Thermal oxidation of two malonamides, extractants for minor actinides in nuclear fuel reprocessing

In this study, the thermal destruction of two potential extractants - the N ,N'-dimethyl, N,N'-dibutyl, tetradecyl malonamide (DMDBTDMA) and the N,N'-d imethyl, N,N'-dibutyl, dodecylethoxy malonamide (DMDBDDEMA) - for minor act inides in nuclear fuel reprocessing was compared. The thermal destruction o f a main by-product, the larger monoamide formed, was also studied for each extractant. Experiments were carried out in small reactors (closed or open ) with different oxidising atmospheres. The recuperation of the syrupy degr adation mixture, with ethyl acetate, was the first step of analysis before separation by Gas Chromatography (GC). The quantitation of separated by-pro ducts was performed with a Flame Ionisation Detector (FID) and the identifi cation was realized by both Fourier Transform Infrared Spectroscopy (FTIR) and Mass Spectrometry (MS). Several by-products are identified and are obta ined by cleavages of covalent bond and/or oxidation. After 1 h at 250 degre esC, under oxygen flow in an open reactor, residual levels of DMDBTDMA and DMDBDDEMA pure solutions are 10 and 12% respectively, showings that the beh aviour of both diamides seems similar. However, the destruction level of in itial molecule do not inform on the global degradation of such complex stru ctures. Initial diamides can lose their methyl and/or butyl groups and then give another malonamides. Its can also lead to monoamide by cleavage of C- CO malonamide bond. One of these monoamide, the major by-product, can repre sent 13% of conversion from initial diamide with conditions described previ ously. An index of degradation based on the molecular weight of residual pr oducts, was calculated so as to compare the efficiency of thermal oxidation . Behaviour of diamides was also followed by thermal differential analysis (TDA) and thermal gravimetric analysis (TGA) coupled with FTIR. These techn iques has shown that carbon dioxide can be produced at temperature around 2 50 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.