Scram reduction by relaxing set points. An analysis of C-E PWRs with analog controls using RETRAN-02. Final report, November 1985

A number of scram setpoints were analyzed relative to reducing scram frequency. The thermal hydraulic analyses were performed based on a Combustion Engineering plant with analog controls for the reactor protection system. The results show that by relaxing these setpoints, most unnecessary scrams can be eliminated. The steam generator low level trip setpoint can be relaxed by lowering it 48 inches from its existing value for operations at less than 50% of full power. By this means, 0.25 scrams/reactor-year can be eliminated. Also, the power below which a reactor scram on a turbine trip is bypassed may be increased from 15% to at least 35%. The thermal margin/low pressure trip may be bypassed below 65% power, provided that the base low pressure value in the trip logic is increased to 2100 psia. Modifying these trips should help to reduce the frequency of spurious scrams at low power. The combination of setpoint relaxation and instrumentation and control modification could reduce the number of automatic scrams by about 1.4 scrams per reactor year. (author)