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Evaluation of remote maintenance schemes by plasma equilibrium analysis in Tokamak DEMO reactor
Fusion Engineering and Design 89 (2014) 2588–2593
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Evaluation of remote maintenance schemes by plasma equilibrium analysis in Tokamak DEMO reactor Hiroyasu Utoh ∗ , Kenji Tobita, Nobuyuki Asakura, Yoshiteru Sakamoto Japan Atomic Energy Agency, Obuchi, Rokkasho-mura, Aomori-ken 039-3212, Japan
h i g h l i g h t s • The remote maintenance schemes in DEMO reactor were evaluated by the plasma equilibrium analysis. • Horizontal sector transport maintenance scheme requires the largest total PF coil current. • The difference of total PF coil current for MHD equilibrium in between the large segmented divertor maintenance and the segmentalized divertor maintenance was about 10%.
a r t i c l e
i n f o
Article history: Received 15 November 2013 Received in revised form 16 June 2014 Accepted 18 June 2014 Available online 19 July 2014 Keywords: Remote maintenance Plasma equilibrium Divertor maintenance Poloidal field coil DEMO
a b s t r a c t The remote maintenance schemes in a DEMO reactor are categorized by insertion direction, blanket segmentation, and divertor maintenance scheme, and are quantitatively evaluated by analysing the plasma equilibrium. The positions of the poloidal field (PF) coil are limited by the size of the toroidal field (TF) coil and the maintenance port layout of each remote maintenance scheme. Because the PF coils are located near the larger TF coil and far from the plasma surface, the horizontal sector transport maintenance scheme requires the largest part of total PF coil current, 25% larger than that required for separated sector transport using vertical maintenance ports with segmented divertor maintenance (SDM). In the unsegmented divertor maintenance (UDM) scheme, the total magnetic stored energy in the PF coils at plasma equilibrium is about 30% larger than that stored in the SDM scheme, but the time required for removal and installation of all the divertor cassettes in the UDM scheme is roughly a third of that required in the SDM scheme because the number of divertor cassettes in the UDM scheme is a third of that in the SDM scheme. From the viewpoint of simple maintenance operations, the merit of the UDM scheme has more merit than the SDM scheme. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Remote maintenance is a critical issue for a fusion DEMO reactor. To ensure high plant availability, the most time-consuming processes, such as in situ cutting/re-welding and inspection, must to be minimized. In this regard, a promising scheme in power plant concept studies for DEMO and power reactors is hot cell maintenance based on sector transport [1,2] or multi-module segment (MMS) in PPCS [3–5]. SlimCS, designed by the JAEA, adopts the sector transport hot cell maintenance scheme and accounts for (1)
∗ Corresponding author. Tel.: +81 175 71 6662. E-mail addresses: [email protected], [email protected] (H. Utoh), [email protected] (K. Tobita), [email protected] (N. Asakura), [email protected] (Y. Sakamoto). http://dx.doi.org/10.1016/j.fusengdes.2014.06.020 0920-3796/© 2014 Elsevier B.V. All rights reserved.
compatibility with the sector-wide conducting shell, (2) flexibility for access to core components, and (3) high availability [6–9] (Fig. 1). The sector transport maintenance scheme is advantageous for maintaining blanket and divertors without using sophisticated in-vessel remote handling devices, including devices that have a limited lifespan when exposed to the high radiation level of a fusion reactor. Additionally, in the sector maintenance scheme, the number of piping cutting/rewelding points is minimized. However, in the previous sector transport maintenance concepts, the divertor was replaced by a blanket module as a sector. Considering the scenario of remote maintenance, the replacement of divertors should preferably be independent of the replacement of blankets because the two have different replacement cycles. In addition, the sector transport maintenance schemes require a larger toroidal field (TF) coil and a larger maintenance port, especially for horizontal removal. In previous work, the remote-handling system was optimized by using Theory of Inventive Problem Solving (TRIZ) and
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Fig. 2. Overall divertor maintenance for (a) segmentalized divertor maintenance and (b) unsegmented divertor maintenance.
divertor cassettes, as for ITER divertor maintenance, or as unsegmented divertor cassettes, as shown in Fig. 2. The TF coil size is adjusted based on the direction in which blanket segments are inserted, the blanket segmentation, and the divertor-maintenance scheme. The PF coil positions are limited by the TF coil size and the maintenance port layout of each remote maintenance scheme. From the viewpoint of plasma equilibrium and PF coil current, this study identifies the following five cases (Fig. 3). 2.1. Case 1: Sector transport using a limited number of horizontal maintenance ports, SLH (Fig. 3(a))
Fig. 1. Conceptual view of (a) the sector transport using limited number of horizontal maintenance ports, SLH [9] and (b) Multi Module Segment, MMS [3].
