The Iran enriched uranium stockpile is the problem the bombs could not solve. When the United States and Israel struck Iran’s nuclear facilities in June 2025, the goal was to end Tehran’s path to a nuclear weapon. Nine months later, roughly 400 kilograms of uranium enriched to 60 percent purity remain intact somewhere in Iran, and the international community has no way to verify where it is or what is being done with it.
Here is what that means, what the options are, and why this matters to everyone.
What Happened to Iran’s Enriched Uranium Stockpile
Before the June 2025 strikes, the International Atomic Energy Agency confirmed that Iran held 440.9 kilograms of uranium enriched to 60 percent U-235. The U.S. and Israeli bombing campaign targeted enrichment facilities at Natanz, Fordow, and Isfahan, destroying centrifuges and above-ground infrastructure. But the enriched uranium itself, stored in heavy metal cylinders roughly the size of scuba tanks, was not destroyed.
The reason is simple: the material was buried too deep. The Chairman of the Joint Chiefs, General Dan Caine, told Congress that even the largest U.S. conventional bomb could not reach the underground storage area at Isfahan. Instead, the military targeted tunnel entrances to block access.
That did not work for long. Within days of the June bombings, satellite imagery showed Iran clearing at least one tunnel entrance at Isfahan. By early February 2026, satellite images showed that all entrances had been completely sealed with soil, suggesting Iran moved material inside for protection.
Where Is the Material Now?
Nobody outside Iran knows for certain. IAEA Director General Rafael Grossi said on March 9 that the agency estimates roughly 200 kilograms of 60 percent enriched uranium is underground at Isfahan. Some material may remain at Natanz. The rest is unaccounted for.
The IAEA has been locked out. In a February 2026 report, the agency stated it “cannot provide any information on the current size, composition or whereabouts of the stockpile of enriched uranium in Iran.” Iran told the IAEA in a February 2 letter that normal safeguards were “legally untenable and materially impracticable” following the strikes.
Why 400 Kilograms Matters
Uranium enriched to 60 percent is not technically “weapons-grade,” which is usually defined as 90 percent. But experts say the distinction is less meaningful than it sounds. According to nuclear physicist Edwin Lyman of the Union of Concerned Scientists, writing in the Bulletin of the Atomic Scientists, 60 percent enriched uranium is already “weapon usable” and could be fashioned into crude nuclear weapons without any further enrichment.
At 60 percent, Lyman estimates Iran’s stockpile could yield roughly 6 to 7 implosion-type weapons, or fewer but simpler gun-type devices. If Iran chose to enrich the material to 90 percent first, the yield rises to 9 or 10 weapons. Either way, the material already in Iran’s possession represents a serious arsenal.
IAEA Director General Grossi put it bluntly: “At the end of this… the material will still be there, the enrichment capacities will be there.”
What Are the Options?
Military seizure. The Trump administration has discussed sending special operations forces to physically recover the enriched uranium from Isfahan’s tunnels. CNN reported that this would require not just elite commandos but dozens if not hundreds of support troops, specialized nuclear handling equipment, and days on the ground in hostile territory. The material is toxic and can sustain a fission reaction, making transport extremely dangerous.
Negotiated removal. Before the February 28 strikes, Iran had signaled willingness to deal. Foreign Minister Abbas Araghchi told CBS in March that he had offered to “dilute those enriched material, or down-blend them, as they say, into lower percentage.” Deputy Foreign Minister Takht-Ravanchi told the BBC that Tehran was ready to discuss curbs, including on its stockpile, in exchange for sanctions relief. But those talks collapsed when the strikes began.
Continued standoff. The current trajectory. The material stays underground, Iran refuses access, the IAEA cannot verify anything, and the risk of weaponization remains.
The Bigger Picture
The strikes destroyed infrastructure but not the knowledge, the material, or the motivation. As political scientist Rupal Mehta of the University of Nebraska wrote for LSE, the attacks may have “turned Iran from a state with a latent nuclear capability into one with a nuclear grievance.” Before the strikes, nuclear ambiguity was a bargaining chip. After them, a weapon may look like an insurance policy.
Meanwhile, under new Supreme Leader Mojtaba Khamenei, Iran’s leadership has strong ties to the Revolutionary Guard, the body most likely controlling access to the enriched uranium. And the IAEA has confirmed it sees no evidence of a structured weapons program, but has also warned that without access, it cannot rule one out.
The 400 kilograms of enriched uranium were the one thing the strikes needed to neutralize and the one thing they could not reach. Every path forward now runs through that material.
The Iran enriched uranium stockpile remains the unresolved core of the crisis. Nine months after Operation Midnight Hammer and weeks into Operation Epic Fury, the central proliferation risk that justified both campaigns is intact: approximately 400 kilograms of uranium hexafluoride enriched to 60 percent U-235, stored in cylinders across one or more underground sites, inaccessible to international inspectors and resistant to conventional military destruction.
The Iran Enriched Uranium Stockpile: What the IAEA Knows
The IAEA’s September 2025 verification report placed Iran’s 60 percent stockpile at 440.9 kilograms on the eve of the June 2025 strikes. The agency has not been permitted to verify the stockpile since. Its February 27, 2026 report (GOV/2026/8) stated it “cannot provide any information on the current size, composition or whereabouts” of Iran’s enriched uranium. Iran informed the IAEA in a February 2 letter that safeguards were “legally untenable and materially impracticable.”
Grossi’s March 9 assessment placed roughly 200 kilograms at Isfahan’s underground tunnel complex, with additional material possibly at Natanz and potentially at undeclared locations. The IAEA’s “continuity of knowledge” over Iran’s nuclear material has been broken, a situation Grossi has called urgent.
