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Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers
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ARTICLE IN PRESS
FUSION-9041; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
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Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers Masaru Nakamichi ∗ , Jae-Hwan Kim National Institutes for Quantum and Radiological Science and Technology, Aomori, Japan
h i g h l i g h t s • • • •
Prototypic Be12 V pebbles were successfully fabricated by a direct the rotating electrode granulation method. Be12 V pebbles were much more resistant to water vapor than pure Be. A surface BeO layer on Be12 V pebbles acted as a protective barrier preventing the hydrogen generation reaction. The hydrogen generation rate was nearly reduced to background level via surface modification of Be12 V pebbles.
a r t i c l e
i n f o
Article history: Received 29 September 2016 Received in revised form 6 December 2016 Accepted 20 January 2017 Available online xxx Keywords: Beryllide Beryllium Intermetallic compound Neutron multiplier Surface modification Beryllium oxide
a b s t r a c t Prototypic pebbles composed of Be12 V showing no peritectic reaction during cooling were successfully fabricated without homogenization. These Be12 V prototypic pebbles were found to have a good oxidation resistance. BeO layer on the surface of Be12 V pebbles was found to act as a protective barrier preventing the hydrogen generation reaction. Thus, the effect of BeO layer as a surface modification of beryllide was evaluated. Using surface-modified pebbles, hydrogen generation experiments were repetitively (3 times) carried out at 1273 K for 24 h. The hydrogen generation rate was nearly reduced to the background level by this surface modification. Thus, surface-modified Be12 V pebbles showing no-hydrogen generation reaction were successfully fabricated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen generation via oxidation of an existing neutron multiplier such as beryllium (Be) with water vapor at high temperatures may reduce the safety hazard of fusion reactors. Beryllium intermetallic compounds (i.e., beryllides) are the most promising advanced neutron multiplier material having high stability at high temperatures when used in pebble bed blankets of demonstration (DEMO) reactors [1,2]. Fundamental technologies for fabricating beryllides pebbles have been developed by combining plasma sintering synthesis and rotating electrode granulation methods. Thus, beryllides pebbles with 1 mm in diameter were successfully fabricated by the rotating electrode granulation method using a plasma-sintered beryllide electrode.
∗ Corresponding author. E-mail address: [email protected] (M. Nakamichi).
In the case of Be12 Ti beryllide, since it undergoes compositional structure changes during granulation via a peritectic reaction caused by re-melting, an annealing treatment is necessary to homogenize the pebbles to a single phase of Be12 Ti after granulation [3,4]. However, homogenized Be12 Ti pebbles showed larger reactivity as compared to unannealed samples because the homogenization treatment increased the specific surface area while also developing a porous body in the treated pebbles [5]. To prevent the increased H2 generation associated with the increase of the specific surface area, other candidate beryllide compositions without peritectic reaction during cooling have been surveyed. Thus, Be12 V was successfully fabricated in replacement of Be12 Ti without any homogenization treatment. These Be12 V prototypic pebbles showed good oxidation resistance characteristics [6]. The reactivity results suggested that a BeO layer on the surface of Be12 V pebbles acted as a protective barrier preventing hydrogen generation. As the next stage, the effect of this BeO layer in surfacemodified beryllides was evaluated in this study.
http://dx.doi.org/10.1016/j.fusengdes.2017.01.033 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
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Fig. 1. Abridged phase diagram (a) of the Be-V binary system, schematic drawing (b) of the rotating electrode granulation method, and compositional SEM images of (c) the surface and (d) cross-section of the as-granulated Be12 V pebbles.
