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Performance of the TAURO blanket system associated with a liquid-metal cooled divertor
Fusion Engineering and Design 49 – 50 (2000) 559 – 565 www.elsevier.com/locate/fusengdes
Performance of the TAURO blanket system associated with a liquid-metal cooled divertor H. Golfier a,*, G. Aiello a, M.A. Fu¨tterer a, A. Gasse b, L. Giancarli a, Y. Poitevin a, J.F. Salavy a, Y. Severi c, J. Szczepanski a b
a CEA Saclay, DRN/DMT/SERMA, F-91191 Gif-sur-Y6ette Cedex, France CEA Grenoble, DTA-CEREM/DEM/SGM, F-38054 Grenoble Cedex 9, France c CEA Cadarache, DRN/DER/STPI, F-13108 Saint Paul-lez-Durance, France
Abstract Blankets and divertors are the key components of a fusion power plant due to their large impact on the overall plant design, performance and availability, and on the cost of electricity. This paper recalls the main features of the TAURO blanket, a self-cooled Pb–17Li concept using SiCf/SiC composite as structural material, and describes the most recent thermo-mechanical analyses performed on the blanket. It includes an evaluation of a liquid-metal cooled divertor which could be associated with such a blanket. Investigations were carried out to determine the potential and limits of the blanket and of a liquid-metal cooled divertor in terms of maximum acceptable surface heat flux. The high outlet temperature of the Pb–17Li coolant (800°C) leads to an attractive energy conversion efficiency ( \ 47%) assuming helium-coolant for the secondary circuit. Special heat exchangers, using SiCf/SiC tubes are envisaged. Thermo-mechanical analyses have pointed out that maximum surface heat flux on FW and divertor could be about 0.65 and B 5 MW/m2, respectively. It is expected that an improvement of the SiCf/SiC modeling, taking into account non linear behavior presently under way, and the use of design criteria adapted to more advanced composites could allow even higher limits. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Liquid metal divertor; TAURO blanket; Pb – 17Li cooling; Fusion power plant
1. Introduction The TAURO blanket is a self-cooled Pb–17Li blanket using SiCf/SiC structures [1]. The rational of this concept is based on the requirement of passive safety of tokamak-type Fusion Power R * Corresponding author. Tel.: +33-1-69083287; fax: + 331-69089935. E-mail address: [email protected] (H. Golfier).
actors (FPRs). This is obtained by limiting the energy available within the first safety barrier which for tokamaks is generally the vacuum vessel, in order to avoid the possibility of its braking and the subsequent accidental release of radioactive materials. The main objectives of the TAURO blanket design studies are the establishment of the major required improvements of the characteristics of the present-day SiCf/SiC composites; low thermal
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H. Golfier et al. / Fusion Engineering and Design 49–50 (2000) 559–565
conductivity and low neutron irradiation resistance have been identified as the most severe issues [2,3].
2. TAURO breeder blanket concept
2.1. Specifications and design description The design is based on the specifications for the SEAFP study [4], i.e. fusion power of 3000 MW, neutron wall load of 2 MW/m2 and surface heat flux of 0.5 MW/m2. The TAURO blanket is essentially formed by a SiCf/SiC box with an indirectly-cooled First Wall (FW) which acts as a container for the Pb – 17Li. The Pb – 17Li acts as coolant, breeder and multiplier material, and tritium carrier. Each outboard segment is poloidally divided into several straight modules, attached on one common thick back-plate, but cooled independently.
Fig. 1. TAURO blanket design, outboard module.
The feeding pipes are located behind the module. The coolant enters the inlet collector through a single tube and is divided into five sub-flows, one for each sub-module (Fig. 1). Within each sub-module the Pb–17Li flows at first poloidally downward in a thin channel located just behind the FW, makes a U-turn at the bottom into a second channel and flows up, then down into the outlet collector. Toroidal plates (stiffeners) are required for reinforcing the sub-module box against the hydrostatic pressure (assumed to be 1.5 MPa in all calculations) and act as flow separators [5]. Manufacturing of the module depends on the possibility of joining SiCf/SiC components. A promising brazing technique using a braze material compatible with SiC, the Brasic®, is currently under development [6].
