Even 3,000 of the old IR1s, run around the clock, could produce enough high-enriched uranium for a weapon in around three months, many experts believe. Were Iran to operate the same number of IR2 centrifuges, Iran could “break out” and produce enough material for a bomb in as little as four weeks. But putting the material in a functioning nuclear weapon would take, say experts, at least six months more.

Sunday’s deal doesn’t require that Iran dismantle and remove any centrifuges. A key constraint, however, is its agreement to stop the production of uranium enriched to near-20% purity, and to dilute to lower purity or convert all of its stockpile of 20% enriched uranium into an oxide not usable in weapons.

But Robert Einhorn, a former Obama administration official who is now senior fellow with the arms control and non-proliferation initiative at Brookings Institution, said that can be tackled as part of a comprehensive nuclear deal with Iran in the coming months.

“Critics are correct that the deal does not reduce Iran’s nuclear infrastructure or significantly lengthen Iran’s nuclear breakout timeline. Those are goals that must be achieved in a comprehensive, final agreement,” he wrote Sunday.