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Structural analysis and manufacture for the vacuum vessel of experimental advanced superconducting tokamak (EAST) device
Fusion Engineering and Design 81 (2006) 1117–1122
Structural analysis and manufacture for the vacuum vessel of experimental advanced superconducting tokamak (EAST) device Yun tao Song ∗ , Damao Yao, Songata Wu, Peide Weng Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Anhui, Hefei 230031, PR China Received 8 January 2005; received in revised form 27 September 2005; accepted 27 September 2005 Available online 27 December 2005
Abstract The experimental advanced superconducting tokamak (EAST) is an advanced steady-state plasma physics experimental device, which has been approved by the Chinese government and is being constructed as the Chinese national nuclear fusion research project. The vacuum vessel, that is one of the key components, will have to withstand not only the electromagnetic force due to the plasma disruption and the Halo current, but also the pressure of boride water and the thermal stress due to the 250 ◦ C baking out by the hot pressure nitrogen gas, or the 100 ◦ C hot wall during plasma operation. This paper is a report of the mechanical analyses of the vacuum vessel. According to the allowable stress criteria of American Society of Mechanical Engineers, Boiler and Pressure Vessel Committee (ASME), the maximum integrated stress intensity on the vacuum vessel is 396 MPa, less than the allowable design stress intensity 3Sm (441 MPa). At the same time, some key R & D issues are presented, which include supporting system, bellows and the assembly of the whole vacuum vessel. © 2005 Elsevier B.V. All rights reserved. Keywords: Fusion device; Vacuum vessel; Mechanical analyses; Manufacturing of tokamak
1. Introduction The experimental advanced superconducting tokamak (EAST) device is based on a superconducting magnet system, which consists of 16 toroidal field (TF) coils and 14 poloidal field (PF) coils located ∗ Corresponding author. Tel.: +86 551 5593271; fax: +86 551 5591310. E-mail address: [email protected] (Y.t. Song).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.09.042
around the plasma mid-plane symmetrically. The central solenoidal (CS) assembly consists of six PF coils. A vacuum vessel with 16 rectangular horizontal ports and 32 bathtub-shaped vertical ports is located in the bore of the TF coil. A cryostat with two thermal shields at 80 K encloses the superconducting coils, the vacuum vessel and the support structures. The superconducting magnet system, the vacuum vessel and the thermal shields in the cryostat are supported independently. The EAST device has a height of 10 m (with the main support),
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Y.t. Song et al. / Fusion Engineering and Design 81 (2006) 1117–1122
Table 1 Main parameters of EAST device Toroidal field, Bo (T) Plasma current, IP (MA) Major radius, Ro (m) Minor radius, a (m) Aspect ratio, R/a Elongation, Kx Triangularity, dx Pulse length (s)
3.5 1 1.7 0.4 4.25 1.6–2 0.6–0.8 1–1000
a diameter of 7.6 m and a total weight of 360 tons. The main parameters of the EAST device are listed in Table 1. The vacuum vessel is one of the key components for this tokamak device. It must provide an ultra-high vacuum and clean environments for plasma operation. The EAST vacuum vessel consists of 16 sectors. During the final assembly, they are all field welded together to form a torus [1,2].
2. Design of vacuum vessel As one of the key components for the device, the vacuum vessel can provide ultra-high vacuum and clean environment for the plasma operation. It is a torus with “D”-shaped cross-section, double wall, upper vertical ports, lower vertical ports, horizontal ports and flexible supports, as shown in Fig. 1. The vessel is symmetrical about equator. The 316 L stainless steel was chosen as its material. Overall exterior dimensions of the vacuum vessel are 2.63 m in height, with the inner radius of 1.95 m and the outer radius of 2.75 m. The thickness of the inner and outer skins is 8 mm. The torus consists of 16 segments and each segment consists of inner shell, outer shell, ribs and ports. Two poloidal ribs, shown in Fig. 2, separate the outer and the inner shells, and give the required mechanical strength [2]. The ribs are welded to inner and outer shells by skip welding technique. Other two ribs (end ribs) are tightly welded both to inner shell and outer shell at segment end. Segments (one in every second segment) are welded together by end ribs. On the vacuum vessel, there are 16 horizontal ports and 32 vertical ports in total. Three types of horizontal ports and four types of vertical ports with different shapes and size are needed for diagnostics and test specifications. One port is dedicated to tangential neutron beam injection and
Fig. 1. Structure of the EAST vacuum vessel.
