Recent commercial satellite imagery indicates that North Korea is excavating a new tunnel for nuclear testing at the Punggye-ri nuclear test site. This tunnel is in a new area of the site in addition to the three others where the North has either conducted nuclear tests or excavated tunnels in the past. While there are no indications that a nuclear test is imminent, the new tunnel adds to North Korea’s ability to conduct additional detonations at Punggye-ri over the coming years if it chooses to do so.
Figure 1. Portals at North Korea’s Punggye-ri nuclear test site.
The new tunnel lies northwest of the test site’s Main Support Area. A review of imagery over the past year shows significant construction in the area beginning in April, including new covered structures and what appear to be logs for construction along the Changuk stream.
Figure 2. Construction related to a new portal at North Korea’s nuclear test site.
Imagery from October and November 2015 show an additional structure and what appear to be significant tailings, indicating excavation of a new tunnel is underway. The logs are no longer present. One likely explanation is that the logs are being used to support the tunnel during excavation. They may have also been used for construction of the new buildings.
Figure 3. Further evidence of a new portal at North Korea’s nuclear test site.
The new tunnel, which provides access to Mount Musan, is located in a new area at the Punggye-ri test site and represents an addition to the three existing areas. All are arranged around a central support facility and are referred to by their location relative to that facility. They are:
- The East Portal, the site of North Korea’s first nuclear test in 2006, which does not appear to be maintained.
- The North Portal, used for tests in 2009 and 2013 and previously known as the West Portal, but more properly is described as lying North of the main support area, which continues to show signs of activity.
- The South Portal, which has been under construction since 2009.
- The new West Portal—North Korea’s fourth area at which it can conduct nuclear tests.
While some analysts conclude each entrance connects to a single tunnel, it is possible that each portal is an entrance to an underground complex capable of supporting multiple nuclear detonations in branches off a main test tunnel. North Korea conducted nuclear tests in 2009 and 2013, apparently using the same main tunnel. (For a discussion of this possibility, see The Tunnels at Punggye-ri: An Alternative View.)
If this is the case, Pyongyang would be able to conduct additional tests in the future. One limiting factor is the physical size of the mountain—how many branches can be constructed with sufficient overburden to contain nuclear explosions conducted within. A second challenge is so-called tired mountain syndrome—the hypothesis that repeated nuclear explosive tests will weaken the rock in the mountain, leaving it unable to contain nuclear explosions. US nuclear weapons designers debated whether cracks observed at Rainier Mesa at the Nevada Test Site indicated “tired mountain syndrome.” The North Koreans may have similar concerns or uncertainties.
 On October 30, Yonhap reported that North Korea was constructing a new tunnel for nuclear testing at the Punggye-ri nuclear test site. “North Korea appears to be in the process of digging another tunnel,” an anonymous official said citing the “movement of people and cars at the nuclear test site.” See, “N. Korea digging new tunnel at its nuke test site: official,” Yonhap News, October 30, 2015, http://english.yonhapnews.co.kr/northkorea/2015/10/30/64/0401000000AEN20151030002800315F.html.