Structure and chemical durability of glasses for high-level waste immobilization: A solid-state nuclear magnetic resonance spectroscopy study

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Krishnamurthy, Arun
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Borosilicate and phosphate glasses function as immobilization matrices for high-level nuclear waste generated from the reprocessing of spent nuclear fuel rods. The waste is highly radioactive and chemically heterogeneous, with more than thirty elements present in their oxide forms, some in high oxidation states with phase-separation tendencies (for example, Mo6+ and S6+). Phosphate glasses show high affinity for these troublesome cations but suffer from poor physical and chemical properties which can be improved by doping the glass with Al2O3 and B2O3. In this work, a detailed account of the network structure of Mo-doped aluminoborophosphate glasses and the mode of Mo incorporation in phosphate glasses is presented using multinuclear solid-state nuclear magnetic resonance spectroscopy and Pauling bond strength modeling. The above-mentioned cations are sparingly soluble in borosilicate melts, wherein they separate as water-soluble crystalline molybdates and sulfates by sequestering radionuclides from the glass. A new approach to improve their incorporation into the glass structure is presented, which relies on matching the field strengths of waste-cations with the glass network constituents. The utility of this cation field-strength matching principle in successfully vitrifying multiple high field-strength cations is demonstrated. The chemical durability of phosphate-doped borosilicate glasses with enhanced Mo incorporation is evaluated to discern their performance and applicability for waste remediation. Finally, the effect of alkali oxide content on the network structure of borosilicate glasses and their chemical durability is explored with the aim of determining the alkali-loading limit for silicate-based glasses targeted toward the vitrification of low- and intermediate-level nuclear waste.
Solid-state NMR spectroscopy, Glass chemistry, Nuclear waste disposal, Borosilicate glasses, Phosphate glasses, Pauling bond strength, Chemical durability, Molybdenum