WASHINGTON -- Just days before Iran enters its first nuclear talks with the West since the summer, international nuclear inspectors said Thursday that the country had begun installing a new generation of equipment to enrich uranium that could speed its ability to produce nuclear fuel.
The installation, at the Natanz nuclear enrichment center, which is highly vulnerable to bombing, came after a half-decade of delays in production made worse by Western sanctions and sabotage. The new centrifuges are four to five times more powerful than the aging design that Iran has been using for years. The advance has worried American, European and Israeli officials who fear Iran might decide to race toward making fuel for nuclear weapons.
But the evidence collected by the International Atomic Energy Agency, as recently as Tuesday, suggests that the Iranian authorities are deliberately slowing the accumulation of medium-enriched uranium, which could most quickly be converted to bomb fuel. According to the agency, the Iranian nuclear authorities have diverted much of that production to make specialized fuel for a research reactor -- more fuel than they need to power the reactor.
The result is that Iran has delayed the day when it would reach what Israel's prime minister, Benjamin Netanyahu, defined as his latest "red line": the accumulation of enough medium-enriched fuel to make a single nuclear weapon. Mr. Netanyahu spoke of the line -- the point Iran would not be allowed to pass -- at the United Nations last fall, when it appeared that Iran would accumulate that amount -- about 240 or 250 kilograms of uranium enriched to 20 percent purity -- by early this summer. The agency's latest evidence pushes that date to the fall, allowing more time for diplomacy.
Still on Thursday the prime minister's spokeswoman called the new assessment of the inspectors "a very grave report, which proves that Iran is continuing to make rapid progress toward the red line." She added that "the first subject" Mr. Netanyahu will discuss with President Obama during his planned visit to Israel next month is preventing Iran from obtaining nuclear arms.
The report also indicated no evidence of any explosion or other setback at the deep-underground nuclear facility called Fordo, which is regularly visited by inspectors. Reports, fueled by a right-wing Web site with ties to the Iranian opposition, had suggested a major explosion at the site; that report, which appeared in a number of European newspapers, now appears to have been false.
The new centrifuges are known as IR-2, short for Iranian second generation. Iran's president, Mahmoud Ahmadinejad, disclosed research on the equipment more than six years ago, boasting that it would quadruple Iran's enrichment powers.
Testing the new centrifuges at the Natanz site in the Iranian desert began at a pilot plant in early 2008, and reached an advanced stage in 2011. But until recently, a range of technical problems had delayed their introduction into the cavernous underground halls of the production plant, which is roughly half the size of the Pentagon.
One mystery is why Iran is installing the new centrifuges at Natanz. It is just barely underground, and vulnerable to air attack. It is also the plant that was struck by a series of American and Israeli cyberattacks, part of a classified program called Olympic Games that resulted in a temporary setback for the program.
Centrifuges spin extraordinarily fast to capture the rare isotope of uranium that can fuel atom bombs or nuclear reactors.
The IR-2 is based on Pakistan's second-generation model. The rotor of the Pakistani machine, made of superhard steel, can spin much faster than the original model, speeding the pace of enrichment.
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.
Western experts say the IR-2 is roughly half the height of Iran's original centrifuge but spins twice as fast. Its rotor is made not of superstrong steel but carbon fibers, which Iran has also experienced difficulty making and importing from abroad because of Western sanctions.
If they work properly, 1,000 of the new machines would be able to enrich the same amount of uranium as 4,000 to 5,000 of the old model. Iran currently has about 10,000 of the old machines spinning away at Natanz.
This article originally appeared in The New York Times.