It's amassing low-enriched uranium at the rate of 2.75 kilograms per day – enough to give Iran nuclear program enough feedstock for two bombs by February, by one estimate.
Day after day, this amount is added to Iran's existing stockpile of low-enriched uranium (LEU), estimated by the US to total about 1,600 kilograms.
This material can be used as the feedstock for weapons-grade uranium. That is why US experts worry about it so much – and why the West wants Iran to ship most of it abroad, where it can be converted into a much less dangerous form of reactor fuel.
Iran's enrichment program "is the pacing element in determining when [Iran] would be capable of making a nuclear weapon," said Paul Pillar, a former US intelligence officer who is now a professor of security studies at Georgetown University, at a recent Arms Control Association seminar on the Iranian nuclear challenge.
Right now it is far from a sure thing that Iran will agree to the US-backed proposal that it send most of its LEU to Russia for processing.
Iranian officials have sent mixed signals on the proposal. On Monday, the head of the UN nuclear agency urged Iran to clarify its response.