Patent Number: 061987867
Section: description

DETAILED DESCRIPTION FIG. 1 is a schematic diagram of the basic parts of a power generating system 8. The system includes a BWR 10 which contains a reactor core 12. Water 14 is boiled using the thermal power of reactor core 12, passing through a water-steam phase 16 to become steam 18. Steam 18 flows through piping in a steam flow path 20 to a turbine flow control valve 22 which controls the amount of steam 18 entering steam turbine 24. Steam 18 is used to drive turbine 24 which in turn drives electric generator 26 creating electric power. Steam 18 flows to a condenser 28 where it is converted back to water 14. Water 14 is pumped by feedwater pump 30 through piping in a feedwater path 32 back to reactor 10. The above described system is generally referred to as a closed loop system. The equations below show the basic relationships between the generation of power in the reactor core Q, the steam flow rate .omega..sub.s, the feedwater flow rate .omega..sub.FW, the reactor system pressure P.sub.s upstream of turbine control valve 22, the pressure P.sub.cv, downstream of turbine control valve 22, and the pressure P.sub.c in condenser 28. Typically, the pressure in P.sub.c in condenser 28 is considered to be zero. Also, the main turbine control valve flow characteristic C.sub.v changes from a relatively small value to a large value as control valve 22 traverses from a nearly closed position to its wide open position. The flow coefficient of turbine 24 is expressed as C.sub.T which may be considered relatively constant for small changes in steam flow. Typically, the steam flow rate .omega..sub.s is equal to the feedwater flow rate .omega..sub.FW when there are no significant alternate sources of water into the reactor system nor any leakage from the reactor system. The following equations depict the basic steady state relationships were secondary variables, such as heat losses, pumping energy and leakage flows, are ignored. The basic equations for system pressure control by main turbine control valve modulation and core power modulation are developed below: The steam flow .omega..sub.s is a function of turbine control valve 22 position C.sub.v, and the pressure drop cross control valve 22 is the difference between the system pressure P.sub.s and the pressure down stream P.sub.cv of flow control valve 22, which can be expressed as: EQU .omega..sub.s =C.sub.v *P.sub.s -P.sub.cv +L Equation 1 The steam flow through turbine control valve 22 and turbine 24 are equal when there are no shunt flow paths between turbine control valve 22 and the turbine inlet. The steam flow .omega..sub.s is a function of the turbine flow coefficient C.sub.T, and the pressure difference between the pressure down stream P.sub.cv of turbine control valve 22 and condenser 28 pressure P.sub.c may be considered equal to zero relative to the system pressure P.sub.s. The expression is: EQU .omega..sub.s =C.sub.t *(P.sub.CV -0) Equation 2 Equations 1 and 2 can be combined to calculate the system pressure P.sub.s in terms of turbine control valve 22 flow coefficient C.sub.v, the turbine flow coefficient C.sub.T and the pressure down stream P.sub.cv of turbine control valve 22. ##EQU1## The thermal power Q from reactor core 12 is approximately proportional to steam flow .omega..sub.s. The proportionally constant K relates these two parameters. The equation for reactor core power is: EQU Q=K*.omega..sub.S Equation 4 Combining Equations 2 and 4, the thermal power Q out of core 12 can be expressed as: EQU Q+K*C.sub.T *P.sub.CV Equation 5 Solving for the pressure down stream of turbine flow control valve 22 the equation becomes: ##EQU2## Combining Equation 3 and Equation 6 the system pressure P.sub.s can be determined in terms of core 12 thermal power Q, the constant that relates core power to steam flow K, main turbine flow control valve coefficient C.sub.v, and the turbine flow coefficient C.sub.T : ##EQU3## For the variable which controls the system pressure P.sub.s for a conventional method of reactor pressure control by turbine flow control valve modulation, Equation 7 is rearranged to: ##EQU4## The terms ##EQU5## are relatively constant for constant reactor power. This equation shows that the reactor system pressure P.sub.s is proportional to the inverse of the square of the turbine control valve flow coefficient C.sub.v, which is linearly proportional to the position of turbine control valve 22 position as previously discussed. When turbine flow control valve 22 closes in response to a decrease in reactor system pressure, the steam flow decreases in response to this flow control valve position change, thus reducing the steam flow rate which causes the reactor system pressure to increase to the desired value and vice versa. For the variable which controls the system pressure P.sub.s for the method of reactor pressure control by modulation of the reactor power Q in accordance with the present invention, Equation 8 is rearranged to: ##EQU6## The terms ##EQU7## are relatively constant for constant steam flow. This equation shows that the reactor system pressure P.sub.s is proportional to the square of the core power