- Email: [email protected]
Nuclear analyses of solid breeder blanket options for DEMO: Status, challenges and outlook
Fusion Engineering and Design xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Nuclear analyses of solid breeder blanket options for DEMO: Status, challenges and outlook ⁎
Pavel Pereslavtsev , Francisco A. Hernández, Guangming Zhou, Lei Lu, Christian Wegmann, Ulrich Fischer Karlsruhe Institute of Technology, 76344, Eggenstein-Leopoldshafen, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: Breeder blanket Solid breeder Neutronics Design optimization
This work presents results of the neutronic analyses for an innovative Helium Cooled Pebble Bed (HCPB) blanket concept for DEMO. Comprehensive 3D Monte Carlo particle transport simulations were performed with the MCNP5 code making use of a highly detailed HCPB DEMO torus sector model. The advanced HCPB Single Module Segmentation (SMS) blanket was consequently optimized to accomplish out the design requirements. A completely new Molten Lead Ceramic Breeder (MLCB) SMS blanket utilizing a solid breeder and a Pb neutron multiplier together was also investigated and proved to be an alternative option for DEMO design.
1. Introduction
2. Design and geometry model generation
A fusion demonstration reactor (DEMO) is assumed to be a nearterm reactor facility that is able to generate electricity and to operate in a self-sufficient tritium fuel cycle [1]. The development of a breeding blanket is one of the most important and challenging issues in the DEMO project due to its novelty and numerous technological and safety problems to be solved. An important modification affecting all R&D works was applied in the current EUROfusion DEMO baseline design [1] compared to the previous one: a radial size of a breeder blanket zone in the outboard side was reduced from 1.3 m to 1.0 m keeping one in the inboard size unchanged. This paper provides an overview of the neutronic development efforts devoted to the detailed design of a solid breeder blanket for DEMO with an inherent capability of a highly efficient tritium breeding. An innovative design of the Helium Cooled Pebble Bed (HCPB) DEMO blanket [2] was developed based on comprehensive iterative neutronic, thermal-hydraulic and stress analyses. As an alternative a promising Molten Lead Ceramic Breeder (MLCB) DEMO design [3] is analyzed and compared to the HCPB one. The key part of the analyses is the development of the geometry model of the breeder blanket with sufficient details to perform high fidelity nuclear simulations. The tritium breeding performance of the blankets was first assessed, followed by calculations of the nuclear power generation and analyses on the shielding performance.
2.1. HCPB blanket design
⁎
The novel HCPB blanket design is based on the implementation of a Single Module Segmentation (SMS) compared to a Multiple Modules Segmentation (MMS) adopted in previous studies [4]. Blanket dimensions were adjusted to fit a current DEMO baseline design 2017 [1]. The SMS blanket casing consists of poloidal breeder modules each of them is built by a 20 mm thick U-shaped-like first wall (FW) with a 30 mm back wall and back supporting structure (BSS) attached to it [2], Fig. 1. The plasma facing part of the FW is faceted and it is designed to have a roofshaped form with a gradient of ˜2° that results in a loss of the breeder zone volume close to plasma by about 3% compared to a flat FW. Such a shape of the FW deals to protect the edges of the blanket module at the gaps against thermal loads due to plasma contacts [5]. The FW encloses He cooling channels of 11 × 11 mm size and a poloidal pitch of 15 mm. The FW is covered with a 2 mm thick W armor. The radial thickness of the BSS is 35 cm (including 15 cm supporting wall) and 35 cm (including 5 cm supporting wall) at the inboard (IB) and outboard (OB) sides, respectively, being constant in all breeder modules. This results in a variable radial depth (from the FW up to back plate) of the breeder zone (BZ) depending on the poloidal position of the breeder module. It is 41 and 63 cm including 6 cm back wall and gas collectors in the mid plane at the IB and OB sides, respectively. An essential innovation included in the new HCPB blanket design is the arrangement of a breeder ceramic in radial breeder pins attached to
Corresponding author. E-mail address: [email protected] (P. Pereslavtsev).
https://doi.org/10.1016/j.fusengdes.2019.01.023 Received 10 September 2018; Received in revised form 23 November 2018; Accepted 6 January 2019 0920-3796/ © 2019 Published by Elsevier B.V.
