Gadolinium Solubility Limits in Sodium-Alumino-Borosilicate Glasses
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remains as a viable option for immobilization. In this study, we have been determining the solubility of surrogate elements as a prelude to determining the solubility of Pu in glasses. To accomplish this, we are studying the basic science underlying the solubility of lanthanides and Hf [4] in glasses as surrogates for Pu and as neutron absorbers. Few data are available on the solubility of lanthanides in glass that are suitable to the disposal of weapons-usable Pu [5]. The solubility of lanthanides in glass is important because some lanthanide is needed to minimize the likelihood of criticality during the storage of the glass and because some lanthanides can be used as surrogates for Pu, i.e. Gd (III) as a surrogate for Pu (III). The presence of lanthanides as neutron absorbers may also affect the solubility of Pu in the same glass. This paper reflects the results of a study of melting temperature effect and the substitution of La for Gd effect on the solubility of Gd in a sodium-alumino-borosilicate glass, which is a simplified Pul6MC3 glass developed for Pu immobilization [6). Boron effect on the solubility of La is also reported. EXPERIMENT Gadolinium and lanthanum solubility limits in a four-component glass system (Na 20B20 3-AI 20 3-SiO2) were investigated. The solubility limit in this study was defined as the largest 329 Mat. Res. Soc. Symp. Proc. Vol. 556 ®1999 Materials Research Society
concentration of gadolinium or lanthanum (with I mass% Gd 20 3 or La2O3 interval) in the glass above which crystallization or phase separation occurred as determined with optical microscopy or by X-ray diffraction (XRD) analyses. The baseline glass compositions used for boron effect on lanthanum solubility were (in molar composition) xB 20 3-20Na 9O-5AI10 3-60SiO 2 , where x=5, 10, 15, and 20. Only baseline glass 15B 2O3-20Na2 O-5A1,O 3 -60SiO 2 was used to study the effects of melting time at 1450'C, La substitution for Gd, and melting temperature on Gd solubility in the glass. Baseline glasses were first fabricated using well-mixed powders of SiO 2 (Fisher Chemicals), A120 3 (J. T. Baker Inc.), boric acid (EM Science) and sodium carbonate (Fisher 0 Chemicals). They were melted at temperatures between tI IO C and 1400'C, depending on The glasses were then quenched crucible. Pt-10%Rh composition, for 60 minutes in a covered by pouring the melt onto a steel plate. Gd 20 3 (Alfa Chemicals) or/and La 20 3 (Alfa Chemicals) were added to ground baseline glasses. Except for the melting temperature effect study, the glasses were melted at 1450'C by putting the covered Pt-lO%Rh crucible containing the glass
powder into a preheated furnace. Different melting times were studied to determine the proper melting conditions. The crucible cover was sealed to the crucible by use of molten glass. This minimized the losses from volatilization to less than 1 mass%. After melting, the melt was quenched by immerging the crucible in water. Then the resulting glasses were examined under optical microscope (up to 500 magnification) and/or with p
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