The Effects of Na 2 O, Al 2 O 3 , and B 2 O 3 on HfO 2 Solubility in Borosilicate Glass
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HfO2
SOLUBILITY IN BOROSILICATE GLASS
L.L. Davis, L. Li, J.G. Darab, H. Li, and D. Strachan, Pacific Northwest National Laboratory, Box 999, Richland, WA 99352; P.G. Allen, J.J. Bucher, I.M. Craig, N.M. Edelstein, and D.K. Shuh, MS 70A-1 150, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT A single borosilicate glass composition has previously been shown to dissolve 10 and 25 mass% oxidizing and reducing conditions, respectively. A simplified version of this glass has been under PuO2 thoroughly investigated to determine the effect of increasing the alkali:aluminum ratio on the HfO 2 solubility in borosilicate glasses. We are investigating HfO 2 solubility because specific Pu wastes are being considered for disposal in glass, and Hf(IV) serves as a structural surrogate for Pu(IV) and as a neutron absorber in glass. Three series of base glasses were produced using the same initial composition, but varying the oxides B20 3, A12 0 3 , or Na 20 one at a time. In a fourth series of the same initial composition, both Na 20 and A120 3 were varied. Hafnia was added to these glasses and the mixture equilibrated for 2 hours: 1 hour at 1450'C after 1 hour at 1560'C. A wide range of HfO2 additions were made to the base glasses, and the solubility of HfO2 determined to within + I mass%. The highest solubility determined was 14 mol% (35 mass%) HfO 2 in a low-Al glass. We conclude that increasing Na2O/AI20 3 increases the HfO 2 solubility, and increasing the B 20 3 content apparently has little effect on HfO 2 solubility in the borosilicate glasses studied. INTRODUCTION We are examining the effects of bulk composition on the solubility of high-valence cations as Pu surrogates and as neutron absorbers. Here we report the effects of peralkalinity (K 20 + Na 20 > A120 3 molar) on HfO 2 solubility. This study was completed in tandem with a Gd 20 3 solubility study [1]. While our intent is to focus on fundamental solubility issues, the data regarding the effects of glass structure, redox state, and bulk compositional variation on solubility limits are essential to the technical development, evaluation, and selection of waste forms for the safe disposal of Pu and transuranic wastes. Our approach was developed to emulate previous systematic experimental work on the saturation concentration of tri- and tetravalent cations in peraluminous (K 20 +Na 20 +CaO < A120 3 molar) to peralkaline silicate melts relevant to naturally occurring systems [e.g., 2-3], and to build upon these studies by starting with simple compositions relevant to waste glass studies. Using the Hf and Gd solubilities determined, we will dope similar base glasses with equivalent amounts of Pu, thereby testing the use of Hf and Gd as Pu surrrogates. We have systematically examined the effects of increasing Na 20, B 2 0 3, and A120 3 on the solubility of HfO 2 in peralkaline borosilicate glasses. We have also examined the effect of changing Na/(Na + Al) on Hf0 2 solubility. The premise of this approach is that alkalis appear
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