diagnostics. The effective aperture of the tangential port is 970 mm × 528 mm. Prior to operation, the vacuum vessel is to be baked out and discharge cleaned at about 250 ◦ C in order to get an ultra-high vacuum and a clean environment for plasma operation. The 350 ◦ C hot nitrogen gas is used for the vacuum vessel baking and electrical heaters are used for the baking of the ports. The non-uniformity of the temperature distribution on the vacuum vessel due to the difference in heater distribution and velocity of nitrogen gas can cause expansion movement and serious thermal stress of the structure. In order to consider this factor during
Fig. 2. Ribs between the outer and inner shells.
Y.t. Song et al. / Fusion Engineering and Design 81 (2006) 1117–1122
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Table 2 Stiffness coefficients calculated for each port Stiffness coefficient (KN/m)
X direction Y direction Z direction
Fig. 3. Support system of vacuum vessel.
the stage of design and to reduce the structure rigidity, a kind of low rigid structure support system has been designed, introducing two bellows on each port neck, which allow slight movements of the vacuum vessel in radial direction, as shown in Fig. 3. Each leg has 10 pieces of plates with 3 mm gap between them [7]. This structure can accommodate thermal deformations and small displacements caused by bake-out and other reasons so that it can protect the device.
3. Structural analysis of vacuum vessel Considering the symmetrical configuration of vacuum vessel, a model of 1/16 vacuum vessel is used for structure analyses. The cyclic symmetric conditions were applied to the toroidal edges of the sector. All the ports are allowed to move slightly along the vertical or horizontal directions thanks to the bellows installed on the port necks. The bottoms of all supports under the vacuum vessel can move and deform a little bit along the radial direction. Bellows on ports and low stiffness supports are considered as spring elements, as shown in Table 2. At the end of each port, proper rigidity is applied on the analysis model in X (radial), Y (vertical), Z (toroidal) directions. Each port end can rotate
Vertical port (including all of the bellows)
Horizontal port (including all of the bellows)
Supporting system of the vacuum vessel
1680 32 1492
160 2577 2630
146627 58479 1602
freely. The thickness of the vessel shells is 8 mm, ribs are 10 mm thick, ports and supports are 10 mm thick. The 316 L stainless steel is chosen as material. The vessel is double wall, because several different working conditions must be considered. During the vacuum test, the space between the two walls will be vacuum pumped, during plasma operation, it will be filled with shielding water, and during bake-out, hot nitrogen will flow through the interlayer. Being a superconducting tokamak, when the device is in operation both volumes inside and outside the vessel will be vacuum pumped. During plasma breakdown and disruptions, the electromagnetic forces are very strong. The vacuum vessel must withstand these individual and combined loading conditions during vacuum test, normal and off-normal operation. For the calculation of electromagnetic force during the plasma disruption, the plasma current (1 MA) was assumed to be a thin current ring, and the decay time is 3 ms. The maximum eddy current induced by the plasma disruption is 690 KA. The maximum force is 0.38 MPa, and is located near the interface area between the port and shells [3]. The calculation of the electromagnetic force due to the Halo current has been done based on the experimental data from some tokamak such as JT-60U [4] and C-MOD [5]. The total Halo current was assumed as ∼40% of the plasma current just before the current quench with the assumption of toroidal symmetry. The toroidal asymmetry factor was chosen as 2.5 due to the toroidal field ripple, the deformation of vacuum vessel, the setting error between the vacuum vessel and TF and PF coils, the low-n mode during current quench, etc. The maximum vertical force is 3000 KN and is around the area of the lower vertical port [6]. Two critical cases resulted from the analysis, one is the plasma disruption during plasma operation (100 ◦ C
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Y.t. Song et al. / Fusion Engineering and Design 81 (2006) 1117–1122
stress intensity and deformation of the vessel is shown in Fig. 4. Case 2 ((250 ◦ C baking for vacuum vessel)). Due to the baking at 250 ◦ C prior to the plasma operation, thermal expansion of the vacuum vessel produces some deformation. Since the bottom of the vessel support structure is constrained in vertical direction, stress concentration occurs in a small area. Fig. 5 shows the distribution of thermal stress and the deformation of the vessel due to 250 ◦ C baking. The maximum stress is 396 MPa and maximum displacement is 20.2 mm. The maximum stress is mainly appearing in the connecting area between the lower vertical port and shell.
