The effect of mixed modifiers on nuclear waste glass processing, leaching, and Raman spectra
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The effect of mixed modifiers on nuclear waste glass processing, leaching, and Raman spectra S. V. Raman Department of Nuclear Engineering, Idaho National Engineering Laboratory, Lockheed Martin Idaho Technologies, Idaho Falls, Idaho 83402 (Received 20 January 1997; accepted 8 October 1997)
Borosilicate glasses with different waste loadings were prepared by ambient melting, quenching, and annealing. Some melt compositions partially crystallize to durable phases of zircon and forsterite. The coexisting liquid quenches to glass and endures the leach tests. The waste loading dependent leach rate trends of these glasses are reminiscent of the mixed alkali effect. Raman spectra suggest initial increase in durability with increases in the depolymerization of silicate species. Fluorine and hydroxyl ions also contribute to depolymerization. Tetraborate and metaborate rings are identified in the Raman spectra. The durability is enhanced when tetraborate bands are more intense than the metaborate bands.
I. INTRODUCTION
Several sustainable characteristics concern the development of nuclear waste glasses. The nuclear waste glass must resist the thermal and corrosive environments that largely originate from radionuclide decay of fission products and natural water permeation in the repository. It must endure these environments for a long period which is determined by the long half life of fissile elements. It must be processable at lower temperatures like 1200 ±C to prevent possible volatilization of nuclear waste components. Its composition and structure must promote spontaneous melt pourability as well as homogeneous solubility of refractory and volatile components. The liquid, liquidus, and glass transition temperatures must differ such that the melt upon quenching is deprived of devitrification. To meet these demands, the nuclear waste is conventionally blended with glass forming and modifying components. Most of the glasses thus made are borosilicate varieties. They are typically low in alkali and boron contents, as these elements generally tend to decrease the durability, although they benefit processing by suppressing the viscosity and melting temperature. The boron oxide content in such glasses is on the order of 8 to 14 wt. % and the sodium oxide is in the neighborhood of 2 to 5 wt. %.1,2 However, in addition to concentration, the distribution of elements among various silicate and borate structural species governs the mobility of elements in the glass network. For example, the mixed alkali effect leads to a decrease in alkali mobility.3,4 Some nuclear waste compositions contain several glass modifying components. It was of interest in this work to examine the effects of mixing among these components on glass structure and durability. The waste composition for this purpose is shown 8
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in Table I. It is a simulant of the actual calcined waste of the Idaho Chemical Processing Plant.5 Alumina, alkali oxides, alkaline earth oxides, calcium fluori
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