basically characterized by the use of two separate rod controllers that independently maintain the reactor temperature and core power distribution, respectively within preselected bands. In order to achieve stable reactor control over the range of anticipated operating scenarios, the two rod controllers are interdependent in certain aspects. For instance, there is a prioritization scheme for the two rod controllers in scenarios where both controllers determine that rod motion is demanded. In such a case, the controller responsible for maintaining core power (average core temperature) in a specified band is given priority. However, it has been recognized by the inventors hereof that there are certain circumstances where core operation could be further improved by deviating from this strategy. 
     Accordingly, it is an object of the embodiments hereafter described to provide a new operating strategy that will further enhances core stability and fuel performance. 
     SUMMARY 
     These and other objects are achieved by the inventions hereafter claimed which provide for a method of operating a pressurized water reactor that has a core of a plurality of fuel assemblies and at least a first bank of control rods that are primarily moved into and out of selected fuel assemblies in the core to adjust the axial flux difference to substantially maintain or restore the axial flux difference within a target band. Furthermore, the pressurized water reactor has at least a second bank of control rods that are primarily moved into and out of other selected fuel assemblies in the core to adjust the average temperature of the core to substantially maintain or return the average temperature to within a second target band. The operation of the first bank of control rods and the second bank of control rods is such that the first bank of control rods and the second bank of control rods are not moved together. The method gives the second bank of control rods priority of movement when the first bank of control rods and the second bank of control rods receive a demand signal at the same time to move in different directions. Furthermore, the method gives the first bank of control rods priority of movement when the first bank of control rods and the second bank of control rods receive a demand signal at the same time to move in the same direction. In one embodiment, when the first bank of control rods is moving and the second bank of control rods gets a signal instructing the second bank of control rods to move in a different direction, the first bank of control rods will stop moving and the second bank of control rods will take over movement as it was instructed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a simplified schematic of a nuclear reactor system to which the embodiments described hereafter can be applied; 
         FIG. 2  is an elevational view, partially in section, of a nuclear reactor vessel and internal components to which the embodiments described hereafter can be applied; 
         FIG. 3  is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; 
         FIG. 4  is a core map showing the different control rod banks employed by the embodiments described hereafter; 
         FIGS. 5A ,  5 B,  5 C and  5 D are graphical representations showing the effects on power fraction, temperature, AFD and control rod movement of a step power decrease from 100% to 75% employing a standard AP1000 M and AO bank control strategy; 
         FIG. 6  is a graphical representation that illustrates the AFD variations as M banks are inserted into the core; 
         FIG. 7  is a diagrammatic representation of a logic flow chart showing an arrangement of logic gates that will implement the control strategy described herein; and 
         FIGS. 8A ,  8 B,  8 C and  8 D are graphical representations of core parameter changes corresponding to those shown in  FIGS. 5A ,  5 B,  5 C and  5 D, but resulting from the control strategy described herein. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     There are two aspects of reactor control in reactors that employ the AP1000 design. The M control banks (MA, MB, MC, MD, M 1  and M 2 ) automatically regulate the average reactor coolant temperature (T avg ) and the AO bank of rods automatically regulates the core axial flux difference (AFD). A core map which shows the location of each of the banks of control rods is shown in  FIG. 4  and Table 1 identifies the types of rods employed by each of the banks, the number of clusters within each bank and their function. 
     
       
         
               
               
               
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Bank ID 
                 Group Association 
                 Cluster Design Type 
                 # of Clusters 
               
               
                   
               
             
             
               
                 MA 
                 MSHIM Control 
                 Gray (GRCA) 
                 4 
               
               
                 MB 
                   
                   
                 4 
               
               
                 MC 
                   
                   
                 4 
               
               
                 MD 
                   
                   
                 4 
               
               
                 M1 
                   
                 Black (RCCA) 
                 4 
               
               
                 M2 
                   
                   
                 8 
               
               
                 AO 
                 Axial Offset Control 
                   
                 9 
               
               
                 S1 
                 Shutdown 
                   
                 8 
               
               
                 S2 
                   
                   
                 8 
               
               
                 S3 
                   
                   
                 8 
               
               
                 S4 
                   
                   
                 8 
               
             
          
           
               
                 Total 
                 69 
               
               
                   
               
             
          
         
       
     
