Patent Number: 044709480
Section: description

DETAILED DESCRIPTION OF THE EMBODIMENT The apparatus shown in FIG. 1 is nuclear-reactor power apparatus 11. This apparatus includes a nuclear reactor 13, a pressurizer 15, and a steam generator 17. Reactor coolant is pumped through the reactor 13 by a pump 19. The coolant flows in a primary loop 21 extending from the reactor, though the hot leg 23, the inlet plenum 25 of the steam generator 17, the primary U-tubes 27 of the steam generator 17 (or straight through tubes), the outlet plenum 29 of the generator, the pump loop seal 31, the pump 19, the cold leg 33 of the loop, to the reactor. The steam generator 17 has a secondary system in heat-exchange relationship with the tubes 27 containing the reactor coolant which produces and conducts steam to the turbines (not shown) of the nuclear-reactor power apparatus. Instruments 35 and 37 are connected to the hot and cold legs 23 and 33 of loop 21. These instruments are connected to another instrument 39 which produces a measurement of the minimum temperature sensed by instruments 35 and 37. There is also an instrument 41 for measuring the temperature of the loop seal 31 and an instrument 43, connected to the hot leg for measuring the pressure of the coolant. The coolant flows into and out of the pressurizer 15 through a surge line 45 connected to the hot leg 23. There is an adder 46 for producing a signal, .DELTA.T.sub.o, measuring the difference .DELTA.T.sub.o between the temperature, T.sub.SG, of the secondary fluid and the temperature T.sub.RCS, of the coolant. T.sub.RCS is derived from instrument 39 which indicates the minimum temperature measured by instruments 35 and 37. There is also an adder 48 for measuring the difference, .DELTA.T.sub.1, between T.sub.RCS and the temperature T.sub.LS of the pump loop seal measured by instrument 41. Where there are a plurality of steam-generator loops, .DELTA.T.sub.o and .DELTA.T.sub.1, may be derived from each loop and used, with or without auctioneering, to control an equal number of valves or a lesser number of valves. The word auctioneering means the selection of a particular .DELTA.T.sub.o and .DELTA.T.sub.1, which is the most limiting signal. The nuclear reactor power apparatus 11 includes a conventional chemical and volume control system 47 which is connected to the coolant loop 21. The coolant loop 21 is supplemented from this system 47 by charging pump or pumps 49 which is connected to cold leg 33. Excessive coolant in loop 21 is dumped into this system from the loop seal 31 through valve 51 when this valve is opened. Coolant is also pumped into the cold leg 33 when necessary through the safety-injection line 53. A signal indicating when the pump 19 is started up is derived from the pump through line 56. Improper operation of the pump or pumps 49 or valve 51 or inadvertent supply of mass to the coolant through line 53 may produce a malfunction in the water-solid state of the apparatus 11. The pressurizer 15 includes electrical heaters 55 for heating the coolant contained in the pressurizer under normal power conditions. Inadvertent operation of heater 55, while coolant is water solid, can also produce a heat input overpressure transient. There is also a nozzle 57 or a plurality of nozzles near the top of the pressurizer, connected to the cold leg 33 through a valve 59 for spraying coolant into the pressurizer under normal operating conditions. The pressurizer 15 is provided with a plurality of safety valves 61 (only one shown) which are interposed in safety lines 63 connecting the pressurizer to pressurizer relief tank 65. There is also a power-actuable relief valve or valves 67 which is interposed in a relief line 69 between the pressurizer and the tank 65. When the relief valve 67 is opened, steam or water from the pressurizer 15 is dumped into tank 65. Relief valve 67 is actuated by air which is supplied through solenoid valve 71. With solenoid valve 71 closed, air from relief valve 67 is vented through vent 73. When the solenoid is energized, vent 73 is closed and control air is injected through valve 71 to open relief valve 67. Instruments 75 and 77 are connected to the pressurizer to measure its pressure and the level of the coolant therein. The pressure measurement signal is impressed on a conventional proportional plus integral plus differential (PID) controller 79. This controller 79 transmits the conventional commands and also a command to