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 U.N.'s atomic watchdog and other experts say radiological contamination is likely inside a key uranium enrichment site at Natanz that was badly damaged by Israel and then hit by U.S. strikes early Sunday.