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Influence of displacement damages on deuterium retention in reduced activation ferritic/martensitic steels F82H and Eurofer97 Vladimir Kh. Alimov a,∗ , Yuji Hatano a , Kazuyoshi Sugiyama b , Sosuke Kondo c , Tatsuya Hinoki c , Masayuki Tokitani d a
University of Toyama, Toyama 930-8555, Japan Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany c Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan d National Institute for Fusion Science, Toki 509-5292, Japan b
h i g h l i g h t s • • • •
F82H and Eurofer97 samples were irradiated with 20 MeV W and 6.4 MeV Fe ions to various damage levels. The damaged samples were exposed to D2 gas at a pressure of 100 kPa and temperatures of 373, 473 and 573 K. At damage levels above 0.5 dpa, the D concentration in the damage zone does n depend on numbers of dpa. The D concentration in the damage zone decreases with increasing D2 gas exposure temperature.
a r t i c l e
i n f o
Article history: Received 1 November 2015 Received in revised form 22 January 2016 Accepted 17 February 2016 Available online xxx Keywords: Reduced-activation ferritic/martensitic steel Ion-induced damage Deuterium trapping
a b s t r a c t The F82H and Eurofer97 steel samples were irradiated at 300 K with 20 MeV W ions to the damage level of 0.54 displacements per atom (dpa) at the damage peak. Additionally, the F82H steel samples were irradiated at 523 K with 6.4 MeV Fe ions to various damage levels in the range from 0.02 to 12.5 dpa. The damaged samples were exposed to D2 gas at a pressure of 100 kPa and various temperatures in the range from 373 to 573 K for a certain length of time sufficient to fill ion-induced defects with deuterium. Trapping of deuterium at the ion-induced defects was examined by the D(3 He, p)4 He nuclear reaction with 3 He energies between 0.69 and 4.0 MeV allowing determination of the D concentration up to a depth of 7 m. It has been found that (i) at the damage level above 0.5 dpa, the concentration of the ion-induced defects responsible for trapping of diffusing D atoms does not depend practically on the numbers of displacements per atom, and (ii) the saturation value of the D concentration in the damage zone decreases with increasing D2 gas exposure temperature, Texp , and varies from about 10−1 at.% at Texp = 373 K to 10−3 at.% at Texp = 573 K. The deuterium-trap binding energy is estimated to be 0.7 ± 0.2 eV. © 2016 Published by Elsevier B.V.
1. Introduction Reduced activation ferritic/martensitic (RAFM) steels are the primary choice material for first wall and breeding blanket structural application in future fusion power plants [1–3]. As probable blanket structural materials, RAFM steels will be exposed to gaseous tritium bred in a blanket under neutron irradiation [4]. A part of tritium implanted into an armor material from plasma permeates to RAFM steels. In addition, the use of bare RAFM steel as first-wall material in a fusion reactor, at least in selected areas
∗ Corresponding author. E-mail address: [email protected] (V.Kh. Alimov).
of the main chamber, has been proposed in the past [4]. Therefore, the evaluation of tritium retention in RAFM steels under neutron irradiation is an important issue for safety assessment of fusion reactors. One possibility to investigate the influence of neutron-produced defects on the hydrogen isotope inventory is to simulate displacement damage using irradiation with energetic heavy ions (for instance, with MeV-range W ions [5–8]) before loading with deuterium (D). First results on D retention in RAFM steels, namely in F82H, Eurofer97 and Rusfer (EK-181), beforehand irradiated with 20 MeV W ions and then exposed both to D plasma and D2 gas at elevated temperatures were reported in Ref. [6–8]. It has been shown that in the W-ion-irradiated RAFM steels, deuterium decorates the
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Please cite this article in press as: V.Kh. Alimov, et al., Influence of displacement damages on deuterium retention in reduced activation ferritic/martensitic steels F82H and Eurofer97, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.060
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damage profile. The saturation value of the D concentration in the damage zone decreases with increasing exposure temperature. Based on temperature dependence of the D concentrations in the F82H sample beforehand irradiated with 0.8 MeV He or 0.3 MeV H ions and then exposed to low-energy (∼1 eV) D plasma, Takagi et al. [9] have reported that traps observed after irradiation with these high-energy light ions are characterized by the deuteriumtrap binding energy of 0.66 eV [9]. It should be noted that the dependence of the D concentration trapped in the damage zone of RAFM steels on the damage level is not yet studied. This paper describes experiments to determine the damage level in RAFM steel where the concentration of defects responsible for trapping of D atoms saturates. To simulate properly the effects of displacement damage on the bulk retention of diffusing D atoms, heavy-ion-irradiated materials should be loaded with deuterium without formation of additional defects, which could interfere with the ion-induced defects. In this work, the ion-irradiated RAFM steels were exposed to D2 gas at elevated temperatures.
