Abstract:
A method and system of regulating an output voltage of a boiling water reactor nuclear reactor plant recirculation system motor generator are provided. The method includes sensing an alternator output voltage and transmitting an alternator output voltage signal to a voltage regulator circuit, sensing an alternator speed and transmitting an alternator speed signal to the voltage regulator circuit, comparing the alternator output voltage signal to the alternator speed signal with a volts per hertz divider network electrically coupled to the alternator output voltage sensing circuit and the alternator speed sensing device, adjusting a capacitive reactance of the voltage regulator with a lead compensation circuit electrically coupled in series with the volts per hertz divider network, and adjusting a current in a control winding of a saturable reactor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to nuclear reactors, and more particularly to systems and methods stabilizing voltage regulators in nuclear reactor recirculation systems.  
           [0002]    A reactor pressure vessel (RPV) of a boiling water reactor (BWR) has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.  
           [0003]    The core of the reactor includes an array of fuel bundles with square cross section. The fuel bundles are supported from below by a fuel support. Each fuel support supports a group of four fuel bundles. The heat generated in the core can be decreased by inserting control rods into the core, and the generated heat can be increased by retracting control rods from the core. In some known BWR&#39;s, the control rods have a cruciform cross section with blades that can be inserted between the fuel bundles of a group of four.  
           [0004]    Historically, reactors were designed to operate at a thermal power output higher than the licensed rated thermal power level. To meet regulatory licensing guidelines, reactors are operated at a maximum thermal power output less than the maximum thermal power output the reactor is capable of achieving. These original design bases include large conservative margins factored into the design. After years of operation, it has been found that nuclear reactors can be safely operated at thermal power output levels higher than originally licensed. It has also been determined that changes to operating parameters and/or equipment modifications will permit safe operation of a reactor at significantly higher maximum thermal power output (up to and above 120% of original licensed power).  
           [0005]    Reactor plant systems, such as, the reactor recirculation system are evaluated to ensure their capabilities can support the reactor plant&#39;s operation at the higher power output levels. Where appropriate, changes are made to such systems to improve their performance.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0006]    In one aspect, a method of regulating an output voltage of a boiling water reactor nuclear reactor plant recirculation system motor generator is provided. The method includes sensing an alternator output voltage and transmitting an alternator output voltage signal to a voltage regulator circuit, sensing an alternator speed and transmitting an alternator speed signal to the voltage regulator circuit, comparing the alternator output voltage signal to the alternator speed signal with a volts per hertz divider network electrically coupled to the alternator output voltage sensing circuit and the alternator speed sensing device, adjusting a capacitive reactance of the voltage regulator with a lead compensation circuit electrically coupled in series with the volts per hertz divider network, and adjusting a current in a control winding of a saturable reactor.  
           [0007]    In another aspect, a voltage regulator for a boiling water reactor nuclear reactor plant recirculation system is provided. The regulator includes a variable frequency alternator, an alternator output voltage sensing circuit electrically coupled to a control circuit, an alternator speed sensing device electrically coupled to the control circuit, a volts per hertz divider network electrically coupled to the alternator output voltage sensing circuit and the alternator speed sensing device, a lead compensation circuit electrically coupled in series with the volts per hertz divider network, and a saturable reactor including a control winding electrically coupled to the volts per hertz divider network output, and a secondary winding electrically coupled to said alternator exciter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a sectional view of a boiling water nuclear reactor pressure vessel.  
         [0009]    [0009]FIG. 2 is a block diagram of a reactor recirculation system motor-generator set voltage regulator.  
         [0010]    [0010]FIG. 3 is schematic diagram of a control circuit of a motor-generator (MG) voltage regulator.  
         [0011]    [0011]FIG. 4 is a schematic diagram of lead compensation circuit.  
