- Email: [email protected]
IFMIF High Flux Test Module—Recent progress in design and manufacturing
Fusion Engineering and Design 83 (2008) 1484–1489
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
IFMIF High Flux Test Module—Recent progress in design and manufacturing D. Leichtle a,∗ , F. Arbeiter a , B. Dolensky a , U. Fischer a , S. Gordeev a , V. Heinzel a , T. Ihli a , K.-H. Lang a , A. Moeslang b , S.P. Simakov a , V. Slobodchuk a , E. Stratmanns a a b
Association FZK-EURATOM, Forschungszentrum Karlsruhe, Institut f¨ ur Reaktorsicherheit, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany Association FZK-EURATOM, Forschungszentrum Karlsruhe, Institut f¨ ur Materialforschung, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e
i n f o
Article history: Available online 24 June 2008 Keywords: Material irradiation Irradiation rig Manufacturing technology
a b s t r a c t The International Fusion Material Irradiation Facility (IFMIF) is an accelerator driven neutron source for irradiation tests of candidate fusion reactor materials. Within the High Flux Test Module (HFTM) a testing volume of 0.5 l filled by qualified small scale specimens will be irradiated at displacement rates of 20–50 dpa/fpy in structural materials. The Forschungszentrum Karlsruhe (FZK) has developed a HFTM design which closely follows the design premise of maximising the space available for irradiation specimens in the IFMIF high flux zone and in addition allows keeping the temperature nearly constant in the rigs containing the specimen. Complementary analyses on nuclear, thermo-hydraulics and mechanical performance of the HFTM were performed to optimize the design. The present paper highlights the main design characteristics as well as recent progress achieved in this area. The contribution also includes (i) recommendations for the use of container, rig and capsule materials, and (ii) a description of the fabrication routes for the entire HFTM including brazing and filling procedures which are currently under development at the Forschungszentrum Karlsruhe. © 2008 Elsevier B.V. All rights reserved.
1. Design overview The High Flux Test Module has to be designed to provide maximum space for miniaturized irradiation specimens, kept at well-defined temperature levels between 300 and 650 ◦ C. Since the dpa-rate degradates rapidly passing through bulk material, the HFTM structural elements have to be very thin (1–1.5 mm). Cooling and temperature control equipment should claim minimum space within the HFTM. The reference design of the HFTM [1] consists of 12 flat plate rigs housed in 4 compartments of the container, made of austenitic steel (see Fig. 1). The compartments are separated by stiffening walls. The container also integrates reflectors for the improvement of dpa level and gradients in the test section volume. The irradiation rigs have the same outer dimensions of 50 mm × 17 mm; cooling channels of 1 mm at the longer side and 0.6 mm at the smaller side are established by spacers on rig and container walls. The HFTM is cooled by helium at low pressure and low temperature (0.3 MPa, 50 ◦ C at the inlet). The helium flow is guided to the test section by a 180◦ bend segmented by three baffles, which extend into the inlet diffuser. Besides stiffening the structure as intended, they allow for an effective flow distribution in the
∗ Corresponding author. Tel.: +49 7247 82 2557; fax: +49 7247 82 3718. E-mail address: [email protected] (D. Leichtle). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.05.004
inlet tube. The large hydraulic resistance at the entrance to the test section is responsible for an effective smoothing of the flow velocity field. The lateral reflectors are cooled by a bypass of the main helium flow. The arrangement of cooling channels of 1 mm width has been optimized to assure a rather homogeneous temperature distribution in the horizontal plane to avoid deformations. The present design of the container envisages a length of 375 mm, comprising the axial reflectors (bottom 80 mm and top 130 mm), the irradiation rigs (140 mm) and the upper stiffening flange (25 mm). This latter component has been reduced in height to avoid excessive ballooning due to thermal expansion. To stiffen the structure in general the steel wall thickness at front and back side has been increased to 1.5 mm (other walls have 1 mm) and additional spacers have been implemented at the container back wall. The design of rigs and capsules has been finalized (see Fig. 2). The rig has a total height of 140 mm, the heated capsule of 120 mm. The irradiation specimen’s loading has been optimized first with the objective to include as many test specimens as possible, and second to have a uniform loading in all of the rigs. The miniaturized irradiation specimens are inserted in capsules. Sodium or a sodium-potassium eutectic alloy filling will guarantee a defined heat transfer from specimen to specimen and between specimen and capsule wall. Electric heaters in three separate windings are brazed into the capsule wall. They balance the nuclear power distribution and keep the specimen temperature range within ±15 ◦ C
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Fig. 1. High Flux Test Module with main components.
