Yongdok-tong Nuclear High Explosives Test Facility: Part 3

This report is the third of a three-part series on the Yongdok-tong Nuclear High Explosive Test Facility, a critical component in the research, development, test, and evaluation (RDT&E) of North Korea’s nuclear warheads. It also serves as the most comprehensive report of the facility currently available at the unclassified level.¹ Part 1 provides an overview of the facility and its development history from the 1980s to the 2000s. Part 2 discusses the development history from the 2010s to the present, and Part 3 covers the underground facilities at Yongdok-tong and reports of other nuclear high-explosive testing and weapons storage locations elsewhere in North Korea.

Underground Facilities

Over the years, various sources have reported that there are underground facilities at the Yongdok-tong Nuclear High Explosive Test Facility or that the facility was the location of “North Korea’s sole nuclear weapons stockpile site.”² In the preparation of this report, an effort was made to validate these reports and locate any unidentified underground facilities (UGFs). While no new UGFs were identified, this should not be understood as saying there are none. A combination of available sources suggests the presence of three UGF entrances located at the following coordinates:

• 40.030720, 125.307350
• 40.050025, 125.312274
• 40.051356, 125.307506

The first location (40.030720, 125.307350) is located at the southern end of the Yongdok-tong facility, opposite the motor vehicle maintenance and storage area and on the western side of the stream. One of the first readily available, high-resolution commercial satellite images of the area was acquired on March 7, 2002, and it indicated that there was nothing there at that time. Subsequently, commercial satellite imagery acquired on April 24, 2010, and November 10, 2010, showed that a site had been excavated out of the hillside at this location, and a support facility consisting of six structures had been built. Two of these structures appear to be 12-meter-long, steel arch-roof, Quonset-type sheds cut into the hillside.³ These two buildings are the ones that sources claim are entrances to a UGF. Subsequently, in 2020, the site was razed and rebuilt with a retaining wall and a single large building covering the area of the previously identified steel-arch-roof Quonset sheds or UGF entrances.

While it is currently believed that there is a UGF entrance at this location, when available imagery, information, and an understanding of the design and construction process of North Korean strategic UGFs are considered, several incongruities are apparent—none of which are insurmountable. Among these are:

1. None of the readily available satellite imagery shows any significant excavation-related activity between 2002 and 2025, such as heavy vehicle presence, tracks, presence of excavation equipment, waste piles, or spillage of waste, all of which are hallmarks of strategic or military UGF construction.
2. There is rarely a structure constructed immediately in front of strategic or military UGF entrances, as seen here at the Yongdok-tong site. Additionally, the building’s proximity to the retaining wall is only 1.5 meters, which is very close and would present challenges with being blocked from above by dirt and debris.
3. Existing strategic or military UGFs almost always have secondary or tertiary entrances at some distance from the primary entrance to provide alternate access should one entrance become unavailable. However, none of these entrances are visible at this Yongdok-tong site.

The now-disbanded United Nations Security Council Panel of Experts reported in March 2023 that it believes there is a UGF entrance at this location.⁴ Additionally, North Korea is known for its unique approaches with camouflage, concealment, and deception procedures for strategic or military activities, and this may very well be the case here.⁵ The presence of a building immediately in front of the UGF entrance would provide an advantage of easily obstructing overhead observation of any loading, unloading, or movement activities. Regardless, a final determination as to whether a UGF entrance is located here remains elusive.

The second reported location (40.050025, 125.312274) of a UGF entrance is further north, situated in a remote support facility at the upper end of the narrow branch valley located between the explosives storage and fabrication area and the open-air, high-explosive testing area. This location, at the top of the branch valley, was first excavated sometime between 2002 and 2003, leading some to speculate that an entrance to a UGF entrance is located here. However, readily available satellite imagery shows no evidence of any tunneling activity or portal construction in the area. The area bears the characteristics of a small, infrequently used sand, rock, and gravel quarry rather than a UGF entrance.