2. Experimental procedure 2.1. Be–V beryllide electrode fabrication Be (Materion Brush, USA, 99.4 wt%) and V (Kojundo Chemical Laboratory, Japan, 99.9 wt%) powders were mixed at 92.3 at% Be and 7.7 at% V (i.e., the stoichiometric composition of Be12 V). The particle sizes of the Be and V powders were less than 45 m. The Be–V beryllide electrode as a granulation raw material was prepared by mixing its elemental powders. In this process, the starting powder was loaded into a die and punch unit that applies uniaxial pressure. An alternating electric current was applied to create the plasma environment and thereby activate the particle surfaces. The compacted powder was resistance-heated by a direct electric current under continued uniaxial pressure on the material in the die and punch unit used as the sintering mold. A plasma sintering apparatus (KE-PasIII, Kaken, Japan) was used. Plasma sintering was performed at 1073 K for 2.5 min under 50 MPa pressure. For the fabrication of Be–V beryllide pebbles via the rotating electrode granulation method, a Be–V beryllide electrode of 15 mm in diameter and 50 mm in length was used as the raw material. 2.2. Be–V beryllide pebbles fabrication The preparation of Be–V beryllide pebbles was carried out via the rotating electrode granulation method using a plasma-sintered Be-V beryllide electrode. A schematic drawing of the rotating electrode granulation method is shown in Fig. 1. In this granulation method, molten metal particles were produced at the end of a heated metal electrode rotated along its longitudinal axis. The molten metal was centrifugally ejected and formed droplets that solidified into spherical particles. The beryllide pebbles were fabricated in a rotating electrode granulation apparatus (KREP-1500, KAKEN, Japan) under a current
of 60 A and a rotation speed of 6000 rpm for the plasma-sintered Be–V beryllide electrode. Be–V beryllide granulation was performed under pure He at approximately 0.15 MPa. Be–V beryllide pebbles with 1 mm in diameter were successfully fabricated under these granulation conditions. The compositional structures of beryllide pebbles formed via a rotating electrode method (REM) were evaluated by X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) and an electron probe microanalysis (EPMA, JXA-8530F, JEOL, Japan). The XRD and EPMA results revealed that the surface of the asgranulated Be–V pebbles lacking a homogenization treatment was exclusively composed of a Be12 V phase. According to the EPMA results, Be12 V was the only phase identified in the interior of the as-granulated Be-V pebbles (Fig. 1). 2.3. Hydrogen generation reaction of beryllide pebbles To evaluate the reactivity of Be12 V beryllide pebbles, hydrogen generation reaction experiments were performed at 1273 K for 24 h under flowing (300 mL/min) Ar gas containing 10,000 ppm of H2 O by thermal gravimetry (TG, TG-8110, Rigaku, Japan) and gas chromatography (GC, CP-4900, Agilent, USA). A schematic flow diagram of the test apparatus used for the evaluation of the reactivity is shown in Fig. 2. The TG experiments were conducted at heating and cooling rates of 10 and 30 K/min, respectively. The Ar gas containing 10,000 ppm of H2 O was flowed at 1273 K and generated by water vapor generation equipment (HUM-1E, Rigaku, Japan). 3. Hydrogen generation reactivity experiments 3.1. Hydrogen generation measurements using as-granulated Be12 V pebbles The hydrogen generation reactivity of as-granulated Be12 V pebbles with water vapor at 1273 K was evaluated as previously studies
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
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Fig. 2. Schematic flow diagram of the test apparatus used for reactivity evaluation.
Fig. 4. Hydrogen generation cycle tests normalized by the weight of the asgranulated Be12 V pebbles at 1273 K under an Ar-1%H2 O stream.
