North Korea's Kim Jong-un
North Korea is believed to already have an arsenal of 12 powerful nuclear weapons, three of which may have been exploded in underground tests. Reuters file photo

US spy satellites have detected a new nuclear power plant in North Korea that intelligence officials fear could double the country's potential output of uranium nuclear warheads.

The new plant means the notoriously unpredictable Hermit Kingdom could build five atom bombs every year, reports a South Korean newspaper.

The revelations come after heat emissions were detected by infrared cameras on US satellites that show a newly built section of the Yongbyon complex at the heart of the country's nuclear programme.

Thermal images revealed a small uranium facility capable of producing deadly nuclear warheads, the construction of which has gone under the radar previously as the factory is less conspicuous than its large plutonium reactor neighbour.

Nuclear arsenal

Yongbyon is also the location of the plutonium nuclear reactor that has supplied the country with fuel for its arsenal of around 12 bombs, three of which have been exploded in underground nuclear tests.

north korea nuclear
The nuclear reactor in Yongbyon as seen from satellite images. Institute for Science and International Security

The new facility adds to its ability to make uranium – not plutonium – warheads, which produce weaker, though still deadly, explosions and are easier to manufacture.

Thursday's report in the JoongAng Ilbo newspaper quotes an unnamed South Korean intelligence official who says the new plant stands next to an established uranium enrichment plant.

"In 2012 the North started building a new facility next to its old uranium plant, about 120m long and 15m wide", the official told the newspaper, which has since been reported in The Times.

Weapons-grade uranium

The new plant appears to be of a similar scale to its older neighbouring plant, which is known to contain 2000 gas centrifuges, the devices that concentrate weapons-grade uranium.

"If the North operates all 4000 centrifuges year-round, it can produce about 80kg of highly enriched uranium annually. That is enough to build four to five nuclear bombs", the source told the newspaper.

Since 2003 the US has accused North Korea of having a second, supplementary uranium program as well as its plutonium reactor, an accusation that it has previously denied.

Uranium-based nuclear warheads can be used without an easily detectable underground test, such as the ones that the regime staged with its plutonium-based warheads in 2006, 2009 and 2012.