A study of the resistance of some N-heterocycles (BTPS) to radiolysis

Full text of publication follows. One option for the selective extraction of trivalent minor actinides within highly active effluents coming from the PUREX process could be as follows: firstly, to co-extract trivalent actinides [An(III)Am(III) and Cm(III)] and also lanthanides [Ln(III)] from the PUREX raffinates; secondly, to separate An(III) from Ln(III) by means of selective extractants. Although efficient extractants for the co-extraction of An(III) and Ln(III) have been developed, selective extractants for An(III) are still required. The various tetra-alkyl substituted BTP molecules, which have been studied within the FP5 European integrated project PARTNEW have been shown to be too sensitive towards acidic hydrolysis and alpha-radiolysis for the development of an An(III)/Ln(III) separation process. Other extracting molecules have been developed within the framework of the FP6 European Integrated Project EUROPART. These molecules include some nitrogen heterocycles. This paper presents some of the basic research carried out in order to improve the resistance of the chosen molecules towards radiolytic degradation. Attempts to improve their extraction properties by the addition of synergists, which usually avoid third-phase formation and improve the kinetics of extraction, will be outlined. It appears that some reagents are not only more selective towards An(III) than tetra-alkyl substituted BTPs, but also more resistant to radiolysis. The carbon atoms on the alpha positions of their triazinyl rings are fully substituted, thus preventing oxidation and nitration of the alkyl groups. Although none of the presently studied ligands has yet met the requirements of a process development strategy, huge improvements have been made in the field of nitrogen heterocycles since the first reference molecule, tetra-n-propyl-bis-triazinyl-pyridine (nPr-BTP), was first chosen in 1998 to demonstrate the scientific feasibility of An(III)/Ln(III) separation from highly active waste of medium acidity ([HNO3] > 0.5 mol/L). Many of these developments have taken place within European research programmes. (authors)