1. Concept development of
compact DEMO reactor
Kenji Tobita
for DEMO Plant Design Team
Japan Atomic Energy Research Institute
Special thanks: F. Najmabadi (UCSD), C.P.C. Wong (GA),
K. Okano(CRIEPI)
IEA/LT Workshop (W59) combined with DOE/JAERI Technical Planning of Tokamak Experiments (FP1-2)
'Shape and Aspect Ratio Optimization for High Beta Steady-State Tokamak'

2. OUTLINE
1. ABC of Fusion Reactor Study
2. Compact reactor study at JAERI
3. DEMO design study at JAERI
Started in 2003
Focus on the possibility of an economically attractive reactor
in low-A (= 2-2.9), left behind in fusion reactor study previously
- 2 -

3. 1. ABC of Fusion Reactor Study
• Direction of fusion reactor studies
• Necessity to pursue economic fusion energy
- 3 -

4. (A) Reactor study seeks for
an economic reactor concept
Design Year
1995 20001990
0
5
10
15
COE(¢/kWh)
SSTR (16 ¥/kWh)
ARIES-I
ARIES-RS
ARIES-AT
CREST (12.5 ¥/kWh)
Cost-of-Electricity of Fusion
COE of other sources
fission ~5¢/kWh
coal-fired ~6¢/kWh
[1992 JA price basis]
- 4 -

5. (B) In fusion energy,
60~70% of COE is capital cost
COE (¢/kWh) =
Cc + CF + COM
Pe • 8760 (h/yr) • fav
Capital Fuel Operation & maintenance
Capital 53.87 B¥/yr
Fuel 0.04 B¥/yr
Operation 19.77 B¥/yr
Maintenance 17.95 B¥/yr
Costs of CREST (discount rate 2%)
Availabilityoutput
To reduce COE
1) Capital cost
2) Thermal efficiency
3) Availability
- 5 -

6. (C) Much lower construction cost
required for commercialization of FE
Const. Cost Electricity Share
SSTR ~4,500 $/kW
ARIES-RS 3,770 $/kW
Default 3,440 $/kW 0 ~ 6%
Low Cost 2,400 $/kW 4~11%
- 6 -
Fusion share assessment in 2100
4% ~ 1,500 plants
Share depends on
• COE of other sources
• CO2-emission standards, etc.
The estimated fusion cost may not be competitive in market
Tokimatsu (2003)

7. Exploration of compact reactor
USA
Najmabadi (2000)
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JAERI
SSTR (1990)
A-SSTR2 (1999)
Rp = 7 m
Rp = 6.2 m
- 7 -

8. How to compensate for reduced Vp in
compact reactor
 low recirculating power by high bootstrap
 higher thermal efficiency
 higher βN
 higher Bmax
ARIES
JAERI
High β to reduce Bmax
Moderate β at high Bmax
- 8 -

9. 2. Compact reactor study at JAERI
What led us to low-A compact reactor concept?
- 9 -

10. JAERI’s approach toward compact reactor
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Rp = 7 m
Bmax = 16.5 T
βN = 3.5
Rp = 6.2 m
Bmax = 23 T
βN = 4
A-SSTR2
VECTOR
SSTR
Higher Bmax and βN
- 10 -

11. High BT can make it heavy
SSTR
A-SSTR2
7.0 m Rp 6.2 m
16.5 T Bmax 23 T
136 GJ WTFC 181 GJ
11,200 tons TFC Weight 14,640 tons
TFC weight is significant part of reactor: ~ 45% in SSTR
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- 11 -

12. JAERI’s approach toward compact reactor
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Rp = 7 m
Bmax = 16.5 T
βN = 3.5
Rp = 6.2 m
Bmax = 23 T
βN = 4
Rp = 3.5 m
Bmax = 20 T
βN = 5.5
A-SSTR2
VECTOR
SSTR
High Bmax with slim TFC
- 10 -

13. 0 5 10
R (m)
Reduce WTFC by small RTF
ITER
Bmax= 13T
WTFC = 41 GJ
SSTR
Bmax= 16.5T
WTFC = 140 GJ
VECTOR
Bmax= 19T
WTFC = 10 GJ
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High WTFC
Low WTFC
Massive TFC
Slender TFC
RTF
- 12 -

