The International Atomic Energy Agency says Iran has taken a significant step toward building a reactor that Western experts say could provide a second path to producing material for a nuclear bomb.
In a quarterly report obtained by Western news agencies Wednesday, the U.N. nuclear agency said Iran has delivered the reactor vessel to the heavy water plant near the western city of Arak. It said the component has not yet been installed.
Western powers fear the Arak facility could provide Iran with plutonium for nuclear weapons if the reactor's spent fuel is reprocessed.
Iran insists its nuclear ambitions are peaceful. Iranian ambassador to the IAEA Ali Asghar Soltanieh said Wednesday the government is cooperating beyond its requirements under the Nuclear Non-Proliferation Treaty.
"After 10 years of the most robust inspections in the history of the IAEA no evidence of diversion of nuclear material or nuclear activities to prohibited purposes are found. And everything remains peaceful. This is, in fact, a very clear document to prove that all those allegations against Iran are forged and fabricated."
Iran has been denying IAEA inspectors access to nuclear facilities and documents the agency wants to see in order to address concerns about the possible military nature of Iran's nuclear program. In particular, Iran has repeatedly barred the IAEA from visiting the Parchin military site, insisting it is a standard military facility.
The two sides have increased the pace of negotiations since January of last year, but have yet to agree on a framework for addressing the agency's questions.
Iran has defied four rounds of U.N. Security Council sanctions demanding a suspension of uranium enrichment.
Western concerns have focused mostly on two facilities that enrich uranium, a process that can have peaceful and military uses.
Iran's underground Fordo plant has been enriching uranium to purities of 20 percent, a relatively small step away from the higher purity needed for a nuclear bomb.
The new IAEA report said enrichment work has stagnated at Fordo in the past three months. But it said Iran has accelerated enrichment at Natanz, with almost 700 advanced centrifuges installed in the facility by this month, compared to 180 in February.
The advanced equipment allows Iran to enrich uranium more quickly. Most uranium enrichment at Natanz is to the lower level of 5 percent purity, far short of weapons-grade.