2. Experimental RAFM steels F82H (Japan) and Eurofer97 (EU) [10] were used in this work. Rectangular-shape RAFM steel samples, 10 × 10 mm2 in size and 0.8 mm in thickness, were cut from slabs of each material followed by mechanical polishing to a mirror-like finish and cleaning in an ultrasonic bath. The F82H and Eurofer97 samples were irradiated at 300 K with 20 MeV W ions to a fluence of 8 × 1017 W/m2 . As a result, the samples were damaged to the damage level of 0.54 displacements per atom (dpa) at the damage peak situated at a depth of 1.8 m. In addition, the F82H samples were irradiated at 523 K with 6.4 MeV Fe ions to various ion fluences in the range from 9.6 × 1016 to 6 × 1019 Fe/m2 . In so doing, the damage levels were in the range from 0.02 to 12.5 dpa at the damage peak situated at 1.5 m. In what follows the heavy-ion-irradiated steel samples will be designated as “damaged” samples. To reveal damage profiles in the RAFM steel targets, the damage profiles in Fe target irradiated with 6.4 MeV Fe and 20 MeV W ions were calculated using the program SRIM 2008.03 [11], ‘full cascade option’, with a displacement energy of Ed = 40 eV. All the damaged samples were exposed to D2 gas at a pressure of 100 kPa. The F82H and Eurofer97 samples irradiated with 20 MeV W ions were exposed at temperatures of 473 and 573 K for 5 h, whereas at 373 K the exposure time period was 210 h. The F82H samples irradiated with 6.4 MeV Fe ions were initially exposed to D2 gas at 473 K for 5 h, then, after determination of D depth profiles with the help of nuclear reaction analysis, were once more exposed to D2 gas at 373 K for 210 h. For exposure to D2 gas, the sample was placed inside the quartz tube connected to the high-vacuum pumping system and heated in a vacuum with the use of an external ohmic heater. The temperature was monitored using a type K thermocouple contacted directly to the sample inside the tube. As the sample temperature reached the required value, a valve between the tube and the pumping system was closed and the tube was filled with D2 gas. The background pressure inside the tube was measured with an ionization gauge, whereas the D2 gas pressure was controlled with a Baratron capacitance manometer. After reaching required exposure duration, D2 gas evacuation and sample cooling started simultaneously. D2 gas was evacuated in several seconds, while the sample was cooled down in several minutes. The deuterium depth profiles in the steel samples were determined by nuclear reaction analysis (NRA). The D(3 He, p)4 He reaction was utilized, and both the ␣ particles and protons were
Fig. 1. Depth profiles of deuterium retained in F82H (a) and Eurofer97 (b) steels damaged by irradiation at temperature of 300 K with 20 MeV W ions to 0.54 dpa and then exposed to D2 gas at a pressure of 100 kPa. Exposure temperatures and exposure durations are indicated in the legends. In both panels, the damage depth profile in Fe target, as calculated by SRIM 2008.03 [11], is also shown.
analyzed. The ␣-spectrum was transformed into a D depth profile at depths up to ≈0.5 m using the program SIMNRA [12]. To determine the D concentration at larger depths, the energy of the analyzing beam of 3 He ions was varied from 0.69 to 4.5 MeV. The proton yields measured at different 3 He ion energies allow D depth profiles to be measured to depths of up to 8 m [13]. 3. Results and discussion It should be remarked that the surface morphology of the RAFM steel samples is not changed under irradiation with MeV-range heavy ions, and, therefore, retention of deuterium in these damaged samples is dictated solely by concentration of both ion-induced and intrinsic defects. Generation of W-ion-induced displacement damages in the F82H and Eurofer97 samples and subsequent exposure of these samples to D2 gas at temperatures in the range from 373 to 573 K lead to trapping of deuterium over the whole damage zone to a concentration depending on the D2 gas exposure temperature, Texp (Fig. 1). A concentration of deuterium trapped at intrinsic defects beyond the damage zone depends also on the D2 gas exposure temperature and decreases from about 2 × 10−3 at.% at Texp = 373 K to below 10−4 at.% at Texp = 573 K (Fig. 1). It should be noted that major portion of heavy-ion-induced defects responsible for D accumulation in the damaged zone of the RAFM steels are saturable strong traps1 being filled by D atoms diffusing from the surface [6]. Thus, the plateau-like D depth profile indicates that the D concentration in the damage zone reaches saturation at specific D2 gas pressure and exposure temperature.
1 Saturable traps are traps having finite capacity. Traps where D in solution becomes trapped when it encounters unoccupied traps are denoted as strong traps.
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Fig. 3. Maximum D concentration in the damage zone of F82H steel damaged by irradiation with 6.4 MeV Fe ions at temperature of 523 K to various damage levels and then exposed to D2 gas at a pressure of 100 kPa and temperatures of 373 and 473 K for 210 and 5 h, respectively, as a function of the damage level.
Fig. 2. Depth profiles of deuterium retained in F82H steel damaged by irradiation with 6.4 MeV Fe ions at temperature of 523 K to various damage levels and then exposed to D2 gas at a pressure of 100 kPa and temperatures of 373 K (a) and 473 K (b) for 210 and 5 h, respectively. The damage levels are indicated in the legends. In both panels, the damage depth profiles in Fe target for 0.02 and 0.5 dpa, as calculated by SRIM 2008.03 [11], are also shown.
