Kairos wins permits for Hermes 2, a salt-cooled test plant meant to make electricity

November 2024 — The U.S. Nuclear Regulatory Commission issued construction permits CPTR-7 and CPTR-8 to Kairos Power on Nov. 21, 2024, for the two units of its Hermes 2 test reactor facility in Oak Ridge, Tennessee. Here's what happened, what it cost, and what went wrong.

Cost and outcome (as of Nov. 29, 2024)

Outcome: two immediately effective construction permits, issued Nov. 21, 2024. (Federal Register)
Size: two low-power test reactors of 35 MW thermal each, sharing one steam power conversion system. (NRC project page)
Review time: application tendered July 14, 2023; permits issued Nov. 21, 2024. (NRC project page)
Cost: not disclosed.
Status: construction permits only; Kairos must still apply for and win an operating licence. (Kairos Power)

What the NRC decided

The permits authorize Kairos to build the test reactor facility described in its application in Oak Ridge. The NRC found that "the applicable standards and requirements" of the Atomic Energy Act and its rules had been met, and that the facility "can be constructed and operated at the proposed location without undue risk to public health and safety, subject to the conditions listed in the construction permits." (Federal Register)

It also found Kairos "technically and financially qualified" for the authorized activities. The Commission issued its decision on the mandatory hearing on Nov. 20, 2024, and the NRC staff prepared a safety evaluation, an environmental assessment and a finding of no significant impact. (Federal Register)

The design

The NRC describes Hermes 2 as "two low-power test reactors" to support Kairos's fluoride salt-cooled, high-temperature reactor technology. The fuel is TRISO particles embedded in carbon pebbles, using high-assay low-enriched uranium. Containment relies mainly on the fuel particles themselves, which the NRC calls "functional containment." Each unit is rated at 35 MW thermal, and the two share a Rankine-cycle steam system. (NRC project page)

Kairos says Hermes 2 will "demonstrate complete plant architecture at a reduced scale and supply clean electricity to the grid." It calls Hermes 2 "the first electricity-producing Gen IV plant to be approved for construction in the United States." That is the company's framing. (Kairos Power)

How the review went

The NRC's milestone table shows the application tendered July 14, 2023, docketed Sept. 11, 2023, a final safety evaluation on July 19, 2024, a final environmental assessment on Aug. 30, 2024, the hearing on Nov. 20 and the permits on Nov. 21, 2024. (NRC project page)

Kairos credits the pace, "just over one year" for the staff review, to pre-application talks with the NRC dating back to 2018, process changes tried on its first Hermes permit, a simplified mandatory hearing, and the two projects sharing a site. The plant will sit on land Kairos bought in 2021 next to the first Hermes reactor, which was then under construction. (Kairos Power)

Where Hermes 2 fits

Kairos, founded in 2016, says it uses a "rapid iterative development approach" and builds much of its own hardware. Its first Hermes reactor received an NRC construction permit in 2023, which Kairos describes as the first for a non-water-cooled reactor in the U.S. in more than 50 years. (Kairos Power)

"The licensing basis established with both the Hermes and Hermes 2 construction permits will carry forward to future license applications," said Peter Hastings, Kairos vice president of regulatory affairs and quality. (Kairos Power)

What went wrong

Not yet licensed to run. Under the two-step Part 50 process, Kairos must still win an operating licence before start-up. (Kairos Power)
Small output. These are low-power test reactors at 35 MW thermal each, not commercial units. (NRC project page)
Self-described first. The "first electricity-producing Gen IV plant" claim is Kairos's own.
What we don't know yet: the build cost, who is paying, and when Hermes 2 will make electricity. None appear in these sources.

Our follow-up question for Kairos Power

What will Hermes 2 cost to build, and when do you expect to apply for its operating licence?

Sources

Federal Register, “Kairos Power LLC; Hermes 2 Test Reactor Facility; Construction Permits,” Nov. 29, 2024. federalregister.gov
U.S. Nuclear Regulatory Commission, “Hermes 2 – Kairos Application,” project page, accessed Oct. 29, 2026. nrc.gov
Kairos Power, “Nuclear Regulatory Commission Approves Construction Permits for Hermes 2 Demonstration Plant,” Nov. 20, 2024. kairospower.com

Disclosure: [not in copy] Editor placeholder