It is a well-known fact that the organic solvent (both TBP and albeit, to a lesser extent, the diluent) degrades due to the effects of radiolysis, and hydrolysis. These degradation products must be removed prior to solvent recycle, lest they build-up in the organic phase with a concomitant decrease in process performance. Solvent cleanup and treatment, both physical and chemical are an important aspect of PUREX processing. These treatments can include scrubbing or washing the solvent with basic (carbonate and/or hydroxide) solutions to remove acidic degradation products. Carbonate solutions have the ability to form soluble carbonate complexes with residual metal cations in the organic phase, thus precluding any risk of metal cation precipitation during solvent wash. The organic is typically subjected to a subsequent acid rinse operation with dilute HNO3 to remove traces of the basic solution from the organic and slightly re-acidify the organic prior to recycle to the extraction operations. Finally, complementary solvent treat­ment operations involving evaporation and subsequent rectification opera­tions allows recovery of purified TBP solution and diluent. Evaporation makes it possible to remove the heavier, stubborn degradation products such as polymers, while the rectification operation serves to remove the lighter degradation products. These operations are performed on a continu­ous basis with a slip stream on a fraction of the total solvent inventory at the La Hague and Rokkasho plants. The evaporation and rectification cycles are not always performed in continuous mode, but can also be batch type operations preformed on a small fraction of the total solvent inventory. The solvent cleanup operations are carried out as separate unit operations from the various extraction, scrub, and strip cycles.

Liquid-liquid extraction (solvent extraction) is intimately coupled to the PUREX process and has been the workhorse of the nuclear industry for 50+ years. Liquid-liquid extraction plays on the unequal distribution of the components between two immiscible liquid phases, and is therefore highly dependent on the chemistry of those species as well as the extractant molecule(s). Much of this information has been described in the previous discussions for the PUREX process. However, it is important to realize the extension of chemical phenomena is promulgated in the equipment design and operational logistics of a large plant at the industrial scale. While limited mass transfer can be completed in a single, batch equilibrium contact of the two phases, one of the primary advantages of liquid-liquid extraction processes is the ability to operate in a continuous, multistage, countercur­rent flow mode. This allows for very high degree of separation while operat­ing at high processing rates. The aqueous and organic steams flow countercurrently from stage to stage, and the final products are the solvent loaded with the solute(s), and the aqueous raffinate, depleted in solute(s). In this manner, the concentration gradient between the phases remains quite high across all of the stages in the system. This concentration gradient is the motive force for mass transfer and provides the basic phenomena upon which countercurrent solvent extraction is based.

While countercurrent processes could be performed in laboratory glass­ware, their primary advantage is to enable continuous processing at high throughputs. In order to achieve continuous processing, specific equipment is needed that can efficiently mix and separate the two phases in a continu­ous operating mode. In the nuclear industry, specific constraints, such as remote operation and maintenance must be considered, since the solutions processed are highly radioactive. There are three basic types of equipment used in industrial-scale nuclear solvent extraction processes: mixer-settlers, columns, and centrifugal contactors. The basic design and operation of this equipment is well described in the literature (Benedict 1981, Long 1967). It is only noted here that the selection of the type of equipment to be used in large-scale reprocessing hinges on a number of different process param­eters and design considerations, including (but not limited to):

• process foot print and building size/height

• operational flexibility (long-term continuous operation or frequent start/stop operation)

• solvent inventory and in-process volume holdup

• degradation of solvents due to radiolysis/hydrolysis (residence time)

• time required to reach steady-state operation

• potential to operate linked, complex multi-cycle processes

• tolerance to cross-phase entrainment

• tolerance to solids in process solutions

• tolerance to process upsets

• mass transfer kinetics

• process chemistry (e. g. kinetics of valance adjustment)

• remote maintenance capabilities

• criticality constraints.

The equipment type chosen for a particular process application should be based on several factors as indicated in the list above. In-depth reviews and comparisons of pulse columns, mixer-settlers, and centrifugal contactors have culminated with a recent rating of the different equipment designs relative to the criteria aforementioned. Results of a recent review per­formed as part of the United States Department of Energy’s Plutonium Technical Exchange Committee is indicated in Table 6.3 (Todd 1998).

For comparison, the equipment currently used in the world’s major reprocessing facilities is summarized in Table 6.4. Note that the use of pulsed columns and mixer-settlers is far more common than for centrifugal contactors. That trend may change in future applications as additional capacity is brought on-line and the next generation reprocessing plants are designed and built.