Heavy ion induced damage in MgAl2O4, an inert matrix candidate for the transmutation of minor actinides

Radiation Meuuremenls

PERGAMON

RadiationMeasurements31 (1999) 507-514

HEAVY ION INDUCED DAMAGE IN MgAI204, AN INERT MATRIX CANDIDATE FOR THE TRANSMUTATION OF MINOR ACTINIDES T. WISS and Hj. MATZKE European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, Germany ABSTRACT Magnesium aluminum spinel (MgAI204) is a material selected as a possible matrix for transmutation of minor actinides by neutron capture or fmsion in nuclear reactors. To study the radiation stability of this inert matrix, especially against fission product impact, irradiations with heavy energetic ions or clusters have been performed. The high electronic energy losses of the heavy ions in this material led to the formation of visible tracks as evidenced by transmission electron microscopy for 30 MeV C60-Buckminsterfullerenes and for ions of energy close to or higher than fission energy (2°9Bi with 120 MeV and 2.38 GeV energy). The irradiations at high energies showed a pronounced degradation of the spinel. Additionally, Mg,A1204 exhibited a large swelling for irradiation at high fluences with fission products of fission energy (here I-ions of 72 MeV) and at temperatures < 500 *C. These observations are discussed from the technological point of view in the frame of using MgA1204 as an inert matrix for the transmutation of minor aclinides.

KEYWORDS Spinel (MgA1204); heavy ions; tracks; swelling; transmutation; transmission electron microscopy.

INTRODUCTION The elimination of the highly radiotoxic transuranium elements Np, Am and Cm, but also of the long living fission products (e.g. 99Tc, 1291)produced in nuclear fuels is a great challenge for the future of nuclear energy. One possible solution is to modify the reprocessing by separating these radiotoxic elements from the high-level nuclear waste (HLW) using a suitable chemical process (partitioning), followed by their transmutation by fission or neutron capture in power reactors. This is particularly effective if the elements to be transmuted are incorporated into a matrix that does not produce new actinides (inert matrix). The magnesium aluminum spinel, MgAI204, has gained general attention since it fulfils different criteria for inert matrices to be used in nuclear reactors: Spinel shows a thermal conductivity as good or even better than today's nuclear fuel, UO2, a small neutron capture crosssection (< 2.7 barn), a high melting point (2135 °C), a good behaviour under neutron irradiation (Clmard et al., 1984, 1985; Sickafus et al., 1995; Konings et aL, 1998) and a good compatibility with water (coolant) and cladding. The production costs and the reprocessing possibilities have also been considered. Among all these criteria, the radiation stability is one of the most relevant properties. During reactor operation a number of damage sources change the properties of the fuel: The matrix is exposed to important neutron fluxes, to the alpha decay of the incorporated actinides (or of the newly formed ones) and to the fission products resulting from the fission of these actmides. In the present study we have investigated the response of spinel against fission product impact by simulating the effect of these highly energetic ions through irradiations in large accelerators as suggested by Matzke (1994). These irradiations with high electronic energy losses (dE/dx)~ provide some new insights into the macroscopic and microstructural changes in irradiated MgAI204 and extend the available data for 1350-4487/99/$ - see frontmatter© 1999ElsevierScienceLtd.All rightsreserved. PII: S1350-4487(99)00113-4

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neutron irradiated samples and for low- and medium-energy (< 5 MeV) heavy ion irradiations (Turos et al., 1996a, b; Zinkle, 1995; Zinlde et al. 1998a; Matzke, 1996). EXPERIMENTAL

The bulk MgA1204 samples used were either single crystals of <100> or <110> orientation of lxl cm size or sintered pellets (diameter = 6 nurL height = 3 mm). These were polished down to 0.25 Ixm with diamond paste and annealed at 1000 °C in air in order to recover the damage resulting from the mechanical treatment. The fabrication procedure and the mechanical and thermodynamic characterization of the sintered pellets are described by Burghartz et aL (1998). The density of the sintered pellets was 95 % of theoretical. To provide transparent foils for transmission electron microscopy (TEM), 3 nun diameter disks were extracted from thin slices Of spinel single crystals and subsequently dimpled and ion milled (Ar+, 6 kV, 2.5 mA, angle of beam incidence 6 °) before irradiation. Both types of specimens were irradiated using facilities at different large accelerators. Some of the irradiation conditions used are listed in Table 1. The energy range varied from fission energy (72 MeV for 1-ions) up to energies of several MeV/amu (e.g. 2.38 GeV for Bi-ions). Different fluences were also obtained depending on the accelerator type and on the time available for the irradiations. The energy losses (dE/dx)e were calculated using the TRlM96-code (Ziegler et al., 1985) using displacement energies Ed of 30, 30 and 59 eV for Mg, AI and O, respectively, as given by Vladimirov et al. (1998). The evaluation of the linear rate of electronic energy deposition for the C6o clusters was estimated to be the sum of the (dE/dx)o of 60 individual carbon atoms as suggested by Jensen et al. (1998). The irradiated samples were investigated with optical microscopy (WILD M8, LEICA), scanning electron microscopy (SEM PHILIPS 515), transmission electron microscopy (HITACHI 1-1700 STEM), and profilometry measurements were performed using a Hommel tester T10 G-2 from HOMMELWERKE. Table 1. Irradiation conditions of some spinel specimens. Type of specimen

