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Development of the plasma facing components in Japan for ITER
Fusion Engineering and Design 87 (2012) 845–852
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Development of the plasma facing components in Japan for ITER Satoshi Suzuki ∗ , Koichiro Ezato, Yohji Seki, Kensuke Mohri, Kenji Yokoyama, Mikio Enoeda Japan Atomic Energy Agency, Naka, Ibaraki, Japan
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Article history: Available online 13 March 2012 Keywords: ITER Divertor Outer Vertical Target Full-scale prototype Carbon fiber composite Tungsten
a b s t r a c t After the successful completion of the prequalification activity for ITER divertor procurement, Japanese Domestic Agency (JADA) and ITER Organization (IO) have entered into the procurement arrangement of divertor Outer Vertical Target (OVT) in June 2009. In accordance with the arrangement, JADA is going to fully procure the outer target components which correspond to 60 divertor cassettes. JADA has started to manufacture an OVT full-scale prototype in order to pick out/solve technical and quality issues and to establish a rational manufacturing process toward the start of the series of production of the OVT components to be installed in ITER. This paper presents the overview of JADA’s manufacturing activity and the procurement schedule on the divertor outer target procurement. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is one of key issues for the construction of ITER to manufacture robust divertor components which can meet technical and quality requirements. ITER divertor consists of 54 cassettes located at the bottom of the vacuum vessel. Fig. 1 shows the schematic of the ITER divertor. There are three plasma facing components (PFCs), namely inner vertical target (IVT), Outer Vertical Target (OVT) and Dome (DO) in a cassette. Those PFCs are manufactured and inspected by the concerned DAs and delivered to the assembly site in EU. The European Domestic Agency (EUDA), the hosting DA, assembles those PFCs onto a cassette body (CB) which is procured by EUDA and performs final testing to check the integrity of those components. The procurement sharing of the components is as follows1 : IVT: European Domestic Agency (EUDA) OVT: Japanese Domestic Agency (JADA) DO: Russian Domestic Agency (RFDA) CB (including assembly operation): EUDA Toward the start of the procurement of these divertor components, prequalification activity to demonstrate the technical capability of each DA has been made under the close collaboration with ITER Organization (IO) since 2007. As a result of the prequalification activity, all three parties, JADA, EUDA and RFDA, concerning the ITER divertor procurement have successfully been qualified
∗ Corresponding author. Tel.: +81 29 270 7169; fax: +81 29 270 7489. E-mail address: [email protected] (S. Suzuki). 1 High heat flux testing of plasma facing units of IVT, OVT and DO to inspect the thermal performance of the components is performed at RFDA under RFDA’s responsibility. 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.02.035
to start the divertor procurement by IO. Based on this successful result, JADA and IO have entered into the procurement arrangement of divertor OVT in June 2009, and the procurement of OVT has started. The procurement of the OVT components will be made as “staged procurement”. Four procurement stages are defined in the procurement arrangement as follows: -
1st stage: manufacturing of OVT full-scale prototype 2nd stage: 10% series of OVT production 3rd stage: 30% series of OVT production 4th stage: 60% series of OVT production
The total number of OVT components defined in the procurement arrangement is 60.5. Among these 60.5 components, 54 components are installed into ITER, 6 components are served as “spare” for replacement of damaged components. The rest 0.5 components correspond to an OVT full-scale prototype which is equivalent to the poloidally halved OVT components. JADA’s procurement activity is checked by IO at each stage. IO makes permission to JADA to shift to the next stage only if JADA’s product meets the requirements. As a first step of the procurement, JADA has started to manufacture an OVT full-scale prototype. This paper presents the latest activity on the OVT procurement in JADA. 2. Overview of specification of Outer Vertical Target Main structural feature and technical specification of the OVT component are described in this section. Fig. 2 shows a schematic of an OVT component which corresponds to single cassette. The OVT components are mechanically attached to CB with nickel–aluminum–bronze pins which allow rotation of the OVT due to deformation during plasma operation. In addition, such pinned
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Table 1 Main design parameters of the OVT components. Heat flux Design heat flux Critical heat flux margin Materials Armor Bonding interlayer Swirl tape Cooling tube (armored part) Cooling tube (pipe fitting part) Cassette body and steel support structure Pins (mechanical fixation) Coolant Temperature Pressure Allowable coolant flow rate Allowable pressure drop
∼10 MW/m2 (at striking point, steady state)∼20 MW/m2 (transient)∼5 MW/m2 (at baffle region) 1.4 Carbon Fiber Composite, CFC (lower straight part of OVT) Tungsten (upper baffle region of OVT) Copper alloy (pure copper, copper–tungsten) Pure copper CuCrZr-IG (ITER grade) Austenitic stainless steel (SS 316L) Austenitic stainless steel (SS 316L(N)-IG, XM-19) Nickel–Aluminum–Bronze (C63200) 70 ◦ C (at inlet of cassette body) 4.0 MPa (at inlet of cassette body) 870 kg/s (for 54 cassettes) 1.35 MPa (at the cassette pipe stubs, which interface with the Tokamak Cooling Water System)
Fig. 1. Schematic of ITER divertor.
connection enables convenient replacement of damaged components by remote handling devices [1]. The plasma facing surface consists of armor tiles and cooling tubes, namely plasma facing unit (PFU). Lower part of the OVT, which has straight shape, is covered with carbon fiber composite (CFC) armor tiles. Since “striking point” locates just onto this straight part, CFC armor is selected as an armor material of this part thanks to its high thermal conductivity, excellent thermal shock resistance and high sublimation temperature. On the other hand, upper part of the OVT, which has curved shape, is covered with tungsten armor tiles from its low sputtering yield and high thermal conductivity points of view. Since PFUs of the OVT are subjected to the highest heat flux among the PFCs not only at steady state and thermal transient period but also at disruptions and ELMs in ITER, those components are required high refractoriness and reliability. Table 1 shows main design parameters of the OVT components [2,3]. CFC armor tiles around the striking point are subjected to the highest heat flux up to 10 MW/m2 for steady state and to 20 MW/m2 for thermal transient within 10 s. To achieve high heat removal capability, these armor tiles are metallurgically bonded onto cooling tubes. The cross section of OVT has “monoblock” shape such that the armor tiles are skewered by the cooling tubes to avoid falling down of the debonded armor tiles. In addition to this, a twisted tape made of pure copper is inserted into a straight part of the cooling tube of the OVT to enhance heat removal capability and to improve critical heat flux performance of the OVT including “striking point” where the maximum incident heat flux appears. The cross section of the twisted tape used for OVT has parallelogram shape chopped off the sharp edges. Its sheet thickness is thicker than that used for IVT to sustain equivalent critical heat flux performance to that of IVT under lower coolant flow rate than IVT. Striking points of the IVT and the OVT are subjected to the highest heat flux up to 10 MW/m2 for steady state and to 20 MW/m2 for thermal transient within 10 s. 3. Manufacturing of OVT full-scale prototype 3.1. Materials selection and specification Main materials selection and the material specification of the OVT components are described in this section.
