Korea Hydro & Nuclear Power (KHNP) has confirmed the New Wolsong 1 reactor, located in Gyeongju, North Gyeongsang Province, automatically shut down at around 11:00am local time on 19 August as its power supply system did not work properly.
An investigation is underway to learn the exact cause of the malfunction at the one million kilowatt new reactor, which began commercial operation 19 days ago.
The shut down has triggered concerns over power shortage in the season of high electricity demand, however there is no danger of a radiation leak.
A KHNP official was quoted by Korea Times as saying that the International Atomic Energy Agency is level zero shut down, in terms of the nuclear and radiological event scale.
"It has nothing to do with the safety of the nuclear plant or radiation risks," the official said.
The reactor is expected to resume operations following approval by the Nuclear Safety and Security Commission and the ongoing investigation.
The state run utility operates large nuclear and hydroelectric plants and is a subsidiary of the Korea Electric Power Corporation (KEPCO).
An alert had been issued several times by KEPCO in lieu of rising demand of electricity towards the end of August, which is the end of peak vacation season.
South Korea operates 20 nuclear reactors out of total 23, which supply about 30% of the nation's overall electricity consumption.