North Korea’s mountain nuclear test site has collapsed, putting China and other nearby nations at unprecedented risk of radioactive exposure, two separate groups of Chinese scientists studying the issue have confirmed.

The collapse after five nuclear blasts may be why North Korean leader Kim Jong-un declared last Friday that he would freeze the hermit state’s nuclear and missile tests and shut down the site, one researcher said.

The mountain’s collapse and the prospect of radioactive exposure in the aftermath, confirms a series of exclusive reports by the South China Morning Post on China’s fears that Pyongyang’s latest nuclear test had caused a fallout leak.

North Korea’s mountain nuclear test site has collapsed, putting China and other nearby nations at unprecedented risk of radioactive exposure, two separate groups of Chinese scientists studying the issue have confirmed.

The collapse after five nuclear blasts may be why North Korean leader Kim Jong-un declared last Friday that he would freeze the hermit state’s nuclear and missile tests and shut down the site, one researcher said.

The last five of Pyongyang’s six nuclear tests have all been carried out under Mount Mantap at the Punggye-ri nuclear test site in North Korea’s northwest.

A research team led by Wen Lianxing, a geologist with the University of Science and Technology of China in Hefei, concluded the collapse occurred following the detonation last autumn of North Korea’s most powerful thermal nuclear warhead in a tunnel about 700 metres (2,296 feet) below the mountain’s peak.

The test turned the mountain into fragile fragments, the researchers found.

The mountain’s collapse and the prospect of radioactive exposure in the aftermath, confirms a series of exclusive reports by the South China Morning Post on China’s fears that Pyongyang’s latest nuclear test had caused a fallout leak.

A KN-08 missile on a TEL at a military parade in Pyongyang on 15 April 2012. There is no evidence that the KN-08 is operational but North Korea continues to develop its ballistic missile program. North Korea

Radioactive dust could escape through holes or cracks in the damaged mountain, the scientists said.

“It is necessary to continue monitoring possible leaks of radioactive materials caused by the collapse incident,” Wen’s team said in the statement.

The findings will be published on the website of the peer-reviewed journal, Geophysical Research Letters, likely next month.

North Korea saw the mountain as an ideal location for underground nuclear experiments because of its elevation – it stood more than 2,100 metres (6,888 feet) above sea level – and its terrain of thick, gentle slopes that seemed capable of resisting structural damage.

The mountain’s surface had shown no visible damage after four underground nuclear tests before 2017.

But the 100-kilotonne bomb that went off on September 3 vaporised surrounding rocks with unprecedented heat and opened a space that was up to 200 metres (656 feet) in diameter, according to a statement posted on the Wen team’s website on Monday.