For F82H steel irradiated with 6.4 MeV Fe ions at 523 K to various damage levels and then exposed to D2 gas at Texp = 373 and 473 K, deuterium depth profiles does not coincide with the calculated damage profile and indicate that the zone of ion-induced defects extends up to 6–7 m (Fig. 2). Obviously, in the course of irradiation of the RAFM steel with 6.4 MeV Fe ions at 523 K migration of ion-induced vacancies and interstitials beyond the primary damage zone takes place [14]. As would be expected, a concentration of deuterium trapped at the ion-induced defects depends strongly on the D2 gas exposure temperature (Fig. 2a,b). Distinction between shapes of the D depth profiles measured after D2 gas exposure at 373 and 473 K (Fig. 2) could be explained by existence of a fraction of weak traps (likely, single vacancies) among the Fe-ion-induced defects. If at Texp = 373 K these weak traps can be occupied by diffusing D atoms, that at higher D2 gas exposure temperature Texp = 473 K these weak traps cannot already retain deuterium. The damage level dependence of the maximum D concentration in the damage zone of the F82H steel samples beforehand irradiation with 6.4 MeV Fe ions at 523 K and then exposed to D2 gas at Texp = 373 and 473 K is shown in Fig. 3. It should be noted that the maximum D concentration is observed in the damage zone at depths between 1 and 3 m, depending on the D2 gas exposure temperature. As the damage level increases from 0.02 to 0.1 dpa, the maximum D concentration increases insignificantly (by factors of 2.4 for Texp = 373 K and 1.5 for Texp = 473 K), and then, at the damage levels over 0.5 dpa, the maximum D concentration remains unaltered (Fig. 3). Thus, it may be deduced that at the damage levels above 0.5 dpa, a concentration of the Fe-ion-induced defects does not depend on the number of displacements per atom, i.e., the concentration of the defects reaches saturation. In the damaged RAFM steels exposed to the D2 gas at 100 kPa, the maximum D concentration in the damage zone decreases
Fig. 4. Maximum concentration of deuterium retained in the damage zone of F82H and EUROFER97 steels beforehand irradiated with MeV-range heavy ions and then exposed to D2 gas at a pressure of 100 kPa, as a function of the exposure temperature. The conditions of irradiation with Fe and W ions are indicated in the legend.
from about 10−1 to about 10−3 at.% as the exposure temperature increases from 373 to 573 K (Fig. 4). This temperature range of significant decreasing in the D concentration gives possibility to estimate the deuterium-trap binding energy as 0.7 ± 0.2 eV. Thermal desorption of tritium generated through spallation reaction process in F82H irradiated with 580 MeV protons at 418 ± 15 and 498 ± 15 K to damage levels of 6.3 and 9.1 dpa, respectively, has been investigated in Ref. [15]. After proton irradiation at 418 ± 15 K, tritium desorption spectrum demonstrated two peaks at temperatures of about 510 and 670 K, whereas after irradiation at higher temperature of 498 ± 15 K only one desorption peak at about 670 K was observed. Thus, based on these findings, one may assume that the cooling time of the damaged F82H sample after termination of exposure to D2 gas at 473 and 573 K (see Section 2) does not influence strongly on a concentration of deuterium retained in the damage zone (Fig. 4). Analyzing data on the D concentration observed after exposure of the damaged steels to D2 gas at different conditions, it is necessary to keep in mind that at the same concentration of the ion-induced defects, the saturation value of the D concentration in the damage zone depends both on the steel sample temperature and on the concentration of D atoms in solute state maintained under D2 gas exposure [5]. From comparison of maximum D concentration in the RAFM steels damaged by irradiation with heavy-ions at different temperatures, namely at 300 K (20 MeV W ions) and 523 K (6.4 MeV Fe ions), and then exposed to D2 gas at Texp = 373 K (Fig. 4), it becomes
Please cite this article in press as: V.Kh. Alimov, et al., Influence of displacement damages on deuterium retention in reduced activation ferritic/martensitic steels F82H and Eurofer97, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.060
