The outgoing aqueous scrub phase, containing a significant fraction (but not necessarily all) of the TcO4-, then undergoes a “complementary” extraction with fresh organic phase to recover plutonium and uranium inadvertently back-extracted with the Tc. The solvent from this complementary extraction operation is recycled to the main extraction section to maintain a high concentration of U and Pu in the outgoing solvent of the codecontamina­tion step and to avoid undesired losses of uranium and plutonium to process wastes. Obviously, the aqueous product from the Tc scrubbing operation cannot be re-combined directly with the previous scrub or feed streams since the higher concentrations of HNO3 would exacerbate the extraction of Zr in the main extraction section and negate the benefits afforded by the dual scrub approach. Consequently, the aqueous raffinate from the comple­mentary scrub operation is sent directly to the waste in combination with the raffinate from the main extraction (Baron 1993, 2003).

In passing, it should be noted that the aqueous streams from the main extraction and Tc scrubbing and complementary extraction operations are subjected to a diluent scrub. The purpose being to wash traces of TBP, lost from the organic via solubility, to the aqueous phase. Such diluent washing has become a standard feature of the PUREX process for aqueous streams that are concentrated in evaporators, and is a standard safety feature in modern reprocessing facilities. The diluent used in these washing steps is recycled to the process to minimize TBP losses. Diluent washing is noted at multiple points in Fig. 6.2 and will not be further discussed.

The diversity of the impurities targeted in the scrub sections of codecon­tamination often involves a compromise, that, when reduced to practice, leads to the use of several different scrub solutions. Other important design requirements of the first cycle include minimization of U and Pu losses to the liquid waste emanating from the cycle, thereby restricting these losses to very low levels (typically <0.1%). Finally, the rather high concentrations of U, typically approaching 300 g/L in the incoming aqueous feed, allows favorable flow ratios of organic and aqueous to be achieved, as the organic can be heavily loaded with uranium. The flowrate of the organic solvent is slightly in excess of that required for the quantity of U and Pu to be extracted; however, a large excess of organic must be avoided in order to restrict excessive extraction of impurities.

In general, there is a high degree of separation of actinides and fission products after the codecontamination step. Typical decontamination factors reported from UP-3 at La Hague are Ru DF ~104, Cs DF > 107, and a Tc DF > 3 was initially targeted (Baron 1993), but is now typically DFTc~10 or higher (Baron 2003).