14. VECTOR
18.2m
Rp 3.2 m Ip 14 MA
a 1.4 m βN
5.5
A 2.3 HH 1.3
κ 2.35 n/nGW 0.9
Bmax 19 T qMHD 6.5
BT 5 T Pfus 2.5 GW
Physical features
 CS-less
 Low A (~2.3)
high κ, high nGW, high q
- 13 -
Remove CS to shorten
RTF and reduce WTFC
Concept of VECTOR
Slender CS
Low-A

15. Difference between VECTOR and ST
conventional
VECTOR
ST
CS removed
Cu coil
SC coil
A ~ 2.5
A ~ 1.5
Power reactor
VNS
w. n-shield
w/o. n-shield
A = 3-4
- 14 -

16. VECTOR, likely to have economical
and environmental advantages
Reactor weight (t)
Power/Weight(kWth/t)
Low const. cost
Resource-saving
Economical
0
100
200
300
0 10,000 20,000 30,000
ITER
ARIES-RS
ARIES-ST
SSTR
A-SSTR2
DREAM
VECTOR
- 15 -

17. Radwaste of VECTOR, ~4,000 t
• LLW, vulnerable point of
fusion
(usually, ≥ 10,000 t)
• PWR ~ 4,000 t
Clearance Clearance Clearance
Reinforced shield Reinforced shield
Recycle
Reuse
Compactness
Resources(t)
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Disposalwaste(t)
0
10,000
10,000
20,000
20,000
Clearance
Low level
Medium level
SSTR DEMO2001 VECTOR
Reuse
LiPb
TiH2
Recycle
Be12Ti
Li12TiO3
- 16 -

18. Remarks on VECTOR
VECTOR concept on TFC
system breaks new ground
of power reactor design in
low-A
ST
1 2 3 4 5
2
4
6
8
ARIES-ST
ARIES-AT
ARIES-RS
ARIES-I
A-SSTR2
SSTR
PPCS(B)
PPCS(A)
PPCS(C)
PPCS(D)
CREST
VECTOR
VECTOR-opt
conventional
A
βN
What is sure
Open question
Is the optimal design point for
cost-minimum really A ~ 2.3 for
the VECTOR concept?
Assumed parametric dependence of
βN(A) is uncertain.
- 17 -

19. 3. DEMO design study at JAERI
 How to fit VECTOR concept to DEMO
 Three DEMO options
- 18 -

20. JA Strategy for FE commercialization
IFMIFIFMIF
Commercial.Commercial.
DEMODEMO
ITERITER
Tech.R&DTech.R&D
NCTNCT
 1 GWe output
 Year-long continuous op.
 Economical feasibility
• DEMO must be compact and
have high power density
- 19 -

21. Tradeoff between size and feasibility
CL
small as possible
to reduce WTFC
CSRemove Install
Compact Large Rp
More
feasible
+
difficult +
Size
plasma
Based on roles of CS, three DEMO
options are under consideration
VECTOR concept
- 20 -

22. Difficulties caused by CS-less
 Ip rise/control
Ex) CS-less Ip ramp-up Exp.
(JT-60U, etc)
will be resolved
 Shaping
triangularity is limited (δx ~ 0.3)
problematic in
• confinement in high n/nGW
• suppression of giant ELMs
0.6
0.5
0.4
0.3
0.2
0.1
0
3 4 5 6 7
δ
θ95
giant ELM
grassy ELM
JT-60U
- 21 -

23. Best effort to raise δ w/o CS
Rp 5.1 m
a 2.1 m
Ip 17.5 MA
βp
2.5
li 0.8
κup
2.0
δup
0.3
A far distance between
plasma and PF coils makes
the shaping difficult.
- 22 -

24. Three DEMO options
shaping Ip rampCSsize
“Full CS” 1.5 m (dia.)
~30 Vsec
δx ~ 0.45 15 MAlarge
Option C
“CS-less” small − δx ~ 0.3 −
Option A
0.7m (dia.)
~10 Vsec
“Slim CS” δx ~ 0.4 ~ 5 MAmedium
Option B
challenging
conservative
- 23 -

