Iran has removed the core of its Arak heavy water reactor and filled part of it with cement, a crucial step under a soon to be implemented nuclear agreement with world powers.
A spokesman for Iran's Atomic Energy Organisation told state television the work was done on Wednesday, paving the way for UN nuclear inspectors to announce Iran has met its commitments.
Asked on Thursday if the core had been removed from the reactor, spokesman Behrouz Kamalvandi replied: "Yesterday we finished the central part and the openings have been filled with cement."
Such measures mean the reactor will not be able to produce any weapons-grade plutonium from spent nuclear fuel, cutting off a potential pathway to an atomic bomb -- an aim Iran has always denied pursuing.
Kamalvandi's comments came ahead of "Implementation Day" for the nuclear agreement between Iran and six powers -- Britain, China, France, Germany, Russia and the United States.
At that time the deal will come into effect and nuclear-related sanctions on Iran will start to be lifted.
Iranian and US officials have said Implementation Day will come soon, probably by Sunday but possibly earlier.
The UN nuclear watchdog, the International Atomic Energy Agency, must first issue a report confirming Iran has met its side of the deal.
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IAEA inspectors are now in Iran, Kamalvandi said, and they would be sending their report to the watchdog's headquarters in Vienna.
The IAEA has not commented on when it will declare its findings and diplomats in Vienna also said it was not yet certain when the IAEA would report.
Reports emerged Monday that Iran had removed the core at Arak, but a top official later denied this was the case, insisting Tehran was still working on a deal to redesign the reactor.
Although China and the United States were expected to help Iran redesign the reactor core, deputy foreign minister Abbas Araghchi said Wednesday that an Iranian company would modernise it.
Under the deal, Iran has already reduced the number of its active centrifuges -- fast-spinning machines that enrich uranium -- and transferred the bulk of its low-enriched uranium stockpile to Russia.
On Monday, President Hassan Rouhani pledged that Iran was about to enter "a year of economic prosperity", with sanctions lifted, and said his government had delivered on its promises.
"The government is running the country under sanctions not under normal circumstances. God willing, in the coming days we will witness a rolling up of the sanctions scroll in this country," he said.
On the back of the nuclear deal, Rouhani wants to make greater inroads in domestic policy, with even modest social and political reforms in the Islamic republic.
But he has faced criticism from hardline groups about the nuclear accord, with opponents warning it could lead to "infiltration" by the United States in Iran's economy and other arenas.