Analytical Hierarchy Process (AHP) approach [10]. Evaluating the maintenance scheme by analyzing the plasma equilibrium is very important because the maintenance scheme affects the poloidal field (PF) coil design and the plasma equilibrium. Therefore, in the present work, we categorize the remote maintenance schemes for a DEMO reactor according to the insertion direction, blanket segmentation, and divertor-maintenance scheme and evaluate these schemes by analyzing the plasma equilibrium. 2. Various remote maintenance schemes for DEMO reactor This study roughly categorizes the remote maintenance schemes for a DEMO reactor according to (1) the insertion direction of blanket segments, (2) blanket segmentation, and (3) the divertor maintenance scheme. We assume that the divertor is replaced independently of blanket replacement; i.e., the blanket modules are removed after the divertor cassettes are removed. The directions in which the blanket segments are inserted are classified as either horizontal or vertical. The blanket segmentations for replacement are classified as coupled with inboard and outboard, corresponding to previous sector transport maintenance or as a separated sector as for the Multi Module Segment (MMS) maintenance scheme. The divertor maintenance schemes are classified as segmented
For horizontal sector transport in SLH [9], the vacuum vessel has large 16 m high horizontal ports, and the TF coils are larger than those for other maintenance schemes. The sector including the inboard and outboard blanket modules and the back plate (BP) was divided into 22.5◦ sectors (for a total of 16 sectors, which is the number of TF coils) in the toroidal direction. The divertor is segmented into 16 cassettes (cassettes are 22.5◦ wide which results in an equal number of cassettes. 2.2. Case 2: Sector transport using a limited number of vertical maintenance ports, SLV, with the segmented divertor maintenance, SDM (Fig. 3(b)-1) For SLV [11], the sector, including inboard and outboard blanket modules and the BP, is divided into 7.5◦ segments in the toroidal direction for a total of 48 sectors. The sector size is determined by the size of the vertical maintenance port. The divertor is segmented into 48 cassettes, each of which is 7.5◦ wide, as shown in Fig. 2(a). The divertor cassette can be removed and inserted through the lower divertor maintenance ports. In this case, the TF coil size is determined by the allowed plasma ripple, which is 0.5% on the plasma surface. In the vertical sector transport maintenance scheme, the upper maintenance port area is forbidden for PF coils. 2.3. Case 3: SLV with unsegmented divertor maintenance, UDM (Fig. 3(b)-2). In this case, the blanket is segmented into modules of the same size as for case 2. The divertor is segmented into sixteen cassettes (cassettes are 22.5◦ wide, which results in an equal number of
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Fig. 3. Layout of superconducting magnets and maintenance ports in each maintenance schemes.
cassettes and TF coils, and is replaced in a single radial direction, as shown in Fig. 2(b). For maintenance of the large divertor cassettes, the TF coils are designed to be larger than those in the SLV with SDM scheme (case 2), and the vacuum vessel has 5 m high horizontal ports.
2.4. Case 4: Separated sector transport using vertical maintenance ports (SSV) with SDM (Fig. 3(c)-1) In the SSV, the segment including blanket modules and the BP is divided into five segments: two inboard segments and three
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Table 1 Constant parameters for analyzing plasma equilibrium. Major radius, Rp Mainer radius, ap Aspect ratio, A Elongation, Triangularity, ı Plasma current Ip Toroidal magnetic field, BT Poloidal beta, ˇp Internal inductance, li
8.2 m 2.6 m 3.2 1.75 0.36 14.6 MA 6.3 T 1.8 0.9
outboard segments (32 inboard and 48 outboard for a total of 80 segments) in the toroidal direction. The divertor was segmented into 48 cassettes, with each cassettes being 7.5◦ wide. The divertor cassette can be removed and inserted through the lower divertormaintenance ports. In this case, the TF coil size is determined by the allowed plasma ripple, as for case 2. In this vertical maintenance scheme, the forbidden area for PF coils is smaller than that in the SLV because the replaced segments are smaller than those in the SLV. 2.5. Case 5: SSV with UDM (Fig. 3(c)-2) In this case, the blanket is segmented into modules of the same size as in case 4. However, as for case 3, the divertor is segmented into sixteen 22.5◦ cassettes, which is equal to the number of TF coils. For maintenance of the large divertor cassettes, the TF coils are larger than SSV with the SDM scheme and the vacuum vessel has 5-m-high horizontal ports. 3. Analysis of plasma equilibrium 3.1. TOSCA: code for analyzing plasma equilibrium In this study, the plasma equilibrium was analyzed by the twodimensional plasma equilibrium code called “Tokamak equilibrium and operation scenario with closed circuit coil analysis” (TOSCA) [12]. TOSCA is a two-dimensional free-boundary equilibrium code suitable for designing Tokamak experiments. It can calculate the best poloidal-coil current for given reference plasma parameters. 3.2. Dependence on maintenance schemes
Fig. 4. Equilibrium for (a) SLH maintenance scheme.