3. Design criteria and models The design optimization and structural assessment of the TAURO blanket and Pb–17Li divertor were performed in 3D and 2D geometry, respectively assuming square meshes and an orthotropic model of SiCf/SiC. Due account was taken of the poloidal variation in heat transfer, coolant conditions, mechanical loads and thermal loads. Conduction remains the dominant heat transfer mechanism, the thermal conductivity for SiCf/SiC was optimistically assumed as 15 W/mK. The other assumed SiCf/SiC characteristics were those of the industrial Cerasep® N3-1 [2]. New design criteria, specific for SiCf/SiC structures, have been recently defined [5] and applied in the thermo-mechanical evaluations. They are the following: the von Mises stresses in the plane are limited to 145 MPa; the normal stresses through the thickness are limited to 110 MPa; the shear stresses are limited to 45 MPa. These limits have been determined using experimental results obtained several years ago on Cerasep® N2-1 and are, therefore, probably too pessimistic when applied to present-day SiCf/SiC or to future improved composites.
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561
tolerated in the TAURO design assuming that the ratio between surface heat flux and neutron wall loading on the FW remains constant. The performance of the TAURO blanket was estimated by varying the inlet temperature, the heat flux level, and some design parameters.
4.2. Temperature limits The Pb–17Li inlet temperature was modified in the range 275–750°C in order to see the impact on the maximum SiCf/SiC temperature which was found in all cases to be below 1300°C. For a surface heat flux of 0.5 MW/m2, the corresponding Pb–17Li outlet temperature ranges between 500 and 900°C. Fig. 2 shows a typical temperature distribution in the sub-module. (aggrandir les chiffres)
4.3. Stress limits Fig. 2. Temperature distribution in a TAURO sub-module.
Further constraints are: maximum SiCf/SiC temperature of 1300°C (expected for future composites based on HiNicalon fibers); average coolant outlet temperature \ 800°C to achieve a conversion efficiency \ 45%.
4. New evaluations for TAURO blanket New thermal and thermo-mechanical analyses, focused on the outboard blanket in the midplane region where the thermal conditions are the most severe, have been performed with CASTEM2000. The evaluated maximum surface heat flux has been kept along the whole sub-module height.
4.1. Blanket thermo-mechanical limits Among the criteria described in Section 2, those related to stresses are the most severe to meet due to the relatively low thermal conductivity of SiCf/ SiC. It was decided to verify what was the maximum surface heat flux on the FW which can be
The highest stresses appear on the FW. For a Pb–17Li velocity in the first layer of 1 m/s, a FW thickness of 6 mm, a module width of 0.3 m, and a module height of 2.15 m, the maximum von Mises stress in plane is 150 MPa (just above the limit) and the maximum shear stress is 40 MPa (below the limit). However, the maximum normal stress through the FW thickness exceeds the limit of the order of 20% [5]. In order to fit all design rules simultaneously, modifications of the above parameters were attempted. In all cases stresses are dominated by the temperature gradients on the FW (through height and thickness) which can be reduced by a reduction of the height of the module to reduce the DT between top and bottom of FW; increasing the Pb–17Li velocity to attenuate the radial and poloidal DT; reducing the width of the box. The requirement can be satisfied by assuming a Pb–17Li velocity in the first layer of 1.3 m/s, a FW thickness of 3 mm, a module width of 0.2 m, and a module height of 0.8 m, for which the maximum von Mises stress, normal stress and shear stress, become respectively about 110, 105, and 40 MPa well within the limits. The module expands about 3 mm in poloidal direction.
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4.4. Surface heat flux limits Increased power densities are expected to yield reduced electricity costs. Therefore exploratory work was done to identify the limits of the TAURO concept for use in an economically attractive power reactor with higher surface heat flux and corresponding neutron wall load. Fig. 3 shows the evolution of temperature and stress as the power deposition increases. Using the above parameters, the limit of the concept will be reached at 0.56 MW/m2 by exceedingly high normal stress in the first wall, whereas the von Mises limit would be reached only at 1.05 MW/m2. At 0.65 MW/m2 the stress level through the FW thickness would be 10% larger than the corresponding limit. This evaluation gives an idea of the sensitivity of the design to possible improvements of calculation models [7] and/or increase of acceptable stress limits. For a module height of 2 m, the limit will simultaneously be reached at 0.6 MW/m2 for both criteria. For the TAURO blanket, the coolant temperature does not affect the stress limit when increasing the surface heat flux. This suggests that in the conditions considered here, high power densities are, in principle, favorable for higher conversion efficiency. However, the increased demand for improvement of SiCf/SiC characteristics
suggests that a value of surface heat flux of 0.6 MW/m2 is probably the best compromise.
