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Description
The final phase of the nuclear fuel cycle involves a critical task: finding suitable matrices and materials to safely incorporate and immobilize high-level radioactive waste 1. Borosilicate glasses have been widely used for high-level nuclear waste immobilization 2. However, iron phosphate and lead iron phosphate glasses are also functional materials that are considered to be economical alternatives to borosilicate glasses for nuclear waste immobilization 3, because these are good solvents for heavy metal ions and exhibit excellent leaching resistance in both acidic and neutral medium. The chemical durability of phosphate glasses can be enhanced significantly by adding Fe2O3 in the glass network; the addition of these metal oxides replaces the easily hydrated –P–O–P– bonds by more hydrate resistant –M–O–P– bonds, where M are metal cations.
Iron phosphate glasses containing 25 to 40 mol% Fe2O3 were prepared by melt quenching technique. Density increases from 2.8 to 3.2 gcm-3. The glass transition, crystallization and liquidus temperatures were determined by differential scanning calorimetry analysis. The ionic packing fraction show small variation while the glass forming tendency decreases significantly with an increase in Fe2O3 concentration in the phosphate network. The Reverse Monte Carlo simulation of neutron diffraction datasets were used to calculate the partial atomic pair distributions and coordination environments in iron phosphate glasses. The most probable P-O and Fe-O bond distances are 1.50 Å and 1.85 Å respectively. Fe exists mostly in 3+ oxidation state with little or no concentration of Fe2+. It is possible that some results reported in the literature that Fe2+ and Fe3+ co-exist in the iron phosphate network and that the relative concentration of Fe2+ in the final glasses increases with an increase in melting time and temperature are erroneous, and this effect could be due to the incorporation of Al3+ in the melt from the slow leaching of alumina crucibles that were used for glass synthesis by previous investigators.
The bond angle distribution in the glass network shows the peaks in the angle ranges: 75-88º and 95-116º for O—Fe—O and O—P—O linkages, respectively. The O—O—O bond angle distributions has a maximum at 60o. The Fe-O co-ordination is in the range: 3.60-4.57 and P-O co-ordination is in the range: 3.09-3.35.
References:
[1] J.D. Vienna, Nuclear waste vitrification in the United States: recent developments and future options, International Journal of Applied Glass Science 1 (2010) 309-321.
[2] C.P. Kaushik, R.K. Mishra, P. Sengupta, Amar Kumar, D. Das, G.B. Kale, Kanwar Raj, barium borosilicate glass – a potential matrix for immobilization of sulfate bearing high-level radioactive liquid waste, Journal of Nuclear Materials 358 (2006) 129-138.
[3] D. Day, C. Ray, C. Kim, Final Report: Iron Phosphate Glasses: an Alternative for Vitrifying Certain Nuclear Wastes, Project No, DEFG07 e96ER45618, (2004).
