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ORIGINAL RESEARCH
Thermal Analysis of Breeder Unit for Helium Cooled SolidBreeder Blanket of Chinese Fusion Engineering Test Reactor
Guangming Zhou • Min Li • Qianwen Liu •
Shuai Wang • Zhongliang Lv • Hongli Chen •
Minyou Ye
� Springer Science+Business Media New York 2014
Abstract As one of the candidate tritium breeding blan-
kets for Chinese Fusion Engineering Test Reactor, a kind
of helium cooled solid tritium breeder blanket was pro-
posed. The blanket uses the pebbles of lithium ceramics
(Li4SiO4) and beryllium as tritium breeder and neutron
multiplier, respectively. The thermal conditions of breeder
unit directly affect the performance of tritium breeding and
the safety of blanket. Therefore, thermal analysis of the
pebble beds is of vital importance for a reliable blanket
design. State steady thermal hydraulic analysis of the
breeder unit has been performed, showing that the tem-
perature satisfied the corresponding material temperature
limits. State steady thermo-mechanical analysis has also
been carried out. The maximum von Mises stress was
within the allowable stress. Parametric sensitivity studies
have been conducted to investigate the influence of main
parameters (e.g. coolant mass flow rate, inlet temperature
and pebble bed thermal conductivity) on the temperature
distribution of the pebble beds and cooling plates.
Keywords Fusion reactor � Solid breeder blanket �Thermal hydraulic analysis � Temperature distribution
Introduction
Chinese Fusion Engineering Test Reactor (CFETR) as the
next step for magnetic confinement fusion energy devel-
opment in China is being designed [1–3]. Tritium-breeding
in D-T fusion reactor, as in CFETR, is essential. As one of
the candidate tritium breeding blankets for CFETR, a kind
of helium cooled solid tritium breeder blanket was pro-
posed [4]. Lithium ceramic (Li4SiO4) pebble bed (packing
factor: 62 %) is used as breeding material, beryllium
pebble bed (packing factor: 80 %) as neutron multiplier,
and Reduced Activation Ferritic-Martensitic (RAFM) steel
as structural material. Breeding blanket is a key component
in fusion reactors, which directly determines the perfor-
mances of tritium breeding and thermal efficiency of the
fusion reactor. The breeder units (BUs) in solid breeder
blankets consist of ceramic breeders and beryllium pebble
beds separated by cooling plates. The key sub-components
of the breeding blankets are the BUs, where the tritium
breeding takes place and nuclear heat is recovered. The
temperature distribution of the materials (breeder, multi-
plier and structural material) in BUs directly affects the
performance of tritium breeding and the safety of the
blanket. Therefore, the thermal hydraulic analysis of BU is
of great significance. Xu and Hermsmeyer [5] conducted a
thermal–hydraulic analysis and optimization of BU for EU
helium cooled pebble bed blanket. Hernandez et al. [6]
carried out thermal analysis to identify the adherence to
maximum temperature in structural and functional mate-
rials in the BU of EU DEMO HCPB blanket. Xu et al. [7]
performed the steady state thermal analysis of HCPB TBM
breeder unit to obtain the temperature distribution in
breeder unit. Cismondi et al. [8] built a 3D CFD model of
EU solid test blanket module to obtain the temperature
fields in the structural and functional materials in breeder
unit. Hernandez et al. [9] assessed the maximum temper-
ature in structural and functional materials of a solid TBM
BU mock-up with respect to the design codes and stan-
dards. Lei et al. [10] performed thermal hydraulic analysis
to assess the temperature distribution in CFETR HECLIC
G. Zhou (&) � M. Li � Q. Liu � S. Wang � Z. Lv � H. Chen �M. Ye
School of Nuclear Science and Technology, University of
Science and Technology of China, Hefei 230027, Anhui, China
e-mail: gmzhou@mail.ustc.edu.cn
123
J Fusion Energ
DOI 10.1007/s10894-014-9798-y
blanket breeder unit, showing that the maximum temper-
ature were within the design temperature limits. Qi et al.
[11] did the thermal hydraulic analyses to measure the
thermal performance of the designed BU of helium-cooled
pebble bed blanket for future fusion reactor.
In this paper, the state steady thermal hydraulic analysis
has been performed. Results showed that the temperature
of the different materials in BU satisfied the corresponding
temperature limits, which indicated that the thermal
hydraulic design of BU was reliable. Thermo-mechanical
analysis has been carried out by using fluid–solid coupling
method. The maximum stress was within the allowable
stress. Those results verified the feasibility of the design. In
addition, parametric sensitivity studies have been per-
formed to study the influence of the main parameters (e.g.
coolant mass flow rate, inlet temperature and pebble bed
thermal conductivity) on the temperature distribution in
BU by CFD code ANSYS FLUENT.
