Chemical durability of peraluminous glasses for nuclear waste conditioning

For the handling of high level nuclear waste (HLW), new glass formulations with a high waste capacity and an enhanced thermal stability, chemical durability, and processability are under consideration. This study focuses on the durability of peraluminous glasses in the SiO2-Al2O3-B2O3-Na2O-CaO-La(2)O(3)system, defined by an excess of Al(3+)ions compared with the network-modifying cations Na(+)and Ca2+. To qualify the behavior of such a peraluminous glass in a geological storage situation, its chemical durability was studied in various environments (pure water, groundwater, and alkaline solutions related to a cement environment) and glass alteration regimes (initial rate, residual rate, and resumption of alteration). The alteration solution was characterized by inductively coupled plasma, and the altered glass by scanning electron microscopy, X-ray diffraction and secondary ion mass spectrometry. A comparative study of the chemical durability of these and reference glasses (ISG and SON68) over all timescales highlights the remarkable properties of the former. While their initial dissolution rate is of the same order as the reference glasses, the gel formed under silica saturation conditions is more passivating, making its dissolution rate at least one order of magnitude lower, while its low alkalinity makes it less susceptible to clayey groundwater and highly alkaline solutions. Nuclear glass: Searching for solutions to dissolution How a 'peraluminous' glass-of interest in nuclear waste storage-changes in solution has been studied. The long term behaviour of borosilicate nuclear glasses, used to store high-level nuclear waste, has important safety ramifications and it is known that dissolution and precipitation processes in water can affect their durability. A team, led by Maxime Fournier at the CEA, Marcoule, France, has now studied the durability of a 'peraluminous' glass, which is defined by its excess of Al3+ ions as compared to more standard 'peralkaline' glasses. Such materials are known to form passivating layers on their surface in aqueous solution and the researchers observe that the layer that forms on the surface of the 'peraluminous' glass is more passivating than that on the peralkaline reference glasses used for comparison, resulting in a significantly lower dissolution rate.