Antares Goes Critical At INL

A private advanced reactor went critical on US soil for the first time in four decades, a second commercial unit cleared the NRC's compressed-track SLR, the Three Mile Island restart drew a preliminary FONSI from the NRC, and Utah picked up a third western-state SMR siting — all inside three weeks.

In context: One year on from the May 2025 nuclear executive orders, the EO-track posture has stopped being a thesis and started producing outputs across every layer of the stack: hardware milestones (Antares), fleet life-extension (Hatch), restart-precedent regulatory clearance (Crane), and new-build siting (Holtec). The remaining question is no longer whether the framework works — it is which projects clear it first and at what cost.

This week at a glance

Antares Mark-0 achieves zero-power criticality at INL — first private advanced reactor under DOE Reactor Pilot Program

First non-light-water reactor to go critical on US soil in more than 40 years and the 53rd reactor ever built at INL since 1951. Hits the EO 14301 July 4 criticality target a month early, backed by $140M+ in private capital.
NRC approves Hatch subsequent license renewal in under 12 months — second SLR under EO 14300's compressed track

Both Hatch units extended to 2054 and 2058. Follows Robinson by seven weeks. Nine Mile Point-1 SLR decision targeted April 2027; Cooper accepted in early June with June 2027 target.
NRC issues preliminary EA and draft FONSI for the Crane Clean Energy Center restart

Constellation's TMI-1 restart anchored by the 20-year Microsoft PPA clears its NEPA gate without a full EIS. Public comment open through July 8; final EA expected September; restart decision in 2027.
Holtec SMR-300 selected to anchor the Green River Advanced Nuclear Project in Utah

Adds Utah to the western SMR cluster alongside Wyoming (Natrium Kemmerer) and Texas (Xe-100 Seadrift, Last Energy PWR-5). Another confirmed SMR-300 siting after the Palisades twin-unit pairing.

In depth

Reference — new this issue

AOR — Authority of Reactor Operation; DOE-issued criticality authorization that substitutes for NRC Part 50/52 within the Reactor Pilot Program scope, on DOE land. Antares went critical under an AOR, not an NRC license

DSA — Documented Safety Analysis; the safety case that supports an AOR. Comparable to the FSAR for an NRC operating license. Antares, Aalo, Valar each cleared DSA in May; Oklo Aurora cleared the preliminary DSA on June 5

Technology

Antares Goes Critical at INL — First Non-LWR on US Soil in 40+ Years

Before: EO 14301 stood up the DOE Reactor Pilot Program in May 2025 with a July 4, 2026 target for at least three private-developer advanced-reactor criticalities. Eleven projects were originally selected. By late May, Antares, Aalo, and Valar were the three with approved Documented Safety Analyses, but no private US advanced reactor had yet cleared the DOE-authorized pathway that bypasses NRC Part 50/52 — the entire premise of the pilot.

Now: On June 4 Antares Nuclear's Mark-0 microreactor achieved zero-power criticality at Idaho National Laboratory, becoming the first private advanced reactor authorized and to go critical under the Reactor Pilot Program and the 53rd reactor ever built at the INL site since 1951. The build is backed by $140M+ in private capital and landed a month ahead of the EO deadline. The day after Antares went critical, DOE-Idaho approved Oklo's Preliminary Documented Safety Analysis for Aurora-INL under the same Pilot Program track, queuing Oklo as the next candidate.

→ Aalo and Valar's July 4 criticality attempts; Oklo Aurora's PDSA-to-DSA pace under Jacob DeWitte; whether the first Part 53 commercial-license applicants (X-energy, TerraPower, Kairos) cite Pilot data as precedent

Regulatory

Hatch SLR Clears in Under 12 Months — the Compressed Track Outlives Its Robinson One-Off Phase

Before: We covered Robinson's subsequent license renewal in Issue 5 as the fastest SLR in NRC history — the first to clear the 12-month review window EO 14300 established for the existing fleet. The open question was whether the compressed posture would hold across the broader SLR pipeline or quietly slip back toward the multi-year baseline.

Now: On June 12 the NRC approved Southern Nuclear's SLR for Plant Hatch Units 1 and 2 in under 12 months — the second and third reactors cleared on the compressed track, with Federal Register publication following June 16. The renewals extend Hatch operations to 2054 (Unit 1) and 2058 (Unit 2). The pipeline behind them is now visible: NRC accepted Constellation's Nine Mile Point-1 SLR application in April with a decision target of April 2027, and NPPD's Cooper SLR was accepted in early June with a June 2027 target. The 12-month posture is no longer one utility's one-off — it is the working pace for the whole 60-to-80 extension lane.

