Nuclear glass alteration under alpha-irradiation and environmental conditions

This work intends to provide a better understanding of the alteration of R7T7-type nuclear waste glass in a complex environment such as the deep geological repository. This approach aims to progressively complexify the glass leaching environment in order to better understand the mechanisms involved on the alteration of a radioactive glass in contact with the Callovo-Oxfordian groundwater (CO_x groundwater) and an iron source, simulating the glass carbon steel overpack. First, the study focusses on the impact of groundwater on the glass alteration under the residual rate regime at 70 C. Results show that magnesium present in the Callovo-Oxfordian groundwater contributes, over a pH threshold, to the precipitation of magnesium phyllosilicates consuming the passivating gel and thus increasing the glass alteration. This work confirms previous studies and brings new data at 70 C. Thereafter, an iron source was introduced in the leaching environment (glass - CO_x groundwater) in order to simulate the carbon steel overpack. The experiments were performed in an anoxic environment with a glass monolith and an iron foil, facing each other with an 80 μm gap, and then immersed in the CO_x groundwater at 70 and 90 C. On these conditions, a weak impact of the temperature was perceived on the kinetics and the reactions controlling glass alteration. The iron corrosion led mainly to the precipitation of siderite (an Fe²⁺-carbonate) and aragonite (a calcium carbonate). Furthermore, an increase of glass alteration was observed while using an iron foil. The main cause explaining this phenomenon is the formation of iron silicates, as seen with magnesium silicates, precipitating at the expense of the passivating gel. These phases were identified mainly at the oxidation state III, indicating an iron oxidation near the glass. It should be noted that in this case, magnesium precipitation is suppressed and only iron silicates precipitate. Finally, the influence of irradiation on glass alteration was studied. Water radiolysis induces the production of oxidizing species such as H₂O₂ at the glass/water interface. Therefore, a first study focused on glass alteration in the CO_x groundwater in the presence of H₂O₂ under the residual rate regime. Results showed that this specie decomposition was higher at the surface of the glass grains and seemed to have a very local impact, favouring the magnesium silicate precipitation and the enhancement of glass alteration. This prompt effect was only observed locally on grains at the surface of the reactor. However, globally, the data showed that H₂O₂ does not have a significant impact on glass alteration in this residual rate regime and this geometry. At last, this work came to experiments using a radioactive glass, doped with ²³⁸Pu, altered in the CO_x groundwater at 70 C in the presence of an iron source, and using the same geometry than the one conducted with the non-radioactive glass. The glass characteristics allowed us to focus only on the α-dose rate effect. Comparing with the non-radioactive reference, a higher iron oxidation was noticed, with the precipitation of iron oxides at the oxidation state III, such as goethite and ferrihydrite. This can be explained by the reaction between H₂O₂ and iron, favouring the oxidation of Fe²⁺ to Fe³⁺. From solution analysis and the first SEM observations, it seems that globally, the glass alteration thickness is the same regardless the α-irradiation field. Nevertheless, supplementary data on solid characterisations must be performed in order to understand and be conclusive about the mechanisms involved on the glass alteration mechanisms under these conditions. (author)