Analytical Electron Microscopy Study of Electron Radiation Damage in Iron Phosphate Glass Waste Forms
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Analytical Electron Microscopy Study of Electron Radiation Damage in Iron Phosphate Glass Waste Forms K. Sun, L. M. Wang and R. C. Ewing Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109 ABSTRACT A series of iron phosphate glass waste forms with compositions of 45Fe2O3-55P2O5, 20Fe2O3-80P2O5, and 20Fe2O3-20Na2O-60P2O5, namely FeP-1, FeP-2 and FeP-3, was studied by analytical electron microscopy (AEM). Transmission electron microscopy (TEM) bright-field (BF) imaging showed that under the electron irradiation, phase segregation occurred in both the FeP-1 and FeP-2 samples at high electron doses (3.84×1026 e/m2). In contrast, bubbles formed in the FeP-3 sample, even at a relatively low dose (2.88×1025 e/m2), which may be attributed to the migration of Na under irradiation as in the case in sodium borosilicate glass. Series electron energy-loss spectroscopy (EELS) analysis showed that the glass materials experienced mass-loss and composition variation. No obvious Fe valence state changes under irradiation were observed within the irradiation period.
INTRODUCTION Iron phosphate glasses have potential for vitrifying nuclear wastes with high-contents of phosphorus, fluorite and heavy metal oxides (e.g., UO2, Bi2O3 and CsO2), which are not well suited for borosilicate glasses. Thus, they have been intensively studied during the past several years by a variety of techniques: Mössbauer spectroscopy [1-3], Raman spectroscopy [2], X-ray diffraction (XRD) [1-3], neutron diffraction [1,2], X-ray absorption fine-structure spectroscopy (XAFS) [1,2], X-ray photoelectron spectroscopy (XPS) [1-3], and optical spectroscopy [4]. These studies have greatly enriched our knowledge on the glass forming ability, chemical durability, short-range order of the iron phosphates and their dependence on the Fe valence states and contents in the glasses. The previous results have indicated that glasses containing more than 25 mol% Fe2O3 have an exceedingly good chemical durability. Mössbauer spectroscopy and XPS studies have indicated that both Fe2+ and Fe3+ ions were present in the glasses, and the chemical durability improved with the increasing of Fe3+ concentration. Large amounts of Fe2+ appear to be detrimental to the glass-forming ability of the iron phosphate melts. However, there are few reports on the radiation damage studies of the iron phosphate glass waste forms. Although long-term materials performance is an essential element of establishing compliance with regulatory release or exposure limits, accelerated simulation experiments of radiation effects can only be performed in laboratory and remains an important technique in testing the radiation stability of the nuclear waste glasses. The electron beam used for conventional transmission electron microscopy (CTEM) can be used to simulate β-particle damage [5]. So CTEM has been widely used in the radiation damage studies of potential crystalline waste form materials, such as ceramics [6] and zeolite [7]. Such an experiment helps to improve
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