→ Constellation's Nine Mile-1 and NPPD's Cooper hitting their 2027 targets without slippage; PG&E's Diablo Canyon routing under the EO sunset clock; APS's Palo Verde as the next likely SLR filer in the compressed lane

Regulatory

Crane (TMI-1) Restart Clears NEPA Without a Full EIS — the Long Mott Pathway Goes Mainstream

Before: Three Mile Island Unit 1 shut down in 2019 on economic grounds. Constellation announced the restart in September 2024, backed by a 20-year power-purchase agreement with Microsoft for the unit's roughly 835 MW. The plant was renamed the Christopher M. Crane Clean Energy Center in honor of the late Exelon CEO, restart targeted for 2027, and the open regulatory question was the depth of NRC's environmental review — full Environmental Impact Statement or the lighter EA/FONSI pathway the NRC just used at Long Mott (Issue 7).

Now: On June 8 the NRC released for public comment a draft Environmental Assessment and preliminary Finding of No Significant Impact for the proposed reauthorization of power operations at Crane. NRC staff's preliminary determination is that no EIS is warranted. The 30-day public-comment window runs through July 8, the final EA and FONSI are expected in September, and the final restart decision is expected in 2027. Two days earlier, on June 1, FERC approved Constellation's waiver to transfer interconnection rights to Crane — the parallel grid-side unlock the NRC track depended on.

→ Intervenor filings during the July 8 comment window; the final FONSI hitting its September target; Duane Arnold (now inside NextEra-Dominion's post-merger fleet) as the next restart candidate routing through the same EA-not-EIS pathway

Construction

Holtec SMR-300 Picks Up Green River, Utah — the Western Cluster Adds a Third State

Before: Holtec's SMR-300 small modular reactor had Palisades as its anchor siting, with the twin-SMR pairing tied to the Palisades large-LWR restart. The open question was whether Holtec's design could win utility-grade siting decisions outside the Michigan home base — specifically, in the western US load center where TerraPower (Wyoming), X-energy (Texas), and Last Energy (Texas) had already planted flags.

Now: Holtec announced in late May that its SMR-300 design was selected to anchor the Green River Advanced Nuclear Project in Green River, Utah. The selection adds Utah to a western US nuclear cluster that now spans Wyoming (TerraPower Natrium at Kemmerer), Texas (Xe-100 at Dow Seadrift, Last Energy PWR-5 at Texas A&M-RELLIS), and Utah (SMR-300 at Green River). It follows the Palisades Pioneer 1 & 2 twin-unit pairing and converts the SMR-300 from a single-utility design into a multi-utility one.

→ SMR-300 final NRC design certification timing under Kelly Trice; NuScale's VOYGR and GE-Hitachi's BWRX-300 next siting wins; TVA's Clinch River decision (NuScale 6 GW with ENTRA1) as the bellwether for which design takes the western cluster's next anchor slot

Stat of the week

40+ years

Time since the last non-light-water reactor achieved criticality on US soil before Antares Mark-0 went critical at INL on June 4. The Mark-0 is also the 53rd reactor ever built at the laboratory since 1951.

— Department of Energy, June 2026

Global signal

Japan — METI proposes replacing 11–14 aging reactors by the 2050s, first explicit number since Fukushima

On June 5 Japan's METI circulated a draft policy to replace up to 14 aging reactors by the 2050s — the first hard number Tokyo has put on new builds since Fukushima. Phased: 2–5.5 GW by the 2040s, 12.7–16 GW by the 2050s. For US professionals: the same balance sheet anchoring the March Trump-Takaichi $40B BWRX-300 deal in TN and AL is now committing to its own domestic build-out — and the Japanese reactor-supply pull-through (GE Vernova-Hitachi, Mitsubishi) lands on the same US fabricators as the Pilot Program.

Forward look

Jul 4 DOE Reactor Pilot Program criticality deadline — Aalo and Valar are the two remaining DSA-approved projects with a path to clearing the window; Antares is already done
Jul 8 Crane EA/FONSI public-comment period closes — intervenor filings are the next signal on whether the final FONSI ships on its September target
Aug–Sep Oklo Aurora-INL DSA target — the next Reactor Pilot Program criticality candidate after Antares, with the PDSA approved June 5
Sep Final Crane EA/FONSI expected from NRC — the gate that determines whether the 2027 restart timeline holds

Companion tool: fuel.jrand.net — live UF₆ conversion capacity, by facility.

Until next Thursday,

— Jace Arnold

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