Geometry
In the design, for the typical outboard blanket at the equa-
torial plane, the poloidal height is 960 mm, the radial
thickness is 800 mm, the toroidal width of the first wall
(FW) and the outmost backplate is 1,448 and 1,606 mm,
respectively. Seven radial-toroidal stiffening plates (SPs)
with same interval, shown in Fig. 1, are used to enhance the
blanket mechanically. The spaces delimited by SPs and FW
are used to accommodate the BUs. Therefore, there are 8
BUs in the blanket. The poloidal height of BU is 106 mm.
The breeding zones are enveloped by a trapezium-shaped
first wall structure. The top and bottom of it, encapsulating
the BUs, are closed by two cap plates. The box is closed by a
coolant manifold block containing the coolant/purge gas
supply and collection headers. The breeding zone is subdi-
vided into lithium ceramic tritium breeder and beryllium
neutron multiplier beds, which are separated by flat
U-shaped cooling plates (CPs) with internal channels. In
order to improve the breeding performance and compensate
the neutron losses and parasitic absorptions, the beryllium
pebble beds are designed surrounding the lithium ceramic
pebble beds to increase the amount of neutrons participating
in the breeding reaction with the lithium. The blanket is
actively cooled by high-speed helium flow with a pressure of
8 MPa and a temperature of 300–500 �C. Tritium bred in
BU is purged by purge gas (He ? 0.1 % H2) with a pressure
of 0.12 MPa. There are four layers of Be beds and three
layers of ceramic breeder (CB) beds in BU, shown in Fig. 2.
There are six CPs separating the Be beds from the CB beds.
CPs with cooling channels are used to take away the heat
generated in Be beds, CB beds and CPs. The main design
parameters of the blanket are tabulated in Table 1 [4].
Steady State Analysis
Heat Source
The power density distributions of different materials were
obtained by neutronics analysis [4]. The powers needed to
be removed by CPs are listed in Table 2. Where CP1
represents the CP closest to FW and CP6 is the CP farthest
to FW (see Fig. 2). Volumetric heat sources were
employed on the materials.
Material Properties
The temperature dependent thermal physical properties of
helium are taken from [12]. The thermal physical proper-
ties of RAFM we use are similar with those of F82H.
Purge gas
outlet
Cap
Purge gas
inlet
First wall
Backplates
Helium inlet
Helium outlet
Be bed
Poloidal
Radial Toroidal
Breeder
Fig. 1 3-D structure of the typical outboard blanket
Fig. 2 a Geometry of BU; b the
six cooling plates in BU
J Fusion Energ
123
Therefore, the physical properties of F82H were used in
this investigation [13]. The temperature dependent thermal
physical properties of CB beds and Be beds were taken
from [14].
Boundary Conditions and Models
Considering the symmetry of BU, a geometry model was
built covering the full width, the full radial depth and the
half-height of BU. The symmetric plane cuts through the
middle of BU. Fluid and solid zone were all modeled in
thermal hydraulic analysis. Hexagonal meshes were gen-
erated by using ANSYS Workbench to improve mesh
quality and reduce the number of meshes (see Fig. 3). Mass
flow rates and temperatures were specified at the coolant
inlets and pressure boundary conditions at the outlets (see
Table 2). It was assumed that the coolant was distributed
uniformly into each channel on same CPs. A constant
temperature of 460 �C was employed on the top surface of
BU, which was taken from a conservative finite element
method. The bottom side was employed as symmetry
boundary. A standard k - e turbulence model with stan-
dard wall functions was used in the simulations for its wide
application [15]. For thermo-mechanical analysis, the same
model was adopted excluding the pebble beds. Tempera-
ture fields were easily transferred from thermal hydraulic
analysis results to thermo-mechanical model in ANSYS
Workbench. 8 MPa pressure mounted by coolant helium
was employed on the inner surface of all cooling channels.
Results
The temperature distribution of steady state thermal
hydraulic analysis using nominal values of the main
parameters is shown in Fig. 4. The maximum temperature
of CB, Be, CP were 789, 538, 529 �C respectively, which
were all below the allowable temperature limits (B920 �C
for CB, B650 �C for Be, B550 �C for RAFM) [13, 14].
The peak temperature values of Be and CB bed were
located on the 2# Be bed and 2# CB bed, respectively due
to the relatively higher power density there. Table 3 reports
the maximum, average and minimum temperature of the
pebble beds. The minimum temperature of CB beds and Be
beds was within the temperature window for tritium release
([400 �C for CB, [300 �C for Be) [16].
