Effect of γ irradiation on nuclear bituminized waste products (BWP): X-ray microtomography and rheological characterization

In the last 50 years bitumen has been widely used in France as a matrix for embedding low- and intermediate-level radioactive waste. This organic matrix is subjected to α, β and γ radiation generating radiolysis gases, which consist mainly of hydrogen but also some carbon oxide and lighter hydrocarbons. The gases form bubbles whose growth entails swelling of the bitumen if the radiolytic process of gas generation is more important than the removal process. To assess the influence of γ radiation on the swelling and rheological behavior of the bitumen matrix, external γ irradiation experiments were performed on pure bitumen (PB) and on a model synthesized bituminized waste product (BWP: 60 wt% bitumen + 40 wt% salts). Greater swelling was observed for the BWP samples than for PB. X-ray microtomography shows that the hydrogen bubbles are more numerous in the case of BWP, indicating that the salts may be considered as preferential nucleation sites. Moreover, the rheological characterization of the samples shows that the Newtonian viscosity is higher for the BWP model sample, and that the irradiation enhances this phenomenon. In the case of accelerated irradiation (dose rate exceeding the actual dose rate), swelling increases with the irradiation dose since the gas production rate is higher than the removal process until a plateau is reached at approximately 4–5 MGy, indicating an equilibrium between gas production and release. Finally, the impact of the dose rate on the number, size and shape of bubbles and on macroscopic swelling is discussed on the basis of X-ray microtomography results.