A SELF-COOLED LIQUID BREEDER BLANKET FOR A LASER IFE POWER PLANT WITH MAGNETIC
INTERVENTION
A.R. Raffray1, A. E. Robson2, M. E. Sawan3, G. Sviatoslavsky3, I. N. Sviatoslavsky3 and X. Wang1
1
460 EBU-II, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 90093-0438, USA, rraffray@ucsd.edu
2
Consultant, Naval Research Laboratory, Washington, DC
3
Fusion Technology Institute, University of Wisconsin, Madison, WI
A possible way to address the issue of dry wall
survival in a Laser IFE chamber is to use magnetic
diversion in order to steer away the ions from the
chamber wall (representing ~25-30% of the yield energy).
A cusp magnetic field is imposed on to the chamber; the
ions from the micro-explosion are trapped within the
magnetic field and are directed to more readily accessible
and replaceable dump regions at the equator and poles. A
large fraction of the magnetic energy can be dissipated in
the chamber walls if an electrically resistive structural
material is used. An advanced blanket based on a selfcooled liquid breeder (e.g.Pb-17Li or flibe) and SiCf/SiC
structure has been proposed for this purpose and a
scoping design study performed as part of the High
Average Power Laser program effort
This paper summarizes the results of this scoping
study, and highlights the advantages of such a concept as
well as the key issues that need to be addressed by R&D.
Utilization of a cusp field for such magnetic
diversion has been experimentally demonstrated
previously (e.g. see Ref. 3) and is illustrated in Fig. 1 for a
four-coil arrangement. Following the micro-explosion, the
ions would compress the field against the chamber wall,
the latter conserving the flux. Because of this flux
conservation, the energetic ions would never get to the
wall. The magnetic energy in the compressed plasma can
be dissipated by using resistive chamber blanket walls,
and recovered through the blanket coolant. This would
remove most of the ion energy thereby reducing the ion
load on the chamber wall and/or on the ion collector
plates. Initial estimates indicate that about 70% of the ion
energy can be dissipated in this way and the remaining
30% guided away to the dump regions.
I. INTRODUCTION
The High Average Power Laser (HAPL) program is
carrying out a coordinated effort to develop laser inertial
fusion energy (IFE) based on direct drive targets and a dry
wall chamber1. The dry wall must accommodate the ion
and photon threat spectra from the fusion micro-explosion
over its required lifetime. To avoid the adverse impact of
a buffer gas on target injection, survival and placement,
the current HAPL strategy assumes as baseline a chamber
with no protective gas. The armor/first wall configuration
is based on tungsten and ferritic steel as preferred armor
and structural materials, respectively. For a given target
yield this strategy results in a fairly large chamber to
ensure armor survival; e.g. with a radius of ~10.5 m for a
target yield of 350 MJ (Ref. 1). Thus, a parallel effort is
underway to explore ways of rendering the overall
concept more attractive based on size, design and
performance. A possible option, as proposed by Robson2,
is to use magnetic diversion in order to steer the ions
(representing ~25-30% of the yield energy) away from the
chamber wall. This would also help in avoiding issues of
ion implantation (in particular He) in the chamber armor
possibly leading to early failure.
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Fig. 1
Schematic of cusp field configuration.
An advanced blanket based on a self-cooled liquid
breeder (Pb-17Li or flibe) and SiCf/SiC structure (with the
low electrical conductivity required for resistively
dissipating the magnetic energy) has been considered for
this purpose, and a scoping study performed. This paper
summarizes the results of this study, highlighting the key
findings and issues.
II. CHAMBER AND BLANKET CONFIGURATION
The chamber configuration is illustrated in Fig. 2. A
biconical geometry has been adopted to match the shape
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LIQUID BREEDER BLANKET FOR LASER IFE WITH MAGNETIC INTERVENTION
taken by the expanding plasma in the cusp field. Ion
dump plates are shown at the equator, through which most
of the ions escape. Smaller dumps are provided at the
polar cusps. As an alternate option, the ions could be
guided to separate outer chambers designed for ion energy
accommodation with minimal impact on the main
chamber environment.
to 0.88/0.33 m at the mid cross-section B-B, to 1.06/0.196
m at the top cross-section A-A.
