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Manufacturing aspects in the design of the breeder unit for helium cooled pebble bed blankets
Fusion Engineering and Design 83 (2008) 1258–1262
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Manufacturing aspects in the design of the breeder unit for helium cooled pebble bed blankets夽,夽夽 J. Rey a,∗ , D. Filsinger a , T. Ihli b , C. Polixa a a b
Institut für Kern- und Energietechnik (IKET), Germany Institut für Reaktorsicherheit (IRS), Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e
i n f o
Article history: Available online 2 October 2008 Keywords: Blanket FW SG BU HIP SE U-CP
a b s t r a c t The breeding blanket programme has been the focus of European fusion research for more than two decades. One central goal in this present status is to ensure all manufacturing processes for these blankets to allow functional operation while integrating in ITER. Two kinds of blankets are established for this first run of ITER: One is the helium cooled pebble bed (HCPB) blanket and the other is the helium cooled liquid lead (HCLL) blanket. Both the designs employ three different cooling plate (CP) dimensions, which are forming the test blanket module (TBM). Namely, the first one is the U-shaped first wall (FW) with two similar dimensioned caps. The second type is the cross section of stiffening grids (SG) that separates the equally spaced breeder unit (BU) region. The third type is the U-shaped CP of each BU canister. Different processes for hot isostatic pressing (HIP) are presently focused to be most successful for the fabrication of all cooling plates. It will be shown that the extension of already verified HIP processes is in need of further optimization for the production of real scaled TBM components. This paper will additionally present an alternative manufacturing by spark erosion (SE) process for the U-formed cooling plates (U-CP) of the breeder unit assembly combined with conventional electron beam (EB) welding technology. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Blanket layout
On the way to find the first power reactor design (DEMO) it is foreseen to start a pre-testing of helium cooled pebble bed (HCPB) blankets under reactor-relevant technology in the concept of ITER. Therefore FZK activities started in a great variety of relevant studies for ITER to generate DEMO [1] in-vessel design assembly. On part of the R&D, FZK is focusing on the design and manufacturing of the HCPB canister. The early necessity of real scaled HCPB test blanket modules test blanket module (TBM) up to a time schedule of 3–5 years forces us to think about additional manufacturing strategies for cooling plate systems to allow a high operation safety in respect of manufacturing strategies that are not verified for real scaled cooling plate components.
For the design of a large HCPB blanket module, an adjustment of several aspects for design shown from FZK also seems economically to be attractive [2,3]. In further observation of reactor integration remote-handling focuses a new HCPB design transfer for DEMO to the so-called multi module segmentation (MMS). This aspect is reducing remarkable all in-vessel assembly measures [4]. In the present status for the design of flexible bonding of MMS to manifold structure combined with remote handling attachment on the so-called hot ring shield (HRS) is showing visible feasibility [5]. The aspects of joint assembly and manufacturing requirements of breeder unit and stiffening grid cooling plates requires for the production of MMS an alternative production strategy also for ITER with simplified process adjustments for all kinds of bonding. In all the present designs of HCPB blanket and also in the studies of the helium-cooled lithium lead (HCLL) blanket are in general three different dimension types of cooling plates to realise the operation of a test blanket module. There is the U-formed first wall (FW) which is shielding the blanket structure to the plasma side. The complete canister box is therefore closed with the similar dimensioned caps plates. The back side of blanket is generated by manifold plates which have no further requirement for internal cooling. Follow also Fig. 1 left position. From the side of plate dimensions in the
夽 Please pay attention to the fact that the main author changed from IKET to IRS in 2007. 夽夽 This paper has to be published still in the name of IKET to which the main author belonged. ∗ Corresponding author at: Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany. Tel.: +49 7247 824156; fax: +49 7247 824837. E-mail address: [email protected] (J. Rey). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.08.027
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Fig. 1. Alternative TBM design → BU fixed on 3× back plate → present half scaled U-CP MU.
