Fabrication and First Characterisations of Uranium Carbide Samples

F-BRIDGE Collaborative Project Co-funded by the European Commission under the Euratom Research and Training Programme on Nuclear Energy within the Seventh Framework Programme Contract Number: 211690 Start date: 01/03/2008 Duration: 48 Months www.f-bridge.eu

Fabrication and first characterisations of uranium carbide samples

Guillaume Martin (CEA), Francisco Garcia-Ferre (CEA), Gaëlle Raveu (CEA), Olivier Fiquet (CEA), Patrick Simon (CNRS), Guillaume Guimbretière (CNRS), Pierre Desgardin (CNRS), Marie-France Barthe (CNRS), Gaëlle Carlot (CEA), Hicham Khodja (CEA), Caroline Raepsaet (CEA), Thierry Sauvage (CNRS), Philippe Garcia (CEA)

F-BRIDGE – D-116 – Revision 0 – Issued on 31/07/2012 by CEA

F-BRIDGE – Document D-116 – revision 0

F-BRIDGE project – Contract Number: 211690 Basic Research in support of Innovative Fuels Design for the GEn IV systems EC Scientific Officer: G. Van Goethem

Document title Fabrication and first characterisations of uranium carbide samples Author(s) Guillaume Martin (CEA), Francisco Garcia-Ferre (CEA), Gaëlle Raveu (CEA), Olivier Fiquet (CEA), Patrick Simon (CNRS), Guillaume Guimbretière (CNRS), Pierre Desgardin (CNRS), Marie-France Barthe (CNRS), Gaëlle Carlot (CEA), Hicham Khodja (CEA), Caroline Raepsaet (CEA), Thierry Sauvage (CNRS), Philippe Garcia (CEA) Number of pages 47 Document type Deliverable Work Package WP1-1 Document number D-116 – Revision 0 Issued by CEA Date of completion 31/07/2012 Dissemination level Public

Summary

First UC pellets were fabricated and samples were sliced from them. The carboreduction process was selected since a control over fabrication conditions can be carried out more readily than in the case of other processes. It appears that carrying out the whole production process under controlled atmosphere should decrease the impurity contents inside the sample as it would provide a better control on the stoichiometry of the final product. Nuclear reaction analyses were for the first time applied to scan the oxygen concentrations at the surface of uranium carbide samples. This technique was shown to be relevant for detecting oxygen contents of several at.% of oxygen within actinide carbide compounds. The results obtained show that the fabrication and preparation conditions play a major role in the oxygen levels measured inside the samples produced. Besides, strong variations in the stoichiometry of samples at a micron scale were monitored suggesting the presence in the fabricated material of other phases than UC, such as UC2. This superior carbide compounds appear to be preferentially oxidized. Raman characterizations were used to probe the presence of impurity phases, as UC itself has no first order Raman active modes. Apart some amount of disordered carbon, no Raman signature of any secondary phase could be detected in these samples (especially no fluorine UO2 trace). Preliminary measurements were performed with positron annihilation spectroscopy in order to evaluate the feasibility of using this technique to study the properties of vacancy defects in UC as it has been done in UO2 (see D111 and D115).

Approval

Rev. Date Short description First author WP leader Domain leader Coordinator G. Martin, CEA M.F. Barthe, CNRS R. Konings, JRC-ITU C. Valot, CEA 0 07/12 First issue 26/06/12 26/06/12 02/07/12 20/07/12

Distribution list

Name Organisation Comments G. Van Goethem EC DG RTD All participants F-BRIDGE

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Table of contents

1 Introduction ...... 5

2 Properties of uranium carbide compounds ...... 5 2.1 The uranium-carbon system ...... 5 2.1.1 Crystalline structure of actinide carbide compounds ...... 5 2.1.2 Uranium-carbon phase diagram ...... 6 2.2 The uranium-carbon-oxygen system ...... 9 2.2.1 Uranium monocarbide susceptibility to oxidation ...... 9 2.2.2 Uranium-carbon-oxygen phase diagrams ...... 9

3 Fabrication of dense UC pellets ...... 10 3.1 Fabrication steps ...... 10 3.2 Powder mixing and pressing steps ...... 11 3.3 Carboreduction ...... 12 3.4 UC milling and pressing ...... 14 3.5 Pellet sintering ...... 14

4 UC samples...... 15 4.1 Sample preparation ...... 15 4.2 Sample characteristics ...... 16

5 NRA depth profiling of oxygen ...... 16 5.1 The NRA experimental technique ...... 16 5.1.1 General presentation ...... 16 5.1.2 Detection ...... 17 5.1.3 Energy of the incident beam ...... 18 5.1.4 Energy calibration of the detection chain ...... 19 5.1.5 Description of the experimental configuration ...... 20 5.2 Determination of the sample stoichiometry ...... 21 5.2.1 Presentation of the method ...... 21 5.2.2 Sample composition and scanning conditions ...... 22 5.2.3 C/U cartographies of monitored areas ...... 24 5.3 Oxygen depth profiling ...... 25 5.3.1 Reaction cross section ...... 25 5.3.2 13C signal simulation ...... 26 5.3.3 Simulation of oxygen spectra ...... 26 5.3.4 Integrated oxygen concentrations at the sample surface ...... 27 5.4 Results and discussion ...... 28

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6 Complementary characterizations of uranium carbide: Raman spectroscopy, polarized reflection microscopy and XRD ...... 31 6.1 Introduction ...... 31 6.2 Samples ...... 31 6.3 Raman Experiments ...... 32 6.3.1 Results ...... 33 6.4 Polarized microscopy ...... 35 6.5 XRD characterizations ...... 36 6.6 Conclusions ...... 36

7 Positron annihilation spectroscopy ...... 37 7.1 Experimental details ...... 37 7.2 Results and discussion ...... 39 7.3 Preliminary PAS conclusions ...... 41

8 Conclusions...... 41

9 Bibliography ...... 43

10 Annexes ...... 46 10.1 Annex 1: High energy pile-up distribution ...... 46 10.2 Annex 2: Document approval by beneficiaries’ internal QA ...... 47

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1 Introduction

Mixed uranium-plutonium carbides are candidate fuels for Generation IV nuclear reactors and have been widely studied since the 1940s1. These compounds present several advantages if compared to standard oxide fuels, namely a higher actinide density, a higher fusion temperature and an excellent thermal conductivity, amongst others2. Their properties should lead to better breeding ratios and lower fuel temperatures under normal operating conditions, increasing the margin towards fuel melting. However there are some issues relative to the fabrication process of carbide fuels, notably due to their pyrophoric behaviour, the narrow range of the stoichiometric monocarbide phase-stability3, the low inter-solubility of minor actinide carbide phases or the carburisation of steel claddings at high temperatures4. Also an important aspect of carbide manufacturing is that actinide carbides readily react with both oxygen and nitrogen to form complex phases. This therefore in principle requires a manufacturing process that guarantees materials are kept in an inert environment. Last but not least, one of the most important drawbacks of carbide fuels regards their swelling behaviour under irradiation which should have a considerable impact upon the mechanical performance of advanced fuel pins. In-pile fuel evolution depends strongly on its chemical state which is in turn influenced by the redistribution of fuel constitutive atoms and fission products5.

Our aim here is to provide a status report on the manufacturing process as it stands and our capacity to carry out ion beam experiments and positron annihilation spectroscopy to characterise the samples’ composition and microstructure respectively. Other complementary analyses have been performed by using Raman spectroscopy and XRD. We therefore first describe each step of the manufacturing process based on carbothermic reduction that was applied to manufacture our UC pellets. Preliminary sample characterisation results are provided. Nuclear reaction analyses using a micron size deuteron beam of several UC samples sliced out of pellets are subsequently described. The oxygen concentrations were probed up to 2 µm below the sample surface. The C/U ratio and oxygen concentrations at the sample surface were also monitored at the micron scale. The results emphasize the heterogeneity that results from the current manufacturing and sample preparation processes that appear to have a major effect upon the oxygen content of the samples. Probably as a result of contact with air, the oxygen content was estimated at some points very close to the surface at values close to 30 at.%. Although this points to shortcomings in the manufacturing process and sample conditioning, IBA (Ion Beam Analysis) techniques appear to constitute extremely promising tools for the compositional and microstructural characterisation of uranium carbide samples.

Concerning positron annihilation spectroscopy, it has to be underlined that the annihilation characteristics of UC are unknown and that the opened literature is virgin. The preliminary measurements reported in this document are the first one ever published. Of course supplementary experiments are needed to be able to use this technique to identify vacancy defects and their properties in UC as it has been possible in UO2 (see D111,D115). These first tests confirm the usefulness of the PAS technique and the consistency with the storage conditions of the UC samples developed during this study.

2 Properties of uranium carbide compounds

2.1 The uranium-carbon system

2.1.1 Crystalline structure of actinide carbide compounds

The crystalline structure of uranium monocarbide UC is a NaCl FCC rock salt type structure. The carbon atoms should be much smaller than the actinide atoms and should fill the octahedral holes in the FCC metal

Page 5/47 F-BRIDGE – Document D-116 – revision 0 lattice. This explains why actinide carbide compounds are often considered as interstitial alloys. Uranium dicarbide UC2 has low and high temperature phases. The high temperature phases are of the KCN FCC type, whereas the low temperature phases are of the CaC2 tetragonal type. Uranium sesquicarbide U2C3 is

BCC (Pu2C3 type) and is stable only below 2093 K. Table 1 shows the lattice parameter of several carbide compounds, together with their relative structure, composition, theoretical density (at room temperature) and melting temperature (from reference 1).

Lattice ρ Material T [K] C/M ratio Structure m parameter [nm] [g/cm3] UC 2780 ± 25 1.0 NaCl-FCC a = 0,4961 13.63 a U2C3 2100 ± 25 1.5 Pu2C3-BCC a = 0,80899 12.88 a = 0,3519 CaC -tetragonal UC 2710 ± 25b 1.95 2 c = 0,5979 11.68 2 KCN-FCC a = 0,5488 (2173K) a = 0,4968 (45 %at. PuC 1875 ± 25b 0.86 NaCl-FCC 13.6 1-x C) b Pu2C3 2285 ± 25 1.5 Pu2C3-BCC a = 0,8131 12.7

Table 1: Melting point, composition, structure, lattice parameter and theoretical density of actinide carbide 1 a b compounds according to Matzke ( Transition U2C3 Æ β-UC2 + UC, Liquidus temperature).

The density of uranium monocarbide as a function of temperature has been calculated by Mendez-Penalosa 6 3 and Taylor through thermal expansion measurements and assuming a theoretical density ρth.of 13.63 g/cm at room temperature. The result is shown in Figure 1.

13,8

13,6

13,4

13,2 [g/cm3] ρ 13,0

12,8

12,6 0 200 400 600 800 1000 1200 1400 1600 1800 2000 T [°C]

Figure 1: The density of uranium monocarbide as a function of temperature6.

2.1.2 Uranium-carbon phase diagram

Several studies of the U-C phase diagram have been previously carried out from room temperature examinations of samples which have been annealed at high temperature then quenched7-13. In these conditions, the observed phases can be in a meta-stable state or even unrepresentative of the state of the material at high temperature, if the quenching process is not fast enough. Nevertheless as the decomposition kinetics of these materials is rather slow, three main carbide compounds have been 7 identified: UC, UC2 and U2C3. The U-C phase diagram, as described by R. Benz , is shown in Figure 2.

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Figure 2: U-C phase diagram according to Benz7

UC is thermodynamically stable at room temperature. UC2 is meta-stable below 1516°C and U2C3 should dissociate into UC + C below 850°C. UC2 has been observed in the form of needles (Widmanstätten structure). Typical ceramographic examinations of uranium carbide compounds are shown in Figure 3.