The third and final location (40.051356, 125.307506) of a reported UGF entrance is within the narrow valley that extends eastward from the open-air, high-explosive testing area. Minor excavation activity was first observed here in a March 7, 2002, satellite image, indicating that it began sometime before this date. What the excavation activity was related to could not be readily determined due to the off-nadir angle of satellite imagery, shadows, and trees. However, an image taken on January 30, 2014, clearly shows what appears to be a short, 8-meter-long, steel-arch-roof, Quonset-type structure cut into the side of a hill. Notably, the same image also shows a trail above this structure and extending southeast for approximately 100 meters, suggesting a location for an opening for air handling equipment. Given the size of the excavated waste rock, this appears to be a small tunnel used for storage or potentially underground high-explosive testing. As this is most definitely not a strategic UGF, the use of a short Quonset-type structure as a portal is reasonable for ease of construction and low cost.

Related to the question of potential UGFs at the Yongdok-tong facility, there has been a question of whether this is the facility at which North Korea stores all its operational nuclear warheads and fissile cores. In his 2020 book, Ankit Panda, states that:

“As of late 2017, the U.S. assessment is that North Korea operates a single storage site for its manufactured warheads and their fissile cores: an underground facility known to the United States as Yongdoktong [sic], northeast of the city of Kusong.⁶

The following year, CNN reporters Zachary Cohen and Kylie Atwood cited a U.S. intelligence official, saying that “Yongdoktong [sic] has been previously identified by US intelligence as a suspected North Korean nuclear weapons storage facility and is still believed to be used for that purpose.”⁷

Reports of Other Nuclear High-Explosive Testing and Weapons Storage Locations

Aside from Yongbyon and Yongdok-tong, which U.S. and South Korean government sources have identified as sites for high-explosive detonator tests, North Korean defectors and other sources have occasionally reported additional locations within the country and elsewhere where such tests, or even “nuclear tests,” have been conducted. It is important to note that North Korean defector statements about that nation’s nuclear developments must be viewed with considerable caution until confirmed.⁸

Testing

Among the earliest references to alternate locations are those from three defectors who reportedly held positions with personal knowledge of these events. One of these defectors was Dr. Kim Kwang-pin, who was a scientist and served as director of the No. 38 Research Laboratory, subordinate to the Nuclear Energy General Bureau, before defecting in 2003. In an August 2004 interview concerning North Korea’s nuclear weapons program, he claims:

I think it was in 1974, the year Kim Chong-il was named to succeed Kim Il-song.  North Korea and the Soviet Union jointly conducted a test of a high performance bomb on [sic] a valley that lies between the Anju Coal Mine in South Pyongan Province and the hydraulic power station of the Chongchon River. After this test was conducted, senior colleagues of mine at “No. 38 Research Laboratory” conducted on-the-site inspections of this site several times and requested that Kim Chong-il conduct an on-the-ground test of the bomb once more, but their request was turned down. A crater, created at the test site in 1977, is still guarded by Korean People’s Army troops, and ordinary people are banned from entering the area.⁹

Interestingly, Yi Chun-son, another defector who was reportedly a vice director of the KPA’s Operations Bureau, also mentions the Anju area in 2001 in connection with North Korea’s nuclear program.¹⁰

Yi Chung-kuk, another North Korean defector who worked in the KPA’s General Staff Department’s Nuclear and Chemical Defense Bureau and escaped North Korea in 1993, states in his book, Kim Chong-il’s Nuclear Weapons and Army:¹¹

On 20 October 1993, a test of a triggering device for a nuclear weapon was conducted in the mountains in Sogam-ri, Pyongwon County, South Pyongan Province. At dawn that day, [He] rode in a car with the head of the reconnaissance department after the core members departed.

