The Raman fingerprint of plutonium dioxide: Some example applications for the detection of PuO2 in host matrices

Abstract Some example applications are presented, in which the peculiar Raman fingerprint of PuO2 can be used for the detection of crystalline Pu4+ with cubic symmetry in an oxide environment in various host materials, like mixed oxide fuels, inert matrices and corium sub-systems. The PuO2 Raman fingerprint was previously observed to consist of one main T2g vibrational mode at 478 cm−1 and two crystal electric field transition lines at 2130 cm−1 and 2610 cm−1. This particular use of Raman spectroscopy is promising for applications in nuclear waste management, safety and safeguard.

a b s t r a c t Some example applications are presented, in which the peculiar Raman fingerprint of PuO 2 can be used for the detection of crystalline Pu 4þ with cubic symmetry in an oxide environment in various host materials, like mixed oxide fuels, inert matrices and corium sub-systems. The PuO 2 Raman fingerprint was previously observed to consist of one main T 2g vibrational mode at 478 cm À1 and two crystal electric field transition lines at 2130 cm À1 and 2610 cm À1 . This particular use of Raman spectroscopy is promising for applications in nuclear waste management, safety and safeguard. The Raman spectrum of crystalline plutonium dioxide has been independently studied by some distinct research groups in the last few years, yielding clear and reproducible results. It has been observed to consist of one main vibrational mode (T 2g , at 478 cm À1 ± 2 cm À1 ) and two clear crystal electric field (CEF) transitions at higher energy (2130 cm À1 ± 5 cm À1 and 2610 cm À1 ± 5 cm À1 ), as shown in Fig. 1. The first band, predicted by the Group Theory [1], was observed over forty years ago for the first time in cerium and thorium dioxides [2], and more than twenty years ago in other actinide dioxides, including plutonium dioxide [3]. Although its exact spectral position slightly depends on the chemical nature of the compounds, it can poorly be used to identify single chemical species in complex mixtures. On the other hand, two higher-energy peaks more peculiar of Pu 4þ with cubic symmetry in an oxide matrix have only been identified and studied in the last few years [4e6]. They have recently been established to be related to G 1 /G 5 and G 1 /G 3 crystal electric field (CEF) transitions, respectively [5,6]. Since in PuO 2 these transitions occur within the ground 5 I 4 manifold, unlike the 3 H 4 -> 3 F 2 intermultiplet transitions observed for example in UO 2 (cf. [7]), they take place at a relatively low energy and are therefore easily observable in the standard Raman spectra of plutonium dioxide. Their presence makes the Raman fingerprint of PuO 2 quite particular not only among other similar fluorite-like compounds, but also among many other chemical species that can be found in nuclear materials.
If the Raman modes typical of crystalline plutonium dioxide have already been assessed in previous research, the main and original goal of the present work consists in showing some example applications in which the PuO 2 Raman fingerprint has advantageously been used for the detection of crystalline PuO 2 in various environments, inert matrices, corium sub-systems or nuclear waste vitrification glasses. This application of Raman spectroscopy can thus have a large interest in terms of nuclear waste management, safety and safeguard.
Raman spectra have been measured in the present research with the help of a recently developed technique for the confinement of radioactive materials, described elsewhere [8,9] . Such a technique permitted the measurement of heavy alpha emitters in a standard Raman microscope, offering all the flexibility of a nonnuclearized instrument.
The Raman microscope used in this work is equipped with a long working distance (10.6 mm) which offers a 0.5 numerical aperture with a 50 times magnification allowing the acquisition of spectra both on microscopic (up to approximately 2 mm by 2 mm) and macroscopic spots on the sample surface. The Raman spectrometer is a Jobin-Yvon T 64000 equipped with a 1800 grooves per mm grating and a low noise LN2 cooled Symphony ® CCD detector. Although the microscope can be coupled to the spectrometer in a confocal mode, this option was not used for the present work. Excitation sources are an Ar þ Coherent ® Continuous Wave (CW) laser with main wavelengths at 488 nm or 514.5 nm or a Kr þ Coherent ® CW laser with main wavelengths at 647 nm or 752 nm, both with a controllable nominal power (up to few W depending on the wavelength) and a monochromator as plasma filter. The excitation source wavelength was chosen case by case depending on the investigated material, in order to maximise the signal-to-noise ratio and minimise fluorescence and sample alteration effects. The power impinging the sample surface is lower by a factor 5 approximately. Using the long focal 50Â objective and the single spectrometer mode permits a good spectral resolution (±1 cm À1 ) independently of the surface shape. The spectrograph is calibrated with the T 2g excitation of a silicon single crystal, set at 520.5 cm À1 [10]. The instrument is calibrated on a daily basis prior to measurements.
