Patent Number: 059600497
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 schematically represents an overview 10 of the functional components associated with the reactor power cutback system (RPCS)12. When in service and confronted with certain transients, the reactor power cutback system sends control signals to the control rod drive mechanisms 14, and interacts with the turbine control system 16, to stabilize the power of the NSSS at a reduced, but non-zero level. If the RPCS cannot stabilize the NSSS by a combination of turbine runback and adjustment of the regulating control rod groups, the RPCS trips one or more groups of control rods. For purposes of the present invention, a logic scheme is associated with the conditions under which a reactor power cutback system trip demand signal is generated at demand block 18, for processing in the RPCS 12. The processing in functional block 12 includes, for example, the control rod selection criteria 20, which in turn is dependent on NSSS data 22 supplied by sensors in the plant. Other calculated conditions of the NSSS are supplied from block 24, to the RPCS 12. The RPCS 12 is also subject to action taken at the reactor power cutback control panel 26, and alarms are generated by the RPCS 12 for display at the alarm section of the operator console at 28. FIG. 2 shows the pump selection logic circuit 100 for three main feedwater pumps, A, B, and C. This circuit can be divided into four functional sections: (1) the pump selection and enablement section 102; (2) the feedwater pump trip signal section 104; (3) the logic implementation section 106; and (4) the RPCS trip demand section 18. The function of the demand section 18 in FIG. 2 can, for purposes of the present description, be considered to be the same as the function of block 18 in FIG. 1. The section 102 has, for each main feed water pump A, B, and C, at least one switch 108, and preferably at least one more switch 110, by which the operator can select whether or not that particular pump is intended to be fully operational for normal power production in the plant. When fully operational, pump speed control above e.g., 5% plant power, would automatically be adjusted by an automatic feedwater control system (not shown). When not fully operational, the pump is either on but in standby, or off and out of service. Thus, the operator can designate which, if any, of the pumps have been intentionally disabled from automatically controlled operation. The automatic feed water control system controls the variables of pump speed and valve position to maintain a preset water level in the steam generators. Although not normally utilized in the operation of an NSSS, an operational pump could, under the manual control of the operator, in effect be in a "stand-by" condition, whereby the pump rotates at a minimum recirculation speed corresponding to the flow to the steam generators produced by the other pumps. In the illustrated embodiment, main feed pump A has associated therewith, a switch 108A which can be manually toggled to the start or stop position, at either the main panel in the plant control room, or locally at, for example, the motor control center associated with the feed pump system. "Start" corresponds to selection of the pump as intended for full operation. "Stop" designates disablement from full operation, i.e., "off" or in standby. As used hereinafter, "operational" means "fully operational". The logical condition of each switch 108A, 110A, is delivered to a pair of logical OR gates 112A, 114A, the outputs of which are delivered to a flip-flop circuit 116A. The output Q of the circuit 116A on line 118A, will be a logical "1" when the operator selects the start condition for pump A at either switch 108A or 110A. This logical "1" is delivered to AND gate 120A, which operates as an enabling latch, indicative of whether the particular pump status is intended to be operational. It should be appreciated that each of the main feed water pumps B and C has associated switches 108B and C; 110B and C; 112B and C; 114B and C; 116B and C; 118B and C; and 120B and C. Preferably, the status latches are also arranged with redundancy, such that the logical conditions at outputs 118A, 118B and 118C, are each delivered to a respective second AND gate 122A, 122B, and 122C. Thus, by way of example, if the operator manually selects the start condition for main feedwater pump A via either switch 108A, or 110A, latches 120A and 122A, will both be enabled, redundantly. The feedwater pump trip signal section 104, includes the three main feedwater pumps A, B and C indicated respectively 124A, 124B and 124C, each of which is responsive to inputs from the main control panel, as indicated at 126A, B and C, as well as from one or more feedwater control systems, indicated at 128. The feedwater control system 128 and associated control logic for generating a trip signal, form no part of the present invention. It should be appreciated, however, that for each pump such as 124A, a trip of that pump will result in the generation of two trip signals 130A, 132A, which are delivered to the latches 120A and 122A, respectively. In similar fashion, trip signals 130B, 132B and 130C, 132C are delivered to the latches 120B, 122B, and 120C, 122C, respectively. When the inputs to any particular latch gate 120A, B, C or 122A, B or C are both logical "1", a respective logical "1" output signal is generated on a respective line 134A, B or C, or 136A, B or C. The logic implementation section 106 includes a RPCS trip control gate 138, which under specified conditions, passes a RPCS trip control signal on line 142 to the RPCS system 18. Preferably, another RPCS trip control gate 140 is also present, from which a trip control signal is passed along line 144 to the demand block 18. Thus, in the preferred embodiment, the actual RPCS trip demand signal is not generated at 18 for delivery to the reactor power cut back system 12 (see FIG. 1), unless a trip control signal is present on both lines 142 and 144. The OR gate 138 receives signals from three AND gates 146, 148, and 150. If any one of these AND gates generates a logical "1" output, the gate 138 generates a control signal on line 142. Similarly, as part of the redundancy described above, the OR gate 140 will pass a trip control signal on line 144, if the output of any one of the AND gates 152, 154, or 156 is a logical "1". Each of the AND gates 146-156 will generate a logical "1" output signal, if and only if a logical "1" is input to the AND gate, from signals corresponding to the condition of two different pumps. It should be appreciated that the invention is especially significant in distinguishing between an initial condition wherein only two of the three feedwater pumps are intended to be in operation, from the condition wherein all three of the feedwater pumps are intended to be in operation. The