Patent Number: 048328987
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed generally to safely delaying nuclear power plant protective functions and more specifically to the variable delay of steam generator low water level protective functions, i.e., reactor trip and auxiliary feedwater system actuation. 2. Description of the Prior Art In present nuclear power plant protection systems, a low water level, in any steam generator, will trip the reactor and activate the auxiliary feedwater system. FIG. 1 illustrates one example of auxiliary feedwater system actuation and reactor trip logic in a plant with four steam generators. Low water level indicators associated with each steam generator drive logical OR gate 14 and logical AND gate 15. A low water level indication in one steam generator will cause a signal to be generated which is available at an output of OR gate 14 to thereby initiate a reactor trip and actuate the motor-driven auxiliary feedwater pumps. Likewise, a low water level indication in two or more steam generators will cause a signal to be generated which is available at an output of AND gate 15 to thereby actuate the turbine-driven auxiliary feedwater pump. These actions are intended to protect the nuclear core and to maintain an adequate heat sink for decay heat removal. The most critical need for such protective action would occur while the plant is operating at full power following a total loss of feedwater to all steam generators or a major feedwater line rupture. Therefore, the steam generator low water level protection system logic and setpoints are determined according to the requirements of these limiting postulated conditions. The same protective functions also occur, however, under less limiting conditions such as the termination of feedwater to only one steam generator during plant startup operations. A survey of nuclear power plant operating experience indicates that a large number of all unplanned reactor trips are attributable to low steam generator water level trip signals. The removal of unnecessary conservatisms that are inherent in the accident analysis assumptions and models and that are applied to determine the steam generator low water level reactor trip setpoint would permit the low water level trip setpoint to be selectively lowered during plant operation under less-than-limiting conditions and thereby reduce the likelihood that the steam generator water level would fall below the trip setpoint and unnecessarily trip the reactor. Unfortunately, lowering the steam generator low water level trip setpoint is constrained by the physical design of the steam generator. Accordingly, the need exits for a system which is capable of eliminating unnecessary reactor trips. SUMMARY OF THE INVENTION The present invention is directed to a system for variably delaying reactor trip and auxiliary feedwater system actuation resulting from a low water condition in one or more steam generators. The system includes a plurality of sensors for determining whether a low water condition exists in each steam generator and for determining the power output level of the reactor. The length of the variable time delay is determined as a function of the severity of the steam generator low water level condition, i.e., the number of steam generators experiencing a low water level condition, and the reactor power output level. The reactor is tripped and the auxiliary feedwater system is actuated at the expiration of the delay period if the steam generator water level is not restored. The delay thus provides time for remedial operator action and the natural stabilization of water level transients. Restoration of the water level during such a delay would avoid unnecessary reactor trips. One embodiment of the present invention is directed to a system for variably delaying reactor trip and auxiliary feedwater system actuation resulting from a low water condition in one or more steam generators comprised of a plurality of sensors and accompanying logic circuitry which determine the number of steam generators in which a low water level condition has occurred and whether the reactor power output level has exceeded certain predefined power levels (e.g., ten percent and fifty percent of Rated Thermal Power). The system is additionally comprised of a plurality of timers activated upon the detection of a low water level condition. These timers may correspond, for example, to delays of five seconds, thirty seconds, one hundred fifty seconds and two hundred eighty seconds. The length of the effective delay is determined by a plurality of logic gates combined in such a fashion so as to select one of the timer output signals based upon the severity of the steam generator low water level condition and the reactor power output level. The reactor trip and auxiliary feedwater system actuation are delayed by five seconds if the power level exceeds ten percent of Rated Thermal Power and a low water condition exists in more than one steam generator, by thirty seconds if the power level exceeds fifty percent of Rated Thermal Power and a low water condition exists in any one steam generator, by one hundred fifty seconds if the power level is between ten percent and fifty percent of Rated Thermal Power and a low water condition exists in any one steam generator and by two hundred eighty seconds if the power level is less than ten percent of Rated Thermal Power and a low water condition exists in at least one steam generator. The reactor is tripped and the auxiliary feedwater system is actuated at the expiration of the delay period if the steam generator water level is not restored. Another embodiment of the present invention is directed to a system for variably delaying reactor trip and auxiliary feedwater system actuation resulting from a low water condition in one or more steam generators comprised of a microprocessor which determines the length of the variable delay in response to a plurality of sensors which determine whether a low water condition exists in each steam generator in addition to the power output level of the reactor. The reactor is tripped and the auxiliary feedwater system is actuated at the expiration of the delay period if the steam generator water level is not restored. The system for variably delaying reactor trip and auxiliary feedwater system actuation of the present invention enables the reactor protection system to distinguish between a severe low water transient which requires prompt protective action and a less severe low water transient during which protective action may be safely delayed. The application of these delays allows time for steam generator water level transients to stabilize and recover and for the operator to take corrective measures, thus resulting in the elimination of many unnecessary reactor trips. These and other advantages and benefits of the present invention will become apparent from the description of a preferred embodiment hereinbelow.