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In July, the U.S. and other nations drew up an agreement with Iran to limit the country’s nuclear program, with a goal of preventing Iran from developing nuclear weapons. As a Congressional vote on the deal draws near, the White House is using science to sell the accord. In a video released today, Ernest Moniz—the Secretary of Energy and a bona fide nuclear physicist—explains the agreement’s limits on uranium and centrifuges, and how they would restrict Iran’s ability to build a bomb.

Video courtesy the Office of Science and Technology Policy

The science hinges on uranium enrichment. As it occurs in nature, uranium is a mix (mostly) of two isotopes: U-235 and U-238. Uranium 235 is the isotope that’s fissile—that is, it causes the runaway reaction that powers bombs and reactors alike. Naturally occurring uranium contains less than one percent U-235. The rest is U-238. To make energy, it’s necessary to boost the relative amount of U-235 to about three to five percent, according to the World Nuclear Association. That boosting is done with centrifuges. To make a bomb, the fraction of U-235 needs to be far higher—about 90 percent.

That’s why the constraints on both the size of Iran’s uranium stockpile and the number of centrifuges are crucial. With a smaller stockpile, there will be less uranium overall. But the key second element is that with fewer centrifuges, it would take much longer for Iran to make that uranium weapons grade. And that, the thinking goes, gives inspectors much greater chance of intercepting Iran’s plans before they successfully create a bomb.

The accord has its opponents, who hold valid concerns. First, many worry that the agreement’s limits—which vary by constraint but are about 10 to 15 years—aren’t long enough to do more than delay Iran’s path to a bomb. Second, there are concerns that the inspections aren’t forceful enough, since the U.S. can’t do them unilaterally and the inspectors have to give Iran 24 hours’ notice. Third, with sanctions lifted—and sanctions are the reason Iran is at the bargaining table in the first place—some worry Iran may use its newfound economic capital to cause further (non-nuclear) strife in the Middle East. The politics may be messy, but in an open letter a few weeks ago, 29 high-level U.S. scientists expressed their support for the deal anyway.

Disclosure: This reporter is a relative of one of the authors of the letter signed by 29 scientists.

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