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Thermal behavior of deuterium implanted into nuclear graphite studied by NRA
Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Thermal behavior of deuterium implanted into nuclear graphite studied by NRA M. Le Guillou a,b,⇑, N. Toulhoat a,c,⇑, Y. Pipon a,d, N. Moncoffre a, N. Bérerd a,d, A. Perrat-Mabilon a, R. Rapegno a a
Institut de Physique Nucléaire de Lyon, CNRS/IN2P3, UMR 5822, Université Claude Bernard Lyon 1, Université de Lyon, 4 rue Enrico Fermi, F-69622 Villeurbanne cedex, France Agence nationale pour la gestion des déchets radioactifs, 1-7 rue Jean Monnet, Parc de la Croix-Blanche, F-92298 Châtenay-Malabry cedex, France Commissariat à l’Energie Atomique et aux Energies Alternatives, CEA/DEN, Centre de Saclay, F-91191 Gif-sur-Yvette cedex, France d Institut Universitaire Technologique, Université Claude Bernard Lyon 1, Université de Lyon, 43 boulevard du 11 novembre 1918, F-69622 Villeurbanne cedex, France b c
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Nuclear Reaction Analysis Graphite Deuterium Temperature
a b s t r a c t This paper focuses on the thermal behavior of deuterium, simulating tritium, implanted into virgin nuclear graphite of French gas-cooled reactors, which are being decommissioned. Deuterium ions D+ were implanted into graphite (around 3 at.% at the projected range Rp) at two different depths (around 670 nm and 2.8 lm) and annealed up to about 300 h in a temperature range from 200 °C to 1200 °C under vacuum or argon flow. Before and after heat treatments, D distribution profiles in the samples were followed using the nuclear reaction D(3He,p)4He, with a millimetric beam at the 4 MV Van de Graaff accelerator of IPNL (Institut de Physique Nucléaire de Lyon, France). The results show that the deuterium release becomes significant at temperatures higher than 600 °C and is almost totally completed at 1200 °C. The comparison of the results, obtained for both implantation depths, points out the role of the porosity with respect to deuterium permeation. The release follows two stages: a rapid step where it occurs within a few hours, followed by a much slower step during which the release of deuterium saturates. The initial stage is characterized by an activation energy of 1.3 eV and might correspond to detrapping of D located at crystallite edges and its diffusion at the crystallite surfaces. We assume that the second stage kinetics corresponds to a very slow diffusion of D located inside the crystallites and chemisorbed to carbon atoms through sp2 or sp3 bonds. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermal behavior of hydrogen isotopes in nuclear graphite is an issue frequently discussed in the context of many research areas such as studies on the experimental thermonuclear fusion devices (JET, ITER, etc.) or on issues related to the management and disposal of nuclear waste. In particular, the dismantling of the first generation ‘‘Uranium Naturel-Graphite-Gaz’’ (UNGG) nuclear reactors commissioned in France in the 1960s for civil nuclear power generation will generate approximately 23,000 tons of irradiated graphite waste, mainly from the neutron moderator of these reactors. The French law of June 2006 established in France a research programme on the management solutions of these long-term low-level ⇑ Corresponding authors. Present address: IPNL, Bâtiment Paul Dirac, 4 rue Enrico Fermi, F-69622 Villeurbanne cedex, France. Tel.: +33 4 72 43 10 63 (M. Le Guillou), +33 4 72 44 84 02 (N. Toulhoat). E-mail addresses: [email protected] (M. Le Guillou), nelly.toulhoat@ univ-lyon1.fr (N. Toulhoat).
wastes. The current reference outlet is underground disposal (up to about 200 m) but alternative management solutions such as partial decontamination and deep disposal are also investigated. Nuclear graphite, used as neutron moderator in UNGG reactors, is synthetic polycrystalline graphite consisting of petroleum coke grains crushed, cooked at 800 °C and then impregnated once or more times with coal tar pitch, which acts as binder, in order to increase the bulk density of the material. A graphitization treatment at about 2800 °C is then carried out before purification that permits to obtain nuclear grade graphite. Due to the various carbon compounds used during its synthesis, structural organization of the material is locally heterogeneous at different scales [1,2]. Thus, the presence of intergranular and intragranular porosities in nuclear graphite leads to an apparent bulk density of UNGG graphite of about 1.6– 1.7 g cm3, value lower than that of crystalline graphite (about 2.2 g cm3). Each graphite grain contains stacks of 30–200 nm randomly oriented crystallites that are themselves formed by stacks of graphene planes [1]. Due to the nature of the raw materials used during the synthesis and the various stages of the manufacturing
http://dx.doi.org/10.1016/j.nimb.2014.02.036 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
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M. Le Guillou et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
