South Korea's nuclear regulators are investigating how a reactor in the southwest of the country was not shut down after its output exceeded the limit in its technical specifications.
The Nuclear Safety and Security Commission says the incident occurred on May 10 at the Hanbit Nuclear Power Plant in Jeollanam-do province.
The reactor's thermal output quickly exceeded the 5 percent limit after an unlicensed worker experimentally removed a control rod from Unit One as part of regular inspections.
Output temporarily rose to 18 percent, but the reactor kept operating for about 11 and a half hours before being shut down manually.
The commission says there was no leakage of radioactive material, and output dropped below the limit when the control rod was put back in place.
The plant operator, Korea Hydro and Nuclear Power Company, told NHK there was no safety risk because the reactor is designed to shut down automatically when output reaches 25 percent.
But the commission on Monday ordered KHNP to suspend the reactor's operations, citing violations of the Nuclear Safety Act.
Commission officers have been designated as special judiciary police officers and are examining how the removal of the control rod was monitored.
South Korean media are calling for a thorough investigation and measures to prevent a recurrence, suggesting there could have been a serious accident.