which produces medical isotopes, Iran began using a single cascade to enrich uranium "up to 19.8%", to match the previously foreign supplied fuel. 20% is the upper threshold for low enriched uranium (LEU). Though HEU enriched to levels exceeding 20% is considered technically usable in a nuclear explosive device, this route is much less desirable because far more material is required to achieve a sustained nuclear chain reaction. HEU enriched to 90% and above is most typically used in a weapons development program. In an interview in October 2011, President Ahmadinejad of Iran said: "We have already expressed our views about nuclear bombs. We said those who are seeking to build nuclear bombs or those who stockpile, they are politically and mentally retarded. We think they are stupid because the era of nuclear bombs is over. [Why] for example, should Iran continue its efforts and tolerate all international treasures only to build a nuclear bomb, or a few nuclear bombs that are useless? They can never be used!" On 22 February 2012, in a meeting in Tehran with the director and officials of the Atomic Energy Organization of Iran (AEOI) and nuclear scientists, Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei said: "The Iranian nation has never pursued and will never pursue nuclear weapons. There is no doubt that the decision makers in the countries opposing us know well that Iran is not after nuclear weapons because the Islamic Republic, logically, religiously and theoretically, considers the possession of nuclear weapons a grave sin and believes the proliferation of such weapons is senseless, destructive and dangerous." U.S. stance In 2005, the United States stated that Iran has violated both Article III and Article II of the NPT. The IAEA Board of Governors, in a rare divided vote, found Iran in noncompliance with its NPT safeguards agreement for a 1985–2003 "policy of concealment" regarding its efforts to develop enrichment and reprocessing technologies. The United States, the IAEA and others consider these technologies to be of particular concern because they can be used to produce fissile material for use in nuclear weapons. The United States has argued that Iran's concealment of efforts to develop sensitive nuclear technology is prima facie evidence of Iran's intention to develop nuclear weapons, or at a minimum to develop a latent nuclear weapons capability. Others have noted that while possession of the technology "contributes to the latency