Far more dangerous than U-235 are the elemental products given off when nuclear fuel goes through fission—especially iodine, strontium, and cesium. “These are the things that uranium splits into when it’s working in a reactor or a bomb,” says Smith. “Those fission products are much more densely radioactive than uranium.”

The United States has attacked three sites in Iran, inserting itself into Israel’s war aimed at destroying the country’s nuclear program in a risky gambit to weaken a longtime foe that prompted fears of a wider regional conflict.

President Donald Trump says the U.S. military struck three sites in Iran, inserting itself into Israel’s effort to decapitate the country’s nuclear program in a risky gambit to weaken a longtime foe amid Tehran’s threat of reprisals that could spark a wider regional conflict.