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

Abstract

In this study, the thermal destruction of two potential extractants — the N, N′-dimethyl, N, N′-dibutyl, tetradecyl malonamide (DMDBTDMA) and the N, N′-dimethyl, N, N′-dibutyl, dodecylethoxy malonamide (DMDBDDEMA) — for minor actinides in nuclear fuel reprocessing was compared. The thermal destruction of 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 degradation mixture, with ethyl acetate, was the first step of analysis before separation by Gas Chromatography (GC). The quantitation of separated by-products was performed with a Flame Ionisation Detector (FID) and the identification was realized by both Fourier Transform Infrared Spectroscopy (FTIR) and Mass Spectrometry (MS). Several by-products are identified and are obtained by cleavages of covalent bond and/or oxidation. After 1 h at 250°C, under oxygen flow in an open reactor, residual levels of DMDBTDMA and DMDBDDEMA pure solutions are 10 and 12% respectively, showing that the behaviour of both diamides seems similar. However, the destruction level of initial molecule do not inform on the global degradation of such complex structures. 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 represent 13% of conversion from initial diamide with conditions described previously. An index of degradation based on the molecular weight of residual products, 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 techniques has shown that carbon dioxide can be produced at temperature around 250°C.