speed to economy and efficiency. The cascades were configurable, so they could produce a large amount of slightly enriched product by running them in parallel, or a small amount of highly enriched product through running them in series. By early 1946, with K-27 in operation, the facility was producing per day, enriched to 30 percent. The next step was to increase the enrichment further to 60 percent. This was achieved on 20 July 1946. This presented a problem, because Y-12 was not equipped to handle feed that was so highly enriched, but the Los Alamos Laboratory required 95 percent. For a time, product was mixed with feed to reduce the enrichment to 30 percent. Taking the concentration up to 95 percent raised safety concerns, as there was the risk of a criticality accident. After some deliberation, with opinions sought and obtained from Percival Keith, Norris Bradbury, Darol Froman, Elmer E. Kirkpatrick, Kenneth Nichols and Edward Teller, it was decided that this could be done safely if appropriate precautions were taken. On 28 November 1946, the K-25 plant began producing 94 percent product. At this point, they ran into a serious flaw in the gaseous diffusion concept: enrichment in uranium-235 also enriched the product in the unwanted and fairly useless uranium-234, making it difficult to raise the enrichment to 95 percent. On 6December 1946, production was dropped back to a steady per day enriched to 93.7 percent uranium-235, along with 1.9 percent uranium-234. This was regarded as a satisfactory product by the Los Alamos Laboratory, so on 26 December 1946 enrichment activity at Y-12 was curtailed. The Manhattan Project ended a few days later. Responsibility for the K-25 facility then passed to the new Atomic Energy Commission on 1January 1947. Workers at the plant were represented by the Oil, Chemical and Atomic Workers International Union (OCAW). Closure and demolition K-25 became a prototype for other gaseous diffusion facilities established in the early post-war years. The first of these was the K-27 completed in September 1945. It was followed by the K-29 in 1951, the K-31 in 1951 and the K-33 in 1954. Further gaseous diffusion facilities were built at Paducah, Kentucky, in 1952, and Portsmouth, Ohio, in 1954. The K-25 plant was renamed the Oak Ridge Gaseous Diffusion Plant in 1955. Today, uranium isotope separation is usually done by the more energy-efficient ultra centrifuge process, developed in the Soviet Union after