On a desolate stretch of high desert in the northwestern United States, a fortresslike building stands alone, windowless, its massive concrete walls seemingly guarding a secret. But its secret was revealed long ago. What took place here affected the world like no other technology before it.

Building 105-B, better known as B Reactor, was the world's first full-scale nuclear reactor. It produced not electric power but plutonium, an invaluable atomic-bomb ingredient when the reactor first went into operation at the height of World War II. It was B Reactor that produced the plutonium used in the first man-made nuclear explosion, the Trinity test in the desert north of Alamogordo, N.M., on 16 July 1945. It also produced the plutonium used in the bomb detonated over Nagasaki, Japan, on 9 August 1945.

Built in less than a year during World War II, B Reactor was once surrounded by a vast and sprawling industrial compound. Support facilities included a water treatment plant, pump houses, an electric power station, and a number of office and service buildings. Today, only the main reactor building and a 61-meter-tall exhaust stack remain.

The reactor is part of the Hanford Site, a 1500-square-kilometer plutonium-production complex in the state of Washington. It was established by the Manhattan Project, the U.S. government's secret program that produced the world's first nuclear weapons during World War II.

B Reactor was one of three reactors built during the war, and one of nine eventually constructed at Hanford. They all sit along a 50-kilometer-long crook of the Columbia River. Together, they produced 67.4 metric tons of plutonium, or nearly two-thirds of the total created by the United States before the country ended production in the mid-1990s.

Despite its enormous historical significance, however, the reactor - permanently shut down since February 1968 - now faces an uncertain future.

The U.S. Department of Energy has been laboring for years to clean up the radioactive and chemical contamination at Hanford. The site's several decades of operation resulted in the accumulation of about 500 million curies of radioactivity in the form of atomic wastes dumped into the soil and other nuclear products stored at aging facilities. (For comparison, the Hiroshima and Nagasaki bombs produced a total of less than 5 million curies of radioactivity.) Getting rid of all that mess is expected to cost some US $50 billion and last until 2035.

This year the DOE is starting yet another cleanup initiative at the site. The work, distributed over a long stretch of the Columbia River, includes cocooning four of Hanford's reactors, which basically means demolishing all the reactors' structures but their cores and then sealing and roofing them. The B unit is on the list.

Before sending the wreckers in, however, the DOE will wait for the outcome of a study that will assess the possibility of converting some of the Manhattan Project's historic sites into parks and museums. In arguing for the U.S. Senate version of the bill proposing the study, Senator Maria Cantwell (D.-Wash.) called B Reactor "a stunning feat of engineering" and exhorted her colleagues to "preserve the reactor for future generations, which must learn about the Manhattan Project and its impact on world history."

Although the cost of turning the facility into a museum has not been calculated, Bechtel Hanford, the main contractor at the site, estimates that a full decontamination of B Reactor, plus necessary structural repairs, could cost US $30 million. Several rooms have already been cleaned up, but hazards still exist, including lingering radiation, toxic chemicals, asbestos, heavy metals, and also some sneaky bats and snakes. For many years, occasional visitors were allowed into the decontaminated rooms, where no type of protection is required, though for security reasons access was significantly restricted after September 11, 2001.