Separative Work and Breakout Calculus
The proliferation significance of the 60 percent stockpile is best understood through the lens of separative work. Robert Goldston, a physicist and member of the Council for a Livable World Board, calculated that producing the 440.9 kg stockpile required approximately 55,330 separative work units (SWU). Further enriching that entire stockpile to weapons-grade (90 percent) would require only 564 SWU, roughly 1 percent of the work already invested. Put differently: 99 percent of the enrichment effort needed for weapons-grade material has already been completed.
At the cascade level, a single string of 175 IR-6 centrifuges could produce one weapon’s worth of 90 percent material every 25 days starting from 60 percent feedstock. Multiple cascades would reduce this proportionally. The IAEA lost oversight of Iran’s centrifuge manufacturing in 2021, meaning Tehran may possess undeclared cascades that have never been inspected.
Weapon Usability at 60 Percent
A persistent misconception in public discourse is that “weapons-grade” (90 percent) is a binary threshold. It is not. The IAEA classifies all uranium above 20 percent as “direct use material,” meaning it can be used for the manufacture of nuclear explosive devices without further enrichment.
Edwin Lyman of the Union of Concerned Scientists, writing in the Bulletin of the Atomic Scientists, assessed that Iran’s stockpile could produce roughly 6 to 7 Fat Man-type implosion weaponsA nuclear weapon that detonates by using conventional explosives to compress fissile material inward, triggering a chain reaction. More efficient than gun-type designs. at 60 percent enrichment, compared to 9 to 10 at 90 percent. Alternatively, gun-type devices similar to the Little Boy design used at Hiroshima could be built with larger quantities of 60 percent material, though with reduced yield predictability.
The practical bottleneck is not enrichment but conversion: turning uranium hexafluoride gas into uranium metal. The Isfahan conversion facility was destroyed in the June 2025 strikes. However, as Lyman notes, a small-scale clandestine conversion capability “does not appear nearly as daunting a task, given that Iran already knows how to do it.”
The Isfahan Problem
Isfahan is the epicenter of the crisis. The underground tunnel complex is too deep for even the Massive Ordnance Penetrator, the largest conventional weapon in the U.S. arsenal, according to testimony by the Chairman of the Joint Chiefs. The U.S. instead targeted tunnel entrances in June 2025, but Iran cleared them within days.
By early February 2026, satellite imagery showed all entrances backfilled and sealed with earth, suggesting Iran moved material inside for protection. Former Los Alamos radiochemist Cheryl Rofer estimated the stockpile occupies 30 to 60 cylinders, each requiring careful separation to prevent nuclear criticalityThe condition in a nuclear reactor where each fission event produces exactly one neutron that triggers another fission, sustaining a steady chain reaction..
The material is stored as UF6 gas, which is both toxic and capable of sustaining a fission reaction at 60 percent enrichment. Any recovery or destruction operation would require specialized CBRNE teams, with risks including hydrofluoric acid exposure if cylinders are compromised.
Military Options and Their Limits
The Trump administration has discussed deploying Joint Special Operations Command units, potentially alongside Israeli commandos, to infiltrate Isfahan and either secure or destroy the material. CNN reported that such an operation would require dozens if not hundreds of troops for outer security, logistical support, and specialized nuclear handling, with a mission lasting days in hostile territory several hundred miles from the nearest U.S. naval assets.
The alternative of destroying the material in place carries its own problems: dispersed uranyl fluoride contamination, no reliable damage assessment, and the possibility that Iran could eventually recover usable material from the debris.
IAEA chief Grossi has stated plainly that military force cannot eliminate Iran’s nuclear program. “At the end of this,” he said, “the material will still be there, the enrichment capacities will be there. We will have to go back to some form of negotiation.”
The Diplomatic Track That Was
The irony is that a diplomatic solution was on the table days before Operation Epic Fury began. The Arms Control Association reported that during three rounds of Oman-mediated talks, Iran proposed pausing enrichment, accepting monitoring, and not accumulating enriched uranium gas. Omani mediators described the third round, held February 26, as having achieved “substantial progress.”
Foreign Minister Araghchi publicly stated on March 15 that he had offered to down-blend the 60 percent material to lower enrichment levels under IAEA supervision. Deputy Foreign Minister Takht-Ravanchi told the BBC that Tehran was prepared to discuss its stockpile in exchange for sanctions relief, though zero enrichment remained off the table.
Two days after that “substantial progress,” the United States and Israel struck, killing Supreme Leader Khamenei and collapsing the diplomatic channel. Araghchi subsequently indicated that prospects for renewed talks had collapsed.
Strategic Implications
The strikes have created what political scientist Rupal Mehta describes as a fundamental shift in Iran’s nuclear calculus. As she argued at LSE, Iran has “transitioned from a state with a latent nuclear capability to a state with a nuclear grievance.” The strategic ambiguity that once served as deterrent failed to prevent decapitation strikes, potentially convincing Tehran’s new leadership that an unfinished weapon was the fatal mistake.
Under new Supreme Leader Mojtaba Khamenei, who maintains strong ties to the IRGCIran's Islamic Revolutionary Guard Corps, an elite military and security organization that operates independently from conventional armed forces and oversees external operations and proxy networks., the material is effectively under Revolutionary Guard control. The IAEA has confirmed no evidence of a structured weapons program, but the loss of verification access means that assessment carries diminishing confidence over time.
The fundamental problem remains: the strikes destroyed what was visible and accessible. The Iran enriched uranium stockpile, the one element that converts latent capability into acute proliferation risk, is neither visible nor accessible. Every scenario for resolving this crisis, whether military, diplomatic, or some combination, must account for 30 to 60 cylinders of uranium hexafluoride buried under meters of Iranian soil. None of the current options does so adequately.
