Thermochemistry and Structure of Model Waste Glass Compositions
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THERMOCHEMISTRY AND STRUCTURE OF MODEL WASTE GLASS COMPOSITIONS
ADAM J. G. ELLISON AND ALEXANDRA NAVROTSKY, Geophysical Sciences, Princeton University,
Dept. of Geological and Princeton, NJ, USA 08544
ABSTRACT Enthalpies of mixing of major oxide and waste components of proposed DWPF Several proposed glass compositions have very similar glasses are estimated. Heats of mixing of molar proportions of major structure-related components. major components are predicted to be small (glassy reference states), but may Lanthanides are likely to form regions in the be endothermic or exothermic. glasses rich in R-O-Si bonds but relatively depleted in Si-O-Si bonds. Measured enthalpies of solution of simple lanthanide-bearing glasses resemble those of mechanical mixtures of these end-members, with near-zero enthalpies DWPF of mixing for fairly large variations in lanthanide concentrations. 3 glasses have alkalis in excess of those needed to charge-balance all T + Electropositive +4, +5, and +6 cations cations in tetrahedral coordination. are expected to use these excess alkalis to stabilize their own coordination polyhedra, forming complexes with effectively constant macroscopic stoichiometric ratios which contribute to enthalpies of solution in direct Therefore, variations in the proportion to their concentration. concentrations of high-valence cations in DWPF waste containment glasses are also predicted to result in near-zero enthalpies of mixing.
INTRODUCTION The thermochemical stability of borosilicate glasses is relevant to their This study estimates use for long-term storage of radioactive waste. enthalpies of mixing within the narrow compositional range of proposed waste Contributions to the enthalpies of mixing are divided containment glasses. into those from the mixing properties of major cation oxides and smaller, but The equally important, contributions from the radionuclides themselves. principal cation components of radioactive waste, actinides and lanthanides, have high valences in their natural oxidation states. Glasses containing other high-valence cations are used as "proxies" for the thermochemical behavior of The complex multicomponent glasses the radionuclides in waste glasses. proposed for nuclear waste disposal are not easily subject to thermochemical measurements, but a suitable choice of a small number of fictive components These predictions illustrate the utility of leads to useful inferences. thermochemical measurements as complements to spectroscopic techniques for determining the stability and intermediate-range structure of glasses, and
Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
194
TABLE 1. Proposed composition of Savannah River DWPF "BLEND" disposal of nuclear waste.
Major Components
Wt-%
Trace Components
Mole-%
Si0 2 TiO2 B203 A1203 Fe203 NiO FeO MnO MgO CaO Li20 Na20 K20
50.20 0.90 8.01 3.98 6.95
54.82 0.74 7.55 2.56 2.86
0.89 3.11 2.03 1.35 0.97 4.40 8.73 3.86
0.78 2.84 1.88 2.20 1.13 9.66 9.24 2.69
Total Major
95.38
98.95 Se,
glass for the
Wt-%
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