Studies on synthesis, structural and thermal properties of sodium niobium phosphate glasses for nuclear waste immobilization applications

The structural and thermal properties of sodium niobium phosphate glass are studied to analyse its suitability for immobilizing long-lived radioactive waste from nuclear reprocessing plants. Two series of sodium niobium phosphate glasses such as (A) xNb(2)O(5)-(50-x)Na2O-50P(2)O(5) and (B) xNb(2)O(5)-(50-x/2)Na2O-(50-x/2) P2O5 (x = 0, 10, 20, 30, 40 mol.%) are prepared by the conventional melt-quenching technique. The influence of the incremental addition of Nb2O5 on the structural and thermal properties of the above two series is investigated. The FTIR and Raman spectra of these glasses revealed that the number of [NbO6] octahedral chain increases with Nb content and dominates as the major structural unit over the number of PO4 tetrahedra unit. The glass transition temperature (T-g), crystallization temperature (T-c) and melting temperature (T-m) are measured by using a differential scanning calorimeter (DSC). Other thermal stability parameters such as Hruby (K-H), Angell (K-A), Saad-Poulain (K-SP) and Weinberg (K-W), the total relaxation time of transformation and fragility are evaluated and compared between different composition of glasses. For both series A and B, the glass transition temperature increases with Nb content, suggesting the formation of a larger number of P-O-Nb or Nb-O-Nb linkages compared to the PO4 tetrahedra, resulting in a more rigid network. This is confirmed by Raman spectra which show an increase in the number of Nb-O-Nb bonds with Nb content. The major crystalline phases formed by devitrifying these glasses are identified by XRD and are found to be mainly NaPO3, NbOPO4, Na2Nb6P4O26 and Na4Nb8P4O32.