A nuclear reactor automatically shut down Thursday after a coolant pump stopped working, but no radiation leak was found, the operator of the country's nuclear plants said.

The Reactor-5 at the Hanul Power Plant in Uljin, some 330 kilometers southeast of Seoul, was shut down at around 1:26 a.m., according to the state-run Korea Hydro & Nuclear Power Co.

The Nuclear Safety and Security Commission said it is currently looking into the stoppage to find the exact cause of the automatic shutdown.

The nuclear watchdog said the malfunction of a coolant pump appears to have caused the nuclear reactor to automatically shut down, adding that the radiation level is being maintained at usual levels.

The shutdown comes as the Reactor-5 at Hanul has been in operation since it completed maintenance in July last year. (Yonhap)