Ion

Energy, MeV

(dE/dx)o, keV/nm

Fluence (ions/cm2)

Temperature

Accelerator

MgAI204

84Kr

742

11.7

3-1013

RT

GANIL 1

MgA1204 <110>

1271

72

16.5

1017

RT

TASCC 2

MgAI204

127I

72

16.5

1016

500 °C

TASCC

MgAI204

127I

72

16.5

1016

900 °C

TASCC

MgAl204

1271

72

16.5

1016

1200 °C

TASCC

MgA1204

ll6Sn

403

22.9

3.2.1013

RT

GANIL

MgAl204

129Xe

250

24

8.1013

RT

ISL3

MgAl204 <100>

2°9Bi

120

23.2

5.101°

RT

GSI4

MgA1204 <100>

2°9Bi

2380

33.8

5. l01°

RT

GSI

MgAl204

C6o

30

65

5"1010

RT

TANDEM IPN5

i GANIL, _GrandAcc61&ateur National pour les Ions Lourds, Caen, France 2 TASCC, Tandem Accelerator Superconducting (~yc~otron,Atomic Energy Company Limited, Chalk River, Canada 3 ISL, Ionens_trahl!al~or,Berlin, ~ n a n y 4 GSI, Geselischafl flit Schwerionenforschung , Darmstadt, Germany TANDEM IPN, Tandem _Institutde Physique N_ucl6aire,Orsay, France (see also acknowledgements).

T. gqss, H. Matzke / Radiation Measurements31 (1999) 507-514

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RESULTS a) S E M and optical microscopy

The surfaces of the irradiated bulk specimens were studied with optical and scanning electron microscopy. Figure la-c shows a set of pictures taken by optical microscopy of three sintered spinel samples irradiated with 1016 I-ions/cm2 of 72 MeV at room temperature, 500 °C and 900 °C, respectively. The irradiated circular areas are clearly visible on the 5 mm diameter pellets. The room temperature irradiation caused a large "pop-out" of the implanted area. This "pop-out" decreased in height with increasing temperature during irradiation. All the spinel samples listed in Table 1 irradiated with iodine-ions with an energy typical for a fission fragment showed this type of feature except the one irradiated at 1200 °C. Figure ld is a SEM micrograph of a MgAl204 sample irradiated with 403 MeV Sn-ions to a fluence of 3~2"10~3 ionsdcm2. The irradiated haft of the pellet shows a large degradation, surface roughening and loss of chips. The same surface damage was observed for the 250 MeV Xe-ion irradiation at about the same fluence. However, the specimen irradiated with 742 MeV Kr-ions at 3.10 ~3 ions/cm 2 showed only a dark coloration of the otherwise undamaged surface.

Fig. 1. a, b, c) Optical micrographs of spinel pellets irradiated at a fluence of 1016ions/cm2 with 72 MeV I-ions at room temperature, 500 °C and 900 °C, respectively and d) SEM micrograph of a spinel sinter (6 mm diameter) partially trradiated (right part) with 403 MeV 116Sn-ionsat a fluence of 3.2.1013 ions/cm2.

b) Swelling measurements

The swelling of the samples irradiated with ions of fission energy (72 MeV iodine ions) and at fluences above 10 ~3 ions/cm 2 was measured by profilometry of the sample surface. It can be seen by the naked eye. The swollen area has the shape of the mask used during the irradiation, i.e. a cylinder is emerging from the sample ("pop out"). The swelling was determined from the measured height of the "pop-our' and from the calculations of the ion range using the TRIM96 code, assuming that swelling is uniform throughout the range of the ions.

