EXAFS and XANES Analysis of Plutonium and Cerium Edges From Titanate Ceramics for Fissile Materials Disposal
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The counting time for the data ranged from 2 to 8 seconds per point in the EXAFS region. The experimental spectra were fitted using the program FEFITfrom the University of Washington package, and FEFF,version 8.00, to generate the scattering paths [2]. Table 1. Ceramic compositions in weight percent. Specimen Type Compound A9 surrogate AO 14.52 9.44 9.95 CaO 39.76 34.04 TiO 2 35.87 11.82 10.11 HfO 2 10.65 8.80 7.54 Gd 20 3 7.95 22.48 23.69 U0 2 11.28 11.89 PuO2 25.10 CeO 2 0.50 A120 3 0.44 MgO 0.66 CaCI2 0.57 Ga20 2 0.15 Fe 20 3 0.08 Cr 20 3 0.13 NiO 0.44 CaF 2 K20
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0.32
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Na 20
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MoO 2
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0.14 0.28
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Si0 2 Ta2 0 5 B20 3 W0 2 ZnO
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0.46 0.19 0.17 0.49
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0.07
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Standards for cerium were Ce(III)PO 4 and Ce(IV)0 2, while high-fired PuO 2 served as a plutonium standard. The Ce-L11 XANES from the cerium surrogate ceramic was clearly located between the Ce(III) and Ce(IV) standards (Figure 1). Fitting the edge structure with a linear combination of spectra from the Ce(III) and Ce(IV) standards yielded an oxidation state of 3.7, consistent with earlier studies on cerium-loaded Synroc [3]. Remarkably, the entire edge structure could be reasonably fit, including the edge position, the location and relative heights of the split peaks in the LI, spectrum, and the relative height of the continuum that occurs after the white line (Figure 2). The near-edge structure (XANES) contains information on the formal oxidation state of the absorbing species. The XANES was fitted by a methodology described elsewhere that uses the sum of a Lorentzian and a hyperbolic tangent function [4]. The analysis of the plutonium edge in the prototype ceramics (Figure 3) demonstrates that it is almost entirely in the Pu(IV) oxidation state. The XANES was nearly identical for the AO and A9 samples, both having the edge position and qualitative edge structure much like those of the PuO2 standard [4,5].
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6.1
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6.25
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6.2
6.25
6.3
Energy (keV)
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Energy (keV)
Figure 2. The cerium XANES from the ceramic (solid line) was fitted with a linear combination of 70% Ce(IV)0 2 and 30% Ce(III)PO 4 standards (dotted line).
Figure 1. The cerium in the surrogate ceramic (solid line) has a mixed oxidation state, as illustrated by this comparison with XANES standards (lines with points). For clarity, the intense white line of the Ce (III) standard has been cut off in the figure.
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X-ray Energy (E-1 8055 eV) Figure 3.The plutonium XANES from the pyrochlore ceramic has an oxidation state very near Pu(IV), by comparison with PuO 2 reference data [4]. Shown is the data (hatches) with the fit generated as a sum (dark gray line) of a Lorentzian and hyperbolic tangent (light gray line) functions. For clarity, the Lorentzian function is not separately shown.
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The EXAFS functions (Figure 4) were nearly identical for the AO and A9 samples, indicating a robust preferred crystal chemical environment for the plutonium in the ceramic. The real-space analysis of the AO ceramic
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