Figure 5 shows the von Mises stress distribution of CPs.
The maximum von Mises stress was located at the corner
Table 1 Main design parameters of the blanket [4]
Parameters
Blanket
dimension
960 mm (poloidal) 9 800 mm
(radial) 9 1,448–1,606 mm (toroidal)
First wall Thickness: 28 mm (3/15/5); Cooling channel:
trapezium-shaped, cross section 15 mm 9 15 mm,
pitch 20 mm, fillet radius 2 mm; Armor: thickness
2 mm
Cap plate Thickness: 28 mm (12/4/12); Cooling channel:
W-shaped, cross section 6.5 mm 9 4 mm, pitch
14.5 mm, fillet radius 0.5 mm
Stiffening
plate
Thickness: 8 mm (2/4/2); Cooling channel:
W-shaped, cross section 6.5 mm 9 4 mm, pitch
14.5 mm, fillet radius 0.5 mm
Breeder unit Pebble bed thickness (radial): 20/15/180/30/200/45/
40 mm; BU poloidal: 106 mm
Cooling plate: U-shaped, thickness 5 mm
Cooling channel: cross section 6.1 mm 9 2.6 mm,
5.7 mm 9 2.6 mm, 3.4 mm 9 2.6 mm,
3 mm 9 2.6 mm, 2.2 mm 9 2 mm,
1.8 mm 9 2 mm; pitch 10.1/9.7/6.4/5/17.2/
25.8 mm; fillet radius 0.5 mm
Table 2 Main parameters of
CPs [4]CP1 CP2 CP3 CP4 CP5 CP6
Power to be removed (kW) 103.2 114.5 75.7 49.4 12.6 7.1
Inlet/outlet temperature (�C) 435/506 435/520 435/496 435/472 435/496 435/500
Mass flow rate (kg/s) 0.2789 0.2597 0.2384 0.2596 0.0401 0.0208
Average flow velocity (m/s) 44.1 44.5 43.8 43.1 43.8 43.9
Fig. 3 Mesh of BU
Fig. 4 Temperature distribution in BU (nominal input values)
J Fusion Energ
123
of CP2 due to the small radius of the blend there. The
maximum von Mises stress (about 239 MPa) was less than
the 3Sm (420 MPa@529 �C for RAFM), satisfying the
Structural Design Criteria for ITER In-vessel Components
[13, 17].
Sensitivity Analysis
The thermal stability of BU directly affects the perfor-
mance of tritium breeding and the safety of blanket. The
thermal condition of BU are mainly impacted by the
coolant thermal hydraulic conditions and the material and
structure characteristics of pebble bed including material
properties, pebble size, packing factor and even purge gas.
Sensitivity analysis is necessary for understanding the
influence of variations of the main parameters on the
thermal stability of BU. Different values of coolant mass
flow rate, coolant inlet temperature and thermal conduc-
tivity of Be and CB pebble beds have been considered here.
Coolant Mass Flow Rate
The coolant mass flow inside the CPs is very important to
cool the pebble beds and the structural materials, which
may change during normal and off-normal operations. To
investigate the influence of flow rate change on the tem-
perature of pebble beds and CPs, different coolant mass
flow rates have been given. The maximum temperature on
pebble beds is depicted in Fig. 6.
It was found that the change of coolant mass flow had
more significant influence on the temperature of the pebble
beds closer to FW than that of the pebble beds far away
from FW. The temperature of pebble beds was more sen-
sitive to the decrease of flow rate than to the increase of
flow rate. As the flow rate increased, its influence on pebble
beds decreased gradually. A 20 % decrease of coolant flow
rate could cause a maximum temperature increase of 28 �C
in Be beds and 19 �C in CB beds.
Figure 7 shows the coolant flow rate effect on CPs
maximum temperature. The temperature of CPs close to
FW was more sensitive than the other CPs. A 20 %
decrease of coolant flow rate could cause a maximum
temperature increase of 30 �C in CPs. Its influence on the
CPs temperature decreased as flow rate increased.
