VIENNA - In an underground chamber near the Iranian city of Natanz, a network of surveillance cameras offers the outside world a rare glimpse into Iran's largest nuclear facility. The cameras were installed by U.N. inspectors to keep tabs on Iran's nuclear progress, but last year they recorded something unexpected: workers hauling away crate after crate of broken equipment.

In a six-month period between late 2009 and last spring, U.N. officials watched in amazement as Iran dismantled more than 10 percent of the Natanz plant's 9,000 centrifuge machines used to enrich uranium. Then, just as remarkably, hundreds of new machines arrived at the plant to replace the ones that were lost.

The story told by the video footage is a shorthand recounting of the most significant cyberattack to date on a nuclear installation. Records of the International Atomic Energy Agency (IAEA), the U.N. nuclear watchdog, show Iran struggling to cope with a major equipment failure just at the time its main uranium enrichment plant was under attack by a computer worm known as Stuxnet, according to Europe-based diplomats familiar with the records.

But the IAEA's files also show a feverish - and apparently successful - effort by Iranian scientists to contain the damage and replace broken parts, even while constrained by international sanctions banning Iran from purchasing nuclear equipment. An IAEA report due for release this month is expected to show steady or even slightly elevated production rates at the Natanz enrichment plant over the past year.

"They have been able to quickly replace broken machines," said a Western diplomat with access to confidential IAEA reports. Despite the setbacks, "the Iranians appeared to be working hard to maintain a constant, stable output" of low-enriched uranium, said the official, who like other diplomats interviewed for this article insisted on anonymity to discuss the results of the U.N. watchdog's data collection.

The IAEA's findings, combined with new analysis of the Stuxnet worm by independent experts, offer a mixed portrait of the mysterious cyberattack that briefly shut down parts of Iran's nuclear infrastructure last year. The new reports shed light on the design of the worm and how it spread through a string of Iranian companies before invading the control systems of Iran's most sensitive nuclear installations.

But they also put a spotlight on the effectiveness of the attack in curbing Iran's nuclear ambitions. A draft report by Washington-based nuclear experts concludes that the net impact was relatively minor.

"While it has delayed the Iranian centrifuge program at the Natanz plant in 2010 and contributed to slowing its expansion, it did not stop it or even delay the continued buildup of low-enriched uranium," the Institute for Science and International Security (ISIS) said in the draft, a copy of which was provided to The Washington Post.

The worm's effect



The ISIS report acknowledges that the worm may have undercut Iran's nuclear program in ways that cannot be easily quantified. While scientists were able to replace the broken centrifuge machines this time, Iran is thought to have finite supplies of certain kinds of high-tech metals needed to make the machines, ISIS concluded. In addition, the worm almost certainly exacted a psychological toll, as Iran's leaders discovered that their most sensitive nuclear facility had been penetrated by a computer worm whose designers possessed highly detailed knowledge of Natanz's centrifuges and how they are interconnected, said David Albright, a co-author of the report.

"If nothing else, it hit their confidence," said Albright, ISIS's president, "and it will make them feel more vulnerable in the future."

The creator of the Stuxnet computer malware remains unknown. Many computer security experts suspect that U.S. and Israeli intelligence operatives were behind the cyberattack, but government officials in the United States and Israel have acknowledged only that Iran's nuclear program appears to have suffered technical setbacks in recent months.

While Israel's government has previously said Iran was on the brink of acquiring a bomb, the country's outgoing intelligence chief estimated last month that the Islamic republic could not have a bomb before 2015. Other intelligence agencies have said Iran could obtain nuclear weapons in less than a year if it kicks out U.N. inspectors and launches a crash program. Iran denies it is seeking to build a nuclear weapon.