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Materials Science Plutonium Futures—The Science Materials Science Materials Science 91 Poster Session 92 P u F u t u r e s — T h e S c i e n c e XANES and EXAFS Studies of Plutonium(III, VI) Sorbed on Thorium Oxide The study of plutonium sorption mechanisms on mineral surfaces seems very R. Drot, interesting considering radwaste management and, in particular, for underground E. Ordonez-Regil, disposal after advanced reprocessing. Indeed, radionuclides retention processes E. Simoni on engineered or geological barriers could enhance retardation. Thus, it appears Institut de Physique important to understand such phenomena at both the macroscopic and molecular Nucléaire, Université Paris Sud, Groupe de levels.1,2 In this context, as plutonium is a very radiotoxic element, it is necessary Radiochimie, to model its behavior in the geosphere and its interaction with natural minerals. 91406 Orsay Cedex, Such an investigation is complicated by the existence of several oxidation states France for plutonium. Then, sorption mechanisms could be different considering Pu(III), Ch. Den Auwer, (IV), or (VI). This work deals with the influence of the oxidation state of pluto- Ph. Moisy nium on the structure of the sorbed complex. X-ray absorption spectroscopy has CEA Marcoule, DCC/ been chosen to perform this study. We have considered thorium oxide as a reten- DRRV/SEMP, Valrhô, tion matrix because, on one hand, it is a sparingly soluble material; thus, dissolu- 30207 Bagnols-sur- tion processes of the solid can be neglected during sorption experiments. On the Cèze, France other hand, only one type of sorption site is expected, which allows us to simplify the study. Thorium oxide has been obtained by heating thorium oxalate at 900°C. Its specific 2 -1 surface area measured by the N2-BET method is around 5 m .g . The synthesized compound has been characterized by X-ray powder diffraction. Moreover, a thorium oxide doped plutonium(IV) (Pu/Th around 0.01) has been synthesized and characterized as well. This later compound has been used as a reference solid for extended X-ray absorption fine structure (EXAFS) analysis. In order to avoid Pu(OH)4 colloid formation or precipitation, it is necessary to adjust the pH of the aqueous phase to very low values. But for such conditions, no retention phenomenon can occur. Therefore, we have only performed experiments with Pu(III) and Pu(VI). Plutonium solutions were prepared from a Pu(IV) stock solution. The Pu(III) solution was prepared by reducing Pu(IV) with zinc powder while the Pu(VI) solution was prepared by oxidizing Pu(IV) with ammonium persulfate. Then, these two solutions were respectively adjusted to pH = 5 and 3 for Pu(III) and Pu(VI) with a 0.1 M NaOH solution, and the concentrations were determined by spectrophotometry. The initial concentrations were around 5.10-3 M. Sorption experiments were performed by shaking, for 3 hours, a weighed amount of ThO2 (previously equilibrated with the aqueous phase) with the plutonium solutions. Then the supernatant was removed, and the solid was dried at room temperature. Both samples were pressed into pellets and sealed with Kapton tape, which served as an X-ray transparent window. The spectroscopic investigation was performed at the Stanford Synchrotron Radiation Laboratory. X-ray absorption near-edge structure (XANES) and EXAFS spectra were collected at the Pu LIII absorption edge. EXAFS data were extracted from the raw absorption spectra by standard methods using in-house codes,3 and single scattering approximation was used for the quantitative fitting procedure. The backscattering phases and amplitudes have been extracted from the initially CP532, Plutonium Futures – The Science, edited by K. K. S. Pillay and K. C. Kim, © 2000 American Institute of Physics 1-56396-948-3/00/$17.00 93 Poster Session calculated EXAFS spectrum of the model solid, ThO2 doped Pu (1%), using FEFF7 code.4 The obtained results indicate that the plutonium is sorbed as an inner- sphere complex. By studying the XANES part of the X-ray absorption spectra, it is possible to identify the oxidation state of sorbed plutonium. Previous published 5 works have shown that the position of the Pu LIII white line depends on the oxidation state of the plutonium species. The obtained results show that the plutonium is not sorbed at the same oxidation state as when it was initially introduced in solution. This seems to indicate that, in our experimental condi- tions, no oxidation or reduction phenomena occur in the electrical double layer. References 1. Manceau A. et al., Applied Clay Science, 7, 201 (1992). 2. Drot R. et al., Langmuir, 15, 14, 4820 (1999). 3. Michalowicz A., Thesis, Université du Val de Marne, France, 624 pp. (1990). 4. Ankoudinov A., Thesis, Washington University, 119 pp. (1996). 5. Conradson S. D. et al., Polyhedron, 17, 4, 599 (1998). 