Iranian satellite images show construction at the Natanz nuclear power plant

Iranian authorities have described the mysterious July explosion as enemy sabotage, which destroyed the centrifuge assembly hall at Iran’s main nuclear fuel enrichment facility in Natanz and provoked a defiant response: The wrecked building will be rebuilt in the heart of the mountains, said Iran’s Atomic Energy Agency .

It is not clear to outside observers what progress has been made in advancing this promise, which could protect the facility from an air attack or other threat. But new satellite images are now shedding light on Iran’s plans.

The New York Times Visual Investigations team tracked construction on site using the new images. For the first time, the entrances to the new tunnel needed for underground construction will appear under the ridge, south of the Natanz facility, about 140 miles south of Tehran.

The Times worked with Jeffrey Lewis, an arms control expert at the Middlebury Institute for International Studies in Monterey, California, to interpret the new picture.

“The new facility is likely to be a much safer place to assemble the centrifuge – it is located far from a road and the ridge offers significant overload that will protect the facility from air attack,” Mr Lewis said in his written comments.

The July explosion was not the only recent case that exposed the security flaws in Iran’s nuclear program, which the country insists is limited to peaceful purposes. In late November, a brazen sunny attack killed Iran’s chief nuclear scientist, Mohsen Fakhrizadeh.

Iran has blamed Israel and the United States for the Natanz explosion and the assassination of Mr Fakhrizadeh, all of whom were seen as a serious setback for Iran’s nuclear program.

Mr. Lewis described the clues that underground construction was taking place at the Natanz site.

“There appear to be two tunnel entrances on either side of a large ridge, near a pile from an excavation. The space between the two entrances is large enough to accommodate a facility about the size of a centrifuge assembly building that was destroyed this summer, and Iran has indicated it is rebuilding it in the mountains. “

Viewing satellite imagery over several months allows you to track changes. Even something as simple and unobtrusive as a pile of dirt.

“The most important clue is the pile from the excavation, which was not present in July,” Mr. Lewis said. “Iran has also classified a couple of roads on both sides of the ridge that look like tunnel entrances.”

Allison Puccioni, an image analyst at Stanford University’s Center for International Security and Cooperation, analyzed other telltale signs of excavations near the rubble pile. In comments to the Times, Ms Puccioni said the images between the rubble pile and the excavation site “show a trail of excavated earth, are lighter in color than the existing hard-packed road”.

The Natanz activity occupied by the satellites in recent months includes the construction of new roads and further excavations that began after the explosion. Some analysts say additional tunnels are being built, suggesting they are working on an even larger underground complex.

The destroyed building was built in 2012 and used to collect centrifuges for uranium enrichment machines needed for peaceful purposes – and enriched to higher levels – for bombs. The 2015 nuclear deal between Iran and world powers halted high-level enrichment, but Iran has begun accumulating enriched uranium again after President Trump left the deal two years ago.

The ability to see by satellite what Iran has done since the Natanz explosion partly reflects such a visual quantum leap over the past two decades.

In 2002, analysts uncovered the construction of the then-secret Natanz enrichment facility using commercial high-resolution satellite imagery. Such sharp images only became available in 2000. Then the analysis required Natanz to search the Persian maps in the Library of Congress, fax an order form, and wait weeks to receive the satellite images on CD-ROM. Eighteen years later, in places like Natanz, even the smallest changes can be quickly tracked by analysts and journalists on their laptops.

These observation skills can present their own challenges. The images often collected do not show the completed construction, but work in progress. An initial interpretation of recent changes in Natanz from October it focused on the former shooting range south of the main facility as a possible location for the new underground centrifuge assembly hall. However, these changes proved to be a construction support facility used for road works and tunnel excavations.

In response to Fakhrizadeh’s assassination, Iran passed a law last week that would immediately eliminate uranium enrichment without lifting U.S. sanctions and ban international inspectors by February. The law also requires the installation of advanced centrifuges in its nuclear facilities, including Natanz.

The United States may have known in advance the recent attacks on Iran’s nuclear infrastructure and personnel. How Iran responds to the latest attack could be an early challenge for the incoming Biden administration, which has claimed to want to rejoin the nuclear deal rejected by Mr Trump.