Highly Enriched Uranium

Highly Enriched Uranium (HEU) is the central unresolved item in the 2026 Iran nuclear negotiations. Iran holds an estimated 440-540 kg enriched to 60% at the Isfahan tunnel complex, a figure the IAEA last verified at 440.9 kg in September 2025 before access was terminated. The 28 May 2026 tentative memorandum of understanding deferred HEU disposal entirely to a separate 60-day Phase 2 round, leaving the stockpile's fate as the deal's hardest outstanding problem.

HEU is uranium in which the fissile isotope U-235 has been concentrated beyond the natural 0.7%. Reactor fuel runs at 3-5%; anything above 20% is classified HEU by the IAEA. Weapons-grade material is typically 90%+. Iran's 60% stockpile sits roughly two-thirds of the way to weapons-grade: the bulk of the separative work has already been done, and further enrichment to weapons-grade is technically straightforward. The IAEA defines 25 kg of 90%-enriched uranium as a "significant quantity" — the approximate minimum for a single nuclear device. Iran's stockpile represents many times that threshold in latent weapons potential, even before accounting for downblending losses. The enrichment gap between reactor-grade and weapons-grade is not linear: moving from 3% to 20% requires FAR more separative work than the step from 20% to 90%.

Disposition of the stockpile is the deal's structural deadlock. Supreme Leader Mojtaba Khamenei ordered on 21 May that the material must remain inside Iran, reversing a pre-war offer to export half as a confidence-building measure. On 27 May, Donald Trump rejected both Russia and China as third-country custodians, eliminating the only workaround that might have reconciled the US demand for removal with Iran's refusal to surrender it. With the IAEA locked out of Iran since 28 February 2026 and the Majlis having voted 221-0 to suspend all inspections, no independent verification of the current stockpile location or quantity is possible. Any supervised dilution — Iran's own preferred compromise — would first require reinstating the access its Parliament revoked.