US-based Kairos Power has completed a three-year graphite qualification testing programme that validates the performance of manufactured ET-10 graphite for use in its advanced reactors. Kairo Power’s strategic partner, Ibiden Co Ltd, provides the high-purity, near-isotropic ET-10 graphite used for the reactor’s reflector blocks and internal structural components.
The two companies recently completed a three-year qualification programme to validate the performance of ET-10 graphite. This involved machining nearly 3,000 specimens to extremely tight tolerances and testing them to destruction for tensile strength and other properties to meet ASME safety standards.
The effort is believed to be the first graphite grade qualified under the ASME Section III, Division 5 HHA-III-4100 design code for high-temperature reactor components. The performance data will support the development, licensing, and deployment of Kairos Power’s Hermes reactor series.
Kairos Power is developing Fluoride Salt-Cooled High-Temperature Reactors (KP-FHR) technology, following an iterative approach, moving from non-nuclear engineering test units (ETUs) to demonstration plants and finally a commercial fleet. First concrete for Hermes 1, a 35 MWt non-power reactor designed to demonstrate nuclear heat production, was poured in May 2025.
The US Nuclear Regulatory Commission (NRC) issued a construction permit for the Hermes Low-Power Demonstration Reactor (Hermes 1) in 2023 – the first non-water-cooled reactor design to be approved for construction in the US in more than 50 years. A construction permit was issued by NRC in November 2024 for Hermes 2, a two-unit plant (35 MWt each) and broke ground for the facility in April. Kairos plans deployment of a larger-scale version of the technology (188 MWt) starting in 2030.
Graphite is a fundamental component of the Hermes reactor design, serving several critical roles. The reactor uses TRISO (Tri-structural Isotropic) fuel particles embedded within a graphite matrix to form golf-ball-sized pebbles. This graphite serves as the “moderator” that slows down neutrons to sustain the nuclear reaction.
In addition, the Hermes reactor core is surrounded by a graphite reflector made of large, machined blocks. This assembly reflects neutrons back into the core to improve efficiency and protects the outer metallic reactor vessel from radiation damage. Because graphite maintains its strength at extremely high temperatures and is compatible with the Flibe (fluoride salt) coolant, it is also used for internal safety-related components that provide structural support within the vessel. It is similarly used in the ETUs.
Because nuclear-grade graphite is manufactured differently by each supplier, reactor developers must carefully test and qualify the specific graphite grades they plan to use. To support this effort, Kairos Power conducted an extensive materials testing programme using ET-10 graphite in collaboration with Ibiden.
The programme evaluated a wide range of physical and mechanical properties, including density, thermal conductivity and expansion, and tensile strength, to ensure the graphite will perform as expected.
“These results statistically validate the material that’s needed to support our reactor design and future licensing activities,” said James McGladdery, Lead Engineer for Graphite Materials at Kairos Power. “Completion of this qualification testing ensures we have a strong technical foundation for the graphite components used in our reactor cores.” The majority of the ET-10 graphite specimens were manufactured by Ibiden and the rest were machined at Kairos Power’s Manufacturing Development Campus in Albuquerque, New Mexico, to meet rigorous nuclear testing standards.
Specialized grips were also developed in-house after progressing through several rapid iterative design cycles to effectively conduct tensile strength testing experiments at Kairos Power’s headquarters in Alameda, California. The resulting data will support materials documentation for the Hermes reactor’s upcoming operating license application and inform future reactor designs.
“Our graphite qualification programme demonstrates the depth of engineering expertise and vertical integration we’ve built at Kairos Power,” said Micah Hackett, Vice President for Fuels & Materials at Kairos Power. “By combining strong supplier partnerships with in-house testing capabilities, we’re able to control cost and accelerate materials qualification for our advanced reactors.”
Kairos Power will continue conducting complementary testing programmes to study graphite performance under reactor conditions.
The company has several agreements in place with NRG Pallas in the Netherlands to perform irradiation experiments using the high flux reactor (HFR) in Petten to help engineers understand how graphite and steel components will perform over the lifetime of a reactor core.
NRG Pallas recently announced the start of a second round of ET-10 irradiation creep tests that will help Kairos Power better predict the performance of structural graphite used in its reactors. The first phase of irradiation-creep testing was completed in November 2025, which were the first experiments of their kind for ET-10 graphite.
Materials (in this case graphite) are irradiated in several stages, with each stage representing a portion of the reactor’s lifetime. Between these steps, the properties of the graphite are accurately measured in the hot cell laboratories at NRG Pallas. This provides a detailed picture of how the material behaves over its entire lifetime providing an essential basis for the reactor’s safety case.
Two types of irradiation tests are being carried out. The ATHENA series, in which the graphite samples are allowed to expand and contract freely, began in mid-February. The earlier ATLAS series graphite was irradiated to determine creep effects. These samples have been analysed and are now being irradiated in follow-up ATLAS-2 tests.
In addition to graphite research, Kairos Power and NRG Pallas are also collaborating on test programmes for metal components and nuclear fuel in the form of Triso fuel particles. The first irradiations are expected in 2027.
In April, NRC approved a request by Kairos Power to extend the latest date for completion of the construction of the Hermes test reactor facility (Hermes 1) from 31 December 2026 to 30 April 2029. Kairos performed a review of the Hermes 1 construction schedule and determined that construction would not be completed by the end of 2026 – latest date for completion of construction prescribed in Construction Permit CPTR–6 issued by NRC.
Kairos now expects construction of the Hermes test reactor facility to be completed in 2028. To accommodate this construction schedule and to incorporate conservatism, Kairos asked to extend the latest date for completion of construction to 2029.