Fig. 4
Fig. 2
Example chamber configuration.
The blanket design builds on the self-cooled Pb-17Li
ARIES-AT concept4, and consists of a number of
SiCf/SiC submodules arranged poloidally in the chamber.
Figure 3 illustrates the submodule cross-section,
comprising an outer wall and a floating inner wall
forming an annular gap. The Pb-17Li is first flown
through the annular channel to cool the structure; it then
turns and, in a second pass, flows slowly in the large inner
channel where it is heated to a high temperature by the
neutrons. In this concept, the maximum Pb-17Li
temperature can be decoupled from the maximum
SiCf/SiC temperature (limited to ~1000°C), thereby
resulting in a higher power cycle efficiency.
A possible fabrication process for the SiCf/SiC
submodule is to utilize expendable core forms. An inner
core form is first used to lay up the SiC fibers and
infiltrate the inner submodule wall; next, a two-piece
form is fitted for lay up and infiltration of the outer
submodule wall. The forms are then removed by thermal
or chemical processes. The separately-fabricated
submodule end caps can then be brazed to complete the
submodule fabrication. Finally, five submodules are
brazed together to form a module for assembly in the
different blanket regions of the chamber.
Fig. 5
Fig. 3
Cross-section of a blanket submodule.
The submodule dimensions vary toroidally because
of the conical geometry, as illustrated in Fig. 4 for the
upper mid-blanket region. In this figure, for a biconical
chamber with dimension of 6 m (for both the cone radius
and height), the submodule toroidal /radial dimensions
vary from 0.7/0.47 m at the equatorial cross-section C-C,
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Upper-mid blanket submodule.
Top view of adjacent modules in chamber
illustrating end submodule wall shaping for a
tight fit.
The module fabrication and assembly are designed so
that the side walls of each submodule are toroidally
pressure-balanced by the adjacent submodules to avoid
the otherwise large stresses due to the long radial span of
the submodule in particular at cross-section A-A in Fig. 4.
This requires a tight fit between all the modules after
assembly, which can be created by shaping the end submodule profiles of neighboring modules (and possibly
including a compliant layer), as illustrated in Figure 5 for
3 of the 16 modules in the upper mid-blanket region.
There are some concerns about the possible domino effect
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LIQUID BREEDER BLANKET FOR LASER IFE WITH MAGNETIC INTERVENTION
on all submodules in case of a catastrophic failure of a
submodule. Possible solutions include isolating a limited
number of modules by including structurally independent
wedges and/or using pressure-sensitive valve system to
drain and decompress the coolant under such an accident
scenario.
III. DESIGN ANALYSIS
The power parameters for the 350 MJ class baseline
target case and a 6 m biconical chamber are listed in
Table I. Detailed neutronics analyses were performed in
support of the design and are described in Ref. 5. They
showed: a tritium breeding ratio of ~1.3 for 90% 6Li in
Pb-17Li and a SiCf/SiC first wall thickness of ~1 cm (as
an initial conservative assumption); a blanket lifetime of
~3.3 FPY for a 3% SiC burnup limit; the 0.5 m thick
shield/vacuum vessel region as lifetime component; and
well-protected superconducting cusp coils.
TABLE I. Chamber parameters
Target yield
367 MJ
Neutron/ion/photon energy
0.75/0.24/0.01
partition
Rep rate
5
Fusion power
1837 MW
Energy multiplication factor
1.19
Total thermal power
2080 MW
Cone height/radius
6/6 m
Peak/avg. neutron wall load
6.1/4.3 MW/m2
Peak power density in SiC
31 MW/m3
Peak/avg. photon heat flux on
0.11/0.08
first wall
MW/m2
plane strain (no strain in the third dimension) and plane
stress (no stress in the third dimension) cases are shown.
The actual total stress would be somewhere between the
two but probably closer to the plane strain case. The
results illustrate the sharp increase in total stress as the
wall thickness is increased, indicating the dominating
effect of the increasing thermal stress over the decreasing
pressure stress. For the present scoping design analysis, it
seems reasonable to choose a first wall thickness of ~5
mm; the corresponding total stresses based on plane stress
and plane strain assumptions are ~83 MPa and ~205 MPa,
respectively, compared to the assumed maximum limit of
~190 MPa for SiCf/SiC. If more margin is needed in the
future, a slightly thinner wall of larger chamber could be
used.