thickness of below 40 mm the manufacturing strategy is the same for FW and Caps. As well known the complete canister structure has to be resistant for worst case scenarios of a pressure leak of Helium system against 8 MPa. To guaranty the structure resistance, an additional stiffening grid (SG) is included. The dimension of SG manifold versa SG cooling plate has to be different to realise the thermohydraulic operation data. This fact is leading to the second type of cooling plate dimensions in the range of a material thickness from 11 mm to 20 mm. See also paper [15] of this conference. At least the low-level dimension of CP with the thickness of 5 mm for the cooling plates of breeder unit generates the third level of cooling plate development. 3. Breeder unit layout The pebble bed breeder unit has to be designed to provide sufficient tritium breeding. This has to be achieved while safe operation is ensured. Critical assessment for the expected temperature loads due to neutronic heating is required. For the ceramic breeder as well as for Beryllium in function as neutron multiplier the maximum bulk temperature has to be limited regarding e.g. sintering effects. Simultaneously the contact temperature between the steel wall of the cooling plate and the pebble beds has to maintain below 550 ◦ C for the mechanical integrity to the cooling plate material EUROFER. Essential from the side of manufacturing there was a change of design from meander formed Helium flux in the cooling plate structure [6] to a straight Helium flux in a U-formed correlation of two cooling plates BU of the design work in FZK. Follow also Fig. 1 with BU in middle position. 4. Current R&D for cooling plate fabrication technologies In several R&D programmes it was concluded that the 9% chrome EUROFER [7] shows most resistivity also in case of neutronic treat-
ments. In the present studies about the fabrication of cooling plates two kinds of diffusion welding processes are focused to be most successful for the connection of the ferritic-martensitic structural blanket material. The advantage of diffusion welding can be focused in bonding of extended areas without heat affected zone and restoration measures. The first procedure of these processes is based on a hot isostatic gas pressure (HIP) bonding [8] and [9]. A variation of this HIP process is pre-assemblies of rectangular tubes which are fixed together and additionally covered by two solid bars to realise the FW dimension by HIP operation [10]. The second principal procedure is a uni-axial diffusion welding process (DWP) [11,12]. In both cases of bonding between the two milled halves of the cooling plate is reached by controlled pressure and mostly by two-step heat cycles. The bonding zone of each diffusion process needs to be prepared 100% clean without the use of cooling liquid. Also a certain periodical roughness is tightening the process success. Contaminations of Si, Al, Ca, etc. set off further bonding defects and additional Carbon also initiate crack sites. Yield stress and ultimate tensile stress graphs (dependent from different test temperatures) showed only a slightly scattering from base material in comparison to diffusion-welded samples. It was therefore shown that it is more useful to compare for the qualification of HIP process the temperature-dependent charpy impact test. This causes the comparison for the Upper Self Energy (USE) at high temperature test specimens and Ductile to Brittle Transiation Temperature (DBTT) results at low temperature test specimens [11]. Here is clearly shown that small material or joint parameter variations lead to a large deviation of DBTT and also increased scattering of USE. The HIP process case of rectangular bonded tubes sandwich design [10] showed a decrease of joint impact toughness about 55%. One reason for this could be found by oxide inclusions in the prejoint assembly. In conclusion all authors showed that HIP/DWP is a realistic future process especially for FW components. On the other hand in focus approaching larger, real scaled components
Fig. 2. Total dimensions of 1–2 scaled U-CP mock up with details of channel profile.
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Fig. 4. Pre mock up 1–8 scaled U-CP mock up. Showed details of corner sharpening quality.
5.2. EB process
Fig. 3. Pre mock up 1–8 scaled U-CP mock up.
