Magnetic Fusion Power Plants
Farrokh Najmabadi
MFE-IFE Workshop
Sept 14-16, 1998
Princeton Plasma Physics Laboratory
Fusion should demonstrate that it can be a
safe, clean, & economically attractive option
• Gain Public acceptance:
 Use low-activation and low toxicity material and care in design.
• Have operational reliability and high availability:
 Ease of maintenance, design margins, and extensive R&D.
• Have an economically competitive life-cycle cost of electricity:
 Low recirculating power;
 High power density;
 High thermal conversion efficiency.
Assessment Based on Attractiveness & Feasibility
Utility Input
Present Data base
and Designs
Mission
and Goals
Design Options
Evaluation Based on
Customer Attributes
Attractiveness
Redesign
Characterization
of Critical Issues
Feasibility
Assessment
R &D Needs
Development Plan
For superconducting tokamaks,
It is b/e (i.e.,bR0/a) that is important, not b
• Fusion power density, P ~ b2BT4 = (b/e)2 (eBT2)2
Almost Constant for
BT fixed at the TF coil
MHD Figure of Merit
0.12
eBT2
0.10
0.08
0.06
0.04
0.02
0.00
0.15
0.18
0.2
0.22
0.25
e = a/R
0.28
0.3
Tokamak Research Has Been Influenced by the
Advanced Design Program
Current focus of tokamak research
bA/S ( Plasma b)
“Conventional”
high-b tokamaks
(Pulsed operation)
PU: Pulsed Operation
SS: 2nd Stability
FS: 1st Stability, steady-state
RS: Reversed-shear
Advanced tokamak
(Balanced bootstrap)
bp /A ( Bootstrap current fraction)
2nd Stability
high-b tokamaks
(Too much bootstrap)
Our Vision of Tokamaks Has Improved
Drastically in the Last Decade
80s
physics
Pulsar
Major radius (m)
90s physics
ARIES-I ARIES-RS
9
7
5.5
b
2.3%
1.9%
5%
bN
3
3.2
4.8
Plasma current (MA)
10
COE (c/kWh)
13
10
(68% BS)
9.5
11
(88% BS)
7.5
Key Performance Parameters of ARIES-RS
Design Feature
Performance Goal
Reversed-shear Plasma
Radiative divertor
Li-V blanket with
insulating coatings
610o C outlet (including divertor)
Low recirculating power
Wall load:
5.6/4.0 MW/m2
Surface heat flux:
6.0/2.0 MW/m2
Lifetime
Radiation-resistant V-alloy
200 dpa
Availability
Full-sector maintenance
Simple, low-pressure design
Goal: 1 month
< 1 MPa
Safety
Low afterheat V-alloy
No Be, no water, Inert atmosphere
< 1 rem worst-case off-site
dose (no evacuation plan)
Environmental
attractiveness
Low activation material
Radial segmentation of fusion core
Low-level waste (Class-A)
Minimize waste quantity
Economics
Power Density
Efficiency
46% gross efficiency
~90% bootstrap fraction
ARIES-RS is a conceptual 1000MWe power plant
based on a Reversed-Shear tokamak plasma
The ARIES-RS Replacement Sectors are
Integrated as a Single Unit for High Availability
•
•
•
•
•
Key Features
No in-vessel maintenance
operations
Strong poloidal ring
supporting gravity and
EM loads.
First-wall zone and
divertor plates attached to
structural ring.
No rewelding of elements
located within radiation
zone
All plumbing connections
in the port are outside the
vacuum vessel.
The ARIES-RS Blanket and Shield Are
Segmented to Maximize Component Lifetime
•
•
•
•
Outer blanket detail
Blanket and shield consists
of 4 radial segments.
First wall segment, attached
to the structural ring, is
replaced every 2.5 FPY.
Blanket/reflector segment is
replaced after 7.5 FPY.
Both shield segments are
lifetime components:
* High-grade heat is
extracted from the hightemperature shield;
* Ferritic steel is used
selectively as structure and
shield filler material.
The divertor is part of the replacement module,
and consists of 3 plates, coolant and vacuum
manifolds, and the strongback support structure
The divertor structures fulfill
several essential functions:
1) Mechanical attachment of
the plates;
2) Shielding of the magnets;
3) Coolant routing paths for the
plates and inboard blanket;
4) “superheating” of the
coolant;
5) Contribution to the breeding
ratio, since Li coolant is used.
