Harriman in Roane County, Tennessee — The American South (East South Central)

S-50 Thermal Diffusion Plant

The Longshot in a Three-Way Race to Enrich Uranium

Photographed by Darren Jefferson Clay, February 14, 2026

1. S-50 Thermal Diffusion Plant Marker

Inscription. The S-50 Thermal Diffusion Plant was the third of three competing technologies pursued by the Manhattan Project in an effort to speed up production of enriched uranium for use in Little Boy, the world's first atomic bomb. Manhattan Project administrators first built the Y-12 Electromagnetic Isotope Separation Plant and the K-25 Gaseous Diffusion Plant. S-50 was the last-built and shortest-lived plant. All three plants operated in unison. Uranium would be slightly enriched at S-50 before being enriched further at K-25 and then receive its final enrichment at Y-12. S-50 began enrichment on September 16, 1944, only 69 days after construction began, but at a high cost. It also suffered a higher accident rate than the other plants, possibly due to S-50 beginning operation while most of the plant was still under construction, with both construction and operational crews often working in the same areas. No accidents were fatal.

With the end of World War II, the high cost and inability to compete with the enrichment method of K25, S-50 was put into standby. On September 9, 1945, less than a year after the first

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column came online, S-50 shutdown. The enrichment columns were quickly removed and from May 1, 1946, through December 31, 1951, S-50 housed the Nuclear Energy for the Propulsion of Aircraft Project. S-50 would be demolished when the project moved to Oak Ridge National Laboratory.

[Caption]:
View of building renovation for the S-50 Plant, 1947

Erected by Manhattan Project National Historical Park, National Park Service, Department of the Interior.

Topics. This historical marker is listed in these topic lists: Notable Places • War, World II. A significant historical date for this entry is May 1, 1946.

Location. 35° 55.385′ N, 84° 23.497′ W. Marker is in Harriman, Tennessee, in Roane County. It can be reached from Tennessee 58 north of Perimeter Road, on the right when traveling north. Touch for map. Marker is at or near this postal address: 2013 TN-58, Kingston TN 37763, United States of America. Touch for directions.

Regionally, this marker is in East Tennessee

and in Greater Knoxville.

It is also in the American South,

specifically in the Upper South,

in Appalachia,

and specifically in Southern Appalachia. Globally,

Photographed by Darren Jefferson Clay, February 14, 2026

2. S-50 Thermal Diffusion Plant Marker

it is in North America,

the Western Hemisphere,

the Western World,

and the Anglosphere.

Historically, it finds itself in what was once the original Cherokee Nation,

the territory of the Mississippian Culture,

one of the Confederate States of America,

and the Antebellum South.

Other nearby markers. At least 8 other markers are within 2 miles of this marker, measured as the crow flies: The Wheat Colony (within shouting distance of this marker); Happy Valley (within shouting distance of this marker); K-25 Plant (within shouting distance of this marker); To Have a Job (within shouting distance of this marker); Building K-1000 (approx. 0.3 miles away); Wheat Community (approx. 0.4 miles away); BNFL, Inc. (approx. 0.6 miles away); Poplar Creek Seminary (approx. 1.4 miles away).

Credits. This page was last revised on February 21, 2026. It was originally submitted on February 16, 2026, by Darren Jefferson Clay of Duluth, Georgia. This page has been viewed 49 times since then. Photos: 1, 2. submitted on February 16, 2026, by Darren Jefferson Clay of Duluth, Georgia. • James Hulse was the editor who published this page.