the uncertainty of ensuring uniform HIP/DWP process parameters across the bonding zone increases the risk of defect sources dramatically and therefore, makes it difficult to guarantee the required local bonding penetration over total CP assembly. Therefore an early extension of this process for enlarged machines is planned in the next 2 years to qualify further on this process. 5. Alternative manufacturing strategy for breeder unit cooling plates 5.1. SE process This study will present an alternative manufacturing strategy instead of HIP and DWP process mentioned in Section 4. The premises for this strategy are the reduction of volumetric joint technique and the substitution of joint technique by mechanical processing. This new proposal of CP manufacturing employs a conventional spark erosion (SE) process. Thus the design comprises straight cooling channel structures mostly. Owing to the limits of the SE cutting process the CP has to be assembled from 3× or optional 2× CP plate segments to reach the design relevant U-bended radial length about max. 968 mm for a breeder unit assembly. Fig. 1 on right shows in a width of about 100 mm half scaled and in length real scaled mock up as a part of FZK manufacturing R&D for 2007/2008. In Fig. 2 are visualised all relevant dimensions of this mock up. For minimizing of SE process time a one step SE cut seems to be sufficient for the realisation of the channel section which is 2.6 mm in plate thickness and 4.5 mm in plate width. With a SE wire diameter of 0.3–0.2 mm the average surface roughness is produced about Rz = 12 m. To avoid the longitudinal shrinkage caused by the preparation of the outer CP by milling it is also foreseen to realise the outer plate thickness of 5 mm in one chucking of SE process (Fig. 3). The corner section of each cooling channel is realised by the diameter of wire for SE plus radial process elongation. In case of 0.3 mm or lower SE wire dimensions the corner sharpness is about radius of R = 0.4 mm which is analytical not relevant as shown in Fig. 4.
For the joints of CP electron beam (EB) welding has been used to minimize thermal deformation and seam rising internal of the cooling channel. Positive aspects of this joint process for thin plate materials of EUROFER are also shown in Ref. [13]. A 0.5-mm nozzle outlet Fig. 5 guarantees the parallel adjustment of CP plate bonding. All joint efforts showed maximum raised root seams below 0.16 mm and also from the side of design rule for pressure vessels an acceptable low level of undercuts was observed as shown in Fig. 6. The pressure loss calculation for this is also demonstrated in Section 5.4. The EB joint takes in account to avoid the bonding of all internal bars. Optional gaps between these bars are below 0.1 mm. The total volume of all internal 0.1 mm joint gaps was calculated to be less than 1 cubic centimetre for whole TBM box. The effect of Tritium permeation and binding in these bar gaps seems therefore to be acceptable. It is shown in present running R&D that EB joint of plate materials with thickness mostly upside range of 5 mm causes a sudden recooling of the melting zone. This leads in visualisation of micrographs detection to the building of frozen ␦-ferrite structures in the EUROFER welding seam scenario. These body-centered cubic ferrite-structures are leading to material relevant decrease of creep strength and are not reversal by post welding heat treatment (PWHT), which will also be shown in present running task [14].
Fig. 5. EB seam preparation with 0.5 nozzle overlapping.
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5.4. Pressure losses of new U-CP formation The pressure loss of a channel (Fig. 1, middle section) with a width of 4.5 mm and a height of 2.6 mm and a length dimension of 968 mm was calculated to be 8600 Pa for Helium flow of 400 ◦ C at 8 MPa. A 90◦ bend will contribute with approx. 550 Pa and a step caused by welding bonds (reduced to 4.2 mm width and 2.3 mm height) will contribute with approx. 800 Pa. All these lead to a pressure loss of 11,300 Pa for one cooling plate with two bends and two (welding) steps and this is considerably lower than for meandering grids as proposed in Ref. [6]. 5.5. Post processes of alternative manufacturing
Fig. 6. Pre mock up 1–8 scaled U-CP mock up. EB parameter adjustment and cap assembly.
Therefore further step for EB processing is for instance the accompaniment of pre-heat treatment or the measuring of EB relevant high-energy parameters which could exclude ␦-ferrite formation. In this mentioned work it is also planned to start R&D programmes for real scaled samples of cooling stripes (60 mm × 8.2 mm × 5 mm) with EB bonding for charpy impact test to qualify this process further on.
5.3. Bending process The minimized calculation radius (depending on minimum calculation of Beryllium bed height) of a U-formed CP canister is about R = 15 mm. The ultimate tensile strength of EUROFER is with an average of 668 MPa [16]. In fact, of unexploited strength limits a cold bending is foreseen to realise the mentioned U-formation. To strongly avoid any kind of transversal elongation of the CP bar structures an additional bending metal (→wood, melting point below 70 ◦ C) is filled in section of complete bending structure. First mockup processing of this is shown in Fig. 7. For the submission of this operation the CP channel vessel is evacuated before filling and after the bending process; an additional low temperature treatment takes mostly out all dross. Further measurements are shown in Section 5.5.
