Iranian bomb no easy task

Now, the report said, Iran must replace and repair the broken machines and prepare the cascade for operation. Then comes the really hard part: If all goes well, the Iranians must mass-produce thousands of centrifuges and learn to run them in concert.

Iran is also struggling to turn concentrated uranium ore into the toxic gas fed into the centrifuges for enrichment.

Iran began the conversion effort in the early 1990s, asking China to help build the complex. But in 1997, the Clinton administration persuaded Beijing to stop the deal. The Iranians got blueprints but little else. So they started building on their own.

The plant encountered problems during its first runs in early 2004, its output laced with impurities, in particular molybdenum, a silvery element often found in uranium ore. The contamination, experts say, can ruin delicate centrifuges.

The Iranians are working hard to solve the problem. Mark Hibbs of Nuclear Fuel, an industry publication, who broke the molybdenum story, said most experts believed that the Iranians would ultimately succeed. British intelligence, he said, put the time needed at a year and a half, Israeli analysts at two or three months.