"Although Iran claims that its nuclear program is exclusively for peaceful purposes, it has generated considerable concern that Tehran is pursuing a nuclear weapons program. Indeed, the UN [United Nations] Security Council has responded to Iran's refusal to suspend work on its uranium enrichment and heavy-water nuclear reactor programs by adopting several resolutions which imposed sanctions on Tehran. Despite this pressure, Iran continues to enrich uranium, install additional centrifuges, and conduct research on new types of centrifuges. Tehran has also continued work on its heavy-water reactor and associated facilities. Whether Iran is pursuing a nuclear weapons program is, however, unclear. A National Intelligence Estimate made public in December 2007 assessed that Tehran 'halted its nuclear weapons program,' defined as 'Iran's nuclear weapon design and weaponization work and covert uranium conversion-related and uranium enrichment-related work,' 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.' Intelligence community officials have reaffirmed this judgment on several occasions. Iranian efforts to produce fissile material for nuclear weapons by using its known nuclear facilities would almost certainly be detected by the IAEA [International Atomic Energy Agency]."
CRS Report for Congress, RL34544