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March 17, 2005

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PS11920.tex

Physica Scripta. Vol. T115, 928–930, 2005

XAFS and Molecular Dynamics Study of Natural Minerals, Analogues of Ceramics for Nuclear Waste Storage M. Harfouche1,* , F. Farges1,2 , J. P. Crocombette3 and A. M. Flank4 1 Laboratoire

des G´eomat´eriaux, Universit´e de Marne-La-Vall´ee, CNRS FRE 2455, 77454 Marne-La-Vall´ee cedex 2, France of Geological and Environmental Sciences, Stanford University, CA 94305-2115, USA 3 Commissariat a ` l’Energie Atomique (CEA/Saclay), CEREM, 91191 Gif sur Yvette, France 4 Laboratoire pour l’Utilisation du Rayonnement Electromagn´ etique, Bˆat 209D - B.P. 34 - 91898 Orsay Cedex, France 2 Department

Received June 26, 2003; accepted in revised form November 10, 2003

pacs numbers: 61.10.Ht, 89.60.−k, 61.82.Ms, 61.72.Bb, 83.80.Nb

Abstract

2. Experimental Methods

Natural actinides (U and Th) are harmful for the crystalline structure of natural minerals, due to their irradiation. Natural minerals can then become amorphous to X-ray diffraction (“metamict”) after being irradiated throughout a long period of time (108 years). Then, they are used as natural analogues of ceramics for nuclear waste storage. XAFS studies were performed in zircon, monazite and titanite to understand the effect of radiation damage on the local structure around Th, U, Zr and P and compared to available molecular dynamics (MD) simulations. In zircon, a local expansion around actinides (when substituting for Zr) is found. The radial expansion is a function of the metamictisation degree: up to ∼4 Å in crystalline zircon and larger in the metamict counterparts. Ab-initio calculations (FEFF7) were performed around Zr (∼23 000 sites) and around U (1000 to 3000 sites) in various crystalline and alpha-decay damaged zircon MD simulations. The calculated averaged EXAFS spectra confirms this expansion, which validates the use of the potentials used in the simulations as well as the alpha decay damage model considered in these MD simulations. Tetravalent actinides were found to be 8-coordinated in the undamaged structure, whereas their coordination drops to 7 in the damaged structures. In contrast to zircon, no local expansion around actinides in monazite was detected, despite some polymerization around P is measured (related to radiation damage). Finally, in some phases (such as titanite), actinides are found as oxyde-type clusters (ThO2 , UO2 ). Consequently, actinides do not “systematically” substitute for major actions in these structure, in contrast to the common belief in mineralogy.

A set of natural minerals that suffered radiation damage were investigated, including zircon (ZrSiO4 ), monazite (CePO4 ), zirconolite (CaZrTi2 O7 ) and titanite (CaTiSiO5 ). We have selected various samples of these minerals that contain significant amounts of natural actinides (Th and U from 0.01 to 30 wt.%). These samples (suffering severe radiation damage) are coming from various localities throughout the world (Sri Lanka, Madagascar, Brazil, South Africa, Canada etc.). They were thoroughly characterized using various methods (X-ray diffraction, electron microprobe, etc.) to differentiate between samples that suffered only from radiation damage to that suffered subsequent weathering. XAFS spectra for major elements were collected at the Si K- (zircon), P K- (monazite), Zr K- (zircon) and LIII/II edges (zircon) to help validate the structure of these materials simulated by molecular dynamics. Then, XAFS spectra were collected for minor to trace amounts of actinides (LIII -edges of Th and U) to understand the effect of radiation damage on the local structure of the remaining actinides. XAFS spectra for soft X-ray energies were collected at the SA32 spectrometer (SuperAco, Orsay, France) using a Ge(111) monochromator. We used total electron-yield detection to collect the spectra. Hard X-ray XAFS spectra (Zr K-edges and Th/U LIII -edges) were collected at SSRL (Stanford, USA) on beamline 11-2, using a Si(220) doublecrystal monochromator. We used a Stern-Heald type fluorescence detector (filled with Xe) with Sr filters to filter the elastic scattering. EXAFS spectra were collected with 0.05 Å−1 steps. Molecular dynamics simulations were performed for the zircon structure only, assuming several levels of radiation damage (recoil of 4 and 5 keV, respectively) with various U contents (1 atom, 4% and 12% of Zr atoms replaced by U). Simulations were performed using periodic conditions assuming 139,964 atoms. Details on the calculations (potentials etc.) are given elsewhere [2]. Ab-initio EXAFS calculations were undertaken using the FEFF7 code [3] at the Zr K-, and U LIII -edges on thousands of clusters provided by molecular dynamics and averaged to compare with the experiment.

