TRISO nuclear fuel: history and prospects

TRISO (Tri-Structural Isotropic) nuclear fuel is an advanced type of fuel composed of tiny uranium particles enclosed in protective layers of silicon carbide and boron nitride, which act as a first containment barrier to retain fission products. Used mainly in high-temperature reactors (HTRs), this fuel offers significant advantages in terms of safety and performance.


Development of TRISO fuel began in the 1960s, mainly in the USA and Germany (1). The technology was originally designed for use in Pebble Bed Reactors (PBRs) and Prismatic Block Reactors (PBRs) (2). Since then, TRISO fuel has been tested and used in several high-temperature reactor projects around the world.

TRISO fuel composition

TRISO fuel consists of three layers of different materials surrounding a uranium particle (3) :

  • A pyrolytic carbon (PyC) layer: This first protective layer provides mechanical strength and a barrier against corrosion.
  • A layer of silicon carbide (SiC): This intermediate layer acts as a barrier against product fission and gases, while providing mechanical strength and corrosion resistance.
  • An outer layer of pyrolytic carbon (PyC): This final protective layer acts as a mechanical barrier and confines fission products.

The particles have the appearance of balls, around 1 millimetre thick. They are then compacted in a graphite powder, forming particle “compacts” 2.5 centimetres high and 1.2 centimetres in diameter.

The advantages of TRISO fuel

These three layers give TRISO fuel several advantages over conventional nuclear fuels:

  • Enhanced safety: The multilayer structure of TRISO fuel effectively confines fission products and gases, considerably reducing the risk of radioactivity release in the event of an accident (4). Corrosion resistance: Silicon carbide and pyrolytic carbon layers offer corrosion resistance, extending fuel life and reducing the risk of fuel failure (5). The U.S. Department of Energy goes so far as to call Triso particles “The most robust fuel on earth”, since they can contain over 99.99% of fission products, even in the event of an accident.
  • Heat tolerance: TRISO fuel can withstand extremely high temperatures, up to 1,600 degrees Celsius (6). This heat tolerance enables HTR reactors to operate at higher temperatures, improving their thermodynamic performance and energy efficiency.
  • Reduced nuclear waste: TRISO fuel offers more efficient uranium utilization and better fuel cycle management, which can reduce the volume of nuclear waste produced (7).

Statistics and facts on TRISO technology

According to the International Atomic Energy Agency (IAEA), several nuclear reactor projects using TRISO technology are under development or in operation worldwide (8). The most advanced projects include :

  • The High Temperature Modular Reactor (HTR-PM) in China, which is currently under construction and is scheduled for commissioning by 2023 (9).
  • EDF’s high-temperature reactor (HTR) project in France, scheduled for commissioning by 2030 (10).
  • Terrestrial Energy’s Integral Molten Salt Reactor (IMSR) in Canada, which also uses TRISO fuel and should be operational by the late 2020s (11).

The modular microreactor developed by French start-up Jimmy would also use this fuel.

In addition, several companies and research organizations, such as Oak Ridge National Laboratory (ORNL) in the USA, are carrying out research and development work to improve TRISO technology and explore potential new applications (12).


TRISO nuclear fuel technology offers significant advantages in terms of safety, performance and nuclear waste reduction. As high-temperature reactors gain in popularity and new reactor projects using TRISO fuel are developed, this technology looks promising for the future of nuclear power.

Challenges to the large-scale deployment of TRISO technology include the establishment of a fuel supply chain, the regulation and certification of new reactors, and public acceptance. Nevertheless, if these challenges are successfully met, TRISO fuel could play a key role in the global energy transition to safer, more sustainable energy sources.

[Article to be revised]


  • (1) World Nuclear Association, “Advanced Nuclear Power Reactors,” 2021.
  • (2) Forsberg, C. W., “High-temperature, gas-cooled reactors,” The American Nuclear Society, 2011.
  • (3) B. J. Merrill, “TRISO Fuel Performance: Modeling and Analysis,” Idaho National Laboratory, 2019.
  • (4) Serco Assurance, “Coated Particle Fuel Technology,” 2006.
  • (5) J. W. Moll, “TRISO Coated Fuel Particle Properties and Design Considerations,” 2008.
  • (6) Verfondern, K. & Nabielek, H., “Status of coated particle fuel development in Germany,” Nuclear Engineering and Design, 2008.
  • (7) World Nuclear Association, “Nuclear Fuel Cycle Overview,” 2021.
  • (8) IAEA, “Advanced Reactors Information System (ARIS),” 2021.
  • (9) World Nuclear News, “China’s HTR-PM reactor achieves first criticality,” 2021.
  • (10) EDF, “The High-Temperature Reactor (HTR) project,” 2021.
  • (11) Terrestrial Energy, “Integral Molten Salt Reactor (IMSR),” 2021.
  • (12) Oak Ridge National Laboratory, “TRISO Fuel Research,” 2021.