The first studies based upon high-temperature measurements14-16 have helped deterrmine important features of the U-C phase diagram. A U self-diffusion study15 has provided an accurate determmination of the transition temperature from β-UC2 to α-UC2 and an estimate of the decomposition temperature of the sesquicarbide

U2C3, and has shown the existence of the UC phase field down to a stoichiometric ratio of 0.92 above 16 1400°C. An evaporation study has revealed that the precipitation of β-UC2 can occur at much lower C/U ratios than assumed before. More recently, Chevalier and Fischer17 published a critical assessment of several binary and ternary actinide-carbon systems from all the available experimenttal data relative to phase diagrams and thermodynamic properties. Theey also performed a thermodynamic modelling of the U-C system which has shown a reasonable agreement with the related experiments.

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Figure 3. Optical micrographs of U-C compounds. (a) Substoichiometric UC0,92 with free U metal at grain boundaries. (b) Stoichiometric UC. (c) Hyperstoichiometric UC1,15 with UC2 needles in a Widmanstätten structure. (d) Stoichiometric U2C3. (e) Hypostoichiometric UC2-x (≈UC1,88)

In 2009, Utton et al.18 have presented the RLS (Reflected Light Signal) technique which is based on in situ measurements of a sample surface reflectivity during its laser melting experiments. They have shown that this techniqque constitutes a suitable method for the study of high-temperature phase transitions in uranium carbides, especially for solid-liquid and solid-solid phase transitions. The author’’s results confirmed the previously reported literature data, although with improved accuracy. The melting point of UC was also measured to 2508 ± 4°C. Their results were included in the U-C phase diagram calculated by Chevalier and Fischer17 which extends over C/U ratios from 0.67 to 4: they are shown in Figure 4.

0.67 1 1.5 2.33 4 C/U ratio

Figure 4: U-C phase diagram17 containing experimental data of Utton et al.18. Blue symbols represent the results obtained by laser melting experiments.

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Even more recently Guéneau et al.20 carried out a comprehensive assessment of the U-Pu-O-C system using the CALPHAD method19 and developed a thermodynamic model which also adequately reproduces the binary and ternary subsystems. The results of their model developments have been incorporated into the FUELBASE thermodynamic database21-22.

2.2 The uranium-carbon-oxygen system

2.2.1 Uranium monocarbide susceptibility to oxidation

Due to their high reactivity with many elements, actinide carbides may contain impurities. Hence oxygen and nitrogen can notably be present in the final carbide compounds. Indeed pure uranium monocarbide is difficult to manufacture since it reacts readily with oxygen even at room temperature. The oxidation kinetics of the surface of a sample which has been polished is fast enough to be observed by optical microscopy23, as shown in Figure 5.

Figure 5: Optical observation of the oxidation of a UC sample before and after few minutes air exposures (zoom x10)23

As reported in the literature the oxygen is believed to be incorporated into the UC lattice through oxygen substitution at carbon sites. The resulting monoxycarbide can be therefore seen as a solid solution between UC and the hypothetical compound UO, which has never been obtained as a bulk phase and has only been reported as an oxidation layer at the surface of metallic uranium samples24. The maximum solubility of 25 oxygen in UC is estimated to be ~ 35 at.% UO , the lattice parameter of the phase UC0,65O0,35 being 0.4948 nm between 1100°C and 2200°C26.

Finally a pyrophoric behaviour of actinide carbide compounds can be observed especially when the material is in the form of a powder. As a consequence of the high susceptibility of uranium monocarbide to oxidation, the fabrication and handling steps of uranium carbide compounds must be carried out under a controlled atmosphere (low oxygen and moisture content). In the next section are reported the data relative to the ternary uranium-carbon-oxygen system which can be found in the literature.

2.2.2 Uranium-carbon-oxygen phase diagrams

Although thermodynamics studies confirm the 30 - 35 at.% UO solubility in UC between 1100°C and 1700°C, slightly different U-C-O phase diagrams have been proposed in the literature as reported by Matzke1. The main differences appeared in the region of higher carbon and oxygen contents, i.e. for (C+O)/U ratios greater than 1. There was no agreement on the coexistence of the U2C3, U(C,O) and UO2 phases in this region. Later studies performed by Henry et al.27 at 1700°C, Alcock et al.28-29 at 1400°C, Henecke and Scherff30 between 1350°C and 1500°C and Tagawa and Fujii31 between 1400°C and 1700°C have revealed that the coexistence of the UO2 and U2C3 phases is thermodynamically impossible in the studied temperature ranges, as later demonstrated by Piazza and Sinott32. The U-C-O ternary diagram at 1700°C from Henry et al.27 is shown in Figure 6.

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Figure 6: U-C-O system at 1700°C according to Henry et al.27

In more recent studies25-26 Potter and Spear haave discussed the correspondence between available ternary and binary phase diagrams through the consideration of equilibrium reactions annd CO partial pressures. According to these studies a phase diagram of the form suggested by Henry et al.27 appears to achieve better agreement with the available data than those which have been previously prooposed (see reference 1).

They have also found that the UO2 and U2C3 phases are not thermodynamically compatible between 1300 and 1700°C even if they could be at lower temperatures. They have finally prooposed a U-C-O ternary diagram at 1730°C, which shows the existence of an under-stoichiometric UC2 phase (down to a C/U ratio of

1.9). The notation UC1-xOx used in Figure 6 has been replaced by U(C,O) since there is a priori no connection between oxygen and carbon contents.

3 Fabrication of dense UC pellets

3.1 Fabrication steps

The studies relative to the fabrication of uranium carbide started in the early 1960’s. Three processes were mainly tested: ¾ Reaction of metallic uranium with graphite, U + C Æ UC, by heating pre-pressed mixtures of fine U and C powders.

¾ Reaction of a U powder with propane or methane (e.g. U + CH4 Æ UC + 2H2).

¾ Carbothermic reduction (carboreduction) of uranium oxide according to UO2+x + (3+x)C Æ UC +

(2+x)CO, by heating UO2 and C powders in vacuum above 1300°C.

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Two methods were first selected at CEA to elaborate UC samples, namely arc melting and carboreduction. However, the arc melting process, which consists in the concomitant melting of mixed graphite and metallic uranium powders at temperatures above 1800°C, does not enable a sufficiently accurate control the homogeneity and the stoichiometry of the synthesized carbide compounds. This process usually results in hyper-stoichiometric uranium carbide samples which contain UC and UC2 phases (U2C3 is barely formed as a consequence of its slow formation rate). Therefore, carboreduction was eventually chosen as the reference process to synthesize UC.

The fabrication of UC pellets involves several successive steps. UO2 and C powders are mixed, submitted to ball milled, then blended for a better homogenization and finally pressed before their conversion into uranium monocarbide. The carboreduction reaction occurs at temperatures around 1600°C in vacuum. The produced uranium carbide is then crushed and pressed. The green pellets obtained are finally sintered, usually in a argon atmosphere at temperatures ranging between 1700°C and 1800°C. The whole fabrication process is schematized in Figure 7.

Figure 7: Fabrication steps of dense UC pellets.

3.2 Powder mixing and pressing steps

Carboreduction reaction rates depend strongly both on the specific surface and on the mean size of the UO2 and C powders used. An agglomerate mean size below 15 µm is desirable. In these conditions specific surfaces in the range of 10 - 15 m2.g-1 appear to constitute a good compromise between the desired high reactivity of the powders and a good homogeneity of the blend33.

Particular care must be given to the amount of carbon which is added into the blend as it directly determines the stoichiometry and composition of the final compound. However the accurate determination of this quantity is difficult since UO2 powders with a high specific surface are likely to oxidize during their storage and processing. This is still a major issue in the fabrication process of pure UC samples since the range of stoichiometry over which the monocarbide is thermodynamically stable at room temperature is very narrow, as shown in Figure 2: any slight deviation from stoichiometry shall indeed result in the formation of secondary phases. This is why the initial O/U and C/U ratios of the powders were estimated by thermogravimetric analyses.

Powder blends composed of about 40 grams of UO2 and of the relevant amount of graphite (with a precision of a tenth of milligram) were prepared by milling for 15 minutes. Powder blends were then pressed into

Page 11/47 F-BRIDGE – Document D-116 – revision 0 pellets at about 400 MPa. All milling and pressing steps (see also section 3.4) were carried out in a glove box in an inert N2 atmosphere containing oxygen and water contents between 2 and 100 ppm. Samples were stored under the same conditions. This atmosphere follows the Lorenzelli and Delaroche specifications34

([O2] < 100 ppm and [H2O] < 200 ppm). Over long periods of time, oxidation of UO2 powders could occur even in such conditions.

3.3 Carboreduction

All carboreduction and sintering annealing stages (see also section 3.5) were carried out at CEA/Cadarache in the PROMECE furnace. This setup is coupled to a gas chromatograph used for gas release monitoring during the carbothermic annealing sequence. The carboreduction reaction according to UO2+x + (3+x)C Æ UC + (2+x)CO is typically performed at temperatures near 1600°C - 1650°C. The carboreduction is a mono- variant equilibrium in which the CO partial pressure depends only on the temperature. The Le Chatelier’s law predicts therefore that low CO partial pressures should favour the reaction. It is why it is usually carried out under a vacuum of a few tenths of millibar. A typical carbothermic annealing sequence is shown in Figure 8.

T (°C) P (mbar) 1800 0,6

Tmax = 1624,03°C

1500 0,5

1200 0,4

900 0,3

600 0,2

300 0,1

0 0,0 0246810121416 Time (hours)

Figure 8: A typical carboreduction annealing sequence. The blue line stands for the temperature and the red line for the pressure in PROMECE.

Two successive gas release stages are observed. The first one starts at about 600°C and would correspond to a return of the UO2+x powder back to stoichiometry. The second massive release stage takes place from 1100°C and stems from the carbothermic reaction which produces carbon monoxide. The temperature plateau was adjusted to obtain theoretical mass loss of 18.3% after the carboreduction reaction. However, a mass loss between 19 to 20 % has been occasionally measured, possibly due to a partial evaporation of uranium. This assumption is supported by the step-like microstructure observed at the surface of UC pellets, as revealed by the SEM (Scanning Electron Microscopy) image presented in Figure 9.

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Figure 9: SEM observation of a uranium monocarbide pellet after carboreduction.

XRD analyses were carried out on pellets crushed into powder after the carbothermic annealing sequence.

The only visible phase present on spectra was iin most cases uranium monocarbidee. Traces of residual UO2 were occassionally observed. Figure 10 shows a UC pellet after carboreduction. At this stage the density of the pellet is very low (around 55% of the theoretical density).

2 mm

Figure 10: A UC pellet after carbothermic reaction. The compound density is at thiis stage very low as it contains lots of pores.

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3.4 UC milling and pressing

Following the carboreduction, UC pellets were crushed into powder by ball milling for 15 minutes using a tungsten carbide cell and a single WC sphere. The resulting particle mean size ranged between 1 and 5 μm. As UC is pyrophoric these uranium carbide powders can ignite in air even at room temperature.

Additives have been sometimes used for milling. Stearic acid can be used to decrease the friction between the particles and between the die and the UC pellet once the powder has been pressed as reported by Matzke1. It is also possible to add ethanol or paraffin to facilitate the removal of the UC powder from the die during its extraction after the milling stage. Another potential additive is Ni since it improves the pellet densification and increases the mean grain size during pellet sintering. To this aim an addition of 0.1 wt.% of Ni is suggested in the literature1. This additive should moreover evaporate above 1150°C. Finally, carbon can also be added to compensate for any possible carbon loss. The purity of additives must be very high, since uranium carbide is very sensitive to moisture contamination.

After milling uranium carbide powders were pressed at 500 MPa. The obtained green pellets could then be sintered in the PROMECE furnace (see section 3.3). They were mounted on the PROMECE sample holder

(made of tungsten) then placed in a polyethylene bag to be transferred from the glove box under a N2 atmosphere (see section 3.2) towards the furnace without any direct air exposure.

3.5 Pellet sintering

The pellet densification was carried out in the PROMECE furnace. The annealing sequence is very similar to the carboreduction annealing stage (see Figure 8), except it was carried out under a 1 bar argon atmosphere and not under vacuum to preclude any uranium evaporation (see section 3.3). XRD analysis was performed on crushed samples after they were sintered. The only phase revealed by XRD spectra was uranium monocarbide although optical observations revealed the presence of secondary phases, presumably UC2 and metallic U inclusions, as shown in Figure 11.