process, nuclear graphite also contains small amounts of impurities such as hydrogen, boron, oxygen, halogens and metals, among them lithium [3]. Lithium is the main production source of tritium in graphite moderator during UNGG reactors operation [4]. This impurity produces tritium by the following neutron activation reaction 6 Li(nth,a)3H with thermal neutrons nth (En ffi 0.025 eV). Tritium is one of the main contributors (with 14C) to the initial inventory of UNGG graphite waste [5]. It is therefore crucial to know its location and speciation in UNGG irradiated graphite, in particular regarding the safety of both decommissioning and disposal, but also for issues related to partial decontamination treatments prior to disposal. During reactor operation, two main factors determine the behavior of tritium: temperature and irradiation. This paper aims at studying the thermal behavior of deuterium, used to simulate the presence of tritium. D+ ions are implanted into virgin nuclear graphite in order to simulate tritium displaced from its original structural site through recoil during reactor operation. The effect of thermal treatments on deuterium mobility is investigated in a temperature range varying from 200 °C to 1200 °C. 2. Experimental procedure 2.1. Sample preparation and D+ ion implantation Graphite samples used during this work were cut from a virgin graphite block issued from the EDF (Electricité De France) UNGG reactor Saint-Laurent A2 (SLA2). The surfaces were manually polished to the micrometer under ethanol with diamond pastes. The average size of the sample was about 9 5 mm2 with a 4 mm thickness. This graphite has the same characteristics than that of the moderator stack of SLA2, with a bulk density of about 1.68 g cm3 and an average porosity of about 32%, including 7% of closed pores. The samples were then annealed in high vacuum (P ffi 107 mbar) at 1000 °C for 8 h to desorb most of the gaseous impurities present in the pores and also to anneal the defects induced by polishing, at least partially. Then, D+ ions were implanted under vacuum into the polished surfaces using the 400 kV ion implanter IMIO400 of IPNL. The sample holder was cooled at 15 °C during implantations. The D implantation profiles were subsequently analyzed by Nuclear Reaction Analysis (NRA), using the nuclear reaction D(3He,p)4He. Since the detection limit of this reaction in our experimental conditions is around the atomic percent, it was necessary to implant D+ ions at a minimum fluence of 5 1016 D+ cm2. Two implantation energies of 70 keV and 390 keV, corresponding to respective projected ranges of 670 nm and 2.8 lm, were selected in order to check the effect of the sample surface proximity on deuterium mobility during annealing. It should be noted that these two implantation energies correspond respectively to the upper and lower limits of the ion implanter of IPNL. The implantation fluence of 5 1016 ions cm2 corresponds to respective D concentrations of about 3.4 and 2.5 at.% at the Rp. These implantation settings were calculated using SRIM 2011 software [6] considering a graphite filler grain density around 2.2 g cm3. Finally, we calculated that, despite the relatively high fluence, the maximum amount of defects generated in the graphite matrix is quite low, of around 0.3 displacement per atom (dpa) at the maximum of the defect profile (Rd).
introduced into a tubular furnace Pekly ETF 30-50/15-S. The annealing duration ranged up to about 300 h with a heating ramp of 5 K min1. Annealing in inert gas was carried out using a pure argon 5.0 flow at about 1 bar in a tubular furnace Thermolyne 21100 with a heating ramp of a few minutes for relatively short annealing durations (below 24 h) and temperatures ranging from 200 °C to 1000 °C. Moreover, a tubular furnace Nabertherm RHTH 120-150/ 18 was also used with a heating ramp of 5 K min1 for heat treatments between 1000 °C and 1200 °C in argon flow. Duplicate annealing experiments (that will not be shown in this paper) performed either in argon or in vacuum showed that the behavior of the implanted deuterium did not depend on the annealing conditions (type of inert atmosphere or heating ramp). After annealing, graphite samples were cooled down to room temperature before analysis. 2.3. Depth profiling analysis of implanted deuterium The D profiles in samples were analyzed through the nuclear reaction D(3He,p)4He with a helium-3 millimetric beam before and after annealing using a new analysis chamber implemented on the 4 MV VDG accelerator of IPNL. This chamber is presented in Fig. 1 and a schematic illustration of the experimental set up is given in Fig. 2. For each sample, the 1 mm sized incident beam probed more or less porous zones made of grains and binder. Therefore, the resulting spectra represent an average of all these regions. Incident 3He+ particles of 900 keV and 1.6 MeV were used to probe deuterium respectively implanted at 70 and 390 keV. These incident energies allowed adjusting the maximum of the D(3He,p)4He reaction cross section (r ffi 60 mb sr1 for E3He ffi 620 keV) with the implantation projected range (Rp) at each energy. In these conditions, helium-3 ions react with implanted D, producing 13 MeV protons and 2 MeV 4He+ particles. Beam was normal to the sample surface and only protons were detected at 155° using a 2000 lm thick silicon surface barrier detector with a solid angle of 12 msr. The average beam current was kept at about 40 nA and the average collected charge was 30 lC. Furthermore, a 23 lm thick MylarÒ screen was placed in front of the detector in order to stop the 4He+ emitted particles and the backscattered 3He+ particles. 3. Results and discussion 3.1. Thermal release of deuterium Fig. 3 shows the evolution of the spectra of protons issued from the nuclear reaction on deuterium measured on samples
2.2. Heat treatments Annealing of graphite samples was performed either in high vacuum (P ffi 107 mbar) in the temperature range of 500–1100 °C, or in inert gas flow (Ar) at temperatures ranging from 200 °C to 1200 °C. Annealing in high vacuum was performed using a silica tube
Fig. 1. New ion beam analysis chamber implemented on the 4 MV Van de Graaff accelerator of IPNL.