Certain materials wreak an additional nuclear activation over lifetime of breeder unit assembly. EUROFER itself is designed with low limits of these elements e.g. Nickel, Copper, Aluminium, Zinc, etc. This also has to be focused in the case of surface contamination while processing EUROFER materials. Standard SE cutting wire is mostly produced out of a Cu and Zn alloy. Also metal dust and oxides from wood alloy used for the stabilisation of the bending process are not acceptable for blocking and abrasion effects of He system in ITER. Therefore it is foreseen to eliminate the SE abrasion residues after both procedures in a one step acid bath and additional neutralisation measures. As far as possible it is also taken into account to allow only a restricted limitation of the complete PWHT (980 ◦ C for 0.5–2 h + 730–750 ◦ C for 2 h) after all arc and beam joint processes to bring a low stressed base in materials for the bonding of TBM. The total assembly of 1–2 scaled mock up (Fig. 1, right position) includes a sequence adaptation of back plate for BU canister with joint adjusted TIG seam preparations. Further examination of He leak tightness and pressurisation about 15.5 MPa (dependant from Rp0,2 decrease of EUROFER at operation temperature and 20 ◦ C) will qualify the mock-up assembly for a scaling of a 1:1 BU mock up. 6. Conclusion and outlook SE process in combination with a full remote operating joint technique like EB seems to be a successful new manufacturing strategy for BU cooling plate design. As shown in [15], it is also a goal in the present work of FZK to extend this measurement for Stiffening Grid production. In aspects of ␦-ferrite formation is necessary for a further R&D work of EB process parameters or simultaneous heat treatments. Alternative high beam processes like LASER or full remote translation arc welding process with Tungsten Inert Gas (TIG) can build alternative bonding strategies including low seam rising in root sections. As mentioned earlier 1–2 mock up is also foreseen to realise a pressure drop test and optional further alternation test of this new CP bonding mechanism. With all these results a recent manufacturing design for a 1:1 scaled CP-BU mock up for TBM can be foreseen from FZK up to 2008. It should not be forgotten that in this moment there is no alternative for HIP/DWP process in case of FW. The extension for real scaled dimensions should be focused to solve all present handling troubles. Acknowledgements
Fig. 7. Pre mock up 1–8 scaled U-CP mock up. 180◦ U-bending sample with internal wood metal support.
The described task was made in the framework of the Nuclear Fusion Programme of the Forschungszentrum Karlsruhe and is supported by the European Union inside the European Fusion Technology Programme. The author thanks the whole manufacturing team of BTI-F in FZK and company CFK-SE Center (Frankfurt) for their starting supports to this production work out for spark erosion.
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References [1] G. Janeschitz, L.V. Boccaccini, W.H. Fietz, W. Goldacker, T. Ihli, R. Meyder, A. Moeslang, P. Norajitra, Development of fusion technology for DEMO in Forschungszemtrum Karlsruhe, Fusion Engineering and Design 81 (2006) 2667–2671. [2] S. Hermsmeyer, B. Dolenksy, J. Fiek, U. Fischer, C. Köhly, S. Malang, P. Pereslavtsev, J. Rey, Z. Xu, Revision of the EU helium cooled pebble bed blanket for DEMO, in: Twentieth IEEE/NPSS Symposium on Fusion Engineering (SOFE), San Diego, October 14–17, 2003. [3] S. Hermsmeyer, L.V. Boccaccini, U. Fischer, C. Köhly, J. Rey, D. Ward, Reactor integration of the Helium cooled pebble bed blanket for DEMO, Fusion Engineering and Design 75–79 (2005) 779–783. [4] T. Ihli, V.L. Boccaccini, G. Janeschitz, C. Köhly, D. Maisonnier, Recent progress in DEMO fusion core engineering: improved segmentation, maintenance and blanket concepts, in: SOFT 2006, 2006. [5] D. Filsinger, Ch. Köhly, J. Rey, C. Polixa, D. Nagy, Engineering aspects on the development of a reactor concept for DEMO, in: Second IAEA Technical Meeting on First Generation of Fusion Power Plants: Design and Technology, Vienna, June 20–22, 2007. [6] Z. Xu, R. Meyder, L.V. Boccaccini, Design and validation of breeder unit of helium-cooled pebble bed blanket for demo fusion reactor, Fusion Engineering and Design 81 (2006) 2233–2238.
