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The Limits of Breakout Estimates in Assessing Iran’s Nuclear Program

An Issue Brief by the Arms Control Association Issue Brief

By Kelsey Davenport, Director for Nonproliferation Policy, with Julia Masterson, Research Assistant

WASHINGTON, D.C. (IDN) –Over the past year, Iran has taken several troubling steps to breach the limits that were imposed on its nuclear program by the 2015 nuclear deal, known as the Joint Comprehensive Plan of Action (JCPOA). While Iran’s violations of the accord appear to be carefully calibrated to create leverage in response to the Trump administration’s 2018 withdrawal from the accord and reimposition of sanctions, Iran’s actions have rekindled the debate about how quickly Iran could “breakout,” or produce enough nuclear material for a bomb. [2020–08-10]

Estimates about the length of time it might take for Tehran to breakout have become synonymous with assessing the risk posed by Iran’s nuclear program. During negotiations on the JCPOA, the breakout time established by the nuclear restrictions imposed by the deal became a key metric by which policymakers, particularly members of Congress, judged the value of the accord. Supporters of the JCPOA highlighted the 12-month breakout achieved by the deal as a measure of success in rolling back the country’s nuclear program and creating a buffer that would give the international community time to respond to any Iranian move to try to produce nuclear weapons. Critics of the nuclear deal decried the accord for ‘only’ achieving a 12- month breakout for the first decade of the accord.

The attractiveness of a breakout estimation from a policy-making perspective is clear—it is a quantitative assessment of a country’s capacity to produce fissile material for a bomb that establishes a time frame for intervention.

While breakout estimates can appear to be a quick and easy metric for assessing a proliferation threat, they can also be misleading and oversimplify the complex technical weaponization process and the political factors that influence the decision to develop nuclear weapons. Breakout is often discussed absent a shared understanding of what the term constitutes and the assumptions that go into the calculation. More importantly, breakout is a measure of technical capacity and capability, not intent. Iran’s decision to pursue nuclear weapons—or not—will be based on an array of political considerations, Tehran’s threat perceptions, and its leaders’ cost-benefit calculations.

Establishing limitations and restrictions that extend a country’s breakout time are important for mitigating proliferation risk. However, pairing the limits established by the JCPOA with a strategy for addressing the factors that impact Tehran's decision-making stands a better chance of preventing a nuclear-armed Iran in the long term.

What is Breakout?

“Breakout” commonly refers to the amount of time it would take for a country to produce enough fissile material for one nuclear bomb. Every country has a breakout timeline.

While the amount of fissile material used in nuclear weapons varies considerably, breakout is often estimated using what the International Atomic Energy Agency (IAEA) refers to as a “significant quantity” of weapons useable material: 25 kilograms of highly enriched uranium (HEU) or four kilograms of separated plutonium-239. States that already possess nuclear weapons have developed warheads using less fissile material, but Iran—which has never built or tested a nuclear device—would likely require the IAEA’s estimated significant quantity of fissile material, or more, to account for wastage in the process of manufacturing the weapon.

In discussions about the Iranian nuclear program, breakout is most frequently used to describe the country’s capacity to produce enough HEU for one weapon using gas centrifuges. Iran has an established uranium enrichment and centrifuge development program that could be more readily reconfigured to produce HEU for nuclear weapons than the plutonium route.

Even before negotiations on the JCPOA, Iran lacked a separation facility to remove plutonium-239 from spent reactor fuel. Additionally, its unfinished heavy-water reactor at Arak, which may have initially been designed as a source of plutonium for nuclear weapons, was years away from completion. Currently, provisions of the deal requiring Iran to modify the Arak reactor to a design that would produce less than a kilogram of weapons-usable plutonium yearly (and even that will be shipped out), and forgo reprocessing for 15 years, serve as a further bulwark against a plutonium route to the bomb.

In determining Iran’s breakout time using its uranium enrichment program, several factors impact the calculation, including the types of centrifuges used for enrichment, the efficiency and configuration of those machines, and the size and enrichment levels of Iran’s existing stockpile of uranium.

Often, breakout estimates are based on worst-case scenarios and they can vary significantly. Variances in breakout estimation can depend on the assumptions made about factors that are not established publicly by IAEA reports on Iran’s nuclear program or other sources of data. For instance, the efficiency of Iran’s first-generation IR-1 centrifuge, known as its “separative work unit (SWU)” capacity, is fairly well established by more than a decade of IAEA reports. The New York Times also reported in 2015 that the United States has a cascade of IR-1 centrifuges that it uses to test its performance assumptions in calculating breakout. Iran’s advanced centrifuges, however, do not have the same public operational history and documentation, making estimates about their efficiency more imprecise. 

Longer or shorter breakout estimates may also be attributed to differing judgments on how quickly Iran could reconfigure its centrifuges to enrich to higher levels, assumptions about how many/few machines will break during that process, the rate at which Iran could install additional centrifuges, and how much material Iran would need to produce to account for wastage in the process.