25. Preliminary design parameters
CS-less Slim CS Full CS
Rp (m) 5.1 5.5 6.5
a (m) 2.15 2.1 2.1
A 2.4 2.6 3.1
κ 2.08 2.0 1.9
δ ~0.3 0.4 0.45
BT /Bmax (T) 5.4 / 18.2 6.0 / 16.4 6.8 / 14.6
Ip (MA) 18.1 16.7 15.0
q95 5.7 5.4 5.3
βN
4.6 4.3 4.1
HH 1.3 1.3 1.3
fBS 0.76 0.77 0.79
n/nGW 0.95 0.98 1.0
Pfus (GW) 3.1 3.0 3.0
Pn (MW/m2
) 3.6 3.5 3.0
Q 49 52 54
Weight (tons) 15,700 17,500 23,900
- 24 -

26. Comparison of Options
0
100
200
300
0 10,000 20,000 30,000
ITER
ARIES-RS
ARIES-ST
SSTR
A-SSTR2
DREAM
VECTOREconomical
Low const. cost
Pfus/weight(kW/t)
Reactor weight (t)
Option A
CS-less
Rp~5.1m
Full CS
Rp~6.4m
Option C
shaping, Ip ramp
Slim CS
Rp~ 5.5m
Option B
shaping
Higher Bmax
κ, βN margin
Adv. n-shield
- 25 -

27. Key parameters in reactor design
inboard SOL
Gap
BLK
n-shield
VV
th-insulator
B
R
CL
Rp
RTF
Bmax
∆TF
∆TF
1.3 m
Rule of thumb
TFCCS
Minimum shield thickness
enough to protect TFC
from neutron damage
Four key parameters : Rp, Bmax, RTF, ∆TF
To use BT effectively, the inboard
SOL width should be small
- 27 -

28. ∆SOL
in
, expected to increase with A
∆SOL
in
usually assumed to be 10 cm
but expected to decrease with A.
∆SOL
out
~
∆ΣΟΛ
ιν
∆ΣΟΛ
ουτ
∼
Α +
1
2
1+
1
Α





Λ
Α −
1
2
1−
1
Α





Λ
∼
1+
Λ
2Α
1−
Λ
2Α
Λ= λν8 Α + βπ +
λι
2
−1
Roughly,
defined by the width of heat flux
in SOL (assumed to be 3 cm)
- 28 -

29. Low-A requires a wide inboard
clearance, especially for “CS-less”
 For A~3
∆SOL
in
~ 10 cm, good approx.
 For A < 2.5
must be careful about ∆SOL
in
without CS
25
20
15
10
2 2.5 3.0
A
∆ΣΟΛ
(χµ )
ιν
p= 2 .5
βpp = 2 .5= 2.5
with CS
Determined from the flux surface corresponding to ∆SOL
out
= 3cm
- 29 -

30. RCS
RTF
∆TF a
RP
0
5
10
15
20
0 1 2 3 4
RTF (m)
Bmax (T)
Rcs = 0.7m
Rcs = 0 m
Rcs = 1.5 m
Separate TFC design
Bmax
CS
TFC
Selection of
design parameter
ｓ
- 30 -

31. κ, βN, BT
Selection of
design parameter
ｓ
RCS
RTF
∆TF a
RP
0
5
10
15
20
0 1 2 3 4
RTF (m)
Bmax (T)
Rcs = 0.7m
Rcs = 0 m
Rcs = 1.5 m
Separate TFC design
Bmax
CS
TFC
75% of κ
78% of βN
2 3 4
1.5
2.0
2.5
3
4
5
A
κ
βΝ
Wong’s formula
(κ, βN)
- 30 -

32. Selection of
design parameter
ｓ
κ, βN, BT
RCS
RTF
∆TF a
RP
0
5
10
15
20
0 1 2 3 4
RTF (m)
Bmax (T)
Rcs = 0.7m
Rcs = 0 m
Rcs = 1.5 m
Separate TFC design
Bmax
CS
TFC
2 3 4
1.5
2.0
2.5
3
4
5
A
κ
βΝ
75% of κ
78% of βN
Wong’s formula
(κ, βN)
HH (=1.3)
IP, qψ, VP, Pfus, PCD, fGW, ….
Check consistency
- 30 -

33. 0.5 1.0 1.5 2.0 2.5 3.0
4
5
6
7
18000 t
15000 t
Pfus= 3 GW
2 GW
RTF (m)
Rp (m)
A = 2
A = 2.5
A = 3
18 T0 T 15 T
Pn = 4MW/m2
3 MW/m2
βΝ = 5
ΤΦΧινβοαρδ ωιδτη (µ)
0.5 1.0 1.5 2.00.0
βΝ = 4
Optimal design point (“Slim CS”)
 Pfus = 3GW
← Pe
net
= 1 GWe
 Weight minimum
 Optimal range, rather wide
optimal
–– less dependent on A (or RTF)
fat TFC & high-A
slender TFC & low-A
- 31 -

