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.”

An early intelligence assessment indicated that the U.S. military strikes on three of Iran's nuclear facilities last weekend did not destroy the core components of Tehran's nuclear program and likely only set it back by months,