This type of reactor is used in some places to generate electricity, but it is better known as a means of producing plutonium for weapons that bypasses uranium enrichment and its many technical obstacles. As a result, the presence of a heavy-water reactor is often regarded as a sign that a country is trying to develop a weapon.
American spy satellites had detected construction at Natanz before its existence was made public last year. But the work near Arak had remained secret because the plant under construction looked like any other distillery or similar factory, according to intelligence officials and U.N. authorities.
After exiles revealed Arak's existence, Gholamreza Aghazadeh, the president of Iran's atomic energy organization, informed the IAEA that the planned reactor was strictly meant for research and producing radioisotopes for medical use.
To many experts, however, the project raises another red flag. "For Iran, there is no justification whatsoever to have a heavy-water plant," said Samore of the International Institute for Strategic Studies.
Echoing him, a senior U.N. official said, "The heavy-water plant sticks out like a sore thumb."
Iran first tried to buy heavy-water reactors as turnkey projects from China and India in the mid-1990s, according to a previously undisclosed dossier prepared by a foreign intelligence agency and provided to The Times. Blocked on that front by the United States, according to former U.S. officials, Iran decided to build its own and turned to two Russian institutes.
The United States learned of the cooperation through telephone intercepts and imposed sanctions on the Russian institutes in 1999. The sanctions remain in effect, but officials with foreign intelligence agencies and the CIA said there is evidence that Russian scientists are still providing expertise for the project.
Khlopkov, the Russian nuclear expert, said he thinks it is unlikely that Russian scientists are helping Iran with any of its weapons programs. Still, he said, the recent disclosures about the Iranian program surprised Moscow and might cause Russia to cancel a second planned reactor unless Iran agrees to stricter international inspections of its nuclear facilities.
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'Industrial Scale'
Despite Iran's progress, most experts said it is unlikely to develop a weapon without more outside help, particularly in procuring specialty technology. That is why some said they were alarmed by Iran's recent attempts to buy critical dual-use technology, which has military and civilian applications.