Fig. 3. Hydrogen generated normalized by the weight of pebbles of the asgranulated Be12 V compared with pure Be pebbles at 1273 K under an Ar-1%H2 O stream [6].
with pure Be pebbles [6]. Fig. 3 show the amount of hydrogen generated from the reaction of the as-granulated Be12 V pebbles with water vapor. For comparison, the results are compared with those of pure Be pebbles. In Fig. 3, the amount of hydrogen generated was normalized by the weight of the pebble. The as-granulated Be12 V pebbles demonstrated low hydrogen generation reactivity (Fig. 3). Thus, the hydrogen generation of the as-granulated Be12 V pebbles was two orders of magnitude less than that of pure Be pebbles [6]. Be pebbles swelled upon oxidation and exhibited numerous cracks on the surface of the oxide layer. In contrast, BeO was deposited over the entire surface of the as-granulated Be12 V pebbles, with no swelling or cracking signs. During oxidation of Be, compressive stress induced cracks within the BeO scale due to the difference of the lattice coherence between substrate and scale. On the other hand, during oxidation of beryllide, BeO was formed with fewer stress on the surface of these pebbles because the atomic distances between Be atoms within the BeO scale are similar to the mean atomic distance between Be atoms within the beryllide substrate [7].
Fig. 5. Cross-sectional view of a pebble after the third test oxidation test with water vapor.
These reactivity results clearly indicated that the BeO layer on the surface of as-granulated Be12 V pebbles acted as a protective barrier preventing the H2 generation reaction. For evaluating the protection effect of the BeO surface layer on the as-granulated Be12 V pebbles via water vapor, hydrogen generation experiments were repeated three times.
3.2. Hydrogen generation measurement cycle tests using as-granulated Be12 V pebbles Hydrogen generation measurements were repeated 3 times in order to verify the effects of the BeO surface layer on the as-granulated Be12 V pebbles. The experimental history and the amount of hydrogen generated by the as-granulated Be12 V pebbles with water vapor are shown in Fig. 4.
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
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Fig. 8. Hydrogen generation cycle tests normalized by the weight of the surfacemodified Be12 V pebbles at 1273 K under an Ar-1%H2 O stream.
Fig. 7. Experimental history and cross-sectional photographs of the surfacemodified Be12 V pebbles before and after the first reactivity test.
When compared with the initial value, the hydrogen generation rate decreased by 90 and 70% after the second and third tests, respectively. A cross-sectional view of the pebble after the third test oxidation test with water vapor is shown in Fig. 5. Some defect areas in the BeO surface layer were formed upon the water vapor oxidation process. Since water vapor caused an intense oxidation reaction, the BeO surface layer of beryllides showed some defect areas. Therefore, in order to cover the pebbles with an oxide layer without defects, surface oxidation tests were performed under an oxygen atmosphere in order to promote surface modification of the BeO layer. 4. Results and discussion 4.1. Surface modification of the as-granulated Be12 V pebbles by oxidation treatment Surface oxidation on the as-granulated Be12 V pebbles was carried out to determine the effect of protection barrier. However, as mentioned before, the BeO layer on the surface of Be12 V pebbles
oxidized by water vapor showed some defects. Therefore, as a part of the surface modification examination via deposition of a BeO protective layer, the as-granulated Be12 V pebbles were oxidized under moderate conditions (i.e., oxygen atmosphere). The as-granulated Be12 V pebbles were annealed at 1273 K for 10 h under He containing varying concentrations (i.e., 100, 1000, and 10,000 ppm) of oxygen. Some photographs of Be12 V pebbles after this annealing oxidation treatment are shown in Fig. 6. The oxidation treatment results performed for surface modification revealed the presence of a yellow V2 O5 layer on the surface of Be12 V pebbles oxidized with 10,000 ppm of O2 . On the other hand, no signs of the V2 O5 layer were observed for surface-modified Be12 V pebbles oxidized with 100 ppm of O2 . These latter pebbles were subsequently used for hydrogen generation measurements. 4.2. Hydrogen generation cycle tests using surface-modified Be12 V pebbles via an oxidation treatment With the aim to verify the effect of the BeO protective layer on the surface of Be12 V pebbles, surface-modified Be12 V pebbles by oxidation with 100 ppm of O2 were used for repeated hydrogen generation measurements (i.e., three times). The experimental history and cross-sectional photographs of these surface-modified Be12 V pebbles before and after the reactivity test are shown in Fig. 7, while the results of the hydrogen generation cycle tests are shown in Fig. 8.
