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Mechanical compression tests of beryllium pebbles after neutron irradiation up to 3000 appm helium production
Fusion Engineering and Design 93 (2015) 36–42
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Mechanical compression tests of beryllium pebbles after neutron irradiation up to 3000 appm helium production V. Chakin a,∗ , R. Rolli a , A. Moeslang a , M. Zmitko b a
Karlsruhe Institute of Technology, Institite for Applied Materials, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany The European Joint Undertaking for ITER and the Development of Fusion Energy, c/Josep Pla, no. 2, Torres Diagonal Litoral, Edificio B3, 08019 Barcelona, Spain b
h i g h l i g h t s • Compression tests of highly neutron irradiated beryllium pebbles have been performed. • Irradiation hardening of beryllium pebbles decreases the steady-state strain-rates. • The steady-state strain-rates of irradiated beryllium pebbles exceed their swelling rates.
a r t i c l e
i n f o
Article history: Received 25 April 2014 Received in revised form 12 January 2015 Accepted 11 February 2015 Available online 25 February 2015 Keywords: Beryllium pebble Compression test Neutron irradiation
a b s t r a c t Results: of mechanical compression tests of irradiated and non-irradiated beryllium pebbles with diameters of 1 and 2 mm are presented. The neutron irradiation was performed in the HFR in Petten, The Netherlands at 686–968 K up to 1890–2950 appm helium production. The irradiation at 686 and 753 K cause irradiation hardening due to the gas bubble formation in beryllium. The irradiation-induced hardening leads to decrease of steady-state strain-rates of irradiated beryllium pebbles compared to non-irradiated ones. In contrary, after irradiation at higher temperatures of 861 and 968 K, the steadystate strain-rates of the pebbles increase because annealing of irradiation defects and softening of the material take place. It was shown that the steady-state strain-rates of irradiated beryllium pebbles always exceed their swelling rates. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Beryllium pebbles with a diameter of 1 mm are planned to be used as neutron multiplier in the helium cooled pebble bed (HCPB) tritium breeding blanket of DEMO [1]. A key issue of DEMO relevant HCPB blankets is mechanical integrity of the beryllium pebbles under high dose neutron irradiation. The neutron irradiation leads to formation of high amounts of helium and tritium in beryllium that causes swelling, i.e. an increase of the pebble volume. The beryllium swelling at temperatures relevant to the blanket conditions (673–923 K) can reach 10–15% and even higher values depending on damage dose [2]. The irradiation-induced swelling of beryllium also in combination with different thermal expansions of the beryllium pebble bed and the structural material (Eurofer steel) can cause high thermal stresses in the pebble bed. In principle,
∗ Corresponding author. Tel.: +49 721 608 23639; fax: +49 721 608 24567. E-mail addresses: [email protected], [email protected] (V. Chakin). http://dx.doi.org/10.1016/j.fusengdes.2015.02.019 0920-3796/© 2015 Elsevier B.V. All rights reserved.
thermal creep of the pebbles should reduce the stresses. But, it is known, that under neutron irradiation degradation of mechanical properties of beryllium occurs which is expressed in hardening and embrittlement [3]. Therefore, only direct mechanical compression tests of beryllium pebbles irradiated at the DEMO blanket relevant conditions can provide the necessary data needed to evaluate a potential compensation of the swelling by the creep deformation. This paper presents results of compression tests of beryllium pebbles with diameters of 1 and 2 mm irradiated in the High Flux Reactor (HFR), Petten, The Netherlands within the HIDOBE-01 experiment [4]. 2. Experimental The beryllium pebbles with diameters of 1 and 2 mm were fabricated by NGK, Japan using the rotating electrode method (REM) [5]. The HIDOBE-01 irradiation campaign was in the HFR on 2005–2007. The compression tests included mechanical loading of a single pebble at a constant loading value during 80 h. The parameters
V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
37
Table 1 Irradiation and mechanical test parameters of beryllium pebbles from HIDOBE-01. Pebble diameter (mm)
Damage dose (dpa)
4
Tirr (K)
Ttest (K)
Loading (N)
1
11.3
1890
686
698
13.9
2300
753
798
16.3
2680
861
923
18.1
2950
968
1023
11.3
1890
686
698
13.9
2300
753
798
16.3
2680
861
923
18.1
2950
968
1023
60 100 150 48 70 90 40 55 70 10 15 20 240 400 600 210 280 360 80 100 160 20 30 40
2
He accumulation (appm)
of the irradiation and mechanical tests are presented in Table 1. The maximum helium accumulation in beryllium was 2950 appm corresponding to a displacement damage dose of 18.1 dpa. The irradiation temperatures of beryllium pebbles were 686, 753, 861, and 968 K. The compression tests were performed at 698, 798, 923, and 1023 K with three loading values to each testing temperature. It was foreseen to adjust the testing temperatures to the corresponding irradiation temperatures. Fig. 1 shows the scheme of loading of a beryllium pebble in the mechanical testing machine. Before start of the compression test, the pebble (1) is placed on a supporting bottom (2). Then, the pebble is heated up to testing temperature. After reaching the testing temperature, the loading of the pebble is performed by a loading piston (3). The mechanical tests are carried out in a glove box filled by pure nitrogen. The mechanical testing machine is able to test pebbles up to temperatures of 1273 K with loading values up to 1000 N. The pebble deformation is measured by a strain gauge transducer with accuracy of ±1 × 10−6 m. At this study, the testing time was limited to 80 h, hence, the loading values for the tests were selected to obtain clearly visible steady-state strain-rates inside this time. Optical observations of the beryllium pebbles were performed before and after compression tests. The pebbles after the tests
Fig. 1. The scheme of loading of a beryllium pebble during the compression test in the mechanical testing machine: (1) beryllium pebble; (2) supporting bottom; (3) loading moveable piston.
