That quantity is too small to make a bomb, but large enough to use in experiments on how to make a bomb.
Iran last year reported to the International Atomic Energy Agency (IAEA) that it had the 19.8 kilograms (43.6 pounds) in its stocks. But when the IAEA proceeded to weigh the stocks, it found that quantity missing.
Iran has yet to give an explanation for how it measured and reported the 40 pounds that are not there.
The United States has expressed concern the material may have been diverted to suspected weapons-related research .
“The discrepancy remains to be clarified,” said the latest IAEA quarterly report.
IAEA inspectors sought information from Iran after their inventory last August of natural uranium metal and process waste at a research facility in Tehran measured less than the laboratory’s count. It has been seeking answers from Iran for six months without success.
In discussions last month about the discrepancy at the Jabr Ibn Hayan Multipurpose Research Laboratory (JHL) in Tehran, the IAEA said it had requested access to records and staff involved in uranium metal conversion experiments from 1995 to 2002.
“Iran indicated that it no longer possessed the relevant documentation and that the personnel involved were no longer available,” the IAEA report said.
The IAEA said Iran had suggested the discrepancy may have been caused by a higher amount of uranium in the waste than had been measured by the UN inspectors.
“In light of this, Iran has offered to process all of the waste material and to extract the uranium contained therein,” it said. The IAEA said it had also begun taking additional analysis samples of the material involved.
Iran’s envoy to the IAEA, Ali-Asghar Soltanieh, last year dismissed the reported discrepancy as “absolutely not an issue.”
But a senior US official said in November it required “immediate” resolution.