South Korea has cleared a major milestone for its next-generation nuclear fleet, bringing the Saeul Unit 3 reactor closer to operation.

The country’s Nuclear Safety and Security Commission (NSSC) confirmed it has completed all required inspections before the reactor’s first criticality, a key step toward commercial power generation.

The regulator said the results show the reactor can safely achieve a sustained nuclear chain reaction.

That milestone marks the transition from construction and testing to active operation.

Final checks before criticality

The NSSC began pre-operational inspections after issuing an operating license on Dec. 30 last year.

These checks focused on systems required before the reactor reaches criticality.

“Since the operating licence was issued last year, the NSSC has been conducting pre-operational inspections (5 stages) on Saeul unit 3,” the regulator said, as reported by World Nuclear News.

“During this process, inspections were conducted on items that must be performed before criticality, such as nuclear fuel loading inspections and high-temperature functional tests. As a result, it was confirmed that reactor criticality can be safely achieved.”

Officials reviewed nuclear fuel loading, equipment integrity, and high-temperature performance.

They verified that all systems met regulatory standards.

The commission also confirmed completion of all nine mandatory inspections required before first criticality.

These checks ensure the reactor can enter a stable fission state, where neutron production and loss remain balanced.

Long path to completion

Saeul Units 3 and 4 use South Korea’s APR1400 reactor design.

The government approved both units in January 2014, when they were known as Shin Kori 5 and 6.

Construction faced delays from the start. Authorities issued a construction license in June 2016, and work began soon after.

Unit 3 construction started in April 2017 but paused for three months following a change in government.

A public committee later voted 59.5% in favor of resuming construction.

Officials cited power supply stability as a key reason behind the decision. Work resumed in late 2017, and Unit 4 construction began in September 2018.

The project missed its original targets of March 2021 and March 2022 for commercial operation.

Now, Unit 3 is expected to begin operations around August 2026, with Unit 4 following later that year.

The NSSC will now move into the next phase of testing. These include power ascension tests and system validation under operating conditions.

The regulator plans to carry out 31 additional inspections after first criticality.

These tests will continue until just before commercial operation.

“The NSSC said it plans to finally confirm the safety of the unit by conducting follow-up inspections, including power increase tests, from the time Saeul 3 achieves first criticality – a sustained chain reaction – until it enters commercial operation.”

Officials will also verify emergency response systems during commissioning.

They aim to ensure the plant can handle accidents or failures at the same level as active nuclear facilities.

Once operational, Saeul Unit 3 will supply about 1.7% of South Korea’s total electricity.

It will also meet roughly 37% of Ulsan’s regional demand.

South Korea continues to expand its APR1400 fleet, including export success in the United Arab Emirates.

The Saeul project reflects the country’s effort to balance energy security with nuclear safety oversight.

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