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apparent that saturation concentration of the ion-induced defects does not depend practically on the ion irradiation temperature in the range from 300 to 523 K. However, after D2 gas exposure at Texp = 473 K, the maximum D concentration in the F82H steel irradiated with 6.4 MeV Fe ions at 523 K is lower than that in the same steel irradiated with 20 MeV W ions at 300 K (Fig. 4). This observation indicates that types of defects and fraction of those are slightly different between irradiation with 6.4 MeV Fe ions and 20 MeV W ions. Though total concentrations of the ion-induced traps are comparable, the irradiation with 6.4 MeV Fe ions results in higher fraction of weak traps. Detailed study on microstructural analysis is necessary to understand the mechanisms underlying the above-mentioned mechanisms. It is well known that in the course of collisions between high-energy particles and lattice atoms, vacancies and interstitials (Frenkel defects) are created [16]. The development of ion-induced vacancy and interstitial concentrations occurs due to competing processes. These defects can be lost either through recombination of vacancies and interstitials or by reaction with defect sinks (voids, dislocations, dislocation loops, grain boundaries or precipitates) [16]. At elevated temperatures of high-energy particle irradiation, vacancies migrate and, as this takes place, the damage zone extends beyond the calculated damage profile. It has been shown recently that irradiation of low activation martensitic steel with 80 MeV fluorine ions generates eventually mono- and di-vacancies, dislocation and vacancy clusters [17]. Obviously, these defects are capable to capture deuterium at elevated temperatures of D2 gas exposure. 4. Summary To determine the damage level at which a concentration of defects responsible for trapping of diffusing D atoms reaches saturation, F82H steel samples were irradiated at 523 K with 6.4 MeV Fe ions to damage levels in the range from 0.02 to 12.5 dpa at the damage peak. Additionally, F82H and Euerofer97 steel samples were irradiated with 20 MeV W ions at 300 K to the damage level of 0.54 dpa. Under following exposure to D2 gas at a pressure of 100 kPa and temperatures, Texp , in the range from 373 to 573 K, ion-induced defects were occupied by diffusing D atoms. It has been shown that at the damage levels above 0.5 dpa, a concentration of the Fe-ion-induced defects does not depend on the number of displacements per atom, i.e., the concentration of the defects reaches saturation. The saturation concentration of the
ion-induced defects is practically the same for the ion irradiation temperatures of 300 and 523 K. For both steel materials, the saturation value of the D concentration in the damage zone decreases with increasing D2 gas exposure temperature and varies from about 10−1 at.% at Texp = 373 K to 10−3 at.% at Texp = 573 K. The deuterium-trap binding energy is estimated to be 0.7 ± 0.2 eV. Acknowledgments This study was partly performed under the Joint Usage/Research Program on Zero-Emission Energy Research, Institute of Advanced Energy, Kyoto University (ZE27A-16) and NIFS Collaboration Research Program (NIFS14KEMF063). References [1] H. Bolt, V. Barabash, W. Krauss, J. Linke, R. Neu, S. Suzuki, N. Yoshida, ASDEX upgrade team, J. Nucl. Mater. 66 (2004) 329–333. [2] K. Mergia, N. Boukos, J. Nucl. Mater. 373 (2008) 1. [3] B. van der Schaaf, F. Tavassoli, C. Fazio, E. Rigal, E. Diegele, R. Lindau, G. LeMarois, Fusion Eng. Des. 69 (2003) 197. [4] H. Bolt, V. Barabash, G. Federici, J. Linke, A. Loarte, J. Roth, K. Sato, J. Nucl. Mater. 43 (2002) 307–311. [5] V. Kh. Alimov, Y. Hatano, K. Sugiyama, J. Roth, B. Tyburska-Püschel, J. Dorner, J. Shi, M. Matsuyama, K. Isobe, T. Yamanishi, J. Nucl. Mater. 438 (2013) S959. [6] V. Kh. Alimov, Y. Hatano, K. Sugiyama, M. Balden, T. Höschen, M. Oyaidzu, J. Roth, J. Dorner, M. Fußeder, T. Yamanishi, Phys. Scr. T. 159 (2014) 014049. [7] O.V. Ogorodnikova, K. Sugiyama, J. Nucl. Mater. 442 (2013) 518. [8] A.V. Spitsyn, A.V. Golubeva, N.P. Bobyr, B.I. Khripunov, D.I. Cherkez, V.B. Petrov, M. Mayer, O.V. Ogorodnikova, V.Kh. Alimov, N.S. Klimov, A. Putrik, V.M. Chernov, M.V. Leontieva-Smirnova, Yu.M. Gasparyan, V.S. Efimov, J. Nucl. Mater. 455 (2014) 561. [9] I. Takagi, T. Komura, M. Akiyoshi, K. Moritani, T. Sasaki, H. Moriyama, J. Nucl. Mater. 442 (2013) S33. [10] R. Lindau, A. Möslang, M. Rieth, M. Klimiankou, E. Materna-Morris, A. Alamo, A.-A.F. Tavassoli, C. Cayron, A.-M. Lancha, P. Fernandez, N. Baluc, R. Schäublin, E. Diegele, G. Filacchioni, J.W. Rensman, B.v.d. Schaaf, E. Lucon, W. Dietz, Fusion Eng. Des. 75–79 (2005) 989. [11] J.F. Ziegler, SRIM—The Stopping and Range of Ions in Matter: ver. SRIM-2008.3 http://srim.org. [12] M. Mayer, SIMNRA user’s guide Tech. Rep. IPP 9/113, Max-Planck-Institut für Plasmaphysik, Garching, 1997. [13] V. Kh. Alimov, M. Mayer, J. Roth, Nucl. Instr. Meth. B 234 (2005) 169. [14] M. Kiritani, H. Takata, K. Moriyama, F.E. Fijita, Philos. Mag. A 40 (1979) 779, and papers cited therein. [15] H. Nakamura, K. Kobayashi, T. Yamanishi, S. Yokoyama, S. Saito, K. Kikuchi, Fusion Sci. Technol. 52 (2007) 1012. [16] G.S. Was, Fundamentals of Radiation Material Science: Metals and Alloys, Springer, Berlin, Heidelberg, New York, 2007. [17] Y. Zheng, Q. Huang, L. Peng, Y. Zuo, P. Fan, D. Zhou, D. Yuan, Y. Wu, S. Zhu, Plasma Sci. Technol. 14 (2012) 629.
