Thermo-mechanical aspects of SPL b=1 cavity helium vessel and cryomodule interface
Thermo-Hydraulic Analysis of the Optimized Helium Cooled ...
Transcript of Thermo-Hydraulic Analysis of the Optimized Helium Cooled ...
THERMO-HYDRAULIC ANALYSIS OF THE OPTIMIZED HELIUM COOLED SOLID BREEDER BLANKET FOR CFETR
Shijie Cui Xi’an Jiaotong University
Xi’an, Shaanxi, China [email protected]
Dalin Zhang* Xi’an Jiaotong University
Xi’an, Shaanxi, China [email protected]
Jie Cheng Xi’an Jiaotong University
Xi’an, Shaanxi, China
Wenxi Tian Xi’an Jiaotong University
Xi’an, Shaanxi, China
Suizheng Qiu Xi’an Jiaotong University
Xi’an, Shaanxi, China
G.H. Su Xi’an Jiaotong University
Xi’an, Shaanxi, China
ABSTRACT
China Fusion Engineering Test Reactor (CFETR) is
under design recently, in which a conceptual structure of
the helium-cooled solid breeder blanket is proposed as
one of the candidate tritium breeding blankets. In this
concept, three radial arranged U-shaped breeding zones
are designed and optimized for higher Tritium Breeding
Ratio (TBR) and structure simplification. This blanket
uses the Li4SiO4 lithium ceramic pebbles as the breeder,
while beryllium pebbles as the neutron multiplier. In this
paper, the thermal and fluid dynamic analyses of the
optimized typical outboard blanket module are performed
by CFD method, where the nuclear heating rate is
obtained from the preliminary neutronics calculations.
The thermal hydraulic behaviors of the first wall (FW),
the temperature distributions of submodule structure
material, Li4SiO4 pebble bed and Beryllium pebble bed
under normal and critical conditions are calculated,
respectively. The results show that the temperature on the
blanket module can be effectively cooled below
allowable temperature limits of the materials, even if the
FW is suffering the maximum surface heat flux, which
verified the reasonability of the design of the blanket
cooling scheme. In addition, several parametric
sensitivity studies are conducted to investigate the
influences of main parameters (e.g. coolant mass flow
rate, inlet temperature, pebble bed thermal conductivity
and fusion power) on the temperature distributions of the
blanket components.
1 INTRODUCTION
China Fusion Engineering Test Reactor (CFETR) is a new
tokamak device proposed by China National Integration Design
Group for Magnetic Confinement Fusion Reactor. CFETR is a
transition between ITER and fusion DEMO in R&D. It is
designed to demonstrate 50–200 MW fusion power, 30–50%
duty time factor and tritium breeding ratio (TBR) not lower
than 1.2. Relying on existing ITER physics and technical bases,
CFETR explores options for DEMO blanket & divertor with an
easy changeable core by remote handling [1-6]. The objectives
of designing CFETR are to demonstrate a generation of fusion
power and to realize tritium self-sufficiency by installing a
suitable breeding blanket. At present, three kinds of breeding
blanket concepts including helium-cooled solid breeder blanket
[6], water cooled solid breeder blanket [7], and liquid lead-
lithium blanket [8], have been developed for CFETR [1]. As
one of the candidate blankets, a kind of helium cooled solid
breeder blanket was proposed and optimized for CFETR as
shown in Fig.1 [6]. The design schemes of the blanket modules
and preliminary neutronics analyses and optimizations have
been carried out [6, 9-12]. The helium-cooled solid breeder
blanket has some remarkable advantages such as stable
structure, easy realization, good compatibility between selected
materials, and no Magneto Hydro Dynamics (MHD) effects
Proceedings of the 2016 24th International Conference on Nuclear Engineering ICONE24
June 26-30, 2016, Charlotte, North Carolina
ICONE24-60144
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caused by liquid metals [13]. However, its complex structure
design and nonuniform heat deposition require intensive
thermal hydraulic analysis at the design phase.
The helium-cooled solid breeder blanket adopts Reduced
Activation Ferritic/Martensitic (RAFM) steel as structure
material. In addition, the F82H data is used for this calculation
due to the lack of RAFM steel data [14]. Lithium ceramic of
Li4SiO4 with 90% 6Li enrichment is used as a tritium breeder in
the form of pebbles with packing factor about 55%. Beryllium
pebbles are adopted as neutron multiplier with packing factor
about 80%. In this design, binary breeder sizes of diameters 0.5
mm and 1.0 mm are used to increase the filling ratio. Helium
gas of 8 MPa pressure is employed as the only coolant to
extract the deposited heat in the blanket. The tritium produced
in the breeder units (BUs) is taken out by 0.12 MPa purge gas
(He+0.1% vol. H2).