The testing ground was located deep in the mountains where civilians are unable to go. The mock nuclear test was to test the triggering device for a nuclear weapon, which was developed by the Nuclear Defense Department in the nuclear complex in Yongbyon, and to test the plutonium 238 [sic] which was extracted, under the participation of researchers.

The test began at 0900 in the morning and continued until evening. About 30 officers, including the director of each department under the General Staff, participated. They observed the test by wearing special contamination-proof clothes in a concrete-supported underground building about 100 meters from the site of the explosion.

…After the test, researchers and officers who came from Yongbyon expressed satisfaction, saying: “The test was a success.” On his return trip in Choe Yong-kwan’s car, the director of the reconnaissance department, Choe, who was very happy, said to Yi: “When compared with tests held on other occasions, this test was the most successful.”¹²

A subsequent report covering the publication of Yi’s book states that Yi claims:

In July 1992, the DPRK conducted nuclear tests first in Russia and then in Ukraine. After Yongbyon’s nuclear facilities were photographed by U.S. satellites, the DPRK could no longer conduct nuclear tests within North Korea. Pyongyang requested that Russia allow underground nuclear testing but was refused. Thus, Kim Chong-il planned to conduct three nuclear tests, but conducted only two, the first test in Russia and the second in Ukraine.

After that, Yi typed a report stating, “a nuclear test using a total of 0.5 kilotonnes of nuclear material (one fortieth the atomic bomb used in Hiroshima) was successfully conducted.” It was the Nuclear Defense Director’s report on his overseas trip. The test evaluation report was submitted to Kim Chong-il [Kim Jong-il].¹³

In 2004, Dr. Kim Kwang-pin (see above) stated that “North Korea built three nuclear test stations: an on-the-ground test station, an underground test station, and a mid-air test station.” He identifies the “on-the-ground test station” being built within the “No. 88 Factory” in Taechon County, North Pyongan Province, the “underground test station,” as being “…an abandoned tunnel at the Anju coal mine in Anju County, South Pyongan Province,” and the “mid-air test station” as being in Yongbyon County, North Pyongan Province. Exactly what is meant by a “mid-air test station” is unclear. Perhaps, it was intended to conduct tests from a tower. Dr. Kim mentioned that in February 1992, North Korea utilized the “underground test station” to conduct a test more than a kilometer underground. Prior to the test, a warning was issued to people living near the mine of a possible earthquake. These people were subsequently evacuated. Kim further states that “The nuclear warhead used to conduct this test was a very small 3-kiloton bomb.”¹⁴

Both Kim and Yi’s statements allude to what appear to be very low-yield nuclear tests. Regardless, no supporting information has surfaced to support either of their assertions concerning such testing.

Two years later, a November 1998 report mentions the establishment of “a high explosives test site” in the Kusong area and “high explosives” tests conducted there:

North Korea has reportedly secretly built a high explosives test site, which is essential to nuclear weapons development, in a valley between Taechon County and Kusong County in North Pyongan Province and has conducted three to four high explosives tests.¹⁵

A year later, in March 1999, reports again mention the establishment of high-explosives detonator test facility and tests conducted there:

The US Government notified the Japanese Government in 1998 that North Korea had succeeded in a detonating test necessary for developing nuclear weapons. A source concerned with Japan-US military affairs revealed this on 30 January. The source strongly believes that North Korea’s technological development, with its intention to possess nuclear arms, has made rapid progress.

According to the source, a detonating test — a step before a nuclear fission occurs — was conducted in Kusong, northwest of Yongbyon, where nuclear-related facilities are concentrated. In its analysis of the situation, the Japanese Defense Agency has pointed out the possible construction of a detonating test site in Kusong. The United States has reportedly found that North Korea has constructed a detonating test facility and confirmed that at least two detonating tests were conducted in 1998.¹⁶

In 2003, the Director of Central Intelligence Agency (CIA) provided their annual Worldwide Threat Briefing to the U.S. Congress. In answering subsequent questions by Congressional members concerning North Korea’s nuclear threat, the CIA director stated:

