Development of a New Oscillation Power Range Monitor Algorithm Based on Period Standard Deviation

Abstract

The Oscillation Power Range Monitors (OPRM) have been installed in some boiling water reactors (BWRs) to detect core regional oscillation by coupled neutronic/thermal-hydraulic instabilities under certain operating conditions, and to trigger a reactor trip signal before the oscillatory amplitude exceeds the safety limit for the fuel cladding integrity.A typical OPRM algorithm monitors the oscillatory periods for each OPRM cell responses that are constructed by Local Power Range Monitor (LPRM) signals at each OPRM cell corner, and determines the inception of instability events by the number of successive oscillatory periods within a certain tolerance and oscillation amplitude. This method might pile up the counts by picking up noise peaks and minima during stable operation conditions. A small tolerance, however, could delay instability detection, as OPRM signal noise is likely to displace the oscillatory peak intervals beyond the tolerance and cause the count to reset. Therefore setpoints should be determined carefully for reliable detection performance.We studied a new algorithm for the OPRM system to address the above-stated technical challenges. Focusing on the oscillatory period coherency, the new method evaluates the standard deviation for the oscillatory periods instead of monitoring them directly. A trip signal is triggered when the standard deviation remains lower than a setpoint for a certain elapsed time. This method aims at excluding the impact of the perturbation caused by the OPRM noise to the detection performance, utilizing statistics that represents the oscillatory characteristics for the instability criterion.To examine the applicability of the new algorithm, we simulated the OPRM signals by Advanced Boiling Water Reactor (ABWR) instability analysis data with the estimated signal noise superimposed, as there are no test data for the ABWR instability events. The new algorithm showed that the trip signal could be generated for the simulated regionally instable condition before the fuel cladding safety limit was reached.Copyright © 2012 by ASME