Enhancing Understanding of Yongbyon Through Thermal Imagery: Part 3

Key Findings

• Thermal infrared (TIR) analysis via LANDSAT 8 and 9 imagery over two years provides key insights into Yongbyon nuclear facilities' operations, highlighting the intricate strengths and limitations of thermal imagery for operational assessment.
• This part of the series builds upon the foundational methodology and findings presented in Part 1, offering a deeper dive into the operational nuances of the Radiochemistry Lab and the centrifuge plant.
• While the thermal patterns at the Radiochemistry Lab and the centrifuge plant have occasionally aligned with known operational timelines-suggesting active reprocessing and enrichment processes-there are significant periods where thermal data alone provided ambiguous or contradictory signals. Notably, the weak or indistinguishable thermal patterns observed during some months of reported activity caution against sole reliance on thermal imagery for definitive operational assessments.
• The challenges posed by environmental and structural factors at Yongbyon, such as the topography of the valley in which the Radiochemistry Lab is located, and the heat-absorbing properties of facility roofing, further complicate thermal readings. These factors can mask or exaggerate the thermal signatures of operational status, thereby requiring a cautious interpretation of the data.

This report is the third of a three-part series that discusses thermal infrared (TIR) analysis of the Yongbyon Nuclear Research Center using LANDSAT 8 and LANDSAT 9 imagery taken in each month from January 2022 to March 2024. By comparing this thermal analysis with what is already known about the operational status at the Yongbyon Nuclear Research Center during the studied period, the report provides a reflection on whether TIR analysis shows clear indicators of the operational status of the key areas at Yongbyon. By doing so, the report can offer not only an understanding of how TIR analysis can be used in the future but also provide clear words of caution of overreliance on infrared analysis to reach conclusions when assessing the operational status of Yongbyon's key facilities.

For an overview of the findings and methodology of the study, please see part one. For a discussion of TIR analysis over the central section of the Yongbyon Nuclear Research Center, please see part two.

As part three of the series, this report focuses on the southern section of the Yongbyon Nuclear Research Center, and specifically on the Radiochemistry Lab and its associated thermal plant, as well as the centrifuge plant further to the south.

Known Operational Timelines

The Radiochemistry Lab (RCL) is where fuel rods irradiated during the 5MWe reactor's operations are sent for reprocessing to provide additional plutonium designed to expand North Korea's nuclear weapons inventory. Approximately 500 meters southeast of the laboratory is the facility's rail-served thermal plant that provides steam to support the RCL.

Activity such as visible plumes rising from the thermal plant's stack has often been associated with ongoing reprocessing campaigns or other relevant activities. When using multispectral imagery, this plume activity is the primary indicator for activity at the RCL. The UN PoE and the IAEA reported seeing such a plume between April 2022 to October 2022, and again between June 2023 to July 2023, which was assessed to have been related to waste treatment rather than a reprocessing campaign for spent fuels from the 5MWe reactor.¹

TIR imagery analysis provides an additional method of assessing operational status through land surface temperature of the RCL directly, rather than depending on indications of activity at the associated thermal plant. However, it is necessary to be conservative and cautious with assessing operational status through TIR analysis due to caveats related to sunlit surface heating. First, the RCL is situated within a valley, where heated land can radiate heat back into the air, which stays trapped within the depths of the valley compared to an open area. Additionally, the RCL has buildings with darker roofs and surface features which tend to absorb greater thermal energy, which can further contribute to a pocket of heat during calm daytime conditions.

Due to these and other reasons, the RCL is the most challenging out of the areas studied to determine operational status with TIR analysis alone. However, this does not mean TIR analysis over the area does not have value - there are instances where strong and clear thermal patterns over the RCL can contribute to an all-source intelligence analysis to reach an assessment as to its operational status. Some of these examples are provided in the following sections.