[7] M. Schirra, A. Falkenstein, Results of investigations regarding the physical and mechanical properties of martensitic 9%Cr steel Eurofer 97, FZKA Report No. 6707, 2002. [8] E. Rigal, G. Reimann, Development of FM steels diffusion bonding technologies for blanket manufacturing applications, Fusion Engineering and Design 49–50 (2000) 651–656. [9] G. Reimann, P. Norajitra, R. Ruprecht, Diffusion welding tests in hot isostatic presses for manufacturing plate components with internal cooling channels for fusion blankets, TTBB-002-D1 Final Report Fusion No. 185, 2002. [10] E. Rigal, First wall HIPing with open channels EFDA task, TW2-BBTT002b Blanket Manufacturing Technologies, 2006. [11] A.V.D. Weth, H. Kempe, Uniaxial HIP joint development, Final Report on the EFDA Task TW2-TTMS-004-D1, 2003. [12] A.V.D. Weth, Manufacturing of HCPB Cooling Plates by diffusion welding, Final Report on the EFDA Task TW2-TTBB-002b-D5, 2007. [13] M. Rieth, Design limits of welded EUROFER components, EFDA task TW4-TTMS004D1, 2006. [14] M. Rieth, Qualification and improvement of welded joints, EFDA task TW6TTMS-004 D2, 2007. [15] M. Lux, Helium-cooled pebble bed test blanket module: alternative design and fabrication routes, in: ISFNT-8 This Conference PS2-1116, ID No. #265, 2007. [16] A.V.D. Weth, Deterioration of EUROFER plates by serve bending, Journal of Nuclear Materials 359 (2006) 150–154.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Manufacturing aspects in the design of the breeder unit for helium cooled pebble bed blankets夽,夽夽 J. Rey a,∗ , D. Filsinger a , T. Ihli b , C. Polixa a a b
Institut für Kern- und Energietechnik (IKET), Germany Institut für Reaktorsicherheit (IRS), Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e
i n f o
Article history: Available online 2 October 2008 Keywords: Blanket FW SG BU HIP SE U-CP
a b s t r a c t The breeding blanket programme has been the focus of European fusion research for more than two decades. One central goal in this present status is to ensure all manufacturing processes for these blankets to allow functional operation while integrating in ITER. Two kinds of blankets are established for this first run of ITER: One is the helium cooled pebble bed (HCPB) blanket and the other is the helium cooled liquid lead (HCLL) blanket. Both the designs employ three different cooling plate (CP) dimensions, which are forming the test blanket module (TBM). Namely, the first one is the U-shaped first wall (FW) with two similar dimensioned caps. The second type is the cross section of stiffening grids (SG) that separates the equally spaced breeder unit (BU) region. The third type is the U-shaped CP of each BU canister. Different processes for hot isostatic pressing (HIP) are presently focused to be most successful for the fabrication of all cooling plates. It will be shown that the extension of already verified HIP processes is in need of further optimization for the production of real scaled TBM components. This paper will additionally present an alternative manufacturing by spark erosion (SE) process for the U-formed cooling plates (U-CP) of the breeder unit assembly combined with conventional electron beam (EB) welding technology. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Blanket layout
On the way to find the first power reactor design (DEMO) it is foreseen to start a pre-testing of helium cooled pebble bed (HCPB) blankets under reactor-relevant technology in the concept of ITER. Therefore FZK activities started in a great variety of relevant studies for ITER to generate DEMO [1] in-vessel design assembly. On part of the R&D, FZK is focusing on the design and manufacturing of the HCPB canister. The early necessity of real scaled HCPB test blanket modules test blanket module (TBM) up to a time schedule of 3–5 years forces us to think about additional manufacturing strategies for cooling plate systems to allow a high operation safety in respect of manufacturing strategies that are not verified for real scaled cooling plate components.
