Ion irradiation to simulate neutron irradiation in model graphites: Consequences for nuclear graphite

Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx

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Ion irradiation to simulate neutron irradiation in model graphites: Consequences for nuclear graphite N. Galy a,b, N. Toulhoat a,c, N. Moncoffre a,⇑, Y. Pipon a,d, N. Bérerd a,d, M.R. Ammar e, P. Simon e, D. Deldicque f, P. Sainsot g a

Université de Lyon, Université Lyon1, CNRS/IN2P3 (IPNL), 4, rue Enrico Fermi, F-69622 Villeurbanne cedex, France EDF DP2D Lyon, France CEA/DEN – Centre de Saclay, F-91191 Gif-sur-Yvette cedex, France d Université de Lyon, UCBL IUT Lyon 1, département de chimie, 43, boulevard du 11 novembre 1918, F-69622 Villeurbanne cedex, France e CNRS, CEMHTI UPR3079, Université d’Orléans, CS90055, F-45071 Orléans Cedex 2, France f Laboratoire de Géologie de l’Ecole Normale Supérieure, Paris, UMR CNRS ENS 8538, 24 rue Lhomond, F-75231 Paris cedex 5, France g Université de Lyon, Université Lyon 1, LaMCoS, INSA Lyon, CNRS, UMR5259, France b c

a r t i c l e

i n f o

Article history: Received 8 December 2016 Received in revised form 22 May 2017 Accepted 23 May 2017 Available online xxxx Keywords: Graphite HOPG 14 C Raman microspectrometry HRTEM

a b s t r a c t Due to its excellent moderator and reflector qualities, graphite was used in CO2-cooled nuclear reactors such as UNGG (Uranium Naturel-Graphite-Gaz). Neutron irradiation of graphite resulted in the production of 14C which is a key issue radionuclide for the management of the irradiated graphite waste. In order to elucidate the impact of neutron irradiation on 14C behavior, we carried out a systematic investigation of irradiation and its synergistic effects with temperature in Highly Oriented Pyrolitic Graphite (HOPG) model graphite used to simulate the coke grains of nuclear graphite. We used 13C implantation in order to simulate 14C displaced from its original structural site through recoil. The collision of the impinging neutrons with the graphite matrix carbon atoms induces mainly ballistic damage. However, a part of the recoil carbon atom energy is also transferred to the graphite lattice through electronic excitation. The effects of the different irradiation regimes in synergy with temperature were simulated using ion irradiation by varying Sn(nuclear)/Se(electronic) stopping power. Thus, the samples were irradiated with different ions of different energies. The structure modifications were followed by High Resolution Transmission Electron Microscopy (HRTEM) and Raman microspectrometry. The results show that temperature generally counteracts the disordering effects of irradiation but the achieved reordering level strongly depends on the initial structural state of the graphite matrix. Thus, extrapolating to reactor conditions, for an initially highly disordered structure, irradiation at reactor temperatures (200 – 500 °C) should induce almost no change of the initial structure. On the contrary, when the structure is initially less disordered, there should be a ‘‘zoning” of the reordering: In ‘‘cold” high flux irradiated zones where the ballistic damage is important, the structure should be poorly reordered; In ‘‘hot” low flux irradiated zones where the ballistic impact is lower and can therefore be counteracted by temperature, a better reordering of the structure should be achieved. Concerning 14C, except when located close to open pores where it can be removed through radiolytic corrosion, it tends to stabilize in the graphite matrix into sp2 or sp3 structures with variable proportions depending on the irradiation conditions. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Nuclear graphite has found widespread use in many areas of nuclear technology based on its excellent moderator and reflector qualities. Thus, since the sixties, several first generation commercial nuclear reactors using natural uranium fuel, CO2-cooled and ⇑ Corresponding author. E-mail address: [email protected] (N. Moncoffre).

graphite moderated were built. Many of these reactors are now being decommissioned and over 250000 tons of irradiated graphite waste are waiting for management all over the world. Neutron irradiation of graphite results in the production of radionuclides such as 14C, 36Cl or 3H, these radionuclides being a key issue for the management of the irradiated graphite waste. In case of disposal, a particular attention is paid to 14C due to its long half-life (T
5730 years) and as it is a major contributor to the radioactive dose and might be dose determining at the outlet [1]. Thus, what-

http://dx.doi.org/10.1016/j.nimb.2017.05.056 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.

