PUREX process is the most mature solvent extraction separation process in use today. Its principles are described in a great detail in Chapter 6 of this book. PUREX was first implemented in the 1950s, thus its radiation chemistry is the best studied. In general, the PUREX solvent consists of 30 vol % n-tributylphosphate (TBP) in an alkane diluent. In this process, as in its advanced version, UREX (Uranium Extraction), a multistage usage of the solvent is expected. With each use it is exposed to the high radiation field arising from dissolved used nuclear fuel. Considering either the group separation design or the multistep UREX+ process, the PUREX/UREX is the starting process step and hence, TBP obtains the highest radiation dose possible during the processing. The aqueous phase initially contains all fission products, which emit strong beta and gamma radiation, and most of which are much shorter lived than actinides. It has long been recognized that the major products of TBP radiolysis are hydrogen, methane, and dibutylphosphoric acid (HDBP), with monobutylphosphoric acid (H2MBP) and phosphoric acid produced in lesser amounts, but increasing with pro­longed exposure without cleanup. Dibutylphosphoric acid is a major radio­lytic product with ion-exchange extraction properties (Egorov, 1986; Zilberman et al., 2002). The radiation chemistry of TBP was recently reviewed (Mincher et al., 2008; 2009).

Many of the extractants applied in the post-PUREX separation stages of advanced nuclear fuel cycle concepts are functionalized aromatic compounds. Their irradiation in the form of a nitric acid saturated organic solvent and in the presence of aqueous nitric acid may result in aromatic nitration reactions. This may lead to degradation or formation of different bonds and, actually, new coordination chemistry between the metal of inter­est and the ligands. This may have adverse effects on solvent extraction. Dependent on the dose and LET of irradiation, different effects are pos­sible. The degradation of ligand or extracted complexes doesn’t have to be progressive. For extraction of Am3+ with diamidic dipicolinates, it was observed that extraction yields first increased with low radiation doses, then with increased dose significantly dropped (Lapka et al., 2010). Stronger binding also may lead to difficulties in stripping, and in such a way, to lower separation yields than desired.