In the French nuclear waste management strategy, after reprocessing of nuclear spent fuel, Fission Products (FP) and Minor Actinides (MA) are confined in R7T7 borosilicate glass, which can incorporate up to 18.5 wt.% of FP and MA oxides [1]. Glass-ceramic matrices (GCM) could be an alternative way to immobilize higher amounts of high level waste than current borosilicate nuclear glasses and could offer a higher flexibility in the management of various waste streams to be vitrified. GCM have chemical and structural heterogeneities originating from the mixing of glassy and ceramic phases [2]. Their design can be adjusted by varying the material chemical composition, but one of the main potential ceramic phases that can be formed is silicate apatite that can crystallize from the glass melt if the lanthanide and actinide loading factor is high [3]. The development of these new class of conditioning materials requires the study of their long-term behavior, especially their self-irradiation response.
In this context, we followed the evolution under alpha decay self-irradiation of a 241Am doped glass-ceramic containing apatite crystals. For this purpose, a synthesis of an aluminoborosilicate SiO2-B2O3-Na2O-Al2O3-CaO-La2O3-Am2O3 glass-ceramic was carried out in hot cells. The chosen cumulative concentrations of La2O3 and AmO2 (21.83 wt%) above the incorporation limit allowed the crystallization of the apatite phase. Indeed, initial observations show that about 10% of the surface is occupied by the crystals. These crystals exhibit a hexagonal-shaped morphology, with crystals elongated along the c-axis of the hexagonal structure, characteristic of apatite crystals. Their chemical composition and cell parameter were found to be close to those expected for an apatite phase of stoichiometric composition Ca2La4Am4(SiO4)6O2. Within the glassy matrix, a depletion of lanthanum and americium is observed near the apatite crystals, which is more marked for americium, showing a preferential incorporation of this alpha-emitter in the apatite phases.
Structural and microstructural evolutions under alpha-decay self-irradiation were followed for 8 years by regularly analyzing the crystals and the residual glassy matrix. The combination of XRD and Raman spectroscopy data suggest a drastic transformation of the apatite structure with the alpha-decay dose. In a first dose range, the transformation results in the loose of the long range order, characterized by the disappearance of the XRD peaks, which corresponds to a progressive radiation-induced amorphization of the crystals. This amorphization appears to directly occur within the collision cascade of the 241Am recoil nucleus, by elastic-collision process, according to a direct impact model. The fully amorphous state is reached at around 3×1018 α/g of apatite. In a second dose range, between 2.8 and 4.23×1018 α/g of apatite, a very rapid or sharp transformation was observed by Raman spectroscopy. This last transformation, never observed before in silicate minerals, is associated to a modification of the medium range order of the metamict state with further accumulation of alpha-decay dose and is characterized by the increase of the connectivity in SiO4 tetrahedra.
The crystalline-to-amorphous transformation is accompanied by an increase in macroscopic volume (swelling). Raman imaging highlighted an emergence of amorphized crystals from the glassy matrix. This out-of-plane expansion is of around 3 µm. Differential swelling between the apatite crystals and the glassy matrix is a possible reason of this out-of-plane expansion. This macroscopic dimensional changes is also associated to a decohesion of the crystals from the glassy matrix. Indeed, SEM observations reveal the appearance of holes caused by the loosening of some crystals. This decohesion underlines a significant mechanical degradation of the amorphized crystals. However, optical and SEM images of the surface of the glass-ceramic did not reveal any significant cracks in the residual glass induced by self-irradiation.
This study shows that the microcracking of GCM due to differential swelling under self-irradiation ageing can be avoided and is certainly strongly depending on the material microstructure. Some future works are needed to improve our understanding of the conditions that control the microcracking of GCM under irradiation and therefore to develop some mechanical resistant glass-ceramics with respect to their radiation ageing.

[1] Gin, S., et al., Radionuclides containment in nuclear glasses: an overview. Radiochimica Acta, 2017. 105(11): p. 927-959.
[2] Ojovan, M.I., V.A. Petrov, and S.V. Yudintsev, Glass Crystalline Materials as Advanced Nuclear Wasteforms. Sustainability, 2021. 13(8).
[3] Kidari, A., et al., Solubility and partitioning of minor-actinides and lanthanides in alumino-borosilicate nuclear glass. Atalante 2012 International Conference on Nuclear Chemistry for Sustainable Fuel Cycles, 2012. 7: p. 554-558.