Figure 11: Optical observations of a UC sample. The right image would reveal the presence of UC2 whereas the left one would show metallic uranium inclusions between grains and around the central pore.

The optical observation of sintered samples also revealed a radial variation of grain size, the grains at the pellet extremities being larger. As this was expected, the use of Ni as an additive (see section 3.4) resulted in a remarkable increase in grain size from 10 - 50 μm to 500 μm (see Figure 12). However the porosity increased as well leading to a reduced density.

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Figure 12: Comparison between the microstructure of two uranium carbide pellets,, without (left) and with (right) an addition of Ni.

Besides the addition of carbon, dodecane and paraffin had no significant or a negaative impact on the final microstructure, since sometimes large carbon innclusions were observed in the corresponding pellets. Without additive, the pellet density was estimated by helium pycnometry and by immersioon in bromobenzene. A maximum value corresponding to 96.5% of the theoretical density was measured in one case. This first result is encouraging as the reproducibility of produced pellets should be considerably imprroved in the future by an improved control of the conditions under which each step of the fabrication process occurs.

4 UC samples

4.1 Sample preparation

UC samples of circa 1 mm thickness were sliced from the UC pellets obtained from the fabrication process described in section 2. Thin UC samples are veery fragile therefore they were coated in Epoxy resin with one face of each sample which was positioned near the surface of the Epoxy block. They were then polished on their face which was intentionally let unrecoveered by the resin. This last polishing stage is necessary for sample characterisation since a minimal planarity of sample surface is usually required for most experimental techniques, and in particular those which have been implemented here (NRA, PAS, RAMAN and XRD). The last micron polishing step was performed using a paraffin oil suspension. In one caase however an aqueous suspension was used in order to observe the possible oxidation induced by such a preparation procedure. A sample was also kept unpolished. Thereafter samples were stored in paraffin, whicch is reputed to prevent them from oxidising for at least several weeks. An as-prepared UC sample is shown in Figure 13.

Epoxy

UC

2 mm 250 µm

Figure 13. Picture of a UC sample after it has been coated in Epoxy and polished ((right) and microscopy image of its surface (left).

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4.2 Sample characteristics

Few UC samples were available for this first batch of experimental characterizations. Furthermore the samples which had been stored for several months were mechanically unstable as they could easily turn into powder under normal handling. Only five samples were therefore selected for further characterizations. These samples have been submitted to a variety of fabrication and preparation conditions which are presented in Table 2. The date at which they were polished indicates how long their storage lasted before they have been analysed (end 2011 - early 2012). Apparent densities were estimated by measuring the weight and the dimensions of as-fabricated pellets. Samples UC_3a, UC_3b and UC_3c were sliced from the same pellet.

Carbo Ball milling Sintering Polishing reduction Sample Apparent Annealing N glove box Annealing Used Date 2 Used additive name density temperature impurities sequence(s) suspension (2011) 88.63 15 ppm O + UC_3a 1650°C 2 - 1690°C/5H in Ar Aqueous 04/04 %/ρth. 3 ppm H2O 88.63 15 ppm O + UC_3b 1650°C 2 - 1690°C/5H in Ar Paraffin 11/10 %/ρth. 3 ppm H2O 88.63 15 ppm O + UC_3c 1650°C 2 - 1690°C/5H in Ar Unpolished - %/ρth. 3 ppm H2O 74.7 30 ppm O + 1700°C/5H in UC_6 1700°C 2 Paraffin Paraffin 04/12 %/ρth. 4 ppm H2O vacuum 1660°C/12min in 69.57 1 ppm O + UC_10 1720°C 2 Paraffin vacuum then Paraffin 05/13 %/ρ 86 ppm H O th. 2 1630°C/5H in Ar

Table 2: UC sample fabrication and preparation conditions

5 NRA depth profiling of oxygen

5.1 The NRA experimental technique

5.1.1 General presentation

SIMS (Secondary Ion Mass Spectrometry) can be a powerful technique for depth profiling oxygen in oxide fuels35. However, this technique requires in general the use of standards in order to be quantitative. In addition, the analysis of oxygen in polycrystalline uranium carbide samples could turn out to be rather complicated since SIMS sputtering rates in UC are not known and may be strongly dependent upon the grain orientation and the sample composition. The homogeneity in composition and structure of the analysed samples is therefore of crucial importance. Moreover quantitative SIMS experiments would require the use of a reference sample in which the oxygen depth profile is well known. Also the technique is semi-destructive since the sample surface is partially sputtered.

IBA (Ion Beam Analysis) techniques can be little sensitive to the sample structure because energy and spatial distributions of incident ions usually depend at first order upon the areal density of atoms. NRA (Nuclear Reaction analysis) techniques are known to be quantitative techniques for light elements characterization under the assumption that the cross section of the used nuclear reaction has been accurately measured (see section 5.3.1). NRA 3He and 18O depth profiling techniques have already been successfully carried out on polycrystalline uranium dioxide samples36.

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The nuclear reactions which can be implemented to detect 16O in usual light ions aaccelerators are namely the 16O(d,p) and 16O(d,α) reactions. 16O(d,p) could be used to analyse oxygen37, however the depth resolution of the technique should appear to be relatively poor essentially because the energy loss of emitted protons is lower than that of α particles in the analysed depth. Since the aim is to measure the depth profile of oxygen concentrations in UC over a few micrrons, the non resonant 16O(d,α) nucllear reaction is the most suitable.

The 16O(d,α) reaction was previously used to determine the oxygen concentration profile at the surface of thermally oxidized silicon samples38 but was never implemented in a carbide compound because the concomitant 12C(d,p) nuclear reactions induced by the deuteron analysis beam covers up the oxygen signal. Through the use of a specially manufactured detector (see section 5.1.2) it was nevertheless possible to depth profile oxygen concentrations present at the surface of uranium carbide samples. In addition a micrometer size deuteron beam was applied at the CEA-CNRS/SIS2M in order to determine the 2D oxygen distribution (or cartography) at the surface of the samples (see section 5.3.4). The C/U variations at the surface of samples were also estimated (see section 5.2.3). The size of the micrometric deuteron beam was measured by applying the procedure described in 39 to ~ 2.5×2.5 µm2. The used experimental configuration is schematized in Figure 14.

Special annular detector at 180°: Detected active depth ~ 60 µm backscattered particles Rasterred µ-beam UC sample of 2H + in Epoxy resin 2

Screen to limit the incident RBS signal RBS and NRA signals

Figure 14: Scheme of the experimental configuration implemented at CEA--CNRS/SIS2M

5.1.2 Detection

The oxygen concentrations at the surface of UC samples were monitored through the detection of alpha particles emitted from the 16O(d,α) reactions. Hoowever, since protons from 12C(d,p) reactions are emitted at higher energies than the alpha particles from 16O(d,α) reactions, there should be carbon signals in the oxygen spectra whatever the incident beam ennergy is. For instance for incident deuterons of 1 MeV the kinematics of the 12C(d,p) and 16O(d,α) nuclear reactions predict the emission of 2.997 MeV protons and 2.58 MeV α particles at a scattering angle of 170°. The cross section of the 12C(d,p) reaction is moreover relatively important and it is expected the oxygen signals would be hardly detectable under those conditions.

The stopping powers and therefore the trajectory of alpha particles and protons of same energy will, however, be very different in the semiconductor detector. The average depth at which protons and alpha particles will stop in pure silicon (called projeected range) calculated using the TRIM-2011 freeware40 is shown as a function of particle energy in Figure 15. At energies of a few MeV, whiich are representative of nuclear reaaction products, protons will slow down in the semiconductor detector over a distance far larger than alpha particles at the same energy.

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Figure 15: Penetration depth of protons and alpha particles in Si as a function of their energy from TRIM- 201140 calculations.

A detector presenting a limited active depth could therefore detect the full kinetic energy of incoming alpha particles but only a part of the energy of MeV protons as those ions would be transmitted through the active detection layer. In such conditions, the only disturbing signals should theoretically come from the low NRA signals from the 13C(d,α) reactions (indeed 1.1 at.% of the carbon should be in the form of 13C).

An annular detector of 300 mm2 was therefore designed by CANBERRA® to separate the 12C and 16O signals. It is made of a high resistivity epitaxial semiconductor layer of ~ 60 µm which has been grown on a low resistivity semiconductor substrate. Since only the upper layer is active for particle detection, the energy of incoming protons is reduced to low levels such that they do not hinder the oxygen detection. The counterpart of the complex structure of this special device compared to a usual PIPS (Passivated Implanted Planar Silicon) detector is that its energy resolution is lower (given by the supplier to be 40 keV for alpha particle emitted from from the decay of 241Am, instead of less than 20 keV for a PIPS). Its relatively poor energy resolution directly impacts the depth resolution of the technique (see section 5.3.3).

5.1.3 Energy of the incident beam

Based on the reaction cross sections (see section 5.3.1), the sensitivity of the technique is expected to increase with increasing incident deuteron energies. From this consideration a 2 MeV deuteron beam was first used. However the massive pile-up induced by the U(d,d) RBS (Rutherford Backscattering Spectroscopy) signal, which extends up to almost 4 MeV under these conditions (see section 10.1), precluded any oxygen detection. A pile-up rejecter was implemented but it only divided by a factor 2 the pile- up signal intensity. The incident deuteron energy was therefore shifted to 960 keV. Figure 16 shows a typical experimental spectrum obtained by sending a 960 keV deuteron beam on sample UC_3b for 7h27.

Low nitrogen concentrations present in the samples explain the presence of high energy counts from the 14N(d,α) reactions. From this weak signal the nitrogen content was roughly estimated to 4 ± 2 at.% at the surface of UC samples (within the first few microns). This nitrogen no doubt comes from the fabrication process since milling and pressing stages are realized in a glove box under a N2 atmosphere (see section 3.2). This relatively low nitrogen content was neglected throughout the spectra simulation process.

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Figure 16: Raw spectrum obtained by analysing sample UC_3b with a 960 keV deuteron beam.

5.1.4 Energy calibration of the detection chain

The pulse heights resulting from particle detection are amplified into a signal the amplitude of which is an increasing function of the energy deposited in the detector. The resulting distribution of detected particles is distributed over a finite number of channels (1024 here). The channel number is proportional to the time- integrated amplitude of the detector signal. In depth profiling and particularly when using a non resonant reaction, it is necessary to convert channel numbers into energy40. This operation is called the energy calibration of the detector and is an essential stage of the depth profiling exercise.

Concerning the detection of ions, the relationship between the energy of a detected particle and the channel number it appears at is often linear. From the kinematics of the reactions between the incident particles of energy and reacting atoms contained in different targets, it is possible to calculate the energies which emitted particles possess at the sample surface. Figure 17 shows the raw RBS surface front signal obtained from a 960 keV incident deuteron beam normal to the surface of a UC sample.

Figure 17: U(d,d) RBS signal which was used for energy calibration purposes

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The highest channel number hence energy at which particles are detected corresponds in this case to the deuterons backscattered at the surface of the sample. From the analysis of different targets and the detection of particles produced from various reactions, the relationship between channel number and detected particle energy can be established. In the particular case of this study there was an unexpected lack of reference samples so the energy calibration was carried out essentially from the U and 16O surface signals of analysed UC samples. Figure 18 shows the calibration equation which has been used here.

Figure 18: Energy calibration of the detection chain

5.1.5 Description of the experimental configuration

Most part of the detector surface was screened to limit the detected particle flux to acceptable values below 4000 particules.s-1. To this end the beam current was also reduced to 0.7 nA (for a beam of 2H+ ions, see section 5.1..5). The backscattering yield is indeed particularly high due to the presence of uranium in the analysed samples. The detector and its screen are visible in Figure 19.