For the design of a large HCPB blanket module, an adjustment of several aspects for design shown from FZK also seems economically to be attractive [2,3]. In further observation of reactor integration remote-handling focuses a new HCPB design transfer for DEMO to the so-called multi module segmentation (MMS). This aspect is reducing remarkable all in-vessel assembly measures [4]. In the present status for the design of flexible bonding of MMS to manifold structure combined with remote handling attachment on the so-called hot ring shield (HRS) is showing visible feasibility [5]. The aspects of joint assembly and manufacturing requirements of breeder unit and stiffening grid cooling plates requires for the production of MMS an alternative production strategy also for ITER with simplified process adjustments for all kinds of bonding. In all the present designs of HCPB blanket and also in the studies of the helium-cooled lithium lead (HCLL) blanket are in general three different dimension types of cooling plates to realise the operation of a test blanket module. There is the U-formed first wall (FW) which is shielding the blanket structure to the plasma side. The complete canister box is therefore closed with the similar dimensioned caps plates. The back side of blanket is generated by manifold plates which have no further requirement for internal cooling. Follow also Fig. 1 left position. From the side of plate dimensions in the
夽 Please pay attention to the fact that the main author changed from IKET to IRS in 2007. 夽夽 This paper has to be published still in the name of IKET to which the main author belonged. ∗ Corresponding author at: Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany. Tel.: +49 7247 824156; fax: +49 7247 824837. E-mail address: [email protected] (J. Rey). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.08.027
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Fig. 1. Alternative TBM design → BU fixed on 3× back plate → present half scaled U-CP MU.
thickness of below 40 mm the manufacturing strategy is the same for FW and Caps. As well known the complete canister structure has to be resistant for worst case scenarios of a pressure leak of Helium system against 8 MPa. To guaranty the structure resistance, an additional stiffening grid (SG) is included. The dimension of SG manifold versa SG cooling plate has to be different to realise the thermohydraulic operation data. This fact is leading to the second type of cooling plate dimensions in the range of a material thickness from 11 mm to 20 mm. See also paper [15] of this conference. At least the low-level dimension of CP with the thickness of 5 mm for the cooling plates of breeder unit generates the third level of cooling plate development. 3. Breeder unit layout The pebble bed breeder unit has to be designed to provide sufficient tritium breeding. This has to be achieved while safe operation is ensured. Critical assessment for the expected temperature loads due to neutronic heating is required. For the ceramic breeder as well as for Beryllium in function as neutron multiplier the maximum bulk temperature has to be limited regarding e.g. sintering effects. Simultaneously the contact temperature between the steel wall of the cooling plate and the pebble beds has to maintain below 550 ◦ C for the mechanical integrity to the cooling plate material EUROFER. Essential from the side of manufacturing there was a change of design from meander formed Helium flux in the cooling plate structure [6] to a straight Helium flux in a U-formed correlation of two cooling plates BU of the design work in FZK. Follow also Fig. 1 with BU in middle position. 4. Current R&D for cooling plate fabrication technologies In several R&D programmes it was concluded that the 9% chrome EUROFER [7] shows most resistivity also in case of neutronic treat-
ments. In the present studies about the fabrication of cooling plates two kinds of diffusion welding processes are focused to be most successful for the connection of the ferritic-martensitic structural blanket material. The advantage of diffusion welding can be focused in bonding of extended areas without heat affected zone and restoration measures. The first procedure of these processes is based on a hot isostatic gas pressure (HIP) bonding [8] and [9]. A variation of this HIP process is pre-assemblies of rectangular tubes which are fixed together and additionally covered by two solid bars to realise the FW dimension by HIP operation [10]. The second principal procedure is a uni-axial diffusion welding process (DWP) [11,12]. In both cases of bonding between the two milled halves of the cooling plate is reached by controlled pressure and mostly by two-step heat cycles. The bonding zone of each diffusion process needs to be prepared 100% clean without the use of cooling liquid. Also a certain periodical roughness is tightening the process success. Contaminations of Si, Al, Ca, etc. set off further bonding defects and additional Carbon also initiate crack sites. Yield stress and ultimate tensile stress graphs (dependent from different test temperatures) showed only a slightly scattering from base material in comparison to diffusion-welded samples. It was therefore shown that it is more useful to compare for the qualification of HIP process the temperature-dependent charpy impact test. This causes the comparison for the Upper Self Energy (USE) at high temperature test specimens and Ductile to Brittle Transiation Temperature (DBTT) results at low temperature test specimens [11]. Here is clearly shown that small material or joint parameter variations lead to a large deviation of DBTT and also increased scattering of USE. The HIP process case of rectangular bonded tubes sandwich design [10] showed a decrease of joint impact toughness about 55%. One reason for this could be found by oxide inclusions in the prejoint assembly. In conclusion all authors showed that HIP/DWP is a realistic future process especially for FW components. On the other hand in focus approaching larger, real scaled components
Fig. 2. Total dimensions of 1–2 scaled U-CP mock up with details of channel profile.
