J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Experimental Investigation of Thermal Properties of the Li4SiO4 Pebble Beds
Yongjin FENG, Kaiming FENG, Yang LIU, Baoping Gong, Yinfen CHENG
Southwestern Institute of Physics, P.O. Box 432, Chengdu, Sichuan, China(Received: 23 September 2014 / Accepted: 19 January 2015)

The features of solid breeder blanket concept proposed by the China are the tritium breeding ceramic as pebble
beds in several submodules. The thermal properties of ceramic pebble beds are important input parameters for thethermo-mechanical design of solid breeder blankets. The objective of this study is to measure the thermal
parameters of Li4SiO4 pebble beds using the transient plane source method (TPS). The thermal conductivity,
thermal diffusivity and specific heat capacity are simultaneously determined from a single measurement process.
The thermal parameters of the bed with non-compressed load were measured at temperatures ranging from room
temperature to 600Υ. The packing fraction was about 63% for the single size pebble beds. Helium at atmosphericpressure was used as a filling gas. The effective conductivity was measured as a function of temperature. The
experimental results showed that the effective thermal conductivity increased with the increase of the average bed
temperature.
Keywords: Pebble bed; Thermal properties; Test blanket module.

concentrated on uncompressed pebble beds andcompressed pebble beds. The measuring methods havebeen classified as the following three categories [4]: (i)steady state and transient methods, (ii) radial and axial
methods, and (iii) absolute and comparative methods. In
the steady state method, temperature distribution isgenerated in the pebble bed to give the observation of thetemperature gradient. Ali Abou-Sena et al [5] built anexperimental apparatus to measure the effective thermal
conductivity of Li2TiO3 pebble bed based on the principlesof the steady state and the axial heat flow methods. Thepebble bed has pebbles of 1.7-2.0 mm diameter and a
packing fraction of 61%. The hot wire method is a standardtransient experimental technique, used widely to measure
the thermal conductivity of pebbles beds. Remiann andHermsmeyer [6] measured the thermal conductivity ofcompressed ceramic breeder pebble beds using the pulsed
hot wire method combined with a uniaxial compression
tests. A distinct increase of the thermal conductivity with
bed deformation was found. Enoeda et al [7,8] measured
the effective thermal conductivity of the binary pebble
beds by hot wire method. The measured value of theeffective thermal conductivity of Li2O 1mm single packing
pebble bed is almost not dependent on the bed temperature.
Tanigawa et al. [9] measured the effective thermal
conductivity of a compressed Li2TiO3 pebble beds using
the hot wire method at temperatures ranging from 673K to973K. At all temperatures, increases of effective thermal
conductivity due to the compressive deformation wereconfirmed. M. Dalle Donne et al. [10-12] measured theLi4SiO4 pebble beds temperature distribution in axial, andcalculated the heat transfer parameters of the bed. The

1. Introduction

Lithium ceramics breeding blanket is considered as
one of the most promising fusion blankets and worldwide
efforts have been devoted to its R&D. The helium-cooled
ceramic breeder (HCCB) with the pebble bed concept wasselected in Chinese test blanket module (TBM) design [1].
In the HCCB TBM, Lithium orthosilicate (Li4SiO4) is
considered as the first candidate tritium breeder andberyllium is used as neutron multiplication. The
material-form options for the tritium breeders and neutron
multiplier are pebble bed. Using pebble beds in breeder
blanket has several advantages. First, bed characteristics
can be tailored to obtain the required thermalcharacteristics. Second, the effective thermal conductivity
can be controlled by adjusting the bed characteristic. Third,tritium produced in the pebble beds can be easily removedby the purge gas. In the thermal mechanical design of theHCCB TBM with pebble beds, the thermal properties ofthe pebble beds is one of the most important design
parameters which decides the optimum breeder andmultiplier arrangement to keep the appropriate temperature
ranges of the breeder and multiplier materials. The pebbles
are surrounded by flowing helium which carries away thetritium produced in pebbles. However the helium velocity
is so small that the effective thermal conductivity of thebed is not affected by the helium flow and the bed behaves
like a stagnant bed [2]. The heat produced in the pebbles iscarried away by means of cooling tubes containing highpressure helium (≈8MPa) [3].
At present, over the past ten years, several studies
have been made on the heat transfer parameters
author’s e-mail: fengyj@swip.ac.cn49©2015 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchY. Feng et al., Experimental Investigation of Thermal Properties of the Li4SiO4 Pebble Beds

diameter and 50mm high, see Fig. 3. The packing ratio,
which is the ratio of the total volume of pebbles to thevolume of the container, is about 63%.

published data on thermal properties of Li4SiO4 pebble bed
are insufficient and more data are still needed.
In the present paper, a new technique (Transient PlaneSource method) has been applied to measure the thermal
parameters of tritium breeder pebble bed. Then,
temperature dependence and thermal properties, including
thermal diffusivity, thermal conductivity and specific heat,
were investigated.