TABLE II. SiCf/SiC properties6
Density
Density Factor
Young's Modulus
Poisson's ratio
Thermal Expansion Coefficient
Thermal conductivity through thickness
Maximum allowable combined stress
Maximum allowable operating
temperature
Maximum allowable SiC burnup
Electrical conductivity
3200 kg/m3
0.95
360 GPa
0.16
4.4 ppm/°C
15 W/m-K
~190 MPa
1000°C
3%
500 (ohm-m)-1
III.A. Stress Analysis
2-D ANSYS analyses were performed to evaluate the
pressure and thermal stresses in the submodule walls for
the maximum heat loads shown in Table I. These occur at
the middle points of the upper and lower chamber conical
regions; the blanket pressure load is higher in the lower
region because of the higher Pb-17Li hydrostatic pressure
(~0.74 MPa for a vertical height of about 9 m from the top
of the chamber to the middle point of the lower chamber
region). When including a blanket pressure drop of ~0.2
MPa and some margin for additional losses in the overall
circuit, the total pressure would be of the order of 1 MPa.
The assumed SiCf/SiC properties are shown in Table II
(Ref. 6).
The pressure and total stresses in the first wall of a
lower mid blanket submodule at the section with
maximum heat loads (roughly equivalent to section B-B
in Fig. 4) are shown in Figure 6 as a function of the first
wall thickness for the 6 m biconical chamber. Both 2-D
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Fig. 6
Stresses in the first wall at section B-B as a
function of first wall thickness for a 1 MPa Pb17Li pressure and a 6 m biconical chamber.
III.B. Thermal-Hydraulic Analysis
The blanket is coupled to a He-driven Brayton cycle
through a heat exchanger (HX). The cycle includes threestage compression with two intercoolers and a high
efficiency recuperator with the following assumed
parameters7:
• Lowest He temperature in the cycle = 35 °C
• Turbine efficiency = 93%
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•
•
•
LIQUID BREEDER BLANKET FOR LASER IFE WITH MAGNETIC INTERVENTION
Compressor efficiency = 89%
Recuperator effectiveness = 95%
He fractional pressure drop = 0.05
Pb-17Li/He temperature difference in HX = 30°C
The compression ratio is set to optimize the cycle
efficiency while maintaining a reasonable He temperature
level at the heat exchanger inlet (which in turn sets the
power core Pb-17Li inlet temperature and influences the
maximum material temperatures).
Fig. 7
Fig. 8
Blanket Pb-17Li inlet and outlet temperatures
and maximum FW SiCf/SiC temperature as a
function of the chamber dimension for a fusion
power of 1836 MW and a maximum SiC/Pb17Li interface temperature limit of 950°C.
Fig. 9
Brayton cycle efficiency, Pb-17Li pressure drop
and pumping power as a function of the SiC/Pb17Li interface temperature for a 6 m biconical
chamber and a fusion power of 1836 MW.
Brayton cycle efficiency, Pb-17Li pressure drop
and pumping power as a function of the chamber
dimension for a fusion power of 1836 MW and a
maximum SiC/Pb-17Li interface temperature
limit of 950°C.
The Pb-17Li flow will be affected by the magnetic
field from the cusp coil (~1 T in the blanket), with typical
transverse Hartmann numbers ranging from ~100 in the
annular channel to ~104 in the inner channel. Even
though the SiCf/SiC provides insulated walls thereby
minimizing this effect, the thermal-hydraulic analysis
conservatively assumed MHD-laminarized flow of the Pb17Li in the blanket and heat transfer by conduction only.
The effect on the pressure drop is small and the analysis
was based on the Dittus-Boelter correlation for rough
tubes8. Figure 7 shows an example of the parametric
results for the Brayton cycle efficiency, Pb-17Li pressure
drop and pumping power as a function of the conical
chamber dimension for the given target yield. The
corresponding Pb-17Li inlet and outlet temperatures are
shown in Figure 8. These calculations assume maximum
SiCf/SiC temperature and SiC/Pb-17Li interface
temperature limits of 1000°C and 950°C, respectively; the
first wall thickness is also optimized around ~5 mm to
maintain the same total stresses in all cases. It is
interesting to observe from Fig. 7 that both the pressure
drop and pumping power show minima at about a conical
chamber dimension of 6 m, corresponding to the largest
temperature difference between the Pb-17Li inlet and
outlet temperatures (and lowest flow rate), as shown in
Fig. 8. This is due to a combination of factors including
606
the higher flow rate required to accommodate the SiC/Pb17Li interface temperature limit for smaller chambers, and
the SiCf/SiC temperature limit for larger chambers, and
the increase in flow length associated with increasing
chamber dimensions.
