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

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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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