UN nuclear watchdog to visit 2nd Iran site in 'a few days'
Iran's refusal to allow the International Atomic Energy Agency (IAEA) access to the two sites had caused a diplomatic row for months.
"The second one will happen in a few days," he told reporters after opening the agency's 35-member board of governors' meeting.
He said analysing environmental samples collected at the first site would take "not less than a couple of months, two or three months maybe".
Tehran announced in late August that it would allow the IAEA access to the two sites, following a visit to Tehran by Grossi.
The denial had prompted the IAEA's board of governors to pass a resolution in June urging Tehran to comply with the agency's requests.
In its latest report, the IAEA said Iran's stockpile of enriched uranium now stands at more than ten times the limit set down in the 2015 deal.
Iran is also continuing to use more advanced centrifuges to enrich uranium than is permitted under the deal.
However, the level of enrichment has not gone above that which would be needed for use in a nuclear weapon.
The head of Iran's atomic agency, Ali Akbar Salehi, said Sunday that 1,044 centrifuges were active at the Fordow uranium enrichment plant.
The remaining parties to the accord -- Britain, China, France, Germany and Russia -- have called on Iran to return to its commitments, but Tehran insists the steps can be reversed once it realises the economic benefits from the deal.
Tensions between Tehran and Washington have spiralled following a US push to extend an arms embargo on Iran that starts to progressively expire in October as well as reimposing UN sanctions on the Islamic republic.