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
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Fig. 2. Schematic illustration of the depth profiling analysis of implanted deuterium by NRA.
implanted at 70 keV (Rp ffi 670 nm) after 4 h annealing at temperatures ranging from 200 °C to 1200 °C. The annealing results in a decrease of the peak areas with increasing temperature. This implies that the main process driving the thermal migration of deuterium is release through the graphite porous structure. The release begins roughly at 600 °C and is completed at 1200 °C. Over 700 °C, an enlargement of the peak is observed which could correspond to a slight diffusion towards the surface of the graphite. However, due to the technique resolution around 100 nm and considering the as implanted peak FWHM lower than 200 nm (from SRIM 2011 [6]), this diffusion process cannot be quantified. Moreover, at 800 °C the peak seems to split into two peaks. This phenomenon could not be observed at higher temperature due to the large deuterium release and is currently not well understood. In order to investigate the time dependence of the D migration in graphite samples, the release curves as a function of annealing time for temperatures ranging from 500 °C to 1200 °C have been plotted in Fig. 4 for the 70 keV implantation energy. Because of the negligible release of deuterium observed at 500 °C, even for annealing times up to more than 300 h, we did subsequently not take into account data obtained below this temperature. For all samples and at any temperature, the data evidence two major release stages with distinct kinetics. At each annealing temperature, an important and rather rapid step, where the release occurs within a few hours (ranging from 30 min at high temperature to around 24 h at low temperature), is followed by a much slower release step (up to
Fig. 3. Proton spectra taken from the analysis of graphite samples implanted with deuterium at 70 keV (Rp ffi 670 nm), and then annealed for 4 h in argon flow up to 1200 °C.
3
Fig. 4. Deuterium release curves as a function of annealing time at different temperatures from 500 to 1200 °C for graphite samples implanted with deuterium at 70 keV (Rp ffi 670 nm), and then annealed from 30 min to 300 h in argon flow or high vacuum.
around 300 h) during which the release of deuterium saturates. It is interesting to notice that the higher the temperature, the more rapidly the saturation of the release is reached. (i) The first release stage has been modeled according to Fick’s law using the Eq. (1):
@C ðx; t Þ ¼ k C ðx; t Þ @t
ð1Þ
which solving gives the following Eq. (2):
CT ¼ exp ðk t Þ C0
ð2Þ
where CT (in ppm) is the atomic D concentration in a sample annealed at a temperature T, C0 (in ppm) the atomic concentration of D present in an as implanted sample, k the kinetic release constant in s1 and t the annealing time in s. In our case, we replaced the concentration ratios by the peak area ratios. At each temperature, the release constant k has been calculated from the evolution of the logarithm of the ratio CT/C0 (peak areas) as a function of annealing time. Finally, the release activation energy could be estimated from the Arrhenius Law:
Ea k ¼ k0 exp kB T
ð3Þ
where the release constant k is in s1, the initial release constant k0 in s1, the activation energy Ea in eV, the Boltzmann constant kB ffi 8.62 105 eV K1 and the annealing temperature T in K. The Arrhenius plot issued from the above relation (3) between 500 and 900 °C and represented in Fig. 5 enabled us to calculate an activation energy around 1.3 eV for the initial rapid release stage. It is interesting to note that an activation energy of 1.3 eV was also reported by Atsumi et al. [7–9] in studies on hydrogen isotopes bulk absorption and desorption processes in several brands of isotropic graphite synthetized by Toyo Tanso Co. Ltd. (IG110U, IG-430U, ISO-88 and ISO-880U) and Union Carbide Corp. (ATJ). In these works, the 5 33 1 mm3 graphite samples were charged with hydrogen by gas exposure in a pressure range of 0.02–40 kPa for 0.5–30 h, or by 20 keV D+2 ion implantation at room temperature with a total ion dose ranging from 5 1016 D+2 cm2 to 5 1018 D+2 cm2. TDS experiments were performed in vacuum below 106 Pa with a constant heating rate of 10 K min1 [9]. According to a model originally proposed by Kanashenko et al. [10] and Chernikov et al. [11], and then adapted and used by
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
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M. Le Guillou et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
Fig. 5. Arrhenius plot between 500 and 900 °C for the rapid release stage in graphite samples implanted with deuterium at 70 keV (Rp ffi 670 nm), and then annealed from 30 min to 300 h in argon flow or high vacuum.
Atsumi et al. [7–9], this activation energy would correspond to the molecular diffusion of deuterium with sequences of dissociation and recombination along crystallite edges in the graphite filler grains. In our study, D release observed in the first stage might correspond to both molecular diffusion (with sequences of dissociation and recombination) and gas permeation through open pores, the limiting factor being the diffusion process. (ii) During the second stage, the release of D is drastically slowed down at any temperature. This means that the remaining D is probably trapped at sites requiring higher energies (i.e. higher temperatures) to be detrapped. The remaining D is most probably located inside the crystallites and chemisorbed to carbon atoms through sp2 or sp3 bonds [9,12]. The thermal activation of these sites may begin from 700 °C (cf. Fig. 3 where the slight widening of the D peak might be at least partly due to the diffusion of deuterium from these sites). However, temperatures higher than 1200 °C are necessary to lead to the total removal of the deuterium.
