Inspectors from the UN nuclear watchdog arrived in Iran Saturday to visit the still-unfinished Arak heavy water plant for the first time in more than two years, the ISNA news agency reported.
The two inspectors from the International Atomic Energy Agency will begin their work on Sunday at the site, 240 kilometres (150 miles) southwest of the capital, but ISNA did not say how long they would be there.
The inspection comes within the framework of a mid-November agreement between Iran and the IAEA that also allows the IAEA access to a uranium mine in Gachin, in the country's south.
The IAEA regularly visits the works on the reactor, but says it has not received any new design details since 2006. And inspectors have not been to the heavy water plant since August 2011.
The Arak reactor is of concern to the international community because Tehran could theoretically extract weapons-grade plutonium from the its spent fuel.
Under a deal reached last month with world powers in Geneva, Iran agreed that it would not commission the reactor or transfer fuel or heavy water to the site for a period of six months.
However, construction delays have effectively made that a moot point, as the authorities had not hoped to finish work before the end of 2014. Subsequently, they said that target would not be met, without giving a new date.