Please cite this article in press as: V.Kh. Alimov, et al., Influence of displacement damages on deuterium retention in reduced activation ferritic/martensitic steels F82H and Eurofer97, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.060
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Influence of displacement damages on deuterium retention in reduced activation ferritic/martensitic steels F82H and Eurofer97 Vladimir Kh. Alimov a,∗ , Yuji Hatano a , Kazuyoshi Sugiyama b , Sosuke Kondo c , Tatsuya Hinoki c , Masayuki Tokitani d a
University of Toyama, Toyama 930-8555, Japan Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany c Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan d National Institute for Fusion Science, Toki 509-5292, Japan b
h i g h l i g h t s • • • •
F82H and Eurofer97 samples were irradiated with 20 MeV W and 6.4 MeV Fe ions to various damage levels. The damaged samples were exposed to D2 gas at a pressure of 100 kPa and temperatures of 373, 473 and 573 K. At damage levels above 0.5 dpa, the D concentration in the damage zone does n depend on numbers of dpa. The D concentration in the damage zone decreases with increasing D2 gas exposure temperature.
a r t i c l e
i n f o
Article history: Received 1 November 2015 Received in revised form 22 January 2016 Accepted 17 February 2016 Available online xxx Keywords: Reduced-activation ferritic/martensitic steel Ion-induced damage Deuterium trapping
a b s t r a c t The F82H and Eurofer97 steel samples were irradiated at 300 K with 20 MeV W ions to the damage level of 0.54 displacements per atom (dpa) at the damage peak. Additionally, the F82H steel samples were irradiated at 523 K with 6.4 MeV Fe ions to various damage levels in the range from 0.02 to 12.5 dpa. The damaged samples were exposed to D2 gas at a pressure of 100 kPa and various temperatures in the range from 373 to 573 K for a certain length of time sufficient to fill ion-induced defects with deuterium. Trapping of deuterium at the ion-induced defects was examined by the D(3 He, p)4 He nuclear reaction with 3 He energies between 0.69 and 4.0 MeV allowing determination of the D concentration up to a depth of 7 m. It has been found that (i) at the damage level above 0.5 dpa, the concentration of the ion-induced defects responsible for trapping of diffusing D atoms does not depend practically on the numbers of displacements per atom, and (ii) the saturation value of the D concentration in the damage zone decreases with increasing D2 gas exposure temperature, Texp , and varies from about 10−1 at.% at Texp = 373 K to 10−3 at.% at Texp = 573 K. The deuterium-trap binding energy is estimated to be 0.7 ± 0.2 eV. © 2016 Published by Elsevier B.V.
1. Introduction Reduced activation ferritic/martensitic (RAFM) steels are the primary choice material for first wall and breeding blanket structural application in future fusion power plants [1–3]. As probable blanket structural materials, RAFM steels will be exposed to gaseous tritium bred in a blanket under neutron irradiation [4]. A part of tritium implanted into an armor material from plasma permeates to RAFM steels. In addition, the use of bare RAFM steel as first-wall material in a fusion reactor, at least in selected areas
∗ Corresponding author. E-mail address: [email protected] (V.Kh. Alimov).
of the main chamber, has been proposed in the past [4]. Therefore, the evaluation of tritium retention in RAFM steels under neutron irradiation is an important issue for safety assessment of fusion reactors. One possibility to investigate the influence of neutron-produced defects on the hydrogen isotope inventory is to simulate displacement damage using irradiation with energetic heavy ions (for instance, with MeV-range W ions [5–8]) before loading with deuterium (D). First results on D retention in RAFM steels, namely in F82H, Eurofer97 and Rusfer (EK-181), beforehand irradiated with 20 MeV W ions and then exposed both to D plasma and D2 gas at elevated temperatures were reported in Ref. [6–8]. It has been shown that in the W-ion-irradiated RAFM steels, deuterium decorates the
http://dx.doi.org/10.1016/j.fusengdes.2016.02.060 0920-3796/© 2016 Published by Elsevier B.V.
Please cite this article in press as: V.Kh. Alimov, et al., Influence of displacement damages on deuterium retention in reduced activation ferritic/martensitic steels F82H and Eurofer97, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.060
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damage profile. The saturation value of the D concentration in the damage zone decreases with increasing exposure temperature. Based on temperature dependence of the D concentrations in the F82H sample beforehand irradiated with 0.8 MeV He or 0.3 MeV H ions and then exposed to low-energy (∼1 eV) D plasma, Takagi et al. [9] have reported that traps observed after irradiation with these high-energy light ions are characterized by the deuteriumtrap binding energy of 0.66 eV [9]. It should be noted that the dependence of the D concentration trapped in the damage zone of RAFM steels on the damage level is not yet studied. This paper describes experiments to determine the damage level in RAFM steel where the concentration of defects responsible for trapping of D atoms saturates. To simulate properly the effects of displacement damage on the bulk retention of diffusing D atoms, heavy-ion-irradiated materials should be loaded with deuterium without formation of additional defects, which could interfere with the ion-induced defects. In this work, the ion-irradiated RAFM steels were exposed to D2 gas at elevated temperatures.
