Structural behaviour of vanadium ions in alkali borosilicate glass for nuclear waste storage

In Japan, it is planned to dissolve high level nuclear waste in molten alkali borosilicate, and then to dispose of the resultant glass 300 m underground with shielding materials. An appropriate design for the glass composition is required to increase the immobilising capacity for nuclear waste without yellow phase precipitation, mainly consisting of an alkali molybdate, We have focused on the structural role of vanadium ions in the borosilicate glass used for nuclear waste storage, because the addition of vanadium to the glass has been empirically found to suppress yellow phase precipitation in nuclear waste storage, although the microscopic mechanism for this has not been clarified. Various kinds of structural analyses, including XAFS, MAS NMR, and Raman spectroscopy, were performed on the synthesised borosilicate glass to determine the structural behaviour of vanadium ions in the glass, particularly in the presence of molybdenum ions. From the results of the structural analyses, it was confirmed that molybdenum ions exist as Mo(VI)O-4(2-) in the glass, and that these molybdate complex ions preferentially associate with alkali ions compared to the other structural units. In addition, it was revealed that vanadium ions occupy V(V)O-4 tetrahedra in the glass, and their introduction can suppress the preference of alkali ions for only Mo(VI)O-4(2-). Instead, alkali ions are proposed to preferentially coordinate with V(V)O-4 tetrahedra, which may result in suppressing the precipitation of alkali molybdate.