THE ENGINEERING DESIGN OF ARC: A COMPACT, HIGH FIELD, FUSION NUCLEAR SCIENCE FACILITY AND DEMONSTRATION POWER PLANT

Authors: B.N. Sorbom, J. Ball, T.R. Palmer, F.J. Mangiarotti, J.M. Sierchio, P. Bonoli, C. Kasten, D.A. Sutherland, H.S. Barnard, C.B. Haakonsen, J. Goh, C. Sung, and D.G. Whyte Massachusetts Institute of Technology, Plasma Science and Fusion Center

Acknowledgements

References

2015 IEEE Symposium of Fusion Energy • May 31 – June 4 • Austin, Texas

We thank Leslie Bromberg, Charles Forsberg, Martin Greenwald, Amanda Hubbard, Zach Hartwig, Brian LaBombard, Bruce Lipschultz, Earl Marmar, Joseph Minervini, Geoff Olynyk, Michael Short, Peter Stahle, Makoto Takayasu, Stephen Wolfe, and Stephen Wukitch for conversations and comments that improved this work. BNS is supported by U.S. DOE Grant No.DE-FG02-94ER54235 and Cooperative Agreement No. DE-FC02-99ER54512 This work originated from a MIT Nuclear Science and Engineering graduate course. DGW acknowledges the support of the NSE Department and the PSFC.

Temp (K)

1. Mangiarotti, F., and J. Minervini. "Advances on the Design of Demountable Toroidal Field coils with REBCO superconductors for a Aries-I class Fusion Reactor." (2014).

2. Iter.org, http://www.iter.org/album/media/7%20-%20technical#2044

3. E. Lord, Superpower Inc, Private communication (July 2013)

4. D. Meade, “A Comparison of Unit Costs for FIRE and ITER,” presented at ITER Cost Review Session July 9, 2002

5. Kim, J. M., et al, 26-T/35-mm No-Insulation Multi-Width All-REBCO Magnet: Design, Construction, and 4.2-K Operation, April 10, 2015

6. L. Bromberg, M. Tekula, L. El-Guebaly, R. Miller, Options for the use of high temperature superconductor in tokamak fusion reactor designs, Fusion Engineering and Design 54 (2) (2001) 167

7. Williams, D. F., L. M. Toth, and K. T. Clarno. Assessment of candidate molten salt coolants for the advanced high temperature reactor (AHTR). United States. Department of Energy, 2006.

This poster based on the paper, “ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets” Submitted to Fusion Engineering and Design, Sept. 2014. Preprint available at:

http://arxiv.org/abs/1409.3540

ITER2 (Nb3Sn, 5.3 T, 500 MW)

ARC (REBCO, 9.2 T, 525 MW)

Higher fields enable smaller designs Abstract

The affordable, robust, compact (ARC) reactor conceptual design study aims to reduce the size, cost, and complexity of a combined fusion nuclear science facility (FNSF) and demonstration fusion pilot power plant. ARC is a ~200 MWe tokamak reactor with a major radius of 3.3 m, a minor radius of 1.1 m, and an on-axis magnetic field of 9.2 T. ARC is designed to use rare earth barium copper oxide (REBCO), a type of high-temperature superconductor (HTS), for its toroidal field coils. The use of HTS technology offers many advantages over traditional superconductors when applied to tokamak designs. REBCO superconductors in particular have orders of magnitude higher critical current density than traditional superconductors such as Nb3Sn at local fields greater than 20 T, enabling much higher fields to be used in the tokamak. The large allowable temperature range (up to ~90 K) of HTS allows the use of coolants other than liquid helium and makes possible the design of joints in the toroidal field coils. This allows the vacuum vessel to be replaced quickly, lowering first wall survivability concerns and reducing the cost and operational implications of vessel failure during the experimental phase of the reactor.

External current drive for ARC is provided by two inboard (high-field side) RF launchers using 25 MW of lower hybrid and 13.6 MW of ion cyclotron fast wave power. The resulting efficient current drive provides a robust, steady state core plasma far from disruptive limits. ARC uses an all-liquid blanket, consisting of low pressure, slowly flowing fluorine lithium beryllium (FLiBe) molten salt. The liquid blanket is low-risk technology and provides effective neutron moderation and shielding, excellent heat removal, and a tritium breeding ratio ≥ 1.1. The large temperature range over which FLiBe is liquid permits blanket operation at ~900 K with single phase fluid cooling and a high-efficiency Brayton cycle, allowing for net electricity generation when operating ARC as a pilot power plant. When coupled with a demountable compact reactor design, the immersion blanket allows the vacuum vessel to be a replaceable component, eliminating the need for complex sector maintenance. The modular design of ARC allows a single machine to initially serve as an experiment and then transition to a demonstration commercial reactor.

