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Iran is enriching uranium and breaking the limit set by the nuclear deal. Here’s what that means.

Uranium enrichment is a critical step in making nuclear energy and nuclear weapons.

Iran has exceeded the uranium enrichment level of 3.67 percent set in the 2015 nuclear deal it made with world powers, a spokesman for Iran’s nuclear agency said, according to reports Monday.

The deal put tight restrictions on Iran’s nuclear program in exchange for the loosening of some international sanctions on the country. The 3.67 percent limitation on uranium enrichment purity was one of many limits in the deal meant to keep Iran from gathering enough material to build an atom bomb in a year if it chose to (Iran has never officially said it wants a nuclear weapon).

On Sunday, Iran’s deputy foreign minister, Abbas Araghchi, said Iran would begin enriching uranium to 4.5 percent for its Bushehr power plant.

“This is to protect the nuclear deal, not to nullify it. ... This is an opportunity for talks. And if our partners fail to use this opportunity, they should not doubt our determination to leave the deal,” Araghchi said.

The 4.5 percent enrichment is still well below the 90 percent considered weapons-grade. But the violation of the deal is a move meant “to pressure Europe to reset the terms of the nuclear agreement following a US withdrawal from the pact last year,” according to the Washington Post.

Given the extraordinary destructive power of a nuclear weapon, keeping a close eye on enrichment around the world is critical to global security. But in the decades since the Manhattan Project, the enrichment process has gone from a massive, power-hungry, brute-force operation to a sophisticated and potentially clandestine affair.

Since it’s immensely important in international diplomacy right now, it’s worthwhile to understand what goes into enriching nuclear material, how the nuclear process works, and the strategies for keeping it in check.

Uranium and plutonium are the key elements in a nuclear reaction

Like the spark that ignites a fire, a nuclear chain reaction can propagate from a very small input. And like a controlled flame, a nuclear reaction can provide useful energy. Unharnessed, it can destroy cities.

However, specific starting materials, most commonly uranium and plutonium, must be processed or enriched to drive a chain reaction.

Here are some of the basics: Uranium is the heaviest naturally occurring element in the periodic table, with an atomic number of 92, representing the number of protons in its nucleus.

It’s scattered in trace amounts in “virtually all soil, rock and water,” according to the US Environmental Protection Agency. Some countries have tried to extract uranium from seawater, but right now it’s far more cost-effective to mine it in mineral deposits.

According to the International Atomic Energy Agency and the Nuclear Energy Agency, about 55,000 metric tons of uranium are mined every year. The top producers are Kazakhstan, Canada, and Australia — which account for two-thirds of the world’s uranium mining.

Plutonium, on the other hand, is a synthetic element. It has an atomic number of 94 and is formed in nuclear reactors as a byproduct of neutrons being captured by uranium. Plutonium can be acquired from reprocessing spent fuel from conventional nuclear power plants, or reactors can be designed specifically to produce plutonium for use in weapons.

But making plutonium usually requires a reactor to begin with, so uranium remains the choke point for both uranium-based and plutonium-based weapons.

The nuclear reaction is the same for weapons and energy. The desired outcome is different.

So you have your uranium (or plutonium). Can you now make a bomb?

Not quite. Let’s wade into the history and science of splitting atoms to set the stage for nuclear negotiations today.

Researchers found since the 1930s that they could bombard uranium with neutrons to create heavier isotopes and form new elements that have never before been seen in nature, like plutonium.

An isotope is a variety of an element with the same chemical structure but a different internal composition. In comparing isotopes of an element like uranium, the atomic number stays the same, but the isotope number — the sum of the protons and neutrons in a nucleus — can differ. Uranium-235 (U-235), for example, has three fewer neutrons than uranium-238 (U-238), but they undergo the same chemical reactions.

In their experiments, German scientists Otto Hahn, Lise Meitner, and Fritz Strassmann in 1938 found another curious result. Among the atoms resulting from neutron bombardment were much smaller atoms like barium, which has an atomic number of 56. Meitner, along with Austrian scientist Otto Frisch, realized that this was the result of splitting the uranium atom into smaller atoms, a phenomenon that also emits a huge amount of energy. The finding marked the dawn of the nuclear age.

