Abstract
A series of calcium borosilicate glasses with varying [B2O3], [MoO3], and [CaO] were prepared and subjected to 92 MeV Xe ions used to simulate the damage from long-term α-decay in nuclear waste glasses. Modifications to the solubility of molybdenum, the microstructure of separated phases, and the Si–O–B network topology were investigated following five irradiation experiments that achieved doses between 5 × 1012 and 1.8 × 1014 Xe ions/cm2 in order to test the hypotheses of whether irradiation would induce, propagate, or anneal phase separation. Using electron microscopy, EDS analysis, Raman spectroscopy, and XRD, irradiation was observed to increase the integration of MoO42− by increasing the structural disorder within and between heterogeneous amorphous phases. This occurred through Si/B-O-Si/B bond breakage and reformation of boroxyl and 3/4-membered SiO4 rings. De-mixing of the Si–O–B network concurrently enabled cross directional Ca and Mo diffusion along defect created pathways, which were prevalent along the interface between phases. The initiation and extent of these changes was dependent primarily on the [SiO2]/[B2O3] ratio, with [MoO3] having a secondary effect on influencing the defect population with increasing dose. Microstructurally, these changes to bonding caused a reduction in heterogeneities between amorphous phases by reducing the size and increasing the spatial distribution of immiscible droplets. This general increase in structural disorder prevented crystallization in most cases, but where precipitation was initiated by radiation, it was re-amorphized with increasing dose. These outcomes suggest that internal radiation can alter phase separation tie lines, and can therefore be used as a tool to design certain structural environments for long-term encapsulation of radioisotopes.
Highlights
The critical concern with nuclear power remains the growing need to deal with significant amounts of high-level radioactive waste (HLW) [1]
The calcium borosilicates synthesized in this study showed a unique heterogeneous structure of embedded immiscibility
Calcium borosilicate glasses with systematically varied [MoO3] and [B2O3] were irradiated with 92 MeV Xe ions to determine how radiation damage would affect the incorporation of molybdenum and the propagation or remediation of phase separation in heterogeneous calcium borosilicates
Summary
The critical concern with nuclear power remains the growing need to deal with significant amounts of high-level radioactive waste (HLW) [1] Owing to their ability to incorporate a wide variety of radioisotopes, as well as showing radiation resistance and chemical durability in aqueous environments, immobilization of radioisotopes from HLW in a borosilicate or phosphate-based glass is the current worldwide practice [2,3,4,5]. In order to prevent yellow phase precipitation, increased incorporation of molybdenum into an alternative glass composition or selective and controlled formation of water-durable CaMoO4 [13] is currently of industrial interest [9, 14,15,16]
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