Please cite this article as: Pereslavtsev, P., Fusion Engineering and Design, https://doi.org/10.1016/j.fusengdes.2019.01.023
Fusion Engineering and Design xxx (xxxx) xxx–xxx
P. Pereslavtsev et al.
Fig. 1. Horizontal cut of the CAD HCPB blanket model (left) and the HCPB SMS blanket (right).
were then integrated into the MCNP generic model as independent universes utilizing proper transformation matrices. Third, separate geometry models were developed on the MCNP platform for the inner space of the blanket and for the FW cooling channels using the repeated structure feature of the MCNP, Fig.2. These models were also integrated into the empty blanket casings of the generic model using transformation cards for each breeder module of the IB and OB blankets. Such a hybrid approach enabled development of the MCNP geometry model with the fully heterogeneous HCPB SMS blankets, Fig.2. The vacuum vessel (VV) in the inboard side has a radial thickness of 60 cm including 6 cm thick SS316 steel walls. The cooling of the VV is
the FW and the back plate. The breeder pin consists of three coaxial tubes: the central 8/14 mm (inner / outer radius) serves for He inlet, a channel between second (30/32 mm) and third (35/39 mm) tubes serves for He outlet, Fig. 1. The annular volume between the first and the second tubes is filled with the breeder ceramic pebbles containing mixture of Li4SiO4 plus 37 mol.% of Li2TiO3 (60% 6Li enrichment in both compounds) with 0.64 package factor. The breeder pins build a radial hexagonal lattice with a regular pitch of 125 mm. The pitch between pins was optimized to provide the maximum tritium breeding ratio (TBR). Be12Ti pebbles, used as neutron multiplier, fill the space around pins with a packing factor of 0.64. All structural elements of the blanket are assumed to be manufactured with Eurofer steel. The SMS blanket is cooled with a He of ˜80 bar and He of ˜2 bar is used as a purge gas. 2.2. HCPB DEMO geometry model The MCNP geometry model of the HCPB DEMO was developed in three steps. First, from the CAD DEMO model 2017 developed in the PPPT, Table 1 [1], a 11.25° toroidal segment was extracted accounting for the symmetry of the model. This segment represents half of one of the 16 toroidal DEMO sectors and includes empty volumes for one IB, one full OB and a half of the central OB blankets keeping 20 mm toroidal gaps between them. This toroidal segment was converted into the MCNP generic model making use of the McCad conversion tool [6]. Such a model has a simple void description and it was used as a basis for the further geometry models build-up. Second, CAD models of three empty SMS HCPB blanket casings were converted separately with McCad into three stand-alone MCNP geometry models. These models Table 1 Main parameters of the DEMO reactor. Major radius, (m) Minor radius, (m) Plasma elongation Plasma triangularity Fusion power, (MW) Net electric power, (MW)
8.938 2.883 1.650 0.333 1998.0 500.0
Fig. 2. MCNP geometry model of the HCPB DEMO. A, B — horizontal cuts in OB and IB sides, C — vertical cut in OB side in radial-poloidal direction, D — vertical cut in OB side in toroidal-poloidal direction. 2
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ensured by water and an ITER-like SS304B7 steel structure, Fig. 2, is arranged inside the VV to provide its stiffening and the shielding performance. Toroidal magnetic field coils (TFC) are enclosed in a steel casing. The divertor is modeled as a solid body (60% steel and 40% water) except three layers facing the plasma. The first and third layers are a 5 mm thick tungsten armor with a 15 mm thick tube layer in between filled with a homogenized mixture of 39.5% W, 17% CuCrZr, 13% Cu and 30% water. The equatorial port is plugged with a steelwater homogenized mixture (60% SS-316 steel and 40% water).
Table 2 Nuclear energy generation [MW] in the DEMO. Component
HCPB
MLCB
Blankets Vacuum vessel Divertor Total Global energy multiplication factor
1931 49 170 2150 1.35
1646 77 197 1920 1.20
of Be pebbles [5]. The realistic roof-shaped FW results in the loss of TBR up to 3% compared to the flat one. The replacement of Be12Ti pebbles with Be ones could provide TBR = 1.23 compared to the reference TBR = 1.16. The replacement of the detailed FW with a homogeneous material mixture results in ΔTBR=+0.01. The replacement of the detailed BZ with the homogeneous material mixture brings additionally ΔTBR=+0.01. The use of the SMS blanket as well as the principally new breeder arrangement and cooling scheme applied in the blanket increased the tritium breeding efficiency and made it possible to compensate the loss of TBR due to the reduction of the breeder blanket space and the replacement of the Be pebbles with Be12Ti. The breakdown of the nuclear power generation in the new HCPB DEMO reactor is given in the Table 2. The energy deposition in the blankets is high enough to ensure a high energy multiplication factor in the HCPB DEMO even in spite of the very short BZ. Fig. 4 shows the radial nuclear power density profiles in the steel in the mid plane of the IB side for different VV configurations. Such configurations refer to different options to enhance the shielding performances: one uses a homogeneous material