Once the product and waste streams, and feedstocks are defined, the next step is to determine the separations modules needed to accomplish this goal. Options are not limited to solvent extraction; hybrid processes may include ion exchange, precipitation, electrochemistry and others. The overall process is composed of a sequence of separation steps or modules linked to generate a desired set of outputs, whether products or intermediates. Although minimizing the number of steps is always desirable, process reli­ability must take precedence. Caution is needed in assessing the compatibil­ity of different steps. Often the adjustments to intermediate process streams are as critical to the success of the overall process as are the actual separa­tions themselves.

The selection of separations steps requires understanding the fundamen­tal chemistry and engineering involved. A process that looks attractive chemically may prove untenable industrially because of unforeseen chemi­cal interactions, poor process behavior, or instability under process condi­tions (temperature, pH, radiation, etc.). The selection is first done by choosing a candidate chemical system that is known to accomplish the desired separation. Design of a detailed process flowsheet that will yield the desired goal then follows. Optimization of the process flowsheet requires a sound understanding of the interactions between different process vari­ables. Such an understanding can best be gained through high fidelity models of both the chemistry and the unit operations.

Laboratory data are used to develop or refine computer models to simu­late the process. Bench-scale process flowsheets are then designed from these simulations and tested, first with simulants then, if practical, with actual spent fuel or radioactive waste. Quite often the genuine feed per­forms differently than simulants, even when the feed is relatively simple in composition. It is critical that a flowsheet be developed from small-scale tests, preferably on prototypical equipment and under prototypical condi­tions to provide data on process kinetics, hydraulics, parameter selection, and any other effects arising from process variability. The use of prototypi­cal equipment is critical as it reduces the number of variables that must be accounted for during scale-up, thus reducing process uncertainty.

Although the heart of a separations facility is the chemical processing (solvent extraction, ion exchange, etc.), precisely recovering the targeted components and stabilizing the separated constituents is not a trivial opera­tion. In fact, the footprint of the post-separations processing is often larger than that required for the separations themselves. In nuclear systems, the footprint is a major driver of the process economics because of the shielding requirements. As a result, great effort must be made to select additives
and complexants that minimize the potential to adversely affect final stabilization.