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ARTICLE IN PRESSG ModelFUSION-7916; No. of Pages 4Fusion Engineering and Design xxx (2015) xxxxxx

Contents lists available at ScienceDirect

Fusion Engineering and Design

jo ur nal home p age: www.elsev ier .com/ locate / fusengdes

Therm bewith no

Joerg Reia IKET, Karlsruhb KBHF GmbH, c IAM, Karlsruhe Institute of Technology, Karlsruhe, Germany

h i g h l i g h t s

In present Spherical p Thermo-m Uniaxial co

conductivi A small exp Compared

a r t i c l

Article history:Received 12 SeReceived in reAccepted 16 AAvailable onlin

Keywords:Granular mateBerylliumPebble bedsThermal conduCeramic breed

1. Introdu

In preseis used as fairly spherberyllium pare of signi

CorresponE-mail add

http://dx.doi.o0920-3796/ e this article in press as: J. Reimann, et al., Thermo-mechanical screening tests to qualify beryllium pebble beds with non-pebbles, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.04.046

ceramic breeder blankets, pebble-shaped beryllium is used as a neutron multiplier.ebbles are considered as the candidate material, however, non-spherical particles are of economic interest.echanical pebble bed data do merely exist for non-spherical beryllium grades.mpression tests (UCTs), combined with the Hot Wire Technique (HWT) were used to measure the stressstrain relations and the thermalty.erimental set-up had to be used and a detailed 3D modelling was of prime importance.to spherical pebble beds, non-spherical pebble beds are generally softer and mainly the thermal conductivity is lower.

e i n f o

ptember 2014vised form 19 February 2015pril 2015e xxx

rials

ctivityer blanket

a b s t r a c t

In present ceramic breeder blankets, pebble-shaped beryllium is used as a neutron multiplier. Fairlyspherical pebbles are considered as a candidate material, however, non-spherical particles are of eco-nomic interest because production costs are much lower. Yet, thermo-mechanical pebble bed data domerely exist for these beryllium grades, and the blanket relevant potential of these grades cannot bejudged.

Screening experiments were performed with three different grades of non-spherical beryllium pebbles,produced by different companies, accompanied by experiments with the reference beryllium pebble beds.Uniaxial compression tests (UCTs), combined with the Hot Wire Technique (HWT), were performed tomeasure both the stressstrain relation and the thermal conductivity, k, at different stress levels. Becauseof the limited amounts of the non-spherical materials, the experimental set-ups were small and a detailed3D modelling was of prime importance in order to prove that the used design was appropriate.

Compared to the pebble beds consisting of spherical pebbles, non-spherical pebble beds are generallysofter (smaller stress for a given strain), and, mainly as a consequence of this, for a given strain value,the thermal conductivity is lower. This means for blanket operation that the desired increase of thermalconductivity during thermal compression is smaller.

2015 Elsevier B.V. All rights reserved.

ction

nt ceramic breeder blankets, pebble-shaped berylliuma multiplier. The candidate material Be-1 consists ofical pebbles with diameters of d 1 mm. Non-sphericalarticles can be produced much cheaper and, therefore,cant economic interest. There is a large database of

ding author. Tel.: +49 721811810.ress: joerg.reimann@partner.kit.edu (J. Reimann).

thermo-mechanical properties for Be-1, see e.g., [14], but nearlyno data exist for non-spherical grades.

The fundamental characterization of pebble beds consists of (i)blanket relevant lling experiments for measuring the packing fac-tor ( is the ratio of pebble volume to total volume), (ii) uniaxialcompression tests (UCTs) in order to determine the pebble bedstressstrain () relation, (iii) the measurement of the pebblebed thermal conductivity, k.

Three grades of non-spherical beryllium pebbles were inves-tigated: Be-A and Be-C produced by the Bochvar Institute [5],Russia, and Be-D from Materion, USA, see Fig. 1. These grades were

rg/10.1016/j.fusengdes.2015.04.0462015 Elsevier B.V. All rights reserved.o-mechanical screening tests to qualifyn-spherical pebbles

manna,, Benjamin Fretzb, Simone Pupeschic

e Institute of Technology, Karlsruhe, GermanyEggenstein-Leopoldshafen, Germanyryllium pebble beds

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ARTICLE IN PRESSG ModelFUSION-7916; No. of Pages 42 J. Reimann et al. / Fusion Engineering and Design xxx (2015) xxxxxx

manufacturquent grinddimensionsgrades wasbe small, was outlinedJapan, werdifferent be

Results oof the packi[57]. This Wire Techn

2. Experim

2.1. Experim

The expatmosphereexcluded thinstead theset-up withheight Hcylheated heasteel tube: the inner ethe pebble bwith two 0surface.