It is not clear what the maximum SiC/Pb-17Li
interface temperature limit really is as it depends on a
number of conditions. Earlier experimental results
indicated no compatibility problems at 800°C (Ref. 6),
whereas more recent results from the Oak Ridge National
Laboratory indicate a higher limit8. The effect of the
assumed interface temperature limit is illustrated in
Figures 9 and 10. Decreasing the interface temperature
limit from 950°C to 800°C results in a marked reduction
in cycle efficiency from ~59% to ~50%. Interestingly, the
pressure drop and pumping power minima correspond to
an interface limit of 950°C, and both increase
significantly as the interface temperature limit is
decreased and an increase in flow rate is required.
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LIQUID BREEDER BLANKET FOR LASER IFE WITH MAGNETIC INTERVENTION
Blanket parameters for a typical case in a 6 m
chamber are summarized in Table III.
pass through the large inner channel where the flibe is
superheated by the neutrons.
An analysis was carried out of the performance of the
flibe blanket coupled to the Brayton cycle described in
Section III.B. The SiC/flibe interface temperature limit is
not well defined and was kept as a variable. The results
are summarized in Fig.12 for a 6 m chamber and a fusion
power of 1836 MW. The maximum coolant temperature
and cycle efficiency are somewhat lower than for the Pb17Li case due mostly to the poorer heat transfer properties
of flibe. For an interface temperature limit similar to the
SiCf/SiC temperature limit (1000°C), the typical flibe
blanket parameters for a 6 m biconical chamber are:
Fig. 10 Blanket Pb-17Li inlet and outlet temperatures
and maximum FW SiCf/SiC temperature as a
function of the
SiC/Pb-17Li
interface
temperature for a 6 m biconical chamber and a
fusion power of 1836 MW.
-
Flibe inlet/outlet temperatures = 673/1000°C;
Maximum Be temperature = 840 °C
Pressure drop = 0.16 MPa; pumping power = 0.27 MW
Brayton cycle efficiency= 0.57
TABLE III. Summary of Pb-17Li Blanket Parameters
Pb-17Li Inlet Temperature
Pb-17Li Outlet Temperature
Pb-17Li Inlet Pressure
Pb-17Li velocity in 5 mm annular
channel
Average Re in annular channel
Pb-17Li velocity in inner channel
Average Re in inner channel
Pb-17Li blanket pressure drop
Pb-17Li blanket pumping power
630°C
1126°C
1 MPa
~1.5 m/s
1.8x105
0.06-0.09 m/s
4x105
0.11 MPa
0.36 MW
Brayton cycle efficiency
Fig. 11 Blanket submodule with SiCf/SiC as structural
material, flibe as coolant/breeder and a Be plate
for neutron multiplication.
0.59
IV. DESIGN WITH FLIBE AS LIQUID BREEDER
The high electrical resistivity of flibe (about 3 orders
of magnitude higher than Pb-17Li) makes it well-suited
for resistive dissipation of the magnetic energy. In
addition its lower density (1870 kg/m3 compared to 8500
kg/m3 for Pb-17Li) results in lower pressure on the
blanket (~0.5 MPa compared to ~1MPa for the Pb-17Li
case previously analyzed) . Its major drawback is its poor
heat transfer characteristics (thermal conductivity ~1
W/m-K). The submodule design was adapted to assess
the use of flibe as liquid breeder in the SiCf/SiC blanket.
One key change is the addition of a 1-1.5 cm Be layer in
the front of the annular channel to provide for acceptable
tritium breeding and for chemical control of the flibe. The
resulting submodule concept is illustrated in Figure 11.