The helium cooled solid breeder blanket experiences
severe surface heat flux from the plasma and volumetric heat
generated by the neutron wall loading during the CFETR
operation. Because the blanket is designed to operate at
elevated temperatures in order to see tritium breeding capability
and high-grade heat extraction, it is important that the blanket
should be effectively cooled since each material used in the
blanket has an allowable temperature limit. In this paper, the
detailed steady state thermal-hydraulics analysis was carried out
by a commercial Computational Fluid Dynamics (CFD) code,
CFX-11 to obtain the temperature field. Results showed that the
maximum temperatures of the FW, the U-shaped cooling plates
(CPs), the Li4SiO4 pebble bed and the Beryllium pebble bed are
all kept below the allowable temperature limits under both
normal and critical conditions, which indicated that the thermal
hydraulic design of blanket was reliable. In addition, several
parametric sensitivity studies have been performed to study the
influence of the main parameters (e.g. coolant mass flow rate,
inlet temperature, pebble bed thermal conductivity and fusion
power) on the temperature distributions of the blanket
components by ANSYS CFX [15].
2 DESIGN DESCRIPTION OF THE OPTIMIZED TYPICAL OUTBOARD BLANKET MODULE
In the design, for the optimized typical outboard blanket
module, which is located at the outboard of equatorial plane of
the tokamak, the toroidal width of the FW and the outmost
backplate is 1,448 and 1,606 mm, respectively, the radial
thickness is 800 mm and the poloidal height is 960 mm. The
blanket is mainly composed of the FW, caps, stiffening plates
(SPs), breeder units (BUs), backplates and attachment system.
The FW is a U-shaped plate and the front wall is directly facing
plasma. To take away the high heat derived from the plasma, 45
radial-toroidal cooling channels are arranged in the FW
structure in parallel. Helium flows in the channels along radial-
toroidal-radial direction. The coolant inlets and outlets are
staggered in two sides of manifold, which can simplify the
fabrication. To achieve a uniform temperature distribution on
the FW, the coolant helium in the neighboring channels flows in
the opposite direction.
The top and bottom of the FW are welded to a radial-
toroidal cap respectively to form a blanket box. Similar to the
FW, helium will also flow in the internal channels of the caps
for cooling the structure. In the blanket box, seven radial-
toroidal SPs with same intervals are welded to the internal wall
of the FW to enhance the blanket mechanically. The heat
deposited on the SPs is removed by helium flowing in the built-
in coolant channels. The spaces divided by SPs and FW are
used to accommodate the BUs. Therefore, there are totally 8
BUs in the blanket. The poloidal height of BU is 106 mm. The
breeding zones are enveloped by a trapezium-shaped FW
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 subdivided into lithium
ceramic tritium breeder and Beryllium neutron multiplier beds,
which are separated by flat U-shaped CPs with internal cooling
channels. According to Ref. [16], breeding zones parallel to
FW, in which the Beryllium pebble beds are designed
surrounding the lithium ceramic pebble beds, were adopted to
improve the breeding performance, compensate the neutron
losses and acquire a higher TBR. High-speed helium gas of 8
MPa pressure and a temperature of 573-773 K is employed as
the only coolant to extract the deposited heat in the blanket. The
tritium produced in the BUs is taken out by 0.12 MPa purge gas
(He+0.1% vol. H2). There are four layers of Beryllium beds and
three layers of ceramic breeder (CB) beds in BU. 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 parameters of the optimized typical
outboard blanket module are listed in Table 1.
