Mohammad ElBaradei told New York Times that his inspectors' initial findings at the nuclear site near holy city of Qom were "nothing to be worried about."
"The idea was to use it as a bunker under the mountain to protect things," ElBaradei, alluding to Tehran's references to the site as a fallback for its nuclear program in case its larger Natanz enrichment plant were bombed by a foe like Israel.
"It's a hole in a mountain," he said.
Details are expected to be included in the next IAEA report on Iran's disputed nuclear activity due in mid-November, the Iranian labors news agency reported.
The inspectors' goal was to compare engineering designs to be provided by Iran with the actual look of the facility, interview scientists and other employees, and take soil samples.
The Islamic Republic revealed the plant's existence to the Vienna-based UN nuclear watchdog on September 21. It said the site, which remains under construction, would enrich uranium only to the low 5 percent purity suitable for power plant fuel.
After talks with Iran and three world powers, ElBaradei drafted a plan for Iran to transfer its low-enriched uranium (LEU) to Russia and France to turn it into fuel for a Tehran reactor that makes isotopes for cancer treatment.
ElBaradei said "The issue is timing, whether the uranium goes out and then some time later they get the fuel, as we agreed tentatively in Geneva or whether it only goes at the same time as the fuel is delivered".
"There are a lot of ideas. One is to send Iran's uranium to a third country, which could be a friendly country to Iran, and it stays there. Park it in another state for something like a year then bring in the fuel. The issue is to get it out, and so create the time and space to start building trust," he said.