Please cite this article in press as: N. Galy et al., Ion irradiation to simulate neutron irradiation in model graphites: Consequences for nuclear graphite, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.05.056

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N. Galy et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx

ever the management option (purification, storage or disposal), a previous assessment of the radioactive inventory has to be made. This work aims at gaining information on 14C location and speciation in the irradiated graphite. The goal of this paper is to elucidate the effects of neutron irradiation on 14C behavior and on graphite structure modification using ion irradiation. 14C has two main production routes: i) transmutation of nitrogen (14N(n,p)14C) where nitrogen has been mainly adsorbed at the surfaces of the irradiated graphite during maintenance procedures; ii) activation of carbon from the matrix (13C(n,c)14C). Significant quantities of 14C and 14 N have been depleted during reactor operation mainly through radiolytic corrosion at the gas/graphite interfaces. Thus, the remaining 14C results mainly from 13C neutron activation of the carbon matrix [2,3]. In order to get more insight into both 14C behavior and graphite structure modification as well as to elucidate the impact of neutron irradiation, we carried out a systematic investigation of ion irradiation and its coupled effects with temperature. The process of fabrication of graphite results into a complex multiscale organization with locally more or less anisotropic and not completely graphitized zones. At the atomic scale, it is composed of stacked graphene layers forming the basic structural units or coherent domains termed as crystallites when the stacking is quasi perfect. From the nanometer to micrometer scales, crystallites are more or less parallel one to each other within ‘‘orientation domains” forming pore walls. At a larger scale, the texture is characterized by around 80% micrometer sized filler coke grains that are blended with pitch-based binder grains and displays multisized inter-granular micrometer sized pores (around 25% open pores and 7% closed pores) [4,5]. Thus, for simplification purposes, Highly Oriented Pyrolitic Graphite (HOPG) was used as a model material system representative of the nuclear graphite coke grains. Moreover, in order to study the influence of a preexisting initial disorder on graphite behavior under irradiation, the samples were implanted with 13C (allowing simulating the presence of 14C) at different fluences. This allows inducing a high or a low disorder into the graphite matrix which is representative of the multiplicity of the structure states already present in nuclear graphite. It also allows simulating structural differences resulting from early neutron irradiation in high or low flux regions of the reactor. Then, the samples were ion irradiated to simulate neutron irradiation. Ion-beam irradiation has been effectively used to simulate neutron irradiation effects [6,7]. The collision of the impinging neutrons with the graphite matrix carbon atoms induces mainly ballistic damage. However, a part of the recoil carbon atom energy is also transferred to the graphite lattice through electronic excitation [8]. Therefore we simulated the effects of the different irradiation regimes coupled with temperature using ion irradiation by varying Sn(nuclear)/Se(electronic) stopping power. Thus, the samples were irradiated with different ions of different energies at various facilities using dedicated irradiation cells. The structural modifications were followed by Raman microspectrometry and High Resolution Transmission Electron Microscopy (HRTEM).