4. Manufacture for the vacuum vessel Fig. 4. Distribution of stress intensity and deformation of the vacuum vessel (Case 1).
hot wall) and the other is the 250 ◦ C baking for vacuum vessel. Case 1 ((plasma disruption with thermal and pressure load due to normal operation)). The analysis has shown that the maximum stress intensity of 169 MPa including the thermal stress appears near the connecting area between upper vertical port and outer shell. The maximum displacement of 6.87 mm appears in the top of the upper vertical port. The distribution of the
4.1. Bellows on the port Since the thickness of the bellows material is only 0.8 mm, the spring back and distortion of welding is difficult to control. Therefore, a clamp was designed to control the welding distortion. The entire weld is made by the tungsten inert gas (TIG) process. Some interstage stress relieving was performed during welding. The main method of the stress relieving was knocking by manual work and vibration stress relief (VSR). After some tests, the welding parameters were confirmed, i.e. welding current equal to 10–15 A, flux of argon gas equal to 10 L/min. At the same time, the speed of welding must be controlled considering the welding distortion. A checkout, including the stress relieving and distortion controlling, was performed for every welded bellow. The welder also must be trained to be proficient in the whole welding process and to accurately control the speed of the welding. In order to get a good production and insure the quality of the vacuum surface, the welding bellows were cleaned by acetone and burnished by sand paper. The finished product is shown in Fig. 6. 4.2. Supporting system
Fig. 5. Distribution of stress intensity and deformation of the vacuum vessel (Case 2).
In order to simulate the axial and radial deformation of the EAST vacuum vessel for the experimental research during the 250 ◦ C baking and vacuum conditions, a test device was designed. It consists of a control
Y.t. Song et al. / Fusion Engineering and Design 81 (2006) 1117–1122
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Fig. 6. Finished product of the prototypical bellows.
system and a mechanical system [7]. They are shown in Fig. 7. The main function of the control system was to control the deformation of port and bellows by controlling the turn of the transducer, with a precision up to 1/20 mm. The mechanical system includes a movable thrust lever and an unmovable bearing. The thrust lever can impel the supporting plane of vacuum vessel and make the port and bellows bend along the horizontal direction, simulating the displacement of bellows during 250 ◦ C vacuum vessel baking. In the experiment, 87 pieces of strain gauge rosettes were selected. The size of each piece of gauge is 2 mm × 1 mm. The angle between the strain gauge is 45◦ . The main stress and its orientation were calculated for each strain gauge rosette in accordance with the Hookers law. The maximum stress on the bellows is 89 MPa, on the support system is 103 MPa and the bellow stiffness is 1224 N/mm, which are all in a good agreement with the finite element analysis results, in which the error is less than 6%. In addition, there is a good linear relation between
Fig. 7. Testing for the bellows and support system.