Fig. 2. Vertical cut through a HFTM irradiation rig.
even during short beam-off periods. Between the rig and capsule walls a gap filled with stagnant helium acts as thermal insulation. In the case of low temperature rigs (specimen temperature of 300 ◦ C) the capsule wall itself acts as the rig wall.
els in the structure. The thermo-hydraulic simulations showed that helium velocity distributions are only slightly changed compared to the previous design; they remain symmetrically to the center axis. The resulting temperature distribution in the rigs is therefore also similar (i.e. homogeneous) to the previous case, only the electrical power to the heaters has to be adapted for some of the rigs. While previous thermo-hydraulic simulations have been focused on the case of 450 ◦ C samples, recent results for the higher temperature rigs at 650 ◦ C revealed hot spots in the container stiffening walls. The temperatures have a maximum around 220 ◦ C above the heated zone of the irradiation rigs (see Fig. 3, left). The cooling channels on the smaller side of the rigs have the previous reference width of 0.5 mm, the rib structure on the rig walls is acting as a heat source in the channels. Accordingly, the cooling gas is heated continuously along the passage of the heated volume. This fact can be elucidated by looking at the temperature profile along the cooling channel and stiffening plate. As there is no significant temperature gradient within the cooling channel, the stiffening plate is heated due to nuclear heating. Maximum temperatures occur above the top level of the specimen stack due to the hot helium gas. Obviously, the cooling capabilities of these channels are not sufficient. In the present state of the design work we compared the vonMise stresses with yield stresses only to get a rough estimate of the allowable limits. In the future a more stringent assessment will be performed after a thorough design review exercise. The thermo-mechanical analysis revealed that the maximum von-Mise stresses are at a level of 350 MPa (Fig. 3, right), which is well beyond
2. Design optimization Although a quasi reference design of the High Flux Test Module (HFTM) for the International Fusion Material Irradiation Facility (IFMIF) has been elaborated [1,2], a continuous effort is devoted to optimization and refinement of the design. Those originate from the concurrent performance analyses conducted in support of the design work. To mitigate thermal deformations in the helium inlet diffuser ABAQUS simulations have been performed to test several proposed changes. The outcome of those calculations showed that the lateral steel walls of the diffuser have to be 3 mm thick and the stiffening of the duct should be achieved by inserting additional baffles in the diffuser as an extension of the baffles in the 180◦ bend. Because there are 4 compartments in parallel the baffle system should provide four channels, therefore 3 baffles have been used starting already from the inlet of the 180◦ bend (Fig. 1). To maintain the uniform feeding of the rig cooling channels several variants of the baffle system have been suggested. The modified design has been studied further thermo-hydraulically using the CFD code STAR-CD [3]. The main conclusion from the present results is that the large hydraulic resistance of the lower reflector zone to the test section is responsible for an effective smoothing of the flow velocity field. Additional windows, i.e. broad notches, at the top of the baffles are supporting this effect and even improve the stress lev-
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Fig. 3. Compartment stiffening wall (650 ◦ C specimen temperature, 0.5 mm cooling channel): temperature (left) and stress (right) distribution (deformation factor 100×).
the yield strength of the stainless steel. After a series of variant simulation analyses it has been decided to increase the channel width to 0.6 mm. Since the irradiation rigs of the present design have been already manufactured (see Section 4 below) the additional rib height of 0.1 mm has been attributed to the compartment stiffening walls (vertical lines in Fig. 4). With this change the maximum temperature is decreased to 166 ◦ C and the maximum stresses are as low as 175 MPa now below the yield strength of 195 MPa (see Fig. 4). The overall stress distribution within the compartment stiffening walls has been improved significantly by applying the modified channel geometry. A safety margin is provided in addition, since the mass flow rate of helium could be increased accordingly; the results mentioned above assumed a non-modified mass flow rate. Near the junction of helium inlet diffuser to the container there are very small spots of still high peak stresses (above 500 MPa) at the lower end of spacers of the compartments. Similar stress concentrations are observed at the upper end. It was shown that due to plastic deformation up to 1% (ideal plastic behaviour above 0.1% elastic strain) the stress concentrations are levelled out to around 250 MPa. Although this behaviour is regarded as acceptable, a modification of the design has been proposed as follows. The present design requires milling and spark erosion of the container block. A wall thickness of 3.5 mm is retained at the entrance
to the bypass helium flow into the lateral reflectors, whereas the compartment wall thickness is only 1.5 mm. The enforced structure will resist the thermal expansion and stresses are concentrated here. Softening by additional erosion of the 3.5 mm wall near the bypass entrance should allow for modest deformation and hence reduced peak stresses. Thermo-mechanical analyses confirmed this suggestion (Fig. 5). The upper end of the compartment walls have been modified by relieving notches which reduces the peak stress only slightly. Further optimization work in this area is in progress to reduce the maximum stress levels. 3. Global performance analyses The design of the High Flux Test Module has been based on concurrent analyses with respect to nuclear, thermo-hydraulic and mechanical performance. Although further work dedicated to details of the design is still needed, this procedure, in combination with accompanying fabrication tests, gives a sound basis for the reliability of the present design. Dedicated computational tools, data and models have been developed over the past few years for IFMIF neutronics and activation analyses [4]. With regard to the deuterium–lithium neutron source, a new evaluation of the reaction system d + 6,7 Li has been
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Fig. 4. Compartment stiffening wall (650 ◦ C specimen temperature, 0.6 mm cooling channel): temperature (left) and stress (right) distribution (deformation factor 100×). Tiny regions of highest stresses at edges (not visible in the picture). The vertical black lines denote the rib structure on the walls.
