Iran’s nuclear program began during the 1950s. The United States has expressed concern since the mid-1970s that Tehran might develop nuclear weapons. Iran’s construction of gas centrifuge uranium enrichment facilities is currently the main source of proliferation concern. Gas centrifuges can produce both low-enriched uranium (LEU), which can be used in nuclear power reactors, and weapons-grade highly enriched uranium (HEU), which is one of the two types of fissile material used in nuclear weapons.
The United States has assessed that Tehran possesses the technological and industrial capacity to produce nuclear weapons, but has not yet mastered all of the necessary technologies for building such weapons. Whether Iran has a viable design for a nuclear weapon is unclear. A National Intelligence Estimate made public in 2007 assessed that Tehran “halted its nuclear weapons program” in 2003. The estimate, however, also assessed that Tehran is “keeping open the option to develop nuclear weapons” and that any decision to end a nuclear weapons program is “inherently reversible.” U.S. intelligence officials have reaffirmed this judgment on several occasions.
Obtaining fissile material is widely regarded as the most difficult task in building nuclear weapons. As of January 2014, Iran had produced an amount of LEU containing up to 5% uranium-235 which, if further enriched, could theoretically have produced enough HEU for as many as eight nuclear weapons. Iran had also produced LEU containing nearly 20% uranium235; the total amount of this LEU would, if it had been in the form of uranium hexafluoride and further enriched, have been sufficient for a nuclear weapon. After the Joint Plan of Action, which Tehran concluded with China, France, Germany, Russia, the United Kingdom, and the United States (collectively known as the “P5+1”), went into effect in January 2014, Iran either converted much of its LEU containing nearly 20% uranium-235 for use as fuel in a research reactor located in Tehran, or prepared it for that purpose. Iran has diluted the rest of that stockpile so that it contained no more than 5% uranium-235.
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