34. Breakdown of weight
A= 2.2
A= 2.8
0.5 1.0 1.5 2.0 2.5 3.0
4
5
6
7
18000 t
15000 t
Pfus= 3 GW
2 GW
RTF (m)
Rp (m)
A = 2
A = 2.5
A = 3
18 T0 T 15 T
Pn = 4 MW/m2
3 MW/m2
βΝ = 5
βΝ = 4
Weight (t)
light heavy
Higher A Torus comp.
PFC
TFC
Lower A TFC Torus comp.
PFC
5,000 10,000 15,000 20,0000
TFCPFCBLK
Div
Shld
VV
CryoOther
- 32 -

35. Problem in parameter selection:
βN (A) is not sure
Kessel (ARIES-AT, -RS)Wong (based on Miller’s stab.DB)
A κ(A)
βN(A,κ)
δ-dependence hidden
 βN vs κ curve, depends on
δ
 βN, less dependent on κ in
Our conditions
A = 2.0
2.5
3.0
3.5
8
7
6
5
4
βΝ
κ
2.0 3.0
100% BS-driven plasma
Our systems code uses this
- 33 -

36. How does the optimal design point change
when βN is independent on κ?
Original assumption
2
3
1
κ
1.5 2.0 2.5 3.0 3.5 4.0
0
2
4
6
βΝ
Α
1.5 2.0 2.5 3.0 3.5 4.0
2
3
1
0
2
4
6
A
κ
βΝ
Alternative assumption
to check an impact of βN(κ)
Based on Wong’s formula Kessel-like
(but not incl. dependence of βN on A)
- 34 -

37. RTF (m)
0.5 1.0 1.5 2.00.0
1.5 2.0 2.5 3.0 3.5 4.0
2
3
1
0
2
4
6
A
κ
βΝ
ΤΦινβοαρδ ωιδτη (µ)
0.5 1.0 1.5 2.0 2.5 3.0
4
5
6
7
18000τ
15000τ
Α=2 Α=2.5 Α=3
18Τ0Τ 15Τ
4ΜΩ/µ2
Πν= 3ΜΩ/µ2
Πφυσ= 3 ΓΩ
2 ΓΩ
0.5 1.0 1.5 2.0 2.5 3.0
4
5
6
7
18000 t
15000 t
Pfus= 3 GW
2 GW
RTF (m)
Rp (m)
A = 2
A = 2.5
A = 3
18 T0 T 15 T
Pn= 4MW/m2
3MW/m2
βΝ= 4
ΤΦΧινβοαρδ ωιδτη (µ)
0.5 1.0 1.5 2.00.0
1.5 2.0 2.5 3.0 3.5 4.0
2
3
1
0
2
4
6
κ
βΝ
Α
βΝ= 5
A ~ 3 optimum when βN(A,κ) = const
Original design Constant βN
βN = 4
optimal
Optimal
Slight increase in Rp
- 35 -

38. Present understanding on DEMO
• With slim CS, DEMO seems to
succeed in adopting the
VECTOR concept with plasma
shaping capability.
• At the optimum design point,
DEMO can have low-A (= 2.5-3)
which is unexplored A in
previous power reactor study
before VECTOR.
ST
1 2 3 4 5
2
4
6
8
ARIES-ST
ARIES-AT
ARIES-RS
ARIES-I
A-SSTR2
SSTR
PPCS(B)
PPCS(A)
PPCS(C)
PPCS(D)
CREST
VECTOR
VECTOR-opt
conventional
A
βN
DEMO
- 36 -

39. Summary
VECTOR concept
Removes CS to shorten RTF and reduce WTFC ,
leading to slim TFC system compatible with high Bmax
Suggests a possibility of power reactor with A = 2-3
DEMO
• CS will be necessary for shaping.
• “Slim CS”, i.e., modified VECTOR concept, enables us to
envision DEMO with A = 2.5-3
To make the proper footing of DEMO, dependence of βN on
A and κ should be investigated in the range of A = 2.5-4,
hopefully through international cooperation
- 37 -