94 P u F u t u r e s — T h e S c i e n c e Effects of Fission Product Accumulation in Cubic Zirconia* The disposition and disposal of plutonium from the dismantlement of nuclear L.M. Wang, weapons and from the reprocessing of the commercial nuclear fuel have lead to S.X. Wang, S. Zhu increased interest in the possibility of “burning” actinides in non-fertile or so- R.C. Ewing, called inert-matrix fuels (e.g., fuels without 238U).1,2 This strategy prevents the University of Michigan, further production of plutonium by neutron capture reactions and subsequent Ann Arbor, MI 48109, USA beta decay. The recently adopted strategy in selecting materials for inert matrix fuel has been to consider not only the properties of the material as a fuel, but also the waste form properties of the inert-matrix fuel. Such an approach would allow direct disposal without reprocessing after once-through burnup. Yttria stabilized cubic-zirconia (YSZ) is a promising candidate material for both inert matrix fuel and waste form based on its high solubility for actinides, high chemical durability and its reported exceptional stability under radiation.3,4 Because the incorporation of fission and other transmutation products during burnup may significantly affect the radiation response and the chemical durabil- ity of cubic zirconia, the solubility and mobility of the fission product nuclides in the inert matrix fuel at high temperatures (reactor fuel conditions) and low temperatures (repository conditions) are important. In this study, we have investigated the effects of fission product incorporation on the microstructure of YSZ (with 9.5 mol. % of yttria) by ion implantation (using 70-400 keV Cs+, Xe+, Sr+ and I+ ions) and transmission electron microscopy (TEM). The ion implantation was conducted in a temperature range between 300 to 873 K to doses up to 1x1021 ions/m2. In situ TEM was conducted on pre-thinned TEM samples to follow the microstructure evolution during ion implantation using the IVEM-Tandem Facility at Argonne National Laboratory. Cross-sectional TEM was performed after implantation of the bulk samples to reveal the depth-dependent microstructure induced by ion implantation. In situ TEM during the 70 keV Cs+ implantation at the room temperature revealed a high density of defect clusters on the nanometer scale after ~2x1020 Cs/m2. The defect clusters with characteristics of interstitial type dislocation loops are interpreted to be the result of planar precipitates of Zr and/or O interstitials displaced from their original lattice site by the collisional events. Amorphous domains in thin regions of the specimen were observed after 1x1021 Cs/m2 with high resolution TEM (HRTEM) and nanobeam electron diffraction (Fig. 1). Figure 1. Plan-view high resolution TEM micrographs from various regions of a cubic zirconia sample (stabilized by 9.5 mol. % of yttria) after 70 keV Cs+ implantation to 1x1021 Cs/m2 at room temperature. (A) from a region with ~7 at.% Cs; (B) from a region with ~11 at.% Cs. CP532, Plutonium Futures – The Science, edited by K. K. S. Pillay and K. C. Kim, © 2000 American Institute of Physics 1-56396-948-3/00/$17.00 95 Poster Session Cross-sectional TEM of a specimen after 400 keV Cs+ implantation to 1x1021 Cs/m2 at the room temperature has revealed an amorphous band in a depth range where Cs concentration is greater than 8 at.% (Fig. 2). Although the front edge of the amorphous layer overlaps with the displacement damage peak that reached 330 displacement per atom (dpa), we suggest that the amorphization is mainly due to the incorporation of Cs rather than from the displacement damage as the center of the amorphous layer overlaps with the peak Cs concentration. This interpretation is consistent with previous results of radiation damage studies in YSZ that reached damage levels as high as 680 dpa but without amorphization.3 Amorphization of YSZ is caused by the large size incompatibility and low mobil- ity of cesium ions in the YSZ structure at room temperature, reflecting a relatively low solubility of Cs in YSZ. Nevertheless, the Cs concentration at which amorphization of YSZ occurred (~8 at. %) is well above the value that will likely be reached in an inert fuel matrix (~5 at. % assuming a 30 at.% Pu loading). Figure 2. Cross- sectional bright- field TEM micrograph with associated electron diffraction pattern showing the formation of an amorphous band in a cubic zirconia sample (stabilized by 9.5 mol. % of yttria) after 70 keV Cs+ implantation to 1x1021 Cs/m2 at room temperature. The overlay of the Cs concentration profile is based on the results of analytical TEM that has been normalized by the results of a Monte Carlo computer simulation using the SRIM code. 96 P u F u t u r e s — T h e S c i e n c e Various types of nanometer-scaled defect clusters (e.g., dislocation loops or small gas bubbles) were apparent in samples implanted by the other three ions after 1x1020 ions/m2.
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