Coolant Inlet Temperature
In the case of accident in coolant supply system, coolant
inlet temperature may increase. This may cause the
Table 3 Maximum, average and minimum temperature of pebble
beds
Unit (�C) 1# CB 2# CB 3# CB 1# Be 2# Be 3# Be 4# Be
Tmax 698 789 569 524 538 478 480
Tave 579 624 521 482 500 466 463
Tmin 462 462 461 451 452 444 450
Fig. 5 Von Mises stress distribution on CPs (nominal input val-
ues) (unit: MPa)
Fig. 6 Flow rate effect on pebble beds maximum temperature
Fig. 7 Flow rate effect on CPs maximum temperature
J Fusion Energ
123
temperature of BU to exceed the temperature limits. Inlet
temperature in 10, 20, 30, 40, 50 �C greater than the
designed value has been investigated. The maximum
temperature of pebble beds are reported in Fig. 8. The
maximum temperature of pebble beds increased linearly
with the increase of coolant inlet temperature. Since the
temperature margins of pebble beds were large (tempera-
ture margin for CB: 131 �C, for Be: 112 �C), the pebble
beds were not easy to exceed the temperature limits with
the increase of inlet temperature. Even with a 50 �C
increase of inlet temperature, the highest temperature of
CB and Be beds equaled to 833 and 567 �C, far away to
exceed the temperature limits (920 and 650 �C).
Figure 9 shows the inlet temperature effect on CPs
maximum temperature. It was found that CP1, CP2, CP3
and CP4 were more sensitive than CP5 and CP6 to the inlet
temperature increase. With a 30 �C increase of inlet tem-
perature, the temperature of CP1 and CP2 reached to 551
and 553 �C respectively, exceeding the RAFM temperature
limit 550 �C.
Be Bed Thermal Conductivity
The pebble bed thermal properties are determined by sev-
eral parameters (pebble size, packing factor, purge gas,
strain, etc.) [18, 19]. Therefore, the pebble bed thermal
properties are subject to change during operations. Dif-
ferent values of the designed Be bed thermal conductivity
were studied in order to investigate the influence of Be bed
thermal conductivity on pebble beds temperature. Fig-
ure 10 reports the maximum temperature of pebble beds.
The Be bed thermal conductivity had very little influence
on the maximum temperature of CB beds and the 3# Be
and 4# Be bed. The increase of Be bed thermal conduc-
tivity had less effect on the temperature of pebble beds than
the decrease. As the Be bed thermal conductivity increased,
its effect decreased gradually.
Figure 11 shows the Be bed thermal conductivity effect
on CPs maximum temperature. No obvious influence on
the CPs maximum temperature was observed.
Ceramic Breeder Bed Thermal Conductivity
Different values of the designed CB bed thermal conduc-
tivity were studied in order to investigate the sensitivity of
the BU temperature to CB bed thermal conductivity. The
CB bed thermal conductivity effect on the maximum
temperature of pebble beds and CPs is shown in Figs. 12
and 13 respectively.
Fig. 8 Inlet temperature effect on pebble beds maximum temperature
Fig. 9 Inlet temperature effect on CPs maximum temperature
Fig. 10 Be bed thermal conductivity effect on pebble beds maximum
temperature
J Fusion Energ
123
CB thermal conductivity showed a significant influence
on CB temperature, while the maximum temperature of
CPs and Be beds showed a little dependency. As the CB
thermal conductivity increased, its influence on CB
decreased gradually.
Conclusions
Steady state thermal hydraulic analysis of the BU of a
helium cooled solid breeder blanket for CFETR has shown
that the temperature of each material satisfied the corre-
sponding temperature limits. Thermo-mechanical analysis
has been carried out. The maximum von Mises stress was
within the allowable stress. These results indicated that the
design of the BU was feasible. Sensitivity studies consid-
ering the variations of main parameters (e.g. coolant mass
flow rate, inlet temperature and pebble bed thermal con-
ductivity) were also performed. In summary, the tempera-
ture of materials in BU closer to the first wall was more
sensitive to the changes of the mass flow rate and coolant
inlet temperature. CPs were more sensitive to the change of
mass flow rate and coolant inlet temperature than the Be
and CB beds. The maximum temperature of BU increased
linearly with the increase of coolant inlet temperature. The
decrease of mass flow rate had a significant influence on
BU. In order to maintain the thermal stability in BU, the
decrease of mass flow and increase of coolant inlet tem-
perature should be avoided. As the mass flow rate
increased, its effect on the temperature of BU decreased
gradually. As Be (CB) bed thermal conductivity increased,
temperature of the Be (CB) bed decreased. The thermal
conductivity of pebble beds had no obvious influence on
the CPs.
Acknowledgments This work is supported by a scholarship from
China Scholarship Council (File No. 201406340091) and by the
National Special R&D Programme for Magnetic Confinement
Fusion Energy of China funded by Ministry of Science and Tech-
nology of the People’s Republic of China (Grant Nos.
2014GB111005 and 2014GB110001).
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