Analyzing the physics behind the flexibility of Nuclear Power Plants

Nuclear plants are increasingly expected to operate flexibly alongside variable wind and solar generation. Two physical effects are generally cited as the main limits on how fast and how far a reactor can be modulated: thermal-mechanical effects of the fuel and core poisoning after a power variation^{1, 2}. Small Modular Reactors (SMRs), particularly integral and/or soluble-boron-free designs, are widely claimed to be more flexible than large PWRs within this field^3–5, but the reasoning behind that claim is often qualitative rather than quantified. This context has renewed relevance in Belgium given the 2025 reversal of the nuclear phase-out policy and the reopened debate on new nuclear capacity.

The objective of this master thesis is to review and, where feasible, quantify how (thermo-)mechanical and core poisoning constraints limit the load following capabilities, with an emphasis on the important effects for a representative large PWR and a representative SMR (with an integral design), and to translate this into a comparative picture of realistic load-following capability (ramp rate, depth, frequency, and recovery time) for the two reactor types of the same technology. 

The work consists of 3 main parts:

First, the student shall perform a literature and regulatory review for the effects and limitations relevant in nuclear load-following, the physical mechanisms behind ramp limits, and the applicable technical/regulatory framework. 

Second, thermalhydraulic models of the power conversion system with a simplified core response will be built taking into account the reactor component(s) identified in the literature review as most critical for load-following, allowing deeper understanding of their impact. It is foreseen to use the Modelica⁶ language, for example in the Dymola modelling and simulation tool. The number of components will be decided during the thesis, based on their relevance and feasibility. Existing open-source libraries, such as the Euratom TANDEM Modelica library for SMRs, can serve as a starting point.

Thirdly, the obtained models from the previous part are used to simulate a power modulation to quantify the load-following capability for a representative scenario, resulting in a tangible case study and suggestions for design optimisation. 

 

References: 

1. Flachet, F. et al, Core and Fuel Feasibility Study for Improved Flexibility on the Belgian Nuclear Power Plants, September 2014.
2. INTERNATIONAL ATOMIC ENERGY AGENCY, Non-baseload Operation in Nuclear Power Plants: Load Following and Frequency Control Modes of Flexible Operation, IAEA Nuclear Energy Series No. NP-T-3.23, IAEA, Vienna (2018)
3. Chang, C.-k.; Oyando, H.C., Review of the Requirements for Load Following of Small Modular Reactors. Energies 2022, 15, 6327.
4. Zhang, Z.; Jiang, J., On load-following operations of small modular reactors. Progress in Nuclear Energy 2024, 173, 105274.
5. Lim, S.G.; Nam, H.S.; Lee, D.H.; Lee, S.W., Design characteristics of nuclear steam supply system and passive safety system for Innovative Small Modular Reactor (i-SMR). Nuclear Engineering and Technology 2025, 57, 103697.
6. Simonini, G., et al, Integrating small modular reactors into hybrid energy systems: the TANDEM Modelica library, July 2024.

Le niveau de diplôme minimum du candidat

• Master en sciences de l'ingénieur

Les connaissances préexistantes nécessaires

• Electromechanics
• Background in mechanical power engineering, interest in reactor physics

La durée estimée

Integrated in the BNEN programme.