Fig. 6. Pictures of Be12 V pebbles after the annealing oxidation treatment at 1273 K for 10 h under He containing: (a) 100, (b) 1000, and (c) 10,000 ppm of oxygen.
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
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The hydrogen generation cycle tests of the surface-modified Be12 V pebbles revealed a significant reduction in the hydrogen generation rate, which nearly decreased to the background value. Thus, surface-modified Be12 V pebbles with no-hydrogen generation were successfully fabricated. 5. Conclusions Prototypic Be12 V pebbles were successfully fabricated by a direct rotating electrode granulation method using a plasmasintered beryllide electrode without homogenization. Be12 V pebbles were significantly more resistant to water vapor than pure Be. The surface BeO layer on Be12 V pebbles acted as a protective barrier preventing hydrogen generation. The hydrogen generation rate was nearly reduced to the background value via surface modification of Be12 V pebbles. These surface-modified pebbles showing no-hydrogen generation reaction were successfully fabricated by oxidation with 100 ppm of O2 . The stability and durability of the surface oxidation layer over beryllide pebbles shall be evaluated.
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References [1] M. Nakamichi, K. Yonehara, Sintering properties of beryllides for advanced neutron multiplier, J. Nucl. Mater. 417 (1–3) (2011) 765–768. [2] M. Nakamichi, K. Yonehara, D. Wakai, Trial fabrication of beryllides as advanced neutron multiplier, Fusion Eng. Des. 86 (9–11) (2011) 2262–2264. [3] M. Nakamichi, J.H. Kim, K. Yonehara, Novel granulation process of beryllides as advanced neutron multipliers, Fusion Eng. Des. 88 (2013) 611–615. [4] M. Nakamichi, J.H. Kim, Homogenization treatment to stabilize the compositional structure of beryllide pebbles, J. Nucl. Mater. 440 (1–3) (2013) 530–533. [5] M. Nakamichi, J.H. Kim, Fabrication and hydrogen generation reaction with water vapor of prototypic pebbles of binary beryllides as advanced neutron multipliers, Fusion Eng. Des. 98–99 (2015) 1838–1842. [6] M. Nakamichi, J.H. Kim, Fabrication and characterization of advanced neutron multipliers for DEMO blanket, J. Nucl. Mater. Energy (2016) (in press). [7] J.H. Kim, M. Nakamichi, Oxidation behavior of plasma sintered beryllium-titanium intermetallic compounds as an advanced neutron multiplier, J. Nucl. Mater. 438 (2013) 218–223.
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
ARTICLE IN PRESS
FUSION-9041; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers Masaru Nakamichi ∗ , Jae-Hwan Kim National Institutes for Quantum and Radiological Science and Technology, Aomori, Japan
h i g h l i g h t s • • • •
Prototypic Be12 V pebbles were successfully fabricated by a direct the rotating electrode granulation method. Be12 V pebbles were much more resistant to water vapor than pure Be. A surface BeO layer on Be12 V pebbles acted as a protective barrier preventing the hydrogen generation reaction. The hydrogen generation rate was nearly reduced to background level via surface modification of Be12 V pebbles.
