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Characterization of radiation damage in inert matrices for transmutation of nuclear waste

C. Dalmasso(1), P. Iacconi (1), M. Beauvy (2), D. Lapraz (1), E. Balan (3), G. Calas(3)

(1) Laboratoire de Physique Electronique des Solides - Centre de Recherche sur les Solides et leurs Applications (LPES-CRESA) EA 1174,

Université de Nice-Sophia Antipolis, Parc Valrose – 06108 Nice Cedex 2, FRANCE (2) CEA Cadarache (DEN/DEC) – 13108 Saint-Paul-Lez-Durance Cedex, FRANCE (3) Laboratoire de Minéralogie-Cristallographie de Paris (LMCP) CNRS-UMR 7590

Université Pierre et Marie Curie – 4 place Jussieu – Case postale 115 – 75252 Paris cedex 05, FRANCE Email : chrystelle.dalmasso@unice.fr

Abstract – The radiation damage produced by energetic heavy ions in several ceramic oxides was characterized by different methods: thermoluminescence, optical absorption, fluorescence and electron paramagnetic resonance. Following ion irradiation, the thermoluminescence intensity of all the samples is shown to decrease. This can be related to the observed rise of the optical absorption in the whole wavelength range. Moreover, optical absorption and electron paramagnetic resonance measurements highlight the appearance of induced defects. The ones observed in the paramagnetic resonance spectra of irradiated pellets seem to be holes trapped on oxygen-ions. The concentration of these induced defects increases with ion fluence and fluorescence measurements indicate that some pre-existing defects such as F+ and F2

2+ centers in alumina follow the same trend. INTRODUCTION The transmutation of highly radiotoxic minor actinides Np, Am and Cm and living fission products such as 99Tc appears as an effective waste management solution for the future. To develop this technique, the knowledge of the behavior of transmutation targets in reactor is necessary. These targets are composed of nuclear waste embedded in an inert matrix, which is designed for containing the different elements produced by the transmutation and withstanding the energy generated. Therefore, the material selected as possible inert matrix has to fulfill a certain number of criteria such as high melting point, good thermal conductivity, suitable mechanical properties, low neutron cross-section, compatibility with the coolant and the cladding and good behavior under radiation effects [1]. Since they respond to a certain number of these criteria, several ceramic oxides are good candidates. The purpose of this paper is to study the stability of three oxides under irradiation by fission products: alumina α-Al2O3, one of the standard materials for the science of ceramics, magnesium aluminate spinel MgAl2O4 used as inert matrix for the majority of irradiation tests in reactor and magnesia MgO chosen for the future tests in reactor (e.g. experiments ECRIX, MATINA, CAMIX-COCHIX and FUTURIX-TA in Phenix [2], and EFTTRA T5 in HFR [3]). To simulate the impact of fission products, several samples of

these materials have been irradiated with highly energetic heavy ions. The radiation damage, especially point defects, was investigated using different characterization techniques namely thermoluminescence (TL), optical absorption, fluorescence and electron paramagnetic resonance (EPR). EXPERIMENTAL METHODS Irradiation of the Samples The investigated samples were disks of about 1mm height cut off from cylindrical sintered pellets of α-Al2O3, MgAl2O4 and MgO (diameter = 5 mm for magnesia and 8 mm for alumina and magnesium aluminate spinel). These disks were polished on one face (down to 1µm diamond powder) before annealing at 950°C during four hours, and they were irradiated at CIRIL-GANIL (Centre Interdisciplinaire de Recherche Ions Lasers - Grand Accélérateur National d’Ions Lourds, Caen, France) with energetic heavy ions (365 MeV 82Kr and 540 MeV 86Kr). TABLE I. Irradiation conditions of the samples irradiated with 540 MeV 86Kr Fluence (ions/cm²) (dE/dx)e (keV/nm) 5×1010

1×1011

1×1012

1.65×1014

14.6 for Al2O3 13.5 for MgO 13.2 for MgAl2O4

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Irradiation conditions are summarized in Tables I and II. Different fluences were obtained and the energy losses (dE/dx)e were calculated using the TRIM-code (version: SRIM-2000) [4] for each material. TABLE II. Irradiation conditions of the samples irradiated with 365 MeV 82Kr Material Fluence

