Dynamic modelling and control of a Small Modular Reactor in load following by cogeneration mode

The growing penetration of variable renewable sources is shifting the operational paradigm of nuclear power plants, traditionally operated as baseload generators, with flexible operation becoming fundamental not only to complement the non-programmability of renewables but also to ensure economic competitiveness. This study aims at evaluating the flexible operation capabilities of a soluble-boron-free, pressurised water type Small Modular Reactor (SMR). The traditional load following capability of the power plant, adopting a reactor-follows-turbine strategy, is compared to the load following by cogeneration mode. For the latter, two steam extraction points were considered to deliver excess thermal power to a generic non-electric application. The load following capabilities are assessed by means of a dynamic simulator of the nuclear power plant, developed in the object-oriented modelling language Modelica within the Dymola simulation environment. Specifically, the dynamic models of the SMR and the power conversion system are used to test the system’s response to variable load demands. Moreover, a control strategy based on a decentralised feedback approach, with control and process variables paired with the support of the relative gain array technique, is proposed to meet grid requirements. Results demonstrate that both traditional and load following by cogeneration can effectively meet variable demands using the proposed control strategy. Cogeneration mode improves cycle efficiency by using the extracted heat as a valuable output while keeping the reactor at rated conditions. In particular, a higher efficiency is achieved when extracting low-temperature steam, with a power loss factor of 0.32 compared to 0.46 for high-temperature extraction. The transient simulation confirms that load following by cogeneration can meet grid requirements by limiting frequency deviations to less than 40 mHz, compared to the 60 mHz in conventional load following, without the need to modulate the core power through external reactivity insertions. In conclusion, the findings of this study suggest that load following by cogeneration could be a viable solution to meet the increasing flexibility requirements while reducing solicitations to the reactor’s primary loop. The proposed dynamic model could be extended to simulate the response of a comprehensive nuclear hybrid energy system, including the dynamics of the coupled cogeneration systems, to explore the overall system’s response in different scenarios.