With both the K-25 and Y-12 plants suffering setbacks in the spring of 1944, Oppenheimer urged Groves to approve the construction of a thermal diffusion plant. The U.S. Navy had researched this method for three years and was already building a pilot plant of 100 columns in Philadelphia.  After reviewing Oppenheimer’s suggestions, Groves decided in late June 1944 to approve construction for what would become the S-50 Thermal Diffusion Plant.