How Hanford Made Plutonium for the Manhattan Project

Hanford's B Reactor in operation, January 1945. U.S. Department of Energy photo.

The Manhattan Project, the U.S. wartime effort to build an atomic bomb, was so promising and yet so unlikely to succeed that it pursued two separate paths, hoping at least one would produce a weapon that could change the war. One relied on enriched uranium, produced at the Clinton Engineer Works in Oak Ridge, Tennessee. The other relied on plutonium, a recently discovered element that barely exists in nature but can be made in nuclear reactors — the job of the Hanford Engineer Works in southeastern Washington State.

Hanford's process grew out of what Enrico Fermi's team proved in 1942 with the world's first nuclear reactor, a small one in Chicago: in a large enough reactor, the intense flow of neutrons could turn uranium into plutonium.

The site

The Army Corps of Engineers took over about 600 square miles (1,553 square kilometers), including the towns of Hanford, White Bluffs and Richland. The vast, remote site bordered the Columbia River, whose water was essential for cooling the reactors.

Making fuel

Tons of uranium were formed into rods 8.7 inches (22 cm) long and about 1.5 inches (3.81 cm) across, then sealed in aluminum — a step called canning. Tens of thousands of these fuel slugs were made.

Workers stack graphite blocks in the B Reactor. U.S. Department of Energy photo.

The reactor

B Reactor was the first of three plutonium production reactors built at Hanford during the war. Its core was a huge stack of graphite blocks, 36 feet by 36 feet by 28 feet front to back, wrapped in 5 feet of heavy shielding. Running through it front to back were 2,004 aluminum process tubes, loaded with more than 60,000 fuel slugs, with Columbia River water flowing in the narrow gap between slugs and tubes. Fermi supervised the first loading of slugs on September 18, 1944, and the reactor went critical — each fission releasing enough neutrons to keep the reaction going — on September 26, 1944.

The graphite slowed the fast neutrons released when uranium-235 atoms split, so that they could split more U-235 atoms in a controlled, sustained chain reaction. Some neutrons were absorbed by uranium-238, which became uranium-239, then neptunium-239, and finally plutonium-239 — the product Hanford was after.

Separating the plutonium

Every four to six weeks, workers pushed about 10–20% of the now intensely radioactive slugs out the back of the reactor into a water-filled basin to cool, thermally and radiologically, for about two to three months. Shielded, water-filled casks on rail cars then carried them to T Plant, the first chemical separations plant at Hanford, where more than a dozen chemical steps dissolved the aluminum jackets and separated plutonium from uranium and other radioactive byproducts. About 4,000 pounds of uranium yielded 1 pound of plutonium.

T Plant, Hanford's first chemical separations plant. U.S. Department of Energy photo.

The leftover uranium, unwanted radioactive elements and process chemicals became liquid waste stored in underground tanks. During the war the focus was on perfecting plutonium separation, and the waste was left for later. The mix of metals, chemicals and radioactivity has made Hanford's cleanup a serious and very costly task that continues more than seven decades on.

From Hanford to Trinity and Nagasaki

Hanford's plutonium was shipped in secret, in several consignments, to Los Alamos, New Mexico, where scientists, engineers and craft workers built the Gadget, a test of an implosion-design plutonium bomb. Detonated in the Trinity test in New Mexico on July 16, 1945, it was the first human-made nuclear explosion and opened the nuclear age. On August 6 the uranium bomb Little Boy, fueled by Oak Ridge uranium, was dropped on Hiroshima, Japan, the first atomic weapon used in war. On August 9, 1945, the United States dropped Fat Man, fueled with Hanford plutonium, on Nagasaki — the second and, so far, the last atomic bomb used against people.

Sources

• National Park Service, Manhattan Project National Historical Park: "Manhattan Project Science at Hanford," drawing on Richard Rhodes, The Making of the Atomic Bomb (1986), and Henry DeWolf Smyth, Atomic Energy for Military Purposes (1945). The source's metric conversions for the process tubes and for an illustrative comparison do not match their imperial figures and are left out.