Technicians work at the Arak heavy water reactor's secondary circuit, as officials and media visit the site, near Arak, 150 miles (250 kilometers) southwest of the capital Tehran, Iran, Monday, Dec. 23, 2019.

Technicians work at the Arak heavy water reactor's secondary circuit, as officials and media visit the site, near Arak, 150 miles (250 kilometers) southwest of the capital Tehran, Iran, Monday, Dec. 23, 2019. Atomic Energy Organization of Iran via AP

How Quickly Could Iran Get a Nuclear Bomb?

A former National Nuclear Security Administration coordinator explains breakout time — and why it’s getting shorter.

On Sunday, Iran updated its public stance toward the nuclear limits it accepted in 2015 as part of the Joint Comprehensive Plan of Action, or JCPOA. “Iran’s nuclear program no longer faces any operational restrictions, including enrichment capacity, percentage of enrichment, amount of enriched material, and research and development,” its official statement said. This announcement was greeted with the expected questions: 

Is Iran rushing to a nuclear bomb? How quickly could Iran get enough material for a bomb, often referred to as “breakout time”? 

To understand breakout time, and why it matters that Iran is no longer observing these limits, you must first understand uranium enrichment.

Sure, give me Uranium Enrichment 101.

In its natural state, uranium contains less than 1 percent U235, the type of uranium that fuels a nuclear explosion. Enrichment produces one output stream in which the U235 level is higher than in the natural state (“enriched”) and another in which the U235 level is lower (“depleted”). The fuel for nuclear power plants usually contains about 3 to 5 percent U235. Uranium is considered highly enriched, or HEU, above 20 percent, and weapons-grade uranium normally refers to material that is at least 90 percent U235. The same equipment that enriches uranium to 5 percent will take it to 90 percent; it is just a question of how it is operated. Gas centrifuges are the most common technology used to enrich uranium, including by Iran. 

Okay, I get it. So what is breakout time?

The amount of time it would take a country to produce enough nuclear material for its first nuclear weapon. It doesn’t include the time it would take to design, manufacture, or assemble the bomb’s components or the nuclear weapon itself. Producing the nuclear explosive material is the long pole in the tent for producing a weapon, which is why it is so closely scrutinized. 

How is breakout time calculated?

The calculations depend on how much uranium is available, in what form, its enrichment level, and the number and type of gas centrifuges available. One can picture these variables like sliders on different bars. If all go up (more material, at higher enrichment levels, with more installed centrifuges) then breakout time goes down. If they all decrease, breakout time grows. If you want to hold breakout time steady, you can move some up and others down (for example, allow more stockpiled material but reduce the number of centrifuges). 

The calculations also assume the use of all available material and technology. If Iran were to try to produce material using only secret sites that are not under UN monitoring, breakout time would be much longer.

At the beginning of the JCPOA negotiation, Iran’s breakout time was unacceptably short: on the order of weeks. The guidance to the U.S. negotiators was to seek a deal that would push that breakout time to one year, and keep it there for a decade or more. The negotiators accomplished this through a combination of limits on Iran’s enriched uranium stockpile (300 kg), the enrichment level (no more than 3.67 percent) and the number of centrifuges (5,060) which were limited to their first-generation (least capable) design. This marked a significant change from the pre-JCPOA situation: 12,000 kg uranium enriched to as high as nearly 20 percent and 19,000 centrifuges of various types, with ongoing research and development to improve the machines’ efficiency.

Related: The Slow Death of the Iran Nuclear Deal

Related: Deterrence Is Failing — Partly Because Iran Has No Idea What the US Really Wants

Related: Today, Everyone’s a Nuclear Spy

What about plutonium?

Plutonium is produced in the fuel of nuclear reactors; it must be separated from that irradiated fuel to be used in a nuclear weapon. 

Before the JCPOA, Iran was building a nuclear reactor that would have produced plutonium of a very good quality for nuclear weapons. Because the reactor was still under construction, the plutonium pathway to a nuclear weapon was not included in the breakout calculation. 

Under the JCPOA, Iran agreed to change the design of the reactor, with help from China and others, to one that would produce plutonium in small quantities and of a quality not well suited for nuclear weapons. Thankfully, this reactor redesign process is ongoing. The JCPOA also requires Iran to send the used fuel out of the country, making it harder to access the plutonium.

Great. So what is Iran’s breakout time now?

I don’t know, and neither does anyone else who is speculating publicly right now. It is certainly now longer than it was before the Iran deal.

With Iran’s announcement, all the slider bars described above could be moving. In its most recent report, the International Atomic Energy Agency, or IAEA, said the enriched uranium stockpile had increased to more than 550 kg, of which nearly a quarter is above the 3.67 percent limit. While the quantities and enrichment levels are nowhere near the pre-JCPOA levels, the breakout time is starting to tick down. If Iran also begins installing more centrifuges, and possibly deploying more advanced designs, the timeline will shrink even more quickly. The next IAEA quarterly report will report the latest quantities of material, levels of enrichment, and number of installed centrifuges. This will not only give the world a better sense of the breakout timeline now, but the pace at which it is changing. 

How will we know how breakout time is changing?

In this time of so many known unknowns, IAEA monitoring and inspections in Iran are key. The verification that came with the JCPOA – and which Iran has so far said it will continue to implement – means that international inspectors are on the ground, every day, in the facilities for uranium enrichment and material storage. This monitoring mission gives facts about what Iran is doing and not just what it is saying. It also gives the IAEA the right to go anywhere it suspects contains relevant, undeclared activities, an access right that the IAEA has exercised and can do so again. These eyes and ears are the “boots on the ground” we need in Iran to reduce dangerous uncertainties.