goingcritical:kairos-hermes-1United States · Research reactor
Hermes 1
Status: Under construction target criticality 2028 (confidence: low) Reactor status full glossary → Announced Publicly announced — a site and design are proposed, but construction has not begun. Site preparation Early site and civil works are under way; the first safety-related ("nuclear") concrete has not yet been poured. Under construction Formal construction has begun, marked by the first nuclear concrete. Fuel loaded Nuclear fuel has been loaded into the core. The reactor is fuelled but has not yet gone critical. Critical Has reached first criticality — a self-sustaining chain reaction, usually at low power — but is not yet connected to the grid. Grid connected Synchronised to the electrical grid and exporting power for the first time, but still in commissioning and power-ascension testing — not yet in commercial service. In commercial operation — the operator has formally declared the unit in regular commercial service. Shut down Permanently shut down and no longer generating; may be defuelled, undergoing decommissioning, or preserved in place. Canceled Abandoned before completion.
Site: Heritage Center Industrial Park, Oak Ridge, Tennessee
First Gen IV reactor under NRC-licensed construction in the US (construction permit issued Dec 2023); groundbreaking July 2024, safety-related first concrete May 1, 2025
Kairos expects construction completion in 2028; NRC extended the latest construction completion date from Dec 31, 2026 to Apr 30, 2029 (May 2026)
Details
| Design | KP-FHR fluoride salt-cooled high-temperature test reactor (35 MWt, non-power) |
| Capacity | 0 W (thermal where reported) |
Milestone history
- Announced —
- Construction start May 1, 2025
- Fuel load —
- First criticality target 2028