Catherine DillNorth Korea’s Uranium Conversion: The History and Process, Part 2

This is a guest post by my colleague Jamie Withorne. Jamie is a Research Assistant at the James Martin Center for Nonproliferation Studies. Her work focuses on emerging technology, North Korea sanctions evasion, and North Korea’s nuclear fuel cycle.

This is Part Two of a three-part series on North Korea’s uranium conversion program. For Part One, see here.

Part Two: North Korea’s Uranium Conversion During the Six-Party Talks and Beyond (2003-November 2010):

During the 2003-2010 timeframe, North Korea restarted parts of its conversion line, changed some of its conversion processes, and declared uranium hexafluoride (UF₆) production capabilities.

Figure 1: Timeline of North Korea conversion activity from 2002, where the orange highlight indicates a process or operational status change

Timeline of North Korea conversion activity from 2002, where the orange highlight indicates a process or operational status change

North Korea’s nuclear fuel cycle inactivity during the Agreed Framework freeze (from around October 1994 until October 2002) paired with the highly corrosive nature of fluorine (a product necessary for uranium conversion) meant that when the freeze ended, North Korea’s uranium conversion capabilities were damaged. In a safeguards report, the International Atomic Energy Agency (IAEA) noted that some key conversion equipment was removed from the Fuel Fabrication Plant (FFP) prior to the Agreed Framework, and that in 1992 the conversion line was already in falling into disrepair.[1] During the freeze the conversion line deteriorated further, especially parts of the line located in the uranium tetrafluoride (UF₄) production building (Building 2, identified in Figure 2 below). This deterioration suggests that when the Agreed Framework ended in October 2002, North Korea had to spend time making repairs to its conversion line before it would once again be fully operational.

 Figure 2: The process flow at North Korea’s Yongbyon Fuel Fabrication Plant [2]

Uranium conversion process flow at Yongbyon

In early August 2003, North Korea stated it was willing to attend the Six-Party Talks. After several rounds of negotiation, in 2005, North Korea agreed to abandon “all nuclear weapons and existing nuclear programs.”[3] In August 2005 Dr. Siegfried Hecker visited the Yongbyon nuclear site. During this visit, the director of Yongbyon confirmed that parts of the FFP had corroded and collapsed during the Agreed Framework freeze and that North Korea was in the process of refurbishing its conversion capabilities.[4] During another visit in November 2006, the Yongbyon director told Hecker that North Korea was finalizing conversion facility preparations and had a goal of producing new fuel by 2007. The director also claimed that North Korea possessed the required amount of stainless steel necessary for uranium conversion equipment.

On February 13, 2007 North Korea agreed to several nuclear disablement steps to work towards its 2005 declaration. For the conversion line, these steps included: removal and storage of three uranium ore concentrate dissolver tanks; removal and storage of seven uranium conversion furnaces, including storage of refractory bricks and mortar sand; removal and storage of metal casting furnaces and associated vacuum system; removal of eight machining lathes; and storage of approximately five tons of uranium trioxide (UO₃) powder in bags monitored by the IAEA.[5] These disablement measures were implemented on July 15, 2007. The removed equipment mainly came from the fuel fabrication facility (Buildings 4 and 5, identified in Figure 2 above) and was stored in-country. Also in July 2007, the IAEA, “for the first time, observed a small scale research and development (R&D) UF₄ conversion apparatus using a dry process.”[6] In a later report, Hecker states this R&D UF₄ mechanism was located in Building 2 and was primitive.[7]

In February 2008, Hecker was invited back to Yongbyon to verify the disablement steps with photographs.[8] In the trip report, Hecker disclosed that the beginning of the conversion line (Building 1) had been operational and producing uranium dioxide (UO₂) until July 15, 2007.[9] He explained that the fuel fabrication facility (Buildings 4 and 5) was also operational until July 15, but the UF₄ production building (Building 2) was not operational during that time period. Hecker visited Building 2 during this trip and confirmed that it had been abandoned, indicating the previously observed R&D UF₄ line had moved facilities. Hecker stated that the North Korea relocated the dry R&D UF₄ production equipment to “one of the ancillary buildings of the FFP,” though it is unclear if he was able to directly observe the small scale line. This suggests that by 2008 a new type of dry UF₄ production was becoming operational on-site at the Yongbyon FFP, though it is unclear where.

Based on the provided information, it is possible that from October 2002 until July 2007 North Korea spent time repairing damage inflicted on its conversion line. It is generally accepted that from October 2002 until July 2007 North Korea was unable to implement a new full scale dry UF₄ production line. However, based on Hecker’s report, it is likely North Korea was able to repair damage done to Building 1 and resume UO₂ production there prior to the July 2007 disablement, though it is unclear when this activity officially restarted. The July 15 disablement steps removed nearly five tons of UO₃, an intermediate uranium product in UO₂ production. While it is unclear when exactly these five tons of UO₃ were produced, given the length of the Agreed Framework freeze and storage requirements of UO₃, it is possible that production occurred after the Agreed Framework freeze (October 2002) but before the Six-Party Talks disablement steps (July 2007). However, as this was a large amount of UO₃ and there were difficulties with IAEA inspections during the Agreed Framework, it is also possible that North Korea continued UO₃ and UO₂ production covertly, or undetected, throughout the Agreed Framework freeze.[10]

Figure 3: UO₃ Observed by Hecker after the July 2007 disablement [11].

Observed by Hecker after the July 2007 disablement
Photo credit: Siegfried Hecker

It is notable that there was a change in North Korea’s conversion process observed by the IAEA and Hecker during this time period. Specifically, North Korea shifted from a wet to a dry UF₄ production line according to the observations. Conversely, because operations at Building 1 restarted relatively quickly and a large amount of UO₃ was produced, it is possible North Korea did not change their UO₂ production process. This suggests that although North Korea shifted from a wet to a dry UF₄ production line, it is likely they continued to employ wet UO₂ production processes in Building 1 during this time frame.