As a first example, Fig. 2 reports the Raman spectrum recorded in this work on a 20 mol % PuO 2 -containing sodium borosilicate glass, produced at JRC Karlsruhe in order to simulate the behaviour of nuclear waste vitrification matrices, along with the spectrum of a PuO 2 -free glass sample. In addition to some spectral modifications the interpretation of which can be left for further research, the Raman fingerprint of crystalline PuO 2 unambiguously appears, as indicated by the arrows, on the background of the borosilicate glass spectrum for the current PuO 2 concentration. This observation is in line with the precipitation, in the vitreous matrix, of PuO 2 crystallites, indicating that the solubility limit of plutonium dioxide in the glass has been overcome. The analysis can be further refined by extending it to glass samples with various plutonium dioxide contents or to different kind of glasses. This would lead to a more accurate Raman spectroscopy assessment of the PuO 2 solubility in the glass. Moreover, the recently studied Raman spectra characteristic of PuO 2 nano-crystals [11] can be applied to this investigation in order to possibly detect the early appearance of crystalline plutonium-dioxide nano-precipitates, by analysing the T 2g peak width and spectral position. Such analysis refinement will be the subject of further research. Independently, the results of the current Raman investigation of PuO 2 e containing borosilicate glasses can be soundly used to detect the presence of crystalline PuO 2 in glasses used for nuclear waste immobilisation.
Another interesting example concerns the detection, by Raman spectroscopy, of Pu 4þ with cubic symmetry in other oxide matrices of nuclear interest. For example, the black curve in Fig. 3 shows the Raman spectrum of a crystalline zirconium dioxide sample, mixed    3. Raman spectra recorded with a 647-nm excitation source on pure PuO 2 , on pure monoclinic ZrO 2 , and on a PuO 2 -containing monoclinic/tetragonal zirconia matrix. The spectrum of purely tetragonal ZrO 2 has been taken from the literature [12]. Sharp lowintensity peaks in the mixed oxide spectrum are due to oxygen and nitrogen impurities adsorbed on the sample surface. with a macroscopic (>20 mol %) amount of plutonium dioxide. This sample was obtained by laser-melting a pellet of a pre-sintered mixture of ZrO 2 and PuO 2 , in the aim of simulating on a laboratory scale the formation of corium, the melted mixture of nuclear fuel and cladding occurring during a nuclear power plant core meltdown accident. In Fig. 3, the Raman spectrum of PuO 2 is reported together with the spectra of the monoclinic and tetragonal zirconia (the latter one was measured only up to 700 cm À1 [12]). Both ZrO 2 allotropies were observed by XRD in the present PuO 2containing matrix, as it often happens in heat-treated zirconia. It is possible to see, from the Raman spectra of single compounds, that many Raman-active modes overlap in the spectral region between 250 cm À1 and 750 cm À1 . In particular, the T 2g line of PuO 2 at 478 cm À1 overlaps almost exactly with a similar line of monoclinic ZrO 2 , which makes very difficult the detection of the PuO 2 contribution to the combined spectrum in this range. In this case, the presence of cubic Pu 4þ in the zirconia matrix can be easily concluded only with the help of the two clear high-energy CEF transition peaks. The different intensities and the slightly varied spectral positions that the two latter peaks display in the zirconia matrix may be due to polarisation effects and to local constraints between PuO 2 and ZrO 2 crystals. Such constraints can be quite complex in this peculiar case, especially when one considers the coexistence of various allotropic forms of zirconia in the investigated material, and the possibility for a partially epitaxial growth, during solidification of the melt, of different crystallites on top of each other. Deepening these aspects goes beyond the scopes of the present communication, and will be left for further more specific research. Nonetheless, Fig. 3 highlights the importance of the two CEF peaks for the sure and prompt detection of the presence of cubic Pu 4þ in a foreign matrix by Raman spectroscopy, even when the main spectral features of the host matrix largely overlap with the plutonia spectrum at lower energies.