Fig. 6. Release curves as a function of temperature for D+ implantation at 390 keV (Rp ffi 2.8 lm) and 24 h annealing in argon flow or high vacuum, and for D+ implantation at 70 keV (Rp ffi 670 nm) and annealing for 30 min and 24 h. Open squares are experimental data on tritium obtained by Sawicki et al. for HT+ implantation at 40 keV (Rp ffi 580 nm) and 20–40 min annealing in vacuum [13].
comparison of our results with those of Sawicki et al. shows that, even if the implantation fluences are different, the release behavior of implanted tritium in samples annealed for 20–40 min is very close to that of deuterium implanted at a similar implantation depth in samples annealed during roughly the same time. Consequently, the thermal release mechanism of implanted deuterium and tritium should be similar. This results shows that deuterium is appropriate to simulate implanted tritium. 4. Conclusions
3.2. Dependence of deuterium thermal release on implantation depth and representativeness versus tritium Fig. 6 presents the deuterium release curves as a function of temperature for 24 h annealing at both implantation depths, 670 nm (70 keV) and 2.8 lm (390 keV). This figure shows that the global shape of the release as function of temperature is similar whatever the implantation depth. However, it is clear from these curves that the release is enhanced when D is implanted close to the sample surface showing the important role of the internal open porosities of nuclear graphite. As an example, for a 24 h annealing at 800 °C, the D release for implantation at 670 nm is twice that for implantation at 2.8 lm. In this figure, we have also compared the deuterium release curve obtained on the samples implanted at 670 nm and annealed for 30 min to the data obtained by Sawicki et al. [13] on tritium release. Although the implantation fluence for tritium was an order of magnitude smaller than that of the present study, the implantation depths are rather similar and the comparison of both results is interesting. The results of Sawicki et al. were obtained on polycrystalline-fine-grained isotropic graphite synthetized by Le Carbone Lorraine (5890/PT type). In their work, HT+ ions were implanted under a 104 Pa vacuum at room temperature (20–45 °C) into 15 15 1 mm3 graphite plates at 40 keV (Rp ffi 580 nm according to SRIM 2011 [6]) and 5 1015 HT+ cm2 ð½HTRp 0:3 at:%Þ. Graphite samples were then annealed in a 104 Pa vacuum for 20–40 min and analyzed by T(D,a)n nuclear reaction using a 500 keV D+2 ion beam from a Van de Graaff accelerator. The
The purpose of this paper is to investigate, using Nuclear Reaction Analysis, the thermal behavior of deuterium implanted into virgin nuclear graphite. The main results are summarized as follows: (1) The main D thermal migration process is release, occurring from about 600 °C to 1200 °C when it is located close to the surface (less than 1 lm). Moreover, in these conditions, D is almost totally released at 1200 °C. (2) This release follows two stages: a rapid step where it occurs within a few hours is followed by a much slower step during which it saturates. The initial stage is characterized by an activation energy of 1.3 eV and might correspond to detrapping of D located at crystallite edges and its diffusion at the crystallite surfaces. We assume that the second stage kinetics corresponds to a very slow diffusion of D located inside the crystallites and chemisorbed to carbon atoms through sp2 or sp3 bonds. The total release of this strongly bound D requires temperatures higher than 1200 °C. (3) The two implantation energies, i.e. implantation depths, performed during this study have revealed the important role of the internal open porosities of nuclear graphite. This role is highlighted by the fact that the release in enhanced when D is implanted close to the sample surface. (4) The comparison of our results obtained on deuterium with those of the literature on tritium show that both elements have a similar thermal behavior.
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
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From our results obtained in inert atmosphere or in high vacuum, it is shown that at UNGG reactor temperatures, that do not exceed around 500 °C, there is no significant deuterium release. However, complementary experiments are presently conducted in order to check the role of gas containing traces of hydrogen bearing molecules, i.e. UNGG coolant gas, with respect to isotopic exchange. As a matter of fact, the presence of these molecules might enhance the release of deuterium. Moreover in the perspective of graphite decontamination and extrapolating our results to tritium, it should be noted that temperatures around 1200–1300 °C should allow an almost complete removal of tritium. Acknowledgements This work was performed in the frame of a Andra PhD Grant. The authors would like to thank Andra, EDF and the European project CARBOWASTE (Seventh EURATOM Framework Programme) for their financial support. Moreover, this work could not have been carried out without the support of the accelerator staff of IPNL.
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References [1] J.P. Bonal, J.C. Robin, Les réacteurs nucléaires à caloporteur gaz, Un matériau fascinant: le graphite, CEA/DEN, CEA Saclay et Groupe Moniteur (Editions du Moniteur), Paris, 2006. [2] C.E. Vaudey, N. Toulhoat, N. Moncoffre, N. Bérerd, L. Raimbault, P. Sainsot, J.N. Rouzaud, A. Perrat-Mabilon, J. Nucl. Mater. 395 (2009) 62. [3] J.P. Bonal, I. Brésard, S. Parraud, Caractérisation du graphite des réacteurs de la filière UNGG, Proceedings of Matériaux, 2002, Tours, France, http:// www.infotheque.info/cache/6205/pem.utbm.fr/materiaux_2002/index.html. [4] M.C. Perrin, B. Poncet, M.T. Pascal, Livre blanc du tritium, Le tritium dans les déchets des réacteurs d’EDF en cours de démantèlement, ASN, 2010, p. 120 (Chapitre 2). [5] Andra, Etude des scénarios de gestion à long terme des déchets de faible activité massique à vie longue, Andra Technical report, FRPADPG.12.0020/A (2012) 37. [6] J.F. Ziegler, J.P. Biersack, M.D. Ziegler, SRIM, The Stopping and Range of Ions in Matter, SRIM Co., 2008. [7] H. Atsumi, J. Nucl. Mater. 307–311 (2002) 1466. [8] H. Atsumi, J. Nucl. Mater. 313–316 (2003) 543. [9] H. Atsumi, K. Tauchi, J. Alloys Compd. 356–357 (2003) 705. [10] S.L. Kanashenko, A.E. Gorodetsky, V.N. Chernikov, A.V. Markin, A.P. Zakharov, B.L. Doyle, W.R. Wampler, J. Nucl. Mater. 233–237 (1996) 1207. [11] V.N. Chernikov, W.R. Wampler, A.P. Zakharov, A.E. Gorodetsky, J. Nucl. Mater. 264 (1999) 180. [12] D. Stojkovic, P. Zhang, P.E. Lammert, V.H. Crespi, Phys. Rev. B 68 (2003) 195406. [13] J.A. Sawicki, J. Roth, L.M. Howe, J. Nucl. Mater. 162–164 (1989) 1021.