become like barrels. The base diameters were measured to calculate stresses applied to the pebbles under loading. Cross sections of 2 mm non-irradiated beryllium pebbles after mechanical tests were investigated using optical microscope (OM) Olympus GX51. 3. Results 3.1. Mechanical compression tests As an example, Fig. 2 shows results of a typical deformation measurement of 1 mm beryllium pebbles tested at 1023 K under loading of 15 N before and after irradiation at 968 K. Fig. 2a reveals the deformation curve for testing time of 81 h. It consists of two stages. The first stage is completed after 30 h of the test duration and reflects the primary deformation stage. At this stage, the pebble is deformed by a high strain-rate. The deformation curve for first 3 h of this test (Fig. 2b) shows that immediately after loading (during time up to 1 h) the strong increase in strain of the pebble occurs. The pebble is quickly deformed by the formation of two opposed parallel contact zones resulted from the applied loads (Fig. 1). Figs. 3 and 4 represent this process in details. In particular, an irradiated beryllium pebble has a regular round form before applying of loading (Fig. 3a). Under compression test, two indentations with a diameter of d on opposite sides of the pebble are formed (see Fig. 3b and c). The relation between the pebble diameter D, the indentation diameter d and the distance h between two opposite parallel indentations on the pebble can be easily expressed by the Pythagorean theorem (Fig. 4). During the primary deformation stage, redistribution of internal stresses and a microstructure evolution (formation of sub-grains [6]) occur in the pebble. Finally, a balance of applied loading to the strength response of the pebble material is achieved to the end of the primary deformation stage. The second deformation stage names the steady-state strain at which the strain-rate has a constant value. In this study, as a rule, the steady-state strain-rate was measured on deformation curves starting from 30–40 h when the slope of the curve became constant. Fig. 5 shows the steady-state strain-rate versus stress for both non-irradiated and irradiated beryllium pebbles. The neutron
38
V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
Total deformation, x10-6 m
120
a
100 80 60 40
non-irr. irr.
20 0 0
10
20
30
40
50
60
70
80
Time, h Fig. 4. The scheme of a finale shape of a beryllium pebble after the compression test.
60
b
Total deformation, x10-6 m
50
40
30
20
non-irr. irr.
10
applied stresses increase because the radiation hardening of the beryllium pebbles takes place reducing the indentation diameters and, accordingly, increasing the stresses. For two highest irradiation temperatures, the steady-state strain-rates of irradiated 1 mm beryllium pebbles are close to non-irradiated ones at the same loadings. The 2 mm beryllium pebbles (Fig. 5b) manifest the similar behavior to the 1 mm pebbles for all testing temperatures excluding the temperature of 923 K where the strain-rate decreases despite of increase the stresses (as was at lower irradiation temperatures). 3.2. Views and microstructure of tested beryllium pebbles
0 0
1
2
3
Time, h Fig. 2. Total deformation versus testing time for beryllium pebbles with a diameter of 1 mm tested at 1023 K under loading of 15 N of both non-irradiated and irradiated at 968 K states for total time of 81 h (a) and for first 3 h (b).
irradiation results changes in the strain-rates depending on irradiation temperature. In particular, for 1 mm pebbles (Fig. 5a) irradiated at two lowest temperatures of 686 and 753 K the steadystate strain-rate decreases despite of increase the stresses. The
Figs. 6–9 show general views and cross sections of nonirradiated beryllium pebbles with a diameter of 2 mm after compression tests. The mechanical tests at the two lowest testing temperatures are sometimes accompanied by formation of cracks which start from the border of the indentation and are oriented mainly in parallel to the loading axis (Figs. 6a and b and 7a and b). It can be assumed that the use of lower loadings in the mechanical tests would be more preferable to avoid the crack formation. But the time limitation accepted for these tests from practical point of view (∼80 h) does not allow to manifest the steady-state strain-rate stage at lower loadings. Hence, it needs to take into account that the deformation at two lowest testing
Fig. 3. Views of beryllium pebbles with a diameter of 1 mm irradiated at 968 K before loading (a) and after loading at 1023 K by 15 N (b, c) ((b) a view to the indentation, (c) after rotation on 90◦ ).