Fig. 8. The fabrication of a vacuum vessel section.
the stress and strain. After 1000 times fatigue test, the change of the structure is very little. It has been proved that the fatigue capability of bellows is very strong [7]. 4.3. Section of vacuum vessel The vacuum vessel consists of 16 segments. It is not a 16-sides polygon; the surface of the vacuum vessel is a torus with “D” shape cross-section. During fabrication, each segment will be made separately. Along the poloidal direction, inner shell and outer shell split into four pieces. Each piece is press formed to obtain the required shape at room temperature. It is difficult to control spring back of the plates at room temperature. In order to obtain acceptable precision for the shape, a special technology was developed. For the assembly of the shell pieces and ribs, a special technical facility was used. On this facility, first of all the inner shell pieces are welded together to obtain a “D”-shaped ring, then the ribs are welded to the inner shell, the outer shell pieces are welded to ribs and, finally, the outer shell pieces are welded together [6]. At the end, the ports are welded to connect the outer and inner shells of the vacuum vessel. During welding, the special technical facility can control the vessel section distortion, and vibration aging is used for each segment after welding to reduce residual stress. Fig. 8 shows the welded vacuum vessel sections without ports. The dimension error of this section is within 2 mm. The ports, flanges and supports are made separately. After all parts of the device are assembled together, then the ports with flanges are welded to ports connections.
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5. Summary Structural analysis was performed using the three dimensional finite element NASTRAN models for the welded bellows and support system of the EAST vacuum vessel. The results of finite element analysis have shown that the maximum stress intensity is 396 MPa during 250 ◦ C baking for vacuum vessel, which is within allowable design stress intensity 3Sm (441 MPa) based on the criteria of American Society of Mechanical Engineers, Boiler and Pressure Vessel Committee (ASME) [8]. At the same time a prototypical bellows, support system and a suitable mechanical testing system have been designed and fabricated to measure the strain and to verify the function and structure capability from the view of experimental mechanics. The experimental data indicated that the results of finite element analysis were coincident with experimental test results. It has been proved that the present vacuum vessel bellows and support system are reasonable and feasible. All the 16 sections of vacuum vessel have been fabricated and qualified, with dimension assembly error of only 2 mm.
Acknowledgements This work is supported by the National Natural Science Foundation of China, No. 10405024. All work presented herein is the work of the EAST Design Team
in the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). This work is also supported by the Shanghai Jialong Company and the Research & Manufacture Center of ASIPP. The authors would like to express their sincere appreciation to all the members who participated and supported this study.
References [1] S.T. Wu, W.Y. Wu, Y.T. Song, P.D. Weng, Design of the HT7U Tokamk Device 18th IEEE/NPSS Symposium Fusion Engineering, Albuquerque, New Mexico, October 25–29, 1999, pp. 549–552. [2] Y. Wan, Overview of steady state operation of HT-7 and present status of the EAST project, Nucl. Fusion 40 (June (6)) (2000) 1057–1068. [3] S.J. Du, G. Tao, Electrical parameters of the vacuum vessel in HT-7U tokamak, Plasma Sci. Technol. 3 (2) (2001) 703–708. [4] Y. Neyatani, R. Yoshino, T. Ando, Effect of Halo current and its toroidal asymmetry during disruptions in JT-60U, Fusion Technol. 28 (11) (1995) 1634–1643. [5] R.S. Granetz, I.H. Hutchinson, J. Sorci, J.H. Irby, B. Labomard, Disruptions and Halo current in Alcator C-MOD, Nucl. Fusion 36 (5) (1996) 545–556. [6] Design of EAST, ASIPP Report, Institute of Plasma Physics, Chinese Academy of Sciences, October 2003. [7] Y.T. Song, D.M. Yao, S.T. Wu, P.D. Weng, A lower rigid support system for HT-7U, Plasma Sci. Technol. 4 (3) (2002) 1289– 1296. [8] American Society of Mechanical Engineers, Boiler and Pressure Vessel Committee, ASME Boiler and Pressure Vessel Code. Section III, Rules for Construction of Nuclear Vessels, New York American Society of Mechanical Engineers, 1993.
