My footsteps echo across the empty corridor, and fluorescent lights flicker above my head. I'm in a compound of boxy concrete buildings at the Resende nuclear complex in southeastern Brazil. Ahead of me, behind locked doors, is a vast, high-ceilinged hall. I can't go in there, my escort tells me, but I shouldn't take it personally. Even among the employees here, very few have access to that hall, and those who do can't talk about what's in there. Inside is the newest example of one of the most heavily guarded technologies of the industrial age.

This is where Brazil will soon produce enriched uranium in industrial quantities to fuel its two nuclear power reactors. That hall—this much is known—houses hundreds of identical machines. They are slender, upright cylinders topped by a maze of thin pipes, and they sprawl around the room like a thick forest of metal. The machines are uranium centrifuges. They spin at supersonic speeds to accomplish a neat trick: separating two types of uranium atom that are virtually identical except for a minuscule difference in weight. This separation process, known as enrichment, isolates the uranium atoms that are fissile—the useful material capable of sustaining a nuclear chain reaction—from the rest.

All over the world, uranium centrifuges and other enrichment technologies are treated as state secrets and subject to stringent export controls. That's because the same equipment used to enrich uranium into reactor fuel can, with only minor modifications, also enrich it to a far higher level to serve as bomb-grade material. So while enrichment technology provides the lifeblood of the nuclear power industry, it can also be instrumental to the production of nuclear weapons.

Brazil's achievement comes at a time when concern is running high over another enrichment program, in Iran. Both countries are parties to the Nuclear Non-Proliferation Treaty (NPT)—the foundation of the international regime that seeks to limit the spread of nuclear weapons—but Brazil's program is notable for its differences from Iran's: Brazil has consistently fulfilled its obligations under the NPT, and the country has forsworn nuclear weapons ambition since a democratic government replaced the military dictatorship that ruled the country from 1964 to 1985.

With its new Resende plant, Brazil is joining the exclusive club of nations that operate commercial-scale centrifuge facilities. These include Germany, the Netherlands, and the United Kingdom— these three forming the Urenco Ltd. consortium—plus Russia, China, and Japan. The United States and France enrich uranium through a different process called gaseous diffusion, although both countries plan to build centrifuge plants. [See table, "Who Does Enrichment?"]