Characterization of a Glass-Bonded Ceramic Waste Form Loaded With U and Pu
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However, the observed formation of a separate oxide phase rather than occlusion raises an important basic question as to whether zeolite occlusion of actinide salts can occur if the competing reaction with water is suppressed by using sufficiently dry zeolites, and/or artificially large actinide contents. The present study was in part designed to address this question. EXPERIMENTAL Four laboratory scale samples of CWF were produced as a test matrix shown in Table 1. At ANL-East, salts of composition given in Table 2 were blended with wet or dry zeolite 4A for 20 h at 500'C under argon. The salt/zeolite blend had 10.5 wt% salt. Table 1. Overview of experimental matrix showing sample designations zeolite H 2 0(w/0/) U:Pu Ratio 3:1 U:Pu Ratio 1:3 0.12
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Table 2. Approximate composition of actinide bearing 300 driver electrorefiner salt (w%) LiCl-KC1I" NaCI CsCI BaC12 REC13 AcCI3 others 65.0 12.7 2.1 1.0 7.9 10.1 1.2 Eutectic compositon with 47.1 wt%KCI. RE: Rare earths; Ac: Pu and U in 3:1 or 1:3 ratio Following blending, the salt-loaded zeolite was mixed with a borosilicate glass frit in 1:3 proportion by weight (glass:blended zeolite), and sealed in small 1" diameter evacuated HIP cans. At ANL-West, the samples were HIPed at 850'C for I h under 100 MPa pressure. Powder x-ray diffractometry (XRD), scanning and transmission electron microscopies (SEM and TEM) were performed using Scintag Xl (Cu Kcx), Zeiss DSM 960 and JEOL 2010 instruments, respectively. TEM sample preparation was performed using standard dimpling and ion milling techniques. RESULTS a) X-Ray Diffractometry
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Fig. 1. XRD scans of samples 4 (top) and 2 (bottom).
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Fig. I shows XRD scans of samples 2 and 4. The phases identified in the scans are sodalite, nepheline, the mixed actinide oxide (U,Pu)0 2 phase with fluorite structure, and halite. In addition, the broad rise in the background intensity near 250 20 is consistent with the presence of a glass phase. As can be seen, the traces are nearly identical, indicating that the influence of water on the phase content of the final CWF is minor. The only significant variation of phase content caused by the presence of water is an increase in the peaks associated with halite for high water content. For changing U:Pu ratio, the only discemable influence in the XRD trace was on the lattice parameter of the mixed oxide (U,Pu)O2 phase. This varied in a way consistent with solid solutions with U:Pu of 3:1 or 1:3. b) Scanning Electron Microscopy Fig. 2 shows the typical appearance of wet and dry zeolite in SEM, backscattered electron (BSE) mode. The predominant microstructure of the CWF consists of polycrystalline sodalite regions joined by a glass phase. The lightest features in the images are actinide and rare-earth bearing regions. Sodalite regions appear as a more continuous light color and glass regions appear diffuse and darker. In the case of sample 1 made with dry zeolite, the actinide species appear to be more homogeneously distributed within the sample, giving
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