Behaviour of the extractant Me-TODGA upon gamma irradiation: quantification of degradation compounds and individual influences on complexation and extraction
Diglycolamides (DGAs), and in particular N,N,N′,N′-tetraoctyl diglycolamide (TODGA), are well-known candidates for the co-extraction of trivalent actinides (An(III)) and lanthanides (Ln(III)) from highly acidic aqueous solutions of nuclear waste. A derivative of TODGA, the so-called Me-TODGA with the addition of a methyl-substituent on the central part of the TODGA molecule, has been proposed to improve its stability properties and extraction behaviour. This work describes the stability and viability of Me-TODGA by studying the properties of its degradation compounds formed upon gamma irradiation. The main degradation products have been synthesised and studied individually. Particular attention has been paid to their quantification, as well as their complexation and extraction properties, for a better understanding of the degradation pathways and the behaviour of the solvents upon gamma irradiation. The extraction behaviour of irradiated Me-TODGA solvents and their degradation compounds have been studied toward the fission products and lanthanides present in a highly active raffinate (HAR) solution. Binding properties of parent molecules (TODGA and Me-TODGA) and their main degradation compounds with Ln(III) have also been determined in a homogeneous phase. All the results obtained on degradation compounds are compared with those of the parent molecules in order to assess the effects of these compounds on the separation process. Among the radiolytic compounds, 2-hydroxyoctylamides are the most problematic compounds not only because of their high affinity for lanthanides but also for other fission products.