Hermes 1 (Kairos Power test reactor)

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Current state

Notes

Non-power unit: Hermes is a 35 MWt (thermal) fluoride salt-cooled test reactor with no electrical output (Kairos: 'Hermes will not produce electricity'), so its net electrical capacity is recorded as 0 MW and it adds nothing to tracker GW totals. Kairos and DOE describe it as the first non-water-cooled (non-light-water) reactor approved for construction in the US in more than 50 years. Hermes 1 precedes and is co-located with Hermes 2, Kairos's two-unit electricity-producing demonstration plant on the same Oak Ridge campus, which builds on Hermes lessons learned. Reactor type is FHR (fluoride salt-cooled high-temperature reactor): solid TRISO fuel with molten fluoride salt as coolant, not a molten salt reactor with fuel dissolved in the salt. Construction start is the July 17, 2024 excavation start; nuclear safety-related construction began May 1, 2025. Barnard Construction is the heavy-civil site and excavation contractor, not an EPC, so no EPC is recorded. The original target is DOE's December 2023 statement that Kairos planned Hermes to be operational as early as 2026; Kairos's July 2024 release targeted 2027. No current operation year is recorded: the NRC record says Kairos expects construction completion in 2028, and Kairos says only that its Oak Ridge facilities are expected to be operational before the end of the decade. A separate NRC operating license is required before Hermes can operate.

Milestone log

• 2023-12-14 — NRC issued Construction Permit No. CPTR-6 (Docket 50-7513) for the Hermes test reactor facility in Oak Ridge, Tennessee, with December 31, 2026 as the latest date for completion of construction (source)
• 2024-07-17 — Barnard Construction began site work and excavation at the Hermes site; Kairos announced the start of construction on July 30, 2024, with DOE investing up to $303 million through the Advanced Reactor Demonstration Program (source)
• 2025-05-01 — Nuclear safety-related construction, subject to NRC oversight, began with the building foundation; Kairos announced the first safety-related concrete placement on May 8, 2025 (source)
• 2026-05-13 — NRC issued an order amending CPTR-6 to extend the latest date for completion of Hermes construction from December 31, 2026 to April 30, 2029, on Kairos's March 24, 2026 request; Kairos expects construction to be completed in 2028 (source)

The milestone log is append-only: corrections are recorded as new entries, never rewrites. Full history via /api/v1/changes.

Sources

1. https://www.govinfo.gov/content/pkg/FR-2023-12-20/html/2023-27960.htm
2. https://www.kairospower.com/updates/nuclear-regulatory-commission-approves-construction-permit-for-hermes-demonstration-reactor
3. https://www.energy.gov/ne/articles/nrc-approves-construction-hermes-reactor
4. https://www.kairospower.com/updates/kairos-power-begins-construction-on-hermes-low-power-demonstration-reactor
5. https://www.energy.gov/ne/articles/kairos-power-starts-construction-hermes-reactor
6. https://www.kairospower.com/updates/kairos-power-begins-nuclear-safety-related-construction-of-hermes-low-power-demonstration-reactor
7. https://public-inspection.federalregister.gov/2026-07700.pdf
8. https://public-inspection.federalregister.gov/2026-09880.pdf
9. https://www.kairospower.com/locations/tennessee
10. https://wnn.world-nuclear.org/articles/regulator-extends-hermes-1-reactor-construction-deadline