due to the extremely difficult plumbing required to link stages together, the design was developed into a cascade design technique (dubbed helikon), in which 20 separation stages are combined into one module, and all 20 stages share a common pair of axial-flow compressors. a basic requirement for the success of this method is that the axial-flow compressors successfully transmit parallel streams of different isotopic compositions without significant mixing. a typical helikon module consists of a large cylindrical steel vessel housing the 20 separator assemblies, along with two compressors (one mounted on each end), and two water-cooled heat exchangers.