With the above brief explanation of the underlying chemistry, the explana­tion of the codecontamination cycle is more fruitful. The reader’s attention is re-directed to the decontamination cycle in Fig. 6.2 for the ensuing discussion.

The dissolved fuel solution enters the main extraction contactor at mod­erate nitric acid concentration of ~3 M HNO3. As the aqueous flows coun­ter-currently to the organic stream, U and Pu are extracted virtually quantitatively into the organic phase. Under these conditions, Zr, Ru, and Tc are also extracted to varying degrees in accord with the above descrip­tions of their chemistries. Also noteworthy is that the conditions of the dissolved solution favor the formation of neptunium in the higher, i. e., Np(V) and Np(VI), oxidation states; the former is inextractable while the

latter is very extractable by TBP into the organic phase. The proportion — ation between these Np oxidation states is a complex function of the exact NO3-/NO2- concentrations and redox kinetics, which can even be influenced by the type and design of the process equipment. It has been reported that ~25% of the Np is rejected, as Np(V), to the raffinate (waste) in the La Hague plant (Dinh 2008); however, the point is that some fraction of the Np in the dissolved feed is extracted into the organic phase and subse­quently carries through into the downstream processes.