Fig. 2. Schematic of outer vertical target (corresponding 1 cassette).
3.1.1. Carbon fiber composite (CFC) In general, CFC materials has high hydrogen retention characteristic comparing to metal materials. Therefore, divertor target which has CFC armor tiles is planned to be replaced to fully tungsten armored target prior to the start of D-T discharge operation in ITER from the viewpoint of minimization of tritium inventory.
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Table 2 Specification of CFC materials. Chemical composition
Density
C
Ash content (other than C)
>99.99%
<0.01%
>1.65 g/cm3
Thermal conductivity (W/m/K) X-direction Temp. Avg. Min.
Y-direction
25 ◦ C 300 260
800 ◦ C 140 120
25 ◦ C 100 90
Z-direction 800 ◦ C 50 45
25 ◦ C 60 50
800 ◦ C 40 35
Tensile strength at room temperature (MPa) X-direction
Y-direction
Z-direction
30
20
5
X-direction: direction along the axis with maximum thermal conductivity. Z-direction: direction along the axis with minimum thermal conductivity.
On the other hand, CFC has higher melting (sublimation) temperature than metal materials and shows higher thermal conductivity more than pure copper. Table 2 summarizes requirements on CFC materials which is based on two-dimensional CFC material. 3.1.2. Tungsten Tungsten is used as an armor material not only for OVT but also IVT and DO. Tungsten has highest melting temperature (∼3400 ◦ C) among metal materials. However, mechanical performance of tungsten degrades if it is recrystallized at elevated temperature. Since the recrystallization temperature of tungsten is around 1400 ◦ C, it should carefully be used as an armor material of those components by taking into account of their maximum operation temperature. In addition, it is likely for generic powder metallurgy tungsten materials to crack and split off since generic powder metallurgy tungsten has isotropic mechanical characteristics and relatively low mechanical strength. Therefore, rolled tungsten which has rolling direction parallel to the incident heat flux is used as an armor tile of OVT and IVT to avoid splitting off of a part of tungsten armor into inside of the vacuum chamber. Table 3 summarizes requirements on tungsten. 3.1.3. Copper–chromium–zirconium ITER grade (CuCrZr-IG) As for CuCrZr, ITER grade (IG) improved material which has been based on ASTM C18150 is used as structural material of cooling tubes [4]. Table 4 summarizes requirements on CuCrZr-IG. In particular, CuCrZr-IG is discriminative from the view point that the mechanical properties are specified not only for asreceived material but also for the material (=cooling tube) after manufacturing of PFUs. Since CuCrZr is a precipitation hardened material, its mechanical properties are largely affected by the thermal transient due to bonding process. In fact, since PFUs are heated up to around 1000 ◦ C during bonding process, it is necessary for
CuCrZr-IG to be gas-quenched and aged for recovering the mechanical strength. For instance, in case that a cooling tube made of CuCrZr-IG is heated up to 980 ◦ C without gas-quenching, its tensile strength becomes as low as that of pure copper even if proper aging process is performed afterward. Consequently, the mechanical properties after the manufacturing process must be stringently specified as well. The required mechanical strength of CuCrZr-IG after the manufacturing process is about 75% as low as that of as-received material, as shown in Table 4. 3.1.4. Stainless steel 316L ITER grade (316L(N)-IG) Stainless steel 316L ITER grade (316L(N)-IG) is based on standard stainless steel material 316L. However, allowable range of its chemical composition is strictly specified from the viewpoint of reduction of induced radioactivity during DT operation of ITER [5]. In addition, required mechanical strength is higher than that of standard stainless steel 316L as shown in Table 5. Additional requirement on its magnetic permeability (<1.03) is defined to mitigate the impact on the magnetic field which takes place in plasma transient, such as disruptions. Consequently, even if commercial 316L (such as SUS316L in accordance with Japanese Industrial Standard) which can meet the requirements on its mechanical strength is available in the domestic market, it can not be used as a structural material of OVT without additional inspection on magnetic permeability and so on. 3.2. Status of manufacturing of OVT Latest status of manufacturing activity in JADA is described in this section. JADA and IO have agreed and executed procurement arrangement of OVT components in June 2009, and the procurement activity in JADA has started. As a first step of the procurement, JADA has started manufacturing of an OVT full-scale prototype.
Table 3 Specification of tungsten. Chemical composition (Tungsten rolled plate based on ASTM B760-86 (1999)) Component
wt% Allowable range in wt%
W
C
O
N
Fe
Ni
Si
bal. ±0.002
0.1 +10% relative
0.1 +0.0005
0.1 +0.001
0.1
0.1
0.1
Density (g/cm3 )
Grain size
Hardness (Hv30 )
More than 19.0
Size no. 3 or finer
More than 410
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Table 4 Specification of CuCrZr-IG. Chemical composition (Parenthetic reference corresponds to ASTM C18150) Composition
wt%
Cu
Cr
Zr
Impurity
Bal.