provided for the Monte Carlo code McDeLicious. The code as well as the cross-section data has been extensively tested and validated to assure their feasibility and performance for IFMIF applications. To this end, a comprehensive 3D model of the TTC has been developed on the basis of CAD designs. An automatic interface programme to transfer CAD to Monte-Carlo geometry has been tested successfully on the Target and Test Cell model. It will be used in the future to facilitate consistent changes of design and neutronics model. The analyses performed for the HFTM confirmed that the displacement rate is in the range of 20–45 dpa/fpy, and damage correlation factors, like gas production to dpa ratio, are well within the range of parameters anticipated in fusion reactor devices. Detailed nuclear heat distributions have been obtained as input to the thermo-hydraulic layout activities. The CFD code STAR-CD has been used for the thermo-hydraulic analysis of the HFTM. Its suitability for the flow regime at the turbulent-laminar transition in the HFTM has been validated by means of dedicated heat transfer and flow velocity measurements at the ITHEX facility [5]. The individual rig layout has been assessed with a triple heater system. It turned out that at a mass flow rate of 0.083 kg/s (0.3 MPa) the temperature differences in the specimen stack would be below 30 ◦ C (at 450 ◦ C) and 22 ◦ C (at 650 ◦ C). Appropriate choices for an additional isolation gap between rig and capsule wall (in the range of 0.5–0.8 mm) and power to the heaters (about 12–25 kW) are needed to achieve these values. The individual adjustments of the heater power have been calculated for each rig.
Calculations performed with ABAQUS for the present design of the HFTM revealed that the high temperature case (650 ◦ C specimen temperature) will pose the most severe conditions to the container structure. The deformation pattern on the container shows a favorable compensating contraction and elongation in the lateral direction transverse to the beam. This leads to a rather flat inner surface of the lateral reflectors and to an improved contact between rig and container wall. 4. Materials and manufacturing An assessment of a material selection guideline [6] for InTest-Cell components is currently performed with the objective to provide designers with basic guidelines and recommendations for structural material selection based on available experimental data. The proper choice of the structural material depends on the expected operation conditions (which include loading and irradiation conditions, operation temperature and corrosive environment if present). The temperature window for the HFTM container is between 50 and 150 ◦ C where austenitic steels are recommended since loss of ductility is quite moderate, strain to necking is still high and helium embrittlement is low. On the contrary, the irradiation capsules temperatures are between 250 and 650 ◦ C. This regime is not allowed for conventional austenitic steels, but more suitable for ferritic-martensitic steels. The reduced activation FM steels like EUROFER and F82Hmod even show significant improvements in irradiation hardening and toughness. Therefore the present choice for rig and capsule material is a RAFM steel. Fur-
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Fig. 5. Van Mises stresses at the lower part of the container after design modification (650 ◦ C, 0.6 mm cooling channel, max. stress 303 MPa). The wall thickness of 1.5 mm of the container is retained in the diffuser part near the bypass entrance.
