Saewool Unit 3 shutdown caused by operator error, regulator says

A mistaken entry by an operator caused Saewool Unit 3 to shut down during commissioning last month, South Korea’s nuclear regulator said. The Nuclear Safety and Security Commission has since approved the reactor’s restart so it can complete its remaining tests.

The shutdown occurred during a test conducted while the reactor was operating at 80% power. After electricity transmission was cut off, output fell to 33.9%. As operators worked to raise power again, one operator entered the wrong setting for the position limiter on a turbine control valve.

The limiter sets the maximum opening of the valve and prevents a sudden increase in the amount of steam flowing into the turbine. As reactor power rises and steam production increases, the valve must open further to send more steam to the turbine. The operator instead entered a value that moved the valve in the closing direction.

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Based on the investigation, we determined that Korea Hydro & Nuclear Power’s measures in response to the automatic reactor shutdown and its recurrence-prevention plan were appropriate.

Korea Hydro & Nuclear Power said it would revise testing and operating procedures to prevent a repeat. The company will require another check when the limiter is adjusted, reflect the change in the operating system and provide all nuclear plant operators with training based on the incident.

The company recorded one human-error-related accident or malfunction in 2023, two in 2024 and three in 2025. This year’s Saewool Unit 3 shutdown is the only such case recorded so far, and the second case, including this one, to result in an operational stoppage. The regulator also found that an emergency diesel generator had been connected to a safety bus and that one of four essential chillers had been started manually. It said it would continue checking the remaining commissioning tests and implementation of the corrective measures after the restart.

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We will continue to monitor the remaining commissioning tests and the implementation of the recurrence-prevention measures after approval of the restart.