Fig.1. Schematic view of the optimized typical outboard blanket
module
Table 1 Main parameters of the optimized typical outboard
blanket module
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Parameters
Blanket size 960 mm (poloidal) × 800 mm (radial) ×
1448-1606 mm (toroidal)
FW Thickness: 28 mm (3/15/5), channel: U-
shaped, cross section:15 ×15 m, pitch: 20
mm, fillet radius: 2 mm
Tungsten armor: thickness 2mm
BU Pebble bed: radial thickness:
20/15/160/30/180/45/80 mm; toroidal width:
1448/1368/1328/1218/1148/1038/938 mm
Curvature radius of pebble bend:
40/17.5/22.5/22.5/30/30/10 mm
Cooling plate: U-shaped, thickness 5 mm
Channel: cross section 6.1 mm ×2.6 mm,
pitch 10.1 mm; fillet radius: 0.5 mm
Cap
Thickness: 28 mm (12/4/12) channel: W-
shaped, cross section: 6.5 ×4 mm; pitch:
14.5 mm, fillet radius: 0.5 mm
SP Thickness:8 mm (2/4/2) channel: W-shaped,
cross section: 6.5 ×4 mm; pitch: 14.5 mm,
fillet radius: 0.5 mm
Backplate Radial thickness: 35/10/10/10/40 mm
Pipe Diameter of helium inlet/outlet: 80 mm
Diameter of purge gas inlet/outlet: 35 mm
3 STEADY STATE ANALYSIS
3.1 Optimized Neutronics Analysis
The optimized neutronics analyses including TBR and
nuclear heat have been performed by using the Monte Carlo
code MCNP and the nuclear cross-section data from the
FENDL-3.0 and ENDF-B-VII.0/n nuclear data library offered
by IAEA. The total TBR of the optimized typical outboard
blanket module can reach 1.54, which shows that the TBR of
the optimized design is greater than that of the original scheme
and meets the tritium-sufficiency requirement very well.
The power density distribution was calculated for the
equatorial blanket of the optimized scheme and the results were
listed in Table 2, where CP1 represents the CP closet to FW and
CP6 is the CP farthest to FW. As there were 8 layers in the
poloidal direction of outboard blanket, the middle one (5th
layer) was chosen which is closest to the equatorial plane. It can
be seen that the radial distributions of the power deposition in
the different materials were all highly nonuniform, which makes
it difficult to cool the blanket components. Fig. 2 shows the
nuclear heating rate in different components of the optimized
typical outboard blanket module as a function of radial distance
from the FW. The total nuclear power deposition in the blanket
is estimated as 0.854 MW. This amount of volumetric heat
sources were employed on the materials and will be removed
from the blanket by proper design of heat exchangers and
ancillary system. About 79.3% of the heat is attributed to the
breeder and multiplier zones, while the heat deposited in the
FW and CPs is about 20.7%.
Table 2 Nuclear power generation of different components of
CFETR
Blanket Component Power (MW)
Neutrons Photons Total
FW 0.0289 0.1041 0.1330
CPs 0.0055 0.0378 0.0434
Breeder Zone 0.3754 0.0164 0.3918
Multiplier Zone 0.2222 0.0636 0.2858
Total 0.6321 0.2219 0.8539
0 10 20 30 40 50 600
2
4
6
8
10
Po
wer
den
sity
(W
·cm
-3)
Radial distance from the FW (cm)
FW
CP1 outside
CP1 inside
CP2 outside
CP2 inside
CP3 outside
CP3 inside
Be1
Be2
Be3
Be4
Li1
Li2
Li3
Fig.2 Nuclear heating rate of the optimized typical outboard
blanket module as a function of radial distance from the FW
3.2 Material Properties
The temperature dependent thermal physical properties of
helium are taken from [17].
Mass Density of Helium:
1
3
1.248.14 1 0.4446 kg m
p p
T T
(1)
Specific Heats of Helium:
5195 J/kg Kpc (2)
3117 J/kg Kvc (3)
Coefficient of Dynamic Viscosity of Helium:
7 0.73.674 10 Pa sT (4)
Coefficient of Thermal Conductivity of Helium:
4
3 3
0.71(1 2 10 ) -1 -1
2.682 10 (1 1.123 10 )
W m Kp
p
T
(5)
The thermal physical properties of F82H were used for this
calculation due to the lack of the data of RAFM steel [14].
Mass Density of F82H:
37871 kg m (6)
Specific Heats of F82H:
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2
-5 3 -8 4
1390.2 7.8498T+0.022969T -
2.7446 10 T +1.1932 10 T J/kg K
pc
(7)
Coefficient of Thermal Conductivity of F82H:
-6 2
-9 3 -1 -1
28.384-0.011777T-1.0632 10 T
-8.2935 10 T W m K
(8)
The temperature dependent thermal physical properties of
CB beds and Be beds were taken from [18].
