U.S. Lays Out Limits It Seeks in Iran Nuclear Talks VIENNA - As six world powers and Iran race to meet a Monday deadline for an agreement that would constrain Iran's nuclear program, the United States has staked out an ambitious goal for what an accord should accomplish. American officials say the agreement should slow the Iranian nuclear program enough that it would take Iran at least a year to make enough material for a nuclear bomb if it decided to ignore the accord. "Our goal is to shut off each pathway sufficient that we know we have a breakout time of a minimum of a year," Secretary of State John Kerry said last month. The logic behind the goal is clear: to give the United States and its allies the time to react diplomatically or with fresh economic sanctions, leaving the option of using military force as a last resort. But translating this simple goal into a verifiable agreement has been devilishly difficult. The United States long ago dropped the goal of eliminating Iran's enrichment ability, a demand that Israel has long insisted was the surest way to guarantee Iran did not maintain an option to pursue the development of nuclear arms. So the negotiations have been focused on measures that would constrain Iran's ability to quickly produce a nuclear bomb but allow it the ability to maintain what Iran insists is a peaceful program of nuclear power and research. Shortly after arriving here Thursday night to join the talks, Mr. Kerry met for more than two hours with Mohammad Javad Zarif, Iran's foreign minister, and Catherine Ashton, the European Union's envoy to the Iran talks. But there was no word on whether they had made any headway. One critical question is how many and what type of centrifuges Iran will be allowed to keep to continue enriching uranium. Another question is what happens to the nuclear material Iran already possesses, which could be used in making a bomb. A third question is what kind of measures can be taken to limit Iran's ability to produce plutonium, which can also be used in producing a nuclear weapon. Affecting all of these issues is the length of time an accord would be in effect. At the same time, the Western and Iranian negotiators must agree on a schedule for suspending or lifting sanctions, Iran's goal in the talks, while preserving the West's leverage in case the accord begins to fray. Iran has nearly 10,000 operational centrifuges, which Iran insists will be used to make fuel for civilian reactors. But given Iran's current abilities, Mr. Kerry has said that it would not take Iran long to produce enough enriched material for a bomb if it abandoned the temporary freeze it had been observing while talks are underway. "I think it's public knowledge today that we're operating with a time period for a so-called "breakout" of about two months," Mr. Kerry told the Senate Foreign Relations Committee in April. It would take far longer for Iran to take a bomb's worth of fissile material, which can sustain a chain reaction, and fashion it into a weapon that could be delivered by a plane or missile. But since such work might be hard to detect and would be beyond the scope of an agreement, the breakout time to produce fissile material for a weapon has become an important indicator of Iran's intentions. "Enrichment time needs to be pushed to a year," said Gary Samore, a former senior National Security Council official and president of an advocacy group called United Against Nuclear Iran. This is what they need to have in order to sell the deal to Congress and U.S. allies. To achieve an adequate breakout time, American and other international negotiators initially proposed establishing a 1,500 limit on the number of basic centrifuges Iran would be allowed to operate while banning the use of more advanced centrifuges. Iran, however, has steadfastly refused to agree to a major reduction in centrifuges, although recently some Iranians have hinted that the number could be set at 8,000. More recently, negotiators have been exploring a formula in which Iran could have as many as 4,500 first-generation centrifuges if it also agreed to ship much of its low-enriched uranium to Russia or take other offsetting steps. For a considerable fee, Russia would convert the fuel into rods that would be burned in Iran's lone operating commercial power reactor. With the two sides far apart, a number of other compromises have also been explored. The Iranians have suggested they could meet Western concerns by keeping a substantial number of centrifuges but reducing the uranium hexafluoride feed that is put into them to produce enriched uranium. But American experts say such a measure could be easily reversed. American officials have suggested that if the Iranians want to keep more centrifuges they should dismantle the pipes that connect them. But such a step does not appear to have won support on the Iranian side. Iran has 9,000 additional centrifuges that are not in operation, and some way would need to be found to dismantle or disable them, especially the more advanced ones that are several times more efficient than the basic model. Another issue to be resolved involves constraints on Iran's ability to produce plutonium, which can also be used to make a nuclear bomb. That could involve modifying Iran's heavy water reactor at Arak, which is almost complete, in a way that would reduce the amount of plutonium it can produce in a year to less than a kilogram. Constraints on the research and development of new types of centrifuges that would be quickly installed in hidden plants, as well as intrusive inspections and additional reporting requirements, would also be needed to ensure that Iran did not sneak out of the agreement, which is as great a worry as a breakout, Western officials say. A major stumbling block is the question of how long an agreement should last. Iran has argued that it could be seven years or even less. That, Iranian officials say, would enable Iran to install tens of thousands of new centrifuges to enrich its own uranium after a Russian contract to supply nuclear fuel for the reactor at Bushehr, Iran, expires in 2021. But an agreement that would shrink Iran's network of centrifuges only to see it expand exponentially within a decade is a nonstarter for the United States. "I would say about 15 years," said Robert J. Einhorn, a former senior State Department official. There has to be a sustained track record of scrupulous Iranian implementation of a deal before the international community will have confidence that the program is strictly peaceful. If Iran needs reactor fuel, Russia, which recently agreed to build two new nuclear reactors in Iran, could supply it, American officials say. With the Monday deadline approaching, some of the United States" negotiating partners have begun to send signals that the most that may be accomplished is a partial understanding that would codify fresh progress while extending the negotiating deadline yet again. But the Obama administration is loath to talk openly about extending the deadline for fear that it would ease the pressure on Iran to make hard decisions. "We are driving towards what we believe is the outline of an agreement that we think we can have," Mr. Kerry said Thursday.