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.”

Trump says Iran’s nuclear program was obliterated, but questions remain about Iran’s capability and the location of hundreds of pounds of highly enriched 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.