Solid Solubilities of Pu, U, Hf and Gd in Candidate Ceramic Phases for Actinide Waste Immobilization
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Phase Monazite,
REPO4
0.5 [1]
1 [2]
0.25#
Zircon,
ZrSiO 4
1 (coffinite)
0.1 [3]
1 [4]
Pyrochlore,
RE 2Ti20
1 [5]
2 [6]*
Zirconolite,
CaZrTi 20 7
0.7 [7]
Titanite,
CaTiSiO 5
0.05 [8]; 0.02#
Apatite,
Ca 2RE 8(SiO 4)60
Perovskite
CaTiO 3
7
1 [5] 0.7 [7]
0.05#
0.02#
0.02[9];0.5# 0.01[10];>0.1#
0.05 [9] 1 [11]
0.1[12];0.13#
*Forms monoclinic structure[6]; # Present results Many relevant data are already known for U and Pu solid solubilities in the above phases, and these are indicated in Table 1, together with values obtained in the present work. In oxide systems, applicable U and Pu valence states lie in the ranges +4 to +6 and +3 to +6 431 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
respectively, but here we are looking in the first instance at only U 4 ' and tri- and tetravalent Pu. Since solid solubilities of the different U and Pu valence states are not necessarily the same, a further aim was to define the valence states in appropriate systems by looking at the charge compensation schemes deduced from the microanalysis and in one case diffuse reflectance and near-edge X-ray absorption spectroscopies. Data also exist for Hf and Gd incorporation into the candidate phases -see Table 2, and values obtained in the present work are shown. Table 2. Known candidate ceramic phases richest in Gd and Hf, in f.u. Phase
Gd
Hf
Monazite
1
< 0.01#
Zircon Pyrochlore Zirconolite
< 0.1# 2 1.4[13]
1 0.3[13] 1
1 [14] 8[15] 0.3#
< 0.1# 0.5#
Perovskite Apatite Titanite # Values obtained in present work.
1
EXPERIMENTAL Samples were generally prepared by the alkoxide-route (see e.g. [12]), in which alkoxides and aqueous nitrate solutions are mixed thoroughly, stir-dried, calcined at - 750'C in an appropriate atmosphere (argon or air), wet-milled using ZrO 2 media in polystyrene containers, pelletised, and finally sintered at high temperatures in an appropriate atmosphere. Silicon and P were introduced as a 40 wt% colloidal solution (Ludox) and 85% H3P0 4 respectively. SThe main characterisation tool was scanning electron microscopy, using a JEOL 6400 instrument run at 15 keV, and fitted with a NORAN Instrument Voyager IV X-ray microanalysis System (EDS) which utilised a comprehensive set of standards for quantitative work, giving a high degree of accuracy[16]. Powder X-ray diffractometry was carried out with a Siemens D500 instrument, using Co Kct radiation. X-ray absorption near edge spectroscopy (XANES) was carried out on Line 4-2 at the Stanford Synchrotron Research Laboratory. UTi 20 6 and CaUO4 were used as valence standards. Diffuse reflectance spectroscopy was carried out using finely powdered samples in a Cary 5 instrument, and the data were then transformed to Kabelka-Munk plots of absorbance vs. wavelength. PREVIOUS WORK, RESULTS AND DISCUSSION U and Pu solubilities in Perovskite, Titanite, and Apatite U in the Ca site of perovskite, CaTiO3. Although tetravalent U can be accommodated in the B site of perovskites such as BaUO 3, actinide ions only occupy the A
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