ther work on filling gaps in the irradiated properties database is ongoing. Crucial manufacturing techniques for the parts of the HFTM have been tested extensively to assure a reliable and reproducible manufacturing route for the final procurement of the HFTM. The IFMIF HFTM consists of several separate parts, namely 12 rigs, 12 upper reflector elements, 4 lower reflector elements, 1 container with lateral reflectors and 1 Vertical Test Assembly (VTA) plug adapter. Three rig casings, two of them made of AISI 316 (DIN 1.4301) and one of EUROFER steel, respectively, have been produced already. They were manufactured by milling, stress-free annealing and spark erosion of the interior. The outside was directly spark eroded to form the rib structure on the surface. The manufacturing of the capsules started with the coarse and fine milling of the outside of the capsule casing from a massive slug. It followed the milling of the grooves by a 5-axes milling machine. For the embedding of the heater wires into the grooves a special clamping device was required to fix mechanically the wires at the long sides of the capsules. The wires were brazed to the capsule wall with help of a dedicated braze guidance box. The outer surface was ablated down to the original capsule surface by spark erosion. Finally the interior was spark eroded to obtain the specimen volume. The container will be assembled from 4 pieces which are joined by laser welding. The main part consists of the container block with 4 compartments, spark eroded lateral reflector elements, and the upper part of the helium gas duct. Additional separate parts are medium part of the gas duct, lower bend, and helium diffuser horn. In addition, the 4 lower reflector elements are welded into the container block, establishing the bottom support of the irradiation rigs. The machining of the container is restricted by the thin walled structures of 1.5 mm. It starts with the coarse cutting from a slug, milling and spark erosion with allowances. The container block will
Fig. 6. Vertical cut of a braze sample. Empty groove structure (black) on top, the extension to the bottom is filled completely by braze filler material.
be annealed stress-free before machining of the outer contour. The inner flange for the VTA plug adapter as well as the helium bypass inlet to the lateral reflectors are also machined from the block. The interior will be spark eroded; the boreholes in the lateral reflectors have to be closed by filler pieces welded on the structure. Similarly the other container parts will be milled and spark eroded, covers are welded helium-tight on the gas duct parts. Major efforts have been devoted recently to the refinement of the brazing procedure for the heater wires. The main issues are related to very tiny gaps and a significant height for the flowing braze material. Braze sample mock-ups have been fabricated in order to adjust the volume temperature distribution inside the vacuum oven. The mock-ups have been brazed with a modified and shortened procedure with only 25 min arrest time
Fig. 7. Infrared picture of the heated capsule surface during heating up.
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above the melting point. Vertical cuts through the samples supported the excellent result of the optimized procedure (Fig. 6). To avoid brazing of the cold end wires to the structure a braze stop paste has been used which successfully kept the braze material for sufficient time within the narrow gap. The procedure has been applied to the original capsule. A first qualitative analysis by thermography showed the expected success of the brazing with a rather homogeneous temperature distribution on the surface (Fig. 7). A more quantitative analysis will be carried out soon.
Acknowledgements
5. Conclusions
[1] V. Heinzel, F. Arbeiter, B. Dolensky, U. Fischer, S. Gordeev, K.-H. Lang, et al., IFMIF High Flux Test Module and Test Cell—design and design validation, in: Proceedings of the 24th Symposium of Fusion Technology, September 11–15, 2006, Warszaw, Fus. Eng. Des. 82 (2007) 2444–2450. [2] B. Dolensky, S. Gordeev, V. Heinzel, K. Lang, A. Moeslang, V. Slobodchuk, E. Stratmanns, Simulation of the Optimised Design of the HFTM and the Natural Convection Simulation in the IFMIF Test Cell Cavity. Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft, Wissenschaftliche Berichte FZKA 7161, October 2005. ¨ [3] B. Dolensky, S. Gordeev, V. Heinzel, K. Lang, D. Leichtle, A. Moslang, V. Slobodchuk, E. Stratmanns, Thermohydraulics of the HFTM modified design, Internal Report FZKA, December 2005. [4] U. Fischer, Y. Chen, S.P. Simakov, P.P.H. Wilson, P. Vladimirov, F. Wasastjerna, Overview of recent progress in IFMIF neutronics, Fus. Eng. Des. 81 (2006) 1195–2002. [5] F. Arbeiter, S. Gordeev, V. Heinzel, D. Leichtle, E. Stratmanns, Mini-channel flow experiments and CFD validation analyses with the IFMIF Thermo-hydraulic Experimental Facility (ITHEX), in: Proceedings of the 24th Symposium of Fusion Technology, September 11–15, 2006, Warszaw, Fus. Eng. Des. 82 (2007) 2456–2461. [6] P. Vladimirov, Structural materials selection guideline for in-cell components, unpublished report.
The HFTM design has been optimized applying concurrent neutronics, thermo-hydraulic and mechanical analyses. The recent container design changes have been carefully applied to assure the global performance of the component. Based on a dedicated material selection exercise austenitic steel AISI 316 and the RAFM steel EUROFER have been selected as structural materials for container and rig/capsule, respectively. The manufacturing route for the procurement of a full-size HFTM to be tested in the Engineering Validation Engineering Design Activities (EVEDA) Phase has been presented. The brazing procedure for the heater wire windings has been successfully optimized; the liquid metal filling of the specimen stack is under preparation. The design of the container will be investigated further supported by additional fabrications tests and by a series of thermo-hydraulic experiments to be conducted within EVEDA.