a r t i c l e
i n f o
Article history: Received 29 September 2016 Received in revised form 6 December 2016 Accepted 20 January 2017 Available online xxx Keywords: Beryllide Beryllium Intermetallic compound Neutron multiplier Surface modification Beryllium oxide
a b s t r a c t Prototypic pebbles composed of Be12 V showing no peritectic reaction during cooling were successfully fabricated without homogenization. These Be12 V prototypic pebbles were found to have a good oxidation resistance. BeO layer on the surface of Be12 V pebbles was found to act as a protective barrier preventing the hydrogen generation reaction. Thus, the effect of BeO layer as a surface modification of beryllide was evaluated. Using surface-modified pebbles, hydrogen generation experiments were repetitively (3 times) carried out at 1273 K for 24 h. The hydrogen generation rate was nearly reduced to the background level by this surface modification. Thus, surface-modified Be12 V pebbles showing no-hydrogen generation reaction were successfully fabricated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen generation via oxidation of an existing neutron multiplier such as beryllium (Be) with water vapor at high temperatures may reduce the safety hazard of fusion reactors. Beryllium intermetallic compounds (i.e., beryllides) are the most promising advanced neutron multiplier material having high stability at high temperatures when used in pebble bed blankets of demonstration (DEMO) reactors [1,2]. Fundamental technologies for fabricating beryllides pebbles have been developed by combining plasma sintering synthesis and rotating electrode granulation methods. Thus, beryllides pebbles with 1 mm in diameter were successfully fabricated by the rotating electrode granulation method using a plasma-sintered beryllide electrode.
∗ Corresponding author. E-mail address: [email protected] (M. Nakamichi).
In the case of Be12 Ti beryllide, since it undergoes compositional structure changes during granulation via a peritectic reaction caused by re-melting, an annealing treatment is necessary to homogenize the pebbles to a single phase of Be12 Ti after granulation [3,4]. However, homogenized Be12 Ti pebbles showed larger reactivity as compared to unannealed samples because the homogenization treatment increased the specific surface area while also developing a porous body in the treated pebbles [5]. To prevent the increased H2 generation associated with the increase of the specific surface area, other candidate beryllide compositions without peritectic reaction during cooling have been surveyed. Thus, Be12 V was successfully fabricated in replacement of Be12 Ti without any homogenization treatment. These Be12 V prototypic pebbles showed good oxidation resistance characteristics [6]. The reactivity results suggested that a BeO layer on the surface of Be12 V pebbles acted as a protective barrier preventing hydrogen generation. As the next stage, the effect of this BeO layer in surfacemodified beryllides was evaluated in this study.
http://dx.doi.org/10.1016/j.fusengdes.2017.01.033 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
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Fig. 1. Abridged phase diagram (a) of the Be-V binary system, schematic drawing (b) of the rotating electrode granulation method, and compositional SEM images of (c) the surface and (d) cross-section of the as-granulated Be12 V pebbles.
2. Experimental procedure 2.1. Be–V beryllide electrode fabrication Be (Materion Brush, USA, 99.4 wt%) and V (Kojundo Chemical Laboratory, Japan, 99.9 wt%) powders were mixed at 92.3 at% Be and 7.7 at% V (i.e., the stoichiometric composition of Be12 V). The particle sizes of the Be and V powders were less than 45 m. The Be–V beryllide electrode as a granulation raw material was prepared by mixing its elemental powders. In this process, the starting powder was loaded into a die and punch unit that applies uniaxial pressure. An alternating electric current was applied to create the plasma environment and thereby activate the particle surfaces. The compacted powder was resistance-heated by a direct electric current under continued uniaxial pressure on the material in the die and punch unit used as the sintering mold. A plasma sintering apparatus (KE-PasIII, Kaken, Japan) was used. Plasma sintering was performed at 1073 K for 2.5 min under 50 MPa pressure. For the fabrication of Be–V beryllide pebbles via the rotating electrode granulation method, a Be–V beryllide electrode of 15 mm in diameter and 50 mm in length was used as the raw material. 2.2. Be–V beryllide pebbles fabrication The preparation of Be–V beryllide pebbles was carried out via the rotating electrode granulation method using a plasma-sintered Be-V beryllide electrode. A schematic drawing of the rotating electrode granulation method is shown in Fig. 1. In this granulation method, molten metal particles were produced at the end of a heated metal electrode rotated along its longitudinal axis. The molten metal was centrifugally ejected and formed droplets that solidified into spherical particles. The beryllide pebbles were fabricated in a rotating electrode granulation apparatus (KREP-1500, KAKEN, Japan) under a current
