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

概要

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We studied a new detection algorithm for the oscillation power range monitor (OPRM) system to suppress postulated neutronic/thermal-hydraulic instability events in boiling water reactors (BWRs) before exceeding the fuel cladding integrity limit. The new method evaluates the standard deviation for OPRM oscillatory periods instead of monitoring them directly. Instability is detected 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 OPRM noise components for each OPRM cell to the detection performance, utilizing statistics that represent the oscillatory characteristics for the instability criterion. Toshiba's BWR system analysis code TRACT simulated the OPRM signals for an up-rated Advanced Boiling Water Reactor (ABWR) regional instability event to examine the applicability of the new algorithm to this new reactor. Since there are no plant instability data for ABWR, we estimated noise components based on the available stability test data and superimposed them on the simulated oscillation data. 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.