Chemical and mineralogical modifications of simplified radioactive waste calcine during heat treatment

The microstructural and mineralogical changes associated with heating calcined mixtures of Al(NO3)3⋅9H2O–NaNO3 have been studied. This system is a simplified analogue of high-level radioactive waste calcine, one of the raw materials used in the vitrification process employed for waste management. The decomposition (dehydration and denitration) and formation of secondary crystalline phases have been studied by differential thermal and gravimetric analysis (DTA & TGA), and heat-treated products characterized by X-ray diffraction, Raman spectroscopy, Nuclear Magnetic Resonance (NMR) and Transmission Electron Microscopy (TEM). It is found that pure Al(NO3)3⋅9H2O transforms to amorphous Al2O3 at a temperature of ∼180°C, well below that of the calcination process (500°C). This amorphous Al2O3 is highly porous with a high specific surface area, but may in turn convert to denser γ-Al2O3 and α-Al2O3 with increasing temperature. On the other hand, pure NaNO3 remains stable up to ∼880°C, despite a solid–liquid transition at ∼320°C. For Al(NO3)3⋅9H2O–NaNO3 mixtures, the products of calcination at 500°C are found to consist of very fine porous material containing Na, Al and O, in addition to a variable proportion of well-defined crystals consisting of Na, and O. Heating these mixtures to temperatures of up to 1000°C shows that for the case 80% Al(NO3)3⋅9H2O −20% NaNO3 (weight%) a variety of crystalline sodium aluminates is formed (NaAlO2, NaAl11O17, NaAl6O9.5), while for the 50–50 mixture, only NaAlO2 is found. In large amounts, addition of alumina thus leads to the formation of crystalline phases rich in Al2O3 that are responsible for hardening the calcine as the temperature rises. The kinetics of nitrogen loss from NaNO3 are also found to be influenced by the relative proportion of Al(NO3)3⋅9H2O in the calcine, larger amounts of Al leading to denitration at lower temperature. These results constitute the necessary background for understanding chemical reactions between the calcined waste and the glass precursor.