     The T avg  controller moves the M banks into or out of the core during power maneuvers to regulate the coolant temperature and restore it to a +/−1.5° F. deadband around a programmed value which is a function of the turbine load. Similarly, the AFD controller regulates the axial core power distribution and restores it to a +/−1% deadband around a target value. An assumption in the AP1000 reactor design safety analysis requires the T avg  control to have precedence over AFD control. As a result, during a power maneuver, the M banks move first to regulate the T avg . As they move, they cause changes in AFD. When the coolant reaches its +/−1.5° F. control deadband, the M banks stop and the AO bank begins to regulate the AFD. The AO bank will move until the AFD is within its target deadband. The movement of the AO bank may cause the coolant temperature to exceed its control deadband. If this occurs, the AO bank will stop and the M banks will again move to correct the coolant temperature. When this is completed the AO bank will move again to resume the AFD correction. 
       FIG. 5 , which includes the graphs shown in  FIGS. 5A ,  5 B,  5 C and  5 D, shows the AFD, T avg , M and AO bank changes during a typical operation transient. Because the M banks have preference, the T avg  transient is well regulated. The AO bank correction near the end of the transient restores the AFD to within 1% of its target. In this example, the maximum deviation of the AFD from its control band is 8%. For more severe transients or under off normal conditions, the AFD deviation could be large enough to compromise peaking factors or pellet clad interaction limits (values as large as 20-30% have been seen in preliminary calculations). 
     A more detailed understanding of the MSHIM operation and control strategy can be found in a paper entitled ROBUSTNESS OF THE MSHIM OPERATION AND CONTROL STRATEGY IN THE AP1000 DESIGN (Paper No. ICONE17-75314) which was given at the Proceedings of the 17 th  International Conference on Nuclear Engineering, Jul. 12-16, 2009, Brussels, Belgium. 
     The inventors have recognized that allowing the AO bank to regulate the AFD during a plant operational transient, even where the average reactor coolant is outside its deadband, would reduce AFD deviations; but, at first glance, the T avg  control preference requirement imposed by the AP1000 safety analysis would seem to preclude that type of operation. However, close examination of the response characteristics of the M and AO banks does provide an opportunity for AFD correction during a major portion of any power change. Specifically:
         1. Moving either the AO or M banks more deeply in to the core will cause a reduction in T avg  and moving either of them further out of the core will cause T avg  to increase.   2. Moving the AO bank (within its allowed operating band) more deeply into the core will cause the AFD to become more negative and moving it further out of the core will cause the AFD to become more positive.
 
Accordingly, the underlying concepts provided for herein are:
   1. If the M banks are moving into the core to reduce the T avg  and the AO bank has a demand to make the AFD more negative, allowing the AO bank to move will both reduce the T avg  and correct the AFD.   2. Similarly, if the M banks are moving out of the core to increase the T avg  and the AO bank has a demand to make the AFD more positive allowing the AO bank to move will both increase the T avg  and correct the AFD.
 
Implementation of these concepts can be stated as follows: In the AP1000, when the AO and M banks both have a demand to move in the same direction (both in or both out of the core), disable the M banks and let the AO bank move. This will produce the correct movement of the T avg  and the AFD.
       

     The normal observation would be that allowing the AO and M banks to move together (since they both have a demand to move in the same direction) would improve regulation of both T avg  and AFD. This is true for T avg  control. Allowing both banks to move in the same direction would speed up the correction of T avg . However, the same is not true for AFD control. The reactivity worths and overlaps of the M banks in the AP1000 design are such that as the M banks move in one direction (in or out) they alternately cause the AFD to become more negative and more positive. This is shown in  FIG. 6 . Hence, allowing both the AO and M banks to move simultaneously to speed up T avg  control is likely to be detrimental to AFD control. In addition, the design and arrangement of the rod control power supplies may preclude simultaneous movement of the AO and M banks. 
     The fundamental principle underlying the inventions claimed hereafter is the fact that natural core feedbacks, i.e., changes in moderator temperature/density, generally result in consistent responses in T avg  and AFD. For instance, when core power is reduced, reactivity increases resulting in an increased T avg . Coincidentally, AFD also becomes more positive. Both would require rod insertion to compensate. The invention claimed hereafter takes advantage of the fact that the heavy control rods, i.e., the black control rods, used for AFD/A 0  control inherently have higher reactivity worth than the gray rods in the M banks normally used for T avg  control; meaning that the AO bank would compensate for both parameters under such conditions. In other words, the inherent, short-term core feedbacks are found to be naturally consistent and the method claimed hereafter leverages that consistency. This is not necessarily the case for the long-term core feedbacks, e.g., xenon transients, but the time spans are much longer for these affects, such that the “independence” of the two controllers is adequate to control for these long-term effects. 
     A control system logic arrangement that will implement the concepts claimed hereafter is shown in  FIG. 7 . The reactor control system and the AFD control system will generate a demand for M and AO bank movement based on the need for correction of the coolant average temperature (T avg ) or core axial power distribution. The demand to reduce T avg  will move the M banks in except when there is a demand to make the AFD more negative. In this case, the M bank demand will be ignored and the AO bank will move in to reduce the T avg  at the same time make the AFD more negative. The demand to increase T avg  will move the M banks out except when there is a demand to make the AFD more positive. In this case, the M bank demand will be ignored and the AO bank will move out to increase the T avg  and at the same time make the AFD more positive. The demand to make the AFD more negative will move the AO bank in only when there is a corresponding demand to move the M banks in or when the M bank demand is in its control deadband. Similarly, a demand to make the AFD positive will move the AO bank out only when there is a corresponding demand to move the M banks out or when the M bank demand is in its controlled deadband. When the AO bank reaches its deadband and stops movement, the M banks will take over movement if T avg  is not in its deadband. This logic, shown in  FIG. 7 , demonstrates how T avg  control is given preference over AFD control while allowing AFD control for the majority of the time during an operational transient.  FIGS. 8A ,  8 B,  8 C and  8 D show the effect of this control strategy on the same transient previously plotted in  FIG. 5  for the prior art control strategy. The improvement in AFD control, without compromising T avg  control and while giving T avg  preference, is significant. 
     While specific embodiments of the invention 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 embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.