ORNL-5388 - the Molten Salt Energy Technologies Web Site

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To take advantage of the small separation factor discussed above, diffusive flow
must be ensured, not just simple gas flow. Diffusive flow requires not only small pores,
i.e., less than two-millionths of an inch in diameter, but also uniformity of pore size.
Because of the small pore size, literally acres of barrier surface are required in a large
production plant.
Complexity of plant design is increased by the difficulties arising from the
nature of the diffusing gas itself. A volatile compound of uranium must be used, and
the hexafluoride (UF6) is the only known suitable compound. It is a solid at room
temperature; consequently, the diffusion plants must be operated at temperatures and
pressures necessary to maintain the UF6 in gaseous form. Although it is a stable com-
pound, UF6 is extremely reactive with water, very corrosive to most comnon metals, and
not compatible with organics such as lubricating oils. This chemical activity dictates
the use of metals such as nickel and aluminum and means that the entire cascade must
be leak-tight and clean. The corrosiveness of the process gas also imposes added diffi-
culties in the fabrication of a barrier which must maintain its separative quality Over
long periods of time.
The enrichment stage is the basic unit of the gaseous diffusion process. In all
stages gas is introduced as UF6 and made to flow along the inside of the barrier tube.
In the standard case about one-half the gas diffuses through the barrier and is fed to
the next higher stage; the remaining undiffused portion is recycled to the next lower
stage. The diffused stream is slightly enriched with respect to 235U, and the stream
which has not been diffused is depleted to the same degree.
' The basic equipment components vital to the process are the axial flow compressors,
the converter shell and the barrier tubes. Axial flow compressors are used to compress
the UF6 gas to maintain the interstage flow, and electric motors are used to drive the
compressors.
A gas cooler is provided in the converter since gas compression unavoidably
generates heat which must be removed a t each stage. The diffuser, or converter, is the
large cylindrical vessel which contains the barrier material. It is arranged in such a
fashion that the diffused stream and the stream that has not diffused are kept separate.
Groups of stages are coupled to make up operating units and such groups, in turn,
make up the cascade.
and Russia.
Gaseous diffusion plants are in operation in the United States, England, France,
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