5. Liquid metal-cooled divertor concepts
5.1. Description The design of possible self-cooled Liquid Metal (LM) divertor concepts has been preliminarily addressed. The objective is to evaluate if a potential solution exists able to withstand the expected high heat flux ( \ 5 MW/m2). The evaluation has been limited to the 2D-thermo-mechanical analysis of potential concept assuming a neutron-wall load of 1.2 MW/m2 and a variable surface heat flux, on 50 cm length, from 5 MW/m2 to the maximum acceptable value in order to keep the stresses within the limits. An additional hydrostatic pressure of 0.3 MPa has also been assumed. The coolant can either be Pb–17Li, as in the TAURO blanket, or Sn–Li, which has favorable heat transfer properties but a higher melting point. At present, the choice of W-alloy for the armor material has been made because of its favorable characteristics in terms of plasma–wall interaction, tritium related and thermo-mechanical properties. Two designs have been evaluated: (1)
Fig. 3. Evolution of temperature and stress in the FW versus heat flux, and module height.
H. Golfier et al. / Fusion Engineering and Design 49–50 (2000) 559–565
Fig. 4. Castellated square tube made of W–1%La2O3 with SiCf/SiC flow channel insert.
W-alloy castellated monoblock with a W-alloy tube inside; (2) W-alloy square tube with a castellated sacrificial layer towards the plasma (Fig. 4). The properties of W – 1%La2O3 were used, material for which a maximum temperature of 2000°C can be tolerated when used as protective layer (ITER assumption). To minimize MHD pressure drop, SiCf/SiC flow channel inserts are inserted between the liquid metal and the wall. Only conduction has been assumed between LM and walls and, conservatively, also through the LM (no mixing has been allowed).
5.2. Acceptable surface heat flux In the calculations, the assumed parameters are: (1) the incident surface heat flux, (2) the type of LM, either Pb – 17Li or SnLi, (3) the LM velocity, (4) the inlet coolant temperature. The more interesting results are summarised in Table 1. The
563
analyses have been performed with the objective to maximise the acceptable surface heat flux. For a given heat flux, the minimum acceptable LM velocity has been retained. Several cases with different inlet LM temperatures are shown in Table 1 in order to give the sensitivity of this parameter. The ‘square tube’ design being the best performing one, only results concerning such a concept are given. Improvements can be expected from an optimization of the dimensions of the castellation. The improvements could either be (1) an increase of the maximum acceptable heat flux, or (2) an increase of the coolant outlet temperature. A complete 3D-model will also have to be developed in order to release somehow the applied boundary conditions of generalized plain strain in the 2Dmodel.
5.3. Discussion In order to remain in line with the overall TAURO strategy of having low-energy within the in-vessel components the use of LM-cooled divertor is an essential step. It is important to have demonstrated that this possibility exists. However, a second required step is to avoid the possibility that a LOFA leads to unacceptably high temperature in the components. The use of W-alloy, typically showing high after-heat levels, needs to be carefully evaluated in this respect. Future work will be devoted to this evaluation which could lead to substantial modifications of the presently assessed designs. Open issues for this type of
Table 1 Summary of the liquid metal-cooled divertor thermo-mechanical analysis resultsa Liquid metal coolant
Pb–17Li SnLi SnLi Pb–17Li a
Heat flux (MW/m2)
6 6 5 5
Minimum velocity (m/s)
2.5 1.7 1.7 2.0
Coolant T in/out (°C)
360/447 360/497 500/617 500/594
T min/T max (°C)
Margin to 3Sm (MPa)
W-alloy
SiC/SiC
Coolant
392/1757 395/1774 532/1688 531/1708
377/1244 380/1261 518/1262 517/1279
363/983 374/998 512/1042 505/1060
Square tube design; structures/armor W1%La2O3–SiC/SiC flow channel insert.