2. Experimental
2.1 Method

The Transient Plane Source (TPS) method formeasuring the thermal conductivity and thermal diffusivity
has been used in a variety of situation and for a number ofdifferent materials. The experimental principle of the TPS
method is based on the transient temperature response of
an infinite medium to step heating of a disk-shaped planesource. In the measurement of the TPS method, the sensor
serves both as a heater and a temperature detector. The
temperature rise in the sensor surface is accurately
determined through resistance measurement, and also
highly dependent on the thermal transport properties of thetest specimen surrounding the sensor.
In this study, the thermal constant analyzer test system(TPS 2500S, Hot Disk, Sweden) has been employed. The
Mica sensor (a type of sensor for high temperaturemeasurement) was horizontally embedded in the center ofthe pebble bed. The radius of the sensor is 9.719 mm andthe thickness is 0.1 mm. The temperature range of theelectric furnace is from ambient temperature to 600Υ.
Stagnant helium at atmosphere pressure was used as afilling gas. After reaching the steady state for eachexperimental run, the sensor is heated by a constantelectrical current for a short period of time. The generated
heat dissipates from the sensor into the surrounding sample
material, which results a rise in temperature of the sensor
and surrounding sample material.

Fig.1 Photograph of Li4SiO4 pebble with 1.0mm indiameter.

Fig.2 Surface Morphology of pebble.

2.2 Material

The Li4SiO4 pebble with 1.0 mm in diameter used inthis study (shown in Fig. 1), and were fabricated by themelt-spraying technique. These pebbles were conditionedby annealing at 1000 Υ for 2h to obtain thethermodynamically stable phase, lithium orthosilicate andmetasilicate. Using the Hg-porosimetry, a density ofapproximately 94% T.D. (T.D. = 2.40 g/cm3) and an open
porosity of 5.2% were measured; while a closed porosityof 3.2% was measured by He-pycnometry. SEM(Scanning Electron Microscopy) was used to study thesurface microstructure of the pebble, see Fig. 2. Li4SiO4pebbles exhibits the known dendritic solidification
microstructure due to rapid cooling. The pebbles werepacked into a container made of stainless steel and tapped
into place by hand. The packed bed was 45mm inFig.3 Test sensor and Pebble container.

3. Results and Discussion50

Y. Feng et al., Experimental Investigation of Thermal Properties of the Li4SiO4 Pebble Beds

filling gas is high, the heat flux prefers to follow the pathof higher conductivity regions (pebbles and contact areas).
Because the helium conductivity is much lower than thatof pebbles, the contact area between pebbles directly
affects the amount of heat flux across it. It was concluded
that the effect of deformation on thermal conductivity
cannot be neglected for lithium ceramic pebble beds.

Thermal conductivity (W/(m K))

1.3

Experimental data
Fitting curve

1.2
1.1
1.0
0.9
0.8

100

200300400500Figure 5 shows the measured effective conductivityof the Li4SiO4 pebble beds compared with the previous
data. These results indicate the effective thermal
conductivity have the same tendency, thermal
conductivity increase with the temperature increase. But
the measured data are higher than the previous
experimental data especially at temperature 500Υ and600Υ. The reason comes from the different measuremethods. The contact areas between pebbles with sensor
using TPS are larger than hot wire method.
Figure 6 shows the measured the relationshipbetween thermal diffusivity of the Li4SiO4 pebble bed α,
and the bed average temperature. The thermal diffusivityof the Li4SiO4 pebble bed has decreased with thetemperature increase. From the results, the thermal
diffusivity of pebble beds was about 0.53 mm2/s at room
temperature, falling to about 0.33-0.37 mm2/s at 600Υ.
The experimental points are fitted by the following
correlation,
α= 0.54761-4.08679×10-4T+1.95265×10-7T2.

600

0Temperature ( C)

Fig.4 Temperature dependence of thermal
conductivity for Li4SiO4 pebble beds.