Q for the term involving the control valve flow coefficient C.sub.v and linear with power for the term involving the turbine coefficient C.sub.T. The power Q is actually changed by changes in the control rod density in the reactor core or by changes in the flow through the reactor core. When the control rod density decreases or the flow through the reactor core increases in response to a decrease in reactor system pressure, the core power increases, which in turn, causes the reactor system pressure to increase back to the desired value and vice versa. FIG. 2 is a schematic flow diagram illustrating core thermal power modulation pressure control of power generating system 8 in accordance with one embodiment of the present invention. As described above, power generating system 8 includes BWR 10 that produces steam 18. Steam 18 flows from BWR 10 through steam path 20 to and through turbine control valve 22 to turbine 24 then to condenser 28 where steam 18 is converted to liquid water 14. Liquid water 14 then flows back to BWR 10 through feedwater flow path 32. Condenser water flow path 60, containing pump 58, connects condenser 28 with heat sink 62. Condenser water is pumped by pump 58 from condenser 28 to heat sink 62 and back to condenser 28 in closed loop flow path 60. Turbine 24 drives electric generator 21 generating electric power. Bypass valve 54 permits steam to flow directly from BWR 10 to condenser 28 bypassing turbine 24. A control rod drive 34 and control rod controller 36 change control rod density within core 12 of BWR 10 to vary or modulate the thermal output from core 12. Water recirculated through core 12 also is used to control thermal output. A recirculation pump 40 pumps water through piping in a recirculation flow path 42. Typically, recirculation pump 40 is a variable speed pump which provides for control and modulation of the recirculation water flow rate. A flow control valve 44 for controlling recirculation flow rate is also included in recirculation flow path 42. Recirculation controller 38 controls the speed of recirculation pump 40 and the operating open position of flow control valve 44. A pressure sensor 46 measures steam pressure in flow path 20. Operator control station 50 communicates with a pressure controller 48, a turbine valve controller 52 and a core thermal power controller 64. In turbine control valve modulation mode, system steam pressure is controlled by first measuring the steam pressure in steam path 20 with pressure sensor 46 which inputs the reading into pressure controller 48. A pressure setpoint is put into pressure controller 48 by the operator at operator control station 50. If the pressure is higher or lower than the setpoint pressure, a signal is sent to turbine valve controller 52 which in turn sends a signal to the main turbine control valve 22 to open or close. Opening turbine control valve 22 allows more steam into turbine 24 and thus lowers system pressure. Closing turbine control valve 22 creates higher pressure in the system. A boiling water reactor power generation plant may have more than one turbine control valve 22. Typically there are four turbine control valves 22 in the system which operate in either full arc mode where all valves move together, or partial arc mode where one or more valves modulate and the remaining valves stay in a full open position. Also, if a system safety pressure setpoint is exceeded, a signal is sent to bypass valve 54 to open to divert steam directly to condenser 28, bypassing turbine 24, and thereby lowering system pressure. Recirculation flow control 38 sends a signal to either variable speed recirculation pump 40 or to control valve 44 to control recirculation flow rate and thereby maintain a constant thermal output from core 12. Condenser 28 operates by condenser water removing thermal energy from steam 18 flowing from turbine 24 thereby converting steam 18 to water 14. The condenser water is pumped by a condenser pump 58 through piping in a closed loop flow path 60 from condenser 28 to a heat sink 62 and back to condenser 28. Heat sink 62 dissipates the thermal energy from the condenser water before it is recirculated to condenser 28. Typically, changeover from conventional control valve modulation pressure control mode, described above, to core thermal power modulation pressure control mode is effected by the plant operator at operator control station 50. However, changeover to core power modulation mode may be effected automatically when predetermined requirement parameters are satisfied. Steam pressure in steam flow path 20 is measured by pressure sensor 46 which sends an input to pressure controller 48 and core thermal power controller 64. Pressure controller 48 sends a signal to turbine valve controller 52 which in turn sends a signal to main turbine control valves 22 to open to a constant position. Control valves 22 are usually set to wide open, but may be set to any other constant setting. Control valves 22 are typically set to at least 75 percent of wide open. To moderate core thermal power, core thermal power controller may either control core power by moderating control rod density within the reactor or may moderate