By WILLIAM J. BROAD

Barbed wire and antiaircraft guns surround a maze of buildings in the Iranian desert that lie at the heart of the West’s five-year standoff with Tehran over its program to enrich uranium.
It is a place of secrets that Iran loves to boast about, clouding the effort’s real status and making Western analysts all the more eager for solid details and clues. Tehran insists that its plans are peaceful. But Washington and its allies see a looming threat.
The sprawling site, known as Natanz, has drawn attention recently because Iran is testing a new generation of centrifuges there that spin faster and, in theory, can more rapidly turn natural uranium into fuel for reactors or nuclear arms.
The new machines are also meant to be more reliable than their forerunners, which often failed catastrophically.
On April 8, President Mahmoud Ahmadinejad visited the desert site, and Iran released 48 photographs of the tour, providing the first significant look inside the atomic riddle.
“They’re remarkable,” Jeffrey G. Lewis, an arms control specialist at the New America Foundation, a nonprofit research group in Washington, said of the photographs. “We’re learning things.”
Most important, the pictures give the first public glimpse of the new centrifuge, known as the IR-2, for Iranian second generation. Nuclear analysts around the globe are scrutinizing the visual evidence to size up the new machine, its probable efficiency and its readiness for the tough job of uranium enrichment. They see the photos as an intelligence boon.
“This is intel to die for,” Andreas Persbo, an analyst in London at the Verification Research, Training and Information Center, a private group that promotes arms control, said on the blog site Arms Control Wonk.
One surprise of the tour was the presence of Iran’s defense minister, Mostafa Mohammad Najjar. His attendance struck some analysts as odd given Iran’s claim that the desert labors are entirely peaceful in nature.
Nuclear analysts say the tour opened a window into a hidden world previously known only to the Iranians and a few international inspectors.
“I don’t see anything to suggest this is propaganda,” Houston G.
Wood III, a centrifuge expert at the University of Virginia, said . “They seem to be working on an advanced machine.”
Such judgments rest not only on the photographic clues, but also on the Iranian record of successful, if limited, enrichment, as well as the reports of international inspectors, who have tracked Iran’s effort to develop the new centrifuges.
Engineers use centrifuges for many applications other than enriching uranium. In general, the devices spin fast to separate all kinds of objects of differing mass and density - for instance, milk from cream and impurities from wine.
Iran is separating U-235 from U-238. Rare in nature, U-235 easily splits in two to produce bursts of atomic energy. It also has three fewer neutrons than U- 238, making it slightly lighter and thus a candidate for centrifuge separation.
It seems easy. But the centrifuges spin at about the speed of sound, must work day and night for months or years on end and can easily lose their balance, tearing themselves apart.
“Our machines broke down frequently” in the program’s early days, Gholamreza Aghazadeh, the chief of the Iranian Atomic Energy Organization, recalled in a 2006 interview . He said a study had traced the failures to centrifuge assembly when technicians with bare hands inadvertently left behind clusters of microbes.
“This little amount of germs,” Mr. Aghazadeh said, was enough to throw the whirling devices off balance, leaving them in ruins. “When we say a machine is destroyed,” he added, “we mean that it turns into powder.”
In great secrecy, Iran began its centrifuge program in 1985, according to inspectors of the International Atomic Energy Agency. It copied a Pakistani design, known as the P-1. Today, the Iranian version stands more than 1.8 meters tall. Inside, a hollow rotor of aluminum spins the uranium gas to blinding speeds. Iran has installed 3,000 of the temperamental machines at Natanz, and recently began expanding that setup to 9,000.
In recent years, Iran has tried to move ahead in sophistication with a newer centrifuge design based on Pakistan’s second-generation model, known as the P-2. Its rotor is made of superhard steel that can spin faster, speeding the pace of enrichment while lowering the risk of breakdown.
But Iran had great difficulty building the machines and obtaining the special steel. Mostly in secret, it instead developed its own version, the IR-2. It is partly indigenous, signaling that the Iranians have achieved new levels of technical skill. If perfected, the IR-2 could accelerate Iran’s production of fuel for reactors or bombs.
Western experts say demonstration models of the IR-2 stand half the height of the P-1. But they spin twice as fast.
“That’s a lot,” said David Albright, president of the Institute for Science and International Security, a private group in Washington that tracks nuclear proliferation. “It would produce about four times the enrichment.”
Why did Iran release the photos- Analysts cite everything from a spirit of cooperation to a show of disdain.
Dr. Wood of the University of Virginia said the episode seemed like bragging. “It was amazing to me that they put the pictures out there,” he said. “It’s sort of a cocky thing. I would think they had more to gain by keeping their cards close to their chests.”