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 four people briefed on it.

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 analyzed by The New York Times show the extent of damage at the three nuclear facilities hit by the United States. Fordo is Iran’s most critical nuclear enrichment facility, housed deep inside a mountain to shield it from attacks.