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concept are mainly related to material characteristics and behavior under irradiation.
6. External circuits and components
to the global electricity output, which typically receives about 20% of the thermal power.
7. Conclusions
The TAURO blanket could be associated with an external primary circuit for power conversion and tritium recovery. The primary circuit consists of several parallel circuits, each of them conveying about 600 MWth. For Pb – 17Li inlet/outlet temperatures of 450/860°C, each circuit has a Pb–17Li flow rate of about 7900 kg/s. Its principal components are a heat exchanger, a tritium extraction unit and, of course, a Pb – 17Li pump [8]. The tritium extraction unit could be a gas/Pb– 17Li contactor in a by-pass configuration, common for all Pb – 17Li circuits and treating a relatively small Pb – 17Li flow-rate (depending on the tolerable tritium concentration in the Pb– 17Li). At present, a Brayton cycle using helium is foreseen for power conversion which eliminates the potential risk of Pb – 17Li/water interaction although it requires Pb – 17Li/helium heat exchangers the technology for which has yet to be developed. The helium pressure should be high to minimize pressure drop and component sizes [9]. A rupture disk in the Pb – 17Li circuit close to the heat exchanger and a discharge vessel will be required to protect the blanket from accidental pressurization in case of heat exchanger failure. When assuming TAURO coolant inlet/outlet temperatures of 450/860°C, the power conversion efficiency would realistically attain \ 43%, without taking into account the pumping power. Such conversion efficiency could be increased up to 47% if the Pb – 17Li inlet temperature is increased to 650°C. Therefore, the precise determination of the global blanket system efficiency depends on an optimization between blanket module arrangement, pressure drop, Pb – 17Li heat-up and power conversion [10]. Further work is also required to optimize the operating temperatures of the divertor coolant (depending on the choice of liquid metal) so as to maximize the contribution of the divertor power .
The TAURO blanket, in combination with a liquid-metal cooled divertor, offers a high capacity for heat extraction at high coolant temperatures and promises favorable conversion efficiencies. Recently performed thermo-mechanical analyses have shown that a surface heat flux of 0.6 MW/m2 for the blanket and 5 MW/m2 for the liquid-metal cooled divertor could be withstood. The reference design, which assumes a surface heat flux of 0.5 MW/m2, has Pb–17Li inlet/outlet temperatures of, respectively 450 and 860°C that lead to an efficiency greater than 43%. Further work is also required to optimize the operating temperatures and the number of modules in serial (depending on the available volume for TAURO blanket). Significant R&D effort is required both for SiCf/SiC and W-alloy components. The high temperature compatibility between Pb–17Li and SiCf/SiC and W-alloys still needs be confirmed. The possibility of using SnLi coolant has to be further investigated.
References [1] L. Giancarli, J.P. Bonal, A. Caso, G. Le Marois, N.B. Morley, J.F. Salavy, Design requirements for SiC/SiC composites structural material in fusion power reactor blanket, Fus. Eng. Design 41 (1998) 165 – 171. [2] L. Giancarli, A. Caso, et al., R&D issues for SiCf/SiC composites structural material in fusion power reactor blankets, in: IAEA, TC Meeting on FPPD, Culham, 24 – 27 March 1998, Fus. Eng. Design, in press. [3] B. Riccardi, P. Fenici, A. Frias Rebelo, L. Giancarli, G. Le Marois, E. Philippe, Status of the European R&D activities on SiCf/SiC composites for fusion reactors, this conference. [4] I. Cook, Fus. Eng. Design 25 (1994) 179. [5] G. Aiello, et al., SiCf/SiC composites for structural applications in fusion power reactors: the TAURO blanket, in: Proceedings of the 12th International Conference on Composite Materials (ICCM-12), Paris, 5 – 9 July, 1999.
H. Golfier et al. / Fusion Engineering and Design 49–50 (2000) 559–565 [6] A. Gasse, G. Aiello, L. Giancarli, G. Le Marois, J.-F. Salavy, J. Szczepanski, Progress on SiCf/SiC structures joining techniques and modelling for design analyses, Proc. SOFT-20 (1998). [7] G. Aiello, H. Golfier, J.F. Maire, Y. Poitevin, J.F. Salavy, Modeling of SiCf/SiC composite structures for nuclear components, this conference. [8] Y. Severi, First assessment of the power conversion sys-
.
