Sitting squarely in the middle of the now decommissioned Hanford Site, a nuclear production complex on the Columbia River near Richland, Washington, is B Reactor—the world’s first large-scale nuclear reactor ever built. For more than forty years, B Reactor, along with eight others, pumped out enough plutonium to build over sixty thousand nuclear weapons that comprises the majority of America’s vast nuclear arsenal. B Reactor is one of the few facilities constructed during the secretive Manhattan Project that’s still standing and accessible to the public.

The front face of the reactor core. Image credit: Jeff Keyzer/Flickr

Uranium-235 and plutonium-239 are two of the most widely used fissile material in nuclear power plants and nuclear weapons. But they are not very easy to come by. Uranium-235 is scarce, so in order to build a weapon, whatever fissile uranium is naturally available needs to be enriched in a series of costly and difficult processes.

Plutonium-239, on the other hand, does not even exist in nature. The half-life of plutonium-239 is just 24,000 years, so even if any were present when the Earth formed, it would have long ago decayed to other elements. But plutonium-239 can also be produced. If you take uranium-238 and irradiate it with neutrons, some of the uranium nuclei will absorb neutrons and become uranium-239. But this nucleus has too many neutrons to be stable, and will rapidly decay into neptunium (the next element in the Periodic Table), which decays further after a couple of hours into plutonium-239. So all you have to do is take natural uranium, which is abundant on earth, bombard it with neutrons, wait a while, and you’ll end up with plutonium.

Plutonium-239 has another advantage over uranium. It has the smallest critical mass among all fissile materials—only 11 kg (as opposed to 56 kg for uranium-235). This allows engineers to build compact nuclear weapons that can be fitted into the nose of a missile. Plutonium is therefore the fuel of choice in many nuclear power reactors and nuclear weapons.

Building of B Reactor. Image credit: Frank Fujimoto/Flickr

The plan was to build three nuclear piles (or reactors) at the Hanford site about six miles apart on the south bank of the Columbia River. They were to be designated as Piles B, D and F. The site selected was remote, but not barren. There were many farming towns and small settlements along the river, and these were evacuated. Many native American tribes were also displaced.

B Reactor went critical in September 1944, followed by D Reactor in December 1944 and the F Reactor in February 1945. Their designs were identical—the core was a 36-feet by 28-feet graphite box penetrated horizontally by more than two thousand aluminum tubes containing nuclear fuel, in this case uranium, and vertically by control rods. This was surrounded by a shield of cast iron and concrete. Water pumped from the Columbia River was used to cool the reactor, while the discharged water was returned back to the river after letting them lose their radioactivity in the settling basins.

B reactor had an initial thermal power output of 250 megawatt, and was designed to produce sufficient plutonium for a nuclear weapons arsenal by itself, but duplicate D and F Reactors were also authorized to provide additional production capacity. Three months later, the first batch of plutonium was processed and transferred to Los Alamos where it was used in the Trinity Test. In the months that followed, the B, D, and F Reactors produced the plutonium that was used in the “Fat Man” bomb dropped in Nagasaki. Throughout this period, only a handful of people employed at Hanford knew they were working on a nuclear weapons project. The dropping of the bomb in Hiroshima was as much a shock to Hanford's workers as to any other American (or any living soul in the world).

Control panel of B Reactor. Image credit: Jeff Keyzer/Flickr

After World War 2, six more reactors were built at Hanford to produce materials for America’s nuclear weapons program. These reactors operated for over 40 years, producing 63 short tons of plutonium in their lifetime, which accounts for the majority of fissile material used by the 60,000 weapons in the US arsenal.

B Reactor operated for more than 20 years, finally shutting down in 1968. The final reactor was shutdown in 1987. Following the shutdown of the Hanford site, the US government spent over $113 billion hauling away radioactive sludge and cleaning up the site. The process is expected to continue till 2046, costing the government approximately $2 billion per year until its done.

As part of the clean-up operation, all the reactors, with the exception of B Reactor, were entombed in concrete to keep radiation in. B Reactor was turned into a museum. The reactor core, consisting of the irradiated graphite block, is still in place and is one of the highlights of the museum tour. The towering grid with over two thousand protruding fuel rods (without the fuels) is a humbling sight. The fuel rods are 13 meters long, but only their tips are visible. The control room bristles with hundreds of switches, knobs, mechanical gauges and dials. Throughout the building are many original artifacts from rotary phones and typewriters to rubber masks and gloves. Outside the reactor building are the railroad cars used to transport the used fuel to the chemical plants where the plutonium was separated. The cars can be touched and climbed upon.

B Reactor is now part of the Manhattan Project National Historical Park that commemorates the Manhattan Project. It is also a National Historic Landmark.

Image credit: Gary Paulson/Flickr

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Image credit: Kelly Michals/Flickr

Image credit: Kelly Michals/Flickr

The front face of the reactor core. Image credit: Frank Fujimoto/Flickr