V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
10-5
Testing temperature: 698 K, non-irr. 698 K, irr. 798 K, non-irr. 798 K, irr. 923 K, non-irr. 923 K, irr. 1023 K, non-irr. 1023 K, irr.
a Steady-state strain-rate, s-1
39
10-6
10-7
10-8
10-9 102
Steady-state strain-rate, s-1
10
-7
Stress, MPa
103
Testing temperature: 698 K, non-irr. 698 K, irr. 798 K, non-irr. 798 K, irr. 923 K, non-irr. 923 K, irr. 1023 K, non-irr. 1023 k, irr
b
10-8
10-9 102
103
Stress, MPa Fig. 5. The steady-state strain-rate versus stress for beryllium pebbles after compression tests: (a) Ø 1 mm pebbles; (b) Ø 2 mm pebbles.
temperatures includes pseudo-deformation due to expansion of the cracks as well. Cross sections of 2 mm pebbles (Figs. 6c and 7c) after mechanical tests were prepared in orientation when the loading directions are placed in the plane of the image. The feature in the pebble microstructure is formation of numerous twinning planes. Formation of smaller grains and, probably, sub-grains takes place in regions close to the surfaces of loading. The mechanical tests at two highest testing temperatures did not lead to the crack formation on external pebble surface even at maximum loadings (Figs. 8a and b and 9a and b). Small cracks are visible in the pebble microstructure on cross sections only (Figs. 8c and 9c). Disordered orientations of the cracks and a specifically elongated shape of the grains show that, likely, this microstructure was already formed during production of the pebbles. Probably, the deformation under loading of the pebbles covers only regions close to the surfaces of the loading. In this case, the cracks on external surfaces are not formed because the high plastic properties of the beryllium pebbles provide the stress relaxation. The cross sections of beryllium pebbles after irradiation were technically difficult to prepare therefore only general views of the pebbles were analyzed. After compression tests of irradiated pebbles the crack formation took place mainly at two lowest testing
Fig. 6. Views of a non-irradiated beryllium pebble with a diameter of 2 mm after compression test at 698 K by loading of 400 N: (a) a view to the indentation; (b) a view after rotation on 90◦ ; (c) cross section.
temperatures. However, in separate cases when the loading was too high the intensive crack formation and propagation was detected at two highest testing temperatures as well (Fig. 10). Actually, it was the complete destruction of the pebble under excessively high loading. 4. Discussion The closest approach to real blanket conditions would be the testing of sufficiently large pebble beds as it was done for nonirradiated beryllium pebbles e.g. in the HECOP II facility [7]. But this facility has a large cylindrical bed with a diameter of 130 mm
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V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
Fig. 7. Views of a non-irradiated beryllium pebble with a diameter of 2 mm after compression test at 798 K by loading of 360 N: (a) a view to the indentation; (b) a view after rotation on 90◦ ; (c) cross section.
and a height of 55 mm that means more than one hundred gram of beryllium pebbles for one mechanical compression test are needed. A capsule for irradiation of any specimens in a nuclear reactor, in particular, of beryllium pebbles in the HIDOBE-01 in the HFR always has a limited irradiating volume which, in the case of HIDOBE-01, allows to irradiate in one capsule not more than a few grams of beryllium specimens. Therefore, it is necessary to find other ways to obtain data on mechanical properties of irradiated beryllium
Fig. 8. Views of a non-irradiated beryllium pebble with a diameter of 2 mm after compression test at 923 K by loading 160 N: (a) a view to the indentation; (b) a view after rotation on 90◦ ; (c) cross section.
pebbles excluding the full-scale pebble bed compression tests. An alternative is to use the single pebble mechanical compression tests which were made in this study and to try to apply the obtained results to pebble bed conditions. The extrapolation of the single beryllium pebble mechanical test data to a pebble bed is in progress now.
V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
Fig. 9. Views of a non-irradiated beryllium pebble with a diameter of 2 mm after compression test at 1023 K by loading of 40 N: (a) a view to the indentation; (b) a view after rotation on 90◦ ; (c) cross section.
41
The main features of the beryllium pebble microstructure after irradiation in the HIDOBE-01 are gas bubbles and voids in the form of discs of hexagonal shape with diameters from 7.5 to 80 nm and heights from 3 to 18 nm, accordingly [8]. With increase of irradiation temperature, these bubbles increase the sizes and decrease the bulk density. Likely, interaction of moving dislocations with these radiation-induced bubbles defines the main features of plastic deformation and thermal creep of irradiated beryllium pebbles. In particular, the bubbles play a role of barriers hindering to glide of dislocations. During thermal creep the dislocations overcome the microstructure barriers by climb which becomes possible for beryllium at temperatures T ≥ 0.5 Tm ≈ 775 K [6]. Regarding to this study, at two highest testing temperatures (923 and 1023 K) dislocation climb in beryllium is already to be possible. Therefore, the strain-rates of irradiated beryllium pebbles have values close to non-irradiated ones because the dislocations already can overcame the bubbles by means of climb mechanism as well as the bubbles themselves having larger sizes at these higher temperatures cannot be as effective barriers for moving dislocations. At lower testing temperatures the irradiation hardening takes place that expresses in lower steady-state strain-rates comparing to non-irradiated pebbles [9]. In the beginning of