How to Make Sure Iran's One-Year Nuclear Breakout Time Does Not Shrink

The final deal between Iran and the P5+1 needs to specify the total enrichment capacity of Iran's installed centrifuges, mandate a robust verification regime, and include other restrictions to the nuclear program's size and content.

Under the U.S. parameters for Iran's uranium enrichment program announced in Lausanne on April 2, Tehran will decrease its stock of about 19,000 installed centrifuges to just 6,104, with only 5,060 of these designated for enriching uranium. This arrangement will last for ten years, and all of the centrifuges will be first-generation IR-1s. The parameters also state that "Iran will not use its IR-2, IR-4, IR-5, IR-6, or IR-8 models to produce enriched uranium" during this period, and that it will "engage in limited research and development with its advanced centrifuges, according to a schedule and parameters which have been agreed to by the P5+1." In addition, the amount of low-enriched stock that Iran can retain is capped at 300 kilograms of 3.67 percent-enriched uranium for the next fifteen years (i.e., uranium that contains 3.67 percent of the fissile isotope U-235).

The technical details underlying these parameters raise several concerns that go to the heart of the proposed deal's efficacy.

Central to calculations about centrifuges is their efficiency, usually measured in terms of separative work units (SWUs), which relate to both the amount of material processed and the degree of enrichment reached

Iran's IR-1 centrifuges are based on an original Dutch design that Tehran acquired from Pakistan. They are arranged in cascades at two known sites, Natanz and Fordow. According to analysts at the Institute for Science and International Security, Washington estimated the efficiency of an IR-1 at between 0.9 and 1.0 SWUs/year when assessing Iran's breakout capacity, whereas Tehran used a figure of 0.66 SWU/year. Therefore, from the U.S. perspective, Iran's allowed 5,060 IR-1s imply a breakout time of around a year.

The reality is more disturbing. The true performance of the IR-1s remains unknown -- their current capacity may be reduced by some parts of the enrichment cascades being isolated or individual machines being broken and not replaced, so their potential output may have been understated. Data derived from the International Atomic Energy Agency (IAEA) shows that IR-1s occasionally operate at 1 SWU/year or slightly better at Natanz and Fordow. But in August 2014, the head of the Atomic Energy Organization of Iran, Ali Akbar Salehi, stated that IR-1s had "a nominal output of over 3 SWUs, but in practice they yield less than 2 SWUs." He presumably meant SWUs UF6/year, so his figures translate to 1.3 SWUs uranium/year. This suggests that Iran's actual breakout time under the Lausanne parameters could be seven or eight months rather than a full year.

Iran is presumably attempting to develop the most practical and efficient machine -- a goal that requires the optimal balance of centrifuge length, composition, and spin speed. It has already installed at Natanz Fuel Enrichment Plant (FEP) six cascades of the IR-2m , which has an estimated output of 3-5 SWUs uranium/year (or up to five times the efficiency of the IR-1). Another twelve IR-2m cascades are planned, and while none of them has been fed with UF6, the latest IAEA report indicates that two IR-1 cascades at the PFEP are currently enriching uranium. In addition, one IR-2m and one IR-4 cascade are fed with natural uranium, but the resultant enriched uranium and depleted "tails" are promptly remixed back to natural uranium. In total, the potential enrichment capacity of the operating IR-1, IR-2m, and IR-4 cascades at this R&D facility is 1,300-1,700 SWUs/year, so if they are maintained in the final agreement as they are currently, Iran's breakout time will drop to eight months (or perhaps even less if Salehi's previously mentioned efficiency estimates for IR-1s hold true).

Moreover, the IAEA report is heavily caveated with expressions like "declared facilities" (allowing for the possibility of undeclared facilities), and it repeatedly warns that "the full implementation of Iran's obligations is needed to ensure international confidence in the exclusively peaceful nature of its nuclear program." Indeed, major uncertainty persists regarding the potential existence of another secret installation like Fordow, the enrichment facility that Iran built inside a mountain and did not disclose until it was discovered by U.S. and other intelligence services. Estimating detection time for clandestine facilities is difficult at best.

Even without hidden facilities, establishing most any Iranian violation of the agreement would likely take several months. First, the IAEA and respective agencies in Washington would have to come to that technical judgment; toward that end, inspectors would need timely access anywhere at any time to confirm such findings. The next step would be to get the political leadership to accept that judgment, then sell the conclusion to the international community.

In short, close attention to several technical factors is essential to the success of a nuclear deal, including the number and type of installed, operable centrifuges; Iran's inventories of enriched uranium; the dismantling of excess centrifuges; unfettered inspection access; and enhanced intelligence on and enforcement of compliance. To maintain a credible verification and monitoring scheme, Iran's installed centrifuge capacity should not exceed 5,000 SWUs per year. That calculation depends on the efficiency of the centrifuges, not just their number.

Olli Heinonen is a senior fellow at Harvard's Belfer Center and a former deputy director-general for safeguards at the IAEA. Simon Henderson is the Baker Fellow and director of the Gulf and Energy Policy Program at The Washington Institute.