         [0012]    [0012]FIG. 5 is a graph showing three traces of voltage regulator response. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    [0013]FIG. 1 is a sectional view, with parts cut away, of a boiling water nuclear reactor pressure vessel (RPV)  10 . RPV  10  has a generally cylindrical shape and is closed at one end by a bottom head  12  and at its other end by a removable top head  14 . A side wall  16  extends from bottom head  12  to top head  14 . Side wall  16  includes a top flange  18 . Top head  14  is attached to top flange  18 . A cylindrically shaped core shroud  20  surrounds a reactor core  22  and a bypass water region, called a reflector  21 . Core shroud  20  is supported at one end by a shroud support  24  and includes an opposed removable shroud head  26 . A downcomer region  28  is an annulus formed between core shroud  20  and side wall  16 . A pump deck  30 , which has a ring shape, extends between shroud support  24  and RPV side wall  16 . Pump deck  30  includes a plurality of circular openings  32 , with each opening housing a jet pump  34 . Jet pumps  34  are circumferentially distributed around core shroud  20 . An inlet riser pipe  36  is coupled to two jet pumps  34  by a transition assembly  38 . Each jet pump  34  includes an inlet mixer  40 , and a diffuser  42 . Inlet riser  36  and two connected jet pumps  34  form a jet pump assembly  44 .  
         [0014]    Heat is generated within core  22 , which includes a plurality of fuel bundles  46  of fissionable material. Water circulated up through core  22  is at least partially converted to steam. A plurality of steam separators  48  separate steam from water, which is recirculated. A plurality of steam dryers  50  remove residual water from the steam. The steam exits the RPV  10  through a steam outlet  52  near vessel top head  14 .  
         [0015]    The amount of heat generated in core  22  is regulated by inserting and withdrawing a plurality of control rods  54  of neutron absorbing material, for example, hafnium. To the extent that control rod  54  is inserted adjacent fuel bundle  46 , it absorbs neutrons that would otherwise be available to promote the chain reaction which generates heat in core  22 .  
         [0016]    Each control rod  54  couples through a control rod drive tube  56  with a control rod drive mechanism (CRDM)  58  to form a control rod apparatus  60 . CRDM  58  moves control rod  54  relative to a core support plate  64  and adjacent fuel bundles  46 . CRDM  58  extends through bottom head  12  and is enclosed in a control rod drive mechanism housing  66 . A control rod guide tube  56  extends vertically from the control rod drive mechanism housing  66  to core support plate  64 . Control rod guide tubes  56  restrict non-vertical motion of control rods  54  during control rod  54  insertion and withdrawal. Control rod guide tubes  56  can have any number of shapes, for example a cruciform shape, a cylindrical shape, a rectangular shape, a Y-shape, and any other suitable polygonal shape.  
         [0017]    [0017]FIG. 2 is a block diagram of a reactor recirculation system motor-generator set voltage regulator  100  in accordance with an exemplary embodiment of the present invention. A reactor recirculation pump  102  supplies motive power to reactor water in a reactor recirculation system (not shown). Pump  102  is mechanically coupled to a reactor recirculation pump motor  104 . Motor  104  receives variable frequency alternating current (AC) power from a reactor recirculation system motor-generator set (MG)  106 . During normal operation of the recirculation system, the frequency of the AC power varies between a range of  15  cycles per second (Hz) and 60 Hz. The frequency may be as low as 11 Hz during normal starting of MG  106 . MG  106  includes an induction drive motor  108  mechanically coupled to a hydraulic variable speed control  110 . Speed control  110  includes an input power couple  112 , an output power couple  114 , and a speed sensing device  116  mechanically coupled proximate a rotating member of speed control  110 , such that an electrical output of speed sensing device  116  is proportional to a rotational speed of output power couple  114 . In the exemplary embodiment, speed-sensing device  116  is a speed transducer. In another embodiment, device  116  is a tachometer-generator. Output couple  114  is mechanically coupled to a variable frequency alternator  118 , which supplies variable frequency (AC) power to reactor recirculation pump motor  104 . The rotational speed of alternator  118  fixes the frequency of the AC power supplied to motor  104 . The rotational speed of alternator  118  is controlled by speed control  110 . Speed control  110  is a hydraulic device which varies its output speed in response to control inputs from a recirculation system flow control circuit  120  by varying the hydraulic coupling of internal rotating members. An AC brushless exciter  122  is electrically coupled to MG  106  and supplies excitation to MG  106  thereby controlling an output voltage of alternator  118 .  
