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.”

Airstrikes on Iran’s nuclear sites may set its program back by only a few months, according to a report by The New York Times based on a preliminary US intelligence assessment. The classified report,

Uranium ore that miners dig up is relatively harmless. But it contains tiny amounts of a rare radioactive isotope, Uranium 235, that can be used to power nuclear reactors at low levels of enrichment, and to fuel atom bombs at higher levels. The percentage of U-235 in mined uranium is less than 1 percent.