As soon as mid-to-late April, Iran is expected to reach a new dangerous, destabilizing threshold, having enough highly enriched uranium (HEU) to fashion a nuclear explosive, about 40-42 kilograms (kg) of 60 percent enriched uranium (uranium mass). With this quantity, an enrichment level of 60 percent suffices to create a relatively compact nuclear explosive; further enrichment to 80 or 90 percent is not needed. According to the International Atomic Energy Agency (IAEA), 41.7 kg of 60 percent enriched uranium (uranium mass) is a significant quantity, which the IAEA defines as the “approximate amount of nuclear material for which the possibility of manufacturing a nuclear explosive cannot be excluded.”
A common fallacy is Iran would require 90 percent HEU, more commonly called weapon-grade uranium, to build nuclear explosives. Although Iran’s nuclear weapons designs have focused on 90 percent HEU and likely prefer that enrichment, modifying them for 60 percent HEU would be straightforward and well within Iran’s capabilities. Historically, the term highly enriched uranium was developed in the nuclear weapon states to distinguish between enriched uranium able to fuel a practical nuclear weapon versus enriched uranium, labeled low enriched uranium, unable to do so. Their cutoff is at 20 percent enriched uranium. At the least, a device made from 60 percent HEU would be suitable for underground nuclear testing or delivery by a crude delivery system such as an aircraft, shipping container, or truck, sufficient to establish Iran as a nuclear power.
Read the full report at the Institute for Science and International Security.