Fluctuations in Iran’s Breakout

Before implementation of the 2013 Joint Plan of Action, the interim nuclear deal between the P5+1 (China, France, Germany, Russia, the United States, the United Kingdom, and the European Union) and Iran, Tehran’s breakout time was assessed at about 2-3 months. At that time, Iran had produced and stockpiled about 200 kilograms of 20 percent enriched uranium gas—nearly enough that, when further enriched to weapons-grade, could be used to produce enough HEU for a bomb. This stockpile had a significant impact on the breakout estimate because enriching to the 20 percent uranium-235 level constitutes about 90 percent of the effort required to produce 90 percent enriched uranium and would jump-start any weapons effort. At that time, Iran was also operating more than 10,000 IR-1 centrifuges and had produced and stockpiled more than enough uranium gas enriched to 3.67 percent uranium-235 that, when further enriched to weapons-grade, would produce enough HEU for a half dozen nuclear weapons.

As a result of the interim Joint Plan of Action, Iran ceased enriching to 20 percent and diluted or converted its stockpile of that material, increasing the breakout time. The JCPOA further limited Iran to 3.67 percent enrichment for 15 years. Additional limitations put in place by the JCPOA included restricting the stockpile size to the equivalent of 300 kilograms of uranium hexafluoride gas enriched to 3.67 percent uranium-235—not nearly enough for one nuclear weapon—for 15 years and limiting enrichment output to only 5,060 IR-1 centrifuges for 10 years.

In combination, these restrictions on the uranium enrichment program extended the breakout time to about 12 months for the first decade of the agreement, according to the Obama administration. Then-Director for National Intelligence Dan Coats presented a similar assessment in Jan. 2019 when Iran was fully implementing the deal.

Since Iran began taking steps in May 2019 to breach limits set by the accord, the breakout time has slowly decreased. As of June 2020, Iran is now enriching uranium up to 4.5 percent uranium-235 using more than 6,100 IR-1 centrifuges and several hundred advanced machines. Iran has also exceeded the stockpile limit for enriched uranium and possesses 1,088 kilograms of uranium enriched between 2-4.5 percent uranium-235—more than enough to produce a significant quantity of HEU if enriched to weapons-grade.

Given the uncertainties about advanced centrifuge machine performance and variances of enrichment levels within the stockpile, breakout estimates as of the June 2020 IAEA report range from three to six months. If Iran continues to install additional centrifuges or begins enriching to higher levels, the breakout time could decrease further.

Iranian officials have made clear, however, that these steps are not a dash to a bomb and that Tehran will return to compliance with the accord if the other parties to the deal meet their obligations, namely on sanctions relief. Iranian officials have notified the IAEA in advance of its actions to breach the deal and the agency has monitored and reported on the violations. The careful calibration of these transparent steps to gradually decrease breakout and the reversibility of Iran’s actions support the claim by Tehran that this is about pressing the remaining parties to the deal to deliver on sanctions relief. Furthermore, the 2020 State Department report on compliance with arms control and nonproliferation agreements assessed that Iran is not engaged in activities relevant to nuclear weapons development nor has Tehran made the decision to do so.

This demonstrates one of the limitations of the breakout calculation: Iran’s breakout has significantly decreased over the past year, but the country’s political calculus does not appear to have shifted in favor of building a bomb. As such, despite the shorter breakout window, there is still time to influence Iran’s decision making.

What Else Goes into Making a Bomb?

While breakout typically refers to the time necessary to produce enough fissile material for a bomb, the calculation is often conflated or confused with the total time necessary to build an actual nuclear weapon.

Although the production of fissile material is arguably the most resource-intensive and difficult step in building a bomb, there are several additional technical hurdles, including designing and constructing an explosive device and integrating it into a delivery system (most likely a ballistic missile). When breakout is misconstrued as the time to build a weapon or reference to these steps is omitted, it can artificially inflate the immediacy of the proliferation risk.

To build a bomb, there are several additional time-consuming steps following the production of fissile material. After producing enough HEU gas for a bomb, Iran would need to convert the material into powder form, fabricate the metallic core of the weapon from the powder, assemble other weapons components that had been previously developed or acquired on an independent track, and integrate the weapons package into a delivery vehicle. This process could be more easily hidden and progress would be more difficult to quantify, but there would still be a period of time for intervention.

It is likely that if Iran were to produce enough weapons-grade material, the country would be able to build a nuclear weapon. In an unclassified 2007 National Intelligence Estimate on Iran, the U.S. intelligence community concluded that Iran had a nuclear weapons capability—namely, the country developed the technical competencies to build a bomb—but had not made the political decision to follow through.

James Clapper, director for national intelligence from 2010 to 2017, confirmed during testimony in Feb. 2016 that that assessment still held, noting that U.S. intelligence community does not believe Iran faces any “insurmountable technical barriers to producing a nuclear weapon,” but that there was no indication that Tehran intended to pursue nuclear weapons at this time.