         [0018]    Alternator  118  output is electrically coupled to a primary winding of a transformer  124 . A secondary winding of transformer  124  is electrically coupled to a first side of a normally closed contact  126  of a relay  127 . An external 120-volt, 60 Hz power source  128  is electrically coupled to a primary winding of a transformer  130 . A secondary winding of transformer  130  is electrically coupled to a first side of a normally open contact  132  of relay  127 . A second side of contacts  126  and  132  are electrically connected to a power rectifier circuit  134 . Power rectifier circuit  134  includes a full wave rectifier circuit  136  and a firing circuit  138 . The output of firing circuit  138  is electrically connected to a field  140  of exciter  122 .  
         [0019]    Alternator  118  output is also electrically coupled to a primary winding of a three phase transformer  142 . A secondary winding of transformer  142  is electrically connected to a first input  144  of a control circuit  146 . User&#39;s power source  128  is also electrically coupled to a power supply  148 . Power supply  148  is electrically coupled to speed sensing device  116  and provides a bias voltage and to speed sensing device  116  which provides an input to control circuit  146 . Control circuit  146  is magnetically coupled to a negative and positive feedback circuit  154  through a saturable reactor (not shown). An input to negative and positive feedback circuit  154  is electrically connected to field  140  of exciter  122 .  
         [0020]    In operation, voltage regulator  100  controls the output voltage of MG  106  by controlling the excitation of alternator  118 . Input power to rectifier  136  is supplied from one of user&#39;s power source  128  through transformer  130  and alternator  118  output through transformer  124 . The selection of power supply is determined by the state of relay  127 . When relay  127  is in an energized state whereby a coil internal to relay  127  is receiving electrical power, contact  132  is closed and contact  126  is open. In this state rectifier  136  is receiving power from power supply  128 . This is the normal case during startup of the recirculation system. After the recirculation system has been started and MG  106  is running input power to rectifier  136  is switched to alternator  118  output by deenergizing relay  127  which reverses the positions of contacts  132  and  126  such that contact  126  is closed and contact  132  is open.  
         [0021]    Power from one of power supply  128  and alternator  118  output is applied to power rectifier circuit  134  of up to 240 Vac, supplying power to full wave rectifier  136  through an inductive filter. Firing circuit  138  regulates the rectified waveform from rectifier  136  to supply a voltage and current for exciter field  140 .  
         [0022]    A current from negative and positive feedback circuit  154  is supplied to a winding of a saturable reactor to change the system gain (positive feedback) and transient response (negative feedback). Circuit  154  has a base capacitance of 560 microfarads and a switch to add 560 microfarads more capacitance. Test points are included to connect an external potentiometer to dial-in the additional capacitance to minimize a transient. Additional stability of voltage regulator  100  is provided by a lead compensation circuit in control circuit  146 .  
         [0023]    Control circuit  146  compares speed sensing device output voltage at input  152  and the alternator  118  output voltage at input  144  to a volts/hertz adjustment potentiometer setting and provides a current to the control coil of a saturable reactor.  
         [0024]    [0024]FIG. 3 is schematic diagram of a control circuit  146  of voltage regulator  100 . Input  144  includes three phase lines  156 ,  158  and  160 . Phase lines  156 ,  158  and  160  are electrically coupled to nodes  162 ,  164  and  166  respectively of three phase full wave rectifier  167 . Node  162  is electrically coupled to an anode of a rectifier  168  and a cathode of a rectifier  170 . Node  164  is electrically coupled to an anode of a rectifier  172  and a cathode of a rectifier  174 . Node  166  is electrically coupled to an anode of a rectifier  176  and a cathode of a rectifier  178 . A cathode of each of rectifiers  168 ,  172  and  176  is electrically coupled to node  180 . An anode of each of rectifiers  104 ,  174 , and  178  is electrically coupled to node  182 . Node  180  is electrically coupled to a first lead of a resistor  184 . A second lead of resistor  184  is electrically coupled to node  186 . In one embodiment, resistor  184  is a 330 ohm resistor. Node  186  is a positive direct current voltage with respect to node  182 . In the exemplary embodiment, when the AC input voltage to rectifier  167  is for example  230  VAC, the potential difference of node  186  with respect to node  182  is about  260  VDC.  
