Nuclear test site toppled, study finds

BEIJING — A study by Chinese geologists shows the mountain above North Korea’s main nuclear test site has collapsed under the stress of the explosions, rendering it unsafe for further testing and necessitating monitoring for any leaking radiation.

The findings by the scientists at the University of Science and Technology of China come days North Korean leader Kim Jong Un announced that his country was ceasing its testing program.

Nuclear explosions release enormous amounts of heat and energy, and the North’s largest test in September was believed early on to have rendered the site in northeastern North Korea unstable.

The data in the latest Chinese study was collected after the most powerful of the North’s six nuclear device tests, on Sept. 3, that is believed to have triggered four earthquakes over the following weeks.

The yield of the bomb was estimated at more than 100 kilotons of TNT, at least 10 times stronger than anything the North had tested previously.

The bomb the United States dropped on Hiroshima in 1945, for instance, had a yield of about 15 kilotons.

The University of Science and Technology of China paper, written by Tian Dongdong, Yao Jiawen and Wen Lianxing, said the first of those earthquakes that occurred 8½ minutes after the explosion was “an onsite collapse toward the nuclear test center,” while those that followed were an “earthquake swarm” in similar locations.

“In view of the research finding that the North Korea nuclear test site at Mantapsan has collapsed, it is necessary to continue to monitor any leakage of radioactive materials that may have been caused by the collapse,” the authors said in a summary dated Monday and viewed Wednesday on the university’s website.

The study is peer-reviewed and has been accepted for publication by the journal Geophysical Research Letters.