Assumptions, Options and
Risks for an MFE DEMO
D J Ward
CCFE
Culham Science Centre
Abingdon, OX14 3DB, UK
CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority
Overview
• This is a personal view, intended to motivate
discussion.
• Just as systems integration can force changes in
our view of what an optimum design of individual
component looks like, including development
timescale and risk can change our view of what an
optimal DEMO looks like.
• Basic Theme: early, low tech DEMO, designed to
minimise development time, cost and risk, at the
expense of having an increased capital cost
compared to a later, more advanced plant.
Outline
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Balancing Development and Timescale
What is the Lowest Tech Solution?
Pulsed operation?
Cost/Value of Current Drive
Steady state, Advanced Physics/Technology
Conclusions
Balancing Development and Timescale
This assumes that development takes time. It also costs money and
increases risks of failure. Note that this is not a roadmap!
What is the Lowest-Tech Solution?
• A DEMO device without current drive and with low first wall
heat loads, particularly on the divertor, appears at first sight
to minimise the challenges, from the plasma side and heat
flux side. DEMO Gen1.
• What about effects of pulsing on magnets, first wall, blanket
and divertor?
• By reducing stresses and extending the pulse length, it may
be possible to achieve a reasonable fatigue life.
• How much attention should be paid to energy storage to turn
this into a power plant with steady-state electrical output?
Largely neglected here.
Purpose of Low-Tech DEMO Gen1
• This would serve as a starting point for DEMO, to
focus research activities, and measure progress as
developments are made.
• Advantage that it would be deliverable in a shorter
time.
• Risk that it will portray fusion as un-competitive (but
what is the comparator?).
• Risk that it will be an inappropriate focus for
research activities, i.e. may distract from a more
long-term target.
How Many Pulses Expected?
• TF coils need to tolerate cyclic stresses imposed by cycling
PF. Over a 30 year lifetime, an 8 hour pulse length would
give around 30,000 pulses, 1 hour pulse length around
200,000.
• Central solenoid needs to tolerate twice this number of
pulses due to swinging from full current to full, opposite
current. Imposes similar stresses on other coils.
• For a 5 year blanket lifetime and a 75% availability, need the
first wall and blanket to survive 4,000 pulses at 8 hours
pulse length, 30,000 at 1 hour.
• Divertor, 2 year lifetime, target 1,500 at 8 hour pulse length,
12,000 at 1 hour.
Generic First Wall Model
Vizvary et al SOFT 2010
Generic First Wall Model - Temperature Results
0.5MW/m2. Eurofer.
First Results
• The helium cooled cases have higher temperatures
and temperature gradients, leading to higher cyclic
stresses.
• All of the water cases have low enough stresses to
exceed the target lifetime for an 8 hour pulse length
and most even for a 1 hour pulse length.
• Almost none of the helium cases achieve the target
lifetime for an 8 hour pulse length.
• However, the water models as studied have too low
a temperature to avoid radiation embrittlement – as
always reality is more complex!
Systems Study Result - Pulsed
This example assumes that 100 MW of heating is used as current drive in
the flat top, to extend the pulse length. Is it better to achieve pulse length
with inductive flux or can non-inductive support be useful?
Cost of Current Drive
• Many ways of looking at this but here is an
illustration.
• Imagine a pulsed DEMO with 8 hour pulse length
(30,000 lifetime pulses) and some supporting
external current drive.
• What is the marginal cost of driving an extra 1MA
by inductive or by non-inductive means?
Inductive
• An extra 1MA will need around 0.003V extra loop
voltage.
• This will need an extra 100 Vs flux swing.
• If the OH coil were around 3m inner radius then this
would need to increase to around 3.3m to provide
the flux swing.
• This would increase the major radius by around 5%
and the total device cost by around 10%.
• (Results based on range of systems studies, but
intended to be generic).
Non-Inductive
• Under DEMO conditions and with current drive γ of
0.25 – 0.5, a marginal current of 1MA would need
an extra 15-30 MW of CD power.
• Based around ITER costs and systems studies, this
is likely to add 1-2% to the plant cost.