SLH becomes greater than that in the vertical maintenance schemes (b) and (c). The SLH maintenance scheme requires the largest total PF coil current, which is 25% larger than the minimum case (case 4). Fig. 6 indicates clearly that SLH imposes a large burden on the PF coils and power supply. Table 2 shows the total magnetic stored energy in the PF and TF coils for the five maintenance cases for high elongation and high triangularity. The magnetic stored energy for
Table 1 shows the constant parameters for analyzing the plasma equilibrium. All analyses were supplied the position of the common first-wall boundary. In the radial build, a gap of 0.2 m is preserved between the first wall and the separatrix on the inboard side. Since the SOL width at the inboard in depends on the aspect ratio and sol
on the presence of the central solenoid (CS) coil, in must be taken sol into account in the design and must be evaluated for the radial build of the DEMO reactor [13]. The main portion of the heat and particle flow from the core plasma circulates within the magnetic surface where the SOL is 3-cm-wide at the equatorial plane on the outboard side. The value of in must correspond to the SOL width of 3 cm at sol the equatorial plane on the outboard side and is evaluated by the equilibrium calculations. In this analysis, the inboard-SOL width for each maintenance scheme is adjusted to within 10 cm. The result of the equilibrium calculation is shown in Fig. 4 for the (a) SLH maintenance scheme. Fig. 5 shows the PF-coil current for each maintenance scheme. For the SLH, two PF coils (#4 and #5) out of the eight are located far from the equatorial plane, which is compatible with horizontal sector transport maintenance. However, for vertical maintenance schemes [(b) SLV and (c) SSV], two PF coils (#4 and #5) out of the eight are located near the outer equatorial plane (Fig. 3). Therefore, the PF coil current, especially for #4 and #5 in (a)
Fig. 5. PF coil current for each maintenance scheme.
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Table 2 Total magnetic stored energy of TF and PF coils for each maintenance scheme.
TF coil Reference case (Table 1) High elongation case ( ∼ 2.0) High triangularity case (ı ∼ 0.5)
Case 1: SLH
Case 2: SLV-SDM
Case 3: SLV-UDM
Case 4: SSV-SDM
Case 5: SSV-UDM
160 GJ 69 GJ 135 GJ 54 GJ
150 GJ 27 GJ 18 GJ 20 GJ
154 GJ 38 GJ 24 GJ 27 GJ
150 GJ 27 GJ 18 GJ 18 GJ
154 GJ 35 GJ 24 GJ 25 GJ
SLH is more than twice as large as for the other schemes, and the SLH has less flexibility for higher elongation range.
4. Discussion 4.1. Divertor maintenance In this analysis of the plasma equilibrium, we compared SDM with UDM and find that the PF coil current (#5) and the divertor coil current (#7) in the UDM scheme are higher than in the SDM scheme. The total energy stored in the PF coil in the UDM scheme is approximately 30% greater than that stored in the SDM scheme. In addition, the TF coils are smaller and less magnetic energy is stored therein for the SDM scheme than for the UDM scheme. From the viewpoint of plasma equilibrium and designing superconducting magnets, the SDM scheme has more merit. However, considering divertor maintenance, the time required for removal and installation of all divertor cassettes for the UDM scheme is simply a third of that required for the SDM scheme because the number of divertor cassettes in the UDM scheme (16) is a third of that in the SDM scheme (48). In addition, for the UDM scheme, the unsegmented divertor cassette (22.5◦ wide) was transported in a single, radial direction, as shown in Fig. 2. For the UDM scheme, the cutting/rewelding of the divertor coolant pipe is easier to inspect because it is done outside of the reactor. These points are especially important if divertor maintenance is required annually. However, the UDM scheme requires a heavier divertor-cassette maintenance machine to be developed, which would be a key issue in DEMO divertor remote handling, as shown in Ref. [14]. These results show that the divertor maintenance scheme should be determined by the DEMO design concept, with these tradeoffs and issues taken into account.