(ions/cm²) (dE/dx)e (keV/nm)

α-Al2O3 4×1012 (2.0, 2.8, 4.0)×1013

15.9

MgO 4×1012 (2.0, 2.8)×1013

1×1014

14.8

MgAl2O4 4×1012 (2.0, 2.8)×1013

14.4

Characterization Techniques TL Measurements TL measurements have been made between room temperature and 700 K for MgO and MgAl2O4 (high-temperature TL), and from liquid nitrogen temperature (77 K) to 600 K (low-temperature TL) for α-Al2O3. The temperature of the crystal was increased linearly with an heating rate of 0.5 K/s. The TL spectra were recorded with a Philips photomultiplier (XP-2018B for low-temperature TL, XP2262B for high-temperature TL). For each sample, the thermoluminescence curves were recorded prior and after ion bombardment. X-ray irradiations were performed at room temperature using an Inel tube (IRG 3000) operated at 45 kV and 1 mA with a dose rate of 6.48 Gy/min for high-temperature TL. For low-temperature measurements, X-ray irradiations were made with a Philips tube operated at 45 kV and 2 mA at 0.59 Gy/min. Optical Absorption Measurements Optical absorption measurements were performed at room temperature using a Cary 500 spectrophotometer (Varian). The spectra were recorded on the wavelength range 200-800 nm with a scan-rate of 600 nm/min. For all the samples, optical absorption was measured before and after ion bombardment. Each difference curve was obtained by subtracting the absorption spectrum of an unirradiated crystal from the spectrum of the same pellet after krypton bombardment. This enables us to highlight probable induced absorption bands.

EPR Measurements EPR measurements were carried out at LMCP (Laboratoire de Minéralogie-Cristallographie de Paris, France). The paramagnetic resonance spectra were measured at room temperature with a Bruker ESP300E spectrometer operating in the X-band frequency range (~ 9.7 GHz) with a 100 kHz field modulation of 5.10

-4 or 1.10

-3 T. The

microwave power was set at 10 or 40 mW. Additional spectra were recorded at 140 K, using a cooled nitrogen flow device. Magnetic field calibration was performed with the DPPH standard (g = 2.0037 ± 0.0002). RESULTS TL Measurements The comparison of the TL curves recorded before and after krypton bombardment shows a decrease of the main TL glow peak area following ion irradiation. Besides, the more important the fluence is, the more significant the decay of the TL glow peak area is. These results are is illustrated in figure 1.

Fig. 1. Area of the main TL glow peak of α-

Al2O3, MgO and MgAl2O4 samples as a function of Kr ion fluence

Furthermore, this graph also indicates that the variation of the TL glow peak area is almost the same for the three oxides: the area drops sharply for low fluences (5.0×1010 to 1.0×1011 ions/cm²) ; then it reaches a plateau until a krypton fluence of above 2.0×1013 ions/cm² prior to decreasing slightly for high fluences. Optical Absorption and EPR Measurements Optical absorption measurements reveal an increase of the crystal absorption in the whole wavelength range after ion irradiation. Moreover,

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figure 2, representing the difference absorption spectra of several MgO pellets irradiated to different krypton fluences, shows that the absorption increases with the fluence. Besides, the graph highlights the appearance of new optical absorption bands. For instance, three significant induced absorption bands are observed at 265, 355 and 574 nm. These new absorption bands suggest the formation of induced defects.

Fig. 2. Difference absorption spectra of MgO samples irradiated to different Kr fluences ΦKr

(in ions/cm²) Figure 3 represents the room temperature EPR spectra of two MgAl2O4 samples irradiated to 5×1010 and 1.65×1014 ions/cm² respectively. This graph clearly shows a signal related to a radiation induced paramagnetic defect at high fluence. The corresponding gyromagnetic tensor displays an orthorhombic symmetry with principal g values observed at gx=2.008, gy=2.005 and gz=2.023. Concerning alumina and MgO, a new defect with an axial symmetry and g values > 2, characteristic of hole centers, had been identified after irradiation. The concentration of this defect is observed to increase with krypton fluence. DISCUSSION The observed decay of the TL intensity after ion bombardment can be related to an increase of the optical absorption. This suggests that photons produced during radiative recombination and responsible for the TL emission are partially absorbed by induced defects. This hypothesis is borne out by optical absorption measurements. However, other more complicated mechanisms are likely to be involved in the decay of the TL intensity. For instance, Plaksin et al. [5] have measured the radioluminescence (RL) of polycrystalline samples of alumina irradiated