This work, supported by the European Communities under the contract of Association between EURATOM and Forschungszentrum Karlsruhe, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed therein do not necessarily reflect those of the European Commission. References
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
IFMIF High Flux Test Module—Recent progress in design and manufacturing D. Leichtle a,∗ , F. Arbeiter a , B. Dolensky a , U. Fischer a , S. Gordeev a , V. Heinzel a , T. Ihli a , K.-H. Lang a , A. Moeslang b , S.P. Simakov a , V. Slobodchuk a , E. Stratmanns a a b
Association FZK-EURATOM, Forschungszentrum Karlsruhe, Institut f¨ ur Reaktorsicherheit, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany Association FZK-EURATOM, Forschungszentrum Karlsruhe, Institut f¨ ur Materialforschung, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e
i n f o
Article history: Available online 24 June 2008 Keywords: Material irradiation Irradiation rig Manufacturing technology
a b s t r a c t The International Fusion Material Irradiation Facility (IFMIF) is an accelerator driven neutron source for irradiation tests of candidate fusion reactor materials. Within the High Flux Test Module (HFTM) a testing volume of 0.5 l filled by qualified small scale specimens will be irradiated at displacement rates of 20–50 dpa/fpy in structural materials. The Forschungszentrum Karlsruhe (FZK) has developed a HFTM design which closely follows the design premise of maximising the space available for irradiation specimens in the IFMIF high flux zone and in addition allows keeping the temperature nearly constant in the rigs containing the specimen. Complementary analyses on nuclear, thermo-hydraulics and mechanical performance of the HFTM were performed to optimize the design. The present paper highlights the main design characteristics as well as recent progress achieved in this area. The contribution also includes (i) recommendations for the use of container, rig and capsule materials, and (ii) a description of the fabrication routes for the entire HFTM including brazing and filling procedures which are currently under development at the Forschungszentrum Karlsruhe. © 2008 Elsevier B.V. All rights reserved.
1. Design overview The High Flux Test Module has to be designed to provide maximum space for miniaturized irradiation specimens, kept at well-defined temperature levels between 300 and 650 ◦ C. Since the dpa-rate degradates rapidly passing through bulk material, the HFTM structural elements have to be very thin (1–1.5 mm). Cooling and temperature control equipment should claim minimum space within the HFTM. The reference design of the HFTM [1] consists of 12 flat plate rigs housed in 4 compartments of the container, made of austenitic steel (see Fig. 1). The compartments are separated by stiffening walls. The container also integrates reflectors for the improvement of dpa level and gradients in the test section volume. The irradiation rigs have the same outer dimensions of 50 mm × 17 mm; cooling channels of 1 mm at the longer side and 0.6 mm at the smaller side are established by spacers on rig and container walls. The HFTM is cooled by helium at low pressure and low temperature (0.3 MPa, 50 ◦ C at the inlet). The helium flow is guided to the test section by a 180◦ bend segmented by three baffles, which extend into the inlet diffuser. Besides stiffening the structure as intended, they allow for an effective flow distribution in the
∗ Corresponding author. Tel.: +49 7247 82 2557; fax: +49 7247 82 3718. E-mail address: [email protected] (D. Leichtle). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.05.004
inlet tube. The large hydraulic resistance at the entrance to the test section is responsible for an effective smoothing of the flow velocity field. The lateral reflectors are cooled by a bypass of the main helium flow. The arrangement of cooling channels of 1 mm width has been optimized to assure a rather homogeneous temperature distribution in the horizontal plane to avoid deformations. The present design of the container envisages a length of 375 mm, comprising the axial reflectors (bottom 80 mm and top 130 mm), the irradiation rigs (140 mm) and the upper stiffening flange (25 mm). This latter component has been reduced in height to avoid excessive ballooning due to thermal expansion. To stiffen the structure in general the steel wall thickness at front and back side has been increased to 1.5 mm (other walls have 1 mm) and additional spacers have been implemented at the container back wall. The design of rigs and capsules has been finalized (see Fig. 2). The rig has a total height of 140 mm, the heated capsule of 120 mm. The irradiation specimen’s loading has been optimized first with the objective to include as many test specimens as possible, and second to have a uniform loading in all of the rigs. The miniaturized irradiation specimens are inserted in capsules. Sodium or a sodium-potassium eutectic alloy filling will guarantee a defined heat transfer from specimen to specimen and between specimen and capsule wall. Electric heaters in three separate windings are brazed into the capsule wall. They balance the nuclear power distribution and keep the specimen temperature range within ±15 ◦ C
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Fig. 1. High Flux Test Module with main components.
