I think no one was really surprised to learn that the diverging opinions between Iran and the P5+1 on the issue of practical needs for uranium enrichment has the potential of being a showstopper. Indeed, Iran's enrichment plans have always been a serious concern and a source of fear for those wishing for a deal. What is required by the P5+1 is known but what is not known is how far the Iranians are willing to go to save the negotiations. Of course, there is a room for political imagination here and there and face-saving actions on both sides but how will the gap between what the P5+1 is willing to accept and what Iran claims to be its practical needs be bridged?
The main two issues are the quantity and quality of centrifuges and the stockpile of enriched uranium. These two parameters, together would, to the first order, determine the breakout time Iran needs to develop a nuclear weapon if it chooses to. Analysts suggest that a timeline between six and twelve months could be achieved by limiting the number of centrifuges to between 2000 and 6000 first-generation IR-1 Iranian centrifuges (or significantly lower numbers if more advanced IR-2 centrifuges are included) and reducing enriched uranium stocks, especially near the 20% level. The scale of the gap between what is perceived as acceptable by the p5+1 and what Iran's claims to be its practical needs can be easily measured if one estimates the enrichment capacity needed by Iran to fuel the Bushehr reactor only. Such a capacity would make producing HEU for weapon purposes a walk in the park.
Any given breakout time can be achieved using many different combinations of centrifuges and enriched uranium stocks as shown in the above figure. There are trade-offs between constraints on centrifuges and constraints on stocks that will enable negotiators to consider a range of possible paths but whatever numbers and combinations are chosen, lengthening the breakout timeline to between six and twelve months would require substantial reductions in current Iranian centrifuge and stockpile levels.