The North Korean Nuclear Test Site

South Korea reports that Kim Jong Un has offered to close North Korea’s nuclear test site at Punggye-Ri in May. He says he will invite US and South Korean experts to examine the site before its demolition to see that it is still usable.

There have been very definite statements from experts outside North Korea that the site may or may not be usable. We don’t have access to the site, so we must surmise the situation from overhead photos, seismic traces, and experience at other test sites.

The yield of the most recent test was very large, perhaps 250 kilotons. It’s hard to estimate the yields of North Korean nuclear tests because we don’t know enough about the geology of the test site. It was followed by three seismic events of 4.6, 3.5, and 2.9 magnitude, which were not tests.

An underground nuclear test forms  a cavity; the larger the yield, the larger the cavity. As the cavity cools, the ceiling collapses and forms a chimney filled with loose rock. A crater may form at the surface (diagram) and video.

The aftershocks could be the cavity collapsing in stages, or they could be the cavity and tunnels collapsing. Or it could be things happening in the rest of the mountain, as the jolt of the blast destabilizes things. It could be that the blast also fractured rock throughout the mountain. Landslides can be seen around the mountain, and its surface contours have been altered.

None of this is extraordinary for nuclear test sites. To say that it represents the mountain’s collapse is an exaggeration, as is the phrase “Tired Mountain Syndrome.”

A few articles have indulged in scare talk about radioactive material escaping from future tests. That would be a mostly local concern, if indeed the mountain is so fractured. There would be no point to another test where the chimney has formed, and North Korea has additional tunnels in other places in the mountain.

It’s likely that the North Koreans would find such an escape undesirable for other reasons. They have been extremely careful to contain their tests; escape of material would allow other countries insight into the design of their nuclear weapons.

US intelligence officials have said that the test site remains operational.

Closing the site would probably involve dynamiting the tunnel entrances. The tunnels could be opened in the future. North Korea destroyed the cooling tower for their plutonium reactor in 2008, in a similarly symbolic gesture. They built it back later.

Even if the test site were damaged, that is likely a small part of Kim’s calculation in offering a pause in testing, and even a closing of the test site. The larger factor is that he feels that he now has a deterrent against American and South Korean attack.


Cross-posted at Balloon Juice.

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