Samples taken by the UN nuclear watchdog at what Prime Minister Benjamin Netanyahu called a "secret atomic warehouse" in Tehran showed traces of uranium that Iran has yet to explain, two diplomats who follow the agency's inspections work closely say.
The International Atomic Energy Agency is investigating the particles' origin and has asked Iran to explain the traces. But Tehran has not done so, according to the diplomats, stoking tensions between Washington and Tehran. US sanctions have slashed Iranian oil sales and Iran has responded by breaching its 2015 nuclear deal with world powers.
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In a speech a year ago Netanyahu, who vehemently opposed the deal, called on the IAEA to visit the site immediately, saying it had housed 15 kg (33 lb) of unspecified radioactive material that had since been removed.
Reuters first reported in April that the IAEA, which is policing the nuclear deal, had inspected the site and environmental samples taken there were sent off for analysis.
Israeli and US media have since reported that the samples turned up traces of radioactive material or matter.
Those traces were, however, of uranium, the diplomats said – the same element Iran is enriching and one of only two fissile elements with which one can make the core of a nuclear bomb. One diplomat said the uranium was not highly enriched, meaning it was not purified to a level anywhere close to that needed for weapons.
"There are lots of possible explanations," that diplomat said. But since Iran has not yet given any to the IAEA it is hard to verify the particles' origin, and it is also not clear whether the traces are remnants of material or activities that predate the landmark 2015 deal or more recent, diplomats say.
The IAEA did not respond to a request for comment. Iranian officials were not available to comment.
The deal imposed tight restrictions on Iran's nuclear program in exchange for sanctions relief and was based on drawing a line under Iran's past activities.
The IAEA takes environmental samples because they can pick up telltale particles even long after material has been removed from a site. Uranium traces could indicate, for example, the former presence of equipment or material somehow connected to those particles.
Cornel Feruta, the IAEA's acting director general, met Iranian officials on Sunday. An IAEA statement said afterward: "Feruta stressed that these interactions (on its nuclear commitments) require full and timely cooperation by Iran."