2. Experimental 2.1. Sample preparation and implantation HOPG is obtained from SPI Supplies (West Chester, US) through Neyco SA (Paris, France). We received the HOPG samples as 1 mm thick 10  10 mm2 plates. The samples were annealed at 1000 °C1200 °C for 8 h in high vacuum (P ffi 107 mbar) in order to desorb most of the gaseous impurities. Then, they were implanted, using rastering mode, parallel to c axis with 13C at room temperature (RT) under vacuum either using the 400 kV ion implanter IMIO400 of the Institut de Physique Nucléaire of Lyon (IPNL, Villeurbanne,

France) or the 200 kV ion implanter EATON 200MC of the ICube laboratory of the University of Strasbourg, France. The implantation flux was in the 2–3  1012 ions cm2 s1range. In order to create a highly damaged structure, a batch of samples was implanted with 13C at the energy of 150 keV and a fluence of 6  1016 at. cm2. It allows on one hand creating a strong disorder generating around 7 dpa at the projected range Rp (at a depth around 300 nm) as calculated by SRIM [9] assuming a density of 2.2 g cm3 for HOPG using the Quick Kichin-Pease model and assuming a carbon displacement energy of 28 eV. On the other hand, in order to create a lower disorder level, we implanted another batch of samples with 13C at the energy of 150 keV and a fluence of 4  1014 at. cm2. This allows creating around 0.02 dpa. Knowing that after around 11.3 years of full power operation at a fluence of 3.4  1021 n cm2 around 2.6 dpa are created into graphite [4], the lower implantation value might correspond to the very early operation stage while the higher amount is of the same order, even if higher, as that achieved at the reactor operation breakdown. In order to enable the nanometer scale characterization of the structural defects by HRTEM along the implantation depth before and after irradiation (see next paragraphs), ultrathin sections were cut perpendicularly to the sample surfaces. The ultrathin sections were manufactured using the Focused ion Beam (FIB) technique which was shown to be relevant to prepare such implanted graphite samples [10]. The sections were cut to a depth around 3 mm, a length around 10 mm and a thickness below 0.1 mm. Such ultrathin sections are electron transparent and allow using the High Resolution mode of TEM. This preparation method, which is well described in [11], was performed with a FEI Strata DB 235 dual beam FIB at the Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN, Université Lille 1, France). A thin layer of platinum was deposited on the samples beforehand, aiming to protect the HOPG surfaces from Ga+ ion implantation during the milling process. The HOPG thin sections were afterwards transferred to TEM lacey grids. 2.2. Sample irradiation The irradiations were carried out using different ions and ion energies. The aim was to try to decouple electronic excitations and ballistic effects as far as possible in order to be able to study their impact independently. Depending on the irradiation device, the samples could be heated up to 500 °C or even 1000 °C during irradiation. The irradiations were performed with carbon, argon, helium, sulfur or iodine ions using different irradiation devices: i) the 400 keV IMIO ion implantor or the 4 MV VDG accelerator (IPNL, Villeurbanne, France) for 400 keV or 600 keV C+ ions and 800 keV Ar+ ions, ii) the 15MV Tandem (IPN, Orsay, France) for 100 MeV S9+ ions or 200 MeV I13+ ions, iii) the cyclotron accelerator (CEMHTI, Orléans, France) for 15.7 MeV He+. During irradiation, samples were kept at room temperature (RT) or annealed at different temperatures. Table 1 resumes the irradiation conditions. Depending on the irradiation facility, different irradiation cells allowing sample heating were used. The cell used for irradiations carried out in vacuum with the 4 MV VDG and the 15MV Tandem accelerator facility has been fully described by Marchand et al. [12]. The cell used for the irradiations at the cyclotron facility allows sample heating up to a temperature of 500 °C and a contact with a gas simulating the UNGG gas (mainly CO2). Thus, according to the ion energy and mass, irradiations were performed on preimplanted samples, on one hand in domains where nuclear energy loss dominates, leading to displacement of atoms via elastic scattering collisions. This was the case for irradiations carried out with carbon or argon ions that produced respectively around 1 to 4 dpa in the implanted zone. These values are in the same order of magnitude as those achieved in reactors. On the other hand, irradia-

Please cite this article in press as: N. Galy et al., Ion irradiation to simulate neutron irradiation in model graphites: Consequences for nuclear graphite, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.05.056

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N. Galy et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx Table 1 Irradiation conditions of the pre-implanted samples. Ions