mixture of 60% SS304B7 steel and 40% water, heterogeneous model with 9.5 cm thick SS304B7 steel inserts in water and an option with 5 cm thick WC plates inserted in the steel ones. The results obtained with the homogenized VV interior structure significantly overestimate the power density there but it has almost no effect to the total TFC heating. This comes from the intensive particle streaming through the toroidal gaps between blankets and through weakly shielded divertor port. For the current DEMO radial build the TFC heating is close to the design limit of 50 W/m3 [8]. The inclusion of neutron absorbing materials like WC can improve the shielding performance of the VV. The radial profiles of the total neutron flux through the blanket up to the TFC in the mid plane of the IB side are shown in the Fig. 5. The option with the WC inserts ensures a better shielding performance of the VV and can decrease the neutron loads to the TFC magnet by a factor of 3÷5. The high energy neutron flux (En > 0.1 MeV) in the superconducting magnet in this case is ˜7⋅108 n/cm2 s that corresponds a neutron fluence of ˜1⋅1021 n/m2 during full life time of the magnet. This is below the design limit of 1022 n/m2 after 6 full power years
2.3. MLCB DEMO geometry model The alternative theoretic MLCB concept [3] of the solid breeder blanket combines two significant features: the use of the breeder ceramic together with a liquid lead neutron multiplier. This concept represent a trial to replace the very expensive and technologically not well developed Be based pebbles with rather effective Pb neutron multiplier to demonstrate the efficiency of the breeder blankets based on the use of solid breeder ceramic. No circulation of the liquid lead outside the reactor is considered currently as a mainstream in the development of the alternative blanket design. As a highlight, the MLCB concept utilizes the same blanket matrix developed for the HCPB DEMO described above. The lattice pitch of 125 mm used in the model was optimized to reach the maximum tritium breeding. 3. Simulation results 3.1. HCPB DEMO The simulations included an assessment of the main required nuclear responses: TBR, nuclear power generation and shielding performances. The calculations were carried out making use of the geometry models discussed above and the MCNP5-1.60 code [7] with nuclear data from the JEFF-3.2 library. For heavy duty MCNP5 runs such as shielding calculations a weight window variance reduction technique was applied. This ensures results with a good statistics usually not exceeding ˜2% for the cells outside vacuum vessel and < 0.1% for the plasma facing components. Fig. 3 shows the neutron wall loading distribution calculated for the new HCPB SMS blankets. The numeration of the breeder modules is anticlockwise starting from the central one in the mid plane of the OB side. The maxima are 1.12 and 1.34 MW/m2 for central IB and OB breeder modules, respectively, the average neutron wall load being ˜0.93 MW/m2. The most basic nuclear response of the HCPB DEMO, it is TBR, was assessed to be 1.16 (with the 0.01% of statistical uncertainty), the same value was achieved in the previous MMS HCPB design based on the uses
Fig. 3. FW neutron wall loading in the HCPB DEMO.
Fig. 4. Radial power density profiles at inboard mid-plane. 3
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Fig. 7. 3D neutron flux distribution in IB side.
3.2. MLCB DEMO Lead employed in the MLCB blanket is a less efficient neutron multiplier compared to Be or Be12Ti compound [9]. To compensate the lower neutron multiplication, a higher 6Li enrichment (90%) is typically applied. The MLCB breeder blanket uses the same matrix as the HCPB with the pitch of 125 mm. The TBR = 1.07 was calculated for this design. Two further consequently modifications were applied to enhance the tritium breeding: the radial thickness of the ceramic layer was enlarged from 16 to 20 mm and the length of the BZ was increased by 5 cm in the OB and 3 cm in the IB sides (BSS was squeezed respectively). The TBR was found to be TBR = 1.11 and 1.13 in the former and latter cases. Further enlargements of the BZ require changes in the DEMO radial build. As a theoretical option, the effect on the TBR of employing pressurized water as a coolant in the FW was assessed, making use of the last MLCB configuration with enlarged BZ. A TBR = 1.05 was found for the homogenized material mixture and TBR = 1.03 in case of the detailed modelling of the FW. The energy generation in the MLCB DEMO for the option with TBR = 1.13, Table 2, is about 10% lower compared to the HCPB DEMO. This comes from the significantly lower energy generation in the blanket, see Fig. 4 due to the lower neutron multiplication. The shielding of the TFC in the MLCB DEMO is comparable with the HCPB reference assuming WC plugs in the VV for both cases, Figs. 4 and 5. This option with sufficient protection of the TFC is feasible also in the case of the MLCB as shown in the previous Figures. The MLCB blanket shows a comparable shielding performance to the HCPB with Be12Ti neutron multiplier.
Fig. 5. Radial neutron fluxes profiles at inboard mid-plane.
Fig. 6. Radial profiles of the DPA and He-accumulation in the inboard midplane.