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tem associated with TAURO breeding blanket concept, CEA Report, DER/STPI (1999). [9] S. Malang, H. Schnauder, M.S. Tillack, Combination of a self-cooled liquid-metal blanket with a gas turbine power conversion system, Fus. Eng. Design 41 (1998) 561 – 567. [10] M. Fu¨tterer, Power conversion for TAURO blanket, CEA Report, DRN/SERMA (1999).
Performance of the TAURO blanket system associated with a liquid-metal cooled divertor H. Golfier a,*, G. Aiello a, M.A. Fu¨tterer a, A. Gasse b, L. Giancarli a, Y. Poitevin a, J.F. Salavy a, Y. Severi c, J. Szczepanski a b
a CEA Saclay, DRN/DMT/SERMA, F-91191 Gif-sur-Y6ette Cedex, France CEA Grenoble, DTA-CEREM/DEM/SGM, F-38054 Grenoble Cedex 9, France c CEA Cadarache, DRN/DER/STPI, F-13108 Saint Paul-lez-Durance, France
Abstract Blankets and divertors are the key components of a fusion power plant due to their large impact on the overall plant design, performance and availability, and on the cost of electricity. This paper recalls the main features of the TAURO blanket, a self-cooled Pb–17Li concept using SiCf/SiC composite as structural material, and describes the most recent thermo-mechanical analyses performed on the blanket. It includes an evaluation of a liquid-metal cooled divertor which could be associated with such a blanket. Investigations were carried out to determine the potential and limits of the blanket and of a liquid-metal cooled divertor in terms of maximum acceptable surface heat flux. The high outlet temperature of the Pb–17Li coolant (800°C) leads to an attractive energy conversion efficiency ( \ 47%) assuming helium-coolant for the secondary circuit. Special heat exchangers, using SiCf/SiC tubes are envisaged. Thermo-mechanical analyses have pointed out that maximum surface heat flux on FW and divertor could be about 0.65 and B 5 MW/m2, respectively. It is expected that an improvement of the SiCf/SiC modeling, taking into account non linear behavior presently under way, and the use of design criteria adapted to more advanced composites could allow even higher limits. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Liquid metal divertor; TAURO blanket; Pb – 17Li cooling; Fusion power plant
1. Introduction The TAURO blanket is a self-cooled Pb–17Li blanket using SiCf/SiC structures [1]. The rational of this concept is based on the requirement of passive safety of tokamak-type Fusion Power R * Corresponding author. Tel.: +33-1-69083287; fax: + 331-69089935. E-mail address: [email protected] (H. Golfier).
actors (FPRs). This is obtained by limiting the energy available within the first safety barrier which for tokamaks is generally the vacuum vessel, in order to avoid the possibility of its braking and the subsequent accidental release of radioactive materials. The main objectives of the TAURO blanket design studies are the establishment of the major required improvements of the characteristics of the present-day SiCf/SiC composites; low thermal
0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 0 ) 0 0 2 8 1 - 7
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H. Golfier et al. / Fusion Engineering and Design 49–50 (2000) 559–565
conductivity and low neutron irradiation resistance have been identified as the most severe issues [2,3].
2. TAURO breeder blanket concept
2.1. Specifications and design description The design is based on the specifications for the SEAFP study [4], i.e. fusion power of 3000 MW, neutron wall load of 2 MW/m2 and surface heat flux of 0.5 MW/m2. The TAURO blanket is essentially formed by a SiCf/SiC box with an indirectly-cooled First Wall (FW) which acts as a container for the Pb – 17Li. The Pb – 17Li acts as coolant, breeder and multiplier material, and tritium carrier. Each outboard segment is poloidally divided into several straight modules, attached on one common thick back-plate, but cooled independently.
Fig. 1. TAURO blanket design, outboard module.
