Iran's Nuclear Activities, Explained

On June 22, the United States struck three Iranian nuclear sites, entering the Iran-Israel conflict that began nine days prior when Israel launched an attack against Iran’s nuclear infrastructure. While the U.S. entry into the campaign is political and its merits can be debated, understanding the technical realities of Iran’s nuclear infrastructure could make for a more intelligent conversation.

There is no debating that Iran has a nuclear weapons program. But, as of yet, it is not building a nuclear weapon. Iran has enriched uranium far beyond what is necessary for either civilian energy or medical use, but has stayed short of the threshold of a weapon, leaving public uncertainty.

And that uncertainty has produced some confusion within the Trump administration. Tulsi Gabbard, the Director of National Intelligence, testified before the Senate Intelligence Committee on March 25 that the intelligence community “continues to assess that Iran is not building a nuclear weapon,” and President Trump told reporters in June that “I don’t care what she said, I think they were very close to having one.”

But, these two positions don’t actually contradict each other. Iran has intentionally stayed in a gray area on the road to a nuclear weapon—it’s already completed the heavy lifting required, but has not begun the final steps. Whether the U.S. and Israeli strikes have seriously delayed an Iranian effort to complete a warhead is still in question, but we do know the technical reality of Iran’s infrastructural capabilities, giving us some clues as to what might come next.

Iran’s Uranium Campaign

Atomic weapons are a technology of the 1940s. Today the hardest part isn’t the physics, it’s getting the right material.

In nature, uranium is more than 99 percent U-238, a stable isotope that does not easily split. Less than 1 percent is U-235, which is the kind that can sustain a nuclear chain reaction. That’s the type that the weapons builders care about.

To build a weapon, you need to “enrich” uranium to raise the proportion of U-235. The uranium used in power reactors, like the one at Bushehr in southern Iran, only needs to be enriched to about 3-5 percent U-235. That is known as low-enriched uranium, or LEU, and it cannot be used for a nuclear weapon. Weapons-grade uranium, by contrast, is usually enriched to around 90 percent U-235

The International Atomic Energy Agency reports that Iran has enriched uranium to 60 percent, far beyond what’s needed for civilian energy or medical uses. There is a small research reactor in Tehran, but that doesn’t need uranium enriched to high levels. High-enriched uranium can be used to make medical isotopes, but it is not essential for that purpose. Iran does not have any civilian hardware that needs uranium enriched that high. This is either a program to build a weapon or reduce the time needed to build one if the country later decides to do so, or an effort to enhance the country’s power and influence by reaching the edge of the nuclear club.

Enrichment is not the entire story. The uranium that comes out of a mine is a metal, and is typically converted into a compound called uranium hexafluoride to be enriched. Uranium hexafluoride becomes a gas at relatively low temperatures. That gas is spun through high-speed centrifuges to separate out more and more U-235. Iran’s enriched uranium is in this chemical form. To actually make a weapon, that enriched gas must be converted back into solid metal, specifically shaped and machined for detonation.

So, when Gabbard testified that the U.S. intelligence community didn’t see signs of Iran building a weapon, this may have been a simplified way of saying that Iran is not yet taking those steps: pushing the enrichment up to 90 percent and then converting the uranium hexafluoride back into a metal. It’s not just a question of how much enriched uranium Iran has, it’s what physical form it’s in, and what’s being done with it.

But uranium isn't the only path to a weapon. Some countries, like India and Israel, used plutonium instead, a different element which is created in uranium-powered reactors. Unlike uranium, plutonium doesn’t need to be enriched. Certain isotopes of plutonium, especially plutonium-239, split very easily and can sustain a fast chain reaction, making them ideal for weapons use. If a country has a reactor capable of producing plutonium and a facility to chemically extract it from spent fuel, it can bypass the need for enrichment beyond the low levels needed for reactor fuel. The drawback is that this chemical plant is expensive and obvious.

Iran’s Progress: The Heavy Lifting is Done

When people ask whether Iran is “close” to a weapon, they’re usually thinking about the end result: a warhead on a missile. While Iran has not done that yet, by already enriching uranium to 60 percent U-235, Iran’s progress to building a simple nuclear weapon is nearly complete. Enriching uranium to 60 percent may not sound close to the 90 percent typically used in weapons, but in terms of the physics and engineering effort required, the difference between natural uranium and 60 percent is far more significant than the final push to 90 percent. Natural uranium is one part U-235 in 141 parts U-238. Low-enriched uranium is roughly 7 parts in 141, and the enrichment level that the International Atomic Energy Agency says that Iran has reached is about 85 parts in 141. To get from there to 90 percent, or 127 parts in 141, is not nearly as much work as has already been done.

Iran has had about two decades, though, to take that step, and hasn’t done it. This is not for lack of centrifuge capacity. Instead it has gone right up to the line, essentially creating a bargaining chip, and positioning itself to make a nuclear weapon quickly, while still being able to say for almost all the development period that it didn’t have uranium enriched enough to make one. Call it “implausible deniability.”

And combined with its extreme rhetoric against Israel, plus support for three militant groups that have inflicted substantial damage on Israel (Hezbollah, Hamas and the Houthis), Israel and the United States have taken the threat seriously. The Prime Minister of Israel, Benjamin Netanyahu, has been warning for decades that Iran was close, and that, too, does not contradict the facts as we know them at this time.