(a) (b)

Figure 19: Pictures of the special annular detector (a) and of the collimation window (b)

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The detection was affected by intense low energy electronic noise. It was also noticed that the detector energy resolution used to fit the energy spectrum with the SIMNRA41 software was estimated to be ~ 60 keV (notably to reproduce the surface U(d,d) and 16O(d,α) signals40), whereas its specifications mention a resolution of 40 keV (see section 5.1.2). This worse than expected resolution might be caused by the high noise levels.

2 + A 1920 keV molecular beam of H2 was delivered by the accelerator of CEA-CNRS/SIS2M instead of a 960 keV deuteron beam since the current stability and the brightness of the beam spot were improved under those conditions. As molecular deuterium ions should dissociate into 2 deuterons of 960 keV when impacting the target material, both sets of conditions are equivalent and a deuteron beam of 960 keV is mentioned throughout this report for stake of clarity. The experimental configuration adopted is indicated below: ¾ Incident beam angle to the normal of the analysed sample surface: 180° ¾ Detection surface: 15.88 mm2 ¾ Projected distance between the sample and the detector: 36 mm ¾ Detection angle (scattering angle): 170.31° (for a minimum of 169.00° and a maximum of 171.62°) ¾ Detection solid angle: 12.254 msr 2 + ¾ Beam description: 1920 keV H2 molecular beam ¾ µ-beam average dimensions: 2.5×2.5 µm2 ¾ Beam characteristic current: limited to 350 pA (700 pA considering a beam of 2H+, see above) ¾ Pressure inside the analysis chamber: ~ 10-7 mbar.

5.2 Determination of the sample stoichiometry

5.2.1 Presentation of the method

The number of particles used to analyse the target is measured by integrating the electrical current which flows between the target and the earth. The beam charges were collected by polarizing the sample holder (+40 V). Since UC is a rather good electrical conductor this method should provide a good estimate of the beam charge sent on the samples while analysed. A line made of silver paste has been drawn on Epoxy coating to electrically connect carbide samples to the sample holder. Samples were mounted inside a mobile glove box slightly over-pressurized with N2. The direct exposure to air while transferring the samples into the analysis chamber was limited to less than a minute to prevent as much as possible their oxygen and moisture contamination (see section 2.2.1).

However, the number of deuterons backscattered from the uranium carbide sample surface can be considered as an accurate internal indicator of the analysis charge, since the Rutherford backscattering cross sections are accurately modelled. However this supposes knowledge of the areal density of atoms, i.e. the composition of UC samples. Actually only the areal density of heavy atoms is required since at first order only this parameter really affects the trajectory of incoming and outgoing ions. Light impurities such as oxygen atoms even in large concentrations should not strongly modify the respective levels of U(d,d) and 12C(d,p) signals for a given C/U ratio, as shown by the SIMNRA41 simulations presented in Figure 20. The selected ROI (Region of Interest) for these elements (see section 5.2.2) are also reported in this figure.

In principle, Figure 20 indicates that taking into account the oxygen contents within UC samples shall induce small changes in the estimated analyse charge and stoichiometry. Therefore a global iterative multi- parametric procedure should actually be implemented to converge towards a fully coherent set of data, i.e. charge and sample composition. This requires to determine the charge, the C/U ratio and the oxygen composition of samples by means of a system of three implicit equations solved by simulating the three U(d,d), 12C(d,p) and 16O(d,α) experimental signals.

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Figure 20: SIMNRA simulation of U(d,d) (a) and 12C(d,p) (b) signals in a pure UC sample presenting a C/U ratio of 1.040. The simulated signal from a pure uranium carbide sample is compared to the signal from a sample which contains 10 at.% of oxygen.

Such a procedure can be decomposed into the following steps: A. The material which is initially considered contains only U and C atoms. B. Analysis charge and sample stoichiometry are found by simulating the carbon and U surface signals as described in section 5.2.2. C. The oxygen concentration profile is determined by applying the procedure described in section 5.3.3. D. The procedure is repeated by applying the step B on taking into account the oxygen depth profile determined from step C. The procedure ends when the charge and sample composition remain stable for two successive iterations.

This procedure was not fully implemented since it was applied only once from steps A to D. However, it was checked that oxygen concentrations were low enough to leave rather unchanged the charge and the stoichiometry determined from the two first initial iterations. At step B of the second iteration, the analysis charge and C/U ratio were respectively found to be 8.038 µC and 0.897 in the case of the sample UC_6 which contains the higher levels of oxygen (see section 5.3.4). The relative charge correction is therefore of 2.8% (see Table 3), which is anyway less than ~ 10% uncertainty related to the 16O(d,α) cross section (see section 5.3.1) and finally considered as main source of error in the framework of this study.

5.2.2 Sample composition and scanning conditions

Due to the detection setup (see section 5.1.2), the 12C(d,p) signal appears in the high energy part of the pile- up signal as shown in Figure 16. Since the high energy pile-up can be considered as a linear decreasing function of the energy (see appendix 10.1), it was possible to simulate it properly in the 12C ROI of the spectrum. The 12C(d,p) signal was then obtained by deducing the as-simulated pile-up from the total raw spectrum. The applied procedure is described in Figure 21.

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Figure 21: Pile-up simulation procedure and extraction of the 12C(d,p) signal (sample UC_3b).

An iterative procedure (dichotomy) was then applied to simulate simultaneously the uranium RBS signal and the 12C(d,p) signal, by adjusting the analyse charge and C/U ratio in each sample. The estimated charges are given in Table 3. Figure 22 shows the simulation of the uranium and carbon signals using a charge of 11.423 µC and a C/U ratio of 1.040 (sample UC_3a). Only the uranium surface signal has been simulated since SIMNRA could not successfully reproduce the slope in the low energy part of the U(d,d) signal. Standard simulation software are indeed known within the IBA community to poorly simulate low energy RBS heavy elements (d,d) RBS signals.

Figure 22: Simultaneous simulation of the experimental U(d,d) (a) and 12C(d,p) (b) signals from the analyse of the sample UC_3a.

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The simultaneous simulation of the carbon and uranium signals without any consideration for the presence of light impurities (oxygen) at this stage has therefore made it possible to estimate the analysis charge as well as the C/U ratio of analysed samples, irrespective of depth variations. Table 3 presents these estimates and the conditions which have been used to scan small 60×60 µm2 areas at the surface of each sample. 2 + 2 + Analysis charges are given for a beam of H , the real charges in terms of H2 charged molecules being basically twice as low (see section 5.1.5).

Sample C/U ratio of the Analysis Dead Measured Estimated name scanned area time time (%) charge (µC) charge (µC) UC_3a 1.040 9h26 3.63 19.275 11.423 UC_3b 0.722 7h27 12.29 16.389 11.571 UC_3c 0.934 7h16 3.60 14.657 8.556 UC_6 0.786 5h39 4.53 13.879 8.270 UC_10 0.949 7h56 4.41 15.926 11.426

Table 3: Scanning conditions of UC samples

5.2.3 C/U cartographies of monitored areas

Once the analysis charge has been determined (see section 5.2.2), the uranium and carbon signals each constitute a measure of the average C/U ratio within the sample. It is therefore possible to get a picture of the C/U variations at the sample surface at micron scale from both the uranium and carbon mapping. Mapping is obtained by adding the counts inside the respective uranium and carbon ROI (Region of Interest, see Figure 20) at each pixel of the scanned areas. The pile-up has been estimated and deduced from the C signal (see section 5.2.2) but we have also considered that this noise signal varies spatially since it depends upon the local sample composition by means of the U(d,d) signal level, as shown in section 10.1. Figure 23 shows the SIMNRA simulated uranium and carbon integrated signals as a function of the C/U ratio. The resulting data have been used to estimate the spatial fluctuations in sample composition.

Figure 23: Normalized U and C signals as a function of the C/U ratio simulated using the SIMNRA41 software

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One should point out that the C and U signals in any determination of the C/U ratio originate from different depths, respectively of around 0.3 and 2 µm. It is therefore interesting to compare the C/U mapping from each signal since they correspond to different ranges of depth. Potential composition gradients up to 2 µm below the sample surface can be detected this way. It was however noticed that the C/U obtained from mapping integrating respectively 0.3 µm and 2 µm below the sample surface were very similar (see Figure 24). Less noise was however observed on the C/U mapping which were obtained from the U signals. This is due to the relatively poor statistics of the carbon signals (see e.g. Figure 22 for the sample UC_3a) and thus at each pixel of the subsequent mapping.

(a) (b)

Figure 24: Comparison between the 60×60 µm2 C/U mapping obtained from the U(d,d) front signal [0,~0.3 µm] (a) and from the 12C(d,p) signal [0,~2 µm] (b) in sample UC_3a

5.3 Oxygen depth profiling

5.3.1 Reaction cross section

Several cross sections at scattering angles greater than 140° have been published for the 16O(d,α) nuclear reaction38,42-46. They can be found in the IAEA cross section database IBANDL (Ion Beam Analysis Nuclear Data Library). Those measured by Picraux, Amsel and Kim et al.43-45 appear to be poorly sampled in the cross section range below 1 MeV. They are therefore not suitable for the simulation of experiments performed using an incident energy of 960 keV as is the case here. Measurements by Turos et al.38 are not usable since they only extend up to 950 keV. The cross section recently published by Jiang et al.46 has been accurately measured up to 1057 keV. This cross section was measured for a 150° scattering angle.

A comparison between the cross sections published by Kim et al.43 at scattering angles of 142.2° and 164.25° reveals relative differences of up to 20% in the considered energy range. Since the scattering angle was 170.31° in the framework of this study, a relative discrepancy of ~ 20% could therefore been expected from the use of the cross section published by Jiang et al.46. This is why the cross section published by Seiler et al.42 at an angle of 164.25° appears therefore to be the most suitable for this study.

One can, however, still estimate the relative uncertainty associated with this cross section to circa 10% below 1 MeV. This points out to the fact that the nuclear data relative to the 16O(d,α) nuclear reaction near 170° should be experimentally determined as this would significantly improve the accuracy of the oxygen measurements using an annular detector. From these considerations, an uncertainty on the relevant oxygen reaction cross sections has been set to 10%.

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5.3.2 13C signal simulation

As shown in Figure 16 and since carbon is naturally composed of 1.1% of 13C, a low 13C(d,α) signal has to be deduced from the raw spectrum in order to obtain the oxygen spectrum. As samples contain a high level of oxygen at their surface (see section 5.3.4), this signal is insignificantly low relatively to the 16O(d,α) signal (it is two orders of magnitude lower). It even appears to be less than the background noise. This spectrum has nevertheless been simulated using SIMNRA then deduced. The simulated 13C signal which should appear in the oxygen ROI part of the UC_6 spectrum is shown in Figure 25.

Figure 25: 16O(d,α) signal for sample UC_6 and SIMNRA simulation of the 13C signal.

Subtracting this signal from the raw data could nevertheless be a problem when studying low oxygen concentrations within carbide samples. A better way to estimate the 13C(d,α) signal within the oxygen ROI would be to collect the experimental signal from a carbonated reference. This was attempted but the surface of all the reference samples which have been analysed (namely vitreous C and ZrC) was oxidised. A SiC sample could be used to this end since this material is thought to be less affected by oxygen contamination than other carbides.

5.3.3 Simulation of oxygen spectra

Oxygen depth profiles have been determined by carrying out a “numerical experiment” using appropriate simulation software, here SIMNRA41. In the framework of this study, the versions 5.02 and 6.06 were both used and appeared to be equivalent. The relevance of the model is assessed by comparing the results of the simulation with strictly equivalent experimental data. A material model made up of a succession of layers of homogeneous composition (constant C/U ratio as determined in section 5.2.2) has been built. The size of analysed layers must be determined from the depth resolution, which is defined by the uncertainty on the depth location of detected elements40. The depth resolution was then calculated using the RESNRA software47. It was found to be hardly dependent upon the depth, the oxygen content or C/U ratio. Different layer sets have nevertheless been used to simulate each experimental spectrum, but they were subsequently all found to be composed of a close number of layers of similar thickness in terms of areal density. Converting this depth unit into length, depending on the respective UC samples density, the depth

Page 26/47 F-BRIDGE – Document D-116 – revision 0 resolution of the technique was found to range between 100 and 200 nm. It is worth noting here that the density of the probed material was considered as a constant despite the high variations of C/U and oxygen contents in the depth and at the surface of samples (see section 5.4).

