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 has 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. Director of National Intelligence James Clapper testified before Congress in February 2016 that "[w]e do not know whether Iran will eventually decide to build nuclear weapons." 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 has also produced LEU containing nearly 20% uranium-235; 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. 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 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 July 2015 Joint Cooperative Plan of Action (JCPOA), which Tehran concluded with the P5+1. 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, comprise the current legal framework governing Iran's nuclear program. Iran has complied with the JCPOA and resolution. 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 made the decision to do so. At the time, Tehran would have needed two to three months of this time to produce enough weapons-grade HEU for a nuclear weapon. Iran's compliance with the JCPOA has lengthened this time to one year, according to Clapper's February 2016 testimony. 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.