Before thwas densifactors, , a

2.2. HWT: m

The pulsof, primarilis embeddeouter dime

4 diplacemen ttransmitt ers

piston

TC TC

hot wire

D=60mm

pebble be d

rmocc powheatesurro

loga lineg tim

apacindins rel

the ein timr coe

(4)

q is t in ahe prd:

k: Tood

bedFig. 1. Investigated Be pebble grades.

ed by crushing sintered beryllium blocks and subse-ing. Scrap-type pebbles were obtained with largest

of 2.5 mm. Because the available amounts of these limited (120 cm3), the experimental set-ups had tohich is especially unfavourable for the k measurement,

below. Therefore, experiments with Be-1 from NGK,e also carried out and the comparison between theryllium grades is important (screening experiments).f lling experiments showed no signicant differences

ng factor for the spherical and the non-spherical pebblespaper contains results on UCTs combined with the Hotique (HWT) to measure k.

ental and data evaluation

ental

by theelectriof the of the vs. theshow aand lonheat csurroutimes i

Fora certatransfe

k = q/

whereresults

In tfullle

Lowing a gpebblee this article in press as: J. Reimann, et al., Thermo-mechanical screenpebbles, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes

eriments were performed in a glove-box in a helium at ambient temperature. The small pebble bed volumee use the HECOP facility [3] for k measurement and

HWT was chosen. Fig. 2 shows schematically the UCT the cylindrical container (inner diameter Dcyl = 60 mm,= 50 mm) and the HW which consisted of an indirectlyter (outer diameter: 1 mm, thickness of outer stainless0.1 mm, MgO insulator thickness: 0.3 mm, diameter oflectrically heated wire: 0.2 mm, heated length withined: L = 55 mm, position above cylinder bottom: 25 mm).25 mm diameter thermocouples welded on the wire

e container was positioned in the press, the pebble beded by vibration. Table 1 shows the obtained packingnd the maximum uniaxial stresses, , of the UCTs.

odelling and data evaluation

ed HWT is a standard technique for the k measurementy, low k materials [8]. A linear heat source (thin wire)d in the centre of the investigated material with largensions. The temperature rise of the wire is measured

for stronglytemperaturpolynomial

Large coresult in a twhich showAfter t* 0.quantify thwhich descinner struct

The tranthe Finite

Table 1Experimental

Batch

Be-1 Be-1 Be-A Be-A Be-C Be-D Be-D Fig. 2. Experimental set-up.

ouples welded on the wire surface. At time t = 0, theer is switched on. By analyzing the temperature riser over a dened time interval the thermal conductivityunding material can be derived. Typical temperature

rithm of time graphs for the transient hot wire methodar region between two non-linear portions at both shortes. The non-linearity at short times is caused by the HWty and the heat resistance between the wire and theg material, while the non-linearity associated to longated to the boundary effect at the container walls.valuation of k, only the linear region is relevant. Aftere period when the HW heat capacity and the HW heatfcient, HTC, no longer play a role, Eq. (1) holds

ln(t2/t1)/(T2 T1), (1)

he electrical power per unit length, q = Q/L. Then, Eq. (1) straight curve with the slope (T2 T1)/ln(t2/t1).esent tests, two requirements of the HWT are not well

he term (T2 T1) must be sufciently large for achiev-measurement accuracy. This is difcult for berylliums, however, satisfactory results could be obtained even

compressed beryllium pebble beds [9] by tting thee curves in the time range of interest by a 3rd order

and using its derivative in Eq. (1).ntainer dimensions: The small container dimensionsemperature curve without a constant slope, see Fig. 3s the HW temperature as a function of the time t* = log t.5, the slope is still a slight function of time. In order toing tests to qualify beryllium pebble beds with non-.2015.04.046

is effect, a 3D model was developed, as outlined below,ribes in detail the experimental set-up including theure of the HW.sient thermal analyses have been carried out withElement commercial code ANSYS [10]. Because of

parameters.

