A study by a Washington think tank that closely follows the Iranian nuclear program, the Institute for Science and International Security (ISIS), concludes that Iran could have tested a nuclear trigger in a device at the disputed Iranian site of Parchin. Breaking Defense obtained a copy of the draft report.

The Iranian military testing ground of Parchin has emerged as the front line in the continuing struggle between the UN’s nuclear watchdog, the International Atomic Energy Agency, and Iran. Iran has refused to let inspectors from the IAEA visit the site, some 30 kilometers southeast of Tehran, where they suspect experiments related to developing nuclear weapons may have been carried out. These tests were allegedly done in a 19-meter-long metal cylinder, 4.4 meters in diameter but reinforced in the middle with concrete, almost doubling the diameter to 7.6 meters. Questions have been raised about whether the experiments, which are explosions, could have been done in this cylinder and why the Iranians would use an enclosed device, when such testing is often done in the open air.

ISIS has determined that the metal cylinder could have been used for an explosive experiment with non-fissile material into the trigger for an atomic bomb. The purpose of doing this in an enclosed metal container would have been, of course, to avoid detection that would have otherwise been possible if the explosion were done in the open air.

The reason this is so sensitive is that nuclear material, namely natural uranium, may have been used in what would have been a dry run for the trigger, rather than a chain reaction. Proof that Iran has employed uranium in military research would destroy the Islamic republic’s central claim that its program has made only peaceful use of nuclear material. Iran says it seeks atomic power for energy and other civilian ends but the United States and a host of other countries fear it is hiding a drive to build, or to be able to build, nuclear weapons.

The IAEA is meanwhile investigating whether Iran diverted some 20 kilograms of natural uranium “in the form of natural uranium metal and process waste” from the Jabr Ibn Hayan Multipurpose Research Laboratory, according to an IAEA report last November. The IAEA is moving cautiously on this charge,however, as evidence relies on potentially unreliable weighing of nuclear material The IAEA has quizzed Ukrainian scientist Vyacheslav Danilenko about Parchin.

It said in its November report that it had “strong indications” a “foreign expert” had assisted Iran in working on developing a “high explosives” trigger for a bomb. This unnamed expert has been identified as Danilenko, who said he was in Iran to help develop “a facility and techniques for making ultra-dispersed diamonds (‘UDDs’ or ‘nanodiamonds’),” the report said. The cylinder at Parchin can be used for nanodiamonds.

But Danilenko had another expertise. He had worked for the Soviet Union on designing nuclear weapons small enough to fit in a missile, bomb or artillery shell. He “was in Iran from about 1996 to about 2002,” the IAEA said in its report.

The IAEA investigation is part of an international effort to win guarantees that Iran does not seek nuclear weapons. The standoff over Parchin comes at an especially sensitive time as diplomacy is being renewed with a Saturday meeting in Istanbul between six major powers, including the United States and Iran. The United States wants Iran to stop higher-level enrichment of uranium and to close a heavily protected enrichment facility near the holy city of Qom. The IAEA, the watchdog agency for the nuclear Non-Proliferation Treaty, has inspectors monitoring uranium enrichment at both the Fordow site near Qom and at Iran’s other, and larger, enrichment plant in Natanz.

The IAEA had visited another site at Parchin in 2005 but found nothing. Parchin is not actively monitored by the IAEA since no nuclear material has been reported there. But the cylinder has aroused suspicions. ISIS has photographs of the site. A satellite photograph from March 14, 2000 shows, ISIS says, “the foundation where Iran would place a high explosive test chamber later (later) in the year 2000.” This foundation looks like just a hole in the ground. The ISIS report then shows a satellite photograph from August 13, 2004 which it says “shows the building containing a high explosive test chamber.” Neither image shows the alleged metal cylinder, which would have been inside the building and may no longer be there.

At stake is testing the initiation system for a nuclear warhead that would fit inside, as ISIS says, “the payload chamber of the Shahab 3 missile tri-conic nose cone.” The IAEA had reported in November 2011 that the explosive chamber at Parchin could be used to test an implosion trigger for an atomic bomb. This is precisely the sort of trigger needed for a nuclear warhead for a missile.

The test would involve, ISIS said, “hundreds of fiber optic cables … placed in proximity of the inner surface of the high explosive. The other end of the fiber cables go to a fixture for a rotating mirror that is part of a high speed streak camera.” What is key here, according to ISIS, is that “an experiment of the initiation of the R265 system (the implosion trigger) . . . would contain less than 70 kilograms of high explosives and would have been possible to conduct in the chamber at Parchin… Iran’s goal of using this chamber would likely have been to hide its activities from overhead observation.”

The IAEA says satellites have reported activity at Parchin recently. This has raised concern that Iran may be trying to clean up the site ahead of any inspection. Iran has rejected such charges. Foreign Ministry spokesman Ramin Mehmanparast told reporters last month in Tehran that uranium traces cannot be cleaned up, as tiny particles would remain after any effort to sanitize an area, and insisted that only “conventional military” activities are carried out at Parchin.

But the IAEA report in November described two types of experiments that could have taken place at Parchin. The first is hydrodynamic tests, which are simultaneous explosions of an implosion trigger to compact a spherical nuclear core. The second would be explosions to test a neutron initiator, a tiny capsule in the center of the bomb core which bathes the nuclear material with neutrons, thus accelerating a chain reaction. Both these tests could use natural uranium as a surrogate for fissile material. Other surrogates, such as tungsten, could also be used. The IAEA inspectors would test both the cylinder and the area for traces of any of these, if they get access to the cylinder.

ISIS warned that it was not clear “if an IAEA visit would be able to detect whether such an experiment happened.” But it said a visit would still “help establish more transparency of Iran’s alleged nuclear weaponization activities and should be supported both publicly and by governments.”

Michael Adler is a scholar at the Woodrow Wilson International Center for Scholars, writing a book on diplomacy in the Iranian nuclear crisis. Michael covered this extensively for five years while in Vienna, where he reported on the International Atomic Energy Agency.