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Thermal behavior of deuterium implanted into nuclear graphite studied by NRA M. Le Guillou a,b,⇑, N. Toulhoat a,c,⇑, Y. Pipon a,d, N. Moncoffre a, N. Bérerd a,d, A. Perrat-Mabilon a, R. Rapegno a a
Institut de Physique Nucléaire de Lyon, CNRS/IN2P3, UMR 5822, Université Claude Bernard Lyon 1, Université de Lyon, 4 rue Enrico Fermi, F-69622 Villeurbanne cedex, France Agence nationale pour la gestion des déchets radioactifs, 1-7 rue Jean Monnet, Parc de la Croix-Blanche, F-92298 Châtenay-Malabry cedex, France Commissariat à l’Energie Atomique et aux Energies Alternatives, CEA/DEN, Centre de Saclay, F-91191 Gif-sur-Yvette cedex, France d Institut Universitaire Technologique, Université Claude Bernard Lyon 1, Université de Lyon, 43 boulevard du 11 novembre 1918, F-69622 Villeurbanne cedex, France b c
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Nuclear Reaction Analysis Graphite Deuterium Temperature
a b s t r a c t This paper focuses on the thermal behavior of deuterium, simulating tritium, implanted into virgin nuclear graphite of French gas-cooled reactors, which are being decommissioned. Deuterium ions D+ were implanted into graphite (around 3 at.% at the projected range Rp) at two different depths (around 670 nm and 2.8 lm) and annealed up to about 300 h in a temperature range from 200 °C to 1200 °C under vacuum or argon flow. Before and after heat treatments, D distribution profiles in the samples were followed using the nuclear reaction D(3He,p)4He, with a millimetric beam at the 4 MV Van de Graaff accelerator of IPNL (Institut de Physique Nucléaire de Lyon, France). The results show that the deuterium release becomes significant at temperatures higher than 600 °C and is almost totally completed at 1200 °C. The comparison of the results, obtained for both implantation depths, points out the role of the porosity with respect to deuterium permeation. The release follows two stages: a rapid step where it occurs within a few hours, followed by a much slower step during which the release of deuterium saturates. The initial stage is characterized by an activation energy of 1.3 eV and might correspond to detrapping of D located at crystallite edges and its diffusion at the crystallite surfaces. We assume that the second stage kinetics corresponds to a very slow diffusion of D located inside the crystallites and chemisorbed to carbon atoms through sp2 or sp3 bonds. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermal behavior of hydrogen isotopes in nuclear graphite is an issue frequently discussed in the context of many research areas such as studies on the experimental thermonuclear fusion devices (JET, ITER, etc.) or on issues related to the management and disposal of nuclear waste. In particular, the dismantling of the first generation ‘‘Uranium Naturel-Graphite-Gaz’’ (UNGG) nuclear reactors commissioned in France in the 1960s for civil nuclear power generation will generate approximately 23,000 tons of irradiated graphite waste, mainly from the neutron moderator of these reactors. The French law of June 2006 established in France a research programme on the management solutions of these long-term low-level ⇑ Corresponding authors. Present address: IPNL, Bâtiment Paul Dirac, 4 rue Enrico Fermi, F-69622 Villeurbanne cedex, France. Tel.: +33 4 72 43 10 63 (M. Le Guillou), +33 4 72 44 84 02 (N. Toulhoat). E-mail addresses: [email protected] (M. Le Guillou), nelly.toulhoat@ univ-lyon1.fr (N. Toulhoat).