the mechanical compression tests (during first 1 h of a test) the beryllium pebble is deformed by plastic deformation. A pseudo-deformation due to the crack formation and propagation has a contribution to the total deformation as well. However, the crack propagation can give a significant contribution to the total deformation mainly at two lowest testing temperatures (698 and 798 K) where the irradiation hardening takes place. The primary creep stage continues until 30 h. The last part of the tests (up to ∼80 h) is the secondary steady-state creep stage on which the creep rates were measured (see Fig. 5). So, at two highest testing temperatures and at not excessive loading the deformation of the pebbles after 30 h of the tests occurs by means of creep mechanism. It is really the secondary steady-state creep stage. The swelling of 1 and 2 mm beryllium pebbles irradiated in the HIDOBE-01 reaches approximately 2% for the first three irradiation temperatures and 9% for the highest temperature [10]. Taking into account the duration of the HIDOBE-01 experiment (649 days) the linear swelling rates (the same units as the strainrates) can be estimated. For the first three irradiation temperatures the swelling rate is 1.3 × 10−10 s−1 , for the highest temperature is 5.4 × 10−10 s−1 . To compensate the volume increase of the beryllium pebbles in a pebble bed at least the same or higher deformation rates on the pebbles needs to be provided. The steady-state strainrates of irradiated beryllium pebbles with diameters of 1 and 2 mm independent of the irradiation temperatures are in the interval of 2 × 10−6 /2 × 10−9 s−1 (see Fig. 5), that is the measured strain-rates have higher values compared to swelling rates. This means the volume changes of the beryllium pebbles (swelling) can be compensated by deformation of the pebbles due to both the plastic deformation and thermal creep. Moreover, in our opinion, the crack formation and propagation do not give the significant contribution in the deformation on the steady-state strain-rate stage because the cracks were already stopped on the first primary deformation stage. The obtained results giving the information about mechanical behavior of beryllium pebbles irradiated up to the relevant neutron doses are very important for the design of the helium cooled pebble bed (HCPB) tritium breeding blanket of DEMO. It was established for the first time that the swelling volume changes of beryllium pebbles in the pebble bed under operation can certainly be compensated due to the creep deformation of the pebbles at least up to damage dose of 18 dpa and helium production of 2950 appm for irradiation temperatures higher than 798 K.
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V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
Fig. 10. Views of irradiated beryllium pebbles with a diameter of 1 mm after compression tests: Tirr = 753 K, Ttest = 798 K, P = 90 N, a view to an indentation; the same parameters as in (a), a view after rotation on 90◦ ; Tirr = 861 K, Test = 923 K, P = 70 N, a view to an indentation; the same parameters as in (c), a view after rotation on 90◦ .
5. Conclusions
References
Mechanical compression tests of beryllium pebbles with diameters of 1 and 2 mm after neutron irradiation at temperatures of 686–968 K up to 1890–2950 appm helium production have been performed. Decrease of the steady-state strain-rates for both 1 and 2 mm beryllium pebbles irradiated at 686 and 753 K and tested at 698 and 798 K, accordingly, occurs as a result of hardening by irradiation-induced formation of gas bubbles. At higher irradiation temperatures of 861 and 968 K (at testing temperatures of 923 and 1023 K, accordingly), the steady-state strain-rates of the pebbles are approximately the same to that as non-irradiated. After irradiation at temperatures of 686–968 K and at the corresponding testing temperatures of 698–1023 K, the steady-state strain-rates of the irradiated beryllium pebbles always exceed their swelling rates.
[1] Y. Poitevin, L.V. Boccaccini, M. Zmitko, K. Schleisiek, I. Ricapito, J.-F. Salavy, E. Diegele, F. Magnani, H. Neuberger, R. Lässer, L. Guerrini, Tritium breeder blankets design and technologies in Europe: development status of ITER test blanket modules, test & qualification strategy and roadmap towards DEMO, Fusion Eng. Des. 85 (2010) 2340–2347. [2] D.S. Gelles, M. Dalle Donne, G.A. Sernyaev, H. Kawamura, Radiation effects in beryllium used for plasma protection, J. Nucl. Mater. 212–215 (1994) 29–38. [3] I.B. Kupriyanov, V.A. Gorokhov, R.R. Melder, Z.E. Ostrovsky, A.A. Gervash, Investigation of ITER candidate beryllium grades irradiated at high temperature, J. Nucl. Mater. 258–263 (1998) 808–813. [4] J.B.J. Hegeman, J.G. van der Laan, H. Kawamura, A. Möslang, I. Kupriyanov, M. Uchida, K. Hayashi, The HFR Petten high dose irradiation programme of beryllium for blanket application, Fusion Eng. Des. 75–79 (2005) 769– 773. [5] E. Ishitsuka, H. Kawamura, N. Sakamoto, K. Nishida, Process for preparing metallic beryllium pebbles, US Patent No. 5958105, 1999. [6] M.E. Kassner, Fundamentals of Creep in Metals and Alloys, Elsevier Ltd., 2009. [7] J. Reimann, H. Harsch, Thermal creep of beryllium pebble beds, Fusion Eng. Des. 75–79 (2005) 1043–1047. [8] M. Klimenkov, V. Chakin, A. Moeslang, R. Rolli, TEM study of beryllium pebbles after neutron irradiation up to 3000 appm helium production, J. Nucl. Mater. 443 (2013) 409–416. [9] M. Scibetta, A. Pellettieri, L. Sannen, Experimental determination of creep properties of beryllium irradiated to relevant fusion power reactor doses, J. Nucl. Mater. 367–370 (2007) 1063–1068. [10] S. van Til, J.B.J. Hegeman, H.L. Cobussen, M.P. Stijkel, Evolution of beryllium pebbles (HIDOBE) in long term, high flux irradiation in the high flux reactor, Fusion Eng. Des. 86 (2011) 2258–2261.
