BWR stability investigation in Ringhals 1 measurement data from October 26, 1989

A BWR stability investigation has been performed on noise measurements collected at Ringhals 1 in connection with power oscillations which occurred on Oct 26, 1989. During reactor start-up following the planned outage in the summer of 1989, the reactor was encountered by unstable power oscillations after a forced reduction of recirculation pump speed due to an E4 chain violation at 80% power and 35% core flow. The neutronic power started to oscillate in limit cycle at 0.5 Hz. The operating conditions were characterized by low coolant flow and high power. The amplitude of the power oscillation reached 16% of full power. The sequence of events was interrupted by partial scram, initiated by the operator after three minutes. This measure stopped the oscillations. A reduction in feedwater flow temperature is judged to be the reason for the onset of limit cycle oscillation during poor stability condition. The paper emphasizes the necessity of on-line monitoring of the measure of stability. The report also concludes that the reactor protection system should be supplemented by scram conditions for high LPRM since the averaging function in the APRM (Average Power Range Monitor) gives lower oscillation amplitudes than the LPRM when the different LPRMS oscillate out of phase