Analysis of Soluble Boron Control Strategy Based on Mode-K+ and Sliding Mode Observer for APR1400 Load Follow Operation

In large pressurized water reactors, axial xenon oscillations induced by power changes distort the axial power distribution, and compensating for the oscillations solely through control rods leads to reduced control margins and degraded control performance. To address these issues, this study analyzed a control system for APR1400 load follow operation based on Mode-K+ and a sliding mode observer, where soluble boron was utilized as an active control means to complement control rods and share the reactivity burden. 3D whole-core transient analysis was performed using the two-step method with SERPENT/KANT. By applying a piece-wise linear boron control strategy based on short-term xenon prediction, the xenon worth compensation range for stable load follow under the Mode-K+ and sliding mode observer control system was expanded. This approach suppressed excessive control rod insertion and maintained axial shape index control margins. Specifically, compensating 55% of the xenon-induced reactivity change through soluble boron ensured stable load follow for 100-50-100%, 100-20-100%, and mixed scenarios at 90% EOC conditions. These findings provide a practical operational reference for load follow within current design constraints, especially for domestic applications with limited commercial experience.