Specific Heats of Be beds:
-3 -116.443 J mol Kc (9)
Function of Thermal Conductivity of Be beds:
0 ( )K A T (10)
Table 3 shows the coefficient of Thermal Conductivity of
Be beds:
Table 3 The coefficient of Thermal Conductivity of Be beds
T/K K0 A
298 1.22 8.9
453 1.53 7.0
648 1.86 6.2
748 1.86 6.0
Table 4 shows the Specific Heats of CB beds:
Table 4 The Specific Heats of CB beds
Temperature/K Specific heats/J·m-3
·K-1
273 1400.6×103
373 2249.9×103
473 2458.5×103
573 2667.1×103
673 2875.7×103
773 3091.8×103
873 3166.3×103
973 3300.4×103
1073 3412.1×103
1173 3486.6×103
1273 3531.3×103
1373 3561.1×103
1473 3576.0×103
Coefficient of Thermal Conductivity of CB beds: 3 -1 -10.7686+0.4957 10 ( 273) W m KT (11)
3.3 Boundary Conditions and Simplified Model
The thermal-hydraulic calculations were all performed
under steady state condition using CFX. This code can solve
conjugate heat transfer between fluid and structure. Simplified
model of the FW and BU were solved simultaneously with
some assumptions, such as symmetry conditions in this primary
study. Considering the blanket repeated the same structure in
poloidal direction, 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. In this work,
only FW and BU were considered, and the thermal-hydraulic
phenomena of caps, backplates and SPs were not taken into
account.
As shown in Table 5, mass flow rates and temperatures
were specified at the coolant inlets and pressure boundary
conditions at the outlets. And it gave the main thermal-hydraulic
parameters of FW and CPs. It was assumed that the coolant was
distributed uniformly into each channel on same CPs. It could
be seen that the mass flow rate of helium coolant flowed into
each channel of FW was 0.0275 kg/s, with the inlet temperature
of 573 K and the outlet temperature was 653.9 K after it passed
through the FW channel. The total mass flow rates of helium
coolant flowed into each channel of CPs were 0.025 kg/s, with
the inlet temperature of 695 K after it passed through the cap
and SP channels. The temperature rises of the helium in the
CP1-6 were different but the mixing temperature was about
773.9K.
Because the helium is in turbulent regime, turbulence
model should be adopted in the calculation. In this simulation,
standard k–ε turbulence model was used with scalable wall
function. Although scalable wall function is known to be
applied to arbitrarily fine mesh [20], the hexahedral mesh was
carefully constructed so that y+ has a range of 20-120 over the
entire mesh near the wall. The total number of nodes in the
mesh is 8 million and this was solved by a 16-node computer
with parallel computing.
Table 5 Main thermal-hydraulic parameters of FW and CPs
Power to
be
removed
(kW)
Inlet/Outlet
temperatur
e (K)
Mass
flow
rate
(kg/s)
Average
Velocity
(m/s)
Pressure
Drop
(kPa)
FW 133.0 573/653.9 0.0275 18.4 9.0
CP1 17.2 695/771.6 0.0035 39.1 194.0
CP2 14.8 695/778.2 0.0045 49.3 252.3
CP3 5.5 695/771.1 0.002 22.8 50.3
CP4 3.8 695/770.2 0.00185 20.8 42.1
CP5 1.3 695/773.9 0.00045 5.8 2.1
CP6 0.7 695/777.7 0.0002 2.3 0.3
3.4 Computational Results
Fig. 3 shows the temperature contours of the FW, the CPs,
the breeder zone and the multiplier zone under 0.3 MW/m2
surface flux. The maximum temperatures of the FW, CPs,
breeder and multiplier were calculated to be 769.1 K, 789.0 K,
1170.4 K and 892.4 K, respectively, which were all within the
corresponding temperature limits (823.15 K for RAFM,
1193.15 K for CB, 923.15 K for Be) [14, 19].
Fig. 4 shows the temperature contours of the FW, the CPs,
the breeder zone and the multiplier zone under 0.5 MW/m2
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surface flux. The maximum temperatures of the FW, CPs,
breeder and multiplier were calculated to be 807.1 K, 788.9 K,
1169.3 K and 892.7 K, respectively, which were all within the
corresponding temperature limits as before. The maximum
temperature of the FW was about 38 K higher than the normal
condition while the CPs, breeder and multiplier had little
difference. The results show that the temperature on the blanket
module can be effectively cooled below allowable temperature
limits of the materials, even if the FW is suffering the maximum
surface heat flux, which verified the reasonability of the design
of the blanket cooling scheme.
