Kori No. 1, South Korea’s oldest nuclear reactor located in Busan, ceased operation Sunday at midnight after four decades.

Its operator Korea Hydro and Nuclear Power Co. said that it cut the power supply Saturday and began the cooling-down process of the reactor.

It was officially decommissioned, with the temperature of the reactor gradually dropping to 90 degrees Celsius, from its normal operation at 300 degree Celsius, the KHNP said. Officials will then relocate the spent nuclear fuel stored inside the rector to a liquid sodium-cooled reactor for reprocessing.

The actual dismantling of the facilities is expected to start no later than 2022. The KHNP expects that at least 634 billion won ($559 million) is required for the dismantling. They also need to submit a dismantlement plan within five years for the NCCS’ approval.

The state-run Nuclear Safety and Security Commission, which approved earlier this month the permanent shutdown of Kori No. 1, said it will continue to check the safety management of the suspended reactor on a regular basis until the dismantlement.

Following the government’s approval in 2007, Kori No. 1’s operation was extended by 10 years after a 30-year run.

Some experts oppose the planned reprocessing of nuclear waste, saying the technology, though effective in reducing the volume of waste, could complicate waste disposal by creating different types of radioactive waste.

Under President Moon Jae-in, the South Korean government aims to close all nuclear power plants by shutting down aged facilities and eventually phasing out the rest over the next 40 years.

By Kim Da-sol (ddd@heraldcorp.com)