         [0025]    Node  186  is electrically coupled to a lead compensation circuit input  188  and a first lead of a resistor  190 . In the exemplary embodiment, resistor  190  is a 680 ohm resistor. Input  188  is electrically coupled to a first end of lead compensation circuit  192 . A second end of circuit  192  is electrically coupled to output  194 . Output  194  and a second lead of resistor  190  are electrically coupled to node  196 . Node  196  is electrically coupled to a first lead  198  of a potentiometer  200 . Lead  198  is electrically coupled through a resistance  201  to a second lead  202  of potentiometer  200 . In the exemplary embodiment, the amount of resistance between lead  198  and lead  202  is 800 ohms when potentiometer  200  is in a shelf state, i.e. no leads connected to a circuit. In another embodiment, potentiometer  200  is rated for 50 watts. A third lead  204  of potentiometer  200  is electrically coupled to resistance  201  of potentiometer  200  in a variable manner through a wiper  206 , such that when wiper  206  is rotated in a first direction  208 , a value of resistance between lead  204  and lead  198  is substantially zero ohms and the value of resistance between lead  204  and lead  202  is substantially equal to the value of resistance between lead  198  and lead  202 , and when wiper  206  is rotated in a second direction  210 , a value of resistance between lead  204  and lead  202  is substantially zero ohms and the value of resistance between lead  204  and lead  198  is substantially equal to the value of resistance between lead  198  and lead  202 . In another embodiment, potentiometer  200  is a rotary make-before-break switch with a plurality of fixed resistors electrically coupled in series providing the resistance  201  between lead  198  and lead  202  and switch contacts that provide electrical coupling between lead  204  and resistance  201 . Lead  202  is electrically coupled to a first lead of resistor  212  and a second lead of resistor  212  is electrically coupled to node  182 . In one embodiment, resistor  212  is a one thousand ohm resistor.  
         [0026]    Lead  204  is electrically coupled to node  214 . Node  214  is further electrically coupled to a first lead of a resistor  216 . A second lead of resistor  216  is electrically coupled to a cathode of diode  218 . An anode of diode  218  is electrically coupled to node  182 .  
         [0027]    Node  214  is further electrically coupled to test point  220  and to a first lead of a resistor  222 . A second lead of resistor  222  is electrically coupled to a test point  224  and to a first lead of a control coil  226  of a saturable reactor  228 . A second lead of control coil  226  is electrically coupled to an anode of diode  230 . A cathode of diode  230  is electrically coupled to a first lead of a resistor  232 . A second lead of resistor  232  is electrically coupled to node  186 . The first lead of resistor  232  and the cathode of diode  230  are further electrically coupled to a first line  234  of input  152 . A second line  236  of input  152  is electrically coupled to node  182 . Saturable reactor  228  is magnetically coupled to a primary winding (not shown) in power rectifier circuit  134  and a secondary winding (not shown) in negative and positive feedback circuit  154 .  
         [0028]    In operation, control circuit  146  compares speed device  116  voltage at input  152  and alternator  118  outage voltage at input  144  to a setting at potentiometer  200  and develops a current output to control coil  226 . Magnetic flux created in the saturable reactor due to the combined effects of current flow in control coil  226 , primary winding (positive feedback) and secondary winding (negative feedback) controls the firing characteristics of firing circuit  138 . An increase in control coil  226  current reduces the degree of saturation of saturable reactor  228  and thus reduces the output of voltage regulator  100  to exciter  122 . Stability of the voltage regulator  100  system is governed by two feedback adjustments. Capacitance in feedback circuit  154  is adjusted by switching in up to a maximum of approximately 1120 microfarads. Additional stability is added using lead compensation circuit  192 .  
         [0029]    [0029]FIG. 4 is a schematic diagram of lead compensation circuit  192 . Input  188  is electrically coupled to turning point and a first lead of switch  240 . A second lead of switch  240  is electrically coupled to a tuning point  242 , a first lead of resistor  244  and a first lead of a contact  246  of time delay relay  248 . A coil of relay  248  is electrically coupled to the recirculation system control system (not shown). A second lead of contact  246  is electrically coupled to a node  250 . A second lead of resistor  244  is electrically coupled to a tuning point  252  and node  250 . Node  250  is further electrically coupled to a first lead of switch  254 , a first lead of resistor  256 , and a first lead of capacitor  258 . A second lead of resistor  256  is electrically coupled to a second lead of switch  248  and a first lead of capacitor  260 . A second lead of capacitor  258  and a second lead of capacitor  260  are electrically coupled to output  194 .  
