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


Suizheng Qiu Xi’an Jiaotong University


G.H. Su Xi’an Jiaotong University


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

1 Copyright © 2016 by ASME

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


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:


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


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


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.


60% 80% 100% 120% 140% 160% 180%600

700

800

900

1000

1100

1200

1300

1400

1500


Max

imum

Tem

per

ature

/K

Coolant mass flow rate

Be1 Be2 Be3

Be4 CB1 CB2

CB3


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


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


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.


60% 80% 100% 120% 140% 160% 180%770

780

790

800

810

820

830


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


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


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



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%.


80% 85% 90% 95% 100% 105% 110% 115% 120% 125% 130%720

740

760

780

800

820

840


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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