As a result of the relatively higher power density there, the
maximum temperatures of CB and Be bed were located on the
CB1 bed and Be2 bed, respectively. Table 6 shows the
maximum, average and minimum temperature of the pebble
beds. The minimum temperatures of CB beds and Be beds were
all within the temperature window for tritium release (>673 K
for CB bed, >573 K for Be bed) [21].
Table 6 Maximum, average and minimum temperatures of the
pebble beds
Unit (K) Tmax Tave Tmin
CB1 1170.43 932.93 695.42
CB2 1023.66 860.12 696.58
CB3 821.03 761.75 702.47
Be1 776.28 687.39 598.49
Be2 892.35 794.30 696.24
Be3 847.87 772.57 597.28
Be4 789.76 747.57 705.37
Fig.3 Temperature contour: (a) FW; (b) CPs: (c) breeder zone
(d) multiplier zone (normal condition)
Fig.4 Temperature contour: (a) FW; (b) CPs: (c) breeder zone
(d) multiplier zone (critical condition)
4 SENSITIVITY ANALYSIS
The thermal stabilities of BU and FW directly affect the
performance of the tritium breeding, the integrality and the
safety of blanket. The thermal conditions of BU and FW are
mainly influenced by the coolant thermal hydraulic conditions,
the fusion power excursion and the material and structure
characteristics of pebble bed including material properties,
pebble diameter, packing factor and even purge gas. Sensitivity
analysis is necessary to understand the influence of variations of
the main parameters on the thermal stabilities of BU and FW.
Different values of coolant mass flow rate, coolant inlet
temperature, thermal conductivity of CB and Be pebble beds
and fusion power have been considered here.
4.1 Coolant inlet temperature
In the case of accident in helium coolant supply system, the
coolant inlet temperature may rise. This may cause the
temperatures of CB and Be beds to exceed the corresponding
temperature limits. Coolant inlet temperature in 10, 20, 30, 40,
50 K greater than the designed value has been investigated in
this work. Fig. 5 shows the maximum temperatures of pebble
beds. It can be seen that the maximum temperatures of pebble
beds increased linearly with the increase of coolant inlet
temperature. Since we use the most conservative physical
properties of pebble beds in this paper, the temperature margins
of pebble beds were not very large (temperature margin for CB:
22.72 K, for Be: 30.80 K). As a result, the pebble beds tended
to exceed the corresponding temperature limits with the
increase of coolant inlet temperature, and this should be
avoided during the CFETR operation. The highest temperatures
of CB and Be beds reached the corresponding temperature
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limits (1193.15 K and 923.15 K) when the coolant inlet
temperatures increased 36.5 K and 30 K, respectively.
690 700 710 720 730 740 750750
800
850
900
950
1000
1050
1100
1150
1200
1250
Temperature limit for Be bed-923.15 K
Temperature limit for CB bed-1193.15 K
Max
imum
Tem
per
ature
/K
Coolant inlet temperature/K
Be1 Be2 Be3
Be4 CB1 CB2
CB3
Fig.5 Coolant inlet temperature effect on pebble beds maximum
temperature
Fig. 6 shows the coolant inlet temperature effect on CPs
maximum temperature. It could be found that CP2, CP3, CP4,
CP5 and CP6 were more sensitive than CP1 to the coolant inlet
temperature rise. The highest temperatures of CP1, CP2, CP3,
CP4, CP5 and CP6 reached the RAFM temperature limit 823.15
K when the coolant inlet temperatures rised 49.32 K, 34.37 K,
44.64 K, 47.00 K, 40.52 K and 42.33 K, respectively, and it
could be seen that the CP2 was the most dangerous.
690 700 710 720 730 740 750770
780
790
800
810
820
830
840
Temperature limit for RAFM-823.15 K
Max
imum
Tem
per
ature
/K
Coolant inlet temperature/K
CP1
CP2
CP3
CP4
CP5
CP6
Fig.6 Coolant inlet temperature effect on CPs maximum
temperature
4.2 Coolant mass flow rate
The helium coolant mass flow inside the FW and the CPs
is very important to cool the structural materials and the pebble
beds, which may change during normal and off-normal
operations such as loss of off-site power accident, In-vessel
LOCA, Ex-vessel LOCA, In-box LOCA and mechanical
failures, etc. To investigate the influence of helium coolant mass
flow rate change on the maximum temperatures of pebble beds,
FW and CPs, different coolant mass flow rates have been given.
