"This capacity is far larger than needed for a nuclear weapon program, supporting Iran's statement that the facility is aimed at producing low enriched uranium for nuclear power reactors. Nonetheless, such a facility could use a relatively small fraction of its capacity, say 10,000 SWU per year, to make enough highly enriched uranium for three nuclear weapons a year, while using the remaining capacity to produce low enriched uranium. In addition, if a country can make an enrichment plant of this size, it can make enough machines to outfit another secret enrichment plant with a capacity of 10,000 SWU per year involving several thousand machines. IAEA safeguards could detect such clandestine activities, but the IAEA must have far more extensive inspection rights than Iran has been willing to provide the IAEA so far."

http://www.isis-online.org/publications/iran/natanz03_02.html