A record of nuclear reactor criticality

“Going critical” — achieving a self-sustaining nuclear chain reaction — is the moment a reactor comes alive. This is a working record of that moment: every first criticality since 1942, and every reactor on the path to its own, each entry traceable to a primary source.

— days

since a reactor last went critical — Aalo-X, July 4, 2026

criticalities recorded

in the last 12 months

under construction

130

reactors tracked ↓

First criticalities per year, 2015–2026

Recently critical

Aalo-X critical
July 4, 2026 · United States · Microreactor

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
July 1, 2026 · United States · Microreactor

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
June 25, 2026 · China · Power reactor
Valar Atomics Ward 250 critical
June 18, 2026 · United States · Microreactor

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
June 4, 2026 · United States · Microreactor

First reactor to reach criticality under the DOE Reactor Pilot Program (EO 14301); zero-power fueled criticality — no power conversion or heat removal systems
Saeul-3 critical
April 12, 2026 · South Korea · Power reactor

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.
PFBR (Prototype Fast Breeder Reactor) critical
April 6, 2026 · India · Power reactor

India's first commercial-scale fast breeder reactor and the key step in stage two of its three-stage nuclear programme; criticality came roughly 15 years behind original schedule
San'ao-1 critical
February 14, 2026 · China · Power reactor

First of six planned Hualong One units at San'ao and the first Hualong One in operation in the Yangtze River Delta
Taipingling-1 critical
February 3, 2026 · China · Power reactor

First of six planned Hualong One units at Taipingling and the first Hualong One to enter operation in the Greater Bay Area
Zhangzhou-2 critical
November 3, 2025 · China · Power reactor