0.60–0.90 (0.50–1.50)
0.07–0.15 (0.05–0.25)
0.1 or less oxygen: must be as low as possible
Temp. (◦ C)
State
Tensile strength (MPa)
Yield stress (MPa)
Elongation (%)
20
As-received After manufacturing As-received After manufacturing
370 280 280 220
240 175 200 150
17 15 10 14
250
This manufacturing activity covers selection of material source, manufacturing process qualification and quality assurance (QA) on overall manufacturing activity, which have not been deeply focused on during the R&D period of the OVT development. After the successful completion of these qualification and QA activities, series of production of the OVT components to be installed in ITER will be started. An OVT full-scale prototype corresponds to a poloidally half size of the real OVT component (=22 PFUs/cassette), which consists of 11 PFUs and a steel support structure. Fig. 3 shows overall view of the OVT full-scale prototype. The dimension of each PFU is exactly identical to that of a real OVT component. However, the surface of the CFC armor is machined to reduce its thickness so that the maximum surface temperature of the CFC armor is kept below 2000 ◦ C in the high heat flux test. 3.2.1. Qualification of key items Prior to the start of manufacturing of the OVT full-scale prototype components, there are some key manufacturing processes to be qualified in accordance with ITER requirements.
coolant water, qualification of this welded joint is essential. By using small test coupons as shown in Fig. 3, tensile test, rotary bending fatigue test, face bending test, root bending test and helium leak test (before and after rotary bending fatigue test) are essentially performed to validate the soundness of this dissimilar joint. The test coupons showed sufficient strength and ductility in tensile and bending test. In addition, no significant leakage was found in helium leak test. As a result, EBW and TIG welding were used in the manufacturing of the dissimilar joint. 3.2.1.2. Load carrying capability of support legs of PFUs. The load carrying capability of each CFC and W joint to the steel support legs is necessary to be validated since electro-magnetic force during the plasma transition is applied to these joint (see Fig. 4). Tensile test using small test coupons shown in Fig. 5 is done. The bonding method for this joint shall be identical to that to be used for the manufacturing of the OVT components, namely
3.2.1.1. CuCrZr/316L tube-to-tube joint. A transition piece made of nickel-based alloy is used in the dissimilar joint between CuCrZr cooling tubes and stainless steel connection tubes, as shown in Fig. 4. JADA is going to use Alloy 625 (Inconel 625) as a transition piece form the view point of good weldability for these tubes. Electron beam welding (EBW) and TIG welding are utilized. Since this dissimilar joint makes pressure boundary between vacuum and
Fig. 4. Schematic of transition part between CuCrZr tubes and stainless steel connection tubes (left); small test coupon for qualification test (right).
Fig. 3. OVT full-scale prototype (11 PFUs).
Fig. 5. Small test coupon for load carrying capability test.
0.10 or lower – 0.10 or lower – Elongation (%)
45 (SUS316L: more than 40) –
Yield stress (MPa)
220 (SUS316L: more than 177) 135 (SUS316L: not specified)
0.0020 or lower – 0.060 ∼ 0.080 –
525 (SUS316L: more than 481) 415 (SUS316L: not specified)
Tensile strength (MPa)
2.30 ∼ 2.70 2.00 ∼ 3.00
3.2.2. Bonding technology improvement for CFC and tungsten armor 3.2.2.1. Changeover of interlayer material for CFC armor/CuCrZr tube. Based on the past R&Ds, soft copper (pure copper) is specified as an interlayer material for CFC armor/CuCrZr tube joint. However, CFC armor sometimes showed cracking after the manufacturing process, according to infrared thermography test. This is attributable to hardening of CuCrZr tube during aging process. Though the aging process encourages recovering of mechanical strength of CuCrZr after brazing process, pure copper can not sufficiently absorb the mismatch of thermal expansion between CFC armor and CuCrZr tube and then fracture of the CFC armor around the braze interface takes place in some cases. To avoid such situation, copper–tungsten (Cu–W) was selected from the view point of its coefficient of thermal expansion (CTE). Table 7 shows major mechanical properties of these materials. Coefficient of thermal expansion of Cu–W lies midway between that of CFC and CuCrZr, which imposes Cu–W is effective to mitigate the mismatch of CTE between CFC and CuCrZr through the bonding process of the OVT components. Small test specimens were manufactured using Cu–W as an interlayer between CFC armor and CuCrZr tube. Table 8 shows the results taken from infrared thermography test before and after bonding process (brazing and aging processes). As shown in this table, all the specimens with pure copper interlayer obviously showed degradation of their braze interface after the aging process. However, specimens with Cu–W interlayer kept soundness of the braze interface after the aging process. Based on this, the interlayer material of Cu–W has been chosen for the manufacturing of the OVT full-scale prototype [6]. 3.2.2.2. Bonding of pure copper interlayer onto W armor before brazing process. In parallel to the improvement of the CFC bonding technology, JADA performed to improve the bonding method between W armor and pure copper interlayer. JADA has ever carried out to braze W armor/pure copper interlayer and pure copper interlayer/CuCrZr tube at the same time in a vacuum furnace. However, it is more preferable to bond W and pure copper prior to the brazing onto CuCrZr tube from the view point of reduction of the rejection rate due to deficient bonding interface. Three kinds of bonding method, namely HIP bonding [7], uni-axial diffusion bonding and direct copper casting are being in qualification in JADA. Fig. 6 shows example of the W/Cu joint fabricated by using HIP bonding method. A small divertor target mock-up has been manufactured using these W/Cu joint monoblocks. High heat flux test was carried out in an ion beam test facility in JAEA. Fig. 7 shows the mock-up which was heated by hydrogen ion beam. The surface heat flux was 5 MW/m2 for 10 s and the total number of cycles applied was 1000 cycles. Finally, the mock-up survived the cyclic heat flux with no degradation of its heat removal capability.
20 250
3.3. Manufacturing of PFUs for OVT full-scale prototype
Temp. (◦ C)
12.00 ∼ 12.50 12.00 ∼ 15.00 17.00 ∼ 18.00 16.00 ∼ 18.00 0.010 or lower Max. 0.030 0.025 or lower Max. 0.045 0.50or lower 1.00 or lower 1.60 ∼ 2.00 2.00 or lower 0.03 or lower 0.03 or lower 316L(N)-IG SUS316L (JIS)
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brazing (armor/copper interlayer tube, armor/copper interlayer plate and copper interlayer plate/steel support leg). The criteria of the load carrying capability of these joint are tensile loading of 3 kN (CFC/steel support leg) and 8 kN (W/steel support leg), respectively. Table 6 shows the results of load carrying capability test. All the test coupons met the criteria, and then the brazing method has successfully been qualified.
