Patent Number: 048328987
Section: claims

1. A system for variably delaying a reactor trip resulting from a low water condition in a steam generator, comprising: means for determining whether a low water level condition exists in the steam generator;  means for determining the power output level of the reactor;  means for determining the length of a variable time delay in response to said steam generator low water level condition and said power output level; and  means for tripping the reactor in response to said variable delay.  determining whether a low water level condition exists in the steam generator;  determining the power output level of the reactor;  determining the length of a variable time delay in response to said steam generator low water level condition and said power output level; and  tripping the reactor in response to said variable delay. 2. The system of claim 1 further comprising means for actuating the auxiliary feedwater system in response to said variable delay. 3. The system of claim 1 wherein said means for determining said low water level condition include means for determining the number of steam generators in which a low water level condition has occurred. 4. The system of claim 1 wherein said means for determining said power output level include means for comparing the thermal power output level of the reactor to predefined thermal power output levels. 5. The system of claim 4 wherein said predefined power output levels correspond to ten percent of Rated Thermal Power and fifty percent of Rated Thermal Power. 6. The system of claim 1 wherein said means for determining the length of said variable delay include a plurality of timers activated upon the detection of said low water level condition to produce output signals at various times and a plurality of logic gates which select one of said output signals based upon said low water level condition and said power output level. 7. The system of claim 6 wherein said plurality of timers correspond to time delays of five seconds, thirty seconds, one hundred fifty seconds and two hundred eighty seconds. 8. The system of claim 6 wherein said plurality of logic gates include a plurality of AND gates equal in number to said plurality of timers and a plurality of bistables and NOT gates each equal in number to predefined power output levels for producing signals indicative of the thermal power output of the reactor and wherein said AND gates are responsive to said output signals of said timers and said signals produced by said bistables and NOT gates to select one of said output signals. 9. The system of claim 7 wherein said logic gates are configured so that the reactor trip is 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 only 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 only 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. 10. The system of claim 1 wherein said means for determining the length of said variable delay include a microprocessor responsive to said low water level condition and said power output level. 11. The system of claim 10 wherein said microprocessor is programmed such that said variable delay is calculated as a function of said low water level condition and said power output level. 12. The system of claim 10 additionally comprising a table of time delays based upon said low water level condition and said power output level and wherein said microprocessor is programmed such that said variable delay is determined by referencing said table. 13. The system of claim 1 wherein said means for determining the length of said variable time delay determine said variable delay in accordance with the graph of FIG. 5. 14. A method for variably delaying a reactor trip resulting from a low water condition in a steam generator, comprising the steps of: 15. The method of claim 14 further comprising the step of actuating the auxiliary feedwater system in response to said variable delay. 16. The method of claim 14 wherein the step of determining said low water level condition includes the step of determining the number of steam generators in which a low water level condition has occurred. 17. The method of claim 14 wherein the step of determining said power output level includes the step of comparing the power output level of the reactor to predefined power output levels. 18. The method of claim 14 wherein the step of determining the length of said variable delay includes the step of determining a delay of 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, of thirty seconds if the power level exceeds fifty percent of Rated Thermal Power and a low water condition exists in only one steam generator, of 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 only one steam generator and of 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.