Exp. no. (%) max (MPa)

1 61.64 7.02 62.35 4.83 61.79 4.34 63.16 4.75 59.46 3.96 60.90 4.77 62.12 4.4

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ARTICLE IN PRESSG ModelFUSION-7916; No. of Pages 4J. Reimann et al. / Fusion Engineering and Design xxx (2015) xxxxxx 3

0

10

20

30

40

50

-1

T (

C)

symmetriesimplemente

All the sof the contheat resistabetween than HTC, whimechanismregion. At aheater simuheat genera

The numwhich agreea nominal vassumed, athe HTCs atand the calc

An iteratk in such a at the samet* = 1.6. Thisin order to dthe result fofor the non-

3. Results

The achivalue 63.5nicantly la

,0

,5

,0

,5

5,0

21,81,61,41,21

t*= logt(s )

meas. 3D calc.

Exp 2d: nominal value for 3D calc: k=4 W/mK

Fig. 5. Measured and calculated k = f(t*).

Be-1: k cali bration: k = 16,559t*2 65,864t* + 66,839-

2d mea s: k = 1,0442t*2 1,6185t* + 3,9404-21,510,50-0,5

t*= log t [s]

Exp 2d

mea s

3D calc

Fig. 3. Measured and calculated T/I2 = f(t*).

3

3

4

4

k [W

/mK

]

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

k [W

/mK

]e this article in press as: J. Reimann, et al., Thermo-mechanical screenpebbles, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes

Fig. 4. Mesh of 3D model.

, only a quarter of the experimental set-up wasd, see Fig. 4.ymmetrical surfaces, as well as all the outer surfacesainer, have been considered as adiabatic surfaces. Thences between the heater surface and pebble bed ande bed and the container wall were simulated by applyingch enables ANSYS to describe the different heat transfers of pebble beds close to walls compared to the bulk

time t = 0, the internal heat generation is applied to thelating the electric power of the experiment. The internaltion is kept constant for the duration of the simulation.erical modelling aims to generate a T = f(t*) dependences with the measured one, see Fig. 3. For the calculation,alue for the pebble bed thermal conductivity has to bend then, the measured curve is approached by varying

the HW and the container walls. Both the measuredulated k depend on time t*, see Fig. 5.ion procedure is required in order to vary the nominalway that the measured and calculated k-values agree

value of t*. For k = 4 W/mK, the only solution exists at procedure is carried out for several experimental pointsetermine the calibration curve k = f (tcal). Fig. 6 showsr Exp 2 with Be-1. In the same way, the calibration curvespherical grades was obtained.

eved packing factors were generally smaller than the%, considered as reference value, obtained with a sig-rger container [2]. The decrease of with decreasing

0,0

1,0

F

container dume fractiodetails, see

Fig. 7 shbed strain, mediate or With decret*cal 2a2b2d2e2g2iing tests to qualify beryllium pebble beds with non-.2015.04.046

2,01,81,61,41,2t*=log t

ig. 6. Be-1: measured k and calibration curve for Be-1.

imensions is primarily caused by the fact that the vol-ns of the wall layers with d/2 thicknesses increase, for[11].ows the uniaxial stress, , as a function of the pebble, obtained by the UCTs. For most experiments, at inter-maximum values some cycles were also carried out.asing , the pebble beds become softer, that is, for a

Fig. 7. Stress increase and cycling curves.

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ARTICLE IN PRESSG ModelFUSION-7916; No. of Pages 44 J. Reimann et al. / Fusion Engineering and Design xxx (2015) xxxxxx

0

2

4

6

8

10

1,501,251,000,750,500,250,00

therm

al con

du

ctivity k

(W/m

K)

uniaxial strain (% )

Be-1, Exp1 Be-1, Exp2

Be-A, Exp3 Be-A, Exp4

Be-C, Exp5 Be-D, Exp6

Be-D, Exp7 HECOP [2]

HW [9]

Fig. 8. Thermal conductivity for rst stress increase.

0

2

4

6

8

10

0

therm

al con

du

ctivity k

(W/m

K) B

B

B

B

B

B

B

given stressBe-A, showreference co

Fig. 8 suincrease pesurements.again belowvalues are sgrades. Oneof the non-the compresurfaces. Thare also smto Be-1. In athe non-sphbeds are at contact sur

For blanparameter box and pefore, Fig. 8 i

The thermal conductivity was also measured at distinct points ofthe stress decrease curve. As observed previously [24], k decreasesonly to a small extend from the maximum value, measured at thebeginning of the stress decrease cycle. This is caused (i) by the closerpacking of pebbles, (compare the small strain changes in Fig. 7), (ii)larger contact numbers, and by (iii) increased contact zones becauseof plastic deformation.

Pebble size distributions were measured before and after theUCT tests. No measurable quantities of fractured particles werefound.