0.30 or lower –
Ti Ni Cr S P Si Mn C
Composition (wt%)
Chemical composition (Lower stand shows JIS (Japanese Industrial Standard) based material)
Table 5 Specification of 316L(N)-IG.
Mo
N
B
Cu
Nb
S. Suzuki et al. / Fusion Engineering and Design 87 (2012) 845–852
Based on the successful completion of the qualification of key items and improvements on the armor joining technology, JADA has started to manufacture the PFUs for the OVT full-scale prototype. As a final exercise of the manufacturing trial, a fullscale PFU, namely “Pre-prototype”, has been manufactured. The
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S. Suzuki et al. / Fusion Engineering and Design 87 (2012) 845–852
Table 6 Results of load carrying capability test (unit: kN). ID No.
1
2
3
4
5
CFC armor W armor
3.56 31.9
6.08 41.6
4.70 38.1
4.44 23.8
7.10 37.0
Table 7 Major mechanical properties of CFC, copper, copper-tungsten and CuCrZr. Material
Chemical comp.
CTE (x10−6 K–1 )
Young’s modulus (GPa)
Tensile strength (MPa)
CFC (CX-2002U) Pure Cu (annealed) Cu-W (NEL-150) CuCrZr-IG
– – 32Cu-68W see Table 4
0.3 (X)1.4 (Y)5.0 (Z) 16.6 10.6 15.7
10.7 (X) 8.1 (Y) 3.4 (Z) 82.4 379 128
34 (X) 29 (Y) 9 (Z) – 640 370
Table 8 Obtained defect size in infrared thermography test before and after aging processes.
in vacuum environment at the same time followed by aging process. A braze filler of Ni–Cu–Mn with 50 m meter thickness was used to braze CFC/pure copper interlayer, Cu–W interlayer/CuCrZr tube (CFC armored part) and pure copper interlayer/CuCrZr tube (W armored part). Fig. 8 shows temperature evolution of bonding process of OVT pre-prototype. The quench rate during the nitrogen gas feeding just after the brazing was 0.83 K/s which was within our target range of 0.7–0.9 K/s. Fig. 9 shows appearance of the OVT pre-prototype which was installed into a test frame for infrared thermography test. The OVT pre-prototype has 30 CFC armor tiles and 96 armor tiles made of NAK-55 which is a simulant material of tungsten. The OVT pre-prototype has been non-destructive tested in an infrared thermography test facility to verify the soundness of the bonding interface of CFC armored part. As a result of the infrared thermography test [8,9], all the CFC monoblocks showed excellent cooling
manufacturing procedure was exactly identical to those qualified through the qualification activity using small specimens. CFC monoblocks and tungsten monoblocks with pure copper interlayer which was already bonded before the brazing process were brazed
Fig. 7. High heat flux test of a small tungsten-armored OVT mock-up in an ion beam test facility.
Fig. 6. Tungsten/pure copper joint monoblock made by using HIP bonding.
Fig. 8. Temperature evolution of bonding process of OVT pre-prototype.
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After the successful manufacturing of the OVT pre-prototype, the manufacturing of the PFUs of the OVT full-scale prototype have been started. During the manufacturing, formal witnessing of the braze process by IO was carried out in the beginning of February 2011. In addition, a spot check on the QA procedure of welding qualification was performed by IO on 3 March 2011 as well. Fig. 10 shows snapshot of IO’s formal witnessing at our contractor’s premises in Japan. The manufacturing of six PFUs for the OVT full-scale prototype has completed in June 2011 though three months have been delayed due to 3.11 earthquake disaster in Japan. Fig. 11 shows the appearance of the finished PFUs. The manufacturing of the rest 5 PFUs and the steel support structure for the OVT full-scale prototype is scheduled 2011–2013. 4. Conclusion Fig. 9. Appearance of OVT pre-prototype.
In accordance with the execution of the procurement arrangement on the supply of the divertor OVT, manufacturing of an OVT full-scale prototype has started in JAEA/JADA since 2009. The staged procurement of the divertor OVT is in the first stage right now. JADA has been performing the manufacturing of PFUs for the OVT fullscale prototype under close collaboration and strong support by IO. After the successful clearance on the qualification of the manufacturing processes related to the PFUs, JADA has developed an OVT pre-prototype as a final exercise toward the manufacturing of PFUs. And it showed good performance in the infrared thermography inspection. Based on the result from the OVT pre-prototype, the first 6 PFUs have been manufactured by the end of June 2011 according to the schedule. The manufacturing of the rest 5 PFUs is scheduled in 2011–2013. Disclaimer
Fig. 10. Formal witnessing of braze process by ITER organization.
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. Acknowledgments The authors would like to express their acknowledgements to Dr. M. Akiba, Dr. T. Hayashi, Dr. E. Tada, Dr. M. Sugimoto and Dr. Y. Neyatani of JAEA and Dr. R. Yoshino of ITER organization for their encouragement and support to conduct the present activities. The mock-ups and the plasma facing units were fabricated with cooperation of Kawasaki Heavy Industries Ltd., Metal Technology Co. Ltd., Toyo-Tanso Co. Ltd., and Yamatogokin, Co. Ltd. References
Fig. 11. Appearance of the finished PFUs for the OVT prototype.
performance in the infrared thermography test except for one questionable monoblock. Based on this result, the fixation jigs for the CFC monoblocks during the brazing process has been modified to avoid generation of such questionable braze interface of CFC monoblocks.
[1] J. Palmer, M. Irving, J. Järvenpää, H. Mäkinen, H. Saarinen, M. Siuko, et al., The design and development of divertor remote handling equipment for ITER, Fusion Engineering and Design 82 (2007) 1977–1982. [2] M. Merola, D. Loesser, A. Martin, P. Chappuis, R. Mitteau, V. Komarov, et al., ITER plasma-facing components, Fusion Engineering and Design 85 (2010) 2312–2322. [3] S. Suzuki, K. Ezato, Y. Seki, K. Yokoyama, T. Hirose, S. Mori, et al., Recent activities related to the development of the plasma facing components for ITER and fusion DEMO plant, Physica Scripta T138 (2009) 014003. [4] V.R. Barabash, G.M. Kalinin, S.A. Fabritsiev, S.J. Zinkle, et al., Specification of CuCrZr alloy properties after various thermo-mechanical treatments and design allowables including neutron irradiation effects, Journal of Nuclear Materials 417 (2011) 904–907. [5] V. Barabash, The ITER International Team, A. Peacock, S. Fabritsiev, G. Kalinin, S. Zinkle, et al., Materials challenges for ITER – current status and future activities, Journal of Nuclear Materials 367–370 (2007) 21–32. [6] K. Ezato, S., Suzuki, Y. Seki et al., R&D activities on manufacturing plasma-facing unit for prototype of ITER Divertor outer target in JADA, Fusion Engineering and Design, in press.