Carbon

Argon

Helium

Sulfur

Energy (MeV) Flux (ions cm2 s1) Fluence (ions cm2) Temperature (°C) Se (keV/mm) in the 13C implanted zone Sn (keV/mm) in the 13C implanted zone Sn/Se Dpa in the 13C implanted zone

0.4–0.6 1–2  1012 5–6  1016 200–600 585–730 10–15 0.01–0.02 1

0.8 1–2  1012 2  1016 200–1000 980 175 0.18 4.4

15.7 1–2  1012 1–2  1016 500 75 0.003 4  105 0.0001–0.0002

100 6  1010–1  1011 2  1015 RT-1000 3700 1 3  104 0.002

tions were carried out in a domain where electronic stopping power prevails, leading to local ionizations induced by high energy and swift ion irradiation carried out with sulfur ions. For these irradiations, the electronic energy loss value was around 3700 keV/mm and therefore much above those achieved in UNGG reactors that are generally lower than 700 keV/mm. The value for helium irradiation was in the lower limit of the UNGG ones and therefore more representative of the reactor conditions. Fig. 1 resumes the evolution of the stopping powers Sn and Se as well as the damage and

implantation profiles in finction of depth for the different irradiations. 2.3. Sample characterization The graphite structure was investigated using Raman microspectroscopy to evaluate the evolution of the structural disordering level before and after implantation, irradiation/ annealing. Analyses were performed in ambient conditions by using a

Fig. 1. Se, Sn and dpa values according to SRIM 2013 as a function of depth for the samples irradiated with (a) C+ at 400 keV and 5  1016 ions cm2, (b) C+ at 600 keV and 6  1016 ions cm2 (c) Ar+ at 800 keV and 2  1016 ions cm2, (d) He+ at 15.7 MeV and 1  1016 ions cm2, (e) S9+ at 100 MeV and 2  1015 ions cm2 .

Please cite this article in press as: N. Galy et al., Ion irradiation to simulate neutron irradiation in model graphites: Consequences for nuclear graphite, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.05.056

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Renishaw INVIA Reflex spectrometer equipped with an Ar laser source (514.5 nm wavelength, i.e., 2.41 eV), focused through a Leica microscope. The Rayleigh scattering component was removed by an Edge filter, and the Raman scattered light was dispersed by a holographic grating with 1800 lines/mm and detected by a CCD camera. The spectra were collected under microscope (50 objective). A very low incident power (<1 mW) was used to avoid heating effect and possible subsequent structural modifications. The experiments were carried out at ENS, Paris, France or CEMHTI, Orléans, France. The HRTEM experiments were carried out using a JEOL JEM 2011 facility using an operating voltage of 200 kV at the ‘‘Laboratoire de Réactivité de Surface” of the Université Pierre et Marie Curie (UPMC – Paris 6), France. The HRTEM images processing was then carried out in two steps according to a method proposed by Clinard and Rouzaud [13,14] and the processing is well described in [15]. 3. Results and discussion As mentioned in Section 2.1, we shall discuss the effects of irradiation in ballistic and electronic regimes taking into account the role played by the initial graphite structure states induced by implantation i.e. considering on one hand strongly disordered samples and on the other hand moderately disordered ones. 3.1. Ballistic effects: Very disordered versus moderately disordered structures through implantation At least 20 samples have been irradiated at different temperatures from RT up to 1000 °C with carbon or argon ions as detailed in Table 1 and shown in Figs. 2 and 3. Fig. 2 represents the Raman spectra of samples implanted with 13 C at a fluence of 6  1016 ions cm2 displaying therefore a strongly disordered initial structure, evaluated at around 7 dpa, and post-irradiated in ballistic regime. These spectra are compared to virgin, as-implanted and implanted/just annealed HOPG samples. Compared to the virgin HOPG characterized by a unique band centered at around 1580 cm1 corresponding to the ‘‘graphite” band, all other spectra, normalized to the G band of the virgin sample, display additional bands called D for ‘‘Defect” bands. They are known to be characteristic of disordered graphite and change in intensity relatively to the G band with increasing degree of disorder of the graphitic structure [16,17]. The G band corresponds to one-phonon Raman scattering process at the 1st Brillouin zone center and consists of the collective in-plane bond stretching of