4.2. Segmentation of blanket Comparing the SLV maintenance scheme with the SSV maintenance scheme reveals that the total PF coil current for the former is a few percent greater than that for the latter. From the viewpoint of the blanket maintenance operation, less cutting/rewelding of piping is required under the SLV maintenance scheme than under the SSV maintenance scheme because the number of segments for SLV (48) is less than that for SSV (32 inboard and 48 outboard for a total of 80). However, the SLV maintenance scheme raises the engineering problems of the sector support method and the transfer mechanism of sector in the vacuum vessel. In addition, considering the plasma vertical stability control by the PF coils, the position and current of the upper PF coil (#3) would be important. Therefore, the blanket segmentation (maintenance port size) should be determined based on all the viewpoints. 5. Summary The remote maintenance scheme in a DEMO reactor is categorized according to the insertion direction, blanket segmentation, and divertor maintenance scheme, which are evaluated by quantitatively analyzing the plasma equilibrium. For each remotemaintenance scheme, the PF coil positions are limited by the TF coil size and maintenance port layout. The horizontal sector transport maintenance scheme requires the largest total PF coil current: 25% larger than that required for separated sector transport using the vertical maintenance ports with segmented divertor maintenance. The reason for this is that two PF coils (#4 and #5) out of eight are located far from the equatorial plane, and far from the plasma surface, which is near the larger TF coil. The total magnetic stored energy of PF coils for magnetohydrodynamic equilibrium in the UDM scheme is about 30% greater than that under the SDM scheme. However, the time required to remove and install all divertor cassettes under UDM is a third of that required under SDM because the number of divertor cassettes for UDM (16) is a third of that for SDM (48). From the viewpoint of simple maintenance, the UDM scheme has more merit than the SDM scheme. However, the UDM scheme requires developing a heavier divertor-cassette maintenance machine, so the divertor-maintenance scheme should be determined by the DEMO design concept, taking into account these tradeoffs and issues. For a vertical maintenance scheme, the total PF coil current for magnetohydrodynamic equilibrium weakly depends on maintenance port size. The blanket segmentation (maintenance port size) should be determined by considering all the viewpoints, the reliability of the maintenance operation, and the control of the plasma vertical stability by the PF coils. Acknowledgment This work was partly supported by the Broader Approach DEMO Design Activity. References
Fig. 6. PF coil stored energy for each maintenance scheme.
[1] S. Malang, F. Najmabadi, L.M. Waganer, M.S. Tillack, ARIES-RS maintenance approach for high availability, Fusion Eng. Des. 41 (1998) 377.
H. Utoh et al. / Fusion Engineering and Design 89 (2014) 2588–2593 [2] L.M. Waganer, F. Najmabadi, M. Tillack, X. Wang, L. El-Guebaly, The ARIES Team, Design approach of the ARIES-AT power core and vacuum vessel cost assessment, Fusion Eng. Des. 80 (2006) 181. [3] T. Ihli, L.V. Boccaccini, G. Janeschitz, C. Koehly, D. Maisonnier, D. Nagy, et al., Recent progress in DEMO fusion core engineering: Improved segmentation, maintenance and blanket concepts, Fusion Eng. Des. 82 (2007) 2705. [4] D. Nagy, T. Ihli, D. Filsinger, R. Gottfried, DEMO maintenance scenarios: Scheme for time estimations and preliminary estimates for blankets arranged in multimodule segments, Fusion Eng. Des. 83 (2008) 1870. [5] D. Maisonnier, European DEMO design and maintenance strategy, Fusion Eng. Des. 83 (2008) 858. [6] K. Tobita, S. Nishio, M. Enoeda, M. Sato, T. Isono, S. Sakurai, et al., Design study of fusion DEMO plant at JAERI, Fusion Eng. Des. 81 (2006) 1151. [7] K. Tobita, S. Nishio, M. Enoeda, H. Kawashima, G. Kurita, H. Tanigawa, et al., Compact DEMO, SlimCS: design progress and issues, Nuclear Fusion 49 (2009) 075029.
