From the Document: "Iran's nuclear program has generated widespread concern that Tehran is pursuing nuclear weapons. According to U.S. intelligence assessments, Tehran has the capacity to produce nuclear weapons at some point, but has halted its nuclear weapons program and has not mastered all of the necessary technologies for building such weapons. (For additional information, see CRS Report RL34544, 'Iran's Nuclear Program: Status', by Paul K. Kerr.) Since the early 2000s, Tehran's construction of gas centrifuge uranium enrichment facilities has been the main source of proliferation concern. Gas centrifuges enrich uranium by spinning uranium hexafluoride (UF6) gas at high speeds to increase the concentration of the uranium235 (u-235) isotope. Such centrifuges can produce both low-enriched uranium (LEU), which can be used in nuclear power reactors, and highly enriched uranium (HEU), which is one of the two types of fissile material used in nuclear weapons. Tehran asserts that its enrichment program is meant to produce fuel for peaceful nuclear reactors."
CRS In Focus, IF12106
Congressional Research Service: https://crsreports.congress.gov/