Fig. 2. Raman spectra of the virgin sample, the samples implanted with 13C at a fluence of 6  1016 ions cm2: as implanted sample, sample just annealed at 1000 °C, and samples irradiated at RT and different temperatures respectively with C+ or Ar+.

Fig. 3. Raman spectra of samples implanted with 13C at a fluence of 4  1014 ions cm2: as implanted sample and samples irradiated respectively with C+ or Ar+ at different temperatures.

the polyaromatic carbon atoms (E2g symmetry). The D1 band, located at 1350 cm1, corresponds to the breathing of the polyaromatic carbon atoms (A1g symmetry). The D2 band which appears as a shoulder on the G band is assigned to phonons near the Brillouin zone center. The activation process of these defect-induced bands involves peculiar electron-phonon interaction mediated by defects [18,19]. It can be noticed that the implantation process results in a strong increase of the defect bands reflecting a strong disordering of the graphite structure tending to amorphization. The disordering results also in an increase of the G band FWHM reaching values around 200 cm1 for the as implanted sample. In the spectrum corresponding to the implanted/just annealed sample at 1000 °C, D1 and G bands can be again distinguished showing a reordering of the graphite structure and consequently FWHM of G band decreases approaching the value of the virgin HOPG (around 15 cm1). Similar behavior has been evidenced for neutron irradiated samples [5]. Indeed, neutron irradiation leads to graphene layers shortening, tilting and twisting. In extreme cases, the lamellar nanostructure is lost and the structure becomes nanoporous. Similarly to implantation or irradiation, the degradation of the structure leads to the occurrence of defect bands on the Raman spectra as well as to an increase of FWHM of G band. However, both D band intensities and G band FWHM decrease with increasing irradiation temperature, putting in evidence the reordering of the structure induced by the temperature increase [5]. The reordering of implanted graphite induced by temperature annealing has already been shown by [20,10]. The comparison of the spectra corresponding to carbon or argon irradiated samples show that 1) irradiation + temperature reorder the graphite leading to the occurrence of two well separated D1 and G bands due the presence of an increasing amount of small polyaromatic structures [8]. Knowing that interstitials and vacancies become increasingly mobile with temperature above 200-500 °C [21,22], this effect is increased at higher temperature; 2) irradiation carried out with argon at higher dpa level (
4 dpa) compared to carbon irradiation at lower dpa level (
1 dpa) result in a better reordering reflecting thereby the synergistic effects of ballistic irradiation and temperature on graphite structure reordering. This fact might explain the stabilization of the implanted 13C into new carbon structures as stated by Galy [23]. This author showed that 13C is not released below 1000 °C meaning that 13C is most probably incorporated into new forming polyaromatic structures, contrary to chlorine that is released from low temperatures [20,24]. Fig. 3 represents the Raman spectra for irradiated samples implanted with 13C at a fluence of 4  1014 ions cm2 characterized therefore by a moderately

Please cite this article in press as: N. Galy et al., Ion irradiation to simulate neutron irradiation in model graphites: Consequences for nuclear graphite, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.05.056