Structural analysis and manufacture for the vacuum vessel of experimental advanced superconducting tokamak (EAST) device Yun tao Song ∗ , Damao Yao, Songata Wu, Peide Weng Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Anhui, Hefei 230031, PR China Received 8 January 2005; received in revised form 27 September 2005; accepted 27 September 2005 Available online 27 December 2005
Abstract The experimental advanced superconducting tokamak (EAST) is an advanced steady-state plasma physics experimental device, which has been approved by the Chinese government and is being constructed as the Chinese national nuclear fusion research project. The vacuum vessel, that is one of the key components, will have to withstand not only the electromagnetic force due to the plasma disruption and the Halo current, but also the pressure of boride water and the thermal stress due to the 250 ◦ C baking out by the hot pressure nitrogen gas, or the 100 ◦ C hot wall during plasma operation. This paper is a report of the mechanical analyses of the vacuum vessel. According to the allowable stress criteria of American Society of Mechanical Engineers, Boiler and Pressure Vessel Committee (ASME), the maximum integrated stress intensity on the vacuum vessel is 396 MPa, less than the allowable design stress intensity 3Sm (441 MPa). At the same time, some key R & D issues are presented, which include supporting system, bellows and the assembly of the whole vacuum vessel. © 2005 Elsevier B.V. All rights reserved. Keywords: Fusion device; Vacuum vessel; Mechanical analyses; Manufacturing of tokamak
1. Introduction The experimental advanced superconducting tokamak (EAST) device is based on a superconducting magnet system, which consists of 16 toroidal field (TF) coils and 14 poloidal field (PF) coils located ∗ Corresponding author. Tel.: +86 551 5593271; fax: +86 551 5591310. E-mail address: [email protected] (Y.t. Song).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.09.042
around the plasma mid-plane symmetrically. The central solenoidal (CS) assembly consists of six PF coils. A vacuum vessel with 16 rectangular horizontal ports and 32 bathtub-shaped vertical ports is located in the bore of the TF coil. A cryostat with two thermal shields at 80 K encloses the superconducting coils, the vacuum vessel and the support structures. The superconducting magnet system, the vacuum vessel and the thermal shields in the cryostat are supported independently. The EAST device has a height of 10 m (with the main support),
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Y.t. Song et al. / Fusion Engineering and Design 81 (2006) 1117–1122
Table 1 Main parameters of EAST device Toroidal field, Bo (T) Plasma current, IP (MA) Major radius, Ro (m) Minor radius, a (m) Aspect ratio, R/a Elongation, Kx Triangularity, dx Pulse length (s)
3.5 1 1.7 0.4 4.25 1.6–2 0.6–0.8 1–1000
a diameter of 7.6 m and a total weight of 360 tons. The main parameters of the EAST device are listed in Table 1. The vacuum vessel is one of the key components for this tokamak device. It must provide an ultra-high vacuum and clean environments for plasma operation. The EAST vacuum vessel consists of 16 sectors. During the final assembly, they are all field welded together to form a torus [1,2].
2. Design of vacuum vessel As one of the key components for the device, the vacuum vessel can provide ultra-high vacuum and clean environment for the plasma operation. It is a torus with “D”-shaped cross-section, double wall, upper vertical ports, lower vertical ports, horizontal ports and flexible supports, as shown in Fig. 1. The vessel is symmetrical about equator. The 316 L stainless steel was chosen as its material. Overall exterior dimensions of the vacuum vessel are 2.63 m in height, with the inner radius of 1.95 m and the outer radius of 2.75 m. The thickness of the inner and outer skins is 8 mm. The torus consists of 16 segments and each segment consists of inner shell, outer shell, ribs and ports. Two poloidal ribs, shown in Fig. 2, separate the outer and the inner shells, and give the required mechanical strength [2]. The ribs are welded to inner and outer shells by skip welding technique. Other two ribs (end ribs) are tightly welded both to inner shell and outer shell at segment end. Segments (one in every second segment) are welded together by end ribs. On the vacuum vessel, there are 16 horizontal ports and 32 vertical ports in total. Three types of horizontal ports and four types of vertical ports with different shapes and size are needed for diagnostics and test specifications. One port is dedicated to tangential neutron beam injection and
Fig. 1. Structure of the EAST vacuum vessel.
