updated 8/25/2009 6:53:42 PM ET 2009-08-25T22:53:42

Iran has put more machines on line to enrich uranium yet its output is stagnating, diplomats said Tuesday, adding that could mean Tehran may be running out of the ore that serves as the base for nuclear fuel — or the fissile core of warheads.

The diplomats — who demanded anonymity because their information was confidential — emphasized that the possibility Iran was running short of uranium oxide was only one of several possible explanations of why it had not substantially upped production of enriched uranium since May.

But they said it seemed unlikely the Islamic Republic had deliberately decided to curb its production. They noted that, despite the stagnation in output, Iran continued over the past three months to expand its capabilities by installing and running hundreds more of the centrifuges that spin uranium hexafluoride gas derived from uranium oxide into enriched uranium.

With Iran under strict U.N embargoes and on an international watch list meant to cripple its ability to import nuclear materials illegally, it could find it difficult to procure enough uranium oxide to feed its enrichment program. That, in turn, could reflect the success of U.N. sanctions by dealing a blow to its stated goal of expanding enrichment to the point where it can supply fuel for a nationwide nuclear network.

Building on "yellowcake"
The International Atomic Energy Agency and independent experts believe that Iran's rapidly expanding uranium enrichment program has been built on 600 tons of so-called "yellowcake" or uranium oxide imported from South Africa during the 1970s as part of ambitious plans by the former regime of Shah Reza Pahlavi to build a network of nuclear reactors.

In the enrichment process, uranium oxide is processed into uranium hexafluoride, which then is spun and re-spun to varying degrees of enrichment, with low enriched uranium used for nuclear fuel and upper-end high enriched uranium for nuclear weapons.

While Iran has built up a large stockpile of uranium hexaflouride, it would not be able to replenish its supply if it is running out of ore. That could account any decision to stop expanding output at Natanz in an effort to use up the supply of uranium gas more slowly.

Iran denied it was running out of yellowcake.

"It is not true," said Ali Asghar Soltanieh, Tehran's chief delegation to the IAEA, while adding that he could not make a substantial comment before the agency publishes its next report on his country's nuclear program sometime this week.

"This is a technical project with its ups and downs," he told the AP, when asked whether output was stagnating. "But everything is going according to plan."

On the pretense of peace
The existence of a secret Iranian enrichment program built on black market technology was revealed seven years ago. Since then the country has continued to expand it with only a few interruptions as it works toward its aspirations of a 50,000-centrifuge enrichment facility.

Iran is under three sets of U.N. Security Council sanctions for refusing to freeze uranium enrichment. These were imposed out of fears Iran is using the pretext of building a peaceful nuclear program — including enrichment to low levels suited for making nuclear fuel — to eventually make weapons-grade enriched uranium.

In its last report on Iran, in June, the International Atomic Energy Agency said that nearly 5,000 centrifuges were operating at Iran's cavernous underground enriching facility at Natanz by May. Diplomats said Tuesday that had expanded to about 6,000 by last month.

The Washington-based Institute for Science and International Security has estimated that even taking the 5,000-centrifuge figure as a basis, Iran could accumulate enough material to produce weapons-grade uranium for two warheads by February 2010.

Iran refuses to stop enriching despite the U.N sanctions. But it is believed to depend on rapidly diminishing outside sources of uranium oxide, with domestic mining yielding only relatively small quantities of inferior ore.

Extrapolating on statistics contained in the IAEA's November report on Iran, ISIS — the Washington-based think tank — said these indicated that by that date, Iran had used up just under three-quarters of its original South African supply.

"The next six months stand to be revealing," said the ISIS study, noting that Iran is likely to reserve perhaps as much as 100 tons of its South African uranium stock for its Arak heavy water nuclear reactor, which will be fueled by natural uranium once completed.

Quoting from the relevant U.N. Security Council sanctions on Iran, ISIS said they ban exports to the Islamic Republic of all "items, materials, equipment, goods and technology" that could contribute its enrichment activities, including uranium ore.

In addition, transfers of uranium ore in quantities greater than 500 kilograms — 1,100 pounds — annually are subject to close scrutiny of the Nuclear Supplies Group of countries exporting atomic technology.

"These restrictions effectively close the door on legal imports by Iran of significant quantities of uranium ore," said ISIS.

'International obligations'
One of the diplomats — who, like the others who commented for this article, is accredited to the IAEA and closely follows the enrichment issue — said that the international community has for months been closely watching Iran in anticipation it will try to secure uranium oxide through illicit channels.

The IAEA next reports on Iran, ahead of a meeting of its 35-nation board starting Sept. 7.

At the State Department, spokesman Ian C. Kelly declined to comment on the report, except to say that the U.S. remains intent on stopping Iran from getting a nuclear weapon.

"We are very concerned about the refusal of Iran to adhere to its international obligations," Kelly said.

The spokesman said the Obama administration is awaiting the formal report from the IAEA and will comment on it then.

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Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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