• The device has to increase slightly in size to
provide the extra power needed for the current
drive system but, overall, external CD is around 5
times cheaper than inductive current drive, at least
in this marginal example. (More work needed to
check how general this).
Result of Systems Study – Supported Pulse
10
Major radius (m)
9
8
7
6
0
20
40
60
80
100
Percentage of Full Current Drive System included
Allowing increasing amounts of CD power suggests that size (and cost)
reduces as the current drive power is increased. (Suggests that a steadystate device is generally the better option.)
Alternatively a fixed size device can have its pulse length varied by adding
some current drive power. This may be the solution to achieve sufficient
fatigue life.
Use Current Drive to Extend Pulse Length
Noninductive
current
fraction
60%
70%
80%
Major
radius (m)
9.6
9.6
9.6
Plasma
current
(MA)
18
18
18
Fusion
Power
(GW)
2.7
2.7
2.7
CD Power
(MW)
83
116
147
Pulse
length
(hours)
5.5
7.2
10.7
Lifetime
pulses
36,000
28,000
19,000
Fixed device size but
use current drive
power to extend the
pulse length and
achieve the required
fatigue life.
Divertor Heat Load
Div heat load (MW/m2)
18
16
14
12
10
8
6
4
2
0
0
20
40
60
80
100
120
Percentage of Full Current Drive System
Using current drive to support the pulse length, reducing the device size
increases the divertor challenge.
Mitigating action needed, e.g. impurity seeding. What if a related device were
taken to full steady-state?
Systems Study Example – Steady State
This is an example of high current drive (~200MW) steady-state rather than an AT
option. Could turn it into a supported pulse version. High Zeff is due to impurity
seeding to protect the divertor. Study as a later plant DEMO Gen2?
Materials
• If we had to use materials available in the short
term how would that constrain the design?
• Is there a deliverable design, which meets our
objectives, within such a constraint (e.g. 4 MWa/m2
2 MWa/m2)?
• More correctly, design DEMO around the available
materials not specify the materials by DEMO
design.
• In order to maintain availability with poorly
performing materials, there is an additional drive
towards a larger, lower power density, device.
Advanced Plant
• It is well known that the problem of high current
drive in a steady-state device can be addressed by
a high bootstrap fraction device.
• Operating at high shape, higher βN, higher q (lower
current) reduces the current drive power but does
not necessarily lead to a smaller device, if the
fusion power is fixed. AT power plant designs are
usually smaller because of improved technology
(efficiency).
• Clearly a good option but relies on more
development of scenarios, control and potentially
higher risk.
Non-Tokamak Solutions
• These appear to fall into the same category as an
advanced DEMO. They may have advantages in
delivering technical solutions but they also have the
disadvantages of longer development time,
development cost and risk.
• In my opinion, these should continue to be explored
in an accompanying R&D programme, but not as
the main design concept for DEMO.
Conclusions - 1
• Development timescale and risk are additional influences on
the optimum design of a DEMO plant.
• The development of an integrated conceptual design of an
early or low-tech DEMO Gen 1 is intended to focus research
activities, allow an analysis of gaps and serve as a
reference against which to measure progress.
• Present thinking in the EU is that this could be a pulsed
device (although this is not unanimous of course).
• To achieve a reasonable fatigue life, a long pulse length is
desirable and it is cheaper to do this using external current
drive than a very large solenoid, if such a system can be
reliable enough. OPEN ISSUE.
• Work on a more advanced version of DEMO Gen 2, steadystate (possibly AT), smaller size, highlights the advantages
and development needs to move beyond a low tech
reference.
Conclusions - 2
• DEMO Gen1 is intended to be buildable on the shortest
timescale and is intended to trade-off increased capital cost
against reduced development time, development cost and
development risk. The early use of outputs from other
programmes such as materials, ITER DT etc provides
external constraints.
• DEMO Gen2 relies on more development and is therefore
expected to be a later option. Although more time is
available for development there are greater development
costs and greater risks that the developments needed, for
instance on efficient current drive or precise measurement
and control systems in a nuclear environment, will not be
successful.
