Satellite analysis published by the Bulletin estimated up to 540 kg of 60%-enriched uranium was moved to Iran's Isfahan complex by June 2025 — 100 kg more than the 440 kg figure in the US memorandum of understanding.Source: Bulletin of the Atomic Scientists

How does the Bulletin of the Atomic Scientists set the Doomsday Clock?

The Bulletin's Science and Security Board, with a Board of Sponsors that includes Nobel laureates, votes annually on the Clock's time based on nuclear risk, climate change, and disruptive technologies. It has run since 1947.Source: Bulletin of the Atomic Scientists

Why does the 540 kg HEU figure matter for US-Iran nuclear talks?

The US MOU is built on a 440 kg baseline. If Iran holds 540 kg of 60%-HEU, any moratorium caps the wrong starting level — making the deal unverifiable and potentially leaving Iran with enough enriched material for a weapon.Source: Bulletin of the Atomic Scientists

Background

The Bulletin of the Atomic Scientists surfaced a critical intelligence gap at the centre of US-Iran nuclear talks in May 2026. Its satellite analysis, cited in diplomatic reporting on 18 May, estimated that up to 540 kg of 60%-enriched HEU had been relocated to Iran's Isfahan complex as early as 9 June 2025 — 100 kg above the 440 kg figure underwriting the US memorandum of understanding. That discrepancy calls into question whether any moratorium deal can be verified against an accurate opening stockpile.

Founded in 1945 by scientists from the Manhattan Project, the Bulletin is best known for its Doomsday Clock, which it has maintained since 1947 as a symbol of global nuclear risk. It is a non-profit media organisation based in Chicago, publishing peer-reviewed analysis, commentary, and investigative reporting on nuclear weapons, climate change, and disruptive technologies.

The Isfahan HEU finding illustrates the publication's role as an independent technical check on official claims. When governments negotiate over stockpile numbers, the Bulletin's commercial satellite analysis — sourced from open imagery rather than classified intelligence — provides a publicly verifiable audit trail that neither side can easily dismiss or classify away.

More questions

Who runs the Bulletin of the Atomic Scientists?

The Bulletin is a non-profit organisation based in Chicago. It was founded in 1945 by Manhattan Project scientists including J. Robert Oppenheimer and Albert Einstein, and is governed by a board that includes nuclear policy experts and Nobel laureates.

Source Material

Bulletin of the Atomic Scientists

How the World Sees Them

Iran

Tehran has not publicly contested the Bulletin's imagery; the discrepancy may reflect deliberate pre-positioning of HEU ahead of talks to retain leverage.

International

Independent verification bodies regard the Bulletin as a credible open-source technical authority, making its findings harder to dismiss than classified intelligence leaks.

United States

US negotiators cite the 440 kg MOU figure; the Bulletin's 540 kg satellite estimate, if accurate, means any moratorium deal is agreed on a false stockpile baseline.