OR gates 160-182 interposed between the AND gate arrays 120, 122 and 146-156, play a role in, for example, the generation of a RPCS trip demand signal if one of only two operational pumps is tripped, while inhibiting the generation of a trip demand signal, if only one of three operational pumps trips. In essence, the AND gates 146-156 require a two out of three pump trip condition in order to pass a logical "1" signal to OR gate 138, 140. If, for example, main feed pump A is not in operational service, i.e., is being used as a spare, it is considered equivalent to a tripped pump in a configuration where three pumps are intended to be in operation. Therefore, when both switches 108A, 110A are in the stop condition, the Q output signal at flip flop 116A is a logical "1" on line 158A. In this condition, the output signal Q is a logical zero, and therefore gate 120A is not enabled. Nevertheless, in order to achieve the desired generation of a trip control signal on line 142 (and line 144) when one of either pump B or C trips, the logic section 106 must produce the same output, as it would under the conditions of pumps A and B tripping during plant operation for which all three pumps A, B and C are intended to be operational. Therefore, the logical "1" from the Q output of the flip flop 116A is delivered to the OR gate 160, such that the logical "1" can be passed to the AND gate 146. If, in the example of a trip of pump B, the AND gate 120B passes a logical "1" through the OR gate 162 to the AND gate 146, gate 146 will pass a logical "1" to the OR gate 138, and on to the demand section 18 via line 142. In this manner, gate 146 is responsive to the condition of both pumps A and B. Due to the redundancy described above, gate 152 is similarly influenced by the condition of pumps A and B. Output of gates 146 and 152, will be a logical "1" if, and only if, (a) pump A or B is considered non-operational as a result of the "stop" settings in section 102, and a trip of pump B or A occurs, respectively; or, (b) pumps A and B are both intended to be operational as indicated by the "start" settings on the switches in section 102, and trip signals from both pumps A and B are generated. It can also be appreciated that, if all the pumps that are intended to be in operation, trip coincidentally, neither of the gates 138 or 140 passes a RPCS trip control signal to the trip demand section 18. Under this condition, the reactor will fully trip on low steam generator level, thereby reducing the power from fission essentially to zero, rather than merely cutting the power back to a lower but non zero value, as a result of the actuation of the RPCS 12. In NSSS with three main feedwater pumps, each provides about 33.3% of the feed water required for the steam generators, but with a maximum capacity of at least 50% each. The Reactor Power Cutback System according to the invention is preferably used in the following manner: ______________________________________ Power RPCS Plant Level Feedwater Status Status Operator Actions Response ______________________________________ 0 to One FW pump Out of Prior to raising If the 40% ON (running at service power above 5% operating operating speed, place Feedwater feedwater supplying water Control System in pump trips to the steam automatic. will likely generators); Operator will result in a second pump ON make selection of plant trip. but at standby pumps in (running at operation or minimum speed, standby via pump not supplying selection logic. water to the steam generators); third pump OFF. 40% Two feedwater Placed At approximately If one to pumps are ON in 40% power, the feedwater 70% running at service second FW pump pump at operating speed at will be placed in operating and the third ON approxi- service. The speed is in standby. mately operator will tripped, the 50% to choose the pumps speed of the 60% in operation at the second power. FW system operating control panel and feedwater the choices are pump will be recognized by the increased by RPCS selection the logic. feedwater control system. Depending on the initial power level, the RPCS will generate a trip demand signal to cut back power by dropping rods to quickly reduce power and will initiate turbine runback. If both feedwater pumps trip, the reactor will trip on low level in steam generator. 40% Two feedwater Placed At approximately If one pump to pumps ON and In 40% power the trips, RPCS 100% third OFF and service second pump will will generate out of service at be placed in an RPCS trip approxi- service. The demand mately operator will signal. If 50% to choose the pumps both FW 60% in operation at the pumps trip, power FW system the plant will control panel and trip. the choices are recognized by the RPCS selection logic. 70% All three In At about 70%, the If one pump to feedwater pumps service third feedwater trips, the 100% are ON and pump is placed in other two running at service and this is pumps will operating speed. recognized by compensate RPCS selection for the loss logic. of the third pump. If two pumps are lost, RPCS will generate an RPCS trip demand signal. ______________________________________ It can thus be appreciated from the foregoing description, that the RPCS is in service only when at least two feedwater pumps are in full operation and the feed water control system is in the automatic mode. The conventional RPCS (inventive RPCS) is always in service when the reactor power is at least 50% (50%-60%), and the feedwater control system is in the automatic mode. The feedwater control system is always on, when power is at least 50%. This relationship between the reactor power cut back system and the feedwater control system can be implemented in a variety of ways. Preferably, the RPCS will automatically sense (without operator intervention) from the feedwater control system, which of the feedwater pumps are intended to be operational. Therefore, the switch means for manually selecting which of the pumps are intended for pumping operation and designating which of the pumps have been disabled from pumping operation, can be the same switch means used by the operators at the control panel, for placing pumps in service. As the power level passes 50% and the RPCS is placed in service, the RPCS will "read" the pump status as previously selected by the operator. Alternatively, the RPCS could be designed such that as the power level increases through approximately 50%, and the RPCS is placed in service, the operator manually selects via switches dedicated to the RPCS, which of the feedwater pumps are intended to be operational. It should also be understood that the functionality described above can be implemented in a variety of ways that would be readily available to one of ordinary skill in this field of technology. For example, programmable logic controllers, or other programmed logic via computer software, may be substituted.