Study on Characteristics of ΔI During Power Reduction Transients at Various Burnup Levels in CPR1000 Nuclear Power Plants

Axial power offset (ΔI) is a key safety parameter characterizing the core power distribution in Pressurized Water Reactor (PWR) nuclear power plants, with its operational limits clearly defined in technical specifications. To investigate the variation characteristics of ΔI during power reduction transients at different burnup depths and provide references for preemptive prediction and proactive control of ΔI, this study conducts simulations of power reduction transients on a domestic CPR1000 unit using a simulation platform. The simulated scenarios cover transient processes where the unit's power decreases from full power to 20% rated power via control rod insertion, during which only routine operator actions are simulated (without specific control measures targeting ΔI). A systematic analysis was performed on three burnup states—beginning-of-life (BOL), middle-of-life (MOL), and end-of-life (EOL)—and four power reduction rates: 50 MW/min, 40 MW/min, 30 MW/min, and 20 MW/min, to examine the transient characteristics of ΔI. The results indicate that as burnup deepens, the variation pattern of ΔI during power reduction evolves from a “double-peak” shape at BOL to a “single-peak” shape at MOL, and eventually flattens out at EOL. The power reduction rate shows no significant influence on the trend of ΔI variation. Additionally, the negative peak of ΔI gradually shifts in the positive direction with increasing burnup. The results of this study offer a valuable reference for operators to predict changes in ΔI and take proactive measures, ensuring it remains within the specified operational limits.