The same cooling configuration is used, with the flibe
flowing in two-pass: a first pass through the annular
channel to cool the structure and Be; and a slow second
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Fig. 12 Brayton cycle efficiency, flibe pressure drop and
pumping power as a function of the SiC/flibe
interface temperature for a 6 m biconical
chamber and a fusion power of 1836 MW.
V. SUMMARY AND CONCLUSIONS
A scoping design analysis has been performed of a
self-cooled Pb-17Li + SiCf/SiC blanket concept for use in
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LIQUID BREEDER BLANKET FOR LASER IFE WITH MAGNETIC INTERVENTION
the magnetic-intervention cone-shaped chamber. The
design is based on a simple submodule geometry; it
allows for draining and accommodation of the 40
rectangular laser ports with vertical aspect ratio. The
submodule side walls are pressure-balanced to avoid the
large stresses associated with the long side wall span;
only the first wall and back wall are designed to
accommodate the loads.
The blanket exhibits good tritium breeding5 and
power production performance, with the possibility of a
cycle efficiency of 50-60% depending on chamber size
and SiCf/SiC properties and temperature limits. The high
coolant temperatures could also be used for H2
production. However, it must be noted that SiCf/SiC is an
advanced material requiring substantially more R&D than
more conventional structural material (such as ferritic
steel). Also, issues of what outside coolant tube and heat
exchanger material(s) to use at these high temperatures
need to be further investigated.
The submodule design can be adapted to flibe as
breeder/coolant by adding a layer of Be to ensure a tritium
breeding ratio of 1.1 and to provide for chemistry control.
The high electrical resistivity of flibe makes it well-suited
for resistive dissipation of the magnetic energy and its
lower density results in lower stresses on the structure; its
major drawback is its poor heat transfer properties.
However, the resulting cycle efficiency, although
somewhat lower than for the Pb-17Li case, is still
appreciable.
3. R.E. PECHACEK et al, "Measurement of Plasma
width in a Ring Cusp" Phys Rev Lett 45, 256-259 (1980) .
4. A. R. RAFFRAY, L. EL-GUEBALY, S. MALANG,
I. SVIATOSLAVSKY, M. S. TILLACK, X. WANG, and
the ARIES Team, “Advanced Power Core System for the
ARIES-AT Power Plant,” Fusion Engineering & Design,
80, 79-98 (2006).
5. M.
E.
SAWAN,
C.
S.
APLIN,
G.
SVIATOSLAVSKY, I.N. SVIATOSLAVSKY, and A. R.
RAFFRAY, "Neutronics Analysis of a Self-Cooled
Blanket for a Laser Fusion Plant with Magnetic
Diversion," these proceedings.
6. A. R. RAFFRAY, R. JONES, G. AIELLO, M.
BILLONE, L. GIANCARLI, H. GOLFIER, A.
HASEGAWA, Y. KATOH, A.KOHYAMA, S. NISHIO,
B. RICCARDI, and M. S. TILLACK, “Design and
Material Issues for SiCf/SiC-Based Fusion Power Cores,”
Fusion Engineering and Design, 55(1), 55-95 (2001).
7. R. SCHLEICHER, A. R. RAFFRAY, and C. P.
WONG, "An Assessment of the Brayton Cycle for High
Performance Power Plant," Fusion Technology, 39 (2),
823-827 (2001).
8. B. PINT, private communication (November 2006).
ACKNOWLEDGMENTS
This work has been performed through grants from
the Naval Research Laboratory as part of DOE’s funded
HAPL program.
REFERENCES
1. J. D. SETHIAN, A. R. RAFFRAY, J. LATKOWSKI,
J. P. BLANCHARD, L. SNEAD, T. J. RENK, and S.
SHARAFAT, “An Overview of the Development of the
First Wall and Other Principal Components of a Laser
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(3), 161-177 (2005).
2. A. E. ROBSON, "Physics of, and rationale for
magnetic intervention," presented at the HAPL meeting,
Livermore CA, 20-21 June 2005, available at
http://aries.ucsd.edu/HAPL/MEETINGS/0506HAPL/program.html. See also A. R. RAFFRAY, et al.,
“Impact of Magnetic Diversion on Laser IFE Reactor
Design and Performance,” Proceedings, Inertial Fusion
Sciences and Applications 2005, Biarritz, France, Journal
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