Acknowledgements This work was supported by Fusion for Energy under the grant contract No. F4E-2009-GRT-030-03. The views and opinions expressed herein reflect only the author’s views. Fusion for Energy is not liable for any use that may be made of the information contained therein.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Mechanical compression tests of beryllium pebbles after neutron irradiation up to 3000 appm helium production V. Chakin a,∗ , R. Rolli a , A. Moeslang a , M. Zmitko b a
Karlsruhe Institute of Technology, Institite for Applied Materials, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany The European Joint Undertaking for ITER and the Development of Fusion Energy, c/Josep Pla, no. 2, Torres Diagonal Litoral, Edificio B3, 08019 Barcelona, Spain b
h i g h l i g h t s • Compression tests of highly neutron irradiated beryllium pebbles have been performed. • Irradiation hardening of beryllium pebbles decreases the steady-state strain-rates. • The steady-state strain-rates of irradiated beryllium pebbles exceed their swelling rates.
a r t i c l e
i n f o
Article history: Received 25 April 2014 Received in revised form 12 January 2015 Accepted 11 February 2015 Available online 25 February 2015 Keywords: Beryllium pebble Compression test Neutron irradiation
a b s t r a c t Results: of mechanical compression tests of irradiated and non-irradiated beryllium pebbles with diameters of 1 and 2 mm are presented. The neutron irradiation was performed in the HFR in Petten, The Netherlands at 686–968 K up to 1890–2950 appm helium production. The irradiation at 686 and 753 K cause irradiation hardening due to the gas bubble formation in beryllium. The irradiation-induced hardening leads to decrease of steady-state strain-rates of irradiated beryllium pebbles compared to non-irradiated ones. In contrary, after irradiation at higher temperatures of 861 and 968 K, the steadystate strain-rates of the pebbles increase because annealing of irradiation defects and softening of the material take place. It was shown that the steady-state strain-rates of irradiated beryllium pebbles always exceed their swelling rates. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Beryllium pebbles with a diameter of 1 mm are planned to be used as neutron multiplier in the helium cooled pebble bed (HCPB) tritium breeding blanket of DEMO [1]. A key issue of DEMO relevant HCPB blankets is mechanical integrity of the beryllium pebbles under high dose neutron irradiation. The neutron irradiation leads to formation of high amounts of helium and tritium in beryllium that causes swelling, i.e. an increase of the pebble volume. The beryllium swelling at temperatures relevant to the blanket conditions (673–923 K) can reach 10–15% and even higher values depending on damage dose [2]. The irradiation-induced swelling of beryllium also in combination with different thermal expansions of the beryllium pebble bed and the structural material (Eurofer steel) can cause high thermal stresses in the pebble bed. In principle,
∗ Corresponding author. Tel.: +49 721 608 23639; fax: +49 721 608 24567. E-mail addresses: [email protected], [email protected] (V. Chakin). http://dx.doi.org/10.1016/j.fusengdes.2015.02.019 0920-3796/© 2015 Elsevier B.V. All rights reserved.
thermal creep of the pebbles should reduce the stresses. But, it is known, that under neutron irradiation degradation of mechanical properties of beryllium occurs which is expressed in hardening and embrittlement [3]. Therefore, only direct mechanical compression tests of beryllium pebbles irradiated at the DEMO blanket relevant conditions can provide the necessary data needed to evaluate a potential compensation of the swelling by the creep deformation. This paper presents results of compression tests of beryllium pebbles with diameters of 1 and 2 mm irradiated in the High Flux Reactor (HFR), Petten, The Netherlands within the HIDOBE-01 experiment [4]. 2. Experimental The beryllium pebbles with diameters of 1 and 2 mm were fabricated by NGK, Japan using the rotating electrode method (REM) [5]. The HIDOBE-01 irradiation campaign was in the HFR on 2005–2007. The compression tests included mechanical loading of a single pebble at a constant loading value during 80 h. The parameters
V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
37
Table 1 Irradiation and mechanical test parameters of beryllium pebbles from HIDOBE-01. Pebble diameter (mm)
Damage dose (dpa)
4
Tirr (K)
Ttest (K)
Loading (N)
1
11.3
1890
686
698
13.9
2300
753
798
16.3
2680
861
923
18.1
2950
968
1023
11.3
1890
686
698
13.9
2300
753
798
16.3
2680
861
923
18.1
2950
968
1023
60 100 150 48 70 90 40 55 70 10 15 20 240 400 600 210 280 360 80 100 160 20 30 40
2
He accumulation (appm)
of the irradiation and mechanical tests are presented in Table 1. The maximum helium accumulation in beryllium was 2950 appm corresponding to a displacement damage dose of 18.1 dpa. The irradiation temperatures of beryllium pebbles were 686, 753, 861, and 968 K. The compression tests were performed at 698, 798, 923, and 1023 K with three loading values to each testing temperature. It was foreseen to adjust the testing temperatures to the corresponding irradiation temperatures. Fig. 1 shows the scheme of loading of a beryllium pebble in the mechanical testing machine. Before start of the compression test, the pebble (1) is placed on a supporting bottom (2). Then, the pebble is heated up to testing temperature. After reaching the testing temperature, the loading of the pebble is performed by a loading piston (3). The mechanical tests are carried out in a glove box filled by pure nitrogen. The mechanical testing machine is able to test pebbles up to temperatures of 1273 K with loading values up to 1000 N. The pebble deformation is measured by a strain gauge transducer with accuracy of ±1 × 10−6 m. At this study, the testing time was limited to 80 h, hence, the loading values for the tests were selected to obtain clearly visible steady-state strain-rates inside this time. Optical observations of the beryllium pebbles were performed before and after compression tests. The pebbles after the tests
Fig. 1. The scheme of loading of a beryllium pebble during the compression test in the mechanical testing machine: (1) beryllium pebble; (2) supporting bottom; (3) loading moveable piston.
