South Korea's new President Moon Jae-In has vowed to scrap all plans to build new nuclear reactors as he seeks to steer Asia's fourth-largest economy clear of atomic power.

Moon, who swept to power with a landslide election win last month, campaigned on promises to phase out atomic energy and embrace what he says are safer and more environmentally- friendly power sources including solar and wind power.

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The Fukushima nuclear meltdown in Japan sparked by a powerful earthquake in March 2011 sparked widespread public concern in neighbouring South Korea over its own aged atomic plants.

"We will dump our atomic-centric power supply and open the door to the post-nuclear era," Moon said in a speech marking the decommissioning of the country's first nuclear reactor, the Kori-1.

"I will scrap all preparations to build new reactors currently underway and will not extend lifespan of current reactors," he said.

Many reactors are located dangerously close to residential areas in the densely-populated nation, Moon said, warning of "unimaginable consequences" in case of a nuclear meltdown.

"South Korea is not safe from the risk of earthquake, and a nuclear accident caused by a quake can have such a devastating impact," he said.

South Korea currently operates 25 nuclear reactors, which generate about 30 per cent of the country's power supply.

Many of them will see their lifespans expire between 2020 to 2030, with decisions on whether to extend some of their operations set to be made during Moon's 2017-2022 term.

Moon, during his presidential campaign, vowed to try to eventually shut down all nuclear power plants across the country, although doing so will likely take decades.

Major corruption scandals involving state nuclear power agencies in recent years and a series of earthquakes last year further fanned public distrust and concerns over the safety of the plants.

Moon today also vowed to decommission "as soon as possible" another aged atomic plant in the southeast, whose original 30-year lifespan had been extended by another decade to 2022.

He also vowed to introduce "post-coal" policy in line with his campaign promise to abandon coal power to ease air pollution in the country, which has the highest level of small air pollutant particles among OECD member nations.

But experts say shutdown of coal power plants could dramatically hike utility cost in the country where coal power generates about 40 per cent of entire power needs.(AFP)