Diamex process for minor actinide partitioning: Hydrolytic and radiolytic degradations of malonamide extractants

This paper deals with the hydrolysis and radiolysis of malonamides, which are used as extractants in the DIAMEX process for partitioning minor actinides from high-level liquid radioactive wastes. The general formula of the malonamides included in this study is as follows: (R(R')NCO)(2)CHR" (where R, R', and R" are alkyl or oxyalkyl groups). Qualitative tests were carried out to identify malonamide degradation products using gas chromatography (GC) coupled to Fourier transform infrared spectroscopy (FTIR) or mass spectrometry (MS). The main degradation products were identified in organic phases after radiolysis or hydrolysis in the presence of the nitric acid aqueous phase and quantified using potentiometric titrations and gas chromatography. From the experimental data, a simplified degradation scheme for malonamide radiolysis and hydrolysis is proposed. The extracting properties of solvents made of malonamide solutions in an aliphatic diluent vs. actinide(III) and lanthanide(III) nitrates were determined after radiolysis and hydrolysis. The presence of malonamide degradation products leads to a decrease of the M(III) distribution ratios. Experiments carried out with synthetic organic solutions show that the decrease in solvent extraction properties can be explained by the decrease of the malonamide concentration and by the presence of the main malonamide degradation products. The hydrolytic and radiolytic stabilities of several malonamides are compared.