This allows for the separation and collection of the isotopes in receivers. Because EMIS is a batch process, with each unit requiring a long time to produce small amounts of U-235, many units were used to produce the first fissionable uranium. The rest is scattered through the calutron and is difficult to recover. At Oak Ridge, two groups of calutrons were used to produce weapons-grade uranium. As difficulties with the gaseous diffusion were overcome, the EMIS process was phased out.
Thermal diffusion uses heat transfer across a thin layer of liquid or gas to separate isotopes.
Cooling a vertical film on one side and heating it on the other produces convection currents, an upward flow on the hot surface and a downward flow along the cooler side. Under these conditions, lighter 235 UF 6 molecules will diffuse toward the warmer surface and heavier 238 UF 6 toward the cooler side. This was a simple, relatively low-cost process, but it consumed much more energy than the gaseous diffusion. A plant containing 2,100 columns, each about 15 meters long, was operated at Oak Ridge to provide the initial enrichment of uranium for the gaseous diffusion and EMIS units. It was closed after about a year of operation.