Fuel and Core
Research into nuclear reactor cores and fuel is critical for advancing the safety, efficiency, and longevity of reactors. Within the extreme conditions of a reactor core—characterized by high temperatures, radiation, and pressures—nuclear fuel undergoes complex interactions and transformations.
Fuel performance analysis, encompassing thermal, mechanical, and chemical aspects, is essential to ensure optimal performance under these conditions. Core design activities focus on optimizing the structure, composition, and refueling operations, aiming to enhance both safety margins and economic efficiency. This research is vital for maintaining reactor performance and meeting stringent operational standards.
The fuel and core of a nuclear reactor are crucial for its safety and efficiency. Nuclear fuel, usually consisting of uranium dioxide or mixed oxide (MOX), is organized into fuel rods that are bundled to form fuel assemblies. These assemblies are carefully positioned within the reactor core, the central part where nuclear fission takes place. Core optimization aims to reduce fuel costs while adhering to safety and operational regulations.
Nuclear fuel undergoes complex interactions and transformations within the extreme conditions of a reactor core, which include high temperatures, radiation, and pressures. Fuel is designed to perform under such conditions; however, if the latter changes, the design must be modified to maintain acceptable performance. Therefore, comprehensively studying fuel performance becomes imperative to ensure the safety, efficiency, and longevity of nuclear reactors. Fuel performance studies encompass a broad spectrum of research areas, including thermal, mechanical, and chemical aspects.
Fuel performance analysis is one of the subjects of the core design activities. In the core, many different physics strongly interact with each other, and it is mandatory to assess the behavior of the core during normal operation and postulated transients. In core design activities, the objective is to optimize the design of structure, compositions and refueling operations so that safety margins are guaranteed in any condition, and so that the economical performance of the reactor are improved with respect to currently existing power plants.
The research within Fuel and Core are split into the three projects:
- CaNel – Calibration of Fuel Performance Codes
- Core design and fuel design optimization for Light Water SMR
- Fuel performance studies supporting SMR utilization in Sweden
CaNel – Calibration of Fuel Performance Codes
The project CaNel – Calibration of Fuel Performance Codes addresses key challenges in the calibration of fuel-performance codes, which are essential for predicting the thermomechanical behaviour of nuclear fuel and ensuring fuel safety during normal reactor operation, anticipated operational occurrences and accident conditions. These challenges included strongly coupled models, biased and sparse experimental data, input uncertainties, high computational costs and model inadequacy. Reliable inverse uncertainty quantification is needed to establish safe operating limits, reduce the risk of fuel-cladding failure and support efficient reactor operation and fuel utilisation while limiting waste and costs.
A central focus of CaNel is model inadequacy, which arises when a model cannot fully reproduce the underlying physical reality. If not properly accounted for, model inadequacy can significantly affect safety-related predictions. The project developed a methodology for quantifying the effects of model inadequacy on calibrated parameters and propagating the resulting uncertainties to subsequent predictions. The methodology is based on Markov chain Monte Carlo sampling and uses Gaussian-process surrogate models to reduce computational costs.
The project also developed machine-learning-based surrogate models for sequential data, supporting faster fuel-performance predictions and improved methods for calibration and uncertainty quantification. These approaches are relevant not only to fuel-performance modelling but also to other computationally demanding applications in nuclear technology. The applications focused on calibrating models for cladding oxidation, hydrogen uptake and fission-gas release, all of which are important for assessing nuclear-fuel performance and safety. The research used both proprietary data provided by Westinghouse and openly available experimental data.
The quality of the research and thesis work conducted within CaNel was recognised through Sigvard Eklund’s Prize, awarded by the Swedish Centre for Nuclear Technology, SKC, for outstanding bachelor’s, master’s and doctoral theses in nuclear technology. Gustav Robertson received the prize for Best PhD Thesis in 2025, and Wenhan Zhou received the prize for Best Bachelor’s Thesis in 2023. A complete list of recipients is available on the SKC winners page.