2. Experimental RAFM steels F82H (Japan) and Eurofer97 (EU) [10] were used in this work. Rectangular-shape RAFM steel samples, 10 × 10 mm2 in size and 0.8 mm in thickness, were cut from slabs of each material followed by mechanical polishing to a mirror-like finish and cleaning in an ultrasonic bath. The F82H and Eurofer97 samples were irradiated at 300 K with 20 MeV W ions to a fluence of 8 × 1017 W/m2 . As a result, the samples were damaged to the damage level of 0.54 displacements per atom (dpa) at the damage peak situated at a depth of 1.8 m. In addition, the F82H samples were irradiated at 523 K with 6.4 MeV Fe ions to various ion fluences in the range from 9.6 × 1016 to 6 × 1019 Fe/m2 . In so doing, the damage levels were in the range from 0.02 to 12.5 dpa at the damage peak situated at 1.5 m. In what follows the heavy-ion-irradiated steel samples will be designated as “damaged” samples. To reveal damage profiles in the RAFM steel targets, the damage profiles in Fe target irradiated with 6.4 MeV Fe and 20 MeV W ions were calculated using the program SRIM 2008.03 [11], ‘full cascade option’, with a displacement energy of Ed = 40 eV. All the damaged samples were exposed to D2 gas at a pressure of 100 kPa. The F82H and Eurofer97 samples irradiated with 20 MeV W ions were exposed at temperatures of 473 and 573 K for 5 h, whereas at 373 K the exposure time period was 210 h. The F82H samples irradiated with 6.4 MeV Fe ions were initially exposed to D2 gas at 473 K for 5 h, then, after determination of D depth profiles with the help of nuclear reaction analysis, were once more exposed to D2 gas at 373 K for 210 h. For exposure to D2 gas, the sample was placed inside the quartz tube connected to the high-vacuum pumping system and heated in a vacuum with the use of an external ohmic heater. The temperature was monitored using a type K thermocouple contacted directly to the sample inside the tube. As the sample temperature reached the required value, a valve between the tube and the pumping system was closed and the tube was filled with D2 gas. The background pressure inside the tube was measured with an ionization gauge, whereas the D2 gas pressure was controlled with a Baratron capacitance manometer. After reaching required exposure duration, D2 gas evacuation and sample cooling started simultaneously. D2 gas was evacuated in several seconds, while the sample was cooled down in several minutes. The deuterium depth profiles in the steel samples were determined by nuclear reaction analysis (NRA). The D(3 He, p)4 He reaction was utilized, and both the ␣ particles and protons were
Fig. 1. Depth profiles of deuterium retained in F82H (a) and Eurofer97 (b) steels damaged by irradiation at temperature of 300 K with 20 MeV W ions to 0.54 dpa and then exposed to D2 gas at a pressure of 100 kPa. Exposure temperatures and exposure durations are indicated in the legends. In both panels, the damage depth profile in Fe target, as calculated by SRIM 2008.03 [11], is also shown.
analyzed. The ␣-spectrum was transformed into a D depth profile at depths up to ≈0.5 m using the program SIMNRA [12]. To determine the D concentration at larger depths, the energy of the analyzing beam of 3 He ions was varied from 0.69 to 4.5 MeV. The proton yields measured at different 3 He ion energies allow D depth profiles to be measured to depths of up to 8 m [13]. 3. Results and discussion It should be remarked that the surface morphology of the RAFM steel samples is not changed under irradiation with MeV-range heavy ions, and, therefore, retention of deuterium in these damaged samples is dictated solely by concentration of both ion-induced and intrinsic defects. Generation of W-ion-induced displacement damages in the F82H and Eurofer97 samples and subsequent exposure of these samples to D2 gas at temperatures in the range from 373 to 573 K lead to trapping of deuterium over the whole damage zone to a concentration depending on the D2 gas exposure temperature, Texp (Fig. 1). A concentration of deuterium trapped at intrinsic defects beyond the damage zone depends also on the D2 gas exposure temperature and decreases from about 2 × 10−3 at.% at Texp = 373 K to below 10−4 at.% at Texp = 573 K (Fig. 1). It should be noted that major portion of heavy-ion-induced defects responsible for D accumulation in the damaged zone of the RAFM steels are saturable strong traps1 being filled by D atoms diffusing from the surface [6]. Thus, the plateau-like D depth profile indicates that the D concentration in the damage zone reaches saturation at specific D2 gas pressure and exposure temperature.
1 Saturable traps are traps having finite capacity. Traps where D in solution becomes trapped when it encounters unoccupied traps are denoted as strong traps.
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Fig. 3. Maximum D concentration in the damage zone of F82H steel damaged by irradiation with 6.4 MeV Fe ions at temperature of 523 K to various damage levels and then exposed to D2 gas at a pressure of 100 kPa and temperatures of 373 and 473 K for 210 and 5 h, respectively, as a function of the damage level.
Fig. 2. Depth profiles of deuterium retained in F82H steel damaged by irradiation with 6.4 MeV Fe ions at temperature of 523 K to various damage levels and then exposed to D2 gas at a pressure of 100 kPa and temperatures of 373 K (a) and 473 K (b) for 210 and 5 h, respectively. The damage levels are indicated in the legends. In both panels, the damage depth profiles in Fe target for 0.02 and 0.5 dpa, as calculated by SRIM 2008.03 [11], are also shown.