diagnostics. The effective aperture of the tangential port is 970 mm × 528 mm. Prior to operation, the vacuum vessel is to be baked out and discharge cleaned at about 250 ◦ C in order to get an ultra-high vacuum and a clean environment for plasma operation. The 350 ◦ C hot nitrogen gas is used for the vacuum vessel baking and electrical heaters are used for the baking of the ports. The non-uniformity of the temperature distribution on the vacuum vessel due to the difference in heater distribution and velocity of nitrogen gas can cause expansion movement and serious thermal stress of the structure. In order to consider this factor during
Fig. 2. Ribs between the outer and inner shells.
Y.t. Song et al. / Fusion Engineering and Design 81 (2006) 1117–1122
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Table 2 Stiffness coefficients calculated for each port Stiffness coefficient (KN/m)
X direction Y direction Z direction
Fig. 3. Support system of vacuum vessel.
the stage of design and to reduce the structure rigidity, a kind of low rigid structure support system has been designed, introducing two bellows on each port neck, which allow slight movements of the vacuum vessel in radial direction, as shown in Fig. 3. Each leg has 10 pieces of plates with 3 mm gap between them [7]. This structure can accommodate thermal deformations and small displacements caused by bake-out and other reasons so that it can protect the device.
3. Structural analysis of vacuum vessel Considering the symmetrical configuration of vacuum vessel, a model of 1/16 vacuum vessel is used for structure analyses. The cyclic symmetric conditions were applied to the toroidal edges of the sector. All the ports are allowed to move slightly along the vertical or horizontal directions thanks to the bellows installed on the port necks. The bottoms of all supports under the vacuum vessel can move and deform a little bit along the radial direction. Bellows on ports and low stiffness supports are considered as spring elements, as shown in Table 2. At the end of each port, proper rigidity is applied on the analysis model in X (radial), Y (vertical), Z (toroidal) directions. Each port end can rotate
Vertical port (including all of the bellows)
Horizontal port (including all of the bellows)
Supporting system of the vacuum vessel
1680 32 1492
160 2577 2630
146627 58479 1602
freely. The thickness of the vessel shells is 8 mm, ribs are 10 mm thick, ports and supports are 10 mm thick. The 316 L stainless steel is chosen as material. The vessel is double wall, because several different working conditions must be considered. During the vacuum test, the space between the two walls will be vacuum pumped, during plasma operation, it will be filled with shielding water, and during bake-out, hot nitrogen will flow through the interlayer. Being a superconducting tokamak, when the device is in operation both volumes inside and outside the vessel will be vacuum pumped. During plasma breakdown and disruptions, the electromagnetic forces are very strong. The vacuum vessel must withstand these individual and combined loading conditions during vacuum test, normal and off-normal operation. For the calculation of electromagnetic force during the plasma disruption, the plasma current (1 MA) was assumed to be a thin current ring, and the decay time is 3 ms. The maximum eddy current induced by the plasma disruption is 690 KA. The maximum force is 0.38 MPa, and is located near the interface area between the port and shells [3]. The calculation of the electromagnetic force due to the Halo current has been done based on the experimental data from some tokamak such as JT-60U [4] and C-MOD [5]. The total Halo current was assumed as ∼40% of the plasma current just before the current quench with the assumption of toroidal symmetry. The toroidal asymmetry factor was chosen as 2.5 due to the toroidal field ripple, the deformation of vacuum vessel, the setting error between the vacuum vessel and TF and PF coils, the low-n mode during current quench, etc. The maximum vertical force is 3000 KN and is around the area of the lower vertical port [6]. Two critical cases resulted from the analysis, one is the plasma disruption during plasma operation (100 ◦ C
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stress intensity and deformation of the vessel is shown in Fig. 4. Case 2 ((250 ◦ C baking for vacuum vessel)). Due to the baking at 250 ◦ C prior to the plasma operation, thermal expansion of the vacuum vessel produces some deformation. Since the bottom of the vessel support structure is constrained in vertical direction, stress concentration occurs in a small area. Fig. 5 shows the distribution of thermal stress and the deformation of the vessel due to 250 ◦ C baking. The maximum stress is 396 MPa and maximum displacement is 20.2 mm. The maximum stress is mainly appearing in the connecting area between the lower vertical port and shell.