(FPY) [9]. The inefficient shielding of the divertor and also the big gap between VV and TFC lead to the neutron streaming and as a result to a slight increase of the neutron flux here. The nuclear damage accumulation in the steel structures at the inboard mid plane of the reactor is given in the Fig. 6. The maximum DPA accumulation in the VVdoes not exceeds ˜1.2 DPA/6 FPY that is below the design limit of the 2.75 DPA. Peaks in the He-production profiles come from intensive reflections of the high energy neutrons from water-steel structure resulting in an amplification of (n,α) reactions. The maximum He accumulation behind the VV does not exceeds the design limit for the pipes welding loccations of 1 ppm. The radial arrangement of the breeder pins in the HCPB blanket presumes some neutron streaming through the He cooling channels. A comparison with the previous MMS HCPB blanket design [7] for the radial distribution of the total neutron flux shows even slightly better shielding performances of the new blanket. In further detailed analyses a 3D picture of the neutron flux distribution was prepared for the reactor IB mid plane, Fig. 7. The main streaming paths in the BZ are the Be12Ti layers around the breeder pins rather than the He cooling tubes. The BSS in the blanket has weak shielding performances and it serves mainly for the scattering of the neutrons. A massive ˜15 cm supporting back wall of the BSS provides certain reduction of the neutron flux. The VV serves as a main neutron-shielding component in the DEMO. The modeling of the heterogeneous structure of the VV is essential to locate possible hot spots in the design. In the present configuration the bulk radial stiffening plate demonstrates even worse neutron shielding compared to the toroidal steel-WC plates.
4. Conclusions In the framework of the PPPT program of EUROfusion, a new HCPB blanket concept was elaborated and supporting comprehensive neutronic analyses were performed. The new blanket design was optimized by means of numerous parametric coupled neutronic-thermal hydraulic simulations. A full heterogeneous MCNP geometry model was produced making use of a hybrid modeling procedure including McCad automated conversions and the application of effective MCNP modelling features. Two innovative solid breeder blankets were developed: the HCPB with Be12Ti and the MLCB with Pb neutron multiplier. Both blankets were demonstrated to have a very effective neutron economy. Both can fulfill the design requirements. The use of the solid breeder with the solid or liquid neutron multiplier and He coolant provides sufficient tritium generation in DEMO. The basic neutronic response TBR = 1.16 and 1.13 were assessed for the HCPB and MLCB DEMOs, respectively, using the realistic SMS blanket design that addresses the latest DEMO requirements on the radial build and FW integrity. The excess of the TBR over the net target value of TBR = 1.05 [9] enables further design studies aiming at the inclusion of various in vessel components reducing tritium breeding in the DEMO. 4
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The shielding performances of the new solid breeder blankets can be enhanced by the inclusion in the blanket matrix shielding inserts such as W, WC, graphite and hydrates. This requires further optimizations efforts in the neutronics, thermal hydraulic, engineering and balance of plant activities. The effect of the heterogeneity was found to be essential for the DEMO design and principal project decisions.
Des. 136 (2018) 729–741. [2] F.A. Hernandez, et al., An enhanced, near-term HCPB configuration as driver blanket for the EU DEMO, Fusion Eng. Des. (2018) this issue. [3] G. Zhou, et al., Progress on the helium cooled Molten Lead Ceramic Breeder concept, as a near-term alternative blanket for EU DEMO, Fusion Eng. Des. (2018) this issue. [4] P. Pereslavtsev, U. Fischer, F. Hernandez, Lei Lu, Neutronic analyses for optimization of the advanced HCPB breeder blanket design for DEMO, Fusion Eng. Des. 124 (2017) 910–914. [5] R. Mitteau, et al., J. Nucl. Mater. 415 (1) (2011) 969–972. [6] L. Lu, U. Fischer, P. Pereslavtsev, Improved algorithms and advanced features of CAD to MC conversion tool McCad, Fusion Eng. Des. 89 (9–10) (2014) 1885. [7] X-5 Monte Carlo Team, MCNP — A General Monte Carlo N-Particle Transport Code Overview and Theory (Version 5, Vol. I), Los Alamos National Laboratory, 2003 Report LA-UR-03-1987, 24 April. (Revised 10/3/05). [8] Ch. Bachmann, et al., Overview over DEMO design integration challenges and their impact on component design concepts, Fusion Eng. Des. 136 (Part A) (2018) 87–95. [9] F.A. Hernandez, P. Pereslavtsev, First principles review on the options for tritium breeder and neutron multiplier materials for breeding blankets in fusion reactors, Fusion Eng. Des. 137 (2018) 243–256.