with 253 MeV krypton (4.3×108 ions/cm²) and also observed a decay of the luminescence after bombardment. They attribute this to charge carrier diffusion and recombination at grain boundaries which may prevent accumulation of these carriers at traps inside the grains. Moreover, oxygen vacancies may diffuse to grains and this may cause the decay of the RL. This diffusion is stimulated by ion irradiation for example due to thermal spikes produced in ion tracks. For Al Ghamdi and Townsend [6], in spite of the fact that new defects are created by ion bombardment, the luminescence efficiency is reduced by either large defect complexes, or stress introduced by implantation.

Fig. 3. Room temperature EPR spectra of MgAl2O4 samples irradiated to 5×1010 and

1.65×1014 ions/cm² The appearance of new optical absorption bands after ion bombardment proves the formation of induced defects. Several of them have been identified. In particular, the 465 nm absorption band in α-Al2O3 spectra was related to F2

2+ centers. This is confirmed by fluorescence measurements since the concentration of F2

2+ centers is shown to increase with ion fluence. The induced absorption band recorded at 255 nm in alumina spectra is due to F+ centers. These are also observed in MgO and MgAl2O4 samples at 265 and 260 nm respectively. Furthermore, the 355 and 574 nm absorption bands were observed by Chen et al. [7] in neutron-irradiated MgO crystals. They attribute them to defects formed in displacement spikes created by the neutrons. Besides, the calculated EPR g values (g > 2) suggest that some of the defects created are electronic hole centers trapped on oxygen-ions, thus forming O- ions, neighboring an impurity or

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a cation vacancy. Nevertheless, further studies are necessary to precise the nature of the defect. Finally, EPR measurements indicate also that the samples contain some transition-metal impurities at relatively low concentration (total impurities <100 ppm). Namely, the six hyperfine structure lines characteristics of Mn2+ ions (S=5/2, I=5/2) have been observed in the MgO spectra. In addition to that, triply-charged state chromium ions have been found in MgO, alumina and probably in spinel. This result confirms TL and fluorescence measurements which also highlight the presence of Cr3+ in all the samples. These impurities are substituting for trivalent Al3+ ions in α-Al2O3 and for divalent Mg2+, likewise Mn2+ impurities, in MgO. The intensity of the measured EPR signals suggests that the impurities are present in concentrations of few to few tens of ppm, and that confirms the high sensitivity of the characterization techniques used. The investigation of the radiation stability will be continued with a focus on the influence of other irradiation parameters (temperature, …). ACKNOWLEDGEMENTS The authors would like to thank GDR Nomade and CNRS-IN2P3 for financial support. REFERENCES 1. N. CHAUVIN, T. ALBIOL, R. MAZOYER,

J. NOIROT, D. LESPIAUX, J. C. DUMAS, C. WEINBERG, J. C. MENARD, J. P. OTTAVIANI, J. Nucl. Mater., 274, 91 (1999).

2. Y. CROIXMARIE, personal communication 2004 (CEA internal Reports 1999-2004).

3. D. WARIN et al., 5th OECD/NEA Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation, SCK-CEN, Mol, Belgium (1998).

4. J.F. ZIEGLER, J.P. BIERSACK, U. LITTMARK, The Stopping and Range of Ions in Solids, Pergamon Press, Oxford (1985).

5. O.A. PLAKSIN, V.A. STEPANOV, P. V. DEMENKOV, P.A. STEPANOV, V.A. SKURATOV, N. KISHIMOTO, Nucl. Instr. and Meth. B, 206, 1083 (2003).

6. A. AL GHAMDI, P.D. TOWNSEND, Nucl. Instr. and Meth. B, 46, 133 (1990).

7. Y. CHEN, R.T. WILLIAMS, W.A. SIBLEY, Phys. Rev., 182, 960 (1969).