Press reports indicate North Korea has been conducting nuclear weapons-related high explosive tests since the 1980s in order to validate its weapon design(s). With such tests, we assess North Korea would not require nuclear tests to validate simple fission weapons.¹⁷

In his 2004 interview, Dr. Kim Kwang-pin (see above) stated that:

…research on uranium-type nuclear warheads in North Korea is being conducted primarily at the on-the-ground facility in Kilchu County. Most of the stockpile of highly concentrated uranium was removed from an underground test station to an underground test station in Kusong City. Researchers are now continuing with small-scale nuclear tests in this underground test station, as well as in an underground test station in Anju, South Pyongan Province. They conduct their tests in the middle of the night.¹⁸

This statement suggests that the Yongdok-tong facility was also conducting design testing of “gun-type” nuclear warheads and that additional tests were being undertaken using the “underground test station” at the Anju Coal Mine. The reference to an “on-the-ground facility in Kilchu County” is likely referring to the Punggye-ri Nuclear Testing Facility.¹⁹

Dr. Kim further stated that an explosive, known as “Chuche [Juche] Gunpowder” or “Victory 4.15” (Victory April 15) gunpowder developed during the late 1980s and early 1990s, is used in “all nuclear warheads and ordinary missiles produced by North Korea.” This would suggest that the high-explosive detonator test conducted at the Yongdok-tong facility used this explosive.²⁰

Storage Location

It is not surprising that the Yongdok-tong facility has some capacity for storing nuclear warheads, given its mission. From an operational perspective, having Yongdok-tong as the “sole” nuclear warhead storage facility, as suggested in some reports, doesn’t seem operationally viable.²¹ The self-proclaimed primary delivery system for North Korea’s nuclear weapons is its ballistic missile force, and that force is widely dispersed around the nations—often in remote northern mountainous areas. Therefore, it is probable that a small number of warhead and fissile core storage facilities are distributed around the nation to maintain operational readiness, flexibility, security, and a creditable threat.

A 1994 report covering a recently published book authored by North Korean Defector Yi Chung-kuk, who worked in the KPA’s General Staff Department’s Nuclear and Chemical Defense Bureau, claims that nuclear weapons are stored at Yongbyon:

On 12 March 1993, tensions were heightened inside North Korea’s military following the DPRK’s announcement of its withdrawal from the Nuclear Nonproliferation Treaty and the declaration of a semi-war status. Yi’s direct superior, Chief Hwang, encouraged them by saying: “We built some fortresses in the valleys along Kuryong River (the river that crosses Yongbyon nuclear complex) and dug tunnels there. Nuclear weapons are being produced and stored in the tunnels. Therefore, you do not have to worry too much about war.” Yi feels certain nuclear weapons are hidden in underground tunnels in the valleys along the Kuryong River.²²

Yi Chun-son, another defector who was reportedly a vice director of the KPA’s Operations Bureau, mentions the Anju area as having a nuclear storage facility, stating during 2001 that nuclear materials are:

…stored at an underground storage for nuclear material in Kyochangdo Valley (between Namchil-ri of Anju and Ipsok-ri [Ipsong-ni] of Mundok County) in South Pyongan Province.²³

This description is unusual as the distance between the two towns of Ipsong-ni (입성리, 39.475059 125.494187) and Namchil-li (남칠리, 39.584167 125.535556) is only 12 kilometers. In addition, the area is relatively flat with no notable valleys, making it a challenging landscape for underground storage of nuclear materials. If Yi’s description is accurate, then the presently unspecified “Kyochangdo Valley” location may be in the hills further to the east of a line drawn between these two towns.

