A nuclear reactor on South Korea's eastern coast was temporarily shut down Monday due to a problem in one of its control rods, operators said.

The reactor unit 1 at Hanwool Nuclear Power Plant in Uljin, 330 kilometers southeast of Seoul, was manually turned off after a control rod dropped, according to an official from the Korea Hydro & Nuclear Power Co., Ltd.

A control rod is used in reactors to control the rate of nuclear fission.

"Nuclear reactors are designed to shut down automatically when there is a major safety issue, but this time, operators had already been monitoring the reactor as they detected a possible problem with the control rod and manually turned off the reactor when the control rod dropped," the official said.

A reactor has more than 50 control rods that are designed to drop into nuclear fuel in case of a power failure or other problems, preventing a meltdown of the reactor.

South Korea operates 23 nuclear reactors throughout the country that generate about 30 percent of its overall electricity supplies. (Yonhap)