become like barrels. The base diameters were measured to calculate stresses applied to the pebbles under loading. Cross sections of 2 mm non-irradiated beryllium pebbles after mechanical tests were investigated using optical microscope (OM) Olympus GX51. 3. Results 3.1. Mechanical compression tests As an example, Fig. 2 shows results of a typical deformation measurement of 1 mm beryllium pebbles tested at 1023 K under loading of 15 N before and after irradiation at 968 K. Fig. 2a reveals the deformation curve for testing time of 81 h. It consists of two stages. The first stage is completed after 30 h of the test duration and reflects the primary deformation stage. At this stage, the pebble is deformed by a high strain-rate. The deformation curve for first 3 h of this test (Fig. 2b) shows that immediately after loading (during time up to 1 h) the strong increase in strain of the pebble occurs. The pebble is quickly deformed by the formation of two opposed parallel contact zones resulted from the applied loads (Fig. 1). Figs. 3 and 4 represent this process in details. In particular, an irradiated beryllium pebble has a regular round form before applying of loading (Fig. 3a). Under compression test, two indentations with a diameter of d on opposite sides of the pebble are formed (see Fig. 3b and c). The relation between the pebble diameter D, the indentation diameter d and the distance h between two opposite parallel indentations on the pebble can be easily expressed by the Pythagorean theorem (Fig. 4). During the primary deformation stage, redistribution of internal stresses and a microstructure evolution (formation of sub-grains [6]) occur in the pebble. Finally, a balance of applied loading to the strength response of the pebble material is achieved to the end of the primary deformation stage. The second deformation stage names the steady-state strain at which the strain-rate has a constant value. In this study, as a rule, the steady-state strain-rate was measured on deformation curves starting from 30–40 h when the slope of the curve became constant. Fig. 5 shows the steady-state strain-rate versus stress for both non-irradiated and irradiated beryllium pebbles. The neutron
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Total deformation, x10-6 m
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applied stresses increase because the radiation hardening of the beryllium pebbles takes place reducing the indentation diameters and, accordingly, increasing the stresses. For two highest irradiation temperatures, the steady-state strain-rates of irradiated 1 mm beryllium pebbles are close to non-irradiated ones at the same loadings. The 2 mm beryllium pebbles (Fig. 5b) manifest the similar behavior to the 1 mm pebbles for all testing temperatures excluding the temperature of 923 K where the strain-rate decreases despite of increase the stresses (as was at lower irradiation temperatures). 3.2. Views and microstructure of tested beryllium pebbles
0 0
1
2
3
Time, h Fig. 2. Total deformation versus testing time for beryllium pebbles with a diameter of 1 mm tested at 1023 K under loading of 15 N of both non-irradiated and irradiated at 968 K states for total time of 81 h (a) and for first 3 h (b).
irradiation results changes in the strain-rates depending on irradiation temperature. In particular, for 1 mm pebbles (Fig. 5a) irradiated at two lowest temperatures of 686 and 753 K the steadystate strain-rate decreases despite of increase the stresses. The
Figs. 6–9 show general views and cross sections of nonirradiated beryllium pebbles with a diameter of 2 mm after compression tests. The mechanical tests at the two lowest testing temperatures are sometimes accompanied by formation of cracks which start from the border of the indentation and are oriented mainly in parallel to the loading axis (Figs. 6a and b and 7a and b). It can be assumed that the use of lower loadings in the mechanical tests would be more preferable to avoid the crack formation. But the time limitation accepted for these tests from practical point of view (∼80 h) does not allow to manifest the steady-state strain-rate stage at lower loadings. Hence, it needs to take into account that the deformation at two lowest testing
Fig. 3. Views of beryllium pebbles with a diameter of 1 mm irradiated at 968 K before loading (a) and after loading at 1023 K by 15 N (b, c) ((b) a view to the indentation, (c) after rotation on 90◦ ).
