North Korea’s nuclear test cavity collapse and subsequent chimney collapse sequence

New Evidence that North Korea Could Still Conduct Nuclear Tests at Punggye-ri

April 30, 2018

Newly acquired synthetic aperture radar data of the Punggye-ri Nuclear Test Site provides additional evidence that the two mountainous areas accessible by the South and West Portals remain viable, and could support future underground nuclear testing if there were to be a political decision to do so. The data also corroborates Kim Jong Un’s publicly reported statement that two tunnels of the site remain in good condition. In short, while Chinese seismological findings that the Mt. Mantap portion of the nuclear test site may no longer suitable for tests due to significant damage and deformation appear correct, claims that the entire site “is wrecked beyond repair” are wrong.

The Chinese Findings

Two Chinese seismological research groups recently released assessments of the September 2017 nuclear test in which they independently concluded that a 4.1 magnitude earthquake detected eight and a half minutes following the nuclear test was caused by a cavity collapse under Mt. Mantap peak. One also suggested that monitoring for “the leakage of radioactive materials” should be continued as a result of this collapse. Nowhere in these findings were any comments made on the viability of the overall test site for future underground nuclear testing, or were any claims made that “the test site ‘collapsed.’”[1]

However, another researcher, Zhao Lianfeng, with the Institute of Earth Science at the Chinese Academy of Sciences in Beijing, was quoted as saying that, “the two studies supported a consensus among scientists that ‘the site was wrecked’ beyond repair.”

The Data Suggests Otherwise

Five of North Korea’s six known underground nuclear tests, including the largest one conducted in September 2017, were conducted under Mt. Mantap and supported by a tunnel system accessible via the North Portal. Commercial satellite imagery has indicated the North Portal seems to have been abandoned since that last test. However, as we have also previously reported, two unused mountainous areas, accessible via the South and West Portals, appear to remain viable in more pristine rock located approximately 2.5 and 3.5 kilometers from Mt. Mantap’s peak.

An interferometrically processed radar image of the Punggye-ri nuclear test site (Figure 1), provided by the European Union Satellite Center (EU SatCen), shows differential changes in elevation, before and after the last nuclear test, demonstrates that point. The red in the figure indicates significant deformation/displacement and likely rock fracturing in the Mt. Mantap area, where the most recent nuclear test occurred. However, the figure also shows that the areas accessible via the South and West Portals have not been similarly disturbed.[2]

It should be noted that there was substantial tunneling at the West Portal beginning shortly after the sixth nuclear test, and up to at least early March 2018, as evidenced by rapid growth of the adjacent spoil pile. At no time was there any observable evidence of a halt to the tunneling due to tunnel collapses as had been also reported in the media. The most recent commercial satellite image of the site showed about a dozen mining carts out on that West Portal spoil pile clearly indicating that tunneling was still being conducted until only quite recently.

Figure 1. EU SatCen interferometrically processed ALOS-2 SAR images from August 29, 2017, September 12, 2017 and September 26, 2017 and overlain them on Google Earth to show the September 3, 2017 nuclear test effects at Punggye-ri.

A.jpgSAR image and processing courtesy of European Union Satellite Center (EU SatCen). Image © Google Earth 2018.

[1] For instance, a Financial Times article’s headline specifically stated, “North Korea nuclear test site said to have ‘collapsed.’”

[2]  Inelastic deformation falls off with distance from the epicenter of the previous event with the distance dependent on the yield of the event. “We can distinguish a zone of inelastic deformations of rock mass (situated at distances to 120-130 m/kt 1/3, and in isolated directions may reach 150 m/kt 1/3)”, which for a test having a yield of ~250 kilotons would be under a kilometer and the “changes in properties of rocks and rock masses in the vicinity of previously conducted underground explosions impose certain requirements on selection of a location for conducting underground tests. The location of a new test must be chosen such that zones of increased fracturing of the rock mass from previously conducted explosions do not intersect with the planned explosion.” https://pubs.usgs.gov/of/2001/0312/report.pdf

Source: A 38 North exclusive with analysis by Frank V. Pabian, Joseph S. Bermudez Jr. and Jack Liu https://www.38north.org/2018/04/punggye043018/

 

What a “Chimney Collapse” Means (and Doesn’t Mean) for Mt. Mantap

Several media reports continue to misconstrue the meaning of “cavity collapse” and “chimney collapse” at the Punggye-ri nuclear test site, concluding either that the entire test site has collapsed or that radiation is leaking into the atmosphere as a result of the September 2017 nuclear test.[1] However, neither a “cavity collapse” nor a “chimney collapse” equates to a test site collapse, as explained in a recent 38 North article.