CaNel formed part of WP6, Advanced Fuel Performance Modelling, within the EU-supported APIS project. Its contribution was the development and delivery of new calibration and uncertainty-quantification methodologies for use by the project partners and other stakeholders.
APIS stands for Accelerated Program for Implementation of Secure VVER Fuel Supply. The programme aimed to strengthen the security of nuclear-fuel supply for Russian-designed pressurised-water reactors operating in the EU and Ukraine by diversifying fuel sources while maintaining full compliance with nuclear-safety requirements. APIS has strengthened European capabilities for supplying VVER-440 fuel by supporting Western fuel designs and fabrication capacity and by promoting cooperation among participating countries in fuel licensing. The programme was co-funded by the European Union through the Euratom Work Programme 2023–2025.
Selected publications from the CaNel project are available in DiVA.
The research initiated within CaNel will continue within ANItA – Academic-industrial Nuclear technology Initiative to Achieve a sustainable energy future. Building on the methods and expertise developed in CaNel, the continued work will further advance fuel-performance calibration, uncertainty quantification and machine-learning-based surrogate modelling in close collaboration between academia and industry. ANItA brings together academic and industrial expertise in nuclear technology.
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Core design and fuel design optimization for Light Water SMR
During the 20th and the beginning of the 21th century, a set of different reactor design have been proposed, studied and constructed all around the world. Historically, the first attempt to categorize the different reactor types is by the neutron flux spectrum, or, in simple words, by the average energy of the neutrons. Fast reactors use neutrons characterized by energies similar to those produced by fission reaction to sustain the fission chain reaction, while thermal reactors make use of neutrons which are in thermal equilibrium with the surrounding media. In order to obtain these energies, there is the need to slow down the fast neutrons and it is necessary a moderating media. Water is one of the best media for this purpose and engineers and physicists soon realized that it is also the perfect coolant of a nuclear reactor. Today the vast majority of the nuclear reactor cores uses thermal spectrum and light water coolant.
During the ‘70s and ‘80s most of the attention was focused to create larger and larger nuclear reactors, but after Three Mile Island accident and Chernobyl accident, and even more recently with the Fukushima accident, the attention of designers has been shifted towards small modular reactors (SMR) in order to limit the potential economical and safety risks associated to reactor construction, operation and future reactor decommissioning operations. By contrast, the reactor dimension has negative effects on the fuel cycle performance since the reduced dimension is responsible for a lower neutron economicity, which can be interpreted as the capability of neutrons to cause a fission reaction without being absorbed by different materials or escape through the reactor boundaries. The reduced neutron economicity has direct negative consequences on the volumes of nuclear wastes produced for unit energy output and for what concerns the economic performance of the fuel cycles. For this reason, ad hoc solutions must be studied for these reactors to improve their performance, including but not limited to fuel assembly design, fuel composition, reflector design and control and shutdown system design.
In this project, as part of the ANItA competence center, we aim to propose and apply modern optimization techniques for fuel composition and loading pattern design for two SMRs: AP300 from Westinghouse and BWRX-300 from General Electric Hitachi. Additionally, extensive studies will be performed on the possible operation of AP300 with boron-free cycles and load following operations for both reactors designs.
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Fuel performance studies supporting SMR utilization in Sweden
Small Modular Reactors (SMRs) have garnered substantial attention in recent years due to their compact size and modular design, allowing for quicker and more cost-effective manufacturing. These reactors, tailored for various end uses, are at different stages of development globally, with some ready for near-term deployment. Many SMR designs are based on established large-scale reactors, primarily Light Water Reactors (LWRs), leveraging operational experience. However, differences in core sizes, composition, and operational modes may impact fuel behavior and operational limits in SMRs. As part of the ANItA competence center, we aim to identify key differences in fuel behavior in near-deployment SMRs. The project will assess these changes through modeling and post-irradiation examination. Additionally, the need for establishing poolside fuel inspection capabilities to support Post Irradiation Examination (PIE) at distributed reactors will be examined to minimize fuel transportation to hot cell laboratories.