Fiber Bragg Grating–Based In-Core Neutron Flux Monitoring with Bias- and Gain-Aware State Estimation in a 13-String Boiling Water Reactors Configuration: A Short Communication

Abstract: Accurate three-dimensional reconstruction of neutron flux in Boiling Water Reactors (BWRs) is critical for safe power operation and thermal margin management. Conventional in-core instrumentation provides limited spatial coverage and is subject to calibration drift, bias, and gain uncertainties that degrade long-term accuracy. This work presents an Ensemble Kalman Filter (EnKF)-based framework for real-time flux estimation in a 13-string × 24-axial-node BWR configuration, integrating flux and gamma thermometer measurements within a void-coupled core dynamics model. The proposed method augments the state vector with adaptive bias and gains parameters for each instrument string, enabling simultaneous core state reconstruction and online sensor self-calibration. Simulation results demonstrate improved flux reconstruction accuracy, reduced measurement residuals, and robust tracking of sensor drift under transient operating conditions. The approach provides a foundation for next-generation digital twin architectures and fiber-optic–based in-core monitoring systems for advanced BWR.