JAEA High-temperature Engineering Test Reactor. Image: Japan Atomic Energy Agency.
The Loss of Forced Coolant (LOFC) joint project is a collaborative international initiative aimed at advancing the safety evaluation of high-temperature gas-cooled reactors (HTGRs). Initiated following recommendations from the CSNI Task Group on Advanced Reactor Experimental Facilities (TAREF), the project addresses a long-standing need for research in this area. The High-Temperature Engineering Test Reactor (HTTR), operated by the Japan Atomic Energy Agency (JAEA), was identified as a unique and essential test platform, being the only operational experimental HTGR within NEA member countries.
The project involves organisations from Czechia, France, Germany, Hungary, Japan, Korea, and the United States, with a shared budget of €3 million.
Following the first test in December 2010, the project was paused after the Great East Japan Earthquake and tsunami of March 2011. After safety enhancements and regulatory approval, the HTTR restarted in July 2021. The second and third tests were conducted in January 2022 and March 2024, respectively.
To further strengthen the scientific value of the project, a code benchmark exercise is being conducted based on one of the LOFC tests. Consequently, the project has been extended through March 2027.
The HTTR facility
The HTTR is a helium-cooled, graphite-moderated prismatic block HTGR with a thermal output of 30 MW. It supports operations at 850°C and 950°C and has demonstrated its ability to reach temperatures exceeding 1 600°C under transient conditions. Equipped with advanced instrumentation and engineered safety systems, the HTTR is a critical platform for validating simulation codes and demonstrating the passive safety capabilities of HTGRs, especially in loss-of-forced-cooling (LOFC) scenarios.
LOFC tests and relevance
The LOFC tests explore the reactor’s behavior during anticipated transients without SCRAM (ATWS) — scenarios involving a full cessation of forced helium flow without triggering automatic reactor shutdown.
Three major test scenarios have been successfully completed:
- 9 MW LOFC with vessel-cooling system (VCS)
- 30 MW LOFC with VCS
- 9 MW LOFC without VCS
In each case, the tests were initiated by tripping all helium gas circulators while disabling reactivity control systems. This allowed the reactor to enter a re-critical state before passive safety features stabilised the system. The resulting data is essential for validating thermal-hydraulic and reactor kinetics models and is directly applicable to licensing and safety evaluations of HTGRs.
Results of safety demonstration tests. Visual: Japan Atomic Energy Agency (JAEA).
Key achievements:
- Execution of three full-scale LOFC experiments: Delivering unprecedented data on HTGR response to loss-of-cooling scenarios.
- Validation of simulation codes: Enhancing the reliability of thermal-hydraulic and kinetics models used in safety assessments and reactor licensing.
- Initiation of an international code benchmark exercise: Enabling rigorous cross-comparison of predictive models against high-fidelity experimental data.
The LOFC project is a cornerstone in the international effort to evaluate and validate the safety case for next-generation nuclear technologies, including small modular reactors (SMRs) and microreactors. The HTTR’s design and performance also support high-temperature applications beyond electricity generation — such as hydrogen production and industrial heat supply — highlighting its relevance to future energy systems.
The LOFC joint project, now extended through 2027, continues to serve as a model of international technical collaboration under the NEA framework. A key milestone was the expert meeting hosted by the US Nuclear Regulatory Commission in March 2025, where recent findings were reviewed. As the project progresses toward completion, its outcomes are expected to significantly bolster international capabilities in advanced reactor modeling, safety analysis, and regulatory decision making.
Background
For nearly seven decades, the OECD Nuclear Energy Agency (NEA) has co-ordinated close to 60 international nuclear safety research projects, significantly advancing nuclear safety and fostering innovation across the sector. These projects demonstrate the enduring commitment of NEA member countries to collaborate in addressing critical safety challenges by pooling expertise, data and resources.
The NEA joint experimental projects are designed to close key knowledge gaps, support the safe operation of nuclear facilities and preserve vital research infrastructure and capabilities. These initiatives engage national safety authorities, industry, and research organisations to generate experimental data that informs international safety standards and regulatory frameworks.