recirculation water flow rate through reactor core 12. To moderate control rod density, a signal is sent by core thermal power controller 64 to control rod drive controller 36. Control rod drive controller 36 then directs control rod drive 34 to either raise or lower the control rods thereby changing or modulating the control rod density in reactor core 12. The core thermal power is inversely proportional to control rod density. For example, as the control rod density increases thermal power decreases, and conversely as control rod density decreases, core thermal power increases. To moderate recirculation flow rate, core thermal power controller 64 sends a signal to recirculation flow controller 38. Controller 38 then causes variable speed pump 40 to change speed thus modulating recirculation flow rate. Alternatively, controller 38 sends a signal to recirculation control valve 44 to modulate the open position of valve 44, thus modulating the recirculation flow rate of water through reactor core 12. Modulating recirculating water flow rate modulates reactor core thermal power output. FIG. 3 is a schematic functional control block diagram illustrating core thermal power modulation pressure control of power generating system 8 in accordance with an exemplary embodiment of the present invention. FIG. 3 illustrates function blocks for a sensed system pressure 200, a steam line pressure sensor 202, a pressure setpoint adjustment 204, and a summer or compensator 206. These function blocks are typically included in a conventional pressure regulation function which provides a steam flow demand signal to the turbine control system 210. As known in the art, turbine control system 210 may typically include a valve position characterizer, a valve position controller, an electric signal to hydraulic flow converter, a hydraulic cylinder, flow control valves, a valve position sensor, and a hydraulic power unit. Turbine control system 210 also includes a turbine load limit setpoint 212 function block and an increase bypass valve close bias 214 function block. If the pressure increases over the turbine load limit setpoint 212, and the increase is over the bypass close bias, the bypass valves will open routing steam directly to the condenser. FIG. 3 also illustrates power regulator 220, power control fault logic 230, power control on 232, neutron flux 234 and power control bias 240 function blocks. Function blocks for recirculation pump variable speed control system 250, recirculation flow control valve position control system 254, and control rod position control system 260 are also illustrated. In operation, steam pressure in pipe 200 is measured by pressure sensor 202 which sends a signal to summer or compensator 206 which compares the pressure to pressure setpoint 204. A signal is then sent to the turbine control system 210 and to summer 208. When core power control 232 is turned on, power control fault logic 230 is activated. Power control fault logic 230 will monitor power control system 220 for control system failures, position of the bypass valves, level of neutron flux 234, and power control system 220 operating parameters for acceptable values. If a variable is out of tolerance or a control system hardware is in a failed condition, power control logic 230 will not allow transfer to core power modulation mode. Also, if the plant is operating in the power control mode, fault logic 230 will automatically transfer back to turbine control valve modulation mode to maintain acceptable system pressure. When power control mode on 232 is turned on, a power control bias 240 will add a set signal to summer 208 which also receives the value of turbine load limit 212. These signals are summed with the pressure error signal from summer 206. The control signal from summer 208 is input to the power regulator 220 which, for example, may be a proportional plus integral controller. The output from the power regulator 220 is provided to one of the power control systems which can be either the recirculation pump variable speed system 250, the recirculation flow control valve position system 254, or the control rod position control system 260. FIG. 4 illustrates an operational relationship between the percent of rated reactor core power versus the percent of rated core flow for a BWR. The operational domain of conventional turbine control valve modulation pressure control 300 has an upper boundary of line 310 which represents the operating power limit for control valve modulation mode. The operational domain of core thermal power modulation pressure control 320 has a lower boundary line 330. Line 330 is based on an acceptable system stability and plant transient behavior during transfer from core thermal power modulation mode to turbine control valve modulation mode. The upper boundary line 340 of domain 320 represents the maximum power generated with thermal power modulation mode pressure control. As illustrated in FIG. 4, the maximum power generated from a BWR using thermal power modulation pressure control is greater than if turbine control valve power modulation pressure control is used. From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.