It's also vital to notice that when the coolant mass flow rate
changes, the coolant temperature rise will also change after it
passes through the FW, Cap and SP. The main thermal-
hydraulic parameters of the FW, Cap and SP under different
coolant mass flow rates are shown in Table 7. Fig. 7 shows the
maximum temperatures on pebble beds.
Table 7 Main thermal-hydraulic parameters of the FW, Cap and
SP under different coolant mass flow rates
Mass Flow
rate
QFW
(kg/s)
TFWinlet
(K)
TFWoutlet
(K)
TCPinlet
(K)
60% 0.0165 573 708.0 776.3
80% 0.022 573 674.3 725.5
100% 0.0275 573 654.0 695.0
120% 0.033 573 640.5 674.7
140% 0.0385 573 630.9 660.1
160% 0.044 573 623.6 649.3
180% 0.0495 573 618.0 640.8
It could be easily found that the change of coolant mass
flow rate had more significant influence on the maximum
temperatures of the pebble beds far away from FW than those
closer to FW, and it also had more significant influence on Be
beds than CB beds. The maximum temperatures of pebble beds
were much more sensitive to the decrease of coolant mass flow
rate than to the increase, which meant that the influence of the
coolant mass flow rate on the pebble beds increased gradually
as it decreased. A 20 % decrease of coolant mass flow rate
could cause a maximum temperature increase of 86.7 K in Be
beds and 76.3 K in CB beds. The highest temperatures of CB
and Be beds reached the corresponding temperature limits
(1193.15 K and 923.15 K) when the coolant mass flow rates
decreased 16% and 14%, respectively.
Fig. 8 shows the coolant mass flow rate effect on FW and
CPs maximum temperature. It could be found that the maximum
temperatures of FW and CPs were much more sensitive to the
decrease of coolant mass flow rate than to the increase, and CPs
were more sensitive than FW. As the coolant mass flow rate
increased, its influence on FW and CPs maximum temperatures
decreased gradually. A 20% decrease of coolant mass flow rate
could cause a maximum temperature increase of 64.0 K in FW
and 87.5 K in CPs. The highest temperatures of FW, CP1, CP2,
CP3, CP4, CP5 and CP6 reached the RAFM temperature limit
823.15 K when the coolant mass flow rates decreased 24.7%,
16.3%, 13.0%, 17.2%, 18.7%, 15.0% and 16.1%, respectively,
and it could be seen that the CP2 was still the most dangerous.
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60% 80% 100% 120% 140% 160% 180%600
700
800
900
1000
1100
1200
1300
1400
1500
Temperature limit for Be bed-923.15 K
Max
imum
Tem
per
ature
/K
Coolant mass flow rate
Be1 Be2 Be3
Be4 CB1 CB2
CB3
Temperature limit for CB bed-1193.15 K
Fig.7 Coolant mass flow rate effect on pebble beds maximum
temperature
4.3 CB pebble bed thermal conductivity
The pebble bed thermal properties are determined by
several parameters (packing factor, purge gas, pebble diameters,
etc.). Therefore, the pebble bed thermal properties are subject
to change during normal and off-normal operations. Different
values of the designed CB pebble bed thermal conductivity
have been studied to investigate the influence of CB pebble bed
thermal conductivity on pebble beds temperature, and this was
shown in Fig. 9. It could be seen that the CB pebble bed
thermal conductivity had very little influence on the maximum
temperatures of Be pebble beds, and the change of CB pebble
bed thermal conductivity had more significant influence on the
maximum temperatures of the CB pebble beds closer to FW
than those far away from FW. The maximum temperatures of
CB pebble beds were much more sensitive to the decrease of
CB pebble bed thermal conductivity than to the increase. As the
CB pebble bed thermal conductivity increased, its influence on
the maximum temperatures of CB pebble beds decreased
gradually. The highest temperature of CB pebble beds reached
the temperature limits (1193.15 K) when the CB pebble bed
thermal conductivity decreased 5.8%, according to the most
conservative physical properties of pebble beds used in this
paper.
60% 80% 100% 120% 140% 160% 180%650
700
750
800
850
900
950
Temperature limit for RAFM-823.15 K
Max
imu
m T
emp
erat
ure
/K
Coolant mass flow rate
FW CP1
CP2 CP3
CP4 CP5
CP6
Fig.8 Coolant mass flow rate effect on FW and CPs maximum
temperature
Figure 10 shows the CB pebble bed thermal conductivity
effect on CPs maximum temperature. It could be seen that the
CB pebble bed thermal conductivity had very little influence on
the maximum temperatures of CPs.
