Microstructure and Chemistry of an Aluminophosphate Glass Waste Form under Electron Beam Irradiation
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0LFURVWUXFWXUHDQG&KHPLVWU\RIDQ$OXPLQRSKRVSKDWH*ODVV:DVWH)RUPXQGHU(OHFWURQ %HDP,UUDGLDWLRQ K. Sun, L. M. Wang and R. C. Ewing Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA $%675$&7 An alkali-containing (mainly sodium) aluminophosphate glass waste form was studied by analytical electron microscopy. Bright-field imaging showed that small bubbles formed under the electron irradiation even at a low electron dose (8×1022e/m2). These bubbles grew with the increase of the electron dose and were finally released at the surface of the sample. At the same time alkali elements were also lost under irradiation. At an electron dose of about 2.2×1026e/m2 (6.6×1011Gy), all the bubbles were released and no bubbles were formed with further irradiation. The glass was finally transformed to a pure aluminophosphate glass. Further irradiation resulted in the phase separation between the Al-rich and the P-rich phases. The electron irradiation damage effects on the aluminophosphate glass are compared to those observed in iron phosphate glasses. ,1752'8&7,21 Two families of glass compositions, borosilicates and iron phosphate, have been considered as nuclear waste form glasses in the United States [1-7]. In Russia, aluminophosphate (AlPO) glass compositions have been used and 12,000 cubic meters HLW have been immobilized in 2,300 metric tonnes (300 million &L) of AlPO glasses. Phosphate glass forms were thought to be ideal for vitrifying nuclear wastes with high-contents of phosphorus, fluorite and heavy metal oxides (e.g., UO2, Bi2O3 and CsO2) [7]. An important issue is the response of different glass compositions to high levels of ionizing radiation. Although phosphate glass has a relatively low chemical stability, the addition of oxides such as Al2O3 and Fe2O3 lead to significant improvement in durability [8,9]. In the literature, the dependence of the chemical durability on the content of aluminum and glass structure has been demonstrated [10,11]. It is clear that there exists an optimized content of aluminum in the phosphate glass in order to strengthen its network. The addition of aluminum into the glass results in the formation of P-O-Al bonds instead of P-O-P bonds. While there are number of reports on the radiation damage in borosilicate glasses [3], only few experiments have been performed on the radiation durability of AlPO glasses. Nishida and Maki studied γ-ray irradiation damage in several Fe, Pb and Ca containing AlPO glasses and AlPO glasses containing 10 stable isotopes (Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Pr, and Nd) using Mossbauer spectroscopy [12]. They concluded that AlPO glasses containing Fe and Pb have a high radiation-stability in addition to high thermal stability and low leach rate. The electron irradiation induced damage in a material can be used to simulate the effects of ionization and electron excitations from β-particles and γ-rays in a radioactive glass. Techniques available in a transmission electron microscope (TEM) have been widely used fo
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