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Fig. 4. Pre mock up 1–8 scaled U-CP mock up. Showed details of corner sharpening quality.
5.2. EB process
Fig. 3. Pre mock up 1–8 scaled U-CP mock up.
the uncertainty of ensuring uniform HIP/DWP process parameters across the bonding zone increases the risk of defect sources dramatically and therefore, makes it difficult to guarantee the required local bonding penetration over total CP assembly. Therefore an early extension of this process for enlarged machines is planned in the next 2 years to qualify further on this process. 5. Alternative manufacturing strategy for breeder unit cooling plates 5.1. SE process This study will present an alternative manufacturing strategy instead of HIP and DWP process mentioned in Section 4. The premises for this strategy are the reduction of volumetric joint technique and the substitution of joint technique by mechanical processing. This new proposal of CP manufacturing employs a conventional spark erosion (SE) process. Thus the design comprises straight cooling channel structures mostly. Owing to the limits of the SE cutting process the CP has to be assembled from 3× or optional 2× CP plate segments to reach the design relevant U-bended radial length about max. 968 mm for a breeder unit assembly. Fig. 1 on right shows in a width of about 100 mm half scaled and in length real scaled mock up as a part of FZK manufacturing R&D for 2007/2008. In Fig. 2 are visualised all relevant dimensions of this mock up. For minimizing of SE process time a one step SE cut seems to be sufficient for the realisation of the channel section which is 2.6 mm in plate thickness and 4.5 mm in plate width. With a SE wire diameter of 0.3–0.2 mm the average surface roughness is produced about Rz = 12 m. To avoid the longitudinal shrinkage caused by the preparation of the outer CP by milling it is also foreseen to realise the outer plate thickness of 5 mm in one chucking of SE process (Fig. 3). The corner section of each cooling channel is realised by the diameter of wire for SE plus radial process elongation. In case of 0.3 mm or lower SE wire dimensions the corner sharpness is about radius of R = 0.4 mm which is analytical not relevant as shown in Fig. 4.
For the joints of CP electron beam (EB) welding has been used to minimize thermal deformation and seam rising internal of the cooling channel. Positive aspects of this joint process for thin plate materials of EUROFER are also shown in Ref. [13]. A 0.5-mm nozzle outlet Fig. 5 guarantees the parallel adjustment of CP plate bonding. All joint efforts showed maximum raised root seams below 0.16 mm and also from the side of design rule for pressure vessels an acceptable low level of undercuts was observed as shown in Fig. 6. The pressure loss calculation for this is also demonstrated in Section 5.4. The EB joint takes in account to avoid the bonding of all internal bars. Optional gaps between these bars are below 0.1 mm. The total volume of all internal 0.1 mm joint gaps was calculated to be less than 1 cubic centimetre for whole TBM box. The effect of Tritium permeation and binding in these bar gaps seems therefore to be acceptable. It is shown in present running R&D that EB joint of plate materials with thickness mostly upside range of 5 mm causes a sudden recooling of the melting zone. This leads in visualisation of micrographs detection to the building of frozen ␦-ferrite structures in the EUROFER welding seam scenario. These body-centered cubic ferrite-structures are leading to material relevant decrease of creep strength and are not reversal by post welding heat treatment (PWHT), which will also be shown in present running task [14].
Fig. 5. EB seam preparation with 0.5 nozzle overlapping.