4. Summary and conclusions

Screening experiments were performed to investigate thethermo-mechanical behaviour of beryllium pebble beds consistingof non-spherical pebbles. For comparison, experiments with spher-ical pebbles, considered as reference material, were performed as

ompared to the reference pebble beds, the thermal con-ity fobed bi) thegula

resut invthermThesexperrada

is ac

nces

eimaneder aeimanble beeimanductieiman1

e-1, Exp1

e-1, Exp2

e-A , Exp3

e-A , Exp4

e-C, Exp5

e-D, Exp6

e-D, Exp7

2

uniaxial stress

3

(MP a)

4 5

Fig. 9. Thermal conductivity as a function of stress.

, , value, a larger strain, , occurs. Be-1, and Exp 4 with the stiffest behaviour but the values are still below therrelation [2].mmarizes the k measurements for the rst pressureriod by keeping the stress constant during the HW mea-

well. Cductivsofter and, (inon-re

Thepresento the beds. vant ethe degstacks)

Refere

[1] J. Rbre

[2] J. Rpeb

[3] J. Rcon

[4] J. Re this article in press as: J. Reimann, et al., Thermo-mechanical screenpebbles, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes

The results for the rather spherical Be-1 pebbles are the previously proposed correlations. However, the

till signicantly above the results for the non-spherical reason for this can be the softer pebble bed behaviourspherical grades, see Fig. 7, because for a given value,ssion is smaller and with this the increase of contactis argument does not hold for Exp 4, where the k valuesaller although the stressstrain dependence is similar

graph k = f(), Fig. 9, the differences between Be-1 anderical grades are smaller but still the candidate pebble

the upper bound which can indicate that the generatedfaces are smaller for the scrap-type pebbles.ket operation, the pebble bed strain is the primarybecause the constrained expansions between blanketbble beds are the reason for the stress build-up. There-s of prime relevance.

7579 (2[5] I.B. Kupri

Zmitko, DapplicatioBarcelon

[6] J. Reimanmatic con

[7] J. Reimanpebble beBeryllium

[8] K.D. Magerty Meas

[9] J. ReimansuremenYamawakTokyo, 20

[10] www.ans[11] J. Reiman

of concaving tests to qualify beryllium pebble beds with non-.2015.04.046

r non-spherical pebble beds is lower caused by (i) theehaviour (smaller stress for a given strain value),

generation of smaller contact surfaces because of ther pebble shape.lts from previous lling experiments [57] and the

estigations are only considered as a rst step in respectomechanical characterization of non-spherical pebble

investigations should be repeated with more rele-imental set-ups when it has been demonstrated thattion under irradiation of these pebbles (or small pebbleceptable.

n, L. Boccaccini, M. Enoeda, A.Y. Ying, Thermomechanics of solidnd Be pebble bed materials, Fus. Eng. Des. 6162 (2002) 319331.n, G. Piazza, H. Harsch, Thermal conductivity of compressed berylliumds, Fus. Eng. Des. 81 (2006) 449454.n, G. Piazza, Z. Xu, A. Goraieb, H. Harsch, Measurements of the thermalvity of compressed beryllium pebble beds, FZKA-7096 (2005).n, H. Harsch, Thermal creep of beryllium pebble beds, Fus. Eng. Des.005) 10431046.yanov, G.N. Nikolaev, S.K. Zavjalov, L.A. Kurbatova, N.E. Zabirova, M.evelopment of beryllium pebbles with ne grain structure for fusionn, in: 11th IEA Int. Workshop on Beryllium Technology (BeWS 2013),

a, E, September 1213, 2013.n, A. Abou-Sena, R. Nippen, P. Tafforeau, Pebble bed packing in pris-tainers, Fus. Eng. Des. 88 (2013) 23432347.n, A. Abou-Sena, E. Brun, B. Fretz, Packing experiments of berylliumds for the fusion reactor HCPB blanket, in: 11th IEA Int. Workshop on

Technology (BeWS 2013), Barcelona, E, September 1213, 2013.lic, A. Cezairliyan, V. Peletsky, Compendium of Thermophysical Prop-urement Methods, vol. 2, Plenum Press, New York, 1984, pp. 161184.n, S. Hermsmeyer, G. Piazza, G. Wrner, Thermal conductivity mea-ts of deformed beryllium pebble beds by hot wire method, in: M.i (Ed.), CBBI-9, Toki, Japan, September 2729, 2000, Univ. of Tokyo,02, pp. 199214.ys.comn, E. Brun, C. Ferrero, J. Vicente, Pebble bed structures in the vicinitye and convex walls, Fus. Eng. Des. (2015) (submitted for publication).