V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
10-5
Testing temperature: 698 K, non-irr. 698 K, irr. 798 K, non-irr. 798 K, irr. 923 K, non-irr. 923 K, irr. 1023 K, non-irr. 1023 K, irr.
a Steady-state strain-rate, s-1
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b
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Stress, MPa Fig. 5. The steady-state strain-rate versus stress for beryllium pebbles after compression tests: (a) Ø 1 mm pebbles; (b) Ø 2 mm pebbles.
temperatures includes pseudo-deformation due to expansion of the cracks as well. Cross sections of 2 mm pebbles (Figs. 6c and 7c) after mechanical tests were prepared in orientation when the loading directions are placed in the plane of the image. The feature in the pebble microstructure is formation of numerous twinning planes. Formation of smaller grains and, probably, sub-grains takes place in regions close to the surfaces of loading. The mechanical tests at two highest testing temperatures did not lead to the crack formation on external pebble surface even at maximum loadings (Figs. 8a and b and 9a and b). Small cracks are visible in the pebble microstructure on cross sections only (Figs. 8c and 9c). Disordered orientations of the cracks and a specifically elongated shape of the grains show that, likely, this microstructure was already formed during production of the pebbles. Probably, the deformation under loading of the pebbles covers only regions close to the surfaces of the loading. In this case, the cracks on external surfaces are not formed because the high plastic properties of the beryllium pebbles provide the stress relaxation. The cross sections of beryllium pebbles after irradiation were technically difficult to prepare therefore only general views of the pebbles were analyzed. After compression tests of irradiated pebbles the crack formation took place mainly at two lowest testing
Fig. 6. Views of a non-irradiated beryllium pebble with a diameter of 2 mm after compression test at 698 K by loading of 400 N: (a) a view to the indentation; (b) a view after rotation on 90◦ ; (c) cross section.
temperatures. However, in separate cases when the loading was too high the intensive crack formation and propagation was detected at two highest testing temperatures as well (Fig. 10). Actually, it was the complete destruction of the pebble under excessively high loading. 4. Discussion The closest approach to real blanket conditions would be the testing of sufficiently large pebble beds as it was done for nonirradiated beryllium pebbles e.g. in the HECOP II facility [7]. But this facility has a large cylindrical bed with a diameter of 130 mm
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Fig. 7. Views of a non-irradiated beryllium pebble with a diameter of 2 mm after compression test at 798 K by loading of 360 N: (a) a view to the indentation; (b) a view after rotation on 90◦ ; (c) cross section.
and a height of 55 mm that means more than one hundred gram of beryllium pebbles for one mechanical compression test are needed. A capsule for irradiation of any specimens in a nuclear reactor, in particular, of beryllium pebbles in the HIDOBE-01 in the HFR always has a limited irradiating volume which, in the case of HIDOBE-01, allows to irradiate in one capsule not more than a few grams of beryllium specimens. Therefore, it is necessary to find other ways to obtain data on mechanical properties of irradiated beryllium
Fig. 8. Views of a non-irradiated beryllium pebble with a diameter of 2 mm after compression test at 923 K by loading 160 N: (a) a view to the indentation; (b) a view after rotation on 90◦ ; (c) cross section.
pebbles excluding the full-scale pebble bed compression tests. An alternative is to use the single pebble mechanical compression tests which were made in this study and to try to apply the obtained results to pebble bed conditions. The extrapolation of the single beryllium pebble mechanical test data to a pebble bed is in progress now.
V. Chakin et al. / Fusion Engineering and Design 93 (2015) 36–42
Fig. 9. Views of a non-irradiated beryllium pebble with a diameter of 2 mm after compression test at 1023 K by loading of 40 N: (a) a view to the indentation; (b) a view after rotation on 90◦ ; (c) cross section.
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The main features of the beryllium pebble microstructure after irradiation in the HIDOBE-01 are gas bubbles and voids in the form of discs of hexagonal shape with diameters from 7.5 to 80 nm and heights from 3 to 18 nm, accordingly [8]. With increase of irradiation temperature, these bubbles increase the sizes and decrease the bulk density. Likely, interaction of moving dislocations with these radiation-induced bubbles defines the main features of plastic deformation and thermal creep of irradiated beryllium pebbles. In particular, the bubbles play a role of barriers hindering to glide of dislocations. During thermal creep the dislocations overcome the microstructure barriers by climb which becomes possible for beryllium at temperatures T ≥ 0.5 Tm ≈ 775 K [6]. Regarding to this study, at two highest testing temperatures (923 and 1023 K) dislocation climb in beryllium is already to be possible. Therefore, the strain-rates of irradiated beryllium pebbles have values close to non-irradiated ones because the dislocations already can overcame the bubbles by means of climb mechanism as well as the bubbles themselves having larger sizes at these higher temperatures cannot be as effective barriers for moving dislocations. At lower testing temperatures the irradiation hardening takes place that expresses in lower steady-state strain-rates comparing to non-irradiated pebbles [9]. In the beginning of the mechanical compression tests (during first 1 h of a test) the beryllium pebble is deformed by plastic deformation. A pseudo-deformation due to the crack formation and propagation has a contribution to the total deformation as well. However, the crack propagation can give a significant contribution to the total deformation mainly at two lowest testing temperatures (698 and 798 K) where the irradiation hardening takes place. The primary creep stage continues until 30 h. The last part of the tests (up to ∼80 h) is the secondary steady-state creep stage on which the creep rates were measured (see Fig. 5). So, at two highest testing temperatures and at not excessive loading the deformation of the pebbles after 30 h of the tests occurs by means of creep mechanism. It is really the secondary steady-state creep stage. The swelling of 1 and 2 mm beryllium pebbles irradiated in the HIDOBE-01 reaches approximately 2% for the first three irradiation temperatures and 9% for the highest temperature [10]. Taking into account the duration of the HIDOBE-01 experiment (649 days) the linear swelling rates (the same units as the strainrates) can be estimated. For the first three irradiation temperatures the swelling rate is 1.3 × 10−10 s−1 , for the highest temperature is 5.4 × 10−10 s−1 . To compensate the volume increase of the beryllium pebbles in a pebble bed at least the same or higher deformation rates on the pebbles needs to be provided. The steady-state strainrates of irradiated beryllium pebbles with diameters of 1 and 2 mm independent of the irradiation temperatures are in the interval of 2 × 10−6 /2 × 10−9 s−1 (see Fig. 5), that is the measured strain-rates have higher values compared to swelling rates. This means the volume changes of the beryllium pebbles (swelling) can be compensated by deformation of the pebbles due to both the plastic deformation and thermal creep. Moreover, in our opinion, the crack formation and propagation do not give the significant contribution in the deformation on the steady-state strain-rate stage because the cracks were already stopped on the first primary deformation stage. The obtained results giving the information about mechanical behavior of beryllium pebbles irradiated up to the relevant neutron doses are very important for the design of the helium cooled pebble bed (HCPB) tritium breeding blanket of DEMO. It was established for the first time that the swelling volume changes of beryllium pebbles in the pebble bed under operation can certainly be compensated due to the creep deformation of the pebbles at least up to damage dose of 18 dpa and helium production of 2950 appm for irradiation temperatures higher than 798 K.