As regards to the high variations of oxygen levels within the samples, the simulated oxygen profiles could not be expressed as a linear combination of each layer spectrum. Strong variations in the oxygen concentration in a surface layer will indeed modify the trajectory of incoming and outgoing ions and therefore should impact the signal coming from deeper layers. The automated simulation procedure described in 47 involving a numerical minimisation of the difference between the experimental and simulated spectra has therefore not been applied. The oxygen concentrations were indeed manually adjusted so that the simulated spectra reproduced the experimental signals adequatly. The general shape of the solution was nevertheless provided by running an automated simulation since this procedure appeared somehow to converge towards a quasi acceptable depth profile. Figure 26 shows the SIMNRA simulation of the experimental spectrum measured in the sample UC_3a. The corresponding depth profile is visible in Figure 29.

Figure 26: SIMNRA simulation of the experimental oxygen spectrum of sample UC_3a.

5.3.4 Integrated oxygen concentrations at the sample surface

The average oxygen content inside the scanned areas was calculated by integrating the oxygen depth profiles48. This was then possible to draw the oxygen lateral distributions at the surface of the 60×60 µm2 scanned areas. The results of the oxygen analyses performed and especially the global oxygen content found in the samples are shown in Table 4.

Sample C/U ratio of the Probed Average Surface 16O content In-depth 16O content name scanned area depth 16O content (for z < 0.2 µm) (for z > 0.2 µm) UC_3a 1.040 1.89 µm 5.74 at.% 15.86 at.% 4.25 at.% UC_3b 0.722 1.40 µm 6.34 at.% 11.81 at.% 5.32 at.% UC_3c 0.934 1.72 µm 10.93 at.% 20.27 at.% 9.24 at.% UC_6 0.786 1.42 µm 12.60 at.% 26.00 at.% 9.58 at.% UC_10 0.949 1.72 µm 7.88 at.% 16.33 at.% 6.61 at.%

Table 4: Measured oxygen contents in UC samples

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The surface and in-depth oxidation levels were also defined by averaging oxygen concentrations between the surface and 200 nm and between 200 nm and the effectively probed depth.

5.4 Results and discussion

The C/U and 16O mapping of samples UC_3b, UC_3c, UC_6 and UC_10 are presented in Figure 27 and Figure 28. It is worth noting here that the composition of 60×60 µm2 scanned areas should be unrepresentative of the analysed samples. Measured C/U and oxygen distributions are visibly correlated (the oxygen mapping of UC_3a was found to be homogeneous so is not presented here).

(a) C/U (a’) [16O] UC_3b UC_3b

(b) C/U (b’) [16O] UC_3c UC_3c

Figure 27: Respective 60×60 µm2 C/U mapping [0,~0.3 µm] and oxygen surface 2D distributions [0,~1.5 µm] of samples UC_3b (a and a’) and UC_3c (b and b’).

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(a) C/U (a’) [16O] UC_6 UC_6

(b) C/U (b’) [16O] UC_10 UC_10

Figure 28: Respective 60×60 µm2 C/U mapping [0,~0.3 µm] and oxygen surface 2D distributions [0,~1.5 µm] of samples UC_6 (a and a’) and UC_10 (b and b’).

The presented mapping reveals high C/U areas which could correspond to small uranium dicarbide inclusions. Oxygen contents are higher inside these inclusions. This suggests that the oxidation is facilitated in uranium carbide compounds with increasing carbon content. This could be expected under the assumption that the oxygen incorporates within actinide carbides through carbon atom substitution (see section 2.2.1). Moreover some very local high C/U could be related to the observation of diamond particles with a size of 3-5 µm by using Raman spectroscopy (see section 6.1).

Samples UC_3a, UC_3b and UC_3c sliced from a same UC pellet have been polished in different conditions. Figure 29 presents the oxygen depth profiles which have been obtained from their NRA analysis. The sample UC_3b which was polished using a paraffin oil suspension shows the lower oxygen contents at the sample surface. The sample UC_3a polished in an aqueous suspension shows an unexpectedly low oxygen contents in the bulk, even slightly lower than in sample UC_3b. In this sample oxygen concentrations are nevertheless greater in layers closest to the surface (see Table 4) which indicates that the use of water during the last polishing step tends to oxidize a thin surface layer of few hundreds of nm deep. However its relatively low bulk oxidation as regards to the used polishing solution could be explained by the fact that it was polished only a few weeks before the NRA analysis, whereas this is not the case of the sample UC_3b (see Table 4). Finally the unpolished sample was found to be the far more oxidised. Oxygen concentrations measured in-depth (beyond 200 nm) are approximately twice those measured in the polished samples.

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Figure 29: Oxygen depth profiles of samples UC_3a, UC_3b and UC_3c.

These results indicate that the polishing stage has a protective role relatively to sample oxidation which is likely to occur during their storage. Polishing is indeed expected to drastically reduce their specific surface. The polishing solution seems eventually to play a secondary role as regards to the sample contamination beyond the first few hundreds of nm, but it is difficult at this stage to draw definitive conclusions.

Figure 30 shows the oxygen depth profiles corresponding to samples UC_3b, UC_6 and UC_10. These samples were manufactured from powders crushed and pressed (see section 3.4) under N2 atmospheres containing different oxidising impurity levels. The oxygen and moisture content of the atmosphere seem here to have a direct impact on the detected oxygen concentrations, particularly beyond 200 nm below the sample surface.

Figure 30: Oxygen depth profiles of samples UC_3b, UC_6 and UC_10

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This suggests that a large part of the oxygen present within the polished samples is somehow introduced during the fabrication process. The milling and pressing atmosphere may be important with regard to the introduction of impurities. These samples were also sintered under various atmospheres (see Table 2) and the pellet sintering conditions may also play a major role relatively to their bulk oxidation. A strong influence of the use of paraffin as additive is less probable since oxygen levels are very different in samples UC_6 and UC_10 for which the same additive and same polishing treatments were used. Although it is difficult at this stage to determine whether the oxygen concentration in the bulk is a result of the sintering atmosphere or another stage in the sample fabrication process, as expected, the manufacturing conditions appear to have a major impact upon the oxygen levels eventually measured.

6 Complementary characterizations of uranium carbide: Raman spectroscopy, polarized reflection microscopy and XRD

6.1 Introduction

Raman spectroscopy is a technique used to study vibrational, rotational, and other low-frequency modes in a sample. It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser source. The Laser light interacts with molecular vibrations, phonons or other excitations in the sample, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the sample. This technique is a non-destructive one.

The aim of these experiments, performed at the CEMHTI facility, CNRS laboratory in Orléans, was to try to identify any second phase in the uranium carbide samples. Complementary characterizations were also done by performing polarized reflection microscopy and X-ray diffraction (XRD) on the same samples.

6.2 Samples

Only eight samples were selected for Raman characterizations. These samples have been submitted to a variety of fabrication and preparation conditions which are presented in. The date at which they were polished indicates how long their storage lasted before they have been analysed (end 2011 - early 2012). Apparent densities were estimated by measuring the weight and the dimensions of as-fabricated pellets. Samples UC_3a, UC_3b and UC_3c were sliced from the same pellet. The last polishing stage is necessary for sample characterisation since a minimal planarity of sample surface is required for RAMAN experimental technique, especially for mapping characterizations. The last micron polishing step was performed using a paraffin oil suspension. In one case however an aqueous suspension was used in order to observe the possible oxidation induced by such a preparation procedure. A sample was also kept unpolished. Thereafter samples were stored in paraffin, which is reputed to prevent them from oxidising for at least several weeks. For the transport, a thin layer of paraffin oil covered with tape was deposited at the surface of samples to prevent them from oxidising, and then they were placed in membranes boxes. Finally, the boxes were putted in double vinyl bags packaged in an inert nitrogen atmosphere glove box to avoid contact with air. At the CEMHTI laboratory, a glove box was set up in order to prepare the samples before spectroscopy and to store them. All samples were cleaned, before analysis, with trichloroethylene and then putted in an airtight cell. The same preparations were used before performing XRD and polarized microscopy.

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Carboreduction Ball milling Sintering Polishing Sample Apparent Annealing N2 glove box Used Annealing Used Date name density temperature impurities additive sequence(s) suspension (2011) 88.63 15 ppm O + UC_3a 1650°C 2 - 1690°C/5H in Ar Aqueous 04/04 %/ρth. 3 ppm H2O 88.63 15 ppm O + UC_3b 1650°C 2 - 1690°C/5H in Ar Paraffin 11/10 %/ρth. 3 ppm H2O 88.63 15 ppm O + UC_3c 1650°C 2 - 1690°C/5H in Ar Unpolished - %/ρth. 3 ppm H2O 65.44 30 ppm O + UC_6a 1700°C 2 Dodecane 1700°C/5H in vacuum Paraffin 04/12 %/ρth 4 ppm H2O 74.7 30 ppm O + UC_6b 1700°C 2 Paraffin 1700°C/5H in vacuum Paraffin 04/12 %/ρth. 4 ppm H2O 33 ppm O2 + UC_7a %/ρth 1720°C Paraffin 1660°C/5H in vacuum Paraffin 05/13 12 ppm H2O 33 ppm O2 + UC_7a2 %/ρth 1720°C Paraffin 1660°C/5H in vacuum Paraffin 11/17 12 ppm H2O 1660°C/12min in 69.57 1 ppm O + UC_10 1720°C 2 Paraffin vacuum Paraffin 05/13 %/ρ 86 ppm H O th. 2 then 1630°C/5H in Ar

Table 5: Particularities in UC sample fabrication and preparation conditions.

This Raman spectroscopy was conducted on a Renishaw Invia Reflex spectrometer. Three monochromatic light laser sources were available that give five different wavelengths (785 (infrared), 633, 514, 488 and 457 nm). The laser beam is focused through a microscope onto the sample surface, and the scattered light is passed into a spectrometer, which disperses the light onto a charge-coupled-device (CCD) detector. An automatic X-Y device on the microscope table allows to acquiring Raman maps on the sample surface. In this configuration the 1-2 first µm of the samples is probed.

6.3 Raman Experiments

The first step of the experiment was to make an optical microscopic examination of each sample. Most of the samples present a microstructure with different colours (blue, brown, white and black) grains as shown in Figure 32. Only two samples (UC_7a and UC_7a2) seem to have a uniform metallic appearance.

Figure 31: Microstructure of UC_3a sample Figure 32: Microstructure of UC_7a2 sample

Measurements were performed by mapping samples in more or less extended areas in an attempt to observe differences between the microstructural zones. To validate the existence of a peak or not, the maps were made contiguously. That means to validate the existence of a peak, the peak must be found on two consecutive spectra. Measurements were tried with the whole set of exciting laser lines available on the CEMHTI Raman facility. Results do not exhibit any privileged laser excitation.

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One important trend of Raman characterization in UC is that no first order Raman line is expected on pure UC, due to its crystalline symmetry. This lack of Raman information on UC itself beecomes an advantage as 49 the method is highly sensitive to any perturbation or impurity in UC, as UO2 , UC2, U2C3.

6.3.1 Results

In the sample UC_3a, a thin peak at 1332 cm-1 was observed (see Figure 33). This frequency is well-known as due to diamond. In fact, the last polishing step was made with a diamond suspension. So, it is possible that one grain of diamond had taken place in a pore of the sample.

Figure 33: Raman Spectra for the UC_3a sample showing diamond peak (excciting line: 633 nm)

Raman peaks are positioned at the same relative positions to the laser wavelength. In contrary, luminescence peaks are absolute positioned. So, if a peak is found for different wavelength at the same position it is a Raman signal. The results presennted below take into account this consideration.

