WASHINGTON: UN nuclear inspectors are pursuing leads to make the case that Iran is working on the bomb, a month after a report that lacked the “smoking gun” needed to brand Iran guilty of seeking nuclear weapons.

If Iran used nuclear material to do weapons work, the country would be in direct violation of the Non-Proliferation Treaty, which Tehran has signed. Failing to report that work to the watchdog International Atomic Energy Agency — as they are required to – would also put Iran at odds with the international community. In any case, a large measure of ambiguity about Iran’s nuclear work would be removed. IAEA inspectors were careful to avoid “extrapolating” in the November report, in the hope that Iran would answer their questions. The result is that important implications were not spelled out in the detailed allegation of a comprehensive and sustained Iranian nuclear weapons program.

The United States has estimated that Iran stopped its weaponization activities in 2003 but the IAEA report states that some work has continued since then. These activities may have included neutron initiator testing using natural uranium, or natural uranium transformed into uranium deuteride. A neutron initiator is a key trigger to setting off a nuclear explosion. The tiny initiator is placed in the center of a nuclear core to produce a burst of neutrons, initiating a fission chain reaction.

The test would have required a dry-run explosion.

Even if the Iranians claim the test, believed to have taken place before 2003, was for peaceful purposes it still would have been in violation of UN rules. Those rules state that all use of nuclear material must be reported to IAEA. The test would also violate the NPT, which bans nuclear-weapon-related work.

Iran denies that it seeks nuclear weapons, claiming the IAEA allegations are based on forged documents from U.S. intelligence. But international tension about a nuclear Iran — and its potential threat to Israel — is mounting. The United States and Israel say Iran’s desire for a nuclear weapon is clear. Iran says its nuclear work is a peaceful effort to develop a fuel cycle and power reactors in order to generate electricity. Yet Tehran is under four rounds of UN sanctions for failing to come clean about its atomic work.

The report said member states notified the IAEA that Iran “had undertaken work to manufacture small capsules for use as containers of a component containing nuclear material” and may also “have experimented with such components in order to assess their performance in generating neutrons” for initiating a fission chain reaction.

Iran allegedly tried to cover its tracks on that work. “The location where the experiments were conducted was said to have been cleaned of contamination after the experiments had taken place,” the report stated. The design of the capsule and the material used was consistent with information Iran is believed to have obtained from a “clandestine nuclear supply network,” a clear reference to one run by A.Q. Khan, the father of Pakistan’s atomic bomb.

U.S. officials have repeatedly noted that almost 20 kilograms of uranium metal has gone missing in Iran, according to the IAEA report. Officials from Western states have linked this discrepancy to the report’s claims that “kilogram quantities of natural uranium metal were available to the AMAD plan,” which is Iran’s alleged weaponization program supposedly run by the military.

Allocating uranium metal to a military program would be monumental, no matter how the Iranians had used it, since this would take away Iran’s justification of its nuclear work. At this point, the IAEA “continues to verify the non-diversion of declared nuclear material” in Iran, according to the report. This finding seemingly supports Tehran’s claims that its nuclear program is a peaceful one.

Yet the IAEA is still investigating since it is “unable to provide credible assurance about the absence of undeclared nuclear material and activities in Iran, and therefore to conclude that all nuclear material in Iran is in peaceful activities.” This is why the missing 20 kilograms (actually 19.8 kilograms) are so important.

To be fair, Iran has cleared up other issues in the past, such as contamination by enriched uranium particles of equipment that turned out to be from used centrifuge parts imported from Pakistan, and not from secret Iranian work. But Iran is refusing to answer IAEA questions about possible military dimensions of its nuclear program.

Robert Kelley, a former director for IAEA inspections in Iraq, said that “natural uranium metal diverted from the Jabr Ibn Hayn Multipurpose Research Laboratory could be an indicator of doing experiments on the neutron initiator or uranium metal shells for hydrodynamic testing. These experiments are well known to us from the work of AQ Khan and the Iraqi nuclear weapons program that ended in 1991.”

U.S. officials have raised concerns about other possible uses of natural uranium by Iran. An atomic bomb would use highly enriched uranium. Simulation tests, without an explosion through a chain reaction, are crucial to developing an actual bomb. Natural uranium is perfect for such tests since its metallurgical properties match enriched, weapon-grade uranium but it does not set off a chain reaction. Highly enriched uranium is refined to raise the level of the isotope U-235 from 0.7 percent in natural uranium to over 90 percent.

“In terms of the use of uranium metal, if you’re doing work on nuclear weapons research, at some point you might want to use natural uranium metal as a surrogate material when you’re testing to see whether or not your nuclear design will function properly,” a senior U.S. official said. “You can do this . . . without producing a nuclear yield if you use surrogate material instead of fissile material for the core.”

Iran used tungsten as a surrogate for a fissile core, the IAEA report said, noting that “Iran has manufactured simulated nuclear explosive components using high density materials such as tungsten.” The report also said that Iran has apparently “constructed a large explosives containment vehicle in which to conduct hydrodynamic experiments.” The IAEA claims the vessel was installed at Iran’s Parchin military testing ground in 2000.

Michael Adler is a Public Policy Scholar in the Middle East Program at the Woodrow Wilson International Center for Scholars, specializing in nuclear issues.