control the power-actuable relief valve 67 (FIG. 9). The nuclear power apparatus 11 according to this invention includes control logic 81 for controlling the power-actuable relief valve 67 when the apparatus is in water-solid condition. As indicated, this control logic 81 receives the temperature, pressure, level, pump start up, .DELTA.T.sub.o, .DELTA.T.sub.1 signals from the components of the apparatus which signals serve as criterians for the actuation of relief valve 67. Mass input command A and heat-input command B, derived from these signals, when they are produced, are transmitted to the relief valve control 83. The PID command from PID controller 79 is also impressed on this control 83. Under the command of the highest of signals A, B or PID, the control 83 energizes the solenoid valve 71 and the relief valve 67 with command C. In FIGS. 2 and 3, coolant 85 is represented by cross-hatching and steam 87 by dots. As shown in FIG. 2 there is during normal operation a large steam bubble 87 above the level of the coolant in pressurizer 15. In the water-solid state the pressurizer is filled with coolant 85 as shown in FIG. 3 or the volume of the bubble is very small; i.e., the level of the water is above the setpoint. Typically the reactor 13 supplies a plurality of steam generators. Each steam generator is supplied from a separate loop 21 including the components shown in FIG. 1 except for the pressurizer 15 and its components. The pressurizer is connected to hot leg of only one of the loops. Tyically the control logic 81, the relief-valve control 83 and the PID controller 79 are components of a computer. FIG. 4 shows the region of operation of the reactor apparatus 11 to which this invention is applicable. Coolant temperature, T.sub.RCS, (the minimum coolant temperature) is plotted horizontally and coolant pressure, P.sub.RCS, vertically. The curve C1 marks the coolant-pressure limit of reactor vessel 13 as defined by Appendix G; i.e., the pressure below which it is required that the apparatus operate at a corresponding coolant temperature. The curve C1 can be substantially flat below a temperature T2.sub.RCS. This invention is applicable at all coolant temperatures below the setpoint T1.sub.RCS for which the limit of Appendix G must be observed. Above this temperature the conventional protective devices of the reactor power apparatus take over. FIG. 5 shows the part of the control logic 81 (FIG. 1) from which the command A for actuating the power-actuable valve 67 for mass input is derived. The logic shown in FIG. 5 includes AND 91 which has inputs 92, 94, 96. The coolant-pressure signal is impressed on a time-derivative component 93 which derives the rate of change of pressure level anticipatory of overpressurization. The output of the time-derivative component 93 is impressed on an adder 95. The negative coolant pressure-rate setpoint is also impressed on this adder. If the coolant-pressure rate is positive, the difference between the coolant-pressure rate and the setpoint is impressed on a threshold gate 97 which passes a signal only if this difference becomes zero or exceeds a predetermined threshold. The signal from gate 97 is impressed on input 92 of AND 91 through a timer 99. The timer 99 is set to impress the signal on AND 91 only if it persists for a time interval .tau..sub.1. The time .tau..sub.1 is long enough to prevent actuation of the relief valve 67 for short, spurious transients. This is illustrated in FIG. 6. Time is plotted horizontally and coolant pressure vertically. The curve C2 for the pressure is seen to have a hump manifesting an increase in coolant pressure for short spurious transients. As shown, the interval .tau..sub.1 starts when the pressure starts to increase. The rate of increase is the slope of the line labelled dP/dt. If dP/dt equals the setpoint or exceeds it by a threshold for an interval longer than .tau..sub.1 the signal from gate 97 is impressed on AND 91. For curve C2 as illustrated by FIG. 6, the rate dP/dt is not positive for the interval .tau..sub.1 and the signal would not be impressed on AND 91. For curve C8, (FIG. 6), the rate dp/dt is positive for a time greater than interval .tau..sub.1 and the signal wound be impressed on AND 91. Another signal is impressed on input 94 of AND 91 if the coolant temperature is less than a setpoint. If this temperature exceeds the setpoint, the apparatus is in normal operation and coolant pressure is monitored by