Acknowledgments This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] G. Federici, et al., DEMO design activity in Europe: progress and updates, Fusion Eng.
5
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Nuclear analyses of solid breeder blanket options for DEMO: Status, challenges and outlook ⁎
Pavel Pereslavtsev , Francisco A. Hernández, Guangming Zhou, Lei Lu, Christian Wegmann, Ulrich Fischer Karlsruhe Institute of Technology, 76344, Eggenstein-Leopoldshafen, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: Breeder blanket Solid breeder Neutronics Design optimization
This work presents results of the neutronic analyses for an innovative Helium Cooled Pebble Bed (HCPB) blanket concept for DEMO. Comprehensive 3D Monte Carlo particle transport simulations were performed with the MCNP5 code making use of a highly detailed HCPB DEMO torus sector model. The advanced HCPB Single Module Segmentation (SMS) blanket was consequently optimized to accomplish out the design requirements. A completely new Molten Lead Ceramic Breeder (MLCB) SMS blanket utilizing a solid breeder and a Pb neutron multiplier together was also investigated and proved to be an alternative option for DEMO design.
1. Introduction
2. Design and geometry model generation
A fusion demonstration reactor (DEMO) is assumed to be a nearterm reactor facility that is able to generate electricity and to operate in a self-sufficient tritium fuel cycle [1]. The development of a breeding blanket is one of the most important and challenging issues in the DEMO project due to its novelty and numerous technological and safety problems to be solved. An important modification affecting all R&D works was applied in the current EUROfusion DEMO baseline design [1] compared to the previous one: a radial size of a breeder blanket zone in the outboard side was reduced from 1.3 m to 1.0 m keeping one in the inboard size unchanged. This paper provides an overview of the neutronic development efforts devoted to the detailed design of a solid breeder blanket for DEMO with an inherent capability of a highly efficient tritium breeding. An innovative design of the Helium Cooled Pebble Bed (HCPB) DEMO blanket [2] was developed based on comprehensive iterative neutronic, thermal-hydraulic and stress analyses. As an alternative a promising Molten Lead Ceramic Breeder (MLCB) DEMO design [3] is analyzed and compared to the HCPB one. The key part of the analyses is the development of the geometry model of the breeder blanket with sufficient details to perform high fidelity nuclear simulations. The tritium breeding performance of the blankets was first assessed, followed by calculations of the nuclear power generation and analyses on the shielding performance.
2.1. HCPB blanket design
⁎
The novel HCPB blanket design is based on the implementation of a Single Module Segmentation (SMS) compared to a Multiple Modules Segmentation (MMS) adopted in previous studies [4]. Blanket dimensions were adjusted to fit a current DEMO baseline design 2017 [1]. The SMS blanket casing consists of poloidal breeder modules each of them is built by a 20 mm thick U-shaped-like first wall (FW) with a 30 mm back wall and back supporting structure (BSS) attached to it [2], Fig. 1. The plasma facing part of the FW is faceted and it is designed to have a roofshaped form with a gradient of ˜2° that results in a loss of the breeder zone volume close to plasma by about 3% compared to a flat FW. Such a shape of the FW deals to protect the edges of the blanket module at the gaps against thermal loads due to plasma contacts [5]. The FW encloses He cooling channels of 11 × 11 mm size and a poloidal pitch of 15 mm. The FW is covered with a 2 mm thick W armor. The radial thickness of the BSS is 35 cm (including 15 cm supporting wall) and 35 cm (including 5 cm supporting wall) at the inboard (IB) and outboard (OB) sides, respectively, being constant in all breeder modules. This results in a variable radial depth (from the FW up to back plate) of the breeder zone (BZ) depending on the poloidal position of the breeder module. It is 41 and 63 cm including 6 cm back wall and gas collectors in the mid plane at the IB and OB sides, respectively. An essential innovation included in the new HCPB blanket design is the arrangement of a breeder ceramic in radial breeder pins attached to
Corresponding author. E-mail address: [email protected] (P. Pereslavtsev).
https://doi.org/10.1016/j.fusengdes.2019.01.023 Received 10 September 2018; Received in revised form 23 November 2018; Accepted 6 January 2019 0920-3796/ © 2019 Published by Elsevier B.V.