510

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Figure 2 shows the swelling measurements as a function of temperature and fluence. Swelling is larger for sintered specimens than for single crystals for the same irradiation conditions and it reaches saturation for fluences around 1017 ions/cm2, At 1200 °C, no measurable swelling was observed for an irradiation fluence of 1016 ions/cm 2. The inset a) in Fig. 2 shows the profile recorded for the spinel irradiated at room temperature at a fluence of 1017 ions/cm2, and inset b) shows the corresponding optical micrograph of the pop-out.

40 20 200 100 500 20 500 900 1200

•

>

•

35

•

• []

<~ 30

..=

v o

25

]

°C < 1 1 0 > sc| /

°c <100> sc/ ~ °C °C °C °C °C

m

i

°C < 100> sc/ ~ i ~ : i

.........

< 100> sc Profileof an implantedarea sinter sinter sinter sinter

Optical micrographof an implanted area

r/)

~D

20 15 10 5 0

- ..................................................................................

•

-5

•

1

•

•

unnnmmII

10

•

un

•

•

n m n m II

100

O ...............................

•

•

•

•

unnnnnnuII

nu

l

m n II n n n l l

1000

10000

f l u e n c e , x 1013 i o n s / c m 2 Fig. 2. Swelling of different spinel samples (sc = single crystal) irradiated with 72 MeV I-ions at different temperatures and different fluences. Inset a) shows the recorded profile of the single crystal irradiated to a fluence of 1017 ions/cm2 at room temperature. Inset b) is the optical micrograph showing clearly the ,pop-out" of the same implanted area. c) Microstructure analysis

Tracks of 30 MeV Buckminster fuUerenes have been observed by transmission electron microscopy (see Fig. 3a). The shape of the tracks indicates that the initial fullerene has split frequently into 2 fragments and sometimes into 3, as also observed by Dunlop et al. (1997) in yttrium h-on garnet (YIG). The mean track diameter is about 20 nm corresponding to the juxtaposition of two tracks of 10 nm diameter due to the two fragments. The samples irradiated with Bi-ions also show clearly visible tracks of 3.5 + 0.5 um diameter for both the 2.38 GeV ions and for the 120 MeV ions (see Figs. 3 b to d). In very recent work, Zinkle and Skuratov (1998b) have reported similar track diameters in sintered spinel (2.0 and 2.6 nm for 430 MeV Kr and 614 MeV Xe-irradiations, respectively).

T, if'us, H. Matzke / Radiation Measurements 31 (1999) 507-514

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Fig. 3. TEM bright field images of prethirmed MgA1204 irradiated with a) 30 MeV C60 clusters, b, c) 2.38 GeV Bi-ions and d) 120 MeV Bi-ions. In all three cases the irradiation fluence was 5.10 j° ions/cm2. The irradiations were performed at room temperature with the ion beam normal to the surface. The sample in Fig. 3c was tilted to better illustrate the 3D nature of the tracks.

DISCUSSION Spinel shows a good resistance to void formation under neutron irradiation (for fluences up to 2.3.10 26 n/m2 with ~ > 0.1 MeV at temperature up to 1100 K) as summarised by Konings et al. (1998), and to void swelling (Kinoshita et al., 1995). The physical reasons are understood, e.g. by the formation of faulted loops in spinel acting as less effective traps for interstitials than the unfaulted loops in A1203 which shows unacceptably large swelling (Matzke, 1982). Also Sickafus et al. (1995) reported that interstitial-vacancy recombination is a highly efficient point defect annihilation mechanism in neutron irradiated spinel (fluences > 5.1026 n/m 2, F_~ > 0.1 MeV, T = 1029 K, dpa > 50). Low and medium energy ion irradiations were also performed on spinel. Using ion channeling techniques and MgAI204 single crystals, Turos et al. (1996b) showed that the AI and O sublattices can be heavily disordered whereas the Mg sublattice is more stable (see also Matzke, 1996). R. Devanathan et al. (1996) gave evidence for spinel amorphization by performing irradiations at cryogenic temperature (100 K) with

