A record of nuclear reactor criticality
“Going critical” — achieving a self-sustaining nuclear chain reaction — is the moment a reactor comes alive. This database shows first criticalities since 1942 and forecasts for future reactors coming online soon.
— days since a reactor last went critical — Mochovce-4, August 7, 2026
626criticalities recorded
371GW on the grid now
14in the last 12 months
83under construction
First criticalities per year, 2015–2026
Cumulative reactors to reach first criticality, 1942–present (626 on record)
Recently critical
- Mochovce-4 critical Final unit of a project begun in 1987; sister unit Mochovce 3 started up in 2023
- Groves Isotope Test Reactor critical First advanced reactor project on private land with fully commercially sourced fuel and systems to receive DOE DSA approval; the test platform preceding Atomic Alchemy's planned ~15 MWt VIPR production units, not one of them (Oklo acquired Atomic Alchemy in 2025)
- Changjiang-3 critical First criticality achieved at 18:00 local time on 10 July 2026 (CNNC, via WNN) — the second HPR-1000 unit of Changjiang Phase II; both Phase II units are expected to be fully operational in early 2027
- Aalo-X critical Fourth DOE-authorized reactor to reach criticality (zero-power fueled criticality at 12:20 a.m. MT, July 4, 2026), meeting the EO 14301 Independence Day goal
- Deployable Energy Unity critical Third DOE-authorized criticality under EO 14301; selected under the Nuclear Energy Launch Pad (successor/expansion of the Reactor Pilot Program), not among the original 11 pilot projects; ~150 days from project kick-off to zero-power fueled criticality (DOE announcement implies late June 30)
- Taipingling-2 critical
- Valar Atomics Ward 250 critical First DOE-authorized reactor built outside a national laboratory; DOE-authorized zero-power fueled criticality June 18 (company reported an earlier non-DOE zero-power criticality in Nov 2025); power ascension to 10 kWt on June 22, 2026
- Antares Mark-0 critical First reactor to reach criticality under the DOE Reactor Pilot Program (EO 14301); zero-power fueled criticality — no power conversion or heat removal systems
- ZiaCore critical First heat-pipe, zirconium-hydride-moderated LEU microreactor to reach criticality — a high-temperature (>800°C) zero-power physics validation run over four weeks in April–May 2026 on the Deimos testbed at NCERC in Nevada; a Los Alamos National Laboratory design, announced 30 July 2026
- Saeul-3 critical Formerly Shin-Kori 5; operating licence issued December 2025, NSSC cleared initial criticality 10 April 2026 and it was achieved two days later. Commercial operation expected around August 2026.
Up next
≈76 GW of new electrical capacity is under construction — the reactors closest to first criticality:
Natura MSR-1
target 2026 · low confidence