Negotiators from Iran are furiously working with their counterparts from the so-called E3/EU+3 - that's the European Union, including Britain, France and Germany, plus China, Russia and the United States - to see if they can reach an agreement that will trade sanctions relief for limits on Iran's uranium enrichment program. They have until June 30th, or perhaps a bit longer if they go into extra time.
If they succeed, the deal will be known as the Joint Comprehensive Plan of Action. It will replace the current interim agreement known simply as the Joint Plan of Action, which was agreed for six months in November 2013 and extended twice as negotiators have racked up hotel bills. (For clarity, the current freeze is simply called the "Interim Agreement" here.)
Whether or not the agreement is a good deal or a bad one depends, in part, on the limits that Iran accepts on its uranium enrichment program. While some people say even one centrifuge in Iran is one too many, most experts believe that any agreement that can lengthen the time it takes Iran to build a nuclear weapon to at least a year is worth doing.
The limits under negotiation, combined with other measures, are therefore intended in part to lengthen the hypothetical "breakout time" from a few months today to more than a year under a final agreement. "Breakout" is the term used to describe the time that it would take for Iran to use declared facilities to produce a single "significant quantity" of highly-enriched uranium (25 kilograms). (A "significant quantity" only refers to the amount of material itself; additional time would be necessary to fashion it into a nuclear weapon.) There are, of course, other considerations, but breakout looms large in most debates.
Iran Centrifuge Limits With and Without a Comprehensive Agreement
See the full infographic.
Source: Implementation of the NPT Safeguards Agreement & relevant provisions of Security Council resolutions in the Islamic Republic of Iran, GOV/2015/34 (May 29, 2015); US Department of State, "Parameters for a Joint Comprehensive Plan of Action Regarding the Islamic Republic of Iran's Nuclear Program," (April 2, 2015).
With No Agreement
Assessing any deal requires understanding how many and what kind of centrifuges Iran currently possesses, as well as how many and what kind it is likely to have absent an agreement.
Today, Iran's centrifuge program is limited by the Interim Agreement. Under the Interim Agreement, Iran's program was frozen at current levels for the duration of negotiations. Iran currently has 19,113 centrifuges at two enrichment sites - one near Natanz and another called Fordow (but actually near Qom).
- Iran has installed 16,428 IR-1 and IR-2M centrifuges at the Natanz Fuel Enrichment Plant. Under the Interim Agreement, Iran is feeding uranium hexafluoride into 9,156 IR-1 centrifuges. While most of the centrifuges at Natanz are relatively inefficient first generation IR-1 centrifuges, Iran has installed 1,008 IR-2M centrifuges and completed installation preparations for another 2,016 IR-2Ms. (The IR-2M is a more advanced centrifuge than the IR-1.) Iran plans to eventually install a total of approximately 50,000 centrifuges at Natanz.
- Iran has installed 2,710 IR-1 centrifuges at the Fordow Fuel Enrichment Plant. The facility is designed to hold 2,976 centrifuges. Under the Interim Agreement, Iran is enriching uranium with 696 centrifuges. Before the Interim Agreement, Iran used two sets of two interconnected cascades at Fordow to enrich some uranium to nearly 20 percent, the material of the greatest proliferation concern. 
President Obama has stated that today, under the Interim Agreement, breakout time is about 2-3 months.
If negotiations were to fail, Iran could eventually install the full complement of nearly 50,000 centrifuges at Natanz, including advanced generation machines, and resume production of 20 percent HEU material at Fordow. In such a scenario, the breakout time to produce enough HEU for a nuclear weapon would be only a few days -- functionally zero.
If There is a Comprehensive Agreement
The Obama Administration is seeking terms under a Joint Comprehensive Plan of Action that would stretch breakout time to more than a year. The terms of this agreement, as described in the press and the documents released by the parties, would require Iran to remove the vast majority of centrifuges, including all the more advanced IR-2M centrifuges at Natanz, and place them in storage under IAEA monitoring. Under a final agreement, Iran is expected to be limited to a total of 6,104 IR-1 centrifuges - 5,060 centrifuges at Natanz and 1,044 centrifuges at Fordow. Iran would be prevented from installing additional centrifuges, installing advanced generation centrifuges, or enriching uranium above 3.67 percent for at least 15 years.
- The Comprehensive Agreement would restrict all uranium enrichment activities to the Natanz site. All 5,060 centrifuges at the Natanz Fuel Enrichment Plant will be IR-1-type machines and Iran would not be permitted to enrich material above 3.67 percent for at least 15 years.
- The Comprehensive Agreement also prohibits Iran from enriching uranium at the Fordow Fuel Enrichment Plant. Iran would retain 1,044 installed IR-1 centrifuges at Fordow, with 348 machines (two cascades) in operation. These machines will be modified, however, and used for stable isotope separation, and probably specifically the production of molybdenum for medical and other civil uses.
Research and development of more advanced centrifuge machines will be conducted according to an agreed schedule that will severely limit the testing of new centrifuges. Iran has reportedly agreed to refrain from "lead cascade testing" of centrifuge types beyond the IR-2M.
Combined with other restrictions of research and development, and access provided to the International Atomic Energy Agency, the limitations on Iran's centrifuges under a final agreement are designed to extend the breakout time associated with Iran's declared facilities from a few months today to more than a year.
Break-out time is, of course, only one way to measure the effectiveness of an agreement with Iran. Many experts believe the scenario is too artificial and that Tehran is unlikely to initiate a crisis by using a declared facility to race to a single nuclear weapon. Some experts are more concerned about covert facilities. Nevertheless, break-out is the benchmark the Obama Administration and others have cited for whether a deal with Iran strengthens the security of the United States, as well as its allies and partners.
In terms of the separative work required to enrich uranium to weapons-grade, material that has been enriched to twenty percent requires relatively little additional work to reach weapons grade.
Natanz Fuel Enrichment Plant
As of May 2015:
- 90 IR-1 cascades installed (15,420 centrifuges)*
- 6 IR-2m cascades installed (1,008 centrifuges)
- 54 cascades enriching
Natanz is designed to hold 144 cascades (~50,000 centrifuges).
Under an agreement, Iran would be limited to 30 cascades at Natanz totaling 5,060 centrifuges.**
As of May 2015:
- 16 IR-1 cascades installed (2,710 machines)
- 4 IR-1 cascades enriching (696 machines)
Fordow is designed to hold 16 cascades (as many as 2,976 machines).
Under an agreement, Iran would be limited to 4 cascades at Natanz totaling 1,044 centrifuges, of which only 2 cascades would be operated for to separate stable isotopes (348 centrifuges).
*24 cascades of 164 machines, 66 cascades of 174 machines
**14 cascades of 164 centrifuges and 16 cascades of 174 centrifuges