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 — Tianwan-7, September 28, 2026
628criticalities recorded
374GW on the grid now
14in the last 12 months
82under construction
First criticalities per year, 2015–2026
Cumulative reactors to reach first criticality, 1942–present (628 on record)
Recently critical
- Tianwan-7 critical First VVER-1200 in China to reach criticality — brought to minimum controlled power on 28 September 2026, five years after construction began. Grid connection and a stepped climb to 100% power follow; commercial operation still pending
- 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 Hainan’s first Hualong One unit and the first large PWR built and operated by China Huaneng as controlling shareholder; first criticality at 18:00 local time on 10 July 2026, grid-connected 1 August, commercial operation 17 September 2026. Sister unit Changjiang-4 is still under construction
- 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
Up next
≈74 GW of new electrical capacity is under construction — the reactors closest to first criticality:
Natura MSR-1
target 2026 · low confidence