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.”

Meanwhile, nearly 400 kilograms, or 880 pounds, of uranium enriched to 60% purity is still publicly unaccounted for.

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