All samples show a very wide bump between 13300 and 1600cm-1 (see Figure 34). This very important peak width implies structural disorder. This group is well-known to originate from sp2 carbons: a single peak at 1580 cm-1 (position slightly depending of the exxcitation wavelength) is representatiive 3D – graphite order, called “g” peak for crystalline graphite. In all other carbonaceous materials a seecond peak appears at 1350cm-1, called “d” peak for disordered carbons. In very disordered of amorphous carbon materials, the two peaks collapse in a very broad band characteristic of this. So, in the uranium carbide samples, it is clear that there is some excess carbon in a very disordereed state.

A sample of metallic uranium was also analysed. This sample come from the CEMHTI laboratory and was stored in air, so it should be oxidized at the surface. Only a very important luminesceence was observed.

On several samples, white areas (see optical microscopy observation) exhibit lower intensity on Raman spectra compared with blue and brown areas (ssee Figure 35). This drop of intensitty could be explained by considering these white areas as metal, because the laser beam less penetrates the metal compared to the rest of the non-metallic sample. This loss of intensity was observed for grains but allso for grain boundaries. Nevertheless all these intensities remain very small and at this step, except for carbon signatures, it remains difficult to conclude on the origin of these signals, which be 1st or more probably 2nd order Raman, or low- intensity luminescence.

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Curve 1 1350 308.314 51.3688 72.9502 Mixed 19027.9 Curve 2 1580 177.264 38.4639 100 Mixed 7257.8

Figure 34: Wide bump observed for the UC_3c sample, exhibiting d (1350) andd g (1580cm-1) lines characteristic of dissordered carbonaceaous mater

Figure 35: Raman spectra on (a) white area and (b) brown or blue area

The main conclusion of this studyd is that apaart from the diamond and disordeerred carbon signatures, not any Raman peak was observed in the whole set of samples under consideration. Many systematic studies by Raman mapping were undertaken, nevertheless, no sigignature of any impure phase (as UUO2 UC2 or U2C3) was observed.

It must be underlined that if the samples would present a thin layer of approximately 440 nm of ordered UO2 at their surface. It would be hard but possible to see the characteristic Raman signal. SSo if the surface oxygen content found in NRA (≈ 30 at.%) is in the form of dioxide, the Raman measurements show that the structure is not ordered UO2.

One sample (UC_7a2) was sacrificed in order to observe some surface oxidizing. The first step was to analyze the sample like the other ones in the airtight cell conditioned in the glove box under argon. The second and third steps were to open the airtight cell in air respectively during one aand ten minutes and re- analyze the sample. Changes were observed neither in Raman spectra nor in the surface microstructure. Even after one night in contact with air, the sampple has showed no changes. This saample presentn s a metallic

Page 34/47 F-BRIDGE – Document D-116 – revision 0 lustre. This is perhaps the reason why contact with air has given no results. This part of experiment should be tried with another sample with a different microstructure (for example, the sample UC_3a). Nevertheless this shows that UC samples do not seems very sensitive to surface oxidation, at least for a surface layer thick enough (>40 nm) to be detected by Raman.

6.4 Polarized microscopy

The polarized reflection microscopy was performed directly at the CEMHTI facility. The use of two polarizers can highlight birefringence of an anisotropic crystal. Isotropic crystals, for their part, do not exhibit birefringence. This technique could therefore help to visualize crystals of uranium metal with an orthorhombic crystallization and UC2 with tetrahedral crystallization (in its majority crystallographic form) from the other 2 phases possibly present (UC and U2C3 which crystallize respectively in cubic and face centred cubic form) .

Only two samples have revealed some interesting results in terms of birefringence. The sample UC_7a2 has initially a metallic brightness (Figure 36.a) which turns off completely when the polarizers are rotated by 90° (Figure 36.b ). For the sample UC_3b, only one grain goes out (a and b Figure 37).

a a

b b

Figure 36. UC_7a2 sample: a/ parallel Figure 37. UC_3b sample: a/crossed polarization; polarization; b/ crossed polarization b/ parallel polarization

It is possible that the surface of the UC_7a2 sample is mainly composed of metallic uranium. The grain in the

UC_3b sample could be a grain of UC2.

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6.5 XRD characterizations

Before transport to Orleans, all samples were not characterized by XRD. In the CEMHTI laboratory it was possible to perform this characterization by using XRD diffractometer in a low incidence configuration (5- 10°). Thanks to collimators only the UC surface has been characterized. In these conditions of incidence of the X rays, the first µm of the UC samples can be probed. The results obtained are presented in Figure 38.

70000

60000

F UC 4-10

50000 F UC 4-7a2

40000

F UC 4-7a Lin (Counts) Lin 30000 F UC 4-6a

20000

10000 F UC 4-3a

0 10 20 30 40 50 60 70 80 2-Theta - Scale Temp.: 25 °C (Room) - F UC 4-3a - File: F UC 4-3a.raw - Type: 2Th/Th locked - Start: 10.291 ° - End: 90.202 ° - Step: 0.016 ° - Step time: 370. s - Time Started: 4 s - 2-Theta: 10.291 ° - Theta: 5.000 ° - Chi: 0.00 Temp.: 25 °C (Room) - F UC 4-10 - File: F UC 4-10.raw - Type: 2Th/Th locked - Start: 10.286 ° - End: 90.198 ° - Step: 0.016 ° - Step time: 370. s - Time Started: 13 s - 2-Theta: 10.286 ° - Theta: 5.000 ° - Chi: 0.0 Temp.: 25 °C (Room) - F UC 4-6a - File: F UC 4-6a.raw - Type: 2Th/Th locked - Start: 10.579 ° - End: 90.406 ° - Step: 0.016 ° - Step time: 370. s - Time Started: 12 s - 2-Theta: 10.579 ° - Theta: 5.000 ° - Chi: 0.0 Temp.: 25 °C (Room) - F UC 4-7a2 - File: F UC 4-7a2.raw - Type: 2Th/Th locked - Start: 10.139 ° - End: 90.094 ° - Step: 0.016 ° - Step time: 370. s - Time Started: 7 s - 2-Theta: 10.139 ° - Theta: 5.000 ° - Chi: 0 Temp.: 25 °C (Room) - F UC 4-7a - File: F UC 4-7a.raw - Type: 2Th/Th locked - Start: 10.315 ° - End: 90.219 ° - Step: 0.016 ° - Step time: 370. s - Time Started: 8 s - 2-Theta: 10.315 ° - Theta: 5.000 ° - Chi: 0.00 00-004-0783 (I) - Silver-3C, syn - Ag - Y: 8.64 % - d x by: 1. - WL: 1.5406 - Cubic - a 4.08620 - b 4.08620 - c 4.08620 - alpha 90.000 - beta 90.000 - gamma 90.000 - Face-centered - Fm-3m (225) - 4 - 68.2274 - 03-065-8815 (A) - Uranium Carbide - UC - Y: 29.81 % - d x by: 1. - WL: 1.5406 - Cubic - a 4.96200 - b 4.96200 - c 4.96200 - alpha 90.000 - beta 90.000 - gamma 90.000 - Face-centered - Fm-3m (225) - 4 - 122 01-089-2418 (N) - Uranium Carbide - UC1.86 - Y: 4.44 % - d x by: 1. - WL: 1.5406 - Tetragonal - a 3.52600 - b 3.52600 - c 6.00000 - alpha 90.000 - beta 90.000 - gamma 90.000 - Body-centered - I4/mmm (139)

Figure 38 : XRD spectra measured in low incidence for five samples of uranium carbide

For all analysed samples, the characteristic peaks of UC were observed (highlighted in blue on the bottom axis in Figure 38) and also the characteristic peaks of silver (highlighted in fuchsia on the bottom axis in Figure 38) which come from silver lacquer used for conductive contacts (see section 4.1).

Only one sample (UC_3a) presents between 5 and 10 vol.% of UC2 phase. In none of the samples were observed U2C3, UO2 or U metal.

6.6 Conclusions

Raman spectroscopy has showed some C enriched zones with very disordered carbon in each sample. No signature of UO2 or any oxide compound or of non-stoichiometric uranium carbide (UC2 or U2C3) has been found. But Raman signatures of these species are not unambiguously known today.

The polarized microscopy has given the possibility to identify metallic uranium and the tetragonal UC2 phase in two different samples. Only one grain of UC2 has been highlighted and the Raman spectroscopy on it does

Page 36/47 F-BRIDGE – Document D-116 – revision 0 not give any result. On the other hand, some huge holes has been found (200 µm) with the polarized microscopy because this holes can react like birefringence grains.

The XRD analysis has showed the presence of UC in all samples and a small fraction of the probed region is composed of UC2 only in the UC_3a sample.

With the Raman spectroscopy, no peak is expected for the presence of UC in the samples. The UC2 phase found in UC_3a sample by XRD has not been found with the Raman analysis. Despite a significant scanning of the sample, it could be possible that the areas analysed did not contain UC2.

7 Positron annihilation spectroscopy

In this chapter we report the results of the positron annihilation experimental studies that have been performed in the network of F-BRIDGE on UC samples. Positron annihilation spectroscopy (PAS) has demonstrated its powerful in the identification and determination of fundamental properties (migration, agglomeration, etc.) of the vacancy defects in several materials including UO2 as shown in D111 and D115 of this project or in references 50-56. Very few data are known about the properties of the vacancies in UC as reported in 2. No data have been found in the open literature on the defects induced in UC by irradiation. Matzke has shown that the exposure of UC single crystal to air with various humidity percentage, lead to the formation of a U sublattice disordered layer which becomes thicker when the exposure time and humidity increase57. It must be underlined that PAS technique is also sensitive to the chemical composition of material.

Originally the objectives of this study were to characterize the vacancy defects induced in UC when irradiated with He ions and to investigate their evolution as a function of the annealing temperature. To carry out this kind of study it is first necessary to: 1/ check the quality of the UC samples and determine the preparation conditions (polishing, annealing) to obtain the positron annihilation characteristics of the material with the lowest concentration of vacancy defects in order to be able to identify the defects induced by irradiation. 2/ probe the defects induced by irradiation and try to identify them. 3/ measure the evolution of the positron annihilation characteristics after annealing of the irradiated samples and give some hypothesis on the eventual transformation of detected defects.

Due to the delay in the supply of the UC samples only a few experiments have been possible before the end of the F-BRIDGE project. The objective of these preliminary and exploratory measurements was to check the quality of the UC samples that have been already prepared and also characterized by using complementary techniques such as NRA for oxygen content, Raman spectroscopy and XRD.

7.1 Experimental details

The UC samples measured in this first campaign are listed in the following table (Table 6). They have been wrapped with epoxy and polished in the conditions reported in the same table. For the transportation, a thin layer of paraffin oil covered with tape was deposited at the surface of samples to prevent them from oxidising, and then they were placed in membranes boxes. Finally, the boxes were putted in double vinyl bags packaged in an inert nitrogen atmosphere glove box to avoid contact with air.

At the CEMHTI laboratory, a glove box was set up in order to prepare the samples before spectroscopy and to store them. All samples were cleaned, before analysis, with trichloroethylene and then mounted on the sample holder in air. The mounting operation lasts a few minutes. Then the sample holder is installed in the

Page 37/47 F-BRIDGE – Document D-116 – revision 0 detection chamber used for PAS measurements which is pumped up to a vacuum of a few 10-8 mbar maintained during all the measurements.

Carboreduction Ball milling Sintering Polishing Sample Apparent Annealing N2 glove box Used Annealing Used Date name density temperature impurities additive sequence(s) suspension (2011)

15 ppm O2 + UC_3a 88.63 %/ρth. 1650°C - 1690°C/5H in Ar Aqueous 04/04 3 ppm H2O 33 ppm O2 + UC_7a ?? %/ρth. 1720°C Paraffin 1660°C/5H in vacuum Paraffin 05/13 12 ppm H2O 30 ppm O2 + UC_6 74.7 %/ρth. 1700°C Paraffin 1700°C/5H in vacuum Paraffin 04/12 4 ppm H2O 1660°C/12min in 1 ppm O + UC_10 69.57 %/ρ 1720°C 2 Paraffin vacuum Paraffin 05/13 th. 86 ppm H O 2 then 1630°C/5H in Ar

Table 6: Particularities in UC sample fabrication and preparation conditions.

