South Korea's Saeul 3 reactor achieves first sustained chain reaction

Saeul 3 has crossed the point reactor builders wait for most: first criticality, the moment the core can sustain a controlled chain reaction on its own. Korea Hydro & Nuclear Power said the unit hit that milestone on 12 April 2026 at the Saeul nuclear power plant in Ulju County, near Ulsan, moving the 1,400 MW APR1400 pressurised water reactor closer to full operation under its 60-year design life. The unit was formerly known as Shin Kori Unit 5.

The step came after South Korea’s Nuclear Safety and Security Commission completed all nine pre-operation inspections required before initial criticality on 10 April. That clears the way for the next phase, but it is still not grid-ready in the commercial sense. KHNP said output will be raised gradually over about six months, with final safety confirmation continuing through follow-up inspections and power-increase tests before commercial operation in the second half of 2026.

That slow climb matters because first criticality is only the start of the last stretch. The reactor now has to prove it can hold, ramp and shed power in a controlled way while operators and regulators verify performance across a series of tests. Saeul 3 is the third of four APR1400 reactors planned for the Saeul site, and its licensing path has been a long one. South Korea authorized construction of Saeul units 3 and 4 in January 2014, issued the construction licence in June 2016 and began building unit 3 in April 2017. Work was then suspended for three months after the 2017 change in government before a committee voted 59.5% in favor of resuming the project. Before those delays, commercial operation had been expected in March 2021.

The operating licence finally arrived on 30 December 2025, and KHNP said the project involved about 760 companies across equipment manufacturing, construction and maintenance. Once Saeul 3 enters commercial service, it is expected to supply about 1.7% of South Korea’s total power generation and 37% of Ulsan’s electricity demand. That is a meaningful addition in a country that already runs four operational APR1400s, Saeul 1 and 2 and Shin Hanul 1 and 2, while four more APR1400 units at the Barakah plant in the United Arab Emirates are already in commercial operation.