'Automatic Shutdown at Saeul Unit 3' Traced to Human Error... NSSC Approves Recommissioning

The cause of the automatic shutdown of Saeul Unit 3 during commissioning on August 11 has been identified as human error. On September 4, the Nuclear Safety and Security Commission (NSSC) announced that it had concluded its investigation into the Saeul Unit 3 incident and approved its restart for further commissioning tests.

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According to the NSSC, the investigation found that the incident occurred when an operator incorrectly entered the setting value for the turbine control valve position limiter while restoring reactor output (at that time 33.9%) after a commissioning test that involved shutting off power transmission at 80% of reactor output.

The turbine control valve position limiter is designed to set an upper limit on how much the turbine control valve can open, preventing a sudden surge in the amount of steam entering the turbine. At the time of the incident, the operator inputted the limiter setting to prevent unexpected fluctuations in the turbine generator’s output. However, the value was mistakenly entered in the direction that closes the valve, which led to the automatic shutdown of the reactor.

Korea Hydro & Nuclear Power Co., Ltd. (KHNP) has implemented measures to prevent human errors such as this one, including revising the procedures for relevant tests and operations, as well as providing special training to operators.

The NSSC also stated that during the incident investigation, it confirmed that the emergency diesel generator was connected to the safety bus, and that one of the four essential chillers (safety equipment) was manually started. The committee reviewed the appropriateness of the deployment of the emergency diesel generator, among other points.

The NSSC commented, "Based on these findings, we have confirmed that KHNP’s overall response and recurrence prevention measures regarding this automatic reactor shutdown are appropriate. Even after approving the recommissioning of Saeul Unit 3, we will continue to closely monitor the remaining commissioning tests and the implementation of recurrence prevention measures."

Saeul Unit 3, which received its operating license from the NSSC in December last year, underwent automatic shutdown during trial operations on August 11. The NSSC has been conducting its investigation, and Saeul Unit 3 is aiming to begin commercial operation within this year.

This content was produced with the assistance of AI translation services.