South Korea on Friday started work on two new nuclear reactors despite fears over safety in the aftermath of the radioactive disaster in Japan last year.

The reactors, each with a capacity of 1,400 megawatts, are being built at existing nuclear power plants near Uljin on the east coast, the Ministry of Knowledge Economy said.

The government plans to spend seven trillion won ($6.18 billion) to build the New Uljin 1 and 2 reactors. The first is expected to be online around April 2017 and the second by February 2018.

"Safety has been drastically strengthened as all recommendations for enhancing safety that were made after the Fukushima disaster have been reflected in the design," the ministry said in a statement.

New features include devices that automatically shut down reactors at times of quakes, mobile electricity generators and gas-removing facilities that can operate even during a power failure.

The ministry said the new pressurised reactors will be the first models using purely locally developed key parts and systems.

International environmental group Greenpeace, a vocal critic of the country's nuclear power programme, said it was ironic that South Korea started building two new reactors on the eve of Tokyo shutting down its own reactors.

Japan is scheduled to be nuclear-free Saturday when the last of its 50 reactors goes offline temporarily for maintenance.

"It is unbelievable that on the day before Japan closes all its reactors, President Lee (Myung-Bak) is going in the other direction -- building two new ones in Korea," Lee Hee-Song, a Greenpeace campaign leader in Seoul, said in a statement.

"It is clear that Korea has learned little from the Fukushima disaster, where hundreds of thousands of people are continuing to suffer the consequences."

South Korea relies on 21 nuclear reactors to meet about 35 percent of its electricity needs. It plans to build 11 more, including those started Friday, by 2020.