ICONE15-10489 An Enhanced Detect-and-Suppress Stability Protection Method for Boiling Water Reactors

Abstract

Systems based on the Detect-and-Suppress (D&S) Long Term Stability Solution Option III have been installed in many BWR plants. Such systems issue scram signals upon detecting oscillatory neutron flux signals from Local Power Range Monitors (LPRM) grouped into several Oscillation Power Range Monitor (OPRM) cells. The licensing basis of the existing systems does not address situations leading to highly unstable conditions with multiple interacting instability modes. These conditions are anticipated with flow windows extended beyond the Maximum Extended Load Line Limit Analysis (MELLLA) domain, which are typically associated with power uprate. The Enhanced Option III (EO-III) solution presented in this paper is an evolutionary step relying on the existing methodology and hardware, and introducing measures for addressing the reduced stability associated with extended flow window conditions in general, and the higher probability of single channel hydraulic instability excitation in particular. The new elements introduced as enhancements to the existing Option III solution include the introduction of a calculated exclusion region on the power/flow map, protected by reactor scram, to preclude single channel instabilities. The calculation procedures for supporting the stability protection system are modified consistent with the introduction of the channel instability exclusion region. In particular, the limiting critical power response to power oscillations which is calculated using 3-D system codes, will always be guaranteed to be regular and bounded as a function of the oscillation magnitude when the flow in all the reactor channels oscillate coherently. In addition to the physical basis behind the improvements in the new EO-III, aspects of the engineering application and licensing will be also discussed.