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5.4. Pressure losses of new U-CP formation The pressure loss of a channel (Fig. 1, middle section) with a width of 4.5 mm and a height of 2.6 mm and a length dimension of 968 mm was calculated to be 8600 Pa for Helium flow of 400 ◦ C at 8 MPa. A 90◦ bend will contribute with approx. 550 Pa and a step caused by welding bonds (reduced to 4.2 mm width and 2.3 mm height) will contribute with approx. 800 Pa. All these lead to a pressure loss of 11,300 Pa for one cooling plate with two bends and two (welding) steps and this is considerably lower than for meandering grids as proposed in Ref. [6]. 5.5. Post processes of alternative manufacturing
Fig. 6. Pre mock up 1–8 scaled U-CP mock up. EB parameter adjustment and cap assembly.
Therefore further step for EB processing is for instance the accompaniment of pre-heat treatment or the measuring of EB relevant high-energy parameters which could exclude ␦-ferrite formation. In this mentioned work it is also planned to start R&D programmes for real scaled samples of cooling stripes (60 mm × 8.2 mm × 5 mm) with EB bonding for charpy impact test to qualify this process further on.
5.3. Bending process The minimized calculation radius (depending on minimum calculation of Beryllium bed height) of a U-formed CP canister is about R = 15 mm. The ultimate tensile strength of EUROFER is with an average of 668 MPa [16]. In fact, of unexploited strength limits a cold bending is foreseen to realise the mentioned U-formation. To strongly avoid any kind of transversal elongation of the CP bar structures an additional bending metal (→wood, melting point below 70 ◦ C) is filled in section of complete bending structure. First mockup processing of this is shown in Fig. 7. For the submission of this operation the CP channel vessel is evacuated before filling and after the bending process; an additional low temperature treatment takes mostly out all dross. Further measurements are shown in Section 5.5.
Certain materials wreak an additional nuclear activation over lifetime of breeder unit assembly. EUROFER itself is designed with low limits of these elements e.g. Nickel, Copper, Aluminium, Zinc, etc. This also has to be focused in the case of surface contamination while processing EUROFER materials. Standard SE cutting wire is mostly produced out of a Cu and Zn alloy. Also metal dust and oxides from wood alloy used for the stabilisation of the bending process are not acceptable for blocking and abrasion effects of He system in ITER. Therefore it is foreseen to eliminate the SE abrasion residues after both procedures in a one step acid bath and additional neutralisation measures. As far as possible it is also taken into account to allow only a restricted limitation of the complete PWHT (980 ◦ C for 0.5–2 h + 730–750 ◦ C for 2 h) after all arc and beam joint processes to bring a low stressed base in materials for the bonding of TBM. The total assembly of 1–2 scaled mock up (Fig. 1, right position) includes a sequence adaptation of back plate for BU canister with joint adjusted TIG seam preparations. Further examination of He leak tightness and pressurisation about 15.5 MPa (dependant from Rp0,2 decrease of EUROFER at operation temperature and 20 ◦ C) will qualify the mock-up assembly for a scaling of a 1:1 BU mock up. 6. Conclusion and outlook SE process in combination with a full remote operating joint technique like EB seems to be a successful new manufacturing strategy for BU cooling plate design. As shown in [15], it is also a goal in the present work of FZK to extend this measurement for Stiffening Grid production. In aspects of ␦-ferrite formation is necessary for a further R&D work of EB process parameters or simultaneous heat treatments. Alternative high beam processes like LASER or full remote translation arc welding process with Tungsten Inert Gas (TIG) can build alternative bonding strategies including low seam rising in root sections. As mentioned earlier 1–2 mock up is also foreseen to realise a pressure drop test and optional further alternation test of this new CP bonding mechanism. With all these results a recent manufacturing design for a 1:1 scaled CP-BU mock up for TBM can be foreseen from FZK up to 2008. It should not be forgotten that in this moment there is no alternative for HIP/DWP process in case of FW. The extension for real scaled dimensions should be focused to solve all present handling troubles. Acknowledgements
Fig. 7. Pre mock up 1–8 scaled U-CP mock up. 180◦ U-bending sample with internal wood metal support.
The described task was made in the framework of the Nuclear Fusion Programme of the Forschungszentrum Karlsruhe and is supported by the European Union inside the European Fusion Technology Programme. The author thanks the whole manufacturing team of BTI-F in FZK and company CFK-SE Center (Frankfurt) for their starting supports to this production work out for spark erosion.
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