Commercial satellite imagery of North Korea’s Punggye-ri Nuclear Test Site indicates that work continues throughout the site, with evidence of personnel in both the north and south areas and minor activity around Tunnels 3 and 4. While the support infrastructure and instrumentation around Tunnel No. 3 suggest that it is capable of fielding a test if and when the decision is made, there are no major activities—such as increased vehicle traffic and personnel near the tunnel—that would indicate a test may be imminent.
Tunnel No. 3 (Former South Portal)
In 2018, ahead of the first US-DPRK summit, the test tunnel entrances at Punggye-ri and the majority of its support infrastructure, including the “measurement building,” which would have contained the instrumentation for monitoring and recording data from a test, were demolished (Figure 1). The site remained in caretaker status through the end of 2021.
In March 2022, the first signs of excavation of a new entrance to Tunnel No. 3 were observed, leading to widespread speculation that a nuclear test was being prepared. While no test occurred, excavation at Tunnel No. 3 appeared to be mostly complete by August 2022, along with the buildup of new support infrastructure (Figure 2). Little has changed around this tunnel area since then.
Figure 1a. Before and after demolition of Tunnel No. 3 entrance (South Portal). Image © 2023 DigitalGlobe, Inc.; Figure 1b. Before and after demolition of Tunnel No. 3 entrance (South Portal). Image Pleiades © CNES 2023, Distribution Airbus DS. For media options, please contact [email protected].
The “measurement building,” which would have been used to house instrumentation for recording a test, was destroyed in May 2018, and its previous location was made inaccessible because of the rubble. It appears the North Koreans have opted to accommodate that equipment along the road from the tunnel entrance (Figure 2). If identified correctly, the figure shows the necessary cabling, instrumentation, and monitoring equipment necessary to conduct and assess a test.
This photo from the Nevada Test Site in the United States (from decades ago) illustrates the long length of cabling required to monitor and record an underground nuclear test. While this was a test where the nuclear device was lowered through a vertical shaft as opposed to a horizontal tunnel, as Tunnel No. 3 is believed to be, the monitoring requirements are similar. The photo shows the tower from which the nuclear device and sensors are lowered, the cabling connected to those sensors, and the instrumentation vans at the other end of those cables. In those days, thick coaxial cables were only available, and much thinner and higher capacity fiber optic lines did not yet exist. Presumably, the North Koreans are using fiber optic instrumentation cabling.
Although tunnel restoration may be mostly complete, minor fixes appear to continue. For example, imagery from March 20 captured a probable stack of lumber at the entrance, possibly for shoring parts of the tunnel network to improve safety.
Tunnel No. 4 (Former West Portal)
Since January 2023, cleanup efforts have continued at the demolished entrance to Tunnel No. 4 and widened work area. They are now in a position to dig and access the tunnel network if and when they choose to do so.
Figure 4a. Former Tunnel No. 4 entrance on imagery from May 7, 2018. Image © 2023 DigitalGlobe, Inc.; Figure 4b. Former Tunnel No. 4 entrance on imagery from April 3, 2023. Image Pleiades NEO © Airbus DS 2023. For media options, please contact [email protected].
Underground nuclear test monitoring, regardless of whether with vertical shafts or horizontally with tunnels, requires measuring and recording of pressure, radiation, and temperature to assess that the explosive yield occurred as predicted. The signals must be collected and recorded from sensors that are destroyed within the first few moments of a nuclear detonation. In order to avoid power line noise contamination, the instrumentation cables must be physically separate from power cables. The preamplifiers extract and amplify the pressure, radiation and temperature sensors to specific test equipment that assembles the data samples, digitizes them and records for subsequent analysis. For a more detailed and technical discussion of what and how an underground nuclear test is monitored, see https://sgp.fas.org/othergov/doe/lanl/pubs/las28/mortensen.pdf.