.
60% 80% 100% 120% 140% 160% 180%700
800
900
1000
1100
1200
1300
1400
Temperature limit for CB bed-1193.15 K
Max
imu
m T
emp
erat
ure
/K
CB pebble bed thermal conductivity
Be1 Be2 Be3
Be4 CB1 CB2
CB3
Fig.9 CB pebble bed thermal conductivity effect on pebble beds
maximum temperature
4.4 Be pebble bed thermal conductivity
Different values of the designed Be pebble bed thermal
conductivity have been studied to investigate the influence of
Be pebble bed thermal conductivity on pebble beds maximum
temperature, and this was shown in Fig. 11. It could be seen that
the Be pebble bed thermal conductivity had very little influence
on the maximum temperatures of CB pebble beds, Be1 and Be4
pebble beds. The maximum temperature of Be2 pebble bed was
more sensitive to the increase of Be pebble bed thermal
conductivity than Be3. The maximum temperatures of Be
pebble beds were much more sensitive to the decrease of Be
pebble bed thermal conductivity than to the increase. As the Be
pebble bed thermal conductivity increased, its influence on Be
pebble beds maximum temperatures decreased gradually. The
highest temperature of Be pebble beds reached the temperature
limits (923.15 K) when the Be pebble bed thermal conductivity
decreased 18.0%, according to the most conservative physical
properties of pebble beds used in this paper.
Figure 12 shows the Be pebble bed thermal conductivity
effect on CPs maximum temperature. No obvious influence on
the CPs maximum temperature was observed.
7 Copyright © 2016 by ASME
60% 80% 100% 120% 140% 160% 180%770
780
790
800
810
820
830
Temperature limit for RAFM-823.15 K
Max
imu
m T
emp
erat
ure
/K
CB bed thermal conductivity
CP1
CP2
CP3
CP4
CP5
CP6
Fig.10 CB pebble bed thermal conductivity effect on CPs
maximum temperature
0.6 0.8 1 1.2 1.4 1.6 1.8750
800
850
900
950
1000
1050
1100
1150
1200
Temperature limit for Be bed-923.15 K
Max
imu
m T
emp
erat
ure
/K
Be pebble bed thermal conductivity
Be1 Be2 Be3
Be4 CB1 CB2
CB3
Fig.11 Be pebble bed thermal conductivity effect on pebble
beds maximum temperature
4.5 Fusion power
Plasma instability may happen during the CFETR
operation, which can result in the fusion power excursion.
Different values of the designed fusion power have been studied
to investigate the influence of fusion power on pebble beds
maximum temperature. It's vital to notice that the FW surface
heat flux and the nuclear power deposition in the blanket will
change in direct proportion to the fusion power, while the
coolant mass flow rate still maintains the normal level. Fig. 13
shows the fusion power effect on pebble beds maximum
temperature. It could be seen that the maximum temperatures of
pebble beds all increased linearly with the increase of fusion
power. It could be easily found that the change of fusion power
had more significant influence on the maximum temperatures of
the CB pebble beds closer to FW than those far away from FW,
and the degrees of sensitivity for the maximum temperatures of
Be pebble beds were as below: Be2>Be3>Be1>Be4.
Besides, the change of fusion power also had more significant
influence on CB beds than Be beds. Since we use the most
conservative physical properties of pebble beds in this paper,
the temperature margins of pebble beds were not very large
(temperature margin for CB: 22.72 K, for Be: 30.80 K). As a
result, the pebble beds tended to exceed the corresponding
temperature limits with the increase of fusion power. The
highest temperatures of CB and Be beds reached the
corresponding temperature limits (1193.15 K and 923.15 K)
when the fusion power increased 5.9% and 15.1%, respectively.
