1945

Uranium Enrichment

Imagine the following situation faced by engineers working on the Manhattan Project in 1942. The uranium that comes out of the ground is almost entirely U-238. But mixed in with the U-238 atoms is the occasional U-235 atom (less than 1 percent). The U-235 atoms are what engineers need to build a nuclear bomb. How is it possible to separate the U-235 atoms from the U-238 atoms?

There are lots of processes that engineers use in factories to separate one thing from another. Oil refineries use different boiling and condensation temperatures. Quarries use sieves to separate different sizes of gravel. If salt and sand mix together, water can chemically dissolve the salt to separate it. But separating U-235 from U-238 is difficult because the atoms are nearly identical.

People came up with many different ideas for performing the separation: thermal, magnetic, centrifuge, etc. The method they settled on as the best means of separation at the time is called gaseous diffusion and it involves two steps: Turn solid uranium into a gas called uranium hexafluoride, and let the gas diffuse though hundreds of micro-porous membranes, which have a slight preference for letting U-235 atoms through instead of U-238 atoms.

While this sounds simple, engineering a structure to perform the operation reliably turned out to be a gigantic engineering challenge. The K-25 building—the first full-scale gaseous diffusion plant, at Oak Ridge, Tennessee—came online in 1945, cost $500 million at the time ($8 billion today) and used a noticeable percentage of the nation’s electricity. The building was enormous—one of the largest in the world, with something like fifty enclosed acres holding thousands of diffusion chambers along with their pumps, seals, valves, temperature controls, etc. One of the biggest problems was the highly corrosive nature of uranium hexafluoride. Newly developed materials like Teflon helped block its action.

With an unprecedented level of secrecy and a speed that boggles the mind, engineers built K-25 (and other plants) and brought them online to purify the uranium for the first atomic bombs. After World II, the gaseous diffusion process kept purifying uranium until it was replaced by more efficient centrifuges.

SEE ALSO Wamsutta Oil Refinery (1861), Trinity Nuclear Bomb (1945), Light Water Reactor (1946), CANDU Reactor (1971).

Gas centrifuges used to produce enriched uranium. This photograph is of the US gas centrifuge plant in Piketon, Ohio, from 1984.