Ask a Scientist: Iran and the Bomb: Some Factors to Consider

January 6, 2021
Elliott Negin
Senior Writer

The Iranian government just announced that it has resumed enriching uranium up to 20 percent at its Fordo nuclear facility, potentially moving the country closer to being able to build a nuclear bomb.

It was just the latest iteration of tit-for-tat provocations between the United States and Iran since President Trump unilaterally withdrew from the Iran nuclear agreement and reimposed severe economic sanctions. Under the terms of the nuclear deal, Tehran is allowed to enrich uranium to only 3.67 percent—well below the threshold needed for a nuclear device—and no enrichment is allowed at the Fordo plant.

Edwin Lyman, the director of nuclear power safety at UCS, is an internationally recognized expert on nuclear proliferation and nuclear terrorism.

Monday’s announcement did not come as a complete surprise, because the Iranian parliament recently passed a bill that approved enriching uranium to prod Europe to ease sanctions. The move also will put pressure on President-elect Biden, who has said that he is willing to reenter the nuclear deal.

To better understand the ramifications of Iran’s intention to enrich uranium to 20 percent, I ran some questions by Edwin Lyman, a physicist with the Union of Concerned Scientists. Below is a transcript of my conversation with Dr. Lyman.

EN: First, let’s start with the basics. What exactly is enriched uranium?

EL: Uranium that occurs naturally in the environment is primarily composed of two isotopes, which are variants of the same element that have different atomic weights: uranium-238 and uranium-235. Natural uranium, which contains more than 99 percent uranium-238 and less than 1 percent uranium-235, can be used in some types of nuclear reactors, but it cannot be used in the most common type, the light-water reactor. It also cannot be used as fuel for a nuclear weapon.

To obtain uranium that can be used in a light-water reactor or a nuclear weapon, it must undergo a chemical process called enrichment, which increases the relative amount of uranium-235. Light-water reactors typically use uranium that has been enriched to around 3 to 5 percent uranium-235. Nuclear weapons typically use uranium enriched to 90 percent uranium-235. However, it is possible to build a nuclear bomb with much lower levels of uranium-235, perhaps as low as around 10 percent.

Enrichment is a complex and difficult process because it has to separate two isotopes that are very close together in weight. The state-of-the-art enrichment technology today is called the gas centrifuge. Industrial-scale enrichment plants could have tens of thousands of centrifuges coupled together. A civil enrichment plant designed to produce fuel for nuclear power reactors could be easily reconfigured to produce material for nuclear weapons. That’s why such facilities pose nuclear proliferation risks and need to be rigorously safeguarded.

Countries also could use plutonium to make a nuclear bomb, but there is no indication that Iran possesses a significant amount of it.

EN: As you know, it was just reported that Iranian scientists have loaded centrifuges with more than 285 pounds of uranium to be enriched to 20 percent uranium-235. What does that mean? Is it a cause for concern?   

The 20 percent uranium-235 threshold is the dividing line between “low-enriched uranium” (LEU) and “highly enriched uranium” (HEU). Twenty percent is the lowest enrichment that is considered practical for making a nuclear weapon, because the quantity of uranium needed and the difficulty of turning it into a high-yield weapon increases sharply as the enrichment drops below 20 percent. The International Atomic Energy Agency considers HEU to be “direct use” material for nuclear weapons and applies much more stringent safeguards to its production and storage than LEU. But keep in mind, 20 percent is not a magical dividing line between weapon-usable and non-weapon usable material. As I mentioned earlier, it is possible—although likely very difficult—to produce a nuclear bomb with uranium enriched as low as around 10 percent.

Twenty percent is also significant because a country possessing a stockpile of uranium enriched just below 20 percent and an enrichment facility of a fixed size would be able to produce a given quantity of 90 percent HEU nearly four times more quickly than if it only possessed 3.67 percent enriched material, which is what the Iran nuclear agreement—officially called the Joint Comprehensive Plan of Action—permitted Iran to stockpile. But this difference might not be that great in practice, depending on the size of the enrichment facility and what the enrichment goal would be. It is also important to factor in a country’s level of sophistication in bomb design, as well as the number and type of weapons it would want to produce. Again, I want to stress that it isn’t necessary for Iran to enrich uranium 90 percent to make a nuclear weapon, as so many news outlets have misleadingly reported.

EN: So how close is Iran to producing enough enriched uranium for a nuclear device? 

EL: Like any country with an enrichment facility of substantial size, Iran has the capability of producing enriched uranium for a nuclear weapon in a relatively short period of time. One of the main goals of the Iran nuclear agreement was to extend the “breakout” time it would have taken for Iran to produce enough fissile material for an enriched uranium or plutonium nuclear weapon from a matter of weeks to at least one year once it decided to do so. To accomplish that, the agreement limited the number of deployed centrifuges, their capacities, and the enrichment and quantity of uranium feedstock that Iran could possess, as well as other limits on its ability to produce plutonium in a nuclear reactor. As Iran systematically breached each of those limitations in response to President Trump’s ill-advised withdrawal from the agreement, it has shortened the breakout time. My guess is that Iran could produce a nuclear device in well under 12 months at this point, if it ever really needed more than a year. But its current plan to enrich 285 pounds of uranium will probably not be sufficient to provide enough HEU for a weapon.

EN: How should the incoming Biden administration deal with this latest development?

EL: Fortunately, it is not too late to defuse the situation. As soon as possible, the Biden administration should rejoin the Iran nuclear agreement and make every effort to reverse the damage that Trump caused by abandoning it. The United States should honor its end of the deal and Iran should resume compliance with its obligations.

About the author

More from Elliott

Elliott writes about UCS-related topics for a range of news organizations. Prior to joining UCS, Elliott was the Washington communications director for the Natural Resources Defense Council, a foreign news editor at National Public Radio, the managing editor of American Journalism Review, and the editor of Nuclear Times and Public Citizen magazines.