Common Questions

What is stored in the Isfahan nuclear bunker?: Approximately 200 kg of 60%-enriched uranium was sealed inside a hardened underground bunker at the Uranium Conversion Facility near Isfahan on 9 June 2025, days before Israeli airstrikes. No neutral inspector has entered since.Source: Associated Press / IAEA
Where is the Uranium Conversion Facility in Iran?: The Uranium Conversion Facility (UCF) is located near Isfahan in central Iran. It converts yellowcake uranium to uranium hexafluoride, the feedstock for enrichment centrifuges.Source: IAEA
How close is 60%-enriched uranium to weapons grade?: Weapons-Grade Uranium is typically enriched to 90% or above; 60% enrichment is significantly above civilian reactor grade (3-5%) and requires relatively modest additional enrichment to reach weapons-capable levels, making it a serious proliferation concern.
Has the IAEA been able to inspect the Isfahan facility since the 2025 strikes?: No. IAEA Director General Rafael Grossi confirmed in an Associated Press interview published 29 April 2026 that no neutral inspector has entered the hardened bunker at the Uranium Conversion Facility since June 2025.Source: Associated Press

Background

The Uranium Conversion Facility (UCF) is located near Isfahan, Iran, and is the principal site in Iran's nuclear fuel cycle for converting yellowcake uranium into uranium hexafluoride (UF6), the feedstock for uranium enrichment centrifuges. The facility has been a focal point of international nuclear inspections and IAEA monitoring since the early 2000s.

The UCF gained new significance in the 2026 conflict when satellite imagery, assessed by an official speaking to the Associated Press in an interview published 29 April 2026, revealed that 18 cargo containers were moved into a hardened underground bunker at the facility on 9 June 2025 — four days before Israeli airstrikes on Iran's nuclear programme began on 13 June 2025. IAEA Director General Rafael Grossi confirmed that approximately 200 kg of 60%-enriched uranium were sealed inside the bunker and that no neutral inspector has entered the facility since June 2025.

The sealed enriched uranium stockpile represents a significant proliferation concern. 60%-enriched uranium is well above civilian reactor-grade levels (3-5%) and close to weapons-grade threshold (90%). The IAEA's loss of access to the site since the June 2025 strikes means the international community cannot verify the material's current state, location within the bunker, or whether further processing has occurred, deepening uncertainty about Iran's post-war nuclear trajectory.

Source Material

IAEA — Iran Nuclear Monitoring

How the World Sees Them

IAEA

Grossi has confirmed the sealed uranium but cannot verify its current state without inspector access; restoring IAEA monitoring at Isfahan is a stated priority for any post-war nuclear deal.

Iran

Iranian officials have not publicly acknowledged the specific quantity or enrichment level of material at the UCF bunker, insisting all nuclear activity complies with national security requirements.

Israel

Israeli intelligence assessed the pre-strike container movement as evidence Iran rushed enriched material into protected storage; the sealed stockpile is treated as a residual proliferation risk.

United States

Washington regards the sealed 60%-enriched stockpile as a central sticking point in any future nuclear talks, requiring verified IAEA re-access before sanctions relief could be considered.