A South Korean nuclear reactor came to a halt Thursday only five days after being restarted, due to lingering problems with its cooling system, its operator said.

The No. 3 reactor at Hanbit Nuclear Power Plant in Yeonggwang, South Jeolla Province, automatically stopped operating when the reactor’s coolant pump ― which produces steam for cooling down the reactor ― suddenly stopped functioning on Thursday afternoon, according to Korea Hydro and Nuclear Power.

The KHNP announced that no radioactive materials were released at the unit and they are investigating the exact cause behind the sudden malfunctioning.

The shutdown came only five days after the nation’s Nuclear Safety and Security Commission gave its approval to restart the reactor, which automatically shut down in October last year due to similar problems.

At the time, the reactor had automatically stopped functioning because metal debris had been blocking the key pathways inside the unit’s steam generator.

Though the authorities were unable to remove all the debris due to technical limitations, the reactor was put back into operation on Sunday, amid an uproar from residents in Yeonggwang.

The protesters had expressed strong opposition to the operator’s claim that the reactor was safe, despite the remaining defects in its steam generator.

By Sohn Ji-young (jys@heraldcorp.com)