Beyond the Six-Party Talks (April 2009-Present)

In September 2008 North Korea began reversing its disablement steps, removing IAEA safeguards equipment and only intermittently allowing international inspectors. In April 2009 North Korea expelled the IAEA and completely removed all safeguards equipment, declaring it would restart its nuclear reactor. By the end of August 2009 the Korea Central News Agency (KCNA) reported that “measures [were] taken to restore nuclear facilities at Yongbyon.”[12]

Since April 2009, there have been limited visits from outsiders to Yongbyon. On November 12, 2010 Hecker was once again invited back to Yongbyon. During this visit, North Korea revealed construction of an experimental light-water reactor and a uranium enrichment facility, suggesting that North Korea had made advances in its nuclear program since resuming unmonitored operations in mid- to late- 2009.[13]

Hecker reported that the uranium enrichment workshop was constructed in what was previously the fuel fabrication facility at the FFP (Buildings 4 and 5). He observed that the enrichment workshop was modern and sophisticated, and that North Korea had begun construction on it as early as April 2009, completing construction a few days prior to Hecker’s visit.

When revealing the new enrichment capabilities, North Korea claimed that it “produce(s) uranium hexafluoride,” and had enough for “sufficient throughput for the size of the centrifuge facility.”[14] Hecker was told that North Korea had an operational anhydrous hydrofluorination (i.e., dry UF₄ production) line, that was located in a building on-site, though the building was not identified nor were process descriptions provided. This visit was also the first time North Korea declared UF₆ production capabilities, but Hecker was not shown or told where or how this production was occurring.

Because the enrichment facility was sophisticated and was constructed in a short time frame, several analysts believe that North Korea developed one or more pilot (or R&D) enrichment facilities elsewhere in the country before 2010. In July 2018 open-source researchers identified a facility just south of Pyongyang, near Chollima, as a potential pilot uranium enrichment site (commonly referred to as “Kangson”).[15] Construction of the identified facility was of interest to US intelligence officials as early as 2007 and appeared complete in available satellite imagery by 2009. In a November 2020 statement, the Director General of the IAEA stated that the Kangson facility was likely a nuclear facility.[16] Given this, it is possible uranium enrichment occurred at Kangson sometime after 2007, but before November 2010. In order to enrich uranium at this facility, North Korea would have required UF₆, potentially suggesting that North Korea was able to produce UF₆ sometime between 2007-2010, prior to its November 2010 UF₆ declaration. Moreover, like with uranium enrichment facilities, it is possible North Korea operated, and continues to operate, more than one uranium conversion site with a UF₆ line; though, if true, it is unknown where or how.

Considering this timeline and historic review, it currently remains unknown precisely where and how North Korea is producing UF₄ and UF₆ at Yongbyon. The number of uranium conversion facilities (and their respective location and processes) North Korea might be operating outside of Yongbyon, if any, also remains unknown.

[1] IAEA, “Application of Safeguards in the Democratic People’s Republic of Korea,” GOV/2011/53-GC(55)/24, September 2, 2011.

[2] This graphic is a reproduction of Figure 9 from Chaim Braun, Siegfried Hecker, Chris Lawrence, Panos Papadiamantis, “North Korea Nuclear Facilities After the Agreed Framework,” Center for International Security and Cooperation at Stanford University, May 27, 2016 <>.

[3] Paul Kerr, “North Korea Increasing Weapons Capabilities,” Arms Control Association, December, 2005, <>.

[4] Siegfried Hecker, “Report on North Korean Nuclear Program,” Center for International Security and Cooperation at Stanford University, November 15, 2006, <>.

[5] Chaim Braun et al., “North Korea Nuclear Facilities After the Agreed Framework.”

[6] IAEA, GOV/2011/53-GC(55)/24.

[7] Chaim Braun et al., “North Korea Nuclear Facilities After the Agreed Framework.”

[8] “Hecker shares findings from North Korea trip,” Center for International Security and Cooperation at Stanford University, February 20, 2008, <>.

[9] Siegfried Hecker, “Report of Visit to the Democratic People’s Republic of North Korea (DPRK),” Center for International Security and Cooperation at Stanford University, March 14, 2008, <>.

[10] See Part One of this series for more information on IAEA limitations during the Agreed Framework.

[11] “Hecker shares findings from North Korea trip,” Center for International Security and Cooperation at Stanford University.

[12] “DPRK Completes Reprocessing of Spent Fuel Rods,” NK News KCNA Watch, November 3, 2009, <>.

[13] Siegfried Hecker, “A Return Trip to North Korea’s Yongbyon Nuclear Complex,” Center for International Security and Cooperation at Stanford University, November 20, 2010, <>.

[14] Ibid.

[15] Ankit Panda, “Exclusive: Revealing Kangson, North Korea’s First Covert Uranium Enrichment Site,” The Diplomat, July 13, 2018, <>.

[16] “IAEA suspects Kangson facility of enriched uranium production,” Hankyoreh, November 20, 2020, <>.

This work is part of an ongoing joint project between the Verification Research, Training and Information Centre (VERTIC), the James Martin Center for Nonproliferation Studies (CNS) at the Middlebury Institute of International Affairs (MIIS), and the Royal United Services Institute (RUSI). The project seeks to update and systematize analysis of North Korea’s nuclear complex, using remote sensing data and industry-standard nuclear fuel cycle modelling to generate independent models of plausible scenarios for that complex, in the past, present and future, and to use those results to assess priorities for control and verification. The project is generously funded by Global Affairs Canada.