	Background

Iran's nuclear program began during the 1950s. Nevertheless, mid-1970s U.S. intelligence reports expressed concern that Iran might pursue a nuclear weapons program. Scope and Purpose of Iran's Nuclear Program

Iranian officials have repeatedly asserted that the country's nuclear program is exclusively for peaceful purposes (see Appendix A ). The main source of proliferation concern generated by Iran's nuclear program has been Tehran's construction of gas centrifuge uranium-enrichment facilities. 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. This program is currently constrained by the JCPOA. Although Tehran says that the reactor is intended for the production of radioisotopes for medical purposes, the reactor previously under construction was a proliferation concern because its spent fuel would have contained plutonium well-suited for use in nuclear weapons. In order to be used in nuclear weapons, however, plutonium must be separated from the spent fuel—a procedure called "reprocessing." Iran has said that it will not engage in reprocessing. Notably, Tehran has implemented various restrictions on, and provided the IAEA with additional information about, its uranium enrichment program and heavy-water reactor program pursuant to the JCPOA. Iran and the IAEA agreed in August 2007 on a work plan to clarify the outstanding questions regarding Tehran's nuclear program, most of which concerned possible Iranian procurement activities and research directly applicable to nuclear weapons development. A December 2015 report to the IAEA Board of Governors from agency Director-General Amano contains the IAEA's "final assessment on the resolution" of these outstanding issues. Iran ratified the nuclear Nonproliferation Treaty (NPT) in 1970. The parties began implementing the JCPOA on January 16, 2016. On that day, all of the previous Security Council resolutions' requirements were terminated. The NPT and U.N. Security Council Resolution 2231 compose the current legal framework governing Iran's nuclear program. According to this report, which provided the most detailed account to date of the IAEA's evidence regarding Iran's suspected nuclear weapons-related activities, the agency has "credible" information that Iran has carried out activities "relevant to the development of a nuclear explosive device," including
acquisition of "nuclear weapons development information and documentation," work to develop "an indigenous design of a nuclear weapon including the testing of components," efforts "to procure nuclear related and dual use equipment and materials by military related individuals and entities," and work to "develop undeclared pathways for the production of nuclear material." Instead, the board is focused on monitoring and verifying Iran's JCPOA implementation "in light of" United Nations Security Council Resolution 2231, which the council adopted on July 20, 2015. The IAEA was (and still is) able to verify that Iran's declared nuclear facilities and materials have not been diverted for military purposes. This quantity of LEU, if it had been further enriched, would have yielded enough weapons-grade HEU for as many as eight nuclear weapons. Enriched Uranium Containing Up To 20% Uranium-235

As noted, Iran argued that it was producing LEU containing nearly 20% uranium-235 for use in research reactors; as of January 20, 2014, when the JPA went into effect, Tehran had used the Natanz pilot facility and the Fordow facility to produce a total of 447.8 kilograms of uranium hexafluoride containing up to 20% uranium-235. If further enriched, this amount of material would have been sufficient for a nuclear weapon. Since that date, Iran has either converted much of that material for use as fuel in the Tehran Research Reactor or prepared it for that purpose. Iran diluted the rest of that stockpile so that it contained no more than 5% uranium-235. Obtaining fissile material is widely regarded as the most difficult task in building nuclear weapons. As noted, the U.S. government assesses that Tehran has not mastered all of the necessary technologies for building a nuclear weapon. Then-Under Secretary of State for Political Affairs Wendy Sherman explained during an October 3, 2013, Senate Foreign Relations Committee hearing that Iran would need as much as one year to produce a nuclear weapon if the government made the decision to do so. At the time, Tehran would have needed two to three months to produce enough weapons-grade HEU for a nuclear weapon. Declared Versus Undeclared Nuclear Facilities

The U.S. estimates described above apparently assume that Iran would use its declared nuclear facilities to produce fissile material for a weapon. On several occasions, the U.S. intelligence community has reaffirmed the 2007 NIE's assessment that Iran halted its nuclear weapons program but is keeping its options open. Some foreign intelligence agencies have apparently concurred with the U.S. assessment that Iran has not yet decided to build nuclear weapons. In June 2006, China, France, Germany, Russia, the United Kingdom, and the United States, collectively known as the "P5+1," presented a proposal to Iran that offered a variety of incentives in return for Tehran taking several steps to assuage international concerns about its enrichment and heavy-water programs. The P5+1 and Iran agreed on a framework for a Joint Comprehensive Plan of Action (JCPOA) on April 2, 2015, and finalized the JCPOA on July 14, 2015.
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.
Is Iran Capable of Building Nuclear Weapons?
The United States has assessed that Tehran possesses the technological and industrial capacity to produce nuclear weapons. But Iran 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% uranium-235; the total amount of this LEU, if it had been in the form of uranium hexafluoride and further enriched, would 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. In addition, Tehran has implemented various restrictions on, and provided the IAEA with additional information about, its nuclear program pursuant to the July 2015 Joint Comprehensive Plan of Action (JCPOA), which Tehran concluded with the P5+1.
Although Iran claims that its nuclear program is exclusively for peaceful purposes, the program has generated considerable concern that Tehran is pursuing a nuclear weapons program. The U.N. Security Council responded to Iran's refusal to suspend work on its uranium enrichment program by adopting several resolutions that imposed sanctions on Tehran. Despite evidence that sanctions and other forms of pressure have slowed the program, Iran continued to enrich uranium, install additional centrifuges, and conduct research on new types of centrifuges. Tehran has also worked on a heavy-water reactor, which was a proliferation concern because its spent fuel would have contained plutonium—the other type of fissile material used in nuclear weapons. However, plutonium must be separated from spent fuel—a procedure called "reprocessing." Iran has said that it will not engage in reprocessing.
Who Is Monitoring Iran's Nuclear Program?
The International Atomic Energy Agency (IAEA) monitors Iran's nuclear facilities and has verified that Tehran's declared nuclear facilities and materials have not been diverted for military purposes. The agency has also verified that Iran's compliance with the JCPOA. On the JCPOA's Implementation Day, which took place on January 16, 2016, all of the previous Security Council resolutions' requirements were terminated. The nuclear Nonproliferation Treaty (NPT) and U.N. Security Council Resolution 2231, which the council adopted on July 20, 2015, compose the current legal framework governing Iran's nuclear program. Iran has continued to comply with the JCPOA and Resolution 2231. Iran and the IAEA agreed in 2007 on a work plan to clarify outstanding questions regarding Tehran's nuclear program, most of which concerned possible Iranian procurement activities and research directly applicable to nuclear weapons development. A December 2015 report to the IAEA Board of Governors from agency Director-General Yukiya Amano contains the IAEA's "final assessment on the resolution" of these outstanding issues.
How Soon Could Iran Produce a Nuclear Weapon?
Then-Under Secretary of State for Political Affairs Wendy Sherman explained during an October 2013 hearing of the Senate Committee on Foreign Relations that Iran would need as much as one year to produce a nuclear weapon if the government decided to do so. At the time, Tehran would have needed two to three months to produce enough weapons-grade HEU for a nuclear weapon. Iran's compliance with the JCPOA has increased that time frame to one year, according to U.S. officials. These estimates apparently assume that Iran would use its declared nuclear facilities to produce fissile material for a weapon. However, Tehran would probably use covert facilities for this purpose; Iranian efforts to produce fissile material for nuclear weapons by using its known nuclear facilities would almost certainly be detected by the IAEA.