IAEA Research Project Strengthens Understanding of Advanced Fuel Behaviour

As countries strive to develop safer and more efficient nuclear fuels, an IAEA research project has bolstered global understanding of advanced technology and accident tolerant fuel behaviour. 

Launched in 2020, the Coordinated Research Project (CRP) on testing and simulation of advanced technology and accident tolerant fuels has helped countries understand and address technical factors influencing the design, fabrication and performance of advanced fuels and cladding materials – the outermost layer of nuclear fuel elements. 

Following the Fukushima Daiichi accident, nuclear fuel developers and research organizations intensified their efforts to design and qualify new generations of accident tolerant fuels (ATFs) capable of withstanding extreme conditions beyond the performance limits of conventional zirconium-based claddings. Among the various concepts under development, chromium-coated zircaloy and iron-chromium-aluminium (FeCrAl) cladding materials have emerged as promising near-term evolutionary technologies. Their enhanced resistance to high-temperature oxidation, improved mechanical stability and favourable behaviour under transient and accident conditions are expected to provide measurable benefits once deployed.

The CRP was structured around four primary technical objectives: 

• conducting experimental studies, to evaluate ATF performance under normal and accident conditions; 
• benchmarking fuel performance and severe accident codes using newly generated datasets and existing data; 
• developing methodologies to evaluate ATF performance during loss of coolant accident (LOCA) scenarios, with the longer-term aim of supporting licensing applications for ATF concepts; and
• supporting the IAEA fuel and material database with ATF experimental data and material properties. 

The project’s first work task focused on experimental testing of 15 types of ATF cladding materials, including chromium-coated zirconium alloys, zirconium alloys with alternative coatings and FeCrAl alloys, with uncoated zirconium alloys serving as reference baselines. 

More than 200 datasets were produced, which provided essential validation points for fuel performance models. Mechanical testing programmes revealed consistent trends: coated zirconium claddings generally showed higher strength but lower elongation – the degree of swelling or expansion within fuel rods. Supplementary tests covering areas including micro-scale coating analysis and thermal property measurements provided valuable insight into coating degradation mechanisms and microstructural performance. Other tests under extended LOCA and beyond design basis accident conditions demonstrated clear benefits for Cr-coated claddings, including by limiting oxidation. However, they also highlighted challenges associated with coating failure during severe conditions.

For the second work task, researchers conducted extensive benchmarking of fuel performance codes against experimental data on fuel behaviour simulation from both this and previous CRPs. Benchmarks against steady-state irradiation data, high-power tests and power ramps demonstrated good alignment between measured and predicted values. Analyses of this data reinforced the importance of uncertainty quantification when validating fuel performance models. 

Work task 3 encompassed LOCA simulations performed using nine fuel rod codes and one system thermal-hydraulic code. Results showed reasonable consistency in predictions of rod internal pressure and burst timing, though significant uncertainty persisted in cladding deformation predictions. Analyses of LOCA cases identified the importance of accurate initial state definitions, improved thermal-hydraulic boundary conditions and refined material models. 

A simplified LOCA evaluation methodology was developed through these activities. Developing this approach further will require more precise boundary conditions, improved uncertainty analysis and enhanced models for high burnup phenomena such as fuel fragmentation, relocation and dispersal.

Work task 4 supported the development of open-access databases within the IAEA Fuel Experimental Database and the IAEA Advanced Material Database. Data from tests were curated for inclusion, ensuring that the results generated through the CRP will become available to the broader scientific community and enable future validation, and model improvement once the CRP final report has been published.

The CRP led to the production, testing and analysis of several ATF coated cladding options, generated extensive experimental datasets and strengthened the capabilities of fuel behaviour modelling through comprehensive benchmarking activities.

Through collaborative research, sharing experimental data and exchange of best practices in fuel behaviour modelling, the CRP advanced understanding of ATF design, fabrication and in-pile behaviour, thereby increasing the technology readiness of candidate ATF materials. The project also delivered critical feedback and validated methodologies that will guide the next stages of ATF qualification and licensing. An IAEA Technical Document detailing the results of the research activities carried out by the CRP partners is under preparation. 

A total of 21 countries participated in this CRP, including Argentina, Belgium, Belarus, Brazil, Bulgaria, Canada, China, the Czech Republic, France, Germany, Hungary, the Islamic Republic of Iran, Italy, Japan, Poland, the Republic of Korea, Russian Federation, Slovakia, Spain, the United Kingdom and the United States of America, as well as the European Commission.

To build on the results of this successful project, another CRP is planned to begin by the end of 2026. The new project will expand the focus to include conditions for high burnup fuels and issues specific to fuels designed for small modular reactors.