Fig. 2. Vertical cut through a HFTM irradiation rig.
even during short beam-off periods. Between the rig and capsule walls a gap filled with stagnant helium acts as thermal insulation. In the case of low temperature rigs (specimen temperature of 300 ◦ C) the capsule wall itself acts as the rig wall.
els in the structure. The thermo-hydraulic simulations showed that helium velocity distributions are only slightly changed compared to the previous design; they remain symmetrically to the center axis. The resulting temperature distribution in the rigs is therefore also similar (i.e. homogeneous) to the previous case, only the electrical power to the heaters has to be adapted for some of the rigs. While previous thermo-hydraulic simulations have been focused on the case of 450 ◦ C samples, recent results for the higher temperature rigs at 650 ◦ C revealed hot spots in the container stiffening walls. The temperatures have a maximum around 220 ◦ C above the heated zone of the irradiation rigs (see Fig. 3, left). The cooling channels on the smaller side of the rigs have the previous reference width of 0.5 mm, the rib structure on the rig walls is acting as a heat source in the channels. Accordingly, the cooling gas is heated continuously along the passage of the heated volume. This fact can be elucidated by looking at the temperature profile along the cooling channel and stiffening plate. As there is no significant temperature gradient within the cooling channel, the stiffening plate is heated due to nuclear heating. Maximum temperatures occur above the top level of the specimen stack due to the hot helium gas. Obviously, the cooling capabilities of these channels are not sufficient. In the present state of the design work we compared the vonMise stresses with yield stresses only to get a rough estimate of the allowable limits. In the future a more stringent assessment will be performed after a thorough design review exercise. The thermo-mechanical analysis revealed that the maximum von-Mise stresses are at a level of 350 MPa (Fig. 3, right), which is well beyond
2. Design optimization Although a quasi reference design of the High Flux Test Module (HFTM) for the International Fusion Material Irradiation Facility (IFMIF) has been elaborated [1,2], a continuous effort is devoted to optimization and refinement of the design. Those originate from the concurrent performance analyses conducted in support of the design work. To mitigate thermal deformations in the helium inlet diffuser ABAQUS simulations have been performed to test several proposed changes. The outcome of those calculations showed that the lateral steel walls of the diffuser have to be 3 mm thick and the stiffening of the duct should be achieved by inserting additional baffles in the diffuser as an extension of the baffles in the 180◦ bend. Because there are 4 compartments in parallel the baffle system should provide four channels, therefore 3 baffles have been used starting already from the inlet of the 180◦ bend (Fig. 1). To maintain the uniform feeding of the rig cooling channels several variants of the baffle system have been suggested. The modified design has been studied further thermo-hydraulically using the CFD code STAR-CD [3]. The main conclusion from the present results is that the large hydraulic resistance of the lower reflector zone to the test section is responsible for an effective smoothing of the flow velocity field. Additional windows, i.e. broad notches, at the top of the baffles are supporting this effect and even improve the stress lev-
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Fig. 3. Compartment stiffening wall (650 ◦ C specimen temperature, 0.5 mm cooling channel): temperature (left) and stress (right) distribution (deformation factor 100×).
the yield strength of the stainless steel. After a series of variant simulation analyses it has been decided to increase the channel width to 0.6 mm. Since the irradiation rigs of the present design have been already manufactured (see Section 4 below) the additional rib height of 0.1 mm has been attributed to the compartment stiffening walls (vertical lines in Fig. 4). With this change the maximum temperature is decreased to 166 ◦ C and the maximum stresses are as low as 175 MPa now below the yield strength of 195 MPa (see Fig. 4). The overall stress distribution within the compartment stiffening walls has been improved significantly by applying the modified channel geometry. A safety margin is provided in addition, since the mass flow rate of helium could be increased accordingly; the results mentioned above assumed a non-modified mass flow rate. Near the junction of helium inlet diffuser to the container there are very small spots of still high peak stresses (above 500 MPa) at the lower end of spacers of the compartments. Similar stress concentrations are observed at the upper end. It was shown that due to plastic deformation up to 1% (ideal plastic behaviour above 0.1% elastic strain) the stress concentrations are levelled out to around 250 MPa. Although this behaviour is regarded as acceptable, a modification of the design has been proposed as follows. The present design requires milling and spark erosion of the container block. A wall thickness of 3.5 mm is retained at the entrance
to the bypass helium flow into the lateral reflectors, whereas the compartment wall thickness is only 1.5 mm. The enforced structure will resist the thermal expansion and stresses are concentrated here. Softening by additional erosion of the 3.5 mm wall near the bypass entrance should allow for modest deformation and hence reduced peak stresses. Thermo-mechanical analyses confirmed this suggestion (Fig. 5). The upper end of the compartment walls have been modified by relieving notches which reduces the peak stress only slightly. Further optimization work in this area is in progress to reduce the maximum stress levels. 3. Global performance analyses The design of the High Flux Test Module has been based on concurrent analyses with respect to nuclear, thermo-hydraulic and mechanical performance. Although further work dedicated to details of the design is still needed, this procedure, in combination with accompanying fabrication tests, gives a sound basis for the reliability of the present design. Dedicated computational tools, data and models have been developed over the past few years for IFMIF neutronics and activation analyses [4]. With regard to the deuterium–lithium neutron source, a new evaluation of the reaction system d + 6,7 Li has been
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Fig. 4. Compartment stiffening wall (650 ◦ C specimen temperature, 0.6 mm cooling channel): temperature (left) and stress (right) distribution (deformation factor 100×). Tiny regions of highest stresses at edges (not visible in the picture). The vertical black lines denote the rib structure on the walls.