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[8] K. Tobita, U. Utoh, S. Kakudate, H. Takase, N. Asakura, Y. Someya, et al., Maintenance concept for the SlimCS DEMO reactor, Fusion Eng. Des. 86 (2011) 2730. [9] H. Utoh, Y. Someya, K. Tobita, N. Asakura, K. Hoshino, M. Nakamura, Critical Design Factors for Sector Transport Maintenance in DEMO, Nuclear Fusion 53 (2013) 123005. [10] G. Di Gironimo, D. Carfora, G. Esposito, C. Labate, R. Mozzillo, F. Renno, et al., Improving concept design of divertor support system for FAST tokamak using TRIZ theory and AHP approach, Fusion Eng. Des. 88 (2013) 3014–3020. [11] H. Utoh, K. Tobita, Y. Someya, H. Takase, Conceptual study of vertical sector transport maintenance for DEMO fusion reactor, Fusion Eng. Des. 87 (2012) 1409–1413. [12] K. Shinya, J. Plasma Fusion Res. 76 (2000) 479. [13] M. Sato, S. Sakurai, S. Nishio, K. Tobita, T. Inoue, Y. Nakamura, et al., Concept of core and divertor plasma for fusion DEMO plant at JAERI, Fusion Eng. Des. 81 (2006) 1277. [14] D. Nagy, J. Bonnemason, J.-P. Friconneau, Y. Perrot, DEMO divertor maintenance, Fusion Eng. Des. 84 (2009) 1388–1393.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Evaluation of remote maintenance schemes by plasma equilibrium analysis in Tokamak DEMO reactor Hiroyasu Utoh ∗ , Kenji Tobita, Nobuyuki Asakura, Yoshiteru Sakamoto Japan Atomic Energy Agency, Obuchi, Rokkasho-mura, Aomori-ken 039-3212, Japan
h i g h l i g h t s • The remote maintenance schemes in DEMO reactor were evaluated by the plasma equilibrium analysis. • Horizontal sector transport maintenance scheme requires the largest total PF coil current. • The difference of total PF coil current for MHD equilibrium in between the large segmented divertor maintenance and the segmentalized divertor maintenance was about 10%.
a r t i c l e
i n f o
Article history: Received 15 November 2013 Received in revised form 16 June 2014 Accepted 18 June 2014 Available online 19 July 2014 Keywords: Remote maintenance Plasma equilibrium Divertor maintenance Poloidal field coil DEMO
a b s t r a c t The remote maintenance schemes in a DEMO reactor are categorized by insertion direction, blanket segmentation, and divertor maintenance scheme, and are quantitatively evaluated by analysing the plasma equilibrium. The positions of the poloidal field (PF) coil are limited by the size of the toroidal field (TF) coil and the maintenance port layout of each remote maintenance scheme. Because the PF coils are located near the larger TF coil and far from the plasma surface, the horizontal sector transport maintenance scheme requires the largest part of total PF coil current, 25% larger than that required for separated sector transport using vertical maintenance ports with segmented divertor maintenance (SDM). In the unsegmented divertor maintenance (UDM) scheme, the total magnetic stored energy in the PF coils at plasma equilibrium is about 30% larger than that stored in the SDM scheme, but the time required for removal and installation of all the divertor cassettes in the UDM scheme is roughly a third of that required in the SDM scheme because the number of divertor cassettes in the UDM scheme is a third of that in the SDM scheme. From the viewpoint of simple maintenance operations, the merit of the UDM scheme has more merit than the SDM scheme. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Remote maintenance is a critical issue for a fusion DEMO reactor. To ensure high plant availability, the most time-consuming processes, such as in situ cutting/re-welding and inspection, must to be minimized. In this regard, a promising scheme in power plant concept studies for DEMO and power reactors is hot cell maintenance based on sector transport [1,2] or multi-module segment (MMS) in PPCS [3–5]. SlimCS, designed by the JAEA, adopts the sector transport hot cell maintenance scheme and accounts for (1)
∗ Corresponding author. Tel.: +81 175 71 6662. E-mail addresses: [email protected], [email protected] (H. Utoh), [email protected] (K. Tobita), [email protected] (N. Asakura), [email protected] (Y. Sakamoto). http://dx.doi.org/10.1016/j.fusengdes.2014.06.020 0920-3796/© 2014 Elsevier B.V. All rights reserved.
