South Korea's nuclear operator said Tuesday that it manually shut down a reactor to deal with a coolant problem.

The Kori 4 reactor was shut down at 5:11 a.m. after there was a rise in coolant levels in a collection tank, the Korea Hydro and Nuclear Power Co. said. The reactor is located some 400 kilometers southeast of Seoul in northern Busan.

The company said operation of the reactor was gradually reduced in the early morning hours as a precaution after engineers detected the problem. 

KHNP emphasized that there was no radiation leak caused by the incident and that the 950 megawatt reactor and the Kori nuclear power plant are safe.

It said the cause of the problem will be carefully checked.

South Korea operates 23 nuclear reactors that generate about 30 percent of its overall electricity supply. (Yonhap)