The IAEA’s “Significant Quantity:” What It Is and What It Isn’t

December 12, 2025
George Moore

In a recent excellent article in the Bulletin of the Atomic Scientists, author Ed Lyman of the Union of Concerned Scientists considered the proliferation potential of uranium below the 20% breakpoint between Highly Enriched Uranium and High Assay Low Enriched Uranium (HALEU) material. He referred to the IAEA “Significant Quantity” (SQ) of special nuclear material (special fissile material in IAEA parlance) as follows:

The IAEA’s first safeguards glossary, published in 1980, defined high-enriched uranium (enrichment 20 percent or more) as direct-use material, with a “significant quantity” (commonly understood as the approximate amount of material that could be used to build one Nagasaki-like nuclear weapon) of 25 kilograms of contained uranium 235. It also defined low-enriched uranium (enrichment below 20 percent) as indirect-use material, with a significant quantity of 75 kilograms of contained uranium 235. These values remain the basis for the agency’s inspection goals today.¹ (emphasis added)

While it is common that the SQ is referred to as an approximate amount as done in the Lyman article, it is also often referred to as the minimum amount of material needed to build a device. However, these references are not correct.² A more nuanced understanding of the meaning of the SQ and how it interplays with IAEA safeguards established under the Non-Proliferation Treaty (NPT) is important for an informed assessment of the effectiveness of safeguards in the nonproliferation regime.

The IAEA Safeguards Glossary (2022 edition) defines the SQ as follows:

3.19. Significant quantity (SQ). The approximate amount of nuclear material for which the possibility of manufacturing a nuclear explosive device cannot be excluded. SQs take into account unavoidable losses due to conversion and manufacturing processes and should not be confused with critical masses. They are used in establishing the quantity component of the IAEA inspection goal. SQ values currently in use are given in Table 1 from the IAEA’s Glossary shown below.³

TABLE 1. Significant Quantity (SQ) Values Currently in Use

^a For plutonium containing less than 80% ²³⁸Pu.
^b Including low enriched uranium (LEU), natural uranium and depleted uranium.

The U.S. Department of Energy defines and uses different numbers due to the underlying assumptions made about the adversary. For plutonium the “DOE SQ” is 4 kilograms as opposed to the IAEA value of 8 kilograms.⁴

The SQ and the IAEA concept of “timely detection” are translated into detection goals which determine what type of inspections are needed and how frequently they must be accomplished to achieve the goal of inspecting frequently enough that an SQ cannot be diverted between inspections. The frequency of inspections is determined for each specific facility or type of facility based on the facilities ability to generate or store special fissile material and becomes part of the IAEA’s Safeguards Agreement with each NPT state to which safeguards apply.⁵

The important point to understand is that if the IAEA were to change the SQ value the inspection frequency would need to be changed. For example, were the SQ for plutonium changed from 8 kilograms to 4 kilograms, inspections would be required to be about twice as often. Such a change would have a huge impact on the funding requirements for IAEA Safeguards and since it can be said that the IAEA is chronically underfunded, changing the SQ values used by the IAEA becomes a politically charged issue among the member states.

What does adhering to the current IAEA SQ levels mean for nonproliferation assurances based on reliance on IAEA Safeguards? If enough material can be diverted for a weapon without IAEA detection based on its current SQ levels, then there is an arguable “hole” in the system which needs to be recognized,⁶ continually evaluated, and adjusted if the risk associated with not changing any of the SQ values is too great.

Notes:

1. E. Lyman, “Declassified cable reinforces proliferation concerns about high-assay low-enriched uranium fuel,” Bulletin of the Atomic Scientists, November 7, 2015. Available at: https://thebulletin.org/2025/11/declassified-cable-reinforces-proliferation-concerns-about-high-assay-low-enriched-uranium-fuel/#post-heading
2. In all probability the description in the Lyman article was to simplify the SQ without going into rigorous detail.
3. IAEA Safeguards Glossary, Vienna 2022, page 31. Available at: https://www-pub.iaea.org/MTCD/Publications/PDF/PUB2003_web.pdf
4. C. Bathke, “Commonly Overlooked Material Attractiveness Issues,” Los Alamos report LA-UR-21-23091 issued 3/21/2021. Available at: https://srswatch.org/wp-content/uploads/2021/04/Commonly-Overlooked-Material-Attractiveness-Issues-LANL-March-31-2021.pdf
5. https://www.nrc.gov/docs/ML0724/ML072490579.pdf
   https://npolicy.org/adequacy-of-iaeas-safeguards-for-achieving-timely-detection/
6. See, for example the discussion in B, Goddard, A. Solodov, V. Fedchenko, “IAEA “significant quantity” values: time for a closer look?” The Nonproliferation Review 23(5-6:667-689, November 2016. Available via: https://www.researchgate.net/publication/319023807_IAEA_significant_quantity_values_time_for_a_closer_look