Iran maintains that the purpose of its nuclear programme is peaceful
Iran has allowed inspectors from the UN nuclear watchdog into a nearly complete nuclear reactor at Arak after a year without access, Western diplomats say.
They say Iran also agreed on ways to improve monitoring at the Natanz nuclear enrichment plant.
Enriched uranium can be used for civilian nuclear power or to make atomic bombs.
The west fears Iran wants to develop nuclear weapons but Tehran insists its nuclear undertakings are peaceful.
There has been no comment from the International Atomic Energy Agency (IAEA) on the visit to Arak.
The BBC's Bethany Bell in Vienna says that while some observers may see this development as a sign of progress in the standoff over Iran's nuclear programme, others are likely be more cynical and declare that much more co-operation is needed.
In a report on Iran filed in June, the IAEA warned that Iran's refusal to allow its inspectors access to Arak could make it hard for the agency to ensure effective safeguards at the site.
The agency had also been seeking additional cameras and inspections of the Natanz site, about 300 miles (500 kilometres) south of Tehran, to keep track of Iran's continuing enrichment programme.
Enriching uranium is a technology which can be used for peaceful civilian purposes but it can also be used to make atomic bombs, a fact many in the West and Israel fear.
The UN Security Council has called on Iran to suspend any such work, amid fears that Tehran could be developing nuclear weapons in secret.
Iran continues to insist its policy of pursuing a nuclear programme lies on peaceful grounds.
IAEA director Mohamed El Baradei will publish his latest report on Iran's nuclear dealings next week and it will go before the agency's governors some time in September.