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disordered structure, evaluated at around 0.02 dpa. The lower disordering level achieved in this case is clearly evidenced for this asimplanted sample displaying two well defined and distinct D1 and G bands. After subsequent ballistic irradiation, the structure evolution at a defined temperature depends on the induced amount of damage: at 4 dpa a temperature of 200 °C is unable to anneal the disorder induced by irradiation leading to an almost amorphized structure. At higher temperature, the dynamic annealing effects of temperature are clearly visible from 500 °C inducing a strong decrease and narrowing of the D1 band. Indeed, as mentioned before, vacancies and interstitials cannot recombine at low temperature and their relative mobility is progressively enhanced from 200 °C to 500 °C. On the contrary, at 1 dpa, subsequent irradiation leads to a progressive reordering from 200 °C to 500 °C. Thus, there seems to be a ‘‘damage threshold” between 1 and 4 dpa under which temperature cannot counteract the disordering effects of irradiation. Consequently, it seems that the structural modifications induced by irradiation in the ballistic regime depend both on temperature and the initial disorder level of the graphite structure. 3.2. Electronic excitation effects Fig. 4 presents the results obtained both on highly (labeled as ‘‘high disorder”) and moderately disordered (labeled as ‘‘low disorder”) samples for the irradiated and the as implanted graphite. The samples were irradiated at temperatures varying from 500 °C to 1000 °C with sulfur ions for which Se is about 3700 keV/mm in the implanted zone. The Raman spectra obtained on the samples that have been very disordered through implantation are represented by dashed lines whereas those acquired on the samples that have been moderately disordered through implantation are represented by solid lines. We have also represented the spectrum of an initially moderately disordered sample irradiated with helium ions (Se
75 keV/mm) by a dotted blue line. This figure shows that: i) for the irradiation carried out with sulfur ions on the highly disordered graphites and contrary to what was observed for ballistic irradiation, the structure is only slightly reordered even at 1000 °C; ii) for the irradiation carried out on the moderately disordered samples, temperature has a clear reordering effect especially at 1000 °C, put in evidence by the dramatic decrease of the D1 band and narrowing of the G band. This is also true for the irradiation carried out with helium ions at 500 °C as the corresponding spectrum is close to that obtained for sulfur irradiation performed at the same temperature. Thus, there is no clear difference between

Fig. 4. Raman spectra of samples implanted respectively with 13C at a fluence of 6  1016 ions cm2 (labeled as ‘‘high disorder”) or 13C at a fluence of 4  1014 ions cm2 (labeled as ‘‘low disorder”) irradiated with S+ or He+ at different temperatures.

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the data corresponding to both Se values and moreover, if there is any disorder induced by electronic excitation (as it has been shown by Zeng et al. who evidenced sp3 hybridization on the surface of HOPG irradiated by swift heavy ions [25]), it is actually not measurable on the basis of the present results. 3.3. Comparison of ballistic and electronic excitation effects Finally, we performed some HRTEM analyses on graphite irradiated in both regimes. The results of the processing of the images are presented in Fig. 5. This figure shows the graphene interlayer spacing in function of depth measured on virgin and highly disordered as-implanted samples as well as on samples irradiated at 1000 °C with carbon and sulfur ions. The value of the interlayer spacing for the virgin sample is around 0.35 nm at the sample surface and decreases to 0.335 nm around 200–300 nm. This latter value corresponds to the theoretical value of perfectly stacked graphite. The structural degradation at the sample surface might be due to FIB section manufacturing. The highest values are observed for the as-implanted sample with a maximum close to the 13C projected range maximum (about 300 nm, as displayed in Fig. 5). This reflects the high disorder of the graphite structure induced by implantation. The interlayer spacing values obtained for the irradiated samples are between those of the as implanted and the virgin ones but are clearly higher for sulfur irradiated samples in comparison to carbon irradiated ones in the implanted zone. This result is fully in agreement with the Raman data, presented in the previous section, thereby pointing out the synergistic effects of ballistic irradiation and temperature on graphite structure reordering. We also represented the variation of the intensity ratio of the D and G peaks, ID1/IG, in function of temperature for the implanted /just annealed samples and the irradiated ones according to the initial disorder induced by implantation: highly disordered samples (Fig. 6: 6  1016 13C implantation) and moderate disordered ones (Fig. 7: 4  1014 13C). Fig. 6 shows that in case of high initial disorder there is almost no change of the initial structure in the 200 – 500 °C temperature range whatever the irradiation regime. When the initial disorder is lower, a dynamic temperature annealing probably occurs and the evolution trend is close to that of the temperature alone as shown in Fig. 7 (green dashed curve allowing eye guiding) for both irradiation regimes, provided the value of the bal-