the conventional monitoring components of the nuclear power apparatus 11. A third signal is impressed on input 96 of AND 91 if the pressurizer liquid coolant level is above a setpoint; i.e. if a water solid condition potentially exists. If all these signals and signal 92 are impressed, AND 91 outputs a command A to the relief valve control 83 to actuate relief valve 67. FIG. 7 shows the logic, included in control logic 81 (FIG. 1), for actuating the relief valve 67 for excessive heat input (HI) into the coolant. FIGS. 8A and 8B show the effects on coolant temperature and coolant pressure of typical heat input into the coolant during a reactor-coolant pump start-up with secondary fluid temperature at times greater than primary coolant temperatures. In both views time is plotted horizontally. The intersection of any vertical line with the time axes of FIGS. 8A and 8B marks the same instant of time for both graphs. In FIG. 8A temperature is plotted vertically. Curve C3 represents the temperature of the secondary fluid and curve C4 the temperature of the coolant. In FIG. 8B coolant pressure is plotted vertically; the pressure follows curve C5. It is assumed that prior to t.sub.0, the apparatus 11 was shut down and put into a water-solid configuration after the load on this apparatus was removed. In proceeding to complete shut down of the apparatus, the coolant cools at a higher rate than the secondary fluid. Prior to coolant pump startup, at instant t.sub.0, the secondary fluid is at a higher temperature than the coolant as shown by curves C3 and C4 in FIG. 8A. With pump startup, coolant flows into the primary tubes of the warmer steam generator promoting flow of heat from the secondary fluid into the coolant. The pressure of the coolant increases as shown by curve C5 of FIG. 8A. The interchange of heat between the secondary fluid and the coolant continues until the system reaches equilibrium as shown by the ends E1 and E2 of curves C3 and C4 and C5. The invention involves the operation of the apparatus 11 prior to stabilization or equalization of secondary and coolant temperatures. If the pump 19 is enabled at time t.sub.o under the temperature conditions shown in FIG. 8A, there is potential for overpressurization. The apparatus shown in FIG. 7 includes an AND 101 having inputs 105, 107, 109 and 111 and an OR 103 having at least two inputs. AND 101 operates as a gate which can pass its signal, for 107, 109, 111 "high" or 1, only if there is an appropriate signal (a "high" or 1) on input 105. This input 105 receives an appropriate signal 56 (FIG. 1) from pump 19 (FIG. 1) when it starts. The signal is impressed through timer 108. Timer 108 permits the appropriate signal to be impressed only during the interval .tau..sub.2. This interval .tau..sub.2 is the interval between t.sub.0 (FIGS. 8A, 8B), the time of startup and the time when apparatus 11 reaches the equilibrium state represented by ends E1 and E2 of curves C3, C4, C5. Assuming that there is an appropriate signal on input 105, three additional conditions must be met to produce a command B to actuate relief valve 67 responsive to heat input. A signal (1 or "high") is impressed on input 107 if the pressurizer coolant level exceeds the setpoint; i.e., if apparatus 11 is potentially in water-solid state. A signal (1 or "high") is impressed on input 109 through OR 103 either if the temperature difference .DELTA.T.sub.o, namely, the temperature of the secondary fluid (C3 FIG. 8A) less the temperature of the coolant (C4), is greater than a setpoint or if the temperature difference .DELTA.T.sub.1, namely, the temperature of the coolant T.sub.RCS (FIG. 1) less the temperature of the pump loop seal 31, is greater than a setpoint. An appropriate signal is impressed on input 111 if the coolant temperature is less than a setpoint. If inputs 107, 109 and 111 receive appropriate signals while there is an appropriate signal on 105, AND 101 produces a heat input (HI) command B output and relief valve 67 is opened. Commands A or B impress a command to open relief valve 67 through OR 113 and comparator 115 (FIG. 9). Commands A or B or both are impressed on comparator 115. In addition the command from the PID controller is impressed on the comparator. The comparator transmits a command to actuate the relieve valve for the highest command impressed on it. While a preferred embodiment of this invention has been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.