Past verification activities and reports by the International Atomic Energy Agency (IAEA) further support the assessment Iran has retained the technical know-how to adapt a stockpile of HEU for use in a nuclear explosive device. A 2011 IAEA report annex on the possible military dimensions of Iran’s nuclear program referenced Iran’s attempts to convert HEU compounds into metal and to fabricate HEU metal components into a size suitable for a nuclear weapon. That same report confirms that Iran engaged in the development of a detonator, which can be used to ignite the high explosives that surround a weapon’s fissile core.

Beyond constructing a bomb, states developing nuclear weapons have typically conducted multiple, large-scale nuclear test explosions to perfect their warhead designs, particularly the smaller, lighter, and more efficient designs needed for missiles. Iran has not conducted a nuclear test and any attempt to do so would very likely be detected by existing U.S. national means of intelligence and the International Monitoring System established to verify compliance with the Comprehensive Test Ban Treaty.

If Iran chose not to test, Tehran would be staking everything on the perfect performance of one untested system. It is highly improbable that Iran would plan to break out of the nuclear Nonproliferation Treaty by building only one nuclear weapon. Calculating timelines based on a one-device scenario, therefore, compounds the misimpression already left by using a breakout definition that falls short of actually building a weapon.

However, if Tehran were to choose to increase the odds of success by planning to build multiple weapons, it would increase the need for fissile material, thus lengthening the breakout timelines and increasing the chances of international detection and blocking actions. The likelihood of detection is further increased by the more intrusive monitoring and verification mechanism put in place by the JCPOA.

Even if Iran were willing to tolerate the large uncertainties deriving from an untested nuclear weapons design, integrating the warhead with a delivery system could add additional uncertainties and further increase the timeframe. Launching a nuclear weapon using a ballistic missile—which the U.S. intelligence community assessed was Iran’s most likely delivery system—requires miniaturizing a nuclear warhead to ensure the size and weight are compatible with the missile’s capabilities. The annex to the 2011 IAEA report indicated that Iran was studying how to pair a warhead with its Shahab ballistic missiles.

Estimates of how quickly Iran could complete a bomb after producing the fissile material differ. The United States does not provide official estimates, but an annual report from the State Department released in June 2020 noted that “Iran is not currently engaged in key activities associated with the design and development of a nuclear weapon.” Former U.S. officials have said the weaponization process could take about a year. Leaked reports of a threat assessment provided to Israeli Defense Minister Benny Gantz in June 2020 concluded that Iran could produce enough fissile material for a nuclear weapon in six months, but it would take the country about two years to build a bomb if Tehran decided to do so.

The difference between the breakout timeline and the total estimated time to build a nuclear bomb highlights a limitation of focusing too narrowly on breakout as the window for intervention. For one, breakout creates the perception that a state must be stopped before producing a significant quantity of HEU or plutonium, which may be used as a justification for a military strike. Military action, however, will only set back Iran’s nuclear program and may end up spurring Tehran to pursue nuclear weapons to deter future strikes.

Furthermore, breakout assesses the time it would take for Iran to concentrate its nuclear capacity to produce enough nuclear material for one weapon. A single nuclear bomb—particularly given that Iran has not tested a nuclear device in the past to verify its design—has limited security value. It is not an effective deterrent and it is unlikely that Iran would want to face the consequences of breakout—increased international pressure and possible military action—for one nuclear weapon based on an untested design.

The Political Factors Related to Breakout

Overreliance on using breakout estimates also creates the impression that Iran will inevitably pursue nuclear weapons at some point and that there is a technical solution to block what is ultimately a political decision shaped by a country’s threat perception. However, given that the U.S. intelligence community has already assessed that Iran has developed a nuclear weapons capability, restrictions can increase the time it would take to build a bomb, but they cannot undo that knowledge. Ultimately, if Tehran decides to build a bomb, the country has the technical competencies to do so.

But, as Clapper testified in 2012, “we judge Iran’s nuclear decision making is guided by a cost-benefit approach, which offers the international community opportunities to influence Tehran.” To that end, pairing a return to compliance with the 2015 nuclear deal by the United States and Iran with a strategy for active engagement on regional security issues and investment in Iran’s economic development would increase the time it would take to build a bomb while increasing the benefits of compliance. This approach of establishing technical barriers and addressing the factors that impact Tehran’s decision making stands a better chance of preventing a nuclear-armed Iran in the long term.


Limiting and restricting Iran’s breakout timeline is a critical component and benefit of the JCPOA. However, policymakers must focus not only on the technical barriers and the robust inspection regime necessary to prevent Iran from obtaining nuclear weapons but also on a strategy to address the political factors that influence Iranian decision making on security issues.

A key part of any such strategy involves a mutual return to the JCPOA by the United States and Iran. Doing so would be an important first step toward stabilizing the current situation and preventing a new nuclear crisis in the region. A return to full compliance with the nuclear deal would provide a platform for further negotiations on a long-term framework to address the country’s nuclear program and create space to engage with Iran on other areas of concern, such as regional dynamics and the country’s ballistic missile program.[IDN-InDepthNews – 10 August 2020]

Photo: Iranian Petroleum Sector Facilities. Public Domain. (Source: CIA World Fact Book)

IDN is the Flagship Agency of the Non-profit International Press Syndicate.

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