Partial descriptions of the type of equipment used in the gaseous diffusion plants of the U. S. DOE are given in references [Ul] and [Ш]. Figure 14.1 is a schematic plan view of three gaseous diffusion stages. The separating unit on each stage, called a converter, contains

thousands of tubes of diffusion barrier supported by tube sheets at each end. As UF6 gas at the highest process pressure flows along the inside of these tubes, about one-half of it effuses through the tubes into the region at the lowest process pressure outside of the tubes and is thereby slightly enriched in MSUF6. This low-pressure, slightly enriched gas, the stage heads stream, is compressed to an intermediate pressure in the first stage of a horizontally mounted, two-stage, axial-flow compressor of the next higher stage of the cascade. Here it is joined by an equal amount of UF6 at the same pressure and 23SU content representing the tails stream from the second higher stage of the cascade. The combined streams are compressed by the second stage of the compressor to the highest process pressure. The compressed gas flows through a cooler, where the heat of compression is removed by heat exchange against coolant C2F4C12, chosen because it will not react with UF6 should a leak occur. The compressed and cooled gas then flows through the tubes of the converter on the next higher stage of the cascade.

The tails stream from each converter, the gas that has not effused through the holes in the barrier tubes, flows through a control valve and into the intermediate pressure inlet of the compressor on the next lower stage of the cascade. The valve position is adjusted so as to control the pressure level of the converter upstream at the desired level.

In some stages of the U. S. plants the flow sequence is modified with the stage cooler inserted between the converter outlet for the heads stream and the compressor inlet. This permits the converter to operate at the compressor outlet temperature rather than the lower inlet temperature, and improves somewhat the separation performance of the barrier.

Figure 14.2 is a photograph of the process equipment used in the largest stages of the U. S.

Figure 14.3 Eurodif gaseous diffusion stage.

diffusion plants. The large drums in the foreground are the converters, each of which contains a cooler and thousands of barrier tubes. The two-stage axial-flow compressor that recompresses the UF6 that has passed through the barrier and circulates the undiffused gas is at the back of the figure. From 8 to 16 stages such as these are grouped into cells, housed in steel enclosures heated to around 60°C to prevent condensation of UF6. Each cell is the smallest independently operable unit, and is equipped with block and bypass valves to permit shutdown for maintenance.

As Fig. 12.2 showed, about 1270 stages are needed to separate natural uranium into product containing 3 w/o HSU and tails containing 0.2 w/o. The Portsmouth plant of U. S. DOE, which produces uranium enriched to 97 percent 235U, contains 4080 stages.

The large plants of U. S. DOE have operated for 20 years at a capacity factor over 99 percent and attest to the reliability of the gaseous diffusion process.