X-Ray Absorption Fine Structure of Aged, Pu-Doped Glass and Ceramic Waste Forms
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Mat. Res. Soc. Symp. Proc. Vol. 506 01998 Materials Research Society
in July 1982 with 1 wt.% PuO 2 in which the 23Pu/ZPu isotopic composition of plutonium was varied resulting in a factor of 200 variation in the accumulated dose'. The zircon samples were prepared in August 1981 in which 8.1 mole% of zirconium was replaced by 239pu or 238pu 2. The accumulated a -decay dose for these samples is given in Table II. Table I. Composition of defense reference waste glass. oxide
weight %
SiO 2
50.6
B2 0 3 A120
3
CaO Fe2 Oa
oxide LiO
weight %
oxide
weight %
3
4.9
SrO
0.5
7.0
MgO
0.7
ZrO2
0.7
6.0
MnO 2
3.0
Cs 20
0.5
2.0 10t0
Na 20 NiO
9.1 2.0
U3 0 8 PuO 2
3.0 1.0
Table II. Sample isotopic composition and accumulated a-decay dose. sample
Pu isotopic composition
DRG-P1
1.0 wt%
DRG-P2
0.9 wt%
DRG-P3
1.0 wt%
23
9Pu-doped zircon
38
. Pu-doped zircon
239
accumulated dose (a-decays/g)
PuO2
23
9puO 2, 0.1 wt%
2 38
PuO 2
238
10.0 wt% 8.85 wt%
PuO 2
8.8 x 10" 2.0 x 1 0 17 1.9 x 10oi
239
1.2 x
pu
238
23
pu, 1.15 wt% 9pu
1017
2.8 x 10 t9
XAFS measurements were conducted at the Stanford Synchrotron Radiation Laboratory on endstation 4-2 under dedicated operating conditions. Spectra were collected at the Pu and U Lmedges and the Mn, Fe, Sr and Zr K-edges in fluorescence mode at 77 K. The fluorescence signal was collected using a 13-element Ge detector. The phase and amplitude for the cation-oxygen and cation-silicon scattering paths were calculated using the ab initio code FEFF7.02 3'4 . The individual scattering paths were then parameterized and used to curvefit the EXAFS. The crystalline silicates MnSiO 4, Sr2Si0 4, ZrSiO 4, USiO4, and PuSi0 4 were used to approximate the local cation environment in the waste glass and were used as input for FEFF7.02. The scattering paths for tetrahedral Fe in a silicate framework were calculate by substituting Fe for Al in KAISi 30. In addition, the uranium triscarbonato complex was used as the model the 'yl' moiety and multiple scattering contributions, U-0y, and U-ms, respectively. RESULTS Simulated Waste Glass XANES. The XANES of the waste glass samples are shown as insets in Figure 1. Although all the XANES show slight variation in the shape of the XANES or the energy of absorption edge these variations are most likely to be the result of structural changes due to the effects of radiation damage rather than oxidation state changes. The U L0 -edge XANES may be an exception as discussed below. The absorption edge energy in the XANES readily indicates the oxidation state of elements. This particularly useful for elements with multiple oxidation states. In the glass samples, manganese is present as Mn2", iron as Fe 3", strontium as Sr2", zirconium as Zr 4", and plutonium as Pu 4÷. At the 170
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