High-temperature nuclear reactors (HTR)

High-temperature nuclear reactors (HTRs) are an advanced type of reactor that operate at much higher temperatures than conventional nuclear reactors. These reactors offer significant advantages in terms of energy efficiency, safety and application flexibility.

High-temperature nuclear reactors (HTRs) offer significant advantages in terms of energy efficiency, safety and application flexibility. With new HTR projects under development and construction, this technology looks promising for the future of nuclear power, and could play a key role in the global energy transition to safer, more sustainable energy sources.

Challenges to the large-scale deployment of HTRs include the establishment of a fuel supply chain, regulation and certification of new reactors, and public acceptance. Nevertheless, if these challenges are successfully met, HTR reactors could make a significant contribution to diversifying the energy mix and reducing greenhouse gas emissions.


The development of HTR reactors began in the 1950s, with research and development projects in the USA, Germany and the UK (1). The first HTRs were mainly Pebble Bed Reactors (PBRs) and Prismatic Block Reactors (PBRs) (2). Since then, several HTR prototypes and reactors have been built and operated worldwide.

Features of HTR reactors

HTR reactors differ from conventional reactors in several key features:

  • High temperatures: HTR reactors operate at much higher temperatures than conventional reactors, typically between 700 and 1000°C (3). These high temperatures improve thermodynamic efficiency and pave the way for new industrial applications.
  • TRISO fuel: HTR reactors generally use TRISO (Tri-Structural Isotropic) fuel, which offers greater safety, corrosion resistance and heat tolerance than conventional nuclear fuels (4).
  • Passive safety design: HTR reactors are designed with passive safety systems that exploit the natural properties of materials and physical phenomena to ensure safety without the need for active intervention or external energy (5).
  • Flexibility of application: Thanks to their ability to operate at high temperatures, HTR reactors can be used to generate electricity, process heat for industry or produce hydrogen from water (6).

HTR statistics and facts

Several HTR reactors have been built and operated around the world in recent decades. Notable examples include:

  • The AVR experimental reactor in Germany, which operated from 1967 to 1988 and demonstrated the viability of graphite ball HTR reactors (7).
  • The THTR-300 reactor in Germany, which operated from 1983 to 1989 and was designed to use prismatic fuel elements (8).
  • The High Temperature Experimental Reactor (HTTR) in Japan, which was commissioned in 1998 and continues to serve as a test bed for the development of HTR reactors (9).

In addition, several HTR projects are under development or construction:

  • The High Temperature Modular Reactor (HTR-PM) in China, which is currently under construction and is scheduled for commissioning by 2023 (10).
  • EDF’s high-temperature reactor (HTR) project in France, scheduled for commissioning by 2030 (11).

According to the International Atomic Energy Agency (IAEA), more than 20 HTR projects are under development or in operation worldwide (12).

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Sources :

  • (1) World Nuclear Association, “Advanced Nuclear Power Reactors,” 2021.
  • (2) Forsberg, C. W., “High-temperature, gas-cooled reactors,” The American Nuclear Society, 2011.
  • (3) International Atomic Energy Agency (IAEA), “High Temperature Gas Cooled Reactor Fuels and Materials,” 2010.
  • (4) B. J. Merrill, “TRISO Fuel Performance: Modeling and Analysis,” Idaho National Laboratory, 2019.
  • (5) International Atomic Energy Agency (IAEA), “Passive Safety Features of High Temperature Gas Cooled Reactors,” 2018.(6) World Nuclear Association, “Nuclear Process Heat for Industry,” 2021.
  • (7) K. Verfondern and R. Nabielek, “The AVR Pebble Bed Reactor: Summary of Operational Experience,” Nuclear Engineering and Design, 2006.
  • (8) H. J. Rütten and H. Nabielek, “THTR-300: Construction, Operation and Decommissioning,” Nuclear Engineering and Design, 2009.
  • (9) Japan Atomic Energy Agency, “The High Temperature Engineering Test Reactor (HTTR),” 2021.
  • (10) World Nuclear Association, “Nuclear Power in China,” 2021.
  • (11) EDF, “High-Temperature Reactor (HTR) Project,” 2021.
  • (12) International Atomic Energy Agency (IAEA), “Advanced Reactors Information System (ARIS),” 2021.