Thermal conductivity(W/m.K)

1.8

Enoeda et al (2001 ) 0.25~0.63mm PF=62.5%
Dall Done et al (1990) 0.5mm
Dall Done et al (2000) 0.25~0.63mm PF=65%
Dall Done et al (1994) 0.35~0.6mm PF=64.4%
Experimental data

1.6
1.4
1.2
1.0
0.8
0.6
0.40100 200 300 400 500 600 700 800 900
0Temperature( C)Fig.5 Comparison with previous experimental data.
Figure 4 shows the measured effective thermal
conductivity of the Li4SiO4 pebbles bed, λ, versus the bed
average temperature. From this figure, the effective
thermal conductivity of pebble bed increases with theincrease of the pebble bed temperature. In other words,
the effective thermal conductivity increase from 0.86
W/m K at ambient temperature to 1.29 W/m K at 600Υ
The data were correlated in the temperature range ofR.T.-600Υ by the following equation:

Experimental data
Fitting curve

0.55

2

Thermal Diffusivity (mm /s)

0.60

λ= 0.97198+5.04496×10-4T+3.3043210-7T2 .
In the heat transfer process, the pebble beds are
considered as two-phase (pebbles/gas medium). Thermal
conduction through the solid pebbles and thermal
conduction through the contact areas are expected todominate when the filling gas is stagnant and its thermal
conductivity is small compared to the pebbles. Duringinitial packing process, all the pebbles are randomly
packed into a container and the pebbles are almost
point-point contact. With the temperature increase, thepebble beds will generate the thermal deformation. The
point-point contact can evolve to face-face contact. The
effective thermal conductivity of pebble beds are
influenced by many parameters with different extents.
Some of these parameters have significant impact onthermal conductivity of the pebble beds, such as thermal
conductivity of solid pebbles, filling gas, bed deformation
and bed packing fraction. Other parameters have less
impact such as pebble size and surface roughness.
Thermal conductivity of pebbles and filling gas havedirect impact on effective conductivity of the pebble beds.
When the ratio of thermal conductivity of pebbles to0.50
0.45
0.40
0.35
0.30

100200300

4005006000Temp ( C)

Fig.6 Temperature dependence of thermal diffusivity for
Li4SiO4 pebble beds.
Fig.7 shows the measured specific heat of theLi4SiO4 pebble bed versus the bed average temperature.
From the results, the specific heat of Li4SiO4 pebble bed
increased with the increasing temperature. The empiricalequation obtained here is given as following:
cp= 1.57753+0.00179T+2.22244×10-6T2.

51

Y. Feng et al., Experimental Investigation of Thermal Properties of the Li4SiO4 Pebble Beds

measurements of the binary pebble beds by hot wire
method for the breeding blanket, Fusion Technology 34(1998) 877-881.
[8] M. Enoeda, Y. Ohara, N. Roux, A. Ying, G. Piazza, S.
Malang, Effective Thermal Conductivity Measurement ofthe Candidate Ceramic Breeder Pebble Beds by the Hot
Wire Method, Fusion Technology 39 (2001) 612-616[9] H. Tanigawa, T. Hatano, M. Enoeda, M. Akiba, Effective
thermal conductivity of a compressed Li2TiO3 pebble bed,Fusion Engineering and Design 75-79 (2005) 801-805[10] M. Dalle Donne, G. Sordon, Heat Transfer in Pebble Beds
for Fusion Blankets, Fusion Technology 17 (1990)
597-635.
[11] M. Dalle Donne et al. Heat transfer and technologicalinvestigations on mixed beds of beryllium and Li4SiO4pebbles”, Journal of Nuclear Materials 212 (1994)
872-876.
[12] M. Dalle Donne, A. Goraieb, G. Piazza, G. Sordon,“Measurements of the effective thermal conductivity ofLi4SiO4 pebble bed”, Fusion Engineering and Design 49-50(2000) 513-519.

4.0

Experimental data
Fitting curveSpec.Heat (J/(kg K))

3.5

3.0

2.5

2.0

1.5
100200300

400500

6000Temp ( C)

Fig.7 Temperature dependence of specific heat for
Li4SiO4 pebble beds.

4. Conclusion
The transient plane source method has been applied tomeasure the thermal properties of single size Li4SiO4pebble beds. The correlations have been developed which
describe thermal properties as a function of temperature.
It was found that the effective thermal conductivity andeffective specific heat increase with the increase of thetemperature of the bed. While the thermal diffusivity
decreased with the increase of the temperature of the bed.
More experimental studies, with acceptable level ofconfidence and accuracy, are still required especially forcompressed Li4SiO4 pebble beds. These experiments will
be added to the database of the thermal properties ofLi4SiO4 pebble beds which can be used for the design andanalysis of tritium breeder blankets.
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