The Brazilian centrifuges have yet to demonstrate those advantages, but they have already sparked a major controversy. Brazil, as a party to the NPT, has agreed to have its nuclear facilities inspected by the International Atomic Energy Agency, the United Nation's watchdog organization, based in Vienna, Austria. But in 2004, when Brazil and the IAEA began discussing inspections at Resende, they reached a deadlock. Brazil argued that it needed to protect proprietary aspects of its centrifuges; these details apparently included size, shape, materials, and also parts of the control system that are visible. Brazil's proposed solution was to shroud the machines with 2-meter-tall panels. The IAEA had accepted the shielding in the past, for a navy centrifuge pilot plant, but this time the agency insisted it needed full visual access to the Resende installation.

Negotiations dragged on for months, and the impasse prompted an outcry from arms control experts. Some argued that Brazil was undermining the nonproliferation regime by setting a precedent for Iran to demand the same treatment. Others claimed that Brazilian centrifuges were copies of the European design. And yet others speculated about the Resende plant's potential to produce highly enriched uranium—enough for up to six bombs a year, according to one estimate.

Brazilian officials deny these charges. They say that Brazil can't be compared to Iran, known to have violated the NPT, and that there was no illegal transfer of technology. As for Resende's potential to make weapons-grade material, that claim "is baseless speculation," says Odair Gonçalves, president of Brazil's National Commission on Nuclear Energy.

Finally, in October 2004, Brazil and the IAEA agreed on inspection terms. Brazil reduced the size of the masking panels and accepted other procedures to guarantee that no diversion of uranium could take place in the centrifuge hall. Afterward, the IAEA seemed satisfied, and last year, the agency gave its official approval to the plant's first cascade, which is now going into operation.

The episode, however, still puzzles some experts. "The Brazilian case, the only thing that's really strange about it is the secrecy thing—there's no good explanation for it," says physicist Thomas L. Neff, a senior researcher with the Center for International Studies at the Massachusetts Institute of Technology, in Cambridge. "The Brazilians say they have proprietary technology. Well, others have proprietary technology, and they don't seem to think that is a problem. No one else conceals their centrifuges from the IAEA."

Experts like Neff insist that Brazil needs to be more transparent, or suspicions will remain. The problem, they say, is that uranium centrifuges have been tainted by a number of proliferation incidents. In the 1980s, Iraq experimented with centrifuges it acquired from German technicians who had stolen Urenco's designs. Iran, Libya, and North Korea are all believed to have acquired centrifuge technology from a secret smuggling network—exposed in 2004—set up by Abdul Qadeer Khan, the former head of Pakistan's nuclear weapons program, who also obtained classified designs from Urenco.

The sensitivity of enrichment technology became evident during a conference of the treaty signatories this past May to review the NPT. "Without question, improving control over facilities capable of producing weapon-usable material will go a long way toward establishing a better margin of security," said Mohamed ElBaradei, director general of the IAEA, at the opening of the conference in New York City. No such improvements came out of that conference, however. Today, as the standoff with Iran suggests, experts aren't really sure what is the best way—or whether it's even possible—to tighten control over sensitive nuclear technologies, and some are even suggesting that the nonproliferation regime is dangerously eroding.