Other examples in which the presence of Pu 4þ with cubic symmetry can be detected in oxide host matrices are reported in the literature. The work published by B€ ohler et al. on the ThO 2 -PuO 2 system [13] showed that the Raman spectral signature of plutonium dissolved in the fluorite-like crystalline thoria matrix can be detected up to PuO 2 contents in ThO 2 as low as 3 mol %. Like in the previous ZrO 2 -PuO 2 case, also here the high energy electronic lines are somewhat shifted and distorted at high dilution, most probably due the large lattice parameter change induced by thoria in the solid solution, with respect with pure plutonia. B€ ohler et al. noticed that the ratio between the spectral position of the CEF lines and that of the "mixed" T 2g line remained almost constant for all the (Th, Pu) O 2 investigated compositions. This behaviour is normal when it is considered that plutonia is largely soluble in thoria [14], and a distortion of the crystal electric field can be logically expected in the solid solution. Raman spectra have been recorded in this work on similar samples, plutonia-containing thorium dioxide with increasing PuO 2 content, starting from 3 mol % of PuO 2 . They are reported in Fig. 4. It can be noticed that the very intense T 2g line is shifted at different compositions. In all the cases, the presence of Pu 4þ with cubic symmetry in the thoria matrix is obviously detected by the presence of the high-energy CEF transition peaks, indicated by arrows in Fig. 4. The attribution of other Raman lines present in the spectra of Fig. 4 is reported in Ref. [13].
Another interesting example can be found in the investigation performed by Sarsfield et al. [4], in which the Raman fingerprint of PuO 2 reveals the progressive formation of crystalline plutonia, or at least of Pu 4þ with cubic symmetry, upon laser heating-induced decomposition of plutonium (VI) hydroxide. This kind of information, once more accessible with the sole help of Raman spectroscopy and the PuO 2 Raman fingerprint, is useful for the analysis of nuclear waste chemical stability.
It is important to notice here, that distortion and shift of the current CEF Raman peaks are generally observed in samples where cubic Pu 4þ is dissolved in an oxide solid solution or epitaxial crystalline PuO 2 grows on another crystalline matrix, as opposed to the case where pure PuO 2 crystallites precipitate from the material matrix, for example the vitreous one of Fig. 1. The CEF peak shift amounts to a dozen of cm À1 at most for the G 1 /G 5 line and to a couple of dozens of cm À1 for the G 1 /G 3 one. Raman peaks are observed to broaden by a few cm À1 in both modes. In all cases, unambiguous detection of the Raman fingerprint of Pu 4þ with cubic symmetry in an oxide environment is ensured by the simultaneous observation of the T 2g mode close to 478 cm À1 and the two CEF lines around 2130 cm À1 and 2610 cm À1 .
In summary, it is evident, from the recent research results reported here, that the newly assessed Raman fingerprint of crystalline PuO 2 can be conveniently employed for the detection of plutonia and, more in general, of cubic Pu 4þ in different types of oxide environments. In the light of the flexibility offered by Raman spectroscopy, which can be used both as an in-situ and ex-situ analysis technique (cf. [15]), this fingerprint is potentially an extremely useful tool for many applications. It can indeed be employed for the detection of cubic Pu 4þ oxide hotspots in various situations of interest for the nuclear safeguard, the analysis of nuclear waste, the investigation of corium produced in nuclear plant meltdown accidents, the detection of illicit traffic of nuclear materials, etc. Fig. 4. Raman spectra recorded with a 647-nm excitation source mixed ThO 2 e PuO 2 oxides. The spectra are truncated between 900 cm À1 and 2000 cm À1 for a better readability.