Ageing of EUROBITUM bituminised radioactive waste: An ATR-FTIR spectroscopy study

The extent of the physico-chemical processes of concern in the study of the acceptability of Eurobitum bituminised radioactive waste for underground disposal (water uptake by hygroscopic NaNO3 – swelling – pressure build-up – NaNO3 leaching) will depend on the degree of ageing of the bituminous matrix. In the work reported here, the ageing behaviour was studied by comparing the characteristics of 25 years old radioactive Eurobitum with those of 25 years old non-radioactive Eurobitum samples that were heated or gamma-irradiated in the presence or absence of oxygen. Chemical changes in the bitumen structure were followed in the mid-infrared region with Attenuated Total Reflectance Fourier Transform InfraRed Spectroscopy (ATR-FTIR) by measuring the evolution of the band heights at 1700 cm−1 (CO functional groups) and 1600 cm−1 (CC double bonds). Needle penetration depths and ring and ball softening points were determined for some samples. Oxidation of bitumen in the presence of oxygen results in a distinct increase of both the number of CO and CC functionalities, with a positive linear relationship existing between the two groups. The production of CO functional groups seems to promote the generation of CC double bonds. Heating at 130 °C is much more efficient than gamma irradiation at low to moderate dose rates (20–140 Gy/h) to oxidise the bitumen. As the oxygen concentration decreases, for instance by diffusion limitation deeper inside the bitumen, the number of CO and CC functionalities formed per unit of time decreases. A similar behaviour was observed for 25 years old radioactive Eurobitum. In absence of oxygen, gamma irradiation still results in a small increase of the number of CO functional groups, probably by oxygen still adsorbed on the bitumen, and in a relatively higher amount of CC double bonds. The surface layer (<5 cm) of 25 years old radioactive Eurobitum was heavily oxidised. The material had become very hard and brittle, and was full of small fissures. Due to these fissures, radio-oxidation processes took also place deeper inside the waste. The consequences of these observations for the long-term behaviour of Eurobitum in underground disposal conditions are discussed.