wastes. The current reference outlet is underground disposal (up to about 200 m) but alternative management solutions such as partial decontamination and deep disposal are also investigated. Nuclear graphite, used as neutron moderator in UNGG reactors, is synthetic polycrystalline graphite consisting of petroleum coke grains crushed, cooked at 800 °C and then impregnated once or more times with coal tar pitch, which acts as binder, in order to increase the bulk density of the material. A graphitization treatment at about 2800 °C is then carried out before purification that permits to obtain nuclear grade graphite. Due to the various carbon compounds used during its synthesis, structural organization of the material is locally heterogeneous at different scales [1,2]. Thus, the presence of intergranular and intragranular porosities in nuclear graphite leads to an apparent bulk density of UNGG graphite of about 1.6– 1.7 g cm3, value lower than that of crystalline graphite (about 2.2 g cm3). Each graphite grain contains stacks of 30–200 nm randomly oriented crystallites that are themselves formed by stacks of graphene planes [1]. Due to the nature of the raw materials used during the synthesis and the various stages of the manufacturing
http://dx.doi.org/10.1016/j.nimb.2014.02.036 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
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M. Le Guillou et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
process, nuclear graphite also contains small amounts of impurities such as hydrogen, boron, oxygen, halogens and metals, among them lithium [3]. Lithium is the main production source of tritium in graphite moderator during UNGG reactors operation [4]. This impurity produces tritium by the following neutron activation reaction 6 Li(nth,a)3H with thermal neutrons nth (En ffi 0.025 eV). Tritium is one of the main contributors (with 14C) to the initial inventory of UNGG graphite waste [5]. It is therefore crucial to know its location and speciation in UNGG irradiated graphite, in particular regarding the safety of both decommissioning and disposal, but also for issues related to partial decontamination treatments prior to disposal. During reactor operation, two main factors determine the behavior of tritium: temperature and irradiation. This paper aims at studying the thermal behavior of deuterium, used to simulate the presence of tritium. D+ ions are implanted into virgin nuclear graphite in order to simulate tritium displaced from its original structural site through recoil during reactor operation. The effect of thermal treatments on deuterium mobility is investigated in a temperature range varying from 200 °C to 1200 °C. 2. Experimental procedure 2.1. Sample preparation and D+ ion implantation Graphite samples used during this work were cut from a virgin graphite block issued from the EDF (Electricité De France) UNGG reactor Saint-Laurent A2 (SLA2). The surfaces were manually polished to the micrometer under ethanol with diamond pastes. The average size of the sample was about 9 5 mm2 with a 4 mm thickness. This graphite has the same characteristics than that of the moderator stack of SLA2, with a bulk density of about 1.68 g cm3 and an average porosity of about 32%, including 7% of closed pores. The samples were then annealed in high vacuum (P ffi 107 mbar) at 1000 °C for 8 h to desorb most of the gaseous impurities present in the pores and also to anneal the defects induced by polishing, at least partially. Then, D+ ions were implanted under vacuum into the polished surfaces using the 400 kV ion implanter IMIO400 of IPNL. The sample holder was cooled at 15 °C during implantations. The D implantation profiles were subsequently analyzed by Nuclear Reaction Analysis (NRA), using the nuclear reaction D(3He,p)4He. Since the detection limit of this reaction in our experimental conditions is around the atomic percent, it was necessary to implant D+ ions at a minimum fluence of 5 1016 D+ cm2. Two implantation energies of 70 keV and 390 keV, corresponding to respective projected ranges of 670 nm and 2.8 lm, were selected in order to check the effect of the sample surface proximity on deuterium mobility during annealing. It should be noted that these two implantation energies correspond respectively to the upper and lower limits of the ion implanter of IPNL. The implantation fluence of 5 1016 ions cm2 corresponds to respective D concentrations of about 3.4 and 2.5 at.% at the Rp. These implantation settings were calculated using SRIM 2011 software [6] considering a graphite filler grain density around 2.2 g cm3. Finally, we calculated that, despite the relatively high fluence, the maximum amount of defects generated in the graphite matrix is quite low, of around 0.3 displacement per atom (dpa) at the maximum of the defect profile (Rd).
introduced into a tubular furnace Pekly ETF 30-50/15-S. The annealing duration ranged up to about 300 h with a heating ramp of 5 K min1. Annealing in inert gas was carried out using a pure argon 5.0 flow at about 1 bar in a tubular furnace Thermolyne 21100 with a heating ramp of a few minutes for relatively short annealing durations (below 24 h) and temperatures ranging from 200 °C to 1000 °C. Moreover, a tubular furnace Nabertherm RHTH 120-150/ 18 was also used with a heating ramp of 5 K min1 for heat treatments between 1000 °C and 1200 °C in argon flow. Duplicate annealing experiments (that will not be shown in this paper) performed either in argon or in vacuum showed that the behavior of the implanted deuterium did not depend on the annealing conditions (type of inert atmosphere or heating ramp). After annealing, graphite samples were cooled down to room temperature before analysis. 2.3. Depth profiling analysis of implanted deuterium The D profiles in samples were analyzed through the nuclear reaction D(3He,p)4He with a helium-3 millimetric beam before and after annealing using a new analysis chamber implemented on the 4 MV VDG accelerator of IPNL. This chamber is presented in Fig. 1 and a schematic illustration of the experimental set up is given in Fig. 2. For each sample, the 1 mm sized incident beam probed more or less porous zones made of grains and binder. Therefore, the resulting spectra represent an average of all these regions. Incident 3He+ particles of 900 keV and 1.6 MeV were used to probe deuterium respectively implanted at 70 and 390 keV. These incident energies allowed adjusting the maximum of the D(3He,p)4He reaction cross section (r ffi 60 mb sr1 for E3He ffi 620 keV) with the implantation projected range (Rp) at each energy. In these conditions, helium-3 ions react with implanted D, producing 13 MeV protons and 2 MeV 4He+ particles. Beam was normal to the sample surface and only protons were detected at 155° using a 2000 lm thick silicon surface barrier detector with a solid angle of 12 msr. The average beam current was kept at about 40 nA and the average collected charge was 30 lC. Furthermore, a 23 lm thick MylarÒ screen was placed in front of the detector in order to stop the 4He+ emitted particles and the backscattered 3He+ particles. 3. Results and discussion 3.1. Thermal release of deuterium Fig. 3 shows the evolution of the spectra of protons issued from the nuclear reaction on deuterium measured on samples
2.2. Heat treatments Annealing of graphite samples was performed either in high vacuum (P ffi 107 mbar) in the temperature range of 500–1100 °C, or in inert gas flow (Ar) at temperatures ranging from 200 °C to 1200 °C. Annealing in high vacuum was performed using a silica tube
Fig. 1. New ion beam analysis chamber implemented on the 4 MV Van de Graaff accelerator of IPNL.