The feeding pipes are located behind the module. The coolant enters the inlet collector through a single tube and is divided into five sub-flows, one for each sub-module (Fig. 1). Within each sub-module the Pb–17Li flows at first poloidally downward in a thin channel located just behind the FW, makes a U-turn at the bottom into a second channel and flows up, then down into the outlet collector. Toroidal plates (stiffeners) are required for reinforcing the sub-module box against the hydrostatic pressure (assumed to be 1.5 MPa in all calculations) and act as flow separators [5]. Manufacturing of the module depends on the possibility of joining SiCf/SiC components. A promising brazing technique using a braze material compatible with SiC, the Brasic®, is currently under development [6].
3. Design criteria and models The design optimization and structural assessment of the TAURO blanket and Pb–17Li divertor were performed in 3D and 2D geometry, respectively assuming square meshes and an orthotropic model of SiCf/SiC. Due account was taken of the poloidal variation in heat transfer, coolant conditions, mechanical loads and thermal loads. Conduction remains the dominant heat transfer mechanism, the thermal conductivity for SiCf/SiC was optimistically assumed as 15 W/mK. The other assumed SiCf/SiC characteristics were those of the industrial Cerasep® N3-1 [2]. New design criteria, specific for SiCf/SiC structures, have been recently defined [5] and applied in the thermo-mechanical evaluations. They are the following: the von Mises stresses in the plane are limited to 145 MPa; the normal stresses through the thickness are limited to 110 MPa; the shear stresses are limited to 45 MPa. These limits have been determined using experimental results obtained several years ago on Cerasep® N2-1 and are, therefore, probably too pessimistic when applied to present-day SiCf/SiC or to future improved composites.
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561
tolerated in the TAURO design assuming that the ratio between surface heat flux and neutron wall loading on the FW remains constant. The performance of the TAURO blanket was estimated by varying the inlet temperature, the heat flux level, and some design parameters.
4.2. Temperature limits The Pb–17Li inlet temperature was modified in the range 275–750°C in order to see the impact on the maximum SiCf/SiC temperature which was found in all cases to be below 1300°C. For a surface heat flux of 0.5 MW/m2, the corresponding Pb–17Li outlet temperature ranges between 500 and 900°C. Fig. 2 shows a typical temperature distribution in the sub-module. (aggrandir les chiffres)
4.3. Stress limits Fig. 2. Temperature distribution in a TAURO sub-module.
Further constraints are: maximum SiCf/SiC temperature of 1300°C (expected for future composites based on HiNicalon fibers); average coolant outlet temperature \ 800°C to achieve a conversion efficiency \ 45%.
4. New evaluations for TAURO blanket New thermal and thermo-mechanical analyses, focused on the outboard blanket in the midplane region where the thermal conditions are the most severe, have been performed with CASTEM2000. The evaluated maximum surface heat flux has been kept along the whole sub-module height.
4.1. Blanket thermo-mechanical limits Among the criteria described in Section 2, those related to stresses are the most severe to meet due to the relatively low thermal conductivity of SiCf/ SiC. It was decided to verify what was the maximum surface heat flux on the FW which can be
The highest stresses appear on the FW. For a Pb–17Li velocity in the first layer of 1 m/s, a FW thickness of 6 mm, a module width of 0.3 m, and a module height of 2.15 m, the maximum von Mises stress in plane is 150 MPa (just above the limit) and the maximum shear stress is 40 MPa (below the limit). However, the maximum normal stress through the FW thickness exceeds the limit of the order of 20% [5]. In order to fit all design rules simultaneously, modifications of the above parameters were attempted. In all cases stresses are dominated by the temperature gradients on the FW (through height and thickness) which can be reduced by a reduction of the height of the module to reduce the DT between top and bottom of FW; increasing the Pb–17Li velocity to attenuate the radial and poloidal DT; reducing the width of the box. The requirement can be satisfied by assuming a Pb–17Li velocity in the first layer of 1.3 m/s, a FW thickness of 3 mm, a module width of 0.2 m, and a module height of 0.8 m, for which the maximum von Mises stress, normal stress and shear stress, become respectively about 110, 105, and 40 MPa well within the limits. The module expands about 3 mm in poloidal direction.