More importantly, neither type of collapse means that there are cracks or fissures all the way to the surface that would allow radioactive particles and gases to vent into the atmosphere. Instead, what this means is that the chamber where the test took place, under hundreds of meters of granitic rock and volcanic layers, has likely closed in after the explosion. And while this makes the chamber no longer usable, the damaged area is overburied and the damage is unlikely to have reached to the surface.

To illustrate what a “cavity collapse” or “chamber collapse” means, the US Government, Office of Technical Assessment has explained this phenomenon in a publication, entitled “The Containment of Underground Nuclear Explosion”:

The predominantly steam-filled cavity eventually collapses forming a chimney. When collapse occurs, the steam in the cavity is condensed through contact with the cold rock falling into the cavity. Within a few minutes or days after an underground nuclear test, “When the gas pressure in the cavity declines to the point where it is no longer able to support the overlying rock, the cavity may collapse. The collapse occurs as overlying rock breaks into rubble and falls into the cavity void. As the process continues, the void region moves upward as rubble falls downward. The “chimneying” continues until:

the void volume within the chimney completely fills with loose rubble,
– the chimney reaches a level where the shape of the void region and the strength of the rock can support the overburden material. or
– the chimney reaches the surface.

The report also makes clear that,Tunnel tests [the case for all tests at Punggye-ri] are typically overburied. Collapse chimneys do not usually extend to surface.” 

This point is reiterated in well-crafted post by Bruce Tabor on Quora, which was written in response to a question raised about the first post-test tremor following the September 2017 test.

It was a cavity collapse

A magnitude 4.1 aftershock occurred 8.5 minutes after the magnitude 6.3 earthquake caused by the explosion of North Korea’s hydrogen bomb. The media misreported this aftershock as a tunnel collapse, implying tunnels dug for testing purposes collapsed as a result of the test.

In fact the aftershock was most probably a cavity collapse.

The recently reported Chinese findings concur with Bruce Tabor’s assertion. Bruce Tabor also goes on to state:

Explosion cavity

A large underground explosion will melt and deform the surrounding rock and create a hole or cavity in the rock. For a 250 kiloton explosion that cavity will be about 200 metres across. That cavity will be surrounded by a zone of crushed rock about 3 times the diameter of the cavity itself. The roof of this cavity will usually collapse several minutes later, especially for large tests like the NK one. If the explosion is deep enough – deeper than 500–600 metres for a 250 kiloton blast, the collapse will not extend to the surface, i.e. there will be no subsidence at the surface.

Figure 1. A nuclear test cavity collapse and subsequent chimney collapse sequence.

B

Diagram presented by Bruce Tabor.

In the case of the September 2017 test, the minimum depth of burial was at least 700 meters and likely very close to 800 meters, which is more than enough to safely contain that test with no leaks of radionuclides. Moreover, the Comprehensive Test Ban Treaty Organization has not detected or reported that there was any leaked radiation from that test.

As we were about to go to press with this article, we became aware of another reference work that addresses the various issues surrounding the viability of Punggye-ri for additional nuclear testing here-in including the question of the so-called “Tired Mountain Syndrome” with regard to Mt. Mantap, and which provides additional corroborative background information.


  1. [1] See for example: “North Korea test site could be unusable after collapse – Chinese scientists,” BBC News, April 25, 2018, http://www.bbc.com/news/world-asia-43894394.

Source : A 38 North exclusive with analysis by Frank V. Pabian, Joseph S. Bermudez Jr. and Jack Liu https://www.38north.org/2018/04/mtmantap043018/

 

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