For F82H steel irradiated with 6.4 MeV Fe ions at 523 K to various damage levels and then exposed to D2 gas at Texp = 373 and 473 K, deuterium depth profiles does not coincide with the calculated damage profile and indicate that the zone of ion-induced defects extends up to 6–7 m (Fig. 2). Obviously, in the course of irradiation of the RAFM steel with 6.4 MeV Fe ions at 523 K migration of ion-induced vacancies and interstitials beyond the primary damage zone takes place [14]. As would be expected, a concentration of deuterium trapped at the ion-induced defects depends strongly on the D2 gas exposure temperature (Fig. 2a,b). Distinction between shapes of the D depth profiles measured after D2 gas exposure at 373 and 473 K (Fig. 2) could be explained by existence of a fraction of weak traps (likely, single vacancies) among the Fe-ion-induced defects. If at Texp = 373 K these weak traps can be occupied by diffusing D atoms, that at higher D2 gas exposure temperature Texp = 473 K these weak traps cannot already retain deuterium. The damage level dependence of the maximum D concentration in the damage zone of the F82H steel samples beforehand irradiation with 6.4 MeV Fe ions at 523 K and then exposed to D2 gas at Texp = 373 and 473 K is shown in Fig. 3. It should be noted that the maximum D concentration is observed in the damage zone at depths between 1 and 3 m, depending on the D2 gas exposure temperature. As the damage level increases from 0.02 to 0.1 dpa, the maximum D concentration increases insignificantly (by factors of 2.4 for Texp = 373 K and 1.5 for Texp = 473 K), and then, at the damage levels over 0.5 dpa, the maximum D concentration remains unaltered (Fig. 3). Thus, it may be deduced that at the damage levels above 0.5 dpa, a concentration of the Fe-ion-induced defects does not depend on the number of displacements per atom, i.e., the concentration of the defects reaches saturation. In the damaged RAFM steels exposed to the D2 gas at 100 kPa, the maximum D concentration in the damage zone decreases
Fig. 4. Maximum concentration of deuterium retained in the damage zone of F82H and EUROFER97 steels beforehand irradiated with MeV-range heavy ions and then exposed to D2 gas at a pressure of 100 kPa, as a function of the exposure temperature. The conditions of irradiation with Fe and W ions are indicated in the legend.
from about 10−1 to about 10−3 at.% as the exposure temperature increases from 373 to 573 K (Fig. 4). This temperature range of significant decreasing in the D concentration gives possibility to estimate the deuterium-trap binding energy as 0.7 ± 0.2 eV. Thermal desorption of tritium generated through spallation reaction process in F82H irradiated with 580 MeV protons at 418 ± 15 and 498 ± 15 K to damage levels of 6.3 and 9.1 dpa, respectively, has been investigated in Ref. [15]. After proton irradiation at 418 ± 15 K, tritium desorption spectrum demonstrated two peaks at temperatures of about 510 and 670 K, whereas after irradiation at higher temperature of 498 ± 15 K only one desorption peak at about 670 K was observed. Thus, based on these findings, one may assume that the cooling time of the damaged F82H sample after termination of exposure to D2 gas at 473 and 573 K (see Section 2) does not influence strongly on a concentration of deuterium retained in the damage zone (Fig. 4). Analyzing data on the D concentration observed after exposure of the damaged steels to D2 gas at different conditions, it is necessary to keep in mind that at the same concentration of the ion-induced defects, the saturation value of the D concentration in the damage zone depends both on the steel sample temperature and on the concentration of D atoms in solute state maintained under D2 gas exposure [5]. From comparison of maximum D concentration in the RAFM steels damaged by irradiation with heavy-ions at different temperatures, namely at 300 K (20 MeV W ions) and 523 K (6.4 MeV Fe ions), and then exposed to D2 gas at Texp = 373 K (Fig. 4), it becomes
Please cite this article in press as: V.Kh. Alimov, et al., Influence of displacement damages on deuterium retention in reduced activation ferritic/martensitic steels F82H and Eurofer97, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.060