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
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Fig. 2. Schematic illustration of the depth profiling analysis of implanted deuterium by NRA.
implanted at 70 keV (Rp ffi 670 nm) after 4 h annealing at temperatures ranging from 200 °C to 1200 °C. The annealing results in a decrease of the peak areas with increasing temperature. This implies that the main process driving the thermal migration of deuterium is release through the graphite porous structure. The release begins roughly at 600 °C and is completed at 1200 °C. Over 700 °C, an enlargement of the peak is observed which could correspond to a slight diffusion towards the surface of the graphite. However, due to the technique resolution around 100 nm and considering the as implanted peak FWHM lower than 200 nm (from SRIM 2011 [6]), this diffusion process cannot be quantified. Moreover, at 800 °C the peak seems to split into two peaks. This phenomenon could not be observed at higher temperature due to the large deuterium release and is currently not well understood. In order to investigate the time dependence of the D migration in graphite samples, the release curves as a function of annealing time for temperatures ranging from 500 °C to 1200 °C have been plotted in Fig. 4 for the 70 keV implantation energy. Because of the negligible release of deuterium observed at 500 °C, even for annealing times up to more than 300 h, we did subsequently not take into account data obtained below this temperature. For all samples and at any temperature, the data evidence two major release stages with distinct kinetics. At each annealing temperature, an important and rather rapid step, where the release occurs within a few hours (ranging from 30 min at high temperature to around 24 h at low temperature), is followed by a much slower release step (up to
Fig. 3. Proton spectra taken from the analysis of graphite samples implanted with deuterium at 70 keV (Rp ffi 670 nm), and then annealed for 4 h in argon flow up to 1200 °C.
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Fig. 4. Deuterium release curves as a function of annealing time at different temperatures from 500 to 1200 °C for graphite samples implanted with deuterium at 70 keV (Rp ffi 670 nm), and then annealed from 30 min to 300 h in argon flow or high vacuum.
around 300 h) during which the release of deuterium saturates. It is interesting to notice that the higher the temperature, the more rapidly the saturation of the release is reached. (i) The first release stage has been modeled according to Fick’s law using the Eq. (1):
@C ðx; t Þ ¼ k C ðx; t Þ @t
ð1Þ
which solving gives the following Eq. (2):
CT ¼ exp ðk t Þ C0
ð2Þ
where CT (in ppm) is the atomic D concentration in a sample annealed at a temperature T, C0 (in ppm) the atomic concentration of D present in an as implanted sample, k the kinetic release constant in s1 and t the annealing time in s. In our case, we replaced the concentration ratios by the peak area ratios. At each temperature, the release constant k has been calculated from the evolution of the logarithm of the ratio CT/C0 (peak areas) as a function of annealing time. Finally, the release activation energy could be estimated from the Arrhenius Law:
Ea k ¼ k0 exp kB T
ð3Þ
where the release constant k is in s1, the initial release constant k0 in s1, the activation energy Ea in eV, the Boltzmann constant kB ffi 8.62 105 eV K1 and the annealing temperature T in K. The Arrhenius plot issued from the above relation (3) between 500 and 900 °C and represented in Fig. 5 enabled us to calculate an activation energy around 1.3 eV for the initial rapid release stage. It is interesting to note that an activation energy of 1.3 eV was also reported by Atsumi et al. [7–9] in studies on hydrogen isotopes bulk absorption and desorption processes in several brands of isotropic graphite synthetized by Toyo Tanso Co. Ltd. (IG110U, IG-430U, ISO-88 and ISO-880U) and Union Carbide Corp. (ATJ). In these works, the 5 33 1 mm3 graphite samples were charged with hydrogen by gas exposure in a pressure range of 0.02–40 kPa for 0.5–30 h, or by 20 keV D+2 ion implantation at room temperature with a total ion dose ranging from 5 1016 D+2 cm2 to 5 1018 D+2 cm2. TDS experiments were performed in vacuum below 106 Pa with a constant heating rate of 10 K min1 [9]. According to a model originally proposed by Kanashenko et al. [10] and Chernikov et al. [11], and then adapted and used by
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
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Fig. 5. Arrhenius plot between 500 and 900 °C for the rapid release stage in graphite samples implanted with deuterium at 70 keV (Rp ffi 670 nm), and then annealed from 30 min to 300 h in argon flow or high vacuum.
