The US military strikes on three of Iran’s nuclear facilities last weekend did not destroy the core components of the country’s nuclear program and likely only set it back by months, according to an early US intelligence assessment that was described by three people briefed on it.

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.

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.”