compatibility with the sector-wide conducting shell, (2) flexibility for access to core components, and (3) high availability [6–9] (Fig. 1). The sector transport maintenance scheme is advantageous for maintaining blanket and divertors without using sophisticated in-vessel remote handling devices, including devices that have a limited lifespan when exposed to the high radiation level of a fusion reactor. Additionally, in the sector maintenance scheme, the number of piping cutting/rewelding points is minimized. However, in the previous sector transport maintenance concepts, the divertor was replaced by a blanket module as a sector. Considering the scenario of remote maintenance, the replacement of divertors should preferably be independent of the replacement of blankets because the two have different replacement cycles. In addition, the sector transport maintenance schemes require a larger toroidal field (TF) coil and a larger maintenance port, especially for horizontal removal. In previous work, the remote-handling system was optimized by using Theory of Inventive Problem Solving (TRIZ) and
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Fig. 2. Overall divertor maintenance for (a) segmentalized divertor maintenance and (b) unsegmented divertor maintenance.
divertor cassettes, as for ITER divertor maintenance, or as unsegmented divertor cassettes, as shown in Fig. 2. The TF coil size is adjusted based on the direction in which blanket segments are inserted, the blanket segmentation, and the divertor-maintenance scheme. The PF coil positions are limited by the TF coil size and the maintenance port layout of each remote maintenance scheme. From the viewpoint of plasma equilibrium and PF coil current, this study identifies the following five cases (Fig. 3). 2.1. Case 1: Sector transport using a limited number of horizontal maintenance ports, SLH (Fig. 3(a))
Fig. 1. Conceptual view of (a) the sector transport using limited number of horizontal maintenance ports, SLH [9] and (b) Multi Module Segment, MMS [3].
Analytical Hierarchy Process (AHP) approach [10]. Evaluating the maintenance scheme by analyzing the plasma equilibrium is very important because the maintenance scheme affects the poloidal field (PF) coil design and the plasma equilibrium. Therefore, in the present work, we categorize the remote maintenance schemes for a DEMO reactor according to the insertion direction, blanket segmentation, and divertor-maintenance scheme and evaluate these schemes by analyzing the plasma equilibrium. 2. Various remote maintenance schemes for DEMO reactor This study roughly categorizes the remote maintenance schemes for a DEMO reactor according to (1) the insertion direction of blanket segments, (2) blanket segmentation, and (3) the divertor maintenance scheme. We assume that the divertor is replaced independently of blanket replacement; i.e., the blanket modules are removed after the divertor cassettes are removed. The directions in which the blanket segments are inserted are classified as either horizontal or vertical. The blanket segmentations for replacement are classified as coupled with inboard and outboard, corresponding to previous sector transport maintenance or as a separated sector as for the Multi Module Segment (MMS) maintenance scheme. The divertor maintenance schemes are classified as segmented
For horizontal sector transport in SLH [9], the vacuum vessel has large 16 m high horizontal ports, and the TF coils are larger than those for other maintenance schemes. The sector including the inboard and outboard blanket modules and the back plate (BP) was divided into 22.5◦ sectors (for a total of 16 sectors, which is the number of TF coils) in the toroidal direction. The divertor is segmented into 16 cassettes (cassettes are 22.5◦ wide which results in an equal number of cassettes. 2.2. Case 2: Sector transport using a limited number of vertical maintenance ports, SLV, with the segmented divertor maintenance, SDM (Fig. 3(b)-1) For SLV [11], the sector, including inboard and outboard blanket modules and the BP, is divided into 7.5◦ segments in the toroidal direction for a total of 48 sectors. The sector size is determined by the size of the vertical maintenance port. The divertor is segmented into 48 cassettes, each of which is 7.5◦ wide, as shown in Fig. 2(a). The divertor cassette can be removed and inserted through the lower divertor maintenance ports. In this case, the TF coil size is determined by the allowed plasma ripple, which is 0.5% on the plasma surface. In the vertical sector transport maintenance scheme, the upper maintenance port area is forbidden for PF coils. 2.3. Case 3: SLV with unsegmented divertor maintenance, UDM (Fig. 3(b)-2). In this case, the blanket is segmented into modules of the same size as for case 2. The divertor is segmented into sixteen cassettes (cassettes are 22.5◦ wide, which results in an equal number of
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Fig. 3. Layout of superconducting magnets and maintenance ports in each maintenance schemes.
cassettes and TF coils, and is replaced in a single radial direction, as shown in Fig. 2(b). For maintenance of the large divertor cassettes, the TF coils are designed to be larger than those in the SLV with SDM scheme (case 2), and the vacuum vessel has 5 m high horizontal ports.
