US-based Kairos Power has completed Phase 1 of its Engineering Test Unit 3 (ETU 3) hardware demonstration. This advances iterative testing of its Fluoride Salt-Cooled High-Temperature Reactor (KP-FHR) technology in a non-hazardous environment.
In this first phase of the ETU 3 project, a cross-functional team spanning construction, manufacturing, and engineering departments worked with key partners to build a reduced-scale mock-up of the Hermes 2 reactor cavity.
The mock-up will be used to test modular shielding designs and remote maintenance capabilities for the KP-FHR reactor. The data gathered from ETU 3 directly informs the deployment of the Hermes 1 Low-Power Demonstration Reactor and the Hermes 2 Demonstration Plant, which broke ground at the East Tennessee Technology Park in Oak Ridge, Tennessee earlier this year. Eventually, these standardised modular components will be used across Kairos Power’s commercial fleet.
Hermes 1 and Hermes 2 are co-located advanced nuclear demonstration reactors being built by Kairos Power at the oak ridge campus. They use KP-FHR technology, combining TRISO fuel pebbles with a molten Flibe salt coolant to achieve robust inherent safety at low pressure. While they share the same basic architecture, they serve entirely different roles in Kairos Power’s commercialisation timeline
Hermes 1 was approved by the Nuclear Regulatory Commission (NRC) in late 2023 – the first non-water-cooled reactor approved for construction in the US in over 50 years. It began construction in May 2025 and is thermal only (35 MWt) producing no electricity, with heat dissipated through a dump condenser. Its sole purpose is to prove that the KP-FHR nuclear system functions reliably under real nuclear operations. Expected completion date is 2028–2029. In May 2026, NRC extended its construction window to April 2029 to accommodate iterative engineering modifications. It is funded by Kairos Power and the Department of Energy (DOE).
Hermes 2 represents the transition to real-world deployment. It is the first-ever power-producing Generation IV reactor to receive an NRC construction permit. The plant consists of two 35 MWt reactor units that together will supply 50 MWe of electricity to the Tennessee Valley Authority (TVA) grid. It serves as the initial deployment under a larger master agreement with Google, which aims to bring up to 500 MWe of nuclear energy online to power its data centres in Tennessee and Alabama. Construction began in April with operation targeted for 2030.
The ETUs are a series of three reactor-scale, non-nuclear hardware demonstration platforms based on a philosophy of “rapid iterative development”. Kairos Power uses these units to test components, train workers, and iron out construction flaws in a non-hazardous environment before ever loading live nuclear fuel. Each ETU has been built to systematically solve a different engineering challenge.
ETU 1, built at Kairos Power’s manufacturing development campus in Albuquerque (New Mexico), focused on fluid dynamics and chemistry. It completed more than 2,000 hours of pumped Flibe (fluoride-lithium-beryllium) molten salt operations proving startup, salt handling, and online refuelling systems before being decommissioned.
ETU 2, also built in Albuquerque, focused on modular manufacturing to validate the vertical integration strategy by building over 30 separate equipment modules and testing full in-house reactor vessel fabrication. Work was completed earlier this year.
ETU 3, built in Oak Ridge, focuses on civil construction and safety. The team tested 12 modular precast concrete wall structures surrounding the reactor cavity mock-up to assess radiation shielding designs. The unit also served as a proving ground for the remote handling tools needed to maintain and replace reactor components safely. Operating a non-nuclear mock-up allows the team to de-risk interfaces between heavy structures and reactor equipment before active build phases.
Oak Ridge National Laboratory’s (ORNL) Manufacturing Demonstration Facility helped design and produce the 3D-printed polymer composite forms for the precast panels, which were later fabricated at Tindall Corporation in South Carolina and installed by Barnard Construction. The ETU 3 reactor vessel was fabricated in partnership with UK-based Cambridge Vacuum Engineering (CVE) and the University of Sheffield’s Advanced Manufacturing Research Centre (AMRC), using electron beam welding (EBW) technology.
The ETU 3 testing facilities will also host the newly launched Nuclear Center for Advanced Manufacturing and Precast (NuCAMP) initiative, partnering with ORNL and the University of Tennessee to train the local advanced nuclear workforce. The initiative was established under a Memorandum of Understanding (MOU) signed in August to bridge the gap between advanced reactor design and repeatable, real-world construction.
“Completing this initial demonstration with Engineering Test Unit 3 is an important step toward establishing a sustainable maintenance concept for our future fleet,” said Ed Blandford, co-founder and Chief Technology Officer at Kairos Power. “By demonstrating the interfaces between modular shielding structures and reactor equipment, and testing the remote handling tools needed to maintain and replace components, we gain a better understanding of design features that will be standardized in commercial KP-FHR deployments.”