4. Manufacture for the vacuum vessel Fig. 4. Distribution of stress intensity and deformation of the vacuum vessel (Case 1).
hot wall) and the other is the 250 ◦ C baking for vacuum vessel. Case 1 ((plasma disruption with thermal and pressure load due to normal operation)). The analysis has shown that the maximum stress intensity of 169 MPa including the thermal stress appears near the connecting area between upper vertical port and outer shell. The maximum displacement of 6.87 mm appears in the top of the upper vertical port. The distribution of the
4.1. Bellows on the port Since the thickness of the bellows material is only 0.8 mm, the spring back and distortion of welding is difficult to control. Therefore, a clamp was designed to control the welding distortion. The entire weld is made by the tungsten inert gas (TIG) process. Some interstage stress relieving was performed during welding. The main method of the stress relieving was knocking by manual work and vibration stress relief (VSR). After some tests, the welding parameters were confirmed, i.e. welding current equal to 10–15 A, flux of argon gas equal to 10 L/min. At the same time, the speed of welding must be controlled considering the welding distortion. A checkout, including the stress relieving and distortion controlling, was performed for every welded bellow. The welder also must be trained to be proficient in the whole welding process and to accurately control the speed of the welding. In order to get a good production and insure the quality of the vacuum surface, the welding bellows were cleaned by acetone and burnished by sand paper. The finished product is shown in Fig. 6. 4.2. Supporting system
Fig. 5. Distribution of stress intensity and deformation of the vacuum vessel (Case 2).
In order to simulate the axial and radial deformation of the EAST vacuum vessel for the experimental research during the 250 ◦ C baking and vacuum conditions, a test device was designed. It consists of a control
Y.t. Song et al. / Fusion Engineering and Design 81 (2006) 1117–1122
1121
Fig. 6. Finished product of the prototypical bellows.
system and a mechanical system [7]. They are shown in Fig. 7. The main function of the control system was to control the deformation of port and bellows by controlling the turn of the transducer, with a precision up to 1/20 mm. The mechanical system includes a movable thrust lever and an unmovable bearing. The thrust lever can impel the supporting plane of vacuum vessel and make the port and bellows bend along the horizontal direction, simulating the displacement of bellows during 250 ◦ C vacuum vessel baking. In the experiment, 87 pieces of strain gauge rosettes were selected. The size of each piece of gauge is 2 mm × 1 mm. The angle between the strain gauge is 45◦ . The main stress and its orientation were calculated for each strain gauge rosette in accordance with the Hookers law. The maximum stress on the bellows is 89 MPa, on the support system is 103 MPa and the bellow stiffness is 1224 N/mm, which are all in a good agreement with the finite element analysis results, in which the error is less than 6%. In addition, there is a good linear relation between
Fig. 7. Testing for the bellows and support system.