Please cite this article as: Pereslavtsev, P., Fusion Engineering and Design, https://doi.org/10.1016/j.fusengdes.2019.01.023
Fusion Engineering and Design xxx (xxxx) xxx–xxx
P. Pereslavtsev et al.
Fig. 1. Horizontal cut of the CAD HCPB blanket model (left) and the HCPB SMS blanket (right).
were then integrated into the MCNP generic model as independent universes utilizing proper transformation matrices. Third, separate geometry models were developed on the MCNP platform for the inner space of the blanket and for the FW cooling channels using the repeated structure feature of the MCNP, Fig.2. These models were also integrated into the empty blanket casings of the generic model using transformation cards for each breeder module of the IB and OB blankets. Such a hybrid approach enabled development of the MCNP geometry model with the fully heterogeneous HCPB SMS blankets, Fig.2. The vacuum vessel (VV) in the inboard side has a radial thickness of 60 cm including 6 cm thick SS316 steel walls. The cooling of the VV is
the FW and the back plate. The breeder pin consists of three coaxial tubes: the central 8/14 mm (inner / outer radius) serves for He inlet, a channel between second (30/32 mm) and third (35/39 mm) tubes serves for He outlet, Fig. 1. The annular volume between the first and the second tubes is filled with the breeder ceramic pebbles containing mixture of Li4SiO4 plus 37 mol.% of Li2TiO3 (60% 6Li enrichment in both compounds) with 0.64 package factor. The breeder pins build a radial hexagonal lattice with a regular pitch of 125 mm. The pitch between pins was optimized to provide the maximum tritium breeding ratio (TBR). Be12Ti pebbles, used as neutron multiplier, fill the space around pins with a packing factor of 0.64. All structural elements of the blanket are assumed to be manufactured with Eurofer steel. The SMS blanket is cooled with a He of ˜80 bar and He of ˜2 bar is used as a purge gas. 2.2. HCPB DEMO geometry model The MCNP geometry model of the HCPB DEMO was developed in three steps. First, from the CAD DEMO model 2017 developed in the PPPT, Table 1 [1], a 11.25° toroidal segment was extracted accounting for the symmetry of the model. This segment represents half of one of the 16 toroidal DEMO sectors and includes empty volumes for one IB, one full OB and a half of the central OB blankets keeping 20 mm toroidal gaps between them. This toroidal segment was converted into the MCNP generic model making use of the McCad conversion tool [6]. Such a model has a simple void description and it was used as a basis for the further geometry models build-up. Second, CAD models of three empty SMS HCPB blanket casings were converted separately with McCad into three stand-alone MCNP geometry models. These models Table 1 Main parameters of the DEMO reactor. Major radius, (m) Minor radius, (m) Plasma elongation Plasma triangularity Fusion power, (MW) Net electric power, (MW)
8.938 2.883 1.650 0.333 1998.0 500.0
Fig. 2. MCNP geometry model of the HCPB DEMO. A, B — horizontal cuts in OB and IB sides, C — vertical cut in OB side in radial-poloidal direction, D — vertical cut in OB side in toroidal-poloidal direction. 2
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ensured by water and an ITER-like SS304B7 steel structure, Fig. 2, is arranged inside the VV to provide its stiffening and the shielding performance. Toroidal magnetic field coils (TFC) are enclosed in a steel casing. The divertor is modeled as a solid body (60% steel and 40% water) except three layers facing the plasma. The first and third layers are a 5 mm thick tungsten armor with a 15 mm thick tube layer in between filled with a homogenized mixture of 39.5% W, 17% CuCrZr, 13% Cu and 30% water. The equatorial port is plugged with a steelwater homogenized mixture (60% SS-316 steel and 40% water).
Table 2 Nuclear energy generation [MW] in the DEMO. Component
HCPB
MLCB
Blankets Vacuum vessel Divertor Total Global energy multiplication factor
1931 49 170 2150 1.35
1646 77 197 1920 1.20
of Be pebbles [5]. The realistic roof-shaped FW results in the loss of TBR up to 3% compared to the flat one. The replacement of Be12Ti pebbles with Be ones could provide TBR = 1.23 compared to the reference TBR = 1.16. The replacement of the detailed FW with a homogeneous material mixture results in ΔTBR=+0.01. The replacement of the detailed BZ with the homogeneous material mixture brings additionally ΔTBR=+0.01. The use of the SMS blanket as well as the principally new breeder arrangement and cooling scheme applied in the blanket increased the tritium breeding efficiency and made it possible to compensate the loss of TBR due to the reduction of the breeder blanket space and the replacement of the Be pebbles with Be12Ti. The breakdown of the nuclear power generation in the new HCPB DEMO reactor is given in the Table 2. The energy deposition in the blankets is high enough to ensure a high energy multiplication factor in the HCPB DEMO even in spite of the very short BZ. Fig. 4 shows the radial nuclear power density profiles in the steel in the mid plane of the IB side for different VV configurations. Such configurations refer to different options to enhance the shielding performances: one uses a homogeneous material