2.4. Case 4: Separated sector transport using vertical maintenance ports (SSV) with SDM (Fig. 3(c)-1) In the SSV, the segment including blanket modules and the BP is divided into five segments: two inboard segments and three
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Table 1 Constant parameters for analyzing plasma equilibrium. Major radius, Rp Mainer radius, ap Aspect ratio, A Elongation, Triangularity, ı Plasma current Ip Toroidal magnetic field, BT Poloidal beta, ˇp Internal inductance, li
8.2 m 2.6 m 3.2 1.75 0.36 14.6 MA 6.3 T 1.8 0.9
outboard segments (32 inboard and 48 outboard for a total of 80 segments) in the toroidal direction. The divertor was segmented into 48 cassettes, with each cassettes being 7.5◦ wide. The divertor cassette can be removed and inserted through the lower divertormaintenance ports. In this case, the TF coil size is determined by the allowed plasma ripple, as for case 2. In this vertical maintenance scheme, the forbidden area for PF coils is smaller than that in the SLV because the replaced segments are smaller than those in the SLV. 2.5. Case 5: SSV with UDM (Fig. 3(c)-2) In this case, the blanket is segmented into modules of the same size as in case 4. However, as for case 3, the divertor is segmented into sixteen 22.5◦ cassettes, which is equal to the number of TF coils. For maintenance of the large divertor cassettes, the TF coils are larger than SSV with the SDM scheme and the vacuum vessel has 5-m-high horizontal ports. 3. Analysis of plasma equilibrium 3.1. TOSCA: code for analyzing plasma equilibrium In this study, the plasma equilibrium was analyzed by the twodimensional plasma equilibrium code called “Tokamak equilibrium and operation scenario with closed circuit coil analysis” (TOSCA) [12]. TOSCA is a two-dimensional free-boundary equilibrium code suitable for designing Tokamak experiments. It can calculate the best poloidal-coil current for given reference plasma parameters. 3.2. Dependence on maintenance schemes
Fig. 4. Equilibrium for (a) SLH maintenance scheme.
SLH becomes greater than that in the vertical maintenance schemes (b) and (c). The SLH maintenance scheme requires the largest total PF coil current, which is 25% larger than the minimum case (case 4). Fig. 6 indicates clearly that SLH imposes a large burden on the PF coils and power supply. Table 2 shows the total magnetic stored energy in the PF and TF coils for the five maintenance cases for high elongation and high triangularity. The magnetic stored energy for
Table 1 shows the constant parameters for analyzing the plasma equilibrium. All analyses were supplied the position of the common first-wall boundary. In the radial build, a gap of 0.2 m is preserved between the first wall and the separatrix on the inboard side. Since the SOL width at the inboard in depends on the aspect ratio and sol
on the presence of the central solenoid (CS) coil, in must be taken sol into account in the design and must be evaluated for the radial build of the DEMO reactor [13]. The main portion of the heat and particle flow from the core plasma circulates within the magnetic surface where the SOL is 3-cm-wide at the equatorial plane on the outboard side. The value of in must correspond to the SOL width of 3 cm at sol the equatorial plane on the outboard side and is evaluated by the equilibrium calculations. In this analysis, the inboard-SOL width for each maintenance scheme is adjusted to within 10 cm. The result of the equilibrium calculation is shown in Fig. 4 for the (a) SLH maintenance scheme. Fig. 5 shows the PF-coil current for each maintenance scheme. For the SLH, two PF coils (#4 and #5) out of the eight are located far from the equatorial plane, which is compatible with horizontal sector transport maintenance. However, for vertical maintenance schemes [(b) SLV and (c) SSV], two PF coils (#4 and #5) out of the eight are located near the outer equatorial plane (Fig. 3). Therefore, the PF coil current, especially for #4 and #5 in (a)
Fig. 5. PF coil current for each maintenance scheme.
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Table 2 Total magnetic stored energy of TF and PF coils for each maintenance scheme.
