The diplomats, who spoke on condition of anonymity, said the readings were registered by inspectors of the International Atomic Energy Agency looking for evidence of a possible nuclear weapons program.
The diplomats said minute quantities of the substance were found at the Kalay-e Electric Co. Earlier this year, U.N. inspectors found enriched uranium particles at Iran's uranium-enrichment plant at Natanz.
Iran says its nuclear programs are to produce energy and that the traces of weapons-grade material were imported on equipment purchased from abroad. The United States and its allies argue the material is further evidence of an effort to develop nuclear weapons.
The U.N. agency's board of governors earlier this month set an Oct. 31 deadline for Iran to disprove that it has a weapons program.
The board could ask the Security Council to get involved — possibly applying sanctions — in the dispute if at its next meeting it finds that Iran is violating the global treaty banning the spread of nuclear weapons.
Agency officials said they would not comment on ongoing inspections of Iran's nuclear program.
"We are not commenting on the results of samples or ongoing inspections," said IAEA spokesman Mark Gwozdecky.
A team of inspectors from the agency is scheduled to leave for Tehran on Sunday, for "an intensive work program over the next five weeks involving more or less a continuous presence in Iran," Gwozdecky said.
Iran on Monday announced that it would cut back its cooperation with the International Atomic Energy Agency in response to the agency's Oct. 31 deadline — which Tehran says was politically motivated.
The decision, announced by Ali Akbar Salehi, Iran's representative to the IAEA, suggested that that Tehran would cooperate only to the point it was bound to under agreements with the agency.
In recent weeks, agency inspectors had been given access to sites not covered in the agreements, although the IAEA had complained of delays that in some cases appeared to have served to give authorities a chance to cover their tracks.
In August, Iran allowed inspectors to visit Kalay-e after they were turned away two months before when they came to take environmental samples. Iran allegedly had tested centrifuges, which are used to enrich uranium, at the site.