Atsumi et al. [7–9], this activation energy would correspond to the molecular diffusion of deuterium with sequences of dissociation and recombination along crystallite edges in the graphite filler grains. In our study, D release observed in the first stage might correspond to both molecular diffusion (with sequences of dissociation and recombination) and gas permeation through open pores, the limiting factor being the diffusion process. (ii) During the second stage, the release of D is drastically slowed down at any temperature. This means that the remaining D is probably trapped at sites requiring higher energies (i.e. higher temperatures) to be detrapped. The remaining D is most probably located inside the crystallites and chemisorbed to carbon atoms through sp2 or sp3 bonds [9,12]. The thermal activation of these sites may begin from 700 °C (cf. Fig. 3 where the slight widening of the D peak might be at least partly due to the diffusion of deuterium from these sites). However, temperatures higher than 1200 °C are necessary to lead to the total removal of the deuterium.
Fig. 6. Release curves as a function of temperature for D+ implantation at 390 keV (Rp ffi 2.8 lm) and 24 h annealing in argon flow or high vacuum, and for D+ implantation at 70 keV (Rp ffi 670 nm) and annealing for 30 min and 24 h. Open squares are experimental data on tritium obtained by Sawicki et al. for HT+ implantation at 40 keV (Rp ffi 580 nm) and 20–40 min annealing in vacuum [13].
comparison of our results with those of Sawicki et al. shows that, even if the implantation fluences are different, the release behavior of implanted tritium in samples annealed for 20–40 min is very close to that of deuterium implanted at a similar implantation depth in samples annealed during roughly the same time. Consequently, the thermal release mechanism of implanted deuterium and tritium should be similar. This results shows that deuterium is appropriate to simulate implanted tritium. 4. Conclusions
3.2. Dependence of deuterium thermal release on implantation depth and representativeness versus tritium Fig. 6 presents the deuterium release curves as a function of temperature for 24 h annealing at both implantation depths, 670 nm (70 keV) and 2.8 lm (390 keV). This figure shows that the global shape of the release as function of temperature is similar whatever the implantation depth. However, it is clear from these curves that the release is enhanced when D is implanted close to the sample surface showing the important role of the internal open porosities of nuclear graphite. As an example, for a 24 h annealing at 800 °C, the D release for implantation at 670 nm is twice that for implantation at 2.8 lm. In this figure, we have also compared the deuterium release curve obtained on the samples implanted at 670 nm and annealed for 30 min to the data obtained by Sawicki et al. [13] on tritium release. Although the implantation fluence for tritium was an order of magnitude smaller than that of the present study, the implantation depths are rather similar and the comparison of both results is interesting. The results of Sawicki et al. were obtained on polycrystalline-fine-grained isotropic graphite synthetized by Le Carbone Lorraine (5890/PT type). In their work, HT+ ions were implanted under a 104 Pa vacuum at room temperature (20–45 °C) into 15 15 1 mm3 graphite plates at 40 keV (Rp ffi 580 nm according to SRIM 2011 [6]) and 5 1015 HT+ cm2 ð½HTRp 0:3 at:%Þ. Graphite samples were then annealed in a 104 Pa vacuum for 20–40 min and analyzed by T(D,a)n nuclear reaction using a 500 keV D+2 ion beam from a Van de Graaff accelerator. The
The purpose of this paper is to investigate, using Nuclear Reaction Analysis, the thermal behavior of deuterium implanted into virgin nuclear graphite. The main results are summarized as follows: (1) The main D thermal migration process is release, occurring from about 600 °C to 1200 °C when it is located close to the surface (less than 1 lm). Moreover, in these conditions, D is almost totally released at 1200 °C. (2) This release follows two stages: a rapid step where it occurs within a few hours is followed by a much slower step during which it saturates. The initial stage is characterized by an activation energy of 1.3 eV and might correspond to detrapping of D located at crystallite edges and its diffusion at the crystallite surfaces. We assume that the second stage kinetics corresponds to a very slow diffusion of D located inside the crystallites and chemisorbed to carbon atoms through sp2 or sp3 bonds. The total release of this strongly bound D requires temperatures higher than 1200 °C. (3) The two implantation energies, i.e. implantation depths, performed during this study have revealed the important role of the internal open porosities of nuclear graphite. This role is highlighted by the fact that the release in enhanced when D is implanted close to the sample surface. (4) The comparison of our results obtained on deuterium with those of the literature on tritium show that both elements have a similar thermal behavior.
Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036
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From our results obtained in inert atmosphere or in high vacuum, it is shown that at UNGG reactor temperatures, that do not exceed around 500 °C, there is no significant deuterium release. However, complementary experiments are presently conducted in order to check the role of gas containing traces of hydrogen bearing molecules, i.e. UNGG coolant gas, with respect to isotopic exchange. As a matter of fact, the presence of these molecules might enhance the release of deuterium. Moreover in the perspective of graphite decontamination and extrapolating our results to tritium, it should be noted that temperatures around 1200–1300 °C should allow an almost complete removal of tritium. Acknowledgements This work was performed in the frame of a Andra PhD Grant. The authors would like to thank Andra, EDF and the European project CARBOWASTE (Seventh EURATOM Framework Programme) for their financial support. Moreover, this work could not have been carried out without the support of the accelerator staff of IPNL.
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Please cite this article in press as: M. Le Guillou et al., Thermal behavior of deuterium implanted into nuclear graphite studied by NRA, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.036