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[7] H. Takatsu, M. Enoeda, S. Suzuki, T. Hirose, N. Koizumi, K. Takahashi, ITER project–status of procurement activities in domestic agency of Japan and application of HIP technology, in: Proc. of the 2011 Int. Conf. on Hot Isostatic Pressing, Kobe, Japan April. 12–14, 2011. [8] Y. Seki, K. Ezato, S. Suzuki, K. Yokoyama, M. Enoeda, S. Mori, et al., Non-destructive examination with infrared thermography system for
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Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Development of the plasma facing components in Japan for ITER Satoshi Suzuki ∗ , Koichiro Ezato, Yohji Seki, Kensuke Mohri, Kenji Yokoyama, Mikio Enoeda Japan Atomic Energy Agency, Naka, Ibaraki, Japan
a r t i c l e
i n f o
Article history: Available online 13 March 2012 Keywords: ITER Divertor Outer Vertical Target Full-scale prototype Carbon fiber composite Tungsten
a b s t r a c t After the successful completion of the prequalification activity for ITER divertor procurement, Japanese Domestic Agency (JADA) and ITER Organization (IO) have entered into the procurement arrangement of divertor Outer Vertical Target (OVT) in June 2009. In accordance with the arrangement, JADA is going to fully procure the outer target components which correspond to 60 divertor cassettes. JADA has started to manufacture an OVT full-scale prototype in order to pick out/solve technical and quality issues and to establish a rational manufacturing process toward the start of the series of production of the OVT components to be installed in ITER. This paper presents the overview of JADA’s manufacturing activity and the procurement schedule on the divertor outer target procurement. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is one of key issues for the construction of ITER to manufacture robust divertor components which can meet technical and quality requirements. ITER divertor consists of 54 cassettes located at the bottom of the vacuum vessel. Fig. 1 shows the schematic of the ITER divertor. There are three plasma facing components (PFCs), namely inner vertical target (IVT), Outer Vertical Target (OVT) and Dome (DO) in a cassette. Those PFCs are manufactured and inspected by the concerned DAs and delivered to the assembly site in EU. The European Domestic Agency (EUDA), the hosting DA, assembles those PFCs onto a cassette body (CB) which is procured by EUDA and performs final testing to check the integrity of those components. The procurement sharing of the components is as follows1 : IVT: European Domestic Agency (EUDA) OVT: Japanese Domestic Agency (JADA) DO: Russian Domestic Agency (RFDA) CB (including assembly operation): EUDA Toward the start of the procurement of these divertor components, prequalification activity to demonstrate the technical capability of each DA has been made under the close collaboration with ITER Organization (IO) since 2007. As a result of the prequalification activity, all three parties, JADA, EUDA and RFDA, concerning the ITER divertor procurement have successfully been qualified
∗ Corresponding author. Tel.: +81 29 270 7169; fax: +81 29 270 7489. E-mail address: [email protected] (S. Suzuki). 1 High heat flux testing of plasma facing units of IVT, OVT and DO to inspect the thermal performance of the components is performed at RFDA under RFDA’s responsibility. 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.02.035
to start the divertor procurement by IO. Based on this successful result, JADA and IO have entered into the procurement arrangement of divertor OVT in June 2009, and the procurement of OVT has started. The procurement of the OVT components will be made as “staged procurement”. Four procurement stages are defined in the procurement arrangement as follows: -
1st stage: manufacturing of OVT full-scale prototype 2nd stage: 10% series of OVT production 3rd stage: 30% series of OVT production 4th stage: 60% series of OVT production
The total number of OVT components defined in the procurement arrangement is 60.5. Among these 60.5 components, 54 components are installed into ITER, 6 components are served as “spare” for replacement of damaged components. The rest 0.5 components correspond to an OVT full-scale prototype which is equivalent to the poloidally halved OVT components. JADA’s procurement activity is checked by IO at each stage. IO makes permission to JADA to shift to the next stage only if JADA’s product meets the requirements. As a first step of the procurement, JADA has started to manufacture an OVT full-scale prototype. This paper presents the latest activity on the OVT procurement in JADA. 2. Overview of specification of Outer Vertical Target Main structural feature and technical specification of the OVT component are described in this section. Fig. 2 shows a schematic of an OVT component which corresponds to single cassette. The OVT components are mechanically attached to CB with nickel–aluminum–bronze pins which allow rotation of the OVT due to deformation during plasma operation. In addition, such pinned
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Table 1 Main design parameters of the OVT components. Heat flux Design heat flux Critical heat flux margin Materials Armor Bonding interlayer Swirl tape Cooling tube (armored part) Cooling tube (pipe fitting part) Cassette body and steel support structure Pins (mechanical fixation) Coolant Temperature Pressure Allowable coolant flow rate Allowable pressure drop
∼10 MW/m2 (at striking point, steady state)∼20 MW/m2 (transient)∼5 MW/m2 (at baffle region) 1.4 Carbon Fiber Composite, CFC (lower straight part of OVT) Tungsten (upper baffle region of OVT) Copper alloy (pure copper, copper–tungsten) Pure copper CuCrZr-IG (ITER grade) Austenitic stainless steel (SS 316L) Austenitic stainless steel (SS 316L(N)-IG, XM-19) Nickel–Aluminum–Bronze (C63200) 70 ◦ C (at inlet of cassette body) 4.0 MPa (at inlet of cassette body) 870 kg/s (for 54 cassettes) 1.35 MPa (at the cassette pipe stubs, which interface with the Tokamak Cooling Water System)
Fig. 1. Schematic of ITER divertor.