of 60 A and a rotation speed of 6000 rpm for the plasma-sintered Be–V beryllide electrode. Be–V beryllide granulation was performed under pure He at approximately 0.15 MPa. Be–V beryllide pebbles with 1 mm in diameter were successfully fabricated under these granulation conditions. The compositional structures of beryllide pebbles formed via a rotating electrode method (REM) were evaluated by X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) and an electron probe microanalysis (EPMA, JXA-8530F, JEOL, Japan). The XRD and EPMA results revealed that the surface of the asgranulated Be–V pebbles lacking a homogenization treatment was exclusively composed of a Be12 V phase. According to the EPMA results, Be12 V was the only phase identified in the interior of the as-granulated Be-V pebbles (Fig. 1). 2.3. Hydrogen generation reaction of beryllide pebbles To evaluate the reactivity of Be12 V beryllide pebbles, hydrogen generation reaction experiments were performed at 1273 K for 24 h under flowing (300 mL/min) Ar gas containing 10,000 ppm of H2 O by thermal gravimetry (TG, TG-8110, Rigaku, Japan) and gas chromatography (GC, CP-4900, Agilent, USA). A schematic flow diagram of the test apparatus used for the evaluation of the reactivity is shown in Fig. 2. The TG experiments were conducted at heating and cooling rates of 10 and 30 K/min, respectively. The Ar gas containing 10,000 ppm of H2 O was flowed at 1273 K and generated by water vapor generation equipment (HUM-1E, Rigaku, Japan). 3. Hydrogen generation reactivity experiments 3.1. Hydrogen generation measurements using as-granulated Be12 V pebbles The hydrogen generation reactivity of as-granulated Be12 V pebbles with water vapor at 1273 K was evaluated as previously studies
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
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Fig. 2. Schematic flow diagram of the test apparatus used for reactivity evaluation.
Fig. 4. Hydrogen generation cycle tests normalized by the weight of the asgranulated Be12 V pebbles at 1273 K under an Ar-1%H2 O stream.
Fig. 3. Hydrogen generated normalized by the weight of pebbles of the asgranulated Be12 V compared with pure Be pebbles at 1273 K under an Ar-1%H2 O stream [6].
with pure Be pebbles [6]. Fig. 3 show the amount of hydrogen generated from the reaction of the as-granulated Be12 V pebbles with water vapor. For comparison, the results are compared with those of pure Be pebbles. In Fig. 3, the amount of hydrogen generated was normalized by the weight of the pebble. The as-granulated Be12 V pebbles demonstrated low hydrogen generation reactivity (Fig. 3). Thus, the hydrogen generation of the as-granulated Be12 V pebbles was two orders of magnitude less than that of pure Be pebbles [6]. Be pebbles swelled upon oxidation and exhibited numerous cracks on the surface of the oxide layer. In contrast, BeO was deposited over the entire surface of the as-granulated Be12 V pebbles, with no swelling or cracking signs. During oxidation of Be, compressive stress induced cracks within the BeO scale due to the difference of the lattice coherence between substrate and scale. On the other hand, during oxidation of beryllide, BeO was formed with fewer stress on the surface of these pebbles because the atomic distances between Be atoms within the BeO scale are similar to the mean atomic distance between Be atoms within the beryllide substrate [7].
Fig. 5. Cross-sectional view of a pebble after the third test oxidation test with water vapor.
These reactivity results clearly indicated that the BeO layer on the surface of as-granulated Be12 V pebbles acted as a protective barrier preventing the H2 generation reaction. For evaluating the protection effect of the BeO surface layer on the as-granulated Be12 V pebbles via water vapor, hydrogen generation experiments were repeated three times.
3.2. Hydrogen generation measurement cycle tests using as-granulated Be12 V pebbles Hydrogen generation measurements were repeated 3 times in order to verify the effects of the BeO surface layer on the as-granulated Be12 V pebbles. The experimental history and the amount of hydrogen generated by the as-granulated Be12 V pebbles with water vapor are shown in Fig. 4.