TF coil Reference case (Table 1) High elongation case ( ∼ 2.0) High triangularity case (ı ∼ 0.5)
Case 1: SLH
Case 2: SLV-SDM
Case 3: SLV-UDM
Case 4: SSV-SDM
Case 5: SSV-UDM
160 GJ 69 GJ 135 GJ 54 GJ
150 GJ 27 GJ 18 GJ 20 GJ
154 GJ 38 GJ 24 GJ 27 GJ
150 GJ 27 GJ 18 GJ 18 GJ
154 GJ 35 GJ 24 GJ 25 GJ
SLH is more than twice as large as for the other schemes, and the SLH has less flexibility for higher elongation range.
4. Discussion 4.1. Divertor maintenance In this analysis of the plasma equilibrium, we compared SDM with UDM and find that the PF coil current (#5) and the divertor coil current (#7) in the UDM scheme are higher than in the SDM scheme. The total energy stored in the PF coil in the UDM scheme is approximately 30% greater than that stored in the SDM scheme. In addition, the TF coils are smaller and less magnetic energy is stored therein for the SDM scheme than for the UDM scheme. From the viewpoint of plasma equilibrium and designing superconducting magnets, the SDM scheme has more merit. However, considering divertor maintenance, the time required for removal and installation of all divertor cassettes for the UDM scheme is simply a third of that required for the SDM scheme because the number of divertor cassettes in the UDM scheme (16) is a third of that in the SDM scheme (48). In addition, for the UDM scheme, the unsegmented divertor cassette (22.5◦ wide) was transported in a single, radial direction, as shown in Fig. 2. For the UDM scheme, the cutting/rewelding of the divertor coolant pipe is easier to inspect because it is done outside of the reactor. These points are especially important if divertor maintenance is required annually. However, the UDM scheme requires a heavier divertor-cassette maintenance machine to be developed, which would be a key issue in DEMO divertor remote handling, as shown in Ref. [14]. These results show that the divertor maintenance scheme should be determined by the DEMO design concept, with these tradeoffs and issues taken into account.
4.2. Segmentation of blanket Comparing the SLV maintenance scheme with the SSV maintenance scheme reveals that the total PF coil current for the former is a few percent greater than that for the latter. From the viewpoint of the blanket maintenance operation, less cutting/rewelding of piping is required under the SLV maintenance scheme than under the SSV maintenance scheme because the number of segments for SLV (48) is less than that for SSV (32 inboard and 48 outboard for a total of 80). However, the SLV maintenance scheme raises the engineering problems of the sector support method and the transfer mechanism of sector in the vacuum vessel. In addition, considering the plasma vertical stability control by the PF coils, the position and current of the upper PF coil (#3) would be important. Therefore, the blanket segmentation (maintenance port size) should be determined based on all the viewpoints. 5. Summary The remote maintenance scheme in a DEMO reactor is categorized according to the insertion direction, blanket segmentation, and divertor maintenance scheme, which are evaluated by quantitatively analyzing the plasma equilibrium. For each remotemaintenance scheme, the PF coil positions are limited by the TF coil size and maintenance port layout. The horizontal sector transport maintenance scheme requires the largest total PF coil current: 25% larger than that required for separated sector transport using the vertical maintenance ports with segmented divertor maintenance. The reason for this is that two PF coils (#4 and #5) out of eight are located far from the equatorial plane, and far from the plasma surface, which is near the larger TF coil. The total magnetic stored energy of PF coils for magnetohydrodynamic equilibrium in the UDM scheme is about 30% greater than that under the SDM scheme. However, the time required to remove and install all divertor cassettes under UDM is a third of that required under SDM because the number of divertor cassettes for UDM (16) is a third of that for SDM (48). From the viewpoint of simple maintenance, the UDM scheme has more merit than the SDM scheme. However, the UDM scheme requires developing a heavier divertor-cassette maintenance machine, so the divertor-maintenance scheme should be determined by the DEMO design concept, taking into account these tradeoffs and issues. For a vertical maintenance scheme, the total PF coil current for magnetohydrodynamic equilibrium weakly depends on maintenance port size. The blanket segmentation (maintenance port size) should be determined by considering all the viewpoints, the reliability of the maintenance operation, and the control of the plasma vertical stability by the PF coils. Acknowledgment This work was partly supported by the Broader Approach DEMO Design Activity. References
Fig. 6. PF coil stored energy for each maintenance scheme.
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