Abstract
Molybdenum solubility is a limiting factor to actinide loading in nuclear waste glasses, as it initiates the formation of water-soluble crystalline phases such as alkali molybdates. To increase waste loading efficiency, alternative glass ceramic structures are sought that prove resistant to internal radiation resulting from radioisotope decay. In this study, selective formation of water-durable CaMoO4 in a soda lime borosilicate is achieved by introducing up to 10 mol % MoO3 in a 1:1 ratio to CaO using a sintering process. The resulting homogeneously dispersed spherical CaMoO4 nanocrystallites were analyzed using electron microscopy, X-ray diffraction (XRD), Raman and electron paramagnetic resonance (EPR) spectroscopies prior to and post irradiation, which replicated internal β-irradiation damage on an accelerated scale. Following 0.77 to 1.34 GGy of 2.5 MeV electron radiation CaMoO4 does not exhibit amorphization or significant transformation. Nor does irradiation induce glass-in-glass phase separation in the surrounding amorphous matrix, or the precipitation of other molybdates, thus proving that excess molybdenum can be successfully incorporated into a structure that it is resistant to β-irradiation proportional to 1000 years of storage without water-soluble byproducts. The CaMoO4 crystallites do however exhibit a nonlinear Scherrer crystallite size pattern with dose, as determined by a Rietveld refinement of XRD patterns and an alteration in crystal quality as deduced by anisotropic peak changes in both XRD and Raman spectroscopy. Radiation-induced modifications in the CaMoO4 tetragonal unit cell occurred primarily along the c-axis indicating relaxation of stacked calcium polyhedra. Concurrently, a strong reduction of Mo6+ to Mo5+ during irradiation is observed by EPR, which is believed to enhance Ca mobility. These combined results are used to hypothesize a crystallite size alteration model based on a combination of relaxation and diffusion-based processes initiated by added energy from β-impingement and second-order structural modifications induced by defect accumulation.
Highlights
Vitrification into a borosilicate glass is a widely accepted technique to immobilize nuclear waste
We demonstrate in this paper how these radiation-induced structural modifications present themselves in glass ceramic (GC) and investigate subsequent effects on CaMoO4 nucleation and stability
Simplified glass ceramics were successfully synthesized to promote the crystallization of CaMoO4 and prevent the speciation of Na2MoO4 for up to 10 mol % MoO3 by introducing MoO3 in a 1:1 ratio to CaO
Summary
Vitrification into a borosilicate glass is a widely accepted technique to immobilize nuclear waste. One of the limiting factors to waste loading is the concentration of MoO3, as molybdenum has a low solubility in silicate and borosilicate glasses.[5,6] Molybdenum can exist in several oxidation states (Mo6+, Mo5+, Mo4+, Mo3+), but in oxidizing or neutral conditions Mo ions are considered to be primarily hexavalent, taking the form of [MoO4]2− tetrahedra.[7−9] These tetrahedra are found to be unconnected to the glassy framework and are located in nonbridging oxygen (NBO) channels containing alkali and alkaline earth deposits.[9−11] The [MoO4]2− tetrahedral form of molybdenum is found in both the glassy and crystalline phases, which could account for its low solubility in borosilicates.
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