1. Introduction Natural minerals may model long-term radiation effects by receiving very small amounts of radiation during hundreds of million years [1]. These mineral analogues offer a possibility to study the real effect of “in-situ” damage on the structure of ceramics receiving strong amounts of radiation. In the hope of identifying an adequate matrix to contain high activity nuclear wastes, a broad range of minerals of different structure is studied by XAFS methods (experimental and ab-initio XANES and EXAFS methods) and molecular dynamics. We studied the structural environment of several analogues such as zircon, monazite, zirconolite and titanite and compared experimental information to these reconstructed from molecular dynamics to (1) better understand the influence of radiation damage on crystals, (2) help validate the models used in molecular dynamics to simulate radiation damage and (3) provide accurate information on the way disorder effects influence the short-, and medium range environment probed by XAFS methods in these highly disordered materials.

* Corresponding author, e-mail: [email protected]

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3. Results and Discussion The Zr LII/III -edge XANES spectra of metamict zircon show a shift in the first EXAFS oscillation (Fig. 1) as compared to crystalline zircon. This confirms that Zr undergo a 8 → 7 coordination change with increasing radiation damage. Fourier Transforms of the Zr K-edge EXAFS spectra and their models (Fig. 2) show that the Zr-O average interatomic distance  C Physica Scripta 2005

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XAFS and Molecular Dynamics Study of Natural Minerals, Analogues of Ceramics

Normalize absorbance

Zr-O

Zr-Si Zr-Zr

Mud Tank (Australia)

Zr

Ural (Sri Lanka) ~5eV

2220

Zr

Synthetic (crystalline) Mud Tank (crystalline) Madagascar (metamict)

5 metamict

[7]

0

200-300 (Sri Lanka)

Theoritical (FEFF)

Crystalline

Naegy recuit (Japan)

2

Magnitude (FT)

[8]

Zr-Si

Zr-Zr

Naegy (metamict) 0

2240 2260 2280 ENERGY (eV)

0

2

4 6 R+φ (Å)

8

k χ(k)

Fig. 1. Zr LIII -edge XANES (left) and FT’s of Zr K-edge EXAFS (right; with k3 -weighting, k = 1.5–13.5 Å−1 and Kaiser-Bessel window) for crystalline and metamict zircons.

3

EXAFS spectrum

Magnitude (TF)

2 0 -2 4

8 –1 k (Å )

12

k χ(k)

0

3

Experimental Model

2 3

0 -2

2 4

8 –1 k (Å )

1

12 Zircon Naegy

0 0

2

4

6

8

R+φ (Å)

929

near 3.75 Å. The overall good agreement for the EXAFS models computed for the natural samples and the MD-simulations suggests that the MD models are representative of naturally occuring radiation damage. This agreement also confirm that disorder effects in the model of the experimental EXAFS for the next-nearest neighbors were properly taken into account (using cumulant expansion theory). Analysis of the P K-edge EXAFS spectra in monazites suggests that the P-P pairs near 2.5 Å increases significantly in magnitude with radiation damage. Then, some polymerization of the PO4 tetrahedra occurs as a consequence of increasing radiation damage. In contrast, any large structural reorganization due to radiation damage is observed around the other major elements such as Th. Concerning minor, to trace amounts of actinides (Th and U) in natural minerals, we studied first the insertion of these cations in the crystalline minerals before studying the radiation damaged ones (Fig. 3). In crystalline minerals, the analysis of the EXAFS spectra suggests that Th and U are 8-coordinated with Th-O and U-O at 2.41(2) and 2.38(2) Å. Therefore, these cations keep their preferred coordination environment, even if the main site they occupy (e.g., the Zr site in zircon) is not fully adapted (in size and in redox) to these actinides sizes. However, the analysis of the EXAFS for the medium range structure around minor amounts of Th and U (0.05–0.2 wt.%) in zircon shows the presence of relatively longer (Th,U)-Zr bonds (3.7 Å) as compared to the ones in the original zircon (3.6 Å). This suggests the presence of a structurally «expanded» region up to 4 Å from the central actinide. This expansion is due to the insertion of a larger actinide in the site of a smaller Zr (see model on Fig. 4). However, in the 4–5.5 Å range around the central actinide, a relatively «contracted» region is observed, based on the measured EXAFS distances compared to the ones in the original zircon structure. Above 5.5 Å, the undistorted original structure of the host mineral is observed (Fig. 4). These regions around actinides were also observed in the MD simulations, validating then our EXAFS interpretations. These results also help us understanding the point defects that are created in the zircon structure when minor actinides replaces major zirconium atoms

Fig. 2. Fourier transform of the EXAFS spectrum (inset, top) collected at Zr K-edge in a metamict zircon and the data modelling (inset, bottom).