In 2005, a South Korean source identified Jaedok-ri (재덕리) in Hangyongbukto in the northeast corner of the country as having a “nuclear storage facility.”²⁴ This reference appears to be confusing and conflating the town of Jaedok-ri (41.132526 129.164774) with the town of Punggye-ri (41.136229 129.159273), which is approximately 600 meters to the northwest. Punggye-ri serves as the primary rail and logistic support point for the Punggye-ri Nuclear Testing Facility, situated 17.2 kilometers to the north.²⁵ It is at this facility that North Korea has conducted six nuclear tests.

Number of High-Explosive Detonation Tests

One of the earlier reports concerning the actual number of high-explosive detonator tests conducted by North Korea appeared in 1996. Citing Russian sources, it reportedly states that “…plutonium is being stored in a special section in Yongbyon, and that North Korea tested over 70 times high explosive triggering devices for nuclear warheads in Yongbyon and nearby areas from 1991 to 1994.”²⁶ This report stands in contrast to an earlier 1993 Russian Foreign Intelligence Service (SVR) report titled the Proliferation of Weapons of Mass Destruction that makes no mention of any North Korean high-explosive detonation tests.²⁷

Russia has denied all reports of its involvement in North Korea’s nuclear weapons programs.²⁸

A report from 2002 appears to provide some nuanced information concerning the testing:

that the U.S. Defense Intelligence Agency informed Yi Nam-sin, chairman of the South Korean Joint Chiefs of Staff that …North Korea conducted about 70 high detonation tests in the Yongbyon nuclear facility between 1983 and 1993 and three to four tests in an area around Kusong, North Pyongan Province between 1998 and 1999.”²⁹

Subsequently, a South Korean Ministry of National Defense (MND) stated on September 10, 2015:

“It is known that North Korea has conducted more than 100 high explosive experiments since the late 1980s to develop an implosion-type detonator [and that …North Korea has not conducted any high explosive tests this year.”³⁰

While the above reports are interesting, the total number of high-explosive nuclear detonator tests conducted by North Korea, whether at the Yongdok-tong facility or elsewhere, remains publicly unknown. Given the 2015 MND statement, as well as others over the years, it is conceivable that the overall number of high explosive nuclear detonator tests performed by North Korea could potentially be as high as 150 today.

Place Names

The romanization of Korean place names is frequently challenging and often leads to great confusion. For example, references to a high explosive test facility in the Kusong area or specifically to the facility at Yongdok-tong are no exceptions to this phenomenon. Among the commonly used alternative romanizations for names associated with the Yongdok-tong facility are Kusong, Kwisong (an older name for Kusong), Yongdok-dong, Yongdoktong, Yongodong, and Youngdoktong.

These romanization challenges are made even more formidable by the fact that the Yongdok-tong facility is located on the border of two counties—Taegwan-gun (대관군) and Tongchang-gun (동창)—and close to the city of Kusong (구성시).

Disambiguation of references made over the years to a “high explosive test site,” “trigger device testing compound,” “detonating test site,” “uranium processing plant,” etc. in or near “Kusong-si” or “Kwisong” suggests that many are referring to the Yongdok-tong facility.³¹

Finally, the names of small North Korean towns, villages, and hamlets often represent areas rather than precise points. Thus, there may be homes or farms located at both the precise coordinates cited as well as clusters of homes and farms a short distance away, which are also considered to be part of the same named town, village, or hamlet. Additionally, over the years, different sources refer to the same location by various names. For consistency and ease of reading, all places’ names used in this report are those currently accepted by the U.S. National Geospatial-Intelligence Agency.

Gazetteer

This report is the third of a three-part series on the Yongdok-tong Nuclear High Explosive Test Facility, a critical component in the research, development, test, and evaluation (RDT&E) of North Korea’s nuclear warheads. It also serves as the most comprehensive report of the facility currently available at the unclassified level.¹ Part 1 provides an overview of the facility and its development history from the 1980s to the 2000s. Part 2 discusses the development history from the 2010s to the present, and Part 3 covers the underground facilities at Yongdok-tong and reports of other nuclear high-explosive testing and weapons storage locations elsewhere in North Korea.