Isotopes of atoms that can split apart (undergo fission) are described as fissile. When there are enough fissile atoms close together — a quantity known as critical mass — the particles ejected by fission can strike other fissile atoms, triggering more atoms to split apart and so on. The energy released in the process can generate heat to boil water to spin a turbine or wreak devastation from a bomb.

But not all uranium atoms can easily split apart and trigger a chain reaction. In fact, most can’t. In nature, about 99.7 percent of uranium is in the form of the non-fissile isotope U-238.

Only about 0.7 percent of uranium occurs in the fissile form of U-235. And in nature, U-235 is in such a low concentration that even if a stray neutron were to strike it with enough force to break it apart, it’s unlikely that the resulting neutrons would find another U-235 atom nearby to continue the reaction.

Javier Zarracina/Vox

To produce a chain reaction, you need to increase the concentration of U-235 relative to U-238. This is called enrichment.

For plutonium, all isotopes are fissile, but some are easier to use in nuclear weapons than others. Plutonium rich in the isotope Pu-239, called weapons-grade plutonium, poses the fewest technical challenges and can be extracted from nuclear fuel that is only irradiated in a reactor for a short time.

Making uranium and plutonium useful is a major technical challenge

Enrichment is the sorting problem from hell.

Instead of uranium atoms, imagine you have a bag filled with 1,000 marbles, each identical in material, size, shape, color, and texture. However, there are seven marbles in the bag that weigh 1.3 percent less than the others. For 5-gram, 1.5-centimeter diameter marbles, we’re talking about a difference of about 65 milligrams for the light marbles, or the weight of a few grains of sand.

Since it’s tedious to weigh each individual marble, you’ll want to come up with some sort of group sorting mechanism. But weight is the only thing setting them apart and the difference between desired and undesired marbles is small, so the sorting process won’t be perfect and you’ll still have a mixture of light and heavy marbles at the end. So you run the results through the sorter again. And again. And again.

Enrichment is a difficult sorting process.

With each iteration, you have a higher percentage of lighter marbles, but every repetition costs time, money, and energy.

And remember, the marbles in this analogy are atoms, the smallest unit of matter, so they’re that much more difficult to manipulate, and it takes far longer to get the quantities you need when you’re trying to go from atoms of uranium to tons of it.

For a nuclear reactor cooled with ordinary water, you need only about 3 to 5 percent U-235 enrichment, but you need it by the ton. A 1-gigawatt nuclear reactor uses 27 tons of nuclear fuel per year. A comparable coal-fired plant burns 2.5 million tons of coal per year.

Uranium with more than 20 percent U-235 is considered highly enriched. Conversely, the residual uranium with U-235 removed is called depleted (this is the uranium used in armor-piercing ammunition).

A nuclear weapon, on the other hand, requires even higher enrichment, typically around 90 percent, though it needs much less mass than a reactor. The Little Boy bomb dropped on Hiroshima, Japan, used 141 pounds of highly enriched uranium, though only 2 percent actually underwent fission due to inefficiencies in the design of the bomb. The Fat Man bomb dropped on Nagasaki used just 14 pounds of plutonium.

Physicist Harold Agnew holding the plutonium core of the Fat Man bomb dropped on Nagasaki Japan in 1945.
Physicist Harold Agnew holding the plutonium core of the Fat Man bomb dropped on Nagasaki, Japan, in 1945.

The International Atomic Energy Agency defines a “significant quantity” of nuclear material for a weapon to be 55 pounds of U-235 within a quantity of highly enriched uranium, or 17.6 pounds of plutonium.

Some countries with civilian nuclear reactors, like South Korea, don’t bother with the whole enrichment process and have opted instead to buy their nuclear fuel on the open international market. But for others, like France, mastering the fuel cycle is a vital pillar of their energy strategy.