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apparent that saturation concentration of the ion-induced defects does not depend practically on the ion irradiation temperature in the range from 300 to 523 K. However, after D2 gas exposure at Texp = 473 K, the maximum D concentration in the F82H steel irradiated with 6.4 MeV Fe ions at 523 K is lower than that in the same steel irradiated with 20 MeV W ions at 300 K (Fig. 4). This observation indicates that types of defects and fraction of those are slightly different between irradiation with 6.4 MeV Fe ions and 20 MeV W ions. Though total concentrations of the ion-induced traps are comparable, the irradiation with 6.4 MeV Fe ions results in higher fraction of weak traps. Detailed study on microstructural analysis is necessary to understand the mechanisms underlying the above-mentioned mechanisms. It is well known that in the course of collisions between high-energy particles and lattice atoms, vacancies and interstitials (Frenkel defects) are created [16]. The development of ion-induced vacancy and interstitial concentrations occurs due to competing processes. These defects can be lost either through recombination of vacancies and interstitials or by reaction with defect sinks (voids, dislocations, dislocation loops, grain boundaries or precipitates) [16]. At elevated temperatures of high-energy particle irradiation, vacancies migrate and, as this takes place, the damage zone extends beyond the calculated damage profile. It has been shown recently that irradiation of low activation martensitic steel with 80 MeV fluorine ions generates eventually mono- and di-vacancies, dislocation and vacancy clusters [17]. Obviously, these defects are capable to capture deuterium at elevated temperatures of D2 gas exposure. 4. Summary To determine the damage level at which a concentration of defects responsible for trapping of diffusing D atoms reaches saturation, F82H steel samples were irradiated at 523 K with 6.4 MeV Fe ions to damage levels in the range from 0.02 to 12.5 dpa at the damage peak. Additionally, F82H and Euerofer97 steel samples were irradiated with 20 MeV W ions at 300 K to the damage level of 0.54 dpa. Under following exposure to D2 gas at a pressure of 100 kPa and temperatures, Texp , in the range from 373 to 573 K, ion-induced defects were occupied by diffusing D atoms. It has been shown that at the damage levels above 0.5 dpa, a concentration of the Fe-ion-induced defects does not depend on the number of displacements per atom, i.e., the concentration of the defects reaches saturation. The saturation concentration of the
ion-induced defects is practically the same for the ion irradiation temperatures of 300 and 523 K. For both steel materials, the saturation value of the D concentration in the damage zone decreases with increasing D2 gas exposure temperature and varies from about 10−1 at.% at Texp = 373 K to 10−3 at.% at Texp = 573 K. The deuterium-trap binding energy is estimated to be 0.7 ± 0.2 eV. Acknowledgments This study was partly performed under the Joint Usage/Research Program on Zero-Emission Energy Research, Institute of Advanced Energy, Kyoto University (ZE27A-16) and NIFS Collaboration Research Program (NIFS14KEMF063). References [1] H. Bolt, V. Barabash, W. Krauss, J. Linke, R. Neu, S. Suzuki, N. Yoshida, ASDEX upgrade team, J. Nucl. Mater. 66 (2004) 329–333. [2] K. Mergia, N. Boukos, J. Nucl. Mater. 373 (2008) 1. [3] B. van der Schaaf, F. Tavassoli, C. Fazio, E. Rigal, E. Diegele, R. Lindau, G. LeMarois, Fusion Eng. Des. 69 (2003) 197. [4] H. Bolt, V. Barabash, G. Federici, J. Linke, A. Loarte, J. Roth, K. Sato, J. Nucl. Mater. 43 (2002) 307–311. [5] V. Kh. Alimov, Y. Hatano, K. Sugiyama, J. Roth, B. Tyburska-Püschel, J. Dorner, J. Shi, M. Matsuyama, K. Isobe, T. Yamanishi, J. Nucl. Mater. 438 (2013) S959. [6] V. Kh. Alimov, Y. Hatano, K. Sugiyama, M. Balden, T. Höschen, M. Oyaidzu, J. Roth, J. Dorner, M. Fußeder, T. Yamanishi, Phys. Scr. T. 159 (2014) 014049. [7] O.V. Ogorodnikova, K. Sugiyama, J. Nucl. Mater. 442 (2013) 518. [8] A.V. Spitsyn, A.V. Golubeva, N.P. Bobyr, B.I. Khripunov, D.I. Cherkez, V.B. Petrov, M. Mayer, O.V. Ogorodnikova, V.Kh. Alimov, N.S. Klimov, A. Putrik, V.M. Chernov, M.V. Leontieva-Smirnova, Yu.M. Gasparyan, V.S. Efimov, J. Nucl. Mater. 455 (2014) 561. [9] I. Takagi, T. Komura, M. Akiyoshi, K. Moritani, T. Sasaki, H. Moriyama, J. Nucl. Mater. 442 (2013) S33. [10] R. Lindau, A. Möslang, M. Rieth, M. Klimiankou, E. Materna-Morris, A. Alamo, A.-A.F. Tavassoli, C. Cayron, A.-M. Lancha, P. Fernandez, N. Baluc, R. Schäublin, E. Diegele, G. Filacchioni, J.W. Rensman, B.v.d. Schaaf, E. Lucon, W. Dietz, Fusion Eng. Des. 75–79 (2005) 989. [11] J.F. Ziegler, SRIM—The Stopping and Range of Ions in Matter: ver. SRIM-2008.3 http://srim.org. [12] M. Mayer, SIMNRA user’s guide Tech. Rep. IPP 9/113, Max-Planck-Institut für Plasmaphysik, Garching, 1997. [13] V. Kh. Alimov, M. Mayer, J. Roth, Nucl. Instr. Meth. B 234 (2005) 169. [14] M. Kiritani, H. Takata, K. Moriyama, F.E. Fijita, Philos. Mag. A 40 (1979) 779, and papers cited therein. [15] H. Nakamura, K. Kobayashi, T. Yamanishi, S. Yokoyama, S. Saito, K. Kikuchi, Fusion Sci. Technol. 52 (2007) 1012. [16] G.S. Was, Fundamentals of Radiation Material Science: Metals and Alloys, Springer, Berlin, Heidelberg, New York, 2007. [17] Y. Zheng, Q. Huang, L. Peng, Y. Zuo, P. Fan, D. Zhou, D. Yuan, Y. Wu, S. Zhu, Plasma Sci. Technol. 14 (2012) 629.
Please cite this article in press as: V.Kh. Alimov, et al., Influence of displacement damages on deuterium retention in reduced activation ferritic/martensitic steels F82H and Eurofer97, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.060