The structural role of Zn in nuclear waste glasses

Addition of ZnO and CaO to current United Kingdom high level nuclear waste (HLW) glass frits is currently under consideration with the aim of improving processing behavior and aqueous durability. This study was performed in order to better understand the structural role of Zn in model U.K. HLW glasses and, in particular, the role of Zn as a network modifier, intermediate, or former in such materials. Zinc K-edge X-ray absorption spectroscopy was applied to simple soda-lime-silicate glasses doped with ZnO and on model inactive U.K. HLW glasses consisting of simulated Magnox and Blend (25 wt% Magnox + 75 wt% ThORP reprocessing waste) wastes. X-ray absorption near edge and extended X-ray absorption fine structure spectroscopy provided conclusive evidence that Zn occurs in four-fold coordination with Zn–O contact distances of 1.95 ± 0.01 Å in all glasses studied. Analysis of the extended X-ray absorption fine structure data also indicates the presence of short range order with Si next nearest neighbor atoms present at a Zn–Si contact distance of 3.58 ± 0.03 Å.