K-25 Plant

The main building was a four-story U about half a mile long, and its floor space surpassed the Pentagon's. Inside, uranium hexafluoride gas was pushed through barriers fine enough to separate the lighter uranium-235 slightly from uranium-238; the corrosive gas forced a nickel barrier, electroplated so thinly it used a thousandth of the metal a solid one would. Construction peaked at more than 25,000 workers in May 1945, many housed in the camp called Happy Valley. Gaseous diffusion ended in August 1985, and demolition finished in March 2014.

What the plaque says

K-25 Plant As a part of the Manhattan Project, the K-25 plant was designed to house work on separating U-235 from U-238 through the gaseous diffusion process. At the time of its construction, it was the largest industrial complex in history. Plant construction began in 1943 and was completed in 1945. Over 25,000 construction personnel worked on this plant. The main building exceeded 44 acres in size.

Where it stands

Worth the stop nearby

• Fort Southwest Point State Historic Park — 8.8 mi
  A reconstructed 1797 federal fort on a knoll at the meeting of the Clinch and Tennessee rivers, headquarters of Cherokee agent Return J. Meigs, a state park since July 2026.
• American Museum of Science and Energy — 9.9 mi
  The Oak Ridge museum that opened in 1949, when the Manhattan Project city's gates opened, in a town whose Black workers were housed in the Gamble Valley hutments.
• Historic Brushy Mountain State Penitentiary — 13 mi
  The coal-mining prison opened in 1896 after the Coal Creek War ended convict leasing, where James Earl Ray escaped in 1977, now a tourist site and distillery.
• Frozen Head State Park — 15 mi
  A 24,000-acre Cumberland Mountains park around a 3,324-foot peak, made from Brushy Mountain prison lands and the CCC, and home to the Barkley Marathons.

More markers nearby

• Sam Rayburn — 3.8 mi
• X-10 Plant — 4.6 mi
• Oak Ridge — 5.1 mi
• Honoring the Kingston Coal Ash Cleanup Workers — 6.7 mi