provided for the Monte Carlo code McDeLicious. The code as well as the cross-section data has been extensively tested and validated to assure their feasibility and performance for IFMIF applications. To this end, a comprehensive 3D model of the TTC has been developed on the basis of CAD designs. An automatic interface programme to transfer CAD to Monte-Carlo geometry has been tested successfully on the Target and Test Cell model. It will be used in the future to facilitate consistent changes of design and neutronics model. The analyses performed for the HFTM confirmed that the displacement rate is in the range of 20–45 dpa/fpy, and damage correlation factors, like gas production to dpa ratio, are well within the range of parameters anticipated in fusion reactor devices. Detailed nuclear heat distributions have been obtained as input to the thermo-hydraulic layout activities. The CFD code STAR-CD has been used for the thermo-hydraulic analysis of the HFTM. Its suitability for the flow regime at the turbulent-laminar transition in the HFTM has been validated by means of dedicated heat transfer and flow velocity measurements at the ITHEX facility [5]. The individual rig layout has been assessed with a triple heater system. It turned out that at a mass flow rate of 0.083 kg/s (0.3 MPa) the temperature differences in the specimen stack would be below 30 ◦ C (at 450 ◦ C) and 22 ◦ C (at 650 ◦ C). Appropriate choices for an additional isolation gap between rig and capsule wall (in the range of 0.5–0.8 mm) and power to the heaters (about 12–25 kW) are needed to achieve these values. The individual adjustments of the heater power have been calculated for each rig.
Calculations performed with ABAQUS for the present design of the HFTM revealed that the high temperature case (650 ◦ C specimen temperature) will pose the most severe conditions to the container structure. The deformation pattern on the container shows a favorable compensating contraction and elongation in the lateral direction transverse to the beam. This leads to a rather flat inner surface of the lateral reflectors and to an improved contact between rig and container wall. 4. Materials and manufacturing An assessment of a material selection guideline [6] for InTest-Cell components is currently performed with the objective to provide designers with basic guidelines and recommendations for structural material selection based on available experimental data. The proper choice of the structural material depends on the expected operation conditions (which include loading and irradiation conditions, operation temperature and corrosive environment if present). The temperature window for the HFTM container is between 50 and 150 ◦ C where austenitic steels are recommended since loss of ductility is quite moderate, strain to necking is still high and helium embrittlement is low. On the contrary, the irradiation capsules temperatures are between 250 and 650 ◦ C. This regime is not allowed for conventional austenitic steels, but more suitable for ferritic-martensitic steels. The reduced activation FM steels like EUROFER and F82Hmod even show significant improvements in irradiation hardening and toughness. Therefore the present choice for rig and capsule material is a RAFM steel. Fur-
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Fig. 5. Van Mises stresses at the lower part of the container after design modification (650 ◦ C, 0.6 mm cooling channel, max. stress 303 MPa). The wall thickness of 1.5 mm of the container is retained in the diffuser part near the bypass entrance.
ther work on filling gaps in the irradiated properties database is ongoing. Crucial manufacturing techniques for the parts of the HFTM have been tested extensively to assure a reliable and reproducible manufacturing route for the final procurement of the HFTM. The IFMIF HFTM consists of several separate parts, namely 12 rigs, 12 upper reflector elements, 4 lower reflector elements, 1 container with lateral reflectors and 1 Vertical Test Assembly (VTA) plug adapter. Three rig casings, two of them made of AISI 316 (DIN 1.4301) and one of EUROFER steel, respectively, have been produced already. They were manufactured by milling, stress-free annealing and spark erosion of the interior. The outside was directly spark eroded to form the rib structure on the surface. The manufacturing of the capsules started with the coarse and fine milling of the outside of the capsule casing from a massive slug. It followed the milling of the grooves by a 5-axes milling machine. For the embedding of the heater wires into the grooves a special clamping device was required to fix mechanically the wires at the long sides of the capsules. The wires were brazed to the capsule wall with help of a dedicated braze guidance box. The outer surface was ablated down to the original capsule surface by spark erosion. Finally the interior was spark eroded to obtain the specimen volume. The container will be assembled from 4 pieces which are joined by laser welding. The main part consists of the container block with 4 compartments, spark eroded lateral reflector elements, and the upper part of the helium gas duct. Additional separate parts are medium part of the gas duct, lower bend, and helium diffuser horn. In addition, the 4 lower reflector elements are welded into the container block, establishing the bottom support of the irradiation rigs. The machining of the container is restricted by the thin walled structures of 1.5 mm. It starts with the coarse cutting from a slug, milling and spark erosion with allowances. The container block will
Fig. 6. Vertical cut of a braze sample. Empty groove structure (black) on top, the extension to the bottom is filled completely by braze filler material.