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4.4. Surface heat flux limits Increased power densities are expected to yield reduced electricity costs. Therefore exploratory work was done to identify the limits of the TAURO concept for use in an economically attractive power reactor with higher surface heat flux and corresponding neutron wall load. Fig. 3 shows the evolution of temperature and stress as the power deposition increases. Using the above parameters, the limit of the concept will be reached at 0.56 MW/m2 by exceedingly high normal stress in the first wall, whereas the von Mises limit would be reached only at 1.05 MW/m2. At 0.65 MW/m2 the stress level through the FW thickness would be 10% larger than the corresponding limit. This evaluation gives an idea of the sensitivity of the design to possible improvements of calculation models [7] and/or increase of acceptable stress limits. For a module height of 2 m, the limit will simultaneously be reached at 0.6 MW/m2 for both criteria. For the TAURO blanket, the coolant temperature does not affect the stress limit when increasing the surface heat flux. This suggests that in the conditions considered here, high power densities are, in principle, favorable for higher conversion efficiency. However, the increased demand for improvement of SiCf/SiC characteristics
suggests that a value of surface heat flux of 0.6 MW/m2 is probably the best compromise.
5. Liquid metal-cooled divertor concepts
5.1. Description The design of possible self-cooled Liquid Metal (LM) divertor concepts has been preliminarily addressed. The objective is to evaluate if a potential solution exists able to withstand the expected high heat flux ( \ 5 MW/m2). The evaluation has been limited to the 2D-thermo-mechanical analysis of potential concept assuming a neutron-wall load of 1.2 MW/m2 and a variable surface heat flux, on 50 cm length, from 5 MW/m2 to the maximum acceptable value in order to keep the stresses within the limits. An additional hydrostatic pressure of 0.3 MPa has also been assumed. The coolant can either be Pb–17Li, as in the TAURO blanket, or Sn–Li, which has favorable heat transfer properties but a higher melting point. At present, the choice of W-alloy for the armor material has been made because of its favorable characteristics in terms of plasma–wall interaction, tritium related and thermo-mechanical properties. Two designs have been evaluated: (1)
Fig. 3. Evolution of temperature and stress in the FW versus heat flux, and module height.
H. Golfier et al. / Fusion Engineering and Design 49–50 (2000) 559–565
Fig. 4. Castellated square tube made of W–1%La2O3 with SiCf/SiC flow channel insert.
W-alloy castellated monoblock with a W-alloy tube inside; (2) W-alloy square tube with a castellated sacrificial layer towards the plasma (Fig. 4). The properties of W – 1%La2O3 were used, material for which a maximum temperature of 2000°C can be tolerated when used as protective layer (ITER assumption). To minimize MHD pressure drop, SiCf/SiC flow channel inserts are inserted between the liquid metal and the wall. Only conduction has been assumed between LM and walls and, conservatively, also through the LM (no mixing has been allowed).
5.2. Acceptable surface heat flux In the calculations, the assumed parameters are: (1) the incident surface heat flux, (2) the type of LM, either Pb – 17Li or SnLi, (3) the LM velocity, (4) the inlet coolant temperature. The more interesting results are summarised in Table 1. The
563
analyses have been performed with the objective to maximise the acceptable surface heat flux. For a given heat flux, the minimum acceptable LM velocity has been retained. Several cases with different inlet LM temperatures are shown in Table 1 in order to give the sensitivity of this parameter. The ‘square tube’ design being the best performing one, only results concerning such a concept are given. Improvements can be expected from an optimization of the dimensions of the castellation. The improvements could either be (1) an increase of the maximum acceptable heat flux, or (2) an increase of the coolant outlet temperature. A complete 3D-model will also have to be developed in order to release somehow the applied boundary conditions of generalized plain strain in the 2Dmodel.
5.3. Discussion In order to remain in line with the overall TAURO strategy of having low-energy within the in-vessel components the use of LM-cooled divertor is an essential step. It is important to have demonstrated that this possibility exists. However, a second required step is to avoid the possibility that a LOFA leads to unacceptably high temperature in the components. The use of W-alloy, typically showing high after-heat levels, needs to be carefully evaluated in this respect. Future work will be devoted to this evaluation which could lead to substantial modifications of the presently assessed designs. Open issues for this type of
Table 1 Summary of the liquid metal-cooled divertor thermo-mechanical analysis resultsa Liquid metal coolant
Pb–17Li SnLi SnLi Pb–17Li a
Heat flux (MW/m2)
6 6 5 5
Minimum velocity (m/s)
2.5 1.7 1.7 2.0
Coolant T in/out (°C)
360/447 360/497 500/617 500/594
T min/T max (°C)
Margin to 3Sm (MPa)
W-alloy
SiC/SiC
Coolant
392/1757 395/1774 532/1688 531/1708
377/1244 380/1261 518/1262 517/1279
363/983 374/998 512/1042 505/1060
Square tube design; structures/armor W1%La2O3–SiC/SiC flow channel insert.
