As North Korea continues to excavate a new portal (entrance) approximately 50 meters east of the original (now demolished) South Portal, there is wide speculation that North Korea could conduct a nuclear test soon. Based on the terrain, geology, topography, and suspected tunnel schematics, the two drifts at this southern tunnel complex are estimated to be capable of containing explosions around 50 to 120 kilotons, respectively.
Certainly, yield limit estimates do not mean the North Koreans will test to the maximum available capacity or tell us what specifically they may be planning to do next. But it does indicate that this tunnel complex, as depicted in the North Korean schematic map, is incapable of containing explosions the size of the 2017 test. Nonetheless, if Kim Jong Un is working to fully weaponize and proof test North Korea’s tactical nuclear warheads or other battlefield weapons, reaccessing and restoring the depicted tunnels to working conditions would likely be more than sufficient.
Calculating Yield Estimates for the Tunnel No. 3 Complex
The Punggye-ri Nuclear Test Site is comprised of four separate tunnel systems, identified respectively as the East Portal (sometimes referred to as Tunnel No. 1), the North Portal (Tunnel No. 2), the South Portal (Tunnel No. 3) and the West Portal (Tunnel No. 4). Of these four portals, the East Portal was used in 2006 and evidently abandoned shortly after that single use due to contamination. The five subsequent tests were conducted from the North Portal. No tests have yet been conducted at the South Portal, where excavation activity ended in late 2013, or in the West Portal, where excavation activity began in 2017. In May 2018, the North, South and West Portals were all explosively sealed, as witnessed by foreign journalists.
Figure 1. North Korean produced overview schematic of the four portals at the Punggye-ri Nuclear Test Site with projected tunnel layouts (overlain on Google Earth).
Since March 2022, the North Koreans have been excavating a new portal (entrance) approximately 50 meters east of the original (now demolished) South Portal. This has led to wide speculation that North Korea could conduct a nuclear test soon. It also raises key questions about what the effective yield limitations are for conducting a nuclear test via this south tunnel complex and what purpose another test would serve, especially given the success of the previous six tests, including the 2017 test under Mt. Mantap that had an effective yield of roughly 250 kilotons of TNT.
In 2017, prior to that last test, David Coblentz and Frank V. Pabian published an article on 38 North that estimated the test tunnels at the North Portal (Tunnel No. 2) had the capacity to contain underground explosions up to 282 kilotons. That assessment turned out to be quite accurate; the tunnel was able to contain a 250-kiloton blast with no radiation leaks yet detected. The estimate took into account the geologic makeup and terrain of Mt. Mantap, as well as the estimated tunnel length and independently derived schematics. The amount of spoil that had been displaced at the North Portal over the years and the seismically determined epicenters of previous tests for that complex provided a sense of the likely tunnel design and length. From that information, Coblentz and Pabian estimated the longest tunnel could have ended under the highest point of Mt. Mantap, providing an overburden of roughly 800 meters. They wrote:
When we apply the parameters found for Mt. Mantap (which provides 800 meters of overlying rock for a tunnel beginning at the North Portal), to published nominal scaled depth of burial formulae (approximated by the equation: Depth=122(yield in kT)1/3, where depth is measured in meters and yield in kilotons (kT)), then such a depth would allow containment of an underground nuclear explosive event having a yield of up to 282 kilotons.
Using this formula, we can estimate the yield limits of the southern tunnel complex. The geology is assessed to be similar to what is found under Mt. Mantap (Tokureido diorite or Jurassic-age granite). Terrain profiles can be derived from the rough tunnel schematics previously displayed by North Korea, which are reasonably close to our own previously published tunnel projections. Given the known topography of the area and the amount of spoil that was displaced during the original tunnel excavations, it is possible to derive a maximum depth of burial for each of the two tunnel drifts that were shown on the North Korean test tunnel schematic map.
Based on the above nominal scaled depth of burial formula, this means that for the longer tunnel (drift #1, see Figure 2), where the maximum available overburden is about 600 meters, safe containment should be possible for up to roughly 120 kilotons.
Figure 2. Terrain profile of purported South Tunnel projection, drift #1.
In the shorter tunnel section (drift #2, see Figure 3), the maximum available overburden is about 450 meters, which would provide a capability to safely test up to around 50 kilotons. Although, in each case, the potential test yields are significantly less than what was possible at the North Portal, they are still large enough to handle a wide range of fully weaponized nuclear weapons tests, especially tactical nuclear warheads (some of which might range 10-15 kilotons or less). Previous nuclear testing was more likely for the purposes of proving designs and yield-to-mass calculations verification.
Figure 3. Terrain profile of purported South Tunnel projection, drift #2.
Figure 4 shows how it would be possible to reaccess these two tunnel drifts via a parallel by-pass tunnel, one kilometer long, from the newly excavated portal (assuming that the south tunnel was actually damaged to the extent shown in the schematic with three separate detonations). It should be noted, though, that the amount of tunneling required could be significantly less if the tunnels were laid out as we had originally assessed versus what the North Korean schematic showed. Alternatively, it is also possible that not much of the original tunnel was destroyed beyond the original South Portal (despite what was shown on the schematic map), which would then require a much shorter by-pass tunnel. In either case, the amount of time needed to prepare for another underground nuclear test would also be significantly reduced.
Figure 4. Possible by-pass tunnel layout from the new South Portal to access undamaged tunnel sections.
Certainly, estimates of yield limits do not mean the North Koreans will test to the maximum available capacity or tell us what specifically they may be planning to do next. But it does indicate that this tunnel complex, as depicted in the North Korean schematic map, is incapable of containing explosions the size of the 2017 test. Nonetheless, if Kim Jong Un is working to fully weaponize and proof test North Korea’s tactical nuclear warheads or other battlefield weapons, reaccessing and restoring the depicted tunnels to working conditions would likely be more than sufficient.
We should also point out, however, that if additional tunneling were to be conducted in a more westwardly direction, then the North Koreans could achieve a similar maximum overburden as was previously available under Mt. Mantap. A hypothetical drift #3 (see Figure 4) could be extended under the high volcanic plateau, providing a maximum available overburden of about 800 meters, which would provide a capability to safely test up to around 282 kilotons. Additional tunneling would be required, though, and probably observable over time.
Figure 5. North Korean displayed nuclear test tunnel schematic map of Punggye-ri.
For latest imagery and analysis, see Peter Makowsky, Olli Heinonen, and Jack Liu, “Punggye-ri Nuclear Test Site: Work Continues to Restore Tunnel No. 3,” 38 North, April 28, 2022, https://www.38north.org/2022/04/punggye-ri-nuclear-test-site-work-continues-to-restore-tunnel-no-3/; and Joseph S. Bermudez Jr., Victor Cha, and Jennifer Jun, “Punggye-ri Update: Construction and Volleyball,” CSIS Beyond Parallel, April 28, 2022, https://beyondparallel.csis.org/punggye-ri-update-construction-and-volleyball/.