The Nuclear Safety and Security Commission completed its investigation into Unit 3 of the Saeul Nuclear Power Plant, which automatically shut down during commissioning in August, and approved its restart. The investigation found that during the process of restoring reactor power, an operator incorrectly entered a turbine control valve-related setting value, which was found to be the cause of the shutdown.
The Nuclear Safety and Security Commission said on Dec. 4 that it had completed the incident investigation of Saeul Unit 3 and approved a restart for the remaining commissioning tests. Saeul Unit 3 automatically shut down during a commissioning test on Aug. 11.
At the time, Saeul Unit 3 had conducted a test to cut off power transmission at 80% reactor power and was in the process of raising power again. When reactor power had recovered to 33.9%, an operator entered the setting of the turbine control valve position limiter to limit unexpected turbine generator output fluctuations, but the reactor automatically shut down because the value was incorrectly set in the valve-closing direction, the investigation was found. The turbine control valve position limiter is a device that sets the maximum opening range of the valve to limit a rapid increase in the amount of steam flowing into the turbine.
Korea Hydro & Nuclear Power Co. (KHNP) viewed this case as human error that occurred during operations and decided to supplement related test and operation procedures. It will revise procedures to require rechecking the setting value when operating the turbine control valve position limiter and reflect this in the operating system, while also conducting training and education on this case for all nuclear plant operators.
The Nuclear Safety and Security Commission also confirmed during the investigation that the emergency diesel generator was connected to the safety bus and that one of the four essential chillers, which are safety equipment, was started manually. Accordingly, it additionally reviewed whether the process of engaging the emergency diesel generator was appropriate.
The Nuclear Safety and Security Commission reviewed the investigation results and the measures submitted by KHNP and determined that a restart was possible. However, it will continue to check during the remaining commissioning tests after the restart approval and whether the recurrence prevention measures are actually implemented.