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concept are mainly related to material characteristics and behavior under irradiation.
6. External circuits and components
to the global electricity output, which typically receives about 20% of the thermal power.
7. Conclusions
The TAURO blanket could be associated with an external primary circuit for power conversion and tritium recovery. The primary circuit consists of several parallel circuits, each of them conveying about 600 MWth. For Pb – 17Li inlet/outlet temperatures of 450/860°C, each circuit has a Pb–17Li flow rate of about 7900 kg/s. Its principal components are a heat exchanger, a tritium extraction unit and, of course, a Pb – 17Li pump [8]. The tritium extraction unit could be a gas/Pb– 17Li contactor in a by-pass configuration, common for all Pb – 17Li circuits and treating a relatively small Pb – 17Li flow-rate (depending on the tolerable tritium concentration in the Pb– 17Li). At present, a Brayton cycle using helium is foreseen for power conversion which eliminates the potential risk of Pb – 17Li/water interaction although it requires Pb – 17Li/helium heat exchangers the technology for which has yet to be developed. The helium pressure should be high to minimize pressure drop and component sizes [9]. A rupture disk in the Pb – 17Li circuit close to the heat exchanger and a discharge vessel will be required to protect the blanket from accidental pressurization in case of heat exchanger failure. When assuming TAURO coolant inlet/outlet temperatures of 450/860°C, the power conversion efficiency would realistically attain \ 43%, without taking into account the pumping power. Such conversion efficiency could be increased up to 47% if the Pb – 17Li inlet temperature is increased to 650°C. Therefore, the precise determination of the global blanket system efficiency depends on an optimization between blanket module arrangement, pressure drop, Pb – 17Li heat-up and power conversion [10]. Further work is also required to optimize the operating temperatures of the divertor coolant (depending on the choice of liquid metal) so as to maximize the contribution of the divertor power .
The TAURO blanket, in combination with a liquid-metal cooled divertor, offers a high capacity for heat extraction at high coolant temperatures and promises favorable conversion efficiencies. Recently performed thermo-mechanical analyses have shown that a surface heat flux of 0.6 MW/m2 for the blanket and 5 MW/m2 for the liquid-metal cooled divertor could be withstood. The reference design, which assumes a surface heat flux of 0.5 MW/m2, has Pb–17Li inlet/outlet temperatures of, respectively 450 and 860°C that lead to an efficiency greater than 43%. Further work is also required to optimize the operating temperatures and the number of modules in serial (depending on the available volume for TAURO blanket). Significant R&D effort is required both for SiCf/SiC and W-alloy components. The high temperature compatibility between Pb–17Li and SiCf/SiC and W-alloys still needs be confirmed. The possibility of using SnLi coolant has to be further investigated.
References [1] L. Giancarli, J.P. Bonal, A. Caso, G. Le Marois, N.B. Morley, J.F. Salavy, Design requirements for SiC/SiC composites structural material in fusion power reactor blanket, Fus. Eng. Design 41 (1998) 165 – 171. [2] L. Giancarli, A. Caso, et al., R&D issues for SiCf/SiC composites structural material in fusion power reactor blankets, in: IAEA, TC Meeting on FPPD, Culham, 24 – 27 March 1998, Fus. Eng. Design, in press. [3] B. Riccardi, P. Fenici, A. Frias Rebelo, L. Giancarli, G. Le Marois, E. Philippe, Status of the European R&D activities on SiCf/SiC composites for fusion reactors, this conference. [4] I. Cook, Fus. Eng. Design 25 (1994) 179. [5] G. Aiello, et al., SiCf/SiC composites for structural applications in fusion power reactors: the TAURO blanket, in: Proceedings of the 12th International Conference on Composite Materials (ICCM-12), Paris, 5 – 9 July, 1999.
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