A layer of melting snow blankets Hanford's rocky soil and its sparse sagebrush and cheatgrass covering. In the vicinity of B Reactor, the only signs of life on this winter day are a few maintenance workers driving by in pickup trucks and a lonely coyote wandering near the road. It's hard to picture the place as it was 60 years ago, when tens of thousands of workers toiled in scattered facilities all over Hanford, which almost overnight became the third most populous region in Washington State.

As you step through B Reactor's main entrance, pale-green double doors are visible straight ahead, at the end of a short hallway. It's behind those doors that B Reactor's atomic heart resides. In a vast, high-ceilinged hangarlike room, the enormous core looms 12 meters tall, its somber facade covered by protruding metal nozzles.

The core consists of an inner structure [see photos], called the pile, enclosed by thick shielding layers of iron, steel, and Masonite. The pile is a 2000-ton cubical block of pure graphite that 2004 aluminum tubes traverse horizontally. Workers manually filled those tubes with tens of thousands of aluminum-clad uranium cylinders called slugs, each about the size of a large sausage. When enough slugs were in place, they would form a "critical mass," which would initiate the uranium's transformation into plutonium.

Two nuclear reactions took place simultaneously. In one, nuclei of uranium-235, one of the isotopes present in the slugs' natural uranium, started to fission and emit neutrons. These fast neutrons, slowed down by the surrounding graphite, would then hit and split other uranium-235 nuclei in nearby tubes, thereby generating more neutrons, which in turn would split other nuclei, and so on. This fission chain reaction deluged the pile with neutrons. In the other reaction, another isotope in the slugs, uranium-238, would absorb some of the fast neutrons and transmute into plutonium.

The fission reaction released enormous quantities of energy. The reactor, originally rated at a thermal power level of 250 megawatts, would simply have melted down if it weren't for a torrent of Columbia River water directed through its tubes. Located nearby, a water plant large enough to serve a city of 300 000 people pumped 114 000 liters of cooling water through the reactor's seething core every minute. The effluent water would stay in a retention basin for 3 or 4 hours and then flow back into the Columbia River.

Operators adjusted the reactor's power level from a control room separated from the core by a 1-meter-thick concrete wall [see photos]. From there, they could regulate the chain reaction by inserting or retracting one or more of nine motor-driven neutron-absorbing horizontal control rods, which were interspersed perpendicularly among the uranium-filled tubes. In addition, 29 vertical safety rods, suspended by electromagnets, would drop into the core to shut it down immediately if something went awry.

At the control room, operators also kept an eye on a number of gauges and recorders that let them monitor such parameters as the temperature of the pile's shielding and the water pressure of the core's tubes. To see it all today is to drop in on another era - one of dials and knobs instead of liquid-crystal displays and keyboards.