If approved, the deal would commit Iran to temporarily exporting 75 percent of its known stockpile of low-grade nuclear fuel to Russia, or about 2,600 pounds of low-enriched uranium, for additional enrichment. Negotiators say that would prevent the possibility that Iran could turn the fuel into weapons-grade material anytime soon…

But the key to the agreement reached in the talks, if it works, would be in the timing of the shipments — a detail officials were not discussing in Vienna in the hours after the announcement. If Iran actually sends the low-enriched uranium to Russia in a single shipment, as the draft document states, it would have too little fuel on hand to build a nuclear weapon for roughly a year, according to the agency’s experts. If the fuel leaves Iran in batches, the experts warn, Iran would have the ability to replace it almost as quickly as it leaves the country.

Also of concern is the possibility that Iran might have more nuclear fuel than it is letting on.