Today, uranium isotope separation is usually done by the more energy-efficient ultra centrifuge process, developed in the Soviet Union after World War II by Soviet and captured German engineers working in detention. The centrifuge process was the first isotope separation method considered for the Manhattan project, but was abandoned due to technical challenges early in the project. When German scientists and engineers were released from Soviet captivity in the mid-1950s the West became aware of the ultra centrifuge design and began shifting uranium enrichment to this much more efficient process. As centrifuge technology advanced, it became possible to carry out uranium enrichment on a smaller scale without the vast resources that were necessary to build and operate 1940s and '50s "K" and "Y" style separation plants, a development which had the effect of increasing nuclear proliferation concerns.