The nuclear watchdog chief Rafael Mariano Grossi told Fox News there was "no information" about the material's whereabouts.

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.”

Satellite images provide the first glimpses of the aftermath of U.S. strikes on three sites said to be central to Iran’s nuclear program, revealing severe damage at aboveground facilities as well as the entry points left by bombs that burrowed deep underground to target some of the program’s most protected operations.