The top priority of the secret wartime Oak Ridge project was Y-12, the code name for the process considered the best bet for separating weapon-grade uranium-235 (U-235) from U-238. This isotope separation process was the brainchild of Ernest O. Lawrence and his team at the University of California, and the complex machines used there were called calutrons, for California University cyclotrons. Stone & Webster Engineering of Boston served as architect-engineer for this first-of-its-kind plant, whose wartime cost was $478 million. Construction began in February 1943 and required more than 10,000 construction workers and 14,700 tons of silver, valued at $300 million. Silver ingots were borrowed from U.S. Treasury vaults to make thousands of calutron electromagnets because copper was urgently needed for other war uses. Tennessee Eastman Corp. of Kingsport operated the plant, with Frederick Conklin as plant manager. Although Ph. D. physicists were initially thought necessary to run the complex calutrons, Eastman found that local women could be trained to outperform the scientists. Chemical recovery for the calutron project required tremendous effort. In May 1945 Y-12 had 22,400 employees. Despite formidable technical problems in this brand new process, Y-12 succeeded in its wartime mission of producing the vital U-235 that enabled Los Alamos to build the first atomic bomb and helped bring World War II to an end. By the close of 1946, K-25 had taken over the U-235 production task and about 20,000 Y-12 employees had left. In 1947 Y-12 undertook a new and equally vital nuclear defense mission and successfully managed it for the next half century.