Fig. 5. Graphene interlayer spacing values obtained by HRTEM in function of depth measured on virgin and highly disordered as-implanted samples as well as on samples irradiated at 1000 °C with C+ or S+. The dashed and dotted lines allow guiding the eye and the hatched zone corresponds to the one where the value of the implanted 13C Rp is maximum.

Please cite this article in press as: N. Galy et al., Ion irradiation to simulate neutron irradiation in model graphites: Consequences for nuclear graphite, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.05.056

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in ‘‘hot” low reactor flux irradiated zones where the ballistic impact is lower and can therefore be counteracted by temperature, there should be a better reordering of the structure. Concerning the 14 C mobility, except when located close to open pores where it can be removed through radiolytic corrosion, 14C tends to stabilize in the graphite matrix into sp2 or sp3 structures with variable proportions depending on the irradiation conditions. Acknowledgements

Fig. 6. Variation of the ID1/IG ratios in function of irradiation temperature for the highly disordered samples (implantation fluence of 6  1016 13C cm2) for the samples irradiated respectively with C+, Ar+ or S+. The just annealed, as implanted and virgin samples are also represented.

This work was performed in the frame of a EDF Ph.D. grant and was also financially supported by the European (Euratom) Programme FP7/2007-2013 under the grant agreement n° 604779 (CAST 14) as well as the French Programme NEEDS. The authors are very indebted to J.N. Rouzaud for his help throughout the work. They are grateful to Y. Le Gall (ICube) and A. Duranti (IPNL) for ion implantations. They are indebted to D. Troadec from IEMN, Université Lille 1, France, for the FIB sections manufacturing. They also thank the accelerator staffs of IPNL, CEMHTI and Tandem Alto, IPNO for their support during irradiation experiments. The authors are also indebted to Patricia Beaunier from the Laboratoire de Réactivité de Surface (UPMC, Paris 6) for helpful discussions about TEM measurements. References

Fig. 7. Variation of the ID1/IG ratios in function of irradiation temperature for the moderate disordered samples (implantation fluence of 4  1014 13C cm2) for the samples irradiated respectively with C+, Ar+, S+ or He+. The just annealed, as implanted and virgin samples are also represented.

listic damage lies between 1 and 4 dpa. On the contrary, when the ballistic damage exceeds 4 dpa (Argon irradiation in Fig. 7), reordering follows a distinct trend (red dashed curve) and the reordering level cannot reach that of the virgin one even at 1000 °C. This suggests that irradiation of graphite at reactor temperatures results into structure contrasts depending on the initial graphite structure, the temperature as well as the irradiation flux and fluence. 4. Conclusion Using ion irradiation of 13C implanted HOPG graphite, we simulated the effects of neutron irradiation according to different irradiation regimes in synergy with temperature by varying Sn(nuclear)/Se(electronic) stopping power. Thus, the samples were irradiated with different ions of different energies. The structure modifications were followed by HRTEM and Raman microspectrometry. The results show that temperature generally counteracts the disordering effects of irradiation but the achieved reordering level strongly depends on the initial structural state of the graphite matrix. Thus, extrapolating to reactor conditions, it appears that for an initially highly disordered structure, irradiation at reactor temperatures (200–500 °C) induces almost no change of the initial structure. On the contrary, when the structure is initially moderately disordered, there should be a ‘‘zoning” of the reordering. Thus, in ‘‘cold” high reactor flux irradiated zones where the ballistic damage is important, the structure should be poorly reordered;

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Please cite this article in press as: N. Galy et al., Ion irradiation to simulate neutron irradiation in model graphites: Consequences for nuclear graphite, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.05.056