be annealed stress-free before machining of the outer contour. The inner flange for the VTA plug adapter as well as the helium bypass inlet to the lateral reflectors are also machined from the block. The interior will be spark eroded; the boreholes in the lateral reflectors have to be closed by filler pieces welded on the structure. Similarly the other container parts will be milled and spark eroded, covers are welded helium-tight on the gas duct parts. Major efforts have been devoted recently to the refinement of the brazing procedure for the heater wires. The main issues are related to very tiny gaps and a significant height for the flowing braze material. Braze sample mock-ups have been fabricated in order to adjust the volume temperature distribution inside the vacuum oven. The mock-ups have been brazed with a modified and shortened procedure with only 25 min arrest time
Fig. 7. Infrared picture of the heated capsule surface during heating up.
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above the melting point. Vertical cuts through the samples supported the excellent result of the optimized procedure (Fig. 6). To avoid brazing of the cold end wires to the structure a braze stop paste has been used which successfully kept the braze material for sufficient time within the narrow gap. The procedure has been applied to the original capsule. A first qualitative analysis by thermography showed the expected success of the brazing with a rather homogeneous temperature distribution on the surface (Fig. 7). A more quantitative analysis will be carried out soon.
Acknowledgements
5. Conclusions
[1] V. Heinzel, F. Arbeiter, B. Dolensky, U. Fischer, S. Gordeev, K.-H. Lang, et al., IFMIF High Flux Test Module and Test Cell—design and design validation, in: Proceedings of the 24th Symposium of Fusion Technology, September 11–15, 2006, Warszaw, Fus. Eng. Des. 82 (2007) 2444–2450. [2] B. Dolensky, S. Gordeev, V. Heinzel, K. Lang, A. Moeslang, V. Slobodchuk, E. Stratmanns, Simulation of the Optimised Design of the HFTM and the Natural Convection Simulation in the IFMIF Test Cell Cavity. Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft, Wissenschaftliche Berichte FZKA 7161, October 2005. ¨ [3] B. Dolensky, S. Gordeev, V. Heinzel, K. Lang, D. Leichtle, A. Moslang, V. Slobodchuk, E. Stratmanns, Thermohydraulics of the HFTM modified design, Internal Report FZKA, December 2005. [4] U. Fischer, Y. Chen, S.P. Simakov, P.P.H. Wilson, P. Vladimirov, F. Wasastjerna, Overview of recent progress in IFMIF neutronics, Fus. Eng. Des. 81 (2006) 1195–2002. [5] F. Arbeiter, S. Gordeev, V. Heinzel, D. Leichtle, E. Stratmanns, Mini-channel flow experiments and CFD validation analyses with the IFMIF Thermo-hydraulic Experimental Facility (ITHEX), in: Proceedings of the 24th Symposium of Fusion Technology, September 11–15, 2006, Warszaw, Fus. Eng. Des. 82 (2007) 2456–2461. [6] P. Vladimirov, Structural materials selection guideline for in-cell components, unpublished report.
The HFTM design has been optimized applying concurrent neutronics, thermo-hydraulic and mechanical analyses. The recent container design changes have been carefully applied to assure the global performance of the component. Based on a dedicated material selection exercise austenitic steel AISI 316 and the RAFM steel EUROFER have been selected as structural materials for container and rig/capsule, respectively. The manufacturing route for the procurement of a full-size HFTM to be tested in the Engineering Validation Engineering Design Activities (EVEDA) Phase has been presented. The brazing procedure for the heater wire windings has been successfully optimized; the liquid metal filling of the specimen stack is under preparation. The design of the container will be investigated further supported by additional fabrications tests and by a series of thermo-hydraulic experiments to be conducted within EVEDA.
This work, supported by the European Communities under the contract of Association between EURATOM and Forschungszentrum Karlsruhe, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed therein do not necessarily reflect those of the European Commission. References