The number that matters most in the Iran nuclear debate is not a warhead count or a missile range. It is 440.9 kilograms — the amount of uranium enriched to 60% purity that the International Atomic Energy Agency confirmed Iran possessed before the war began. [2] Understanding why that figure alarms nonproliferation experts requires understanding a counterintuitive fact about the physics of enrichment: most of the work happens at the bottom of the ladder, not the top.

Natural uranium contains roughly 0.7% of the fissile isotope U-235. The 2015 Joint Comprehensive Plan of Action limited Iran to enriching uranium to 3.67%, a level suitable for civilian nuclear power. Weapons-grade material requires enrichment to approximately 90%. Iran crossed the 60% threshold in 2022 and has been accumulating material at that level since, a purity with no credible civilian justification. [3]

The physics of "separative work units" — the industry measure of enrichment effort — creates a dynamic that makes the final sprint far easier than the long climb. Enriching natural uranium from 0.7% to 3.67% consumes roughly 70% of the total separative work required to reach 90%. [1] Going from 3.67% to 60% takes most of the remaining effort. The jump from 60% to 90%, by contrast, requires a fraction of the total energy and far fewer centrifuge cascades. In practical terms, the hardest part is already done.

A cascade of approximately 200 advanced IR-6 centrifuges — a modest installation by Iranian standards — could enrich 50 kilograms of feedstock from 60% to 90% in roughly 10 days. [1] That quantity exceeds the approximately 42 kilograms of highly enriched uranium needed for a single implosion-type weapon at the critical mass threshold.

IAEA Director General Rafael Grossi told reporters in Paris that more than 200 kilograms of the 60% stockpile was stored in a tunnel complex at Isfahan, the one facility that appears to have survived the June 2025 U.S.-Israeli strikes largely intact. [2] Natanz and Fordow, Iran's other declared enrichment sites, suffered severe damage. But the Isfahan tunnels — dug 60 to 90 meters underground — held. Satellite imagery shows regular vehicular activity at the site, though the IAEA has been denied physical access since the strikes.

The Arms Control Association estimates that Iran's post-strike breakout time — the period needed to produce enough weapons-grade uranium for a single device — stands at roughly 12 weeks, significantly longer than the pre-strike estimate of one to two weeks. [3] But that calculation assumes Iran is starting from its current, diminished centrifuge capacity. If Tehran has rebuilt or relocated cascades in facilities the IAEA cannot inspect, the real number could be shorter.

The United States has demanded that Iran return to 3.67% enrichment as a precondition for any deal to end the war. Iran has indicated willingness to cap enrichment levels but has refused to stop entirely or surrender its stockpile. The gap between those two positions is not just diplomatic. It is measured in separative work units — and at 60%, Iran has already completed the overwhelming majority of the journey.

The question is no longer whether Iran can build a weapon. It is whether anyone would know in time if it decided to try.

-- KENJI NAKAMURA, Tokyo