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Fig. 10. Views of irradiated beryllium pebbles with a diameter of 1 mm after compression tests: Tirr = 753 K, Ttest = 798 K, P = 90 N, a view to an indentation; the same parameters as in (a), a view after rotation on 90◦ ; Tirr = 861 K, Test = 923 K, P = 70 N, a view to an indentation; the same parameters as in (c), a view after rotation on 90◦ .
5. Conclusions
References
Mechanical compression tests of beryllium pebbles with diameters of 1 and 2 mm after neutron irradiation at temperatures of 686–968 K up to 1890–2950 appm helium production have been performed. Decrease of the steady-state strain-rates for both 1 and 2 mm beryllium pebbles irradiated at 686 and 753 K and tested at 698 and 798 K, accordingly, occurs as a result of hardening by irradiation-induced formation of gas bubbles. At higher irradiation temperatures of 861 and 968 K (at testing temperatures of 923 and 1023 K, accordingly), the steady-state strain-rates of the pebbles are approximately the same to that as non-irradiated. After irradiation at temperatures of 686–968 K and at the corresponding testing temperatures of 698–1023 K, the steady-state strain-rates of the irradiated beryllium pebbles always exceed their swelling rates.
[1] Y. Poitevin, L.V. Boccaccini, M. Zmitko, K. Schleisiek, I. Ricapito, J.-F. Salavy, E. Diegele, F. Magnani, H. Neuberger, R. Lässer, L. Guerrini, Tritium breeder blankets design and technologies in Europe: development status of ITER test blanket modules, test & qualification strategy and roadmap towards DEMO, Fusion Eng. Des. 85 (2010) 2340–2347. [2] D.S. Gelles, M. Dalle Donne, G.A. Sernyaev, H. Kawamura, Radiation effects in beryllium used for plasma protection, J. Nucl. Mater. 212–215 (1994) 29–38. [3] I.B. Kupriyanov, V.A. Gorokhov, R.R. Melder, Z.E. Ostrovsky, A.A. Gervash, Investigation of ITER candidate beryllium grades irradiated at high temperature, J. Nucl. Mater. 258–263 (1998) 808–813. [4] J.B.J. Hegeman, J.G. van der Laan, H. Kawamura, A. Möslang, I. Kupriyanov, M. Uchida, K. Hayashi, The HFR Petten high dose irradiation programme of beryllium for blanket application, Fusion Eng. Des. 75–79 (2005) 769– 773. [5] E. Ishitsuka, H. Kawamura, N. Sakamoto, K. Nishida, Process for preparing metallic beryllium pebbles, US Patent No. 5958105, 1999. [6] M.E. Kassner, Fundamentals of Creep in Metals and Alloys, Elsevier Ltd., 2009. [7] J. Reimann, H. Harsch, Thermal creep of beryllium pebble beds, Fusion Eng. Des. 75–79 (2005) 1043–1047. [8] M. Klimenkov, V. Chakin, A. Moeslang, R. Rolli, TEM study of beryllium pebbles after neutron irradiation up to 3000 appm helium production, J. Nucl. Mater. 443 (2013) 409–416. [9] M. Scibetta, A. Pellettieri, L. Sannen, Experimental determination of creep properties of beryllium irradiated to relevant fusion power reactor doses, J. Nucl. Mater. 367–370 (2007) 1063–1068. [10] S. van Til, J.B.J. Hegeman, H.L. Cobussen, M.P. Stijkel, Evolution of beryllium pebbles (HIDOBE) in long term, high flux irradiation in the high flux reactor, Fusion Eng. Des. 86 (2011) 2258–2261.
Acknowledgements This work was supported by Fusion for Energy under the grant contract No. F4E-2009-GRT-030-03. The views and opinions expressed herein reflect only the author’s views. Fusion for Energy is not liable for any use that may be made of the information contained therein.