connection enables convenient replacement of damaged components by remote handling devices [1]. The plasma facing surface consists of armor tiles and cooling tubes, namely plasma facing unit (PFU). Lower part of the OVT, which has straight shape, is covered with carbon fiber composite (CFC) armor tiles. Since “striking point” locates just onto this straight part, CFC armor is selected as an armor material of this part thanks to its high thermal conductivity, excellent thermal shock resistance and high sublimation temperature. On the other hand, upper part of the OVT, which has curved shape, is covered with tungsten armor tiles from its low sputtering yield and high thermal conductivity points of view. Since PFUs of the OVT are subjected to the highest heat flux among the PFCs not only at steady state and thermal transient period but also at disruptions and ELMs in ITER, those components are required high refractoriness and reliability. Table 1 shows main design parameters of the OVT components [2,3]. CFC armor tiles around the striking point are subjected to the highest heat flux up to 10 MW/m2 for steady state and to 20 MW/m2 for thermal transient within 10 s. To achieve high heat removal capability, these armor tiles are metallurgically bonded onto cooling tubes. The cross section of OVT has “monoblock” shape such that the armor tiles are skewered by the cooling tubes to avoid falling down of the debonded armor tiles. In addition to this, a twisted tape made of pure copper is inserted into a straight part of the cooling tube of the OVT to enhance heat removal capability and to improve critical heat flux performance of the OVT including “striking point” where the maximum incident heat flux appears. The cross section of the twisted tape used for OVT has parallelogram shape chopped off the sharp edges. Its sheet thickness is thicker than that used for IVT to sustain equivalent critical heat flux performance to that of IVT under lower coolant flow rate than IVT. Striking points of the IVT and the OVT are subjected to the highest heat flux up to 10 MW/m2 for steady state and to 20 MW/m2 for thermal transient within 10 s. 3. Manufacturing of OVT full-scale prototype 3.1. Materials selection and specification Main materials selection and the material specification of the OVT components are described in this section.
Fig. 2. Schematic of outer vertical target (corresponding 1 cassette).
3.1.1. Carbon fiber composite (CFC) In general, CFC materials has high hydrogen retention characteristic comparing to metal materials. Therefore, divertor target which has CFC armor tiles is planned to be replaced to fully tungsten armored target prior to the start of D-T discharge operation in ITER from the viewpoint of minimization of tritium inventory.
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Table 2 Specification of CFC materials. Chemical composition
Density
C
Ash content (other than C)
>99.99%
<0.01%
>1.65 g/cm3
Thermal conductivity (W/m/K) X-direction Temp. Avg. Min.
Y-direction
25 ◦ C 300 260
800 ◦ C 140 120
25 ◦ C 100 90
Z-direction 800 ◦ C 50 45
25 ◦ C 60 50
800 ◦ C 40 35
Tensile strength at room temperature (MPa) X-direction
Y-direction
Z-direction
30
20
5
X-direction: direction along the axis with maximum thermal conductivity. Z-direction: direction along the axis with minimum thermal conductivity.
On the other hand, CFC has higher melting (sublimation) temperature than metal materials and shows higher thermal conductivity more than pure copper. Table 2 summarizes requirements on CFC materials which is based on two-dimensional CFC material. 3.1.2. Tungsten Tungsten is used as an armor material not only for OVT but also IVT and DO. Tungsten has highest melting temperature (∼3400 ◦ C) among metal materials. However, mechanical performance of tungsten degrades if it is recrystallized at elevated temperature. Since the recrystallization temperature of tungsten is around 1400 ◦ C, it should carefully be used as an armor material of those components by taking into account of their maximum operation temperature. In addition, it is likely for generic powder metallurgy tungsten materials to crack and split off since generic powder metallurgy tungsten has isotropic mechanical characteristics and relatively low mechanical strength. Therefore, rolled tungsten which has rolling direction parallel to the incident heat flux is used as an armor tile of OVT and IVT to avoid splitting off of a part of tungsten armor into inside of the vacuum chamber. Table 3 summarizes requirements on tungsten. 3.1.3. Copper–chromium–zirconium ITER grade (CuCrZr-IG) As for CuCrZr, ITER grade (IG) improved material which has been based on ASTM C18150 is used as structural material of cooling tubes [4]. Table 4 summarizes requirements on CuCrZr-IG. In particular, CuCrZr-IG is discriminative from the view point that the mechanical properties are specified not only for asreceived material but also for the material (=cooling tube) after manufacturing of PFUs. Since CuCrZr is a precipitation hardened material, its mechanical properties are largely affected by the thermal transient due to bonding process. In fact, since PFUs are heated up to around 1000 ◦ C during bonding process, it is necessary for
CuCrZr-IG to be gas-quenched and aged for recovering the mechanical strength. For instance, in case that a cooling tube made of CuCrZr-IG is heated up to 980 ◦ C without gas-quenching, its tensile strength becomes as low as that of pure copper even if proper aging process is performed afterward. Consequently, the mechanical properties after the manufacturing process must be stringently specified as well. The required mechanical strength of CuCrZr-IG after the manufacturing process is about 75% as low as that of as-received material, as shown in Table 4. 3.1.4. Stainless steel 316L ITER grade (316L(N)-IG) Stainless steel 316L ITER grade (316L(N)-IG) is based on standard stainless steel material 316L. However, allowable range of its chemical composition is strictly specified from the viewpoint of reduction of induced radioactivity during DT operation of ITER [5]. In addition, required mechanical strength is higher than that of standard stainless steel 316L as shown in Table 5. Additional requirement on its magnetic permeability (<1.03) is defined to mitigate the impact on the magnetic field which takes place in plasma transient, such as disruptions. Consequently, even if commercial 316L (such as SUS316L in accordance with Japanese Industrial Standard) which can meet the requirements on its mechanical strength is available in the domestic market, it can not be used as a structural material of OVT without additional inspection on magnetic permeability and so on. 3.2. Status of manufacturing of OVT Latest status of manufacturing activity in JADA is described in this section. JADA and IO have agreed and executed procurement arrangement of OVT components in June 2009, and the procurement activity in JADA has started. As a first step of the procurement, JADA has started manufacturing of an OVT full-scale prototype.
