South Korea’s oldest nuclear reactor will permanently stop its power supply operations as of midnight on Sunday.

The Korea Hydro and Nuclear Power (KHNP) said that as a first step toward the permanent shutdown, it cut off the power supply line of the Kori-1 reactor as of 6 p.m. Saturday.

The KHNP said that the temperature of the reactor is expected to drop to 90 degrees Celsius at midnight Sunday from the current 300, and will gradually go down further.

The Kori-1 reactor near Busan, which started commercial operations in 1978, is the first and oldest nuclear reactor in South Korea.