0.6 0.8 1 1.2 1.4 1.6 1.8770
780
790
800
810
820
830
Temperature limit for RAFM-823.15 K
Max
imu
m T
emp
erat
ure
/K
Be pebble bed thermal conductivity
CP1
CP2
CP3
CP4
CP5
CP6
Fig.12 Be pebble bed thermal conductivity effect on CPs
maximum temperature
80% 85% 90% 95% 100% 105% 110% 115% 120% 125% 130%700
800
900
1000
1100
1200
1300
1400
Temperature limit for Be bed-923.15 K
Temperature limit for CB bed-1193.15 K
Max
imum
Tem
per
ature
/K
Fusion power
Be1 Be2 Be3
Be4 CB1 CB2
CB3
Fig.13 Fusion power effect on pebble beds maximum
temperature
Figure 14 shows the fusion power effect on FW and CPs
maximum temperature. The degrees of sensitivity for the
maximum temperatures of FW and CPs were as below: FW>
CP1>CP2>CP5>CP6>CP3>CP4. It could be seen that the
FW was the most dangerous because it directly faced the
plasma and the work environment was the worst. The highest
temperatures of FW reached the RAFM temperature limit
823.15 K when the fusion power increased 28.0%.
8 Copyright © 2016 by ASME
80% 85% 90% 95% 100% 105% 110% 115% 120% 125% 130%720
740
760
780
800
820
840
Temperature limit for RAFM-823.15 K
Max
imum
Tem
per
ature
/K
Fusion power
FW
CP1
CP2
CP3
CP4
CP5
CP6
Fig.14 Fusion power effect on FW and CPs maximum
temperature
5 CONCLUSIONS
The research presented the optimized neutronics, steady
state thermo-hydraulic and several parametric sensitivity
analyses of the typical outboard blanket for CFETR. The major
results were summarized as follows:
The optimized three-dimensional neutronics analysis has
been performed on the whole blanket. The results presented the
nuclear heating rate distribution in the blanket and the total
TBR of the optimized typical outboard blanket could reach
1.54, which showed that the TBR of the optimized design is
greater than that of the original scheme and meets the tritium-
sufficiency requirement very well.
The detailed steady state thermo-hydraulic behaviors of the
FW, the CPs, the Li4SiO4 pebble beds and the Beryllium pebble
beds under both normal and critical conditions were calculated.
The results showed that the temperature on the whole blanket
could be effectively cooled below allowable temperature limits
of the materials, even if the FW is suffering the maximum
surface heat flux. This verified the reasonability of the design of
the blanket cooling scheme.
Several parametric sensitivity studies were conducted to
investigate the influences of main parameters (e.g. coolant mass
flow rate, coolant inlet temperature, pebble bed thermal
conductivity and fusion power) on the temperature distributions
of the blanket components. In summary, the maximum
temperatures of pebble beds and CPs all increased linearly with
the increase of coolant inlet temperature and fusion power. The
maximum temperatures of pebble beds, FW and CPs were much
more sensitive to the decrease of coolant mass flow rate than to
the increase, and the maximum temperatures of CB pebble beds
were much more sensitive to the decrease of CB pebble bed
thermal conductivity than to the increase, and the maximum
temperatures of Be pebble beds were much more sensitive to
the decrease of Be pebble bed thermal conductivity than to the
increase, as well. The change of coolant mass flow rate had
more significant influence on the maximum temperatures of the
pebble beds far away from FW than those closer to FW. The CB
pebble bed thermal conductivity had very little influence on the
maximum temperatures of Be pebble beds and CPs, and the
change of CB pebble bed thermal conductivity had more
significant influence on the maximum temperatures of the CB
pebble beds closer to FW than those far away from FW. The Be
pebble bed thermal conductivity had very little influence on the
maximum temperatures of CB pebble beds, Be1, Be4 pebble
beds and CPs, and the maximum temperature of Be2 pebble bed
was more sensitive to the increase of Be pebble bed thermal
conductivity than Be3. The change of fusion power had more
significant influence on the maximum temperatures of the CB
pebble beds closer to FW than those far away from FW, and the
degrees of sensitivity for the maximum temperatures of Be
pebble beds were as below: Be2>Be3>Be1>Be4.
Since we use the most conservative physical properties of
pebble beds in this paper, the temperature margins of pebble
beds were not large. As a result, the pebble beds tended to
exceed the corresponding temperature limits with the increase
of coolant inlet temperature, fusion power and the decrease of
coolant mass flow rate, CB pebble bed thermal conductivity. In
order to maintain the thermal stabilities of BU and FW, these
four kinds of operations all should be avoided. The thermo-
hydraulic design of the blanket cooling scheme and the
arrangement of the pebble beds also need optimizing in future
work.
ACKNOWLEDGMENTS
This work was supported by a grant from the evaluation
and research on the thermal-hydraulic design and safety analysis
of Chinese fusion engineering test reactor (CFETR) test blanket
module (No. 2014GB114000).
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