         [0030]    In operation, lead compensation circuit  192  provides capacitive reactance to voltage regulator  100  to improve control stability of regulator  100 . During recirculation system operation, voltage regulator  100  and MG  106  have a stable operating range that limits the maximum MG speed and thus limits the maximum core flow achievable. Operation above such range results in an oscillation of MG  106  output voltage and current. Oscillations of too great a magnitude causes an overcurrent relay to trip, shutting down the recirculation system. Lead compensation circuit  192  is coupled in parallel with resistor  190  and circuit  192 . Resistor  190  and circuit  192  are further coupled in series with potentiometer  200 . During recirculation system operation, lead compensation circuit  192  can be offline meaning switch  240  is open and circuit  192  has no effect on the operation of regulator  100 . To bring circuit  192  online, switch  240  is closed in such a manner to not induce a transient into regulator  100  operation. An external variable resistor (not shown) is coupled to circuit  192  in parallel with switch  240 . The external variable resistor is configured such that maximum resistance is provided between a first lead and a second lead. In the exemplary embodiment the external variable resistor has a maximum resistance of fifty thousand ohms. The first lead of the external resistor is electrically coupled to point  238 . The second lead of the external resistor is electrically coupled to point  242  and switch  240  is closed. The resistance of the external resistance is sufficient to limit current through circuit  192  to a level in which circuit  192  is effectively not yet online. The resistance of the external resistor is slowly removed while observing a response of the recirculation system flow and MG  106  voltage and speed. When all resistance is removed from the external resistor, switch  240  is closed to activate lead compensation circuit  192  and the external resistance is removed from point  238  and point  242 .  
         [0031]    During startup of recirculation system, circuit  192  is offline for a time delay controlled by a setting of time delay relay  248 . Contact  246  is held open to force circuit  192  current through resistor  244 . In the exemplary embodiment, resistor  244  is a ten thousand ohm resistor. Resistor  244  limits current in circuit  192  sufficiently to limit its effect on the operation of regulator  100  during startup of the recirculation system. After the time delay setting of time delay relay  248  times out, contact  246  is closed, bypassing resistor  244  and allowing current to bypass resistor  244 . The stability of regulator  100  is further adjusted by a position of switch  254 . When switch  254  is open, current is forced through resistor  256  thereby limiting the effect of capacitor  260  on the lead compensation circuit. When switch  254  is closed, it bypasses resistor  256  allowing full current flow through capacitor  260  thereby increasing the lead compensation effect in regulator  100 . In the exemplary embodiment, resistor  256  is a 10 K ohm resistor, capacitor  258  is a  560  microfarad capacitor and capacitor  260  is a  640  microfarad capacitor.  
         [0032]    [0032]FIG. 5 is a graph showing three traces of voltage regulator  100  response with different values of negative feedback and lead compensation. A horizontal axis of each trace represents time from a time  286  which represents the beginning of a step change of 5 percent into control circuit  146  to a time  288  which represents a time after time  286  when an output is substantially stable. In the exemplary embodiment, time  286  is approximately 0 seconds and time  288  is approximately 12 seconds. A vertical axis  290  of the traces represents a magnitude of alternator  118  output voltage measured at the secondary winding of transformer  142 . Trace  262  shows a magnitude  292  of system response with  560  microfarads of negative feedback in regulator  100  circuit. Trace  272  shows a magnitude  294  of system response with  1120  microfarads of negative feedback in regulator  100  circuit Trace  282  shows a magnitude  296  of system response with a lead compensation capacitance of 1120 microfarads in addition to 1120 microfarads of negative feedback.  
         [0033]    The above-described lead compensation circuit is cost effective and highly reliable. The lead compensation circuit includes capacitive reactance that facilitates reducing recirculation system oscillations during operation greater than pre-uprate reactor core flow. The lead compensation circuit includes a plurality of capacitors that can be inserted and removed from service while the recirculation system is operating, facilitates operation and maintenance of the system. As a result, the lead compensation circuit facilitates reactor recirculation system operation and maintenance in a cost effective and reliable manner.  
         [0034]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.