Ridge in 1945, all of the calutrons were shut down. The gaseous diffusion technique then became the preferred technique for producing enriched uranium. At the time of their construction in the early 1940s, the gaseous diffusion plants were some of the largest buildings ever constructed. Large gaseous diffusion plants were constructed by the United States, the Soviet Union (including a plant that is now in Kazakhstan), the United Kingdom, France, and China. Most of these have now closed or are expected to close, unable to compete economically with newer enrichment techniques. However some of the technology used in pumps and membranes still remains top secret, and some of the materials that were used remain subject to export controls, as a part of the continuing effort to control nuclear proliferation. Current status In 2008, gaseous diffusion plants in the United States and France still generated 33% of the world's enriched uranium. However, the French plant definitively closed in May 2012, and the Paducah Gaseous Diffusion Plant in Kentucky operated by the United States Enrichment Corporation (USEC) (the last fully functioning uranium enrichment facility in the United States to employ the gaseous diffusion process) ceased enrichment in 2013. The only other such facility in the United States, the Portsmouth Gaseous Diffusion Plant in Ohio, ceased enrichment activities in 2001. Since 2010, the Ohio site is now used mainly by AREVA, a French conglomerate, for the conversion of depleted UF6 to uranium oxide. As existing gaseous diffusion plants became obsolete, they were replaced by second generation gas centrifuge technology, which requires far less electric power to produce equivalent amounts of separated uranium. AREVA replaced its Georges Besse gaseous diffusion plant with the Georges Besse II centrifuge plant. See also Capenhurst Fick's laws of diffusion K-25 Lanzhou Marcoule Molecular diffusion Nuclear fuel cycle Thomas Graham (chemist) Tomsk References External links Annotated references on gaseous diffusion from the Alsos Library Isotope separation Uranium Membrane technology