The International Atomic Energy Agency said that Iran had about 400 kilos of high-enriched uranium hexafluoride, and since the U.S. attack, there has been a lot of speculation about whether that material could have been scattered into the environment. The International Atomic Energy Agency says that there was no detected increase in radiation. There is also the question of whether the material was moved in advance of the bombers. But this may not be a mystery to the military.

Uranium hexafluoride is kept in heavy steel cylinders that can be moved by truck, which would be visible to satellites and aerial surveillance. If Iran moved the cylinders, the Pentagon may know where at least some of them are. Israel may too. So more bombing is a possibility.

An open question is whether Iran has spare centrifuges in some other location. Those might be hard to locate at first, but historically, Iran has had trouble keeping secrets like that.

How Does Iran Get from Here to a Nuclear Weapon?

If Iran resumes enrichment and gets to the 90 percent threshold, the uranium hexafluoride would still have to be “de-converted” back to metal. Those steps could be done in days, depending on the capacity of the chemical plant and assuming that neither the US Air Force or the Israel Defense Forces haven’t taken out all the deconversion plants.

After that comes making a bomb. The secret of nuclear weapons is that there is no secret. The design isn’t difficult; in 1976, a junior at Princeton designed one for his physics homework. And that was for a plutonium bomb; uranium is easier.

The first nuclear weapon used in combat, the Hiroshima bomb, was a simple gun firing a wedge of uranium into a uranium target, into which it fit like a 3-D puzzle. When it hit the target, a critical mass—the minimum amount necessary to sustain a chain reaction—was formed. It was not tested beforehand, because the United States did not have enough U-235 for a test, and because the designers were certain it would work.

The Hiroshima bomb may have been fairly simple, but it was of a size and shape that required delivery by an airplane, letting gravity do its job. Japan at the time had limited air defenses, but Israel is more capable. Using a plane to bomb Israel is something Iran probably can’t do at the moment, because Israel’s air defenses are strong. Iran does, however, make sophisticated missiles, some of which evade Israeli defenses.

But building a uranium warhead that will fit on a missile is much harder. A warhead of the size and shape that would fit atop a missile needs sophisticated electronic switches to control the conventional explosives that compress the uranium into a critical mass. Various countries have those, including North Korea and probably Pakistan. Whether Iran could import them is another question.

And U.S. nuclear weapons have some highly sophisticated enhancements that get more yield out of the uranium. But for Iran, it’s probably not necessary to get the biggest possible bang out of a quantity of uranium. A simple multi-kiloton open-air test in the desert would make the point, no matter what the device's efficiency. Or they may prefer nuclear ambiguity.

Demonstration explosions are the way that India introduced itself into the “nuclear club,” with a “peaceful nuclear device.” Pakistan followed immediately with a series of tests. It’s also the way that the United States introduced the hydrogen bomb.

What Does This Mean for Nuclear Energy?

Understanding the Iranian path towards a nuclear weapon should help us re-calibrate the nuclear proliferation calculation and its relevance to nuclear energy. Experience is showing us that the preferred path to weapons-grade nuclear material is centrifuges (Pakistan, Iran) or research reactors to make plutonium (India, and Israel).

India and Israel did not use power reactors to make plutonium. Iran has one power reactor, but Russia, which supplies the fuel, takes it back after use. Real-world experience is that power reactors do not seem to be attractive choices to would-be nuclear states.

A country that has its own enrichment capability can make weapons-grade uranium. But there is an irony here; if it has a civilian nuclear program, then developing weapons could put the civilian energy program at risk of sanctions. In that sense, developing civilian reactors may be a deterrent to weapons, not an adjunct. Nicholas L. Miller, a nonproliferation researcher and associate professor of government at Dartmouth College, made the case in 2017 that “Although such programs increase the technical capacity of a state to build nuclear weapons, they have important countervailing political effects that limit the odds of proliferation. Specifically, nuclear energy programs increase the likelihood that parallel nuclear weapons programs will be detected and face counterproliferation pressures; they also increase the costliness of nonproliferation sanctions.”

The distinctions are important for nuclear energy production in the United States and around the world, because they concern the fuel cycle for civilian reactors.

For example, one of President Trump’s recent executive orders seeks to encourage reprocessing of spent nuclear fuel, which means using a chemical plant to extract the plutonium produced in ordinary reactor operations, plus the uranium that was not consumed in the reactor.Presidents Ford and Carter banned that technology because they thought it would set a bad example for other countries, which might see that route to a nuclear weapon.

But reprocessing makes good use of components in spent nuclear fuel, and makes the remainder easier to dispose of. In the real world, as Iran shows, it’s not the method of choice for countries that want to enter the nuclear club. The easier route is uranium centrifuges or research reactors.

It follows that exports of power reactors do not appear to be relevant to proliferation. A policy of “energy dominance” that includes robust American exports does not provide a route to nuclear weapons for our friends—or even our friends who later become enemies, a category that includes Iran, which got a research reactor under President Eisenhower’s Atoms for Peace program.

In the short term, whether Iran has civilian nuclear power has little to do with the future of its weapons program. Iran’s choice to remain ambiguous when it comes to nuclear weapons capabilities likely produced some geopolitical benefits, until it prompted a strong military rebuttal from Israel and the United States. Whether or not Iran will choose to continue a strategy of nuclear ambiguity—one in which it remains close to a nuclear weapon, but far enough away to leave space for denials, however implausible—is the question of the day. But its relevance to nuclear energy is limited.