Thermal expansion behavior of a compressed Li2TiO3 pebble bed

CBBI-13 2005/12/1Santa Barbara, USA

H. Tanigawa, S. Suzuki, M. Enoeda and M. Akiba

Blanket Technology Group, JAEA

2

Thermo-Mechanical properties of a pebble bed

Neutron flux

Tritium breeder(Li2TiO3)

NeutronMultiplier(Be alloy)

Temperature distributionThermal expansion

Deformation, Stress

Thermal conductivity

Packing state

Interaction

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Purpose of the study

Thermo-mechanical analysis of the blanket module with pebble bedsusing finite element calculation code

Stress-Strain property （Young’s modulus）

Thermal conductivity

Effective thermo-mechanical properties of Li2TiO3 pebble bed.Thermal expansion

reported inCBBI-11, 12

Empirical continuum model of the pebble bed including following properties should be established.

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Test apparatus

coolant

aluminacontainer

pebblebed

hot wire for measurement ofeffective thermal conductivity

IR furnace

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Load and deformation in apparatusload cell

Load on the bed is measured by the load cell.

Actuator controls the lower loading rod, andcompresses the bed in this test.

The deformation of the bed is obtained by measuring displacement of the lower loading rod.

The measured deformation (nominal deformation) includes thermal expansion of the alumina container and the loading rods when the test section is heated.

actuator

aluminacontainer

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Thermal expansion of the whole system is measured.

Estimation of actual deformation of the bed

pebblebed

Cu rod

A rod made of pure copper is heated instead of the pebble bed.

The obtained data of deformation arecalibrated and actual deformation of the pebble bed can be estimated.

A correlation formula is determined.

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Thermal expansion of the apparatus

y = 0.0041x - 1.1R2 = 0.9997

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

200 400 600 800 1000Temperature / K

Ther

mal

exp

ansi

on /

mm

whole systemapparatuscopper rod

thermal expansion of bed / mm =(nominal deformation)– 0.0041ΔΤ

Temperature of test section, T / K

yapparatus = 0.0041T –1.19R2 = 0.9997

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Experimental conditions

R.T. ～ 973KTest temperature

Atmosphere He; 1atmpurge rate; 30ccm

Sample Li2TiO3 pebble; φ2mm81.1% of T.D.

Initial packing factor 66.8% (hand tapping)

Dimensions of packed bed φ75mm, h60mm; 265.1cm3

0.1MPa (=0.44kN)Holding load on the bedduring heating (friction of O-rings; about 0.03kN)

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Definition of words in the present study

Stress

loadcross section of bed

Packing factor （P.F.）

Vall

defined at R.T.

Vpebble（Mpebble/density）

Thermal expansion coefficient

|dℓ|@973K

ℓ@R.T. × |973-R.T.|

dℓ

ℓ

0.1MPa

@R.T.P.F.= a

dℓ

@973K

dℓË

@R.T.P.F.= a’

ℓ ℓ’

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Thermal expansion of bed without pre-loading

After thermal transient (RT-->973K-->RT),degree of compactionincreased.

Small holding loadof 0.1MPa was present.

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

200 400 600 800 1000Temperature / K

Ther

mal

exp

ansi

on /

mm

heating

cooling

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Thermal expansion of pre-loaded bed at 10 MPa

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

200 400 600 800 1000

Temperature / K

Ther

mal

exp

ansi

on /

mm

heating

cooling

After the measurement of the bed without pre-loading,the bed was compressed at 10 MPa in 5 times. Then, thermal transient (RT --> 973 K --> RT) was loaded.

After thermal transient,degree of compactiondecreased.

Stress and deformation caused by the pre-loading of 10 MPa were relaxed.

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-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

200 400 600 800 1000Temperature / K

Ther

mal

exp

ansi

on /

mm

heating, noloadcooling, nolaodheating, preloadedcooling, preloaded

Change in thermal expansion coefficient and P.F.

①②

③

④

① 67.0

② 67.2

③ 67.9

④ 67.4

compression

thermalexpansioncoefficient / K-1

973K

compression of 10 MPa x 5 times

973K

1.16×10-05

1.31×10-05

1.67×10-05

1.27×10-05

P.F. / %

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Correlation between thermal expansion coefficient and packing factor

1.0E-05

1.1E-05

1.2E-05

1.3E-05

1.4E-05

1.5E-05

1.6E-05

1.7E-05

1.8E-05

66.8 67.0 67.2 67.4 67.6 67.8 68.0

heating

cooling

Packing factor / %

Ther

mal

exp

ansi

on c

oeff

icitn

t/ K

-1

equilibriumP.F.

higherP.F.

lower P.F.

thermaltransient

thermaltransient

largethermalexpansion

smallthermalexpansion

constantthermalexpansion

equilibrium

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Summary

After (several) thermal transient, packing state of the pebblebed reaches equilibrium.

When the bed with higher P.F. than equilibrium one is heated,thermal expansion is large so that P.F. becomes small. In the case of smaller P.F., thermal expansion is small.

These results suggest that residual stress in the bed caused by a compressive load can be annealed when the bed is heated with or without small load.

Thermal expansion behaviour of Li2TiO3 pebble bed was studied.

15

16

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Future works

Determination of correlation between P.F. and thermal expansion coefficient

Analysis of controlling factors for equilibrium packing state

（load on the bed, surface state of the pebbles, atmosphere, etc.）

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Thermal expansion coefficient of bulk Li2TiO3

8.0E-06

1.0E-05

1.2E-05

1.4E-05

1.6E-05

1.8E-05

2.0E-05

2.2E-05

200 400 600 800 1000 1200

73-85%T.D. by Oarai

Ther

mal

exp

ansi

on c

oeff

icie

nt /

K-1

Temperature / K

81.5%T.D. by CEA

present results

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最後の2枚は IEA subtask 用

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Current state for subtask in JA

Preliminary results of effective thermal conductivity ofcompressed pebble beds were obtained for JA Li2TiO3.

Measurement for height of the heated bed was improved.

Correlation between thermal conductivity and packing stateis being analyzed.

The measurement of thermal conductivity for EU material/Osiis in progress.

Thermal expansion coefficient of the bed was estimated.

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Future works for subtask

Difference in materials for the container

Effects of volume or shape of the packed bed

Effects of cyclic loads or thermal creep

Following issues will be checked and correlation betweeneffective thermal conductivity and packing state will bedetermined so that it can be used in the thermomechanicalanalysis.