Three RF Launchers Are Needed for
Current Profile Control in ARIES-RS
ICRF Fast Wave
Location
Plasma axis
Frequency
98 MHz
Power to Plasma (MW)
15.7
Lower Hybrid
Edge plasma
4.6 & 3.0 GHz
13.2 & 19.8
Near qmin surface
1.0 GHz
32.1
High-frequency
Fast Wave
• The ICRF fast wave launcher uses a
folded waveguide cavity with
capacitive diaphragms and coax
feed
• Folded waveguides offer a compact
and robust structure and can be built
out of low-activation material with
thin copper coating.
The ARIES-RS Utilizes An Efficient
Superconducting Magnet Design
•
•
•
•
TF Coil Design
4 grades of superconductor
using Nb3Sn and NbTi;
Structural Plates with
grooves for winding only
the conductor.
TF Strcuture
Caps and straps support
loads without inter-coil
structure;
TF cross section is flattened
from constant-tension shape
to ease PF design.
Alternative Confinement Systems
 No current-drive (low recirculating power):
– Stellarators (SPPS): recent advances bring the size in-line with
advanced tokamaks. Needs coils and components with
complicated geometry.
– No superconducting TF coils
– Spherical tokamaks (ARIES-ST): Potential for high performance
and small size devices for fusion research but requires high beta
and perfect bootstrap alignment. Center-post is a challenge.
– RFP (TITAN): Simple magnets and potential for high
performance. Steady-state operation requires resolution of the
conflict between current-drive and confinement.
Stellarator Power Plant Study focused the US
Stellarator Activity on Compact Stellarators
• Modular MHH configuration
represented a factor of two
improvement on previous
stellarator configuration with
attractive features for power
plants.
• Many critical physics and
technology areas were
identified.
Spherical Tokamak Option
Fusion development devices (e.g., neutron sources):
 Modest size machines can produce significant power;
 Planned experiments should establish the physics basis.
Power Plants:
 Recirculating power fraction (mainly Joule losses in the centerpost) is the driving force: Maximize plasma beta and minimize
the distance between plasma and center-post.
The ARIES-ST Study Has Identified Key
Directions for Spherical Tokamak Research
• Substantial progress is made towards
optimization of ST equilibria with
>95% bootstrap fraction:
 b = 54%, k = 3;
• A feasible center-post design has
been developed;
• Several methods for start-up has
been identified;
• Current-drive options are limited;
• 1000-MWe ST power plants are
comparable in size and cost to
advanced tokamak power plants.
Reversed-Field Pinches
 High engineering beta as the toroidal field in the plasma is mainly
produced by the currents flowing in the plasma.
 TITAN Design:
Major Radius
Minor Radius
Neutron wall loading
Poloidal b
Toroidal field at plasma surface
Plasma current
3.9 m
0.6 m
18 MW/m2
0.22
-0.4 T
18 MA
Reversed-Field Pinches
 Pulsed RFP power plants are not attractive (large formation/startup
voltseconds, high loop voltage).
 Steady-state RFPs require efficient current-drive systems
(bootstrap current is small).
 Helicity injection (e.g., oscillating fields current drive) is an option
but can cause increased transport.
 Requires toroidal divertors, impact on dynamo is unknown.
 Requires a conducting shell for stability.
Advanced Technologies:
High-Temperature Superconductors
 YBCO
– Highly textured tapes. Short tapes is produced
– High current density even at liquid nitrogen temperature as long
as B is parallel to the surface of the tape.
 BSSCO (2212-2223 varieties)
– Wires and tapes have been manufactures (100’s m)
– Easier to manufacture than YBCO but they less impressive
performance.
– Much higher current density and critical field capability
compared to Nb3Sn at 4.2K
Advanced Technologies:
High-Temperature Superconductors
 Physics Implications:
– Operation at higher fields (limited by magnet structures and wall
loading)
– Smaller size, plasma current and current drive requirements.
 Engineering Implications:
– Operation at higher temperatures simplifies cryogenics
(specially is operation at liquid nitrogen temperature is
possible)
– Decreased sensitivity to nuclear heating of cryogenic
environment.
Conclusions
 Customer requirements establish design requirements and attractive
features for a competitive commercial product.
 Progress in the last decade is impressive and indicates that fusion
can achieve its potential as a safe, clean, and economically
attractive power source.
 Additional requirements for fusion research:
– A reduced cost development path
– Lower capital investment in plants.
 For fusion energy objectives, our program must address clearly the
relationship between developing an attractive fusion product, the
cost of an energy R&D pathway, the changing market place, and
quality of environment issues such as global climate change.