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
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Fig. 8. Hydrogen generation cycle tests normalized by the weight of the surfacemodified Be12 V pebbles at 1273 K under an Ar-1%H2 O stream.
Fig. 7. Experimental history and cross-sectional photographs of the surfacemodified Be12 V pebbles before and after the first reactivity test.
When compared with the initial value, the hydrogen generation rate decreased by 90 and 70% after the second and third tests, respectively. A cross-sectional view of the pebble after the third test oxidation test with water vapor is shown in Fig. 5. Some defect areas in the BeO surface layer were formed upon the water vapor oxidation process. Since water vapor caused an intense oxidation reaction, the BeO surface layer of beryllides showed some defect areas. Therefore, in order to cover the pebbles with an oxide layer without defects, surface oxidation tests were performed under an oxygen atmosphere in order to promote surface modification of the BeO layer. 4. Results and discussion 4.1. Surface modification of the as-granulated Be12 V pebbles by oxidation treatment Surface oxidation on the as-granulated Be12 V pebbles was carried out to determine the effect of protection barrier. However, as mentioned before, the BeO layer on the surface of Be12 V pebbles
oxidized by water vapor showed some defects. Therefore, as a part of the surface modification examination via deposition of a BeO protective layer, the as-granulated Be12 V pebbles were oxidized under moderate conditions (i.e., oxygen atmosphere). The as-granulated Be12 V pebbles were annealed at 1273 K for 10 h under He containing varying concentrations (i.e., 100, 1000, and 10,000 ppm) of oxygen. Some photographs of Be12 V pebbles after this annealing oxidation treatment are shown in Fig. 6. The oxidation treatment results performed for surface modification revealed the presence of a yellow V2 O5 layer on the surface of Be12 V pebbles oxidized with 10,000 ppm of O2 . On the other hand, no signs of the V2 O5 layer were observed for surface-modified Be12 V pebbles oxidized with 100 ppm of O2 . These latter pebbles were subsequently used for hydrogen generation measurements. 4.2. Hydrogen generation cycle tests using surface-modified Be12 V pebbles via an oxidation treatment With the aim to verify the effect of the BeO protective layer on the surface of Be12 V pebbles, surface-modified Be12 V pebbles by oxidation with 100 ppm of O2 were used for repeated hydrogen generation measurements (i.e., three times). The experimental history and cross-sectional photographs of these surface-modified Be12 V pebbles before and after the reactivity test are shown in Fig. 7, while the results of the hydrogen generation cycle tests are shown in Fig. 8.
Fig. 6. Pictures of Be12 V pebbles after the annealing oxidation treatment at 1273 K for 10 h under He containing: (a) 100, (b) 1000, and (c) 10,000 ppm of oxygen.
Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033
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The hydrogen generation cycle tests of the surface-modified Be12 V pebbles revealed a significant reduction in the hydrogen generation rate, which nearly decreased to the background value. Thus, surface-modified Be12 V pebbles with no-hydrogen generation were successfully fabricated. 5. Conclusions Prototypic Be12 V pebbles were successfully fabricated by a direct rotating electrode granulation method using a plasmasintered beryllide electrode without homogenization. Be12 V pebbles were significantly more resistant to water vapor than pure Be. The surface BeO layer on Be12 V pebbles acted as a protective barrier preventing hydrogen generation. The hydrogen generation rate was nearly reduced to the background value via surface modification of Be12 V pebbles. These surface-modified pebbles showing no-hydrogen generation reaction were successfully fabricated by oxidation with 100 ppm of O2 . The stability and durability of the surface oxidation layer over beryllide pebbles shall be evaluated.
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Please cite this article in press as: M. Nakamichi, J.-H. Kim, Prevention of hydrogen generation reaction with water vapor by surface modification of beryllides as advanced neutron multipliers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.033