The enrichment process has become easier, which makes controlling nuclear weapons harder

Both Iran and North Korea have developed surreptitious enrichment networks for producing nuclear material. These facilities are hard to detect and easy to reconfigure, so without regular inspections and monitoring, the possibility of a clandestine nuclear weapons program remains.

This wasn’t always the case.

The Manhattan Project marked the first successful effort to enrich uranium for a nuclear weapon. One of the earliest and most primitive enrichment techniques used in this endeavor was gaseous diffusion. Here, uranium is reacted with fluorine to make uranium hexafluoride gas (UF6). The gas is then pumped through membranes, the idea being that lighter isotopes of uranium would diffuse faster than heavier isotopes (fluorine has only one naturally occurring isotope, so any differences in the mass of the gas come from uranium).

But each stage of the process could only separate a tiny amount of uranium, so gaseous diffusion required huge buildings and devoured energy to power the pumps needed to move the gas through the separation stages.

“The original ways of doing it were very inefficient,” said Edwin Lyman, a senior scientist in the Global Security Program at the Union of Concerned Scientists. “They required very large amounts of land, lots of power.”

For example, the K-25 gaseous diffusion building in Oak Ridge, Tennessee, was completed in 1945 at a cost of $500 million. It was half a mile long and 1,000 feet wide, making it the largest building under one roof at the time. The facility employed 12,000 workers at its peak and consumed enough electricity to power 20,000 homes for a year.

An aerial photo of the K-25 gaseous diffusion plant in Oak Ridge, Tennessee.
An aerial photo of the K-25 gaseous diffusion plant in Oak Ridge, Tennessee.
K-25 Virtual Museum

These days, uranium enrichment is much more subtle. The most common tool is the gas centrifuge. This is where uranium hexafluoride gas is fed into a column spinning at upward of 100,000 rotations per minute.

Source: Reuters
Javier Zarracina/Vox

As the centrifuge spins, the heavier isotopes push harder against its wall than the lighter ones. The centrifuge also induces the gas to circulate within the device, further increasing separation. The output of one centrifuge is then fed into another and another in an arrangement called a cascade.


Centrifuges are more energy-efficient than other enrichment techniques and are harder to detect. The centrifuges themselves don’t take up much floor space, so their plants have a much smaller physical footprint than gaseous diffusion facilities. They also don’t draw as much electricity, nor do they leave much of a heat signature.

A declassified 1960 report from a contractor at Oak Ridge National Laboratory noted that “it would not be too difficult to build a relatively small clandestine gas centrifuge plant capable of producing sufficient enriched uranium for a small number of nuclear weapons.”

A uranium enrichment centrifuge cascade.
A uranium enrichment centrifuge cascade.
US Department of Energy/The LIFE Picture Collection/Getty Images

The point is a primitive enrichment apparatus is massive; a modern one is small.

“Centrifuges are the only [enrichment process] today that makes economic sense,” said R. Scott Kemp, director of the Laboratory for Nuclear Security and Policy at MIT. “[A centrifuge plant] capable of producing a weapon can fit in a garage or a small office building, and the energy consumption is less than typical office lighting per square foot.”

That’s why arms control discussions focus so much on centrifuges, and why the Iran nuclear deal — the Joint Comprehensive Plan of Action, or JCPOA — went to great lengths to specify the number and type of centrifuges allowed, as well as how closely they are monitored. Centrifuges are the key variable in how long it takes to enrich a usable quantity of uranium, whether for fuel or for weapons.

To produce nuclear energy, where you need tons of uranium but at low levels of enrichment, an enrichment operation would need many parallel cascades, but only a handful of enrichment stages. For a weapon, which demands kilograms of uranium but at much higher enrichment, it’s almost the reverse: You would only need a few parallel cascades, but those cascades would involve dozens of stages. With enough centrifuges, getting enough usable uranium for either would only take a few weeks.

