A third round of nuclear talks with Iran in as many months ended yesterday with the same disappointing result: no progress. After two days of negotiations in Moscow, Iran was still adamant in rejecting a call to stop enriching uranium up to 20 percent, and to close its provocative enrichment site at Fordow, which is tunneled into a mountain to ward off air attacks. Technical discussions will continue next month, but no high-level follow-up meeting has been scheduled. E.U. foreign policy chief Catherine Ashton concluded that "there are significant gaps" between Iran's position and that of the United States and its partners.

At this point, it makes sense to ask several questions: just where does the most imminent nuclear threat in Iran come from? And what can be done to reduce it? How important, for example, is the enrichment to 20 percent? How important is the site at Fordow? If Iran agreed to give up these items, could the world relax? Could the United States and Europe agree to let up on sanctions?

The following analysis looks at these questions, and comes up with a surprising answer: the most imminent nuclear threats in Iran do not come from Iran's uranium enriched to 20 percent, or from the site at Fordow. Rather, the threats come from the 9,000 gas centrifuges at the Natanz enrichment plant, and from Iran's stockpile of uranium gas enriched to 3.5 percent. And, as it turns out, the best way to reduce these threats is not by bombing. It is by increasing sanctions.

Natanz

Among Iran's known nuclear plants, the one at Natanz presents the greatest immediate risk. If converted to military use, the 9,000 centrifuges installed at Natanz might be capable of producing a bomb's worth of weapon-grade uranium gas in about two months, assuming Iran fed them from Iran's stockpile of uranium gas enriched to 3.5 percent.(1) There is enough of this gas now on hand to fuel approximately four nuclear weapons. Iran could also use natural uranium feed at Natanz to produce weapon-grade uranium gas, though it would take more than twice as long to produce a bomb's worth of this material.

Mitigation: If these centrifuges were converted to military use in the nuclear "breakout" scenario described above, international inspectors probably would detect the change. And if the change were detected, there might be time to attack the Natanz site from the air before a bomb's worth of weapon-grade uranium gas could be produced. The 9,000 centrifuges are located underground, but are vulnerable to air strikes. However, the attack may not destroy all the centrifuges, and probably would start a war. The threat from Natanz could be bounded by persuading Iran to stop expanding the plant's capacity. Increasing the number of centrifuges installed at Natanz, which Iran has been doing steadily for the past several years, decreases the time needed for the plant to produce a bomb's worth of weapon-grade uranium. The surest and most peaceful way to remove the threat from these centrifuges would be to convince Iran to dismantle the Natanz enrichment plant and destroy or export the centrifuges.

Uranium enriched to 3.5 percent

Iran's stockpile of uranium gas enriched to 3.5 percent presents another immediate threat. Iran has produced over six tons of this material at the Natanz plant since the beginning of operations in early 2007, and is estimated to be producing this material at a rate of 6.3 kg per day. Some of the uranium gas has been diverted to another part of Iran's nuclear program for further enrichment, leaving about five tons as of mid-May 2012. This amount, if further enriched to weapon-grade, would be sufficient to fuel at least four nuclear weapons. As stated above, the first bomb's worth of gas could be produced at Natanz in a matter of months. The 3.5 percent enriched uranium, which is stored in cylinders, could also be transported to another plant for enrichment to weapon-grade. After the gas is enriched to weapon-grade, it must be reduced to metal and shaped into spherical form - necessary to achieve a chain reaction. The latter process is estimated by the International Atomic Energy Agency to take a few additional weeks.(2)

Mitigation: The step of enriching this uranium gas to weapon-grade at a declared enrichment plant like Natanz would probably be detected by international inspectors because of the frequency of their visits and the time this conversion would take. However, it is uncertain whether the low-enriched uranium gas, which is stored in moveable cylinders, could be destroyed by bombing. They could be taken to a secret location for storage, or taken to a secret plant for further enrichment. While their removal could be detected, they could not be destroyed by bombing if the location were unknown. Nor could their destruction be assured if they were stored at an underground site sufficiently fortified against air attack. Shipping Iran's stockpile of 3.5 percent uranium gas out of the country would be the best way to remove the proliferation risk.