The centrifuge plant, also known as the uranium enrichment facility or uranium enrichment cascade plant, underwent expansion in 2021 and early 2022, suggesting North Korean ambitions to increase its weapons-grade uranium production.² The centrifuge plant is assessed by the UN PoE to have been continuously operational from at least February 2022 to February 2024, which accounts for a majority of the time covered by this study.³

Similar to the RCL, the centrifuge plant consists of buildings with dark roofs that are close together, which can create pockets of heat. Consistent TIR analysis, however, can suggest an increase in activity that can supplement other sources of intelligence in reaching a conclusion concerning operational status.

Strong Thermal Patterns During Operation

Out of the months studied, 16 of them displayed a strong thermal pattern at the Radiochemistry Lab, and twelve of them at the centrifuge plant.⁴ Below are a few examples when strong patterns that were observed at both locations.

On June 12, 2022, August 31, 2022, and July 1, 2023, normalized surface temperature over the buildings at the RCL and the centrifuge plant area showed strong and concentrated thermal patterns. All three dates also show a thermal pattern at cultivated fields along the eastern shore of the winding river.

During all three months, both the RCL and centrifuge plant were assessed to have been operational by the UN PoE and the IAEA.⁵ Despite the challenges of addressing all the caveats in thermal analysis mentioned in the previous section, TIR analysis supports the UN PoE and IAEA assessment that these facilities were operational during this time. This information can enhance all-source intelligence analysis, helping to draw a likely conclusion about the operational status.

Weak or No Thermal Patterns During Operation

Out of the studied period, weak or close to indistinguishable thermal patterns are observed at the Radiochemistry Lab for 10 months and at the centrifuge plant for 14 months.⁶ Two of these months, January 2023 and December 2023, are examples of when there were weak or indistinguishable thermal patterns at both locations of interest. Open-source information is inconclusive concerning the operational status of the RCL during these months, but the UN PoE assessed the centrifuge plant to have been operational.⁷ This highlights the danger of reaching conclusions concerning operational status using thermal imagery alone, as images are only snapshots of a few days' worth of activity within a month.

Conclusion

This three-part series on the TIR analysis of the Yongbyon Nuclear Research Center culminates with a focused examination of the Radiochemistry Lab and the centrifuge plant, leveraging LANDSAT 8 and LANDSAT 9 imagery. The nuanced understanding gleaned from this detailed study not only augments our knowledge of the operational status of these critical facilities but also highlights the inherent complexities of interpreting thermal imagery in isolation.

Our findings underscore the importance of integrating TIR analysis with other intelligence sources. While the thermal patterns at the RCL and the centrifuge plant have occasionally aligned with known operational timelines-suggesting active reprocessing and enrichment processes-there are significant periods where thermal data alone provided ambiguous or contradictory signals. Notably, the weak or indistinguishable thermal patterns observed during some months of reported activity caution against sole reliance on thermal imagery for definitive operational assessments.

The challenges posed by environmental and structural factors at Yongbyon, such as the valley's topography and the heat-absorbing properties of facility roofing, further complicate thermal readings. These factors can mask or exaggerate the thermal signatures of operational status, thereby requiring a cautious interpretation of the data.

In conclusion, while TIR imagery remains a valuable tool in the arsenal of satellite analysis techniques, its effectiveness is maximized when corroborated by all-source intelligence. This approach not only enriches the analytical process but also enhances the reliability of the conclusions drawn about the operational status of key nuclear facilities at Yongbyon.

Appendix

The following is the chronological collection of the closeup images over the southern section of the Yongbyon Nuclear Research Center discussed in this report. For a chronological collection of the overview images, see part one. For a chronological collection of the closeup images over the central section of Yongbyon, see part two.

Base Image Copyright © 2024 by Maxar Technologies. Image may not be republished without permission. Please contact [email protected].

Joseph S. Bermudez Jr. is senior fellow for Imagery Analysis at the Center for Strategic and International Studies.
Victor Cha is senior vice president for Asia and Korea Chair and the inaugural holder of the Korea Chair at the Center for Strategic and International Studies.
Jennifer Jun is project manager and research associate with the iDeas Lab and Korea Chair at the Center for Strategic and International Studies.
Special thanks to Jisoo Kim for research support.

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