mixture of 60% SS304B7 steel and 40% water, heterogeneous model with 9.5 cm thick SS304B7 steel inserts in water and an option with 5 cm thick WC plates inserted in the steel ones. The results obtained with the homogenized VV interior structure significantly overestimate the power density there but it has almost no effect to the total TFC heating. This comes from the intensive particle streaming through the toroidal gaps between blankets and through weakly shielded divertor port. For the current DEMO radial build the TFC heating is close to the design limit of 50 W/m3 [8]. The inclusion of neutron absorbing materials like WC can improve the shielding performance of the VV. The radial profiles of the total neutron flux through the blanket up to the TFC in the mid plane of the IB side are shown in the Fig. 5. The option with the WC inserts ensures a better shielding performance of the VV and can decrease the neutron loads to the TFC magnet by a factor of 3÷5. The high energy neutron flux (En > 0.1 MeV) in the superconducting magnet in this case is ˜7⋅108 n/cm2 s that corresponds a neutron fluence of ˜1⋅1021 n/m2 during full life time of the magnet. This is below the design limit of 1022 n/m2 after 6 full power years
2.3. MLCB DEMO geometry model The alternative theoretic MLCB concept [3] of the solid breeder blanket combines two significant features: the use of the breeder ceramic together with a liquid lead neutron multiplier. This concept represent a trial to replace the very expensive and technologically not well developed Be based pebbles with rather effective Pb neutron multiplier to demonstrate the efficiency of the breeder blankets based on the use of solid breeder ceramic. No circulation of the liquid lead outside the reactor is considered currently as a mainstream in the development of the alternative blanket design. As a highlight, the MLCB concept utilizes the same blanket matrix developed for the HCPB DEMO described above. The lattice pitch of 125 mm used in the model was optimized to reach the maximum tritium breeding. 3. Simulation results 3.1. HCPB DEMO The simulations included an assessment of the main required nuclear responses: TBR, nuclear power generation and shielding performances. The calculations were carried out making use of the geometry models discussed above and the MCNP5-1.60 code [7] with nuclear data from the JEFF-3.2 library. For heavy duty MCNP5 runs such as shielding calculations a weight window variance reduction technique was applied. This ensures results with a good statistics usually not exceeding ˜2% for the cells outside vacuum vessel and < 0.1% for the plasma facing components. Fig. 3 shows the neutron wall loading distribution calculated for the new HCPB SMS blankets. The numeration of the breeder modules is anticlockwise starting from the central one in the mid plane of the OB side. The maxima are 1.12 and 1.34 MW/m2 for central IB and OB breeder modules, respectively, the average neutron wall load being ˜0.93 MW/m2. The most basic nuclear response of the HCPB DEMO, it is TBR, was assessed to be 1.16 (with the 0.01% of statistical uncertainty), the same value was achieved in the previous MMS HCPB design based on the uses
Fig. 3. FW neutron wall loading in the HCPB DEMO.
Fig. 4. Radial power density profiles at inboard mid-plane. 3
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Fig. 7. 3D neutron flux distribution in IB side.
3.2. MLCB DEMO Lead employed in the MLCB blanket is a less efficient neutron multiplier compared to Be or Be12Ti compound [9]. To compensate the lower neutron multiplication, a higher 6Li enrichment (90%) is typically applied. The MLCB breeder blanket uses the same matrix as the HCPB with the pitch of 125 mm. The TBR = 1.07 was calculated for this design. Two further consequently modifications were applied to enhance the tritium breeding: the radial thickness of the ceramic layer was enlarged from 16 to 20 mm and the length of the BZ was increased by 5 cm in the OB and 3 cm in the IB sides (BSS was squeezed respectively). The TBR was found to be TBR = 1.11 and 1.13 in the former and latter cases. Further enlargements of the BZ require changes in the DEMO radial build. As a theoretical option, the effect on the TBR of employing pressurized water as a coolant in the FW was assessed, making use of the last MLCB configuration with enlarged BZ. A TBR = 1.05 was found for the homogenized material mixture and TBR = 1.03 in case of the detailed modelling of the FW. The energy generation in the MLCB DEMO for the option with TBR = 1.13, Table 2, is about 10% lower compared to the HCPB DEMO. This comes from the significantly lower energy generation in the blanket, see Fig. 4 due to the lower neutron multiplication. The shielding of the TFC in the MLCB DEMO is comparable with the HCPB reference assuming WC plugs in the VV for both cases, Figs. 4 and 5. This option with sufficient protection of the TFC is feasible also in the case of the MLCB as shown in the previous Figures. The MLCB blanket shows a comparable shielding performance to the HCPB with Be12Ti neutron multiplier.
Fig. 5. Radial neutron fluxes profiles at inboard mid-plane.
Fig. 6. Radial profiles of the DPA and He-accumulation in the inboard midplane.