• There remains value in developing and maintaining both (or
more) options at least in the conceptual design phase.
• SPARE SLIDES
Modelling Power Plant Availability
• When availability is modelled, increasing size reduces
power density and increases availability.
• There is potential to trade size against materials
resilience to keep up the materials lifetime
• (Example here with plants of 1.5 GW electrical output)
Plant Availability (%)
85
80
75
70
7
8
9
Major Radius (m)
10
11
The Value and Risks of Fusion Development
• If development continues globally and it is
necessary to reduce reliance on fossil fuels:
– 10% of future world energy market has a NPV in the
region of 5 trillion $ (at present prices)
– If fusion had a 10% chance of making up that market with
a 10% profit, then expectation value of the development
is approximately 50B$.
– Economically worth doing if there is a 10% chance of
making a 10% profit on 10% of the energy market.
– Additional benefit of insurance against price increases or
of failure of other systems to deliver. At the 0.1% level
this seems easy to justify.
Maintenance and Plant Availability
• The maintenance scheme is already a key driver in
the design of DEMO. If we want to design around
low material lifetimes then this is only increased.
• We need more work on the availability with different
approaches, the viability of those schemes and the
dependence on device size:
– Is the shutdown for blanket replacement almost
independent of device size?
– Is a larger device easier to maintain because of access or
harder because of larger weights to be handled?
JET,
MAST,
etc.
Issue
multibeam
ITER
disruption avoidance
2
2
steady-state operation
1
2
divertor performance
1
3
burning plasma Q>10
power plant plasma performance
1
T self-sufficiency
IFMIF
FW/blanket materials lifetime
NB/RF heating systems performance
1
R
2
3
r
R
R
2
3
R
R
2
3
R
R
R
R
R
R
3
2
3
R
3
2
3
R
3
1
3
R
3
2
3
R
R
R
R
R
1
1
3
R
R
R
R
R
R
R
1
1
3
1
2
3
3
1
tritium issues
Key:
Cook et al 2005
R
R
electricity generation at high availability
superconducting machine
3
R
1
1
R
r
1
divertor materials lifetime
Power
Plant
R
1
FW/blanket components lifetime
DEMO
Phase 2
R
3
1
DEMO
Phase 1
3
materials characterisation
plasma-facing surface lifetime
CTF
3
1
Will help to resolve the issue
2
May resolve the issue
3
Should resolve the issue
R
Solution is a requirement
r
Solution is desirable
R
Use Current Drive to Extend Pulse Length
Noninductive
current
fraction
60%
70%
80%
Major
radius
9.6
9.6
9.6
Plasma
current
18
18
18
Fusion
Power
2.7
2.7
2.7
CD Power
83
116
147
Pulse
length
5.5
7.2
10.7
Lifetime
pulses
36,000
28,000
19,000
Double or Single Null Divertor
• In many power plant studies, the tolerable divertor
heat load is a substantial constraint. Double null
operation with 2 divertors seems an obvious
solution.
• The challenge set by controlling the power balance
between the 2 divertors along with the need for
more complex maintenance with reduced access
presents strong arguments against this.
• The jury remains undecided.
Measurement and Control
• Many of the diagnostics that we routinely use will not survive
long in a nuclear environment.
• Work to improve this situation is essential.
• At the same time, work to minimise the reliance on all but
the most basic measurements supports the “low tech”
approach to DEMO.
• Can advanced tokamak operation be sustained in a nuclear
environment?
• Test this in existing devices by reduced set of diagnostics.
– What is the minimum set of diagnostics that can sustain AT
operation?
– Is that set likely to be available in DEMO (or even ITER), after a few
years of DT operation?
Difference Between Fixed and Modelled Availability
•
Neglecting the effect of size on availability means smallest is best
(lowest cost)
Modelling availability can lead to an optimum at larger machine size.
(Here very low neutron resilience <2MWa/m2)
1.2
Modelled availability
Cost of electricity (normalised)
•
Fixed availability
1.1
1
0.9
0.8
7
8
9
Major Radius (m)
10
11