Design Overview

High temperature superconductors enable joints, leading to demountable coils and a

replaceable vacuum vessel

Design Parameter ValueMajor Radius 3.3 m

Minor Radius 1.13 m

Toroidal Field (on axis) 9.2 T

Fusion Power 525 MW

Total Thermal Power 708 MW (accounts for blanket reactions)

Net Electric Power 190 MWe (assumed 40% efficiency)

Plasma Volume 141 m^3

Plasma Current 7.8 MA

Tritium Breeding Ratio 1.1

LHCD Coupled Power 25 MW (~70 MWe wall-plug)

ICRF Fast Wave Coupled Power 13.6 MW (~20 MWe wall-plug)

● ARC's design calls for the TF coil set to be cooled to 20 K, which leaves a significant (~70 K) temperature margin before the REBCO goes normal

● This temperature margin allows a small amount of resistive heating and enables joints to be designed in the coils

● Joints enable a “demountable” TF coilset which allow for vertical maintenance scheme as opposed to traditional sector maintenance

● When combined with an all-liquid blanket (see opposite), a joint-enabled vertical maintenance scheme allows the ARC vacuum vessel to be a single, replaceable component

● COMSOL 3D FEM stress analysis was performed on a 10 degree section of the TF structure, which corresponds to half of the toroidal extent of a single TF coil

● Contact and roller boundary conditions were applied (see ARC paper for details)

● Simulations showed that the CS being energized actually reduced stress on the central column, so steady state results performed without field from CS were taken as the most conservative case

● Steady state results show a maximum von Mises stress of ~660 MPa, approximately 65% of the yield stress of cryogenic SS 316LN

● Further research into demountable TF design [1] moves the joint location to significantly reduce mechanical stress on the joints themselves and represents an attractive choice for the next iteration of ARC

● New joint location was found to have a negligible effect on field uniformity (as compared to ARIES-I continuous coil design)

● “Jumper” concept for joints would allow for faster disassembly times (estimated at 2-3 months for entire coil disassembly)

● Fusion power scales as the magnetic field on axis to the fourth power

● HTS lacks critical current degradation seen at higher fields in low temperature superconductors like Nb

3SN

● This allows access to significantly higher fields

● ARC's critical current density never exceeds 50% of the critical current density limit

● This means that mechanical stress (65% of stress limit), not superconducting physics limits, is the limiting factor to achieving high fields

● This motivates further research into magnet structure design/materials to allow even higher fields on axis

All-liquid molten salt blanket simplifies reactor/FNSF design

● In order to simplify blanket design and complement the vertical maintenance scheme, ARC uses an all-liquid blanket

● Slowly flowing FLiBe (fluorine, lithium, beryllium) molten salt was chosen as blanket material

● FLiBe acts as a multi-purpose single-phase coolant, neutron moderator, and tritium breeder

● FLiBe is an effective neutron moderator and does not have any “cracks” to allow free-streaming neutrons to escape

● When combined with a TiH2 shielding

layer on the inboard side, MCNP5 simulations show a TF lifetime of ~10 FPY (based on 3e18 HTS fluence limit [6])

● FLiBe's large liquid temperature range allows for high blanket temperatures and use of Brayton cycle thermal conversion

Property FliBe [7] Water

Melting Point (K) 732 273

Boiling Point (K) 1700 373

Density (kg/m3) 1940 1000

Specific Heat (kJ/kg/K)

2.4 4.2

Thermal Conductivity(W/m/K)

1 0.58

Viscosity (mPa-s) 6 1

Temperature distribution in double-walled vacuum vessel concept. (note that temperature discontinuities are a plotting artifact in COMSOL, not a real effect)

Conclusions and Future Work

Double-Walled Vacuum Vessel Concept● Design uses externally located centrifugal pumps to circulate FLiBe from

blanket at higher velocity through channels in VV to actively cool the first wall

● Beryllium neutron multiplier layer added to boost channel TBR, increases global TBR to 1.1 (determined by MCNP5 simulation)

● Vacuum vessel designed to be a single-piece, “consumable” component which can be replaced every ~1-2 years

● COMSOL thermal-hydraulic analysis (incorporating surface heat flux from plasma as well as volumetric neutron/photon heating) shows that moderate channel coolant velocity adequately cools first wall and vacuum vessel

2 m/s FLiBe

● Magnetic field strength (which improves plasma performance) and blanket temperature (which improves thermal conversion efficiency) are limited by engineering, not physics constraints

● This warrants more focus on fusion engineering solutions—need more research into materials and structural design

Thermal Conversion Efficiency● A scoping was performed to evaluate the thermal efficiency of three blanket

temperature cases● The analysis assumed a simple, non-ideal Brayton cycle with

turbine/compressor component efficiencies of 95% to obtain the total thermal cycle efficiencies

● Although “aggressive pilot” case slightly exceeds current material temperature limits, it is included as motivation for further high-temperature fusion materials research

Case Blanket Outlet Temp (K)

Brayton Cycle Efficiency

ARC Net Electric Power (MWe)

FNSF 900 40% 190

Conservative Pilot

1100 46% 233

Aggressive Pilot

1200 50% 261

● HTS (REBCO) is already commercially available in convenient tape form

● HTS is not cost-prohibitive—SuperPower, Inc. quoted a price of $100M - $200M for the amount of HTS tape required for ARC [3], which represents only 2% - 3% of the ~$6B estimated total cost of the reactor island using the Meade scaling [4]

● Recent (April 2015) experiments at the National High Magnetic Field Laboratory demonstrated successful operation of an insulation-free REBCO coil at 26 Tesla [5]

Ongoing magnet design work