(FPY) [9]. The inefficient shielding of the divertor and also the big gap between VV and TFC lead to the neutron streaming and as a result to a slight increase of the neutron flux here. The nuclear damage accumulation in the steel structures at the inboard mid plane of the reactor is given in the Fig. 6. The maximum DPA accumulation in the VVdoes not exceeds ˜1.2 DPA/6 FPY that is below the design limit of the 2.75 DPA. Peaks in the He-production profiles come from intensive reflections of the high energy neutrons from water-steel structure resulting in an amplification of (n,α) reactions. The maximum He accumulation behind the VV does not exceeds the design limit for the pipes welding loccations of 1 ppm. The radial arrangement of the breeder pins in the HCPB blanket presumes some neutron streaming through the He cooling channels. A comparison with the previous MMS HCPB blanket design [7] for the radial distribution of the total neutron flux shows even slightly better shielding performances of the new blanket. In further detailed analyses a 3D picture of the neutron flux distribution was prepared for the reactor IB mid plane, Fig. 7. The main streaming paths in the BZ are the Be12Ti layers around the breeder pins rather than the He cooling tubes. The BSS in the blanket has weak shielding performances and it serves mainly for the scattering of the neutrons. A massive ˜15 cm supporting back wall of the BSS provides certain reduction of the neutron flux. The VV serves as a main neutron-shielding component in the DEMO. The modeling of the heterogeneous structure of the VV is essential to locate possible hot spots in the design. In the present configuration the bulk radial stiffening plate demonstrates even worse neutron shielding compared to the toroidal steel-WC plates.
4. Conclusions In the framework of the PPPT program of EUROfusion, a new HCPB blanket concept was elaborated and supporting comprehensive neutronic analyses were performed. The new blanket design was optimized by means of numerous parametric coupled neutronic-thermal hydraulic simulations. A full heterogeneous MCNP geometry model was produced making use of a hybrid modeling procedure including McCad automated conversions and the application of effective MCNP modelling features. Two innovative solid breeder blankets were developed: the HCPB with Be12Ti and the MLCB with Pb neutron multiplier. Both blankets were demonstrated to have a very effective neutron economy. Both can fulfill the design requirements. The use of the solid breeder with the solid or liquid neutron multiplier and He coolant provides sufficient tritium generation in DEMO. The basic neutronic response TBR = 1.16 and 1.13 were assessed for the HCPB and MLCB DEMOs, respectively, using the realistic SMS blanket design that addresses the latest DEMO requirements on the radial build and FW integrity. The excess of the TBR over the net target value of TBR = 1.05 [9] enables further design studies aiming at the inclusion of various in vessel components reducing tritium breeding in the DEMO. 4
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The shielding performances of the new solid breeder blankets can be enhanced by the inclusion in the blanket matrix shielding inserts such as W, WC, graphite and hydrates. This requires further optimizations efforts in the neutronics, thermal hydraulic, engineering and balance of plant activities. The effect of the heterogeneity was found to be essential for the DEMO design and principal project decisions.
Des. 136 (2018) 729–741. [2] F.A. Hernandez, et al., An enhanced, near-term HCPB configuration as driver blanket for the EU DEMO, Fusion Eng. Des. (2018) this issue. [3] G. Zhou, et al., Progress on the helium cooled Molten Lead Ceramic Breeder concept, as a near-term alternative blanket for EU DEMO, Fusion Eng. Des. (2018) this issue. [4] P. Pereslavtsev, U. Fischer, F. Hernandez, Lei Lu, Neutronic analyses for optimization of the advanced HCPB breeder blanket design for DEMO, Fusion Eng. Des. 124 (2017) 910–914. [5] R. Mitteau, et al., J. Nucl. Mater. 415 (1) (2011) 969–972. [6] L. Lu, U. Fischer, P. Pereslavtsev, Improved algorithms and advanced features of CAD to MC conversion tool McCad, Fusion Eng. Des. 89 (9–10) (2014) 1885. [7] X-5 Monte Carlo Team, MCNP — A General Monte Carlo N-Particle Transport Code Overview and Theory (Version 5, Vol. I), Los Alamos National Laboratory, 2003 Report LA-UR-03-1987, 24 April. (Revised 10/3/05). [8] Ch. Bachmann, et al., Overview over DEMO design integration challenges and their impact on component design concepts, Fusion Eng. Des. 136 (Part A) (2018) 87–95. [9] F.A. Hernandez, P. Pereslavtsev, First principles review on the options for tritium breeder and neutron multiplier materials for breeding blankets in fusion reactors, Fusion Eng. Des. 137 (2018) 243–256.
Acknowledgments This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] G. Federici, et al., DEMO design activity in Europe: progress and updates, Fusion Eng.
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