Conference Agenda

With the increasing share of renewable energy generation driven by recent carbon neutrality policies, imbalances in power supply and demand within the electrical grid have become more significant. As one of the practical solutions to these challenges, the demand for load-following operation of nuclear power plants has been increasing. Repeated power variations induce changes in neutron flux within the reactor core, resulting in time-dependent transient behavior of Xe-135, a strong neutron absorber produced from the decay of I-135. Such Xenon transient behavior leads to non-equilibrium Xenon conditions that can influence core reactivity, axial power distribution, and ultimately core operational behavior. In particular, repeated power reduction and power increase processes may cause axial Xenon redistribution depending on the power distribution history, thereby prolonging the duration of non-equilibrium Xenon conditions. Therefore, for the safe and stable operation of advanced pressurized water reactors, it is necessary to quantitatively evaluate Xenon transient behavior under realistic load-following conditions.

In this study, the effects of power maneuver depth and low-power operation duration on Xe-135 transient behavior under various load-following scenarios were systematically investigated using an APR1400 equilibrium core model. Particular emphasis was placed on evaluating the time required for Xe-135 number density to return to equilibrium Xenon conditions following power restoration. Core analyses were performed using the ASTRA (Advanced Static and Transient Reactor Analyzer) code for an APR1400 equilibrium core. ASTRA is a three-dimensional reactor core analysis code developed by KEPCO Nuclear Fuel (KEPCO NF) and has been widely utilized for core design and operational analyses of pressurized water reactors (PWRs). The code employs the Semi-Analytic Nodal Method (SANM) based on the Coarse-Mesh Finite Difference (CMFD) method, providing practical computational efficiency together with three-dimensional depletion and transient analysis capabilities. Owing to these characteristics, ASTRA is well suited for evaluating Xenon transient behavior and power distribution characteristics under load-following conditions.

All load-following scenarios were configured to include power reduction, low-power operation, and subsequent power restoration under initial full-power equilibrium Xenon conditions. The scenarios were categorized into two groups. The first group varied the low-power operation duration while maintaining a fixed power maneuver depth, whereas the second group varied the power maneuver depth while maintaining a fixed low-power operation duration. In addition, a stepwise power maneuver scheme was applied to more realistically simulate actual reactor power changes and control rod bank movement speeds during plant operation. Axial power distribution constraints and control rod operating conditions were also considered during the analyses in order to reflect actual APR1400 operational procedures as realistically as possible.

The results of this study are expected to provide quantitative evaluations of the time and power dependence of Xe-135 behavior under repeated load-following conditions. In particular, the duration of non-equilibrium Xenon conditions and the equilibrium recovery time following power restoration under various operational scenarios will be analyzed. Furthermore, the study is expected to provide technical insights into axial power distribution behavior associated with Xenon redistribution during load-following operation and to serve as technical reference data for future core analyses under non-equilibrium Xenon conditions, power distribution test planning, and the development of load-following operational strategies for APR1400 nuclear power plants.

Nuclear power expansion represents a key pathway for reducing dependence on fossil-fuel-based electricity generation in EU countries. However, nuclear power is most economically viable when operated at a high capacity factor to supply baseload demand. Once baseload requirements are met, further expansion of nuclear capacity to accommodate seasonal variations and peak demand introduces two main challenges: an increase in the levelized cost of electricity (LCOE) and the need for enhanced load-following flexibility. While the integration of renewable energy sources with nuclear generation can partially address system variability, large-scale energy storage remains essential for enabling deeper system flexibility and higher nuclear penetration. In this context, several commercially available energy storage technologies can be considered, including pressurized hot water thermal storage, electrochemical batteries, pumped hydropower storage, and hydrogen production with reconversion pathways. This study evaluates these storage options when integrated with nuclear power plants to enhance the economically viable share of nuclear energy in electricity systems. The economic feasibility is assessed based on the relationship between the levelized cost of electricity and the capacity factor of nuclear power plants under different storage integration scenarios. The results provide insights into strategies for achieving a higher economically sustainable penetration of nuclear power within the EU electricity grid through the coordinated deployment of energy storage technologies.