Fig. 8. The fabrication of a vacuum vessel section.
the stress and strain. After 1000 times fatigue test, the change of the structure is very little. It has been proved that the fatigue capability of bellows is very strong [7]. 4.3. Section of vacuum vessel The vacuum vessel consists of 16 segments. It is not a 16-sides polygon; the surface of the vacuum vessel is a torus with “D” shape cross-section. During fabrication, each segment will be made separately. Along the poloidal direction, inner shell and outer shell split into four pieces. Each piece is press formed to obtain the required shape at room temperature. It is difficult to control spring back of the plates at room temperature. In order to obtain acceptable precision for the shape, a special technology was developed. For the assembly of the shell pieces and ribs, a special technical facility was used. On this facility, first of all the inner shell pieces are welded together to obtain a “D”-shaped ring, then the ribs are welded to the inner shell, the outer shell pieces are welded to ribs and, finally, the outer shell pieces are welded together [6]. At the end, the ports are welded to connect the outer and inner shells of the vacuum vessel. During welding, the special technical facility can control the vessel section distortion, and vibration aging is used for each segment after welding to reduce residual stress. Fig. 8 shows the welded vacuum vessel sections without ports. The dimension error of this section is within 2 mm. The ports, flanges and supports are made separately. After all parts of the device are assembled together, then the ports with flanges are welded to ports connections.
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5. Summary Structural analysis was performed using the three dimensional finite element NASTRAN models for the welded bellows and support system of the EAST vacuum vessel. The results of finite element analysis have shown that the maximum stress intensity is 396 MPa during 250 ◦ C baking for vacuum vessel, which is within allowable design stress intensity 3Sm (441 MPa) based on the criteria of American Society of Mechanical Engineers, Boiler and Pressure Vessel Committee (ASME) [8]. At the same time a prototypical bellows, support system and a suitable mechanical testing system have been designed and fabricated to measure the strain and to verify the function and structure capability from the view of experimental mechanics. The experimental data indicated that the results of finite element analysis were coincident with experimental test results. It has been proved that the present vacuum vessel bellows and support system are reasonable and feasible. All the 16 sections of vacuum vessel have been fabricated and qualified, with dimension assembly error of only 2 mm.
Acknowledgements This work is supported by the National Natural Science Foundation of China, No. 10405024. All work presented herein is the work of the EAST Design Team
in the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). This work is also supported by the Shanghai Jialong Company and the Research & Manufacture Center of ASIPP. The authors would like to express their sincere appreciation to all the members who participated and supported this study.
References [1] S.T. Wu, W.Y. Wu, Y.T. Song, P.D. Weng, Design of the HT7U Tokamk Device 18th IEEE/NPSS Symposium Fusion Engineering, Albuquerque, New Mexico, October 25–29, 1999, pp. 549–552. [2] Y. Wan, Overview of steady state operation of HT-7 and present status of the EAST project, Nucl. Fusion 40 (June (6)) (2000) 1057–1068. [3] S.J. Du, G. Tao, Electrical parameters of the vacuum vessel in HT-7U tokamak, Plasma Sci. Technol. 3 (2) (2001) 703–708. [4] Y. Neyatani, R. Yoshino, T. Ando, Effect of Halo current and its toroidal asymmetry during disruptions in JT-60U, Fusion Technol. 28 (11) (1995) 1634–1643. [5] R.S. Granetz, I.H. Hutchinson, J. Sorci, J.H. Irby, B. Labomard, Disruptions and Halo current in Alcator C-MOD, Nucl. Fusion 36 (5) (1996) 545–556. [6] Design of EAST, ASIPP Report, Institute of Plasma Physics, Chinese Academy of Sciences, October 2003. [7] Y.T. Song, D.M. Yao, S.T. Wu, P.D. Weng, A lower rigid support system for HT-7U, Plasma Sci. Technol. 4 (3) (2002) 1289– 1296. [8] American Society of Mechanical Engineers, Boiler and Pressure Vessel Committee, ASME Boiler and Pressure Vessel Code. Section III, Rules for Construction of Nuclear Vessels, New York American Society of Mechanical Engineers, 1993.