Only the three F-UC/3a, 6b and 10 have been characterized by using NRA (see section 5 in this report) and the results are summarized in the Table 7. It has to be noticed that the C/U ratio and oxygen content are very close for both F-UC/3a and 10 samples whereas their apparent density is very different.

C/U Average Surface In-depth ratio of Probed 16 16 Sample 16O O O Raman Polarized the depth XRD name content content content spectroscopy microscopy scanned (µm) (for z < 0.2 (for z > 0.2 at.% area µm) at.% µm) at.% UC UC_3a 1.040 1.89 5.74 15.86 4.25 at.% 5-10% UC C 2 2 particles U metal UC_7a UC C particles UC_6b 0.786 1.42 12.60 26.00 9.58 at.% UC C

UC_10 0.949 1.72 7.88 16.33 6.61 at.% UC C

C = “defected” Carbon enriched zone

Table 7: Summary of the results obtained with the different characterization techniques (NRA, XRD, Raman) in the F-UC/3a, 6b and 10 samples

In this work slow positron beam coupled with Doppler broadening spectroscopy (DB-SPB) has been used to study the UC samples described in the chapter 4 also examined by using Raman, DRX and NRA.

The positron-electron pair momentum distribution has been measured at 300 K by recording the Doppler broadening of the 511 keV annihilation line characterized by the low S and respectively high W momentum -3 -3 annihilation fraction in the momentum range (0-⎜2.80⎜) x10 m0c and (⎜10.61⎜-⎜26.35⎜)x10 m0c respectively. To investigate the depth dependence of S and W, the curves S(E) and W(E) were recorded as a function of the positron energy E changed in 0.5 keV steps in the 0.5 to 25 keV range using a slow positron beam 55 at the CEMHTI laboratory. The positron mean implantation depth in UC varies from approximately 1 nm to 530 nm (compared to 670 nm in UO2) in this energy range (Figure 39). It is important to underline that positrons at the energy of 25 keV probe the first 1.3 µm under the surface, which represents approximately the same probed region than the one which has been characterized by using NRA, Raman and incidence XRD.

In these study one already characterized UO2 disk has been used as a reference. This AA23 disk has been polished and annealed at 1700°C during 24 hours in humid Ar/H2 atmosphere. It has been measured regularly as a reference sample. In this disk, S and W values remain constant between 5 and 25 keV (SAA23

= 0.3727(5); WAA23 = 0.0783(2)).

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Implantation profile of positrons in UC Makhovian shape P(E,x)= 2x/x 2*exp(-(x/x )2) 14.5 0 0 n x =(1/0.886)*((A/ρ)*E ) 0 -3 A= 29.5, ρ= 13.63 g/cm , n=1.7 11.6

8.7 2.5 keV (/3) 5 keV (/2) 9 keV 5.8 12.5 keV 16 keV 20 keV Implantation Profile * 1e3 2.9 25 keV

0.0 0 300 600 900 1200 1500 Depth [nm]

Figure 39: Positron implantation depth profile in UC at different energies in the range from 2.5 to 25 keV.

The slow positron beam is designed for the measurements of not too thick samples (about 1 mm). It must be noticed that the epoxy support on which the UC samples have been stuck to ensure their integrity during transportations between different laboratories can constitute a drawback for positron annihilation spectroscopy. It can prevent to position the UC sample at a correct distance from a permanent magnet which allows focusing the positron beam on the sample. It has also been necessary to fabricate a specific system to maintain the epoxy support on the sample holder.

7.2 Results and discussion

The variation of the low (high) momentum annihilation fraction S (W, respectively) for several UC samples and the UO2 reference disk is plotted in Figure 40 as a function of the positron energy (S(E) and W(E), respectively) and as a function of the high momentum annihilation fraction W (S(W)) using the positron energy as the running parameter.

0.44 0.44 UO2 AA23 0.42 0.42 AA23 UC F-UC-4/6b 0.40 0.40 F-UC-4/3a

F-UC-4/7a

S parameter S 0.38 0.38

0.36 0.36

0.10 0.050 0.055 0.060 0.065 0.070 0.075 0.080 0.085 0.090 W Parameter

0.09

0.08

0.07 W parameter W

0.06

0.05 0 5 10 15 20 25 Energy (KeV)

Figure 40: Positron annihilation Characteristics in UC samples compared to the ones measured in virgin UO2 disk annealed at 1700°C during 24 hours in a wet ArH2 atmosphere : a) low momentum annihilation fraction S as a function of positron energy , b) high momentum annihilation fraction W as a function of positron energy and c) S as a function of W .

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It can be observed that the S(E) and W(E) profiles depend on the samples.

1/ The profiles obtained in UC samples are clearly different from the one measured for UO2. It can be explained by the various chemical compositions of these uranium compounds. The momentum distribution of electrons which is probed by positrons is indeed expected to be different between carbide and oxide as it is suggested by the position of S,W points measured in U, UO2, and diamond (see Figure 41) 2/ A peak in S(E) and a valley in W(E) can be observed in the positron energy range from 8 to 12 keV for the F-UC-4/7a sample. This dramatic change of annihilation characteristics S and W is probably due to the measure configurations which is not optimized for these UC samples due to the thickness of the epoxy support. As discussed above the magnetic field lines are not optimized for so thick samples and some positrons can annihilate in the epoxy support as it can be suggested by the alignment of the S W points obtained in this energy range with the SW characteristics measured in the epoxy. The data obtained in this energy range will not be taken into account in the following discussion. 3/ The S(E) and W(E) profiles are not the same for all the UC samples characterized, indicating that these samples are not equivalent in the probed region (1-1300 nm). However, common features can be observed. - For all UC samples the S(E) and W(E) variations indicate that the samples are not homogeneous in depth. - This depth heterogeneity changes with the sample. It is possible to distinguish two energy ranges corresponding to two characteristic regions: low energy region 0.5-5 keV for the close to the surface zone (1-100 nm) and high energy region 5.5-25 keV for the “bulk” zone (100-1300 nm). All the 4 samples give different S(E) and W(E) profiles in the close to the surface zone whereas only two tendencies can be observed in the “bulk” zone. - In the “bulk” region, F-UC-4/7a and 6b give comparable S(E) and W(E) profiles clearly different from the ones measured in F-UC-4/3a and 10 which are close together. The S(E) (and respectively W(E)) values are lower (respectively higher) for the second group compared to the first one. It indicates that more vacancy defects are detected in the “bulk” of the F-UC-4/7a and 6b samples. It can be related either to the apparent density of the samples or to the C/U ratio which has been measured by using NRA (see Table 7). Concerning the density it is close to 70% of theoretical value for F-UC-4/6b and 10 and reaches a higher value of 88% for the F-UC-4/3a. Yet if the density can play a role in the change of S and W it is expected to see the S increase and the W decrease when the density decreases. It can be concluded that there is no relation between the density and the S(E) and W(E) behaviour in these 4 samples. The C/U ratio is close for both F-UC-4/3a and 10 samples at a value of about 1 ± 0.05 and dramatically lower for F-UC-4/6b in which it reaches 0.786 for one cartography (on an area of 64*64 µm2) indicating the presence hypostoichiometry zones in this sample, which could appear as a material with high vacancy concentration. One has to notice that with the low number of experiments already performed, it is difficult to assert that this tentative explanation has to be retained. Of course more measurements are planned in various samples prepared in different conditions (for the elaboration and the annealing after polishing). - Close to the surface, all samples show different S(E) and W(E) profiles. These data are very sensitive to the surface quality and its positron annihilation states, to the possibility to form positronium (e+-e- exotic meta-stable atom Ps) which depends on the chemical surface composition. Therefore it has been shown that the Ps fraction can increase dramatically for oxidized metals as it has been shown for Al(111) surfaces where the increase of the oxygen cover lead the Ps fraction increase58. Yet the increase annihilation fraction of Ps should lead to the increase of S and decrease of W. The F-UC- 4/3a present the lowest S(E) values and respectively the highest W(E) ones, whereas S increases and W decreases for F-UC-4/7a, 6b and 10 in correlation with the citation order of these samples. Moreover it can be noticed that there is small differences for 7a and 3a samples. The Ps fraction remains low in this material always lower than 8% for all the samples and it decreases for the various samples with change in the following order 10, 6b, 3a and 7a that means in reverse order compared to the S changes. As expected the S value is the lowest for the lowest Ps fraction. Let’s assume that Ps fraction could increase with the oxygen content at the surface this value is the lowest of about 20 at.%

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for the F-UC-4/3a and increases to 22 and 30 at.% for F-UC-4/10 and F-UC-4/6b, respectively. The Ps fraction changes seem not to be related to the oxygen content at the surface like the S and W values measured close to the surface which are not the reflect of an eventual oxidation. Indeed it can be expected that the SW points measured in an oxygen polluted UC layer would change by being closer to the positron annihilation characteristics of uranium oxides. It is not the case and we can observe exactly the reverse behaviour so it can be concluded that the changes in S and W close to the surface is not directly linked to the oxygen content measured in the UC samples. But it can be explained by the formation of a disordered layer at the surface of UC due to oxidation as H. Matzke et al. have observed by channelling ion beam analysis.

F-UC-4/10 has been characterized twice with 24 hours of storage time in vacuum between the two measurements. The S(E) and W(E) profiles are strictly identical indicating that the sample does not evolve in the bulk or close to the surface.

1.15 Uranium oxyde UO B23 ref (BOUR10) 2 UO AA23 ref (B1EZAA) 2 UO : 3He 1 MeV 1x1017 cm-2 2 1.10 U O (BODX30) 3 7 V complexes in UO Uranium U 2 UN7 : etched + annealed + Etched Ga16 Uranium carbide UC-4/3a (B1EZAB) (UC , 5-10% Vol.)) U 2

S/S UC-4/6b (B1EYAB)

1.05 UC-4/7a (B1EZAC)

U O 1.00 UO 3 7 2

C/ Diamond 0.7 0.8 0.9 1.0 W/W Ga16

Figure 41: Positron annihilation characteristics in various compounds: uranium, uranium oxides (UO2 and U3O7), and diamond samples compared to the UC samples of this study: low momentum annihilation fraction S as a function of high momentum annihilation fraction W.

7.3 Preliminary PAS conclusions

First positron annihilation measurements were performed in UC samples. Our preliminary results suggest that these samples are not equivalent and not homogeneous, which indicates that the vacancy defects distribution is different for each of them. The vacancy defects detected can be due to polishing and/or oxidation. More measurements are planned in various stoichiometric samples prepared in different conditions (for the elaboration and the annealing after polishing).

8 Conclusions

The fabrication of dense monocarbide samples was shown to be difficult due to the complexity of the U-C phase diagram and the susceptibility of actinide carbide compounds to react to oxygen in particular. The carboreduction process has been selected since a control of fabrication conditions can be ensured more readily than with other processes. In particular the sintering behaviour of UC powders is highly dependent upon the C/U ratio and improvements are needed to control this key parameter.

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Despite these difficulties, UC samples were produced and samples prepared from them were analysed by using complementary techniques. µ-NRA has allowed us to probe oxygen concentrations at their surface. A specific experimental configuration was implemented for the first time and this technique was shown to be relevant for characterising the oxygen depth profile at concentrations of a few at.% in our actinide carbide compounds. Further improvements can however be achieved through nuclear reaction cross section measurements and the analysis of reference samples in particular.

The use of other techniques such as Raman spectroscopy and XRD has allowed completing the analysis of the first µm of the UC samples. Therefore it has been found that UC is the major and for 4/5 samples the unique phase found in these first samples. Only one sample shows 5-10% vol. of UC2. Raman and NRA cartographies and positron annihilation spectroscopy demonstrate the heterogeneity of the samples.