Uranium enriched to 20 percent

Iran has also begun to enrich uranium to 20 percent, which - because of the unique characteristics of enrichment - is 90 percent of the way to weapon-grade. If a sufficient quantity of this material were fed into the 9,000 centrifuges at Natanz, a bomb's worth of weapon-grade uranium gas might be produced in less than a month.(3)

But for now, there isn't a sufficient quantity. Iran's stockpile of this material (about 100 kg as of mid-May) is not yet large enough to make a single bomb. The threat from the 20 percent material lies in the future. Iran may have enough of this material for a bomb before the end of 2012, and before the end of 2013, it could have enough for several. Iran is producing this material at a pilot facility located at the Natanz complex and at the Fordow plant near Qum. Combined, the average monthly production rate of this 20 percent enriched uranium gas as of mid-May was about 14 kg. Iran can be expected to increase this production rate, given the spare capacity of the Fordow plant.

Mitigation: Improved monitoring of the withdrawal lines where Iran is producing 20 percent enriched uranium would increase the chance of detecting whether this material were removed in order to pursue higher levels of enrichment at another plant. Even if removal were detected, given the short conversion time at Natanz it is uncertain whether bombing could prevent the production of at least one bomb's worth of weapon-grade uranium gas. And once removed from process lines, it is uncertain whether this uranium, which is stored in moveable cylinders, could be destroyed by bombing. Persuading Iran to stop producing the 20 percent uranium, and persuading Iran to ship its existing stockpile of this material out of the country would eliminate the risk.

Fordow

Iran's enrichment plant at Fordow is scheduled to contain approximately 3,000 centrifuges of the same type as at Natanz. About 700 centrifuges were operating at Fordow as of mid-May. Thus, the risk it poses is in the future. And even then, when compared to the 9,000 centrifuges currently installed at Natanz, its potential for enrichment will be three times smaller. The plant is nevertheless dedicated to the production of 20 percent enriched uranium gas, which is close to weapon-grade. Once it is fully operational, this plant could produce some 480 kg of 20 percent uranium gas each year, enough to fuel up to three nuclear weapons with further enrichment.(4) In addition, the plant is heavily fortified and tunneled into a mountain. It is a risk principally because it may be difficult to bomb.

Mitigation: The risk could be reduced by convincing Iran to produce only 3.5 percent material at the site. The risk could be removed by convincing Iran to dismantle the site, and destroy or export its centrifuges. As with Natanz, it would be difficult to ensure that any material produced at the plant would be destroyed by bombing, because of the plant's fortification and the ease with which any material produced there can be moved.

Conclusion

The centrifuges at Natanz, together with the stockpile of uranium enriched to 3.5 percent, pose the greatest immediate risks. Iran could use this existing capability to produce weapon-grade uranium gas within a couple of months. That will remain true even if Iran agrees to stop enriching uranium to 20 percent, which has been the primary focus of multilateral talks. The benefit of stopping enrichment to 20 percent will be to avoid a future risk. Once a sufficient stockpile of 20 percent material is on hand, the time to produce weapon-grade uranium gas might be reduced to less than a month - a risk well worth avoiding. The plant at Fordow also poses a future risk, mainly because of its resistance to attack.

Bombing would mitigate the immediate risk posed by Natanz, but would not eliminate it. Some centrifuges at the plant might survive an air campaign, and Iran would likely be capable of outfitting a similar plant, using centrifuges produced domestically. International inspectors would probably not have access to such a plant. And bombing would do little to remove the threat posed by the enriched uranium already produced by Iran, because this material is easy to move and to hide. The surest way to remove the threat posed by Natanz and by Iran's stockpile of enriched uranium would be to persuade Iran to give up its enrichment program. Additional sanctions rather than military action appear most suited to achieving this goal.

The risk from unknown sites is difficult to estimate or quantify. As stated above, the only way to remove that risk is to persuade Iran to dismantle and end its entire enrichment program, and to persuade Iran to allow intrusive inspections to be sure Iran has done so. The sanctions applied so far, however, have not been sufficient to achieve that goal.

For additional information on the estimates related to the weapon capacity of Iran's enriched uranium stockpile, see Iran's Nuclear Timetable.