Solid-state NMR characterisation of transition-metal bearing nuclear waste glasses
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Alkali borosilicate glass is used to immobilise high-level radioactive waste generated from the reprocessing of spent nuclear fuel. However, poorly soluble waste products such as molybdenum present issues for long-term storage of the material, whereby phase separation of water-soluble crystalline aggregates truncate the glass network and offer leaching opportunities for the escape of radionuclides into the environment. Solid-state NMR is an effective technique for studying the phase separation in nuclear waste glasses, as it is able to distinguish glassy and crystalline environments using nuclei-specific probes. The presence of paramagnetic transition metal cations within the waste glass compromises the efficacy of NMR, as unpaired electrons can cause broadening and chemical shift effects. This work engages in a multinuclear magnetic resonance characterisation of paramagnetically-doped (by Cr2O3, MnO2, Fe2O3, NiO, or Nd2O3) model nuclear waste glasses and explores the partitioning of transition-metal components. Characterisation of the separated heterogeneous crystalline phase was accomplished principally by 133Cs, 23Na, and 95Mo NMR, where it was determined that Cr substitutes into mixed-alkali molybdates. Furthermore, paramagnetic broadening and relaxation effects were observed and quantified by 11B and 29Si NMR.