Iran allowed the inspectors from the International Atomic Energy Agency (IAEA) to visit the 40-megawatt facility, southwest of the capital Tehran, last week as part of the IAEA drive for more transparency in Iran's controversial nuclear program.
Iran cut off IAEA access to the facility under construction four months ago in protest at UN Security Council sanctions over Tehran's refusal to halt uranium enrichment, which some Western countries suspect is pursuing the creation of nuclear weapons.
Iran has consistently denied the allegations saying it needs the program for peaceful nuclear power generation.
IRNA quoted an informed source as saying the inspection lasted for over four hours, without providing any further details.
Iran has come under pressure for rejecting three consecutive UN resolutions against its nuclear program since last year.
The six countries negotiating the dispute - the UN Security Council permanent members plus Germany - have demanded that Tehran suspend all uranium enrichment before negotiating a solution to the nuclear program.
President Mahmoud Ahmadinejad announced early April the start of uranium enrichment on an industrial level, but in an attempt to avoid tougher sanctions that may follow the two sets already imposed by the Security Council, Tehran has boosted cooperation with the IAEA.