The term of art for the amount of effort required to enrich uranium is a separative work unit, or SWU. It’s built on a complicated formula, and it’s useful for describing the efficiency of a centrifuge cascade. It takes about 120,000 SWU per year to produce enough fuel for a 1-gigawatt nuclear reactor, but it only takes about 5,000 SWU to have enough material for a nuclear weapon. So a country with enough enrichment capacity to sustain a small nuclear energy program theoretically has enough throughput to build dozens of weapons.

And switching between a nuclear fuel centrifuge arrangement and a nuclear weapon arrangement isn’t all that difficult or time-consuming. It’s a matter of changing how pipes are routed, so converting a plant from supplying energy material to supplying weapons material could take no more than a few months.

“That’s the real danger,” the Union of Concerned Scientists’ Lyman said. “Whether or not you can enrich to ‘highly enriched’ just really depends on if you have enough [centrifuges] to string them together.”

Uranium enrichment is the main focus of the Iran nuclear deal

So how do you design an enrichment system that can produce nuclear energy but not a nuclear weapon?

You can’t, really.

The expertise and technology overlap too much. This was the fundamental technical challenge behind the Iran nuclear deal. Iran remains a party to the Non-Proliferation Treaty; North Korea withdrew in 2003. India, Israel, and Pakistan also have nuclear weapons but haven’t signed the NPT.

Under the NPT, countries that don’t currently possess nuclear weapons are prevented from developing or spreading nuclear weapons technologies, but they can pursue nuclear activities for peaceful purposes like research or energy.

In 2003, Iran was found to have violated nuclear activity reporting requirements in the NPT, which spurred the international effort to get Iran to suspend its enrichment work. The US has argued that Iran does not have the right to enrich uranium since it was caught violating some of the safeguards imposed by the NPT, though Iran has not violated the treaty itself.

International Atomic Energy Agency inspectors supervise a centrifuge cascade at the Natanz enrichment facility in Iran.
International Atomic Energy Agency inspectors supervise a centrifuge cascade at the Natanz enrichment facility in Iran.
Kazen Ghane/AFP/Getty Images

The goal of the six countries that signed the JCPOA with Iran in 2015 was to limit what is called “breakout time.” That is, how long it would take Iran to enrich enough material for a nuclear weapon if the country suddenly decided to ditch all international agreements and aggressively ramp up enrichment.

Prior to the agreement, Iran’s breakout time was estimated at four to six weeks. The provisions of the deal (Vox’s Zack Beauchamp put together an excellent explainer on this) aimed to extend this to more than a year, which would give international observers time to detect such a shift and enact countermeasures.

In short, the agreement made Iran limit uranium enrichment to 3.67 percent and decommission about 14,000 of its centrifuges, allowing just roughly 5,000 of Iran’s first-generation units to keep spinning. These IR-1 centrifuges produce between 0.75 and 1 SWU per device, whereas the IR-8 centrifuges Iran was developing at the time of the deal could theoretically manage 24 SWU, making them much more efficient.

Iran also gave up much of its low-enriched uranium stockpile, going from 25,000 pounds to 660 pounds. Iranian officials also agreed to pour concrete into their Arak reactor, a potential source of plutonium for nuclear weapons.

In addition, the JCPOA requires round-the-clock monitoring of Iran’s enrichment facilities in Fordow and Natanz, with only the Natanz facility allowed to operate. These are likely the only places where Iran can enrich uranium for a weapon.

“I think in Iran, we are pretty confident that there is no undeclared plant,” said Alex Glaser, director of the Nuclear Futures Laboratory at Princeton University.

International observers are also monitoring Iran’s uranium mining operations.

As it stands, the agreement effectively eliminates Iran’s prospects for enriching enough uranium for a civilian nuclear program and makes it much more tedious to gather the material required for a weapon. What little enrichment Iran is allowed under the deal is effectively a face-saving measure.

But, critics argue, pausing Iran’s entire nuclear enrichment apparatus only extends the breakout time by a few months since the country could just rebuild or reinstall its centrifuges if it decided to leave the agreement. And it looks like that day may be getting closer: a spokesman for Iran’s Atomic Energy Organization said Monday that the agency may increase the enrichment level to 20 percent or reinstall centrifuges.