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400 keV Xe-ions up to doses of 1016 iouslcm 2. Zinkle and Pells (1998b) measured a step-height swelling of 0.8 % on polycrystalline spinel irradiated with 1017 Ar-ious (EAr = 4 MeV, T = 200 K). However, MgAI204 used as an inert matrix in nuclear reactors will not experience such low temperatures. Rather higher temperatures will be reached, at or above the defect recovery stage occuring at about 500 °C as found by Turos et al. (1996b) for spinel single crystal implanted with 10~5 Kr-ions/cm2 with 190 keV energy. These investigations confirm the good radiation resistance of spinel exposed to neutrons and the relatively good resistance against low and medium energy heavy ions. However, the crucial point for irradiation in fission reactors for transmutation of actinides is the behaviour against fission products (i.e. heavy ions with dE/dx~ values which are large in comparison with those of the above mentioned conditions). SEM examination of the surfaces of spinel samples irradiated with heavy ions at high (dE/dx), (> 16.5 keV/nm, energies > fission energy) at fluences above 10~3ions/cm 2 revealed a poor behaviour (see Fig. ld). The surface roughening of the samples is attributed to Coulomb explosion spikes due to the high charge state of the impinging ions (i.e. 35+ for 116Sn). The specimens irra___diatedwith ions of fission energy (i.e. 72 MeV iodine-ions, charge state +7) were negligibly roughened. However, an important macroscopic effect of these irradiations was the swelling, which reached about 30 % on the sintered specimens irradiated at room temperature for a fluence of 10~6 ions/cm2 (see also Wiss, 1997b). Significant swelling was found already for rather low irradiation fluences (1013 ions/cm2). However, it must be considered that under reactor irradiation conditions (i.e. T _> 500 °C), the swelling can be expected to become more acceptable as indicated by the present experiments with spinel heated to 500, 900 and 1200 °C during the irradiation (see also Wiss and Matzke, 1998). The observed difference in swelling between single crystals and sintered material could be attributed to larger segregation of the created point defects in sinters or to their less effective recombination, the grain boundaries acting as sinks for interstitials (Hobbs, 1994). The answer on how the defects are created along the ion path has been sought by studying the modification of the microstructure of spinel irradiated with swift heavy ions at low fluence. Fig. 3 shows clearly that track formation occurs for (dE/d.x), >_ 23.2 keV/nm. As expected from our experiments, Zinkle, Matzke and Skuratov (1998c) give evidence of track formation in spinel irradiated with 72 MeV I-ions (dE/d,~ =16.5 keV/nm). By increasing the fluence above 1015 ions/cm2, the material turns amorphous along part of the range. If amorphization is the main cause of the swelling, the values shown in Fig. 2 are thus lower limits since uniform swelling along all of the range was assumed when calculating these values. As shown in Fig. 3a, the C6o clusters produced very large tracks. Distinction should be made between these latter and the tracks obtained for single atoms. The velocity of the 30 MeV fullerene ions is about 13 times lower than that of the 120 MeV Bi-ions, while the corresponding (dE/dx), is about three times higher. The velocity ratio reaches about the same value (14x) between the 2.38 GeV Bi-ions and the 120 MeV Bi-ions while the corresponding (dE/dx), differs only by one third. At a given (dE/dx),, the volume density of energy deposited increases while the velocity decreases. This argument can explain that tracks with the same diameter were produced by the Bi-ions at the two mentioned energies. It is even more evident when comparing the large fullerene-tracks with the 2.38 GeV Bi-tracks. The (dE/dx), of the C6o clusters is three times higher than that of the Bi-ions while their velocity is 190 times lower. However, the nature of the track is not yet clear. In the TEM work of Zinlde et al. (1998b, c) there is no evidence whether the track core is amorphous or whether overlapping of tracks is necessary to induce amorphization. The mechanisms for track formation should be carefully further investigated by both high resolution TEM and by theoretical treatments of the existing data. The thermal spike model of Toulemonde et al. (1995) has been shown to quantitatively describe measured track diameters in different insulators following irradiations with swift heavy ions (e.g. in UO2, Wiss et al., 1997a). We will apply this and other models to the present results in continued research. CONCLUSION Evidence was given that spinel undergoes damage under irradiation with ions or clusters in the electronic energy loss regime. Compared to the rather good radiation resistance against neutrons or low energy ions, swelling and track formation caused by fission products of fission energy are at first sight

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an argument against the use of spinel (as an inert matrix) for transmutation of minor actinides. However, the observed temperature effect could be used to design more swelling-resistant fuels. Also, a dispersion of large actinide particles or of actinides incorporated into another type of radiation resistant ceramic particles which then are incorporated into a spinel inert matrix could be a second way to use its resistance against neutron irradiation in an effective way. Work in this direction has been started (Chauvin et al., 1998).

Acknowledgements - The authors thankfully acknowledge the help of J. Vetter and C. Trautmann at GSI Darmstadt, P. Lucuta and R. Verrall at AECL Chalk River, M. Toulemonde at GANIL Caen, A. Dunlop at TANDEM IPN Orsay, and S. Klaumllnzerat ISL Berlin for performingthe irradiations. Thanks are also due to W. Huber and V. Meyritzfor specimenpreparation.

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