Table 3 Specification of tungsten. Chemical composition (Tungsten rolled plate based on ASTM B760-86 (1999)) Component
wt% Allowable range in wt%
W
C
O
N
Fe
Ni
Si
bal. ±0.002
0.1 +10% relative
0.1 +0.0005
0.1 +0.001
0.1
0.1
0.1
Density (g/cm3 )
Grain size
Hardness (Hv30 )
More than 19.0
Size no. 3 or finer
More than 410
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Table 4 Specification of CuCrZr-IG. Chemical composition (Parenthetic reference corresponds to ASTM C18150) Composition
wt%
Cu
Cr
Zr
Impurity
Bal.
0.60–0.90 (0.50–1.50)
0.07–0.15 (0.05–0.25)
0.1 or less oxygen: must be as low as possible
Temp. (◦ C)
State
Tensile strength (MPa)
Yield stress (MPa)
Elongation (%)
20
As-received After manufacturing As-received After manufacturing
370 280 280 220
240 175 200 150
17 15 10 14
250
This manufacturing activity covers selection of material source, manufacturing process qualification and quality assurance (QA) on overall manufacturing activity, which have not been deeply focused on during the R&D period of the OVT development. After the successful completion of these qualification and QA activities, series of production of the OVT components to be installed in ITER will be started. An OVT full-scale prototype corresponds to a poloidally half size of the real OVT component (=22 PFUs/cassette), which consists of 11 PFUs and a steel support structure. Fig. 3 shows overall view of the OVT full-scale prototype. The dimension of each PFU is exactly identical to that of a real OVT component. However, the surface of the CFC armor is machined to reduce its thickness so that the maximum surface temperature of the CFC armor is kept below 2000 ◦ C in the high heat flux test. 3.2.1. Qualification of key items Prior to the start of manufacturing of the OVT full-scale prototype components, there are some key manufacturing processes to be qualified in accordance with ITER requirements.
coolant water, qualification of this welded joint is essential. By using small test coupons as shown in Fig. 3, tensile test, rotary bending fatigue test, face bending test, root bending test and helium leak test (before and after rotary bending fatigue test) are essentially performed to validate the soundness of this dissimilar joint. The test coupons showed sufficient strength and ductility in tensile and bending test. In addition, no significant leakage was found in helium leak test. As a result, EBW and TIG welding were used in the manufacturing of the dissimilar joint. 3.2.1.2. Load carrying capability of support legs of PFUs. The load carrying capability of each CFC and W joint to the steel support legs is necessary to be validated since electro-magnetic force during the plasma transition is applied to these joint (see Fig. 4). Tensile test using small test coupons shown in Fig. 5 is done. The bonding method for this joint shall be identical to that to be used for the manufacturing of the OVT components, namely
3.2.1.1. CuCrZr/316L tube-to-tube joint. A transition piece made of nickel-based alloy is used in the dissimilar joint between CuCrZr cooling tubes and stainless steel connection tubes, as shown in Fig. 4. JADA is going to use Alloy 625 (Inconel 625) as a transition piece form the view point of good weldability for these tubes. Electron beam welding (EBW) and TIG welding are utilized. Since this dissimilar joint makes pressure boundary between vacuum and
Fig. 4. Schematic of transition part between CuCrZr tubes and stainless steel connection tubes (left); small test coupon for qualification test (right).
Fig. 3. OVT full-scale prototype (11 PFUs).
Fig. 5. Small test coupon for load carrying capability test.
0.10 or lower – 0.10 or lower – Elongation (%)
45 (SUS316L: more than 40) –
Yield stress (MPa)
220 (SUS316L: more than 177) 135 (SUS316L: not specified)
0.0020 or lower – 0.060 ∼ 0.080 –
525 (SUS316L: more than 481) 415 (SUS316L: not specified)
Tensile strength (MPa)
2.30 ∼ 2.70 2.00 ∼ 3.00
3.2.2. Bonding technology improvement for CFC and tungsten armor 3.2.2.1. Changeover of interlayer material for CFC armor/CuCrZr tube. Based on the past R&Ds, soft copper (pure copper) is specified as an interlayer material for CFC armor/CuCrZr tube joint. However, CFC armor sometimes showed cracking after the manufacturing process, according to infrared thermography test. This is attributable to hardening of CuCrZr tube during aging process. Though the aging process encourages recovering of mechanical strength of CuCrZr after brazing process, pure copper can not sufficiently absorb the mismatch of thermal expansion between CFC armor and CuCrZr tube and then fracture of the CFC armor around the braze interface takes place in some cases. To avoid such situation, copper–tungsten (Cu–W) was selected from the view point of its coefficient of thermal expansion (CTE). Table 7 shows major mechanical properties of these materials. Coefficient of thermal expansion of Cu–W lies midway between that of CFC and CuCrZr, which imposes Cu–W is effective to mitigate the mismatch of CTE between CFC and CuCrZr through the bonding process of the OVT components. Small test specimens were manufactured using Cu–W as an interlayer between CFC armor and CuCrZr tube. Table 8 shows the results taken from infrared thermography test before and after bonding process (brazing and aging processes). As shown in this table, all the specimens with pure copper interlayer obviously showed degradation of their braze interface after the aging process. However, specimens with Cu–W interlayer kept soundness of the braze interface after the aging process. Based on this, the interlayer material of Cu–W has been chosen for the manufacturing of the OVT full-scale prototype [6]. 3.2.2.2. Bonding of pure copper interlayer onto W armor before brazing process. In parallel to the improvement of the CFC bonding technology, JADA performed to improve the bonding method between W armor and pure copper interlayer. JADA has ever carried out to braze W armor/pure copper interlayer and pure copper interlayer/CuCrZr tube at the same time in a vacuum furnace. However, it is more preferable to bond W and pure copper prior to the brazing onto CuCrZr tube from the view point of reduction of the rejection rate due to deficient bonding interface. Three kinds of bonding method, namely HIP bonding [7], uni-axial diffusion bonding and direct copper casting are being in qualification in JADA. Fig. 6 shows example of the W/Cu joint fabricated by using HIP bonding method. A small divertor target mock-up has been manufactured using these W/Cu joint monoblocks. High heat flux test was carried out in an ion beam test facility in JAEA. Fig. 7 shows the mock-up which was heated by hydrogen ion beam. The surface heat flux was 5 MW/m2 for 10 s and the total number of cycles applied was 1000 cycles. Finally, the mock-up survived the cyclic heat flux with no degradation of its heat removal capability.