Oxygen contents of up to 30 at.% were measured at the sample surface by NRA. No ordered UO2 phase has been found by Raman or XRD probably because of the lower sensitivity of these techniques. The ratio C/U depends on the UC sample. By Raman and NRA C enriched zones have been observed. Finally PAS has shown that these samples are not equivalent and not homogeneous, which indicates that the vacancy defect distributions are different for the various samples. The vacancy defects detected can be due to polishing and/or oxidation. Of course more measurements are planned in various stoichiometric samples prepared in different conditions (for the elaboration and the annealing after polishing).

The results obtained point out that the fabrication conditions play a major role in the oxygen content of as- fabricated UC pellets. Therefore carrying out the whole production process under controlled atmosphere should limit the amount of oxygen or indeed nitrogen within the samples. Particular care should also be given to sample preparation conditions since polishing appears to have a protective effect with regard to surface oxidation of UC samples possibly by decreasing the specific surface. Finally, strong variations in the local stoichiometry of samples on the micron scale have been detected suggesting the presence of other phases such as UC2 in the fabricated material. This superior carbide compound appears to oxidize more readily and warrants additional control of the carbon potential during the carboreduction or sintering stages. The transport and storage conditions developed in this study are conservative.

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9 Bibliography

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14. Laugier, J. and Blum, P.L. Le diagramme métastable UC-UC2. Journal of Nuclear Materials 39, 245-252 (1971).

15. Matzke, H. and Politis, C. Self-diffusion of uranium in uranium dicarbide UC2. Solid State Communications 12, 401-404 (1973). 16. Scherff, H.L. and Springer, A. Redetermination of the carbon-rich phase boundary of uranium monocarbide. Journal of Nuclear Materials 56, 153-160 (1975). 17. Chevalier, P.Y. and Fischer, E. Thermodynamic modelling of the C-U and B-U binary systems. Journal of Nuclear Materials 288, 100-129 (2001). 18. Utton, C.A., De Bruycker, F., Boboridis, K., Jardin, R., Noel, H., Guéneau, C., Manara, D. Laser melting of uranium carbides. Journal of Nuclear Materials 385 (2), 443-448 (2009). 19. Guéneau, C., Chatain, S., Gossé, S., Rado, C., Rapaud, O., Lechelle, J., Dumas, J.C., Chatillon, C. A thermodynamic approach for advanced fuels of gas-cooled reactors. Journal of Nuclear Materials 344 (1-3), 191-197 (2005). 20. Guéneau, C. et al. Thermodynamic modeling of the U-Pu-O-C system using the Calphad method – Intermediate Report. F.BRIDGE WP1-3 report (2010). 21. Guéneau, C. et al. FUELBASE: a thermodynamic database for advanced nuclear fuels. Proceedings of the HTR2006: Third International Topical Meeting on High Temperature Reactor Technology, Johannesburg, South Africa (2006).

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22. Guéneau, C., Dupin, N., Sundman, B., Rado, C. and Konings, R. A progress report on the development of FUELBASE, a thermodynamic database for the description of multicomponent systems for nuclear fuel applications- Part 1, Binary Systems. CEA Technical report 2007 23. Garcia, T. Elaboration de monocarbures d’uranium denses à l’aide des moyens disponibles au LCU. CEA Technical report (2010). 24. Rundle, R.E., Baenziger, N.C., Wilson, A.S. and McDonald, R.A. The Structures of the Carbides, Nitrides and Oxides of Uranium. Journal of the American Chemical Society 70, 99-105 (1948). 25. Potter, P.E. and Spear, K.E. Thermodynamics of Nuclear Materials 1979. IAEA, Vienna 2, 195 (1980). 26. Potter, P.E. The uranium-plutonium-carbon-oxygen systems: The ternary systems uranium-carbon- oxygen and plutonium-carbon-oxygen, and the quaternary system uranium-plutonium-carbon-oxygen. Journal of Nuclear Materials 42, 1-22 (1972). 27. Henry, J.L., Blickensderfer, R., Paulson, D. and Bates, J.L. Hot hardness, thermal diffusivity, and electrical resistivity of uranium oxycarbides. Journal of the American Ceramic Society 617 (1968). 28. Steele, B.C.H., Javed, N.A. and Alcock, C.B. Measurement of the equilibrium oxygen, carbon and uranium activities associated with the uranium oxycarbide phase. Journal of Nuclear Materials 35, 1-13 (1970). 29. Alcock, C.B., Javed, N.A. and Steele, B.C.H. Thermodynamics of solution of oxygen and nitrogen in uranium monocarbide. Bulletin de la société Française de Céramique 99 (1967). 30. Hennecke, J.F.A. and Scherff, H.L. Carbon monoxide equilibrium pressures and phase relations during the carbothermic reduction of uranium dioxide. Journal of Nuclear Materials 38, 285-291 (1971).

31. Tagawa, H. and Fujii, K. Formation of U2C3 in the reaction of UC2 with UO2. Journal of Nuclear Materials 39, 109-114 (1971). 32. Piazza, J.R. and Sinott, M.J.Journal of Chemical Engineering 11, 392 (1966). 33. Louwrier, K.P., Richter, K., Kramer, G. and Lebrun, M. Preparation of a highly reactive plutonium dioxide powder for plutonium-uranium-carbide and nitride fuel. Journal of Nuclear Materials 61, 219-220 (1976). 34. Lorenzelli, R. and Delaroche, P. Fabrication et contrôles de combustibles à base de carbures mixtes (U,Pu)C. CEA Technical report (1980). 35. Garcia, P., Fraczkiewicz, M., Davoisne, C., Carlot, G., Pasquet, B., Baldinozzi, G., Siméone, D. and Petot, C. Oxygen diffusion in relation to p-type doping in uranium dioxide. Journal of Nuclear Materials 400 (2), 112–118 (2010). 36. Martin, G., Garcia, P. and Sauvage, T. Depth profiling. Encyclopedia of Analytical chemistry, John Wiley and Sons (2009). 37. Amsel, G. and Samuel, D. Microanalysis of the stable isotopes of oxygen by means of nuclear reactions. Analytical Chemistry 39, 1689-1697, (1967). 38. Turos, A., Wieluñski, L. and Barcz, A. Use of the nuclear reaction 16O(d, )14N in the microanalysis of oxide surface layers. Nuclear Instruments and Methods 111, 605-610 (1973). 39. Martin, G. Etude et modélisation du comportement de l'He dans le dioxyde d'uranium. Editions Universitaires Européennes, (2010). 40. Ziegler, J.F., Ziegler, M.D. and Biersack, J.P. SRIM – The stopping and range of ions in matter. Nuclear Instruments and Methods B 268 (11-12), 1818-1823 (2010). 41. Mayer, M., SIMNRA User’s Guide. IPP report 9/113, Max-Planck-Institut für Plasmaphysik, Garching, Germany (1997). 42. Seiler, R.F., Jones, C.H., Anzick, W.J., Herring, D.F. and Jones, K.W. The elastic scattering of deuterons by O16 from 0.65 to 2.0 MeV. Nuclear Physics 45, 647-656 (1963).

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16 17 16 1 43. Kim, H.C., Seiler, R.F., Herring, D.F. and Jones, K.W. Cross sections for the O (d,p0)O , O (d,p )O 17 16 14 ∗ and O (d,α0)N reactions from 0.8 to 1.7 MeV. Nuclear Physics 57, 526-530 (1964). 44. Amsel, G. Thesis, Annals of Physics 9, 297 (1964). 45. Picraux, S.T. Low-concentration oxygen depth profiling by the 16O(d,α)14N reaction. Nuclear Instruments and Methods 149 (1-3), 289-294 (1978). 46. Jiang, W., Shutthanandan, V., Thevuthasan, S., McCready, D.E. and Weber, W.J. Oxygen analysis using energetic ion beams. Nuclear Instruments and Methods B 207 (4), 453-461 (2003). 47. Martin, G., Sauvage, T., Desgardin, P., Garcia, P., Carlot,G. and Barthe, M.F. Accurate automated non- 3 resonant NRA depth profiling: application to the low He concentration detection in UO2 and SiC. Nuclear Instruments and Methods B 258, 471-478 (2007). 48. Martin, G., Desgardin, P., Sauvage, T., Garcia, P., Carlot, G., Khodja, H. and Barthe, M.F. A

quantitative µNRA study of helium intergranular and volume diffusion in sintered UO2. Nucl. Instr. and Meth. B 249, 509–512 (2006). 49. Amme M., Renker B., Schmid B., Feth M. P., Bertagnolli H., Döbelin, J. Nucl. Mater. 306, 202 (2002). 50. Labrim H., PhD Thesis, March 2006, Orleans University.

51. Roudil D., Barthe M.F., Jegou C., Gavazzi A., Vella F., "Investigation of Defects in Actinide-Doped UO2 by Positron Annihilation Spectroscopy", J. Nucl. Mater. 420 63 (2012). 52. Barthe M.F., Labrim H., Gentils A., Desgardin P., Corbel C., Esnouf S., Piron J.P., "Positron annihilation

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10 Annexes

10.1 Annex 1: High energy pile-up distribution

Let S(E) a signal of N counts which extends between the energies m and M. For large values of N, the probability of detecting a particle of energy E can be estimated by the distribution function f(E) = ( S(E) / N ) ∀ E ∈ [m,M]. f(E) is considered to be null outside this interval. Considering that for a given fraction of the analysis time two particles will be detected as one, a pile-up signal will appear between the energies 2m and 2M. Assuming that events involving the piling-up of three or more coincident counts are extremely rare and can be neglected, the energy distribution of the appearing pile-up signal is given by:

M p(E) = f (x) f (E − x)dx (1) ∫m It is assumed that the pile-up observed on experimental spectra lies outside S(E): it extends between energies M and 2M.

Pile-up in UC spectra is essentially due to low energy electronic noise and U(d,d) signals (see section 5.1.5), respectively noted B(E) and U(E). S(E) and U(E) are defined over the interval ]0,EU] whereas B(E) is non zero only inside ]0,EB] with EB << EU, therefore S(E) = U(E) inside ]EB,EU].

The high energy pile-up ph(E) is here defined as the signal due to the pile-up counts detected at an energy above ( EU + EB ). Counts coming from the electronic noise cannot generate in these conditions high energy pile-up, and p(E) can be written as:

EU EU ph (E) = f (x) f (E − x)dx = f (x) f (E − x)dx (2) ∫EB ∫ ()E−EU

, since f (E − x) = 0 because E − x > EU ∀x ∈ ]EB , E − EU [

Within ]EB,EU], under the reasonable assumption that the RBS U(d,d) signal U(E) is roughly constant, S(E) will not depend upon the energy and f(E) will be also a constant function over this interval since f(E) ∝ S(E). Writing f(E) = k, it comes:

E U 2 2 ph (E) = k dx = k ()2EU − E (3) ∫ ()E−EU Thus the high energy pile-up distribution in UC spectra will be approximately a linear decreasing function of the energy and should be therefore well estimated by linear regression as shown in section 5.2.2. Moreover writing the level of the RBS U(d,d) signal as S(E) = nU ∀ E ∈ ]EB,EU], it comes k = ( nU / N ) and the pile-up 2 level appears therefore to be proportional to nU at high energy.

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10.2 Annex 2: Document approval by beneficiaries’ internal QA

Fill involved beneficiaries as appropriate

# Name of beneficiary Approved by Function Date

1 Commissariat à l’Energie Atomique C. Valot Partner representative 20/07/12

2 AREVA NP SAS

3 CalculThermo

4 Centre National de la Recherche Scientifique M.F. Barthe Partner representative 26/06/12

6 Nuclear Fuel Experts SA

7 Forschungszentrum Jülich

8 Budapest University of Technology and Economics

9 Imperial College of Science. Technology and Medicine

10 Institute of Solid State Physics

11 Joint Research Centre (Institute for Transuranium Elements)

12 LGI Consulting

13 LISTO bvba

14 Materials Design

15 Nuclear Research and consultancy Group

16 Paul Scherrer Institut

17 Studiecentrum voor Kernenergie - Centre d'Etude de l'Energie Nucléaire

18 Universität Dresden

19 University of Cambridge

20 University of Manchester

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