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3

2

(metamict)

0 –2

Magnitude (FT)

decreases from 2.20 to 2.15 Å with increasing radiation damage, in agreement with [4]. The structure obtained from the MD simulations of the damaged regions in zircon is in agreement with the experiment for metamict zircons. In the damaged region, zirconium is surrounded, on the average, by 6.9 atoms at 2.14 Å (first neighbors are widely distributed from 2.0 to 2.4 Å). The average Zr K-edge ab-initio EXAFS was computed (using FEFF 7) from the MD simulation around Zr. The cumulant expansion model of that FEFF7 spectrum resulted in ∼7 atoms at 2.14 Å, suggesting that disorder effects were properly taken into acccount in the EXAFS models for the natural zircon samples. In addition, next nearest neighbors are observed around Zr in the EXAFS spectra for natural metamict minerals. These contributions can be modeled as (edge-shared) Si second neighbors near 3.10 Å and more distant (despite relatively noisy) Zr-Si contributions (near 3.73 Å). The EXAFS modelisations based on MD simulations for Zr in damaged zircon confirm the presence of Zr-Si contributions (at 3.15 Å) and Zr-Zr contributions

k χ(k)

(crystalline)

2

2

6 –1 k (Å )

10

Zircon Sri Lanka 100-1000 (crystalline)

1 Zircon Sri Lanka 100-1000 (metamict) 0

0

4

8

R+φ (Å) Fig. 3. FT’s of the U LIII -edge EXAFS (inset) spectra for zircons: crystalline (top) and damaged (bottom). Physica Scripta T115

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PS11920.tex

M. Harfouche, F. Farges, J. P. Crocombette and A. M. Flank ThO

3

Magnitude (FT)

k χ(k)

2

Titanite "Capelinha" (Brazil)

10 0

ThO2

5 Titanite "Capelinha" (Brazil)

-10

0

4

-1

k (Å )

Fig. 4. Schematic representation of the local structure around actinides in crystalline zircon, as observed from EXAFS experiments and MD simulations.

8

12

0

0

4

8

R+φ (Å)

Fig. 5. EXAFS spectra collected at Th LIII -edge in a titanite sample from Brazil compared to ThO2 and their Fourier transform.

4. Conclusions (see Fig. 4). The influence of radiation damage on the local structure around these actinides was measured using Th/U-LIII EXAFS spectroscopy and compared to MD simulations (considering a 4 and 5 keV cascade). The agreement between the experimental information and the MD simulations was also excellent, suggesting that the MD methods reproduce faithfully natural radiation damage. We have found, in addition, that in both experiment and MD simulation, tetravalent actinides (Th and U) are 7-coordinated (EXAFS-derived Th-O and U-O distanced averaging anharmonically at 2.37 and 2.35 Å). To our knowledge, this is the first time that such a coordination environment is reported for tetravalent actinides. In some other phases (titanite for instance), we observed that actinides do not replace a major cation in the structure but are present as actinide oxide-type clusters (ThO2 , UO2 ; see Fig. 5). We think that subsequent heat treatment can be at the origin of migration of the actinides from its host matrix (as substituted) to some oxide-type cluster.

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Combination of XAFS and molecular dynamics of radiation damage in minerals analogues of ceramics for nuclear waste suggests that MD simulates adequately radiation damage, consistent with the observation of very old (108 years) metamict minerals. Also, important and unique information about the complex structure of the metamict state was extracted as well as on the sites of actinide in minerals that are geochemically important phases for dating or tracing major geologic events (such as volcanology). Finally, a better understanding on the way disorder affects EXAFS spectra is obtained. References 1. Weber, W. J., Turcotte, R. P. and Roberts, F. P., Radiation Waste Mgmt. 2, 295 (1982). 2. Crocombette, J. P. and Ghaleb, D., J. Nucl. Mater. 257, 282 (1998). 3. Ankudinov, A. L., Ravel, B., Rehr, J. J. and Conradson, S. D., Phys. Rev. B 58, 7565 (1998). 4. Farges, F., Phys. Chem. Miner. 20, 504 (1994).

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