20 250
3.3. Manufacturing of PFUs for OVT full-scale prototype
Temp. (◦ C)
12.00 ∼ 12.50 12.00 ∼ 15.00 17.00 ∼ 18.00 16.00 ∼ 18.00 0.010 or lower Max. 0.030 0.025 or lower Max. 0.045 0.50or lower 1.00 or lower 1.60 ∼ 2.00 2.00 or lower 0.03 or lower 0.03 or lower 316L(N)-IG SUS316L (JIS)
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brazing (armor/copper interlayer tube, armor/copper interlayer plate and copper interlayer plate/steel support leg). The criteria of the load carrying capability of these joint are tensile loading of 3 kN (CFC/steel support leg) and 8 kN (W/steel support leg), respectively. Table 6 shows the results of load carrying capability test. All the test coupons met the criteria, and then the brazing method has successfully been qualified.
0.30 or lower –
Ti Ni Cr S P Si Mn C
Composition (wt%)
Chemical composition (Lower stand shows JIS (Japanese Industrial Standard) based material)
Table 5 Specification of 316L(N)-IG.
Mo
N
B
Cu
Nb
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Based on the successful completion of the qualification of key items and improvements on the armor joining technology, JADA has started to manufacture the PFUs for the OVT full-scale prototype. As a final exercise of the manufacturing trial, a fullscale PFU, namely “Pre-prototype”, has been manufactured. The
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Table 6 Results of load carrying capability test (unit: kN). ID No.
1
2
3
4
5
CFC armor W armor
3.56 31.9
6.08 41.6
4.70 38.1
4.44 23.8
7.10 37.0
Table 7 Major mechanical properties of CFC, copper, copper-tungsten and CuCrZr. Material
Chemical comp.
CTE (x10−6 K–1 )
Young’s modulus (GPa)
Tensile strength (MPa)
CFC (CX-2002U) Pure Cu (annealed) Cu-W (NEL-150) CuCrZr-IG
– – 32Cu-68W see Table 4
0.3 (X)1.4 (Y)5.0 (Z) 16.6 10.6 15.7
10.7 (X) 8.1 (Y) 3.4 (Z) 82.4 379 128
34 (X) 29 (Y) 9 (Z) – 640 370
Table 8 Obtained defect size in infrared thermography test before and after aging processes.
in vacuum environment at the same time followed by aging process. A braze filler of Ni–Cu–Mn with 50 m meter thickness was used to braze CFC/pure copper interlayer, Cu–W interlayer/CuCrZr tube (CFC armored part) and pure copper interlayer/CuCrZr tube (W armored part). Fig. 8 shows temperature evolution of bonding process of OVT pre-prototype. The quench rate during the nitrogen gas feeding just after the brazing was 0.83 K/s which was within our target range of 0.7–0.9 K/s. Fig. 9 shows appearance of the OVT pre-prototype which was installed into a test frame for infrared thermography test. The OVT pre-prototype has 30 CFC armor tiles and 96 armor tiles made of NAK-55 which is a simulant material of tungsten. The OVT pre-prototype has been non-destructive tested in an infrared thermography test facility to verify the soundness of the bonding interface of CFC armored part. As a result of the infrared thermography test [8,9], all the CFC monoblocks showed excellent cooling
manufacturing procedure was exactly identical to those qualified through the qualification activity using small specimens. CFC monoblocks and tungsten monoblocks with pure copper interlayer which was already bonded before the brazing process were brazed
Fig. 7. High heat flux test of a small tungsten-armored OVT mock-up in an ion beam test facility.
Fig. 6. Tungsten/pure copper joint monoblock made by using HIP bonding.
Fig. 8. Temperature evolution of bonding process of OVT pre-prototype.
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After the successful manufacturing of the OVT pre-prototype, the manufacturing of the PFUs of the OVT full-scale prototype have been started. During the manufacturing, formal witnessing of the braze process by IO was carried out in the beginning of February 2011. In addition, a spot check on the QA procedure of welding qualification was performed by IO on 3 March 2011 as well. Fig. 10 shows snapshot of IO’s formal witnessing at our contractor’s premises in Japan. The manufacturing of six PFUs for the OVT full-scale prototype has completed in June 2011 though three months have been delayed due to 3.11 earthquake disaster in Japan. Fig. 11 shows the appearance of the finished PFUs. The manufacturing of the rest 5 PFUs and the steel support structure for the OVT full-scale prototype is scheduled 2011–2013. 4. Conclusion Fig. 9. Appearance of OVT pre-prototype.
In accordance with the execution of the procurement arrangement on the supply of the divertor OVT, manufacturing of an OVT full-scale prototype has started in JAEA/JADA since 2009. The staged procurement of the divertor OVT is in the first stage right now. JADA has been performing the manufacturing of PFUs for the OVT fullscale prototype under close collaboration and strong support by IO. After the successful clearance on the qualification of the manufacturing processes related to the PFUs, JADA has developed an OVT pre-prototype as a final exercise toward the manufacturing of PFUs. And it showed good performance in the infrared thermography inspection. Based on the result from the OVT pre-prototype, the first 6 PFUs have been manufactured by the end of June 2011 according to the schedule. The manufacturing of the rest 5 PFUs is scheduled in 2011–2013. Disclaimer
Fig. 10. Formal witnessing of braze process by ITER organization.
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. Acknowledgments The authors would like to express their acknowledgements to Dr. M. Akiba, Dr. T. Hayashi, Dr. E. Tada, Dr. M. Sugimoto and Dr. Y. Neyatani of JAEA and Dr. R. Yoshino of ITER organization for their encouragement and support to conduct the present activities. The mock-ups and the plasma facing units were fabricated with cooperation of Kawasaki Heavy Industries Ltd., Metal Technology Co. Ltd., Toyo-Tanso Co. Ltd., and Yamatogokin, Co. Ltd. References
Fig. 11. Appearance of the finished PFUs for the OVT prototype.
performance in the infrared thermography test except for one questionable monoblock. Based on this result, the fixation jigs for the CFC monoblocks during the brazing process has been modified to avoid generation of such questionable braze interface of CFC monoblocks.
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