Thermomechanical behaviour of ceramic breeder pebble stacksfor HICU

J.B.J. Hegeman a,*, E.D.L. van Essen a, M. Jong a, J.G. van der Laan a,J. Reimann b

a NRG Petten*/Materials, Monitoring and Inspection, P.O. Box 25, 1755 ZG Petten, The Netherlandsb Forschungszentrum Karlsruhe, Karlsruhe, Germany

Abstract

This paper reports on the thermomechanical behaviour of small pebble beds or pebble stacks in order to lay out

irradiation test specimens. Uniaxial compression tests (UCTs) of the pebble stacks with relatively small diameters are

performed at 27 and 800 8C i.e. the maximum irradiation temperature. The H/D ratio of the specimens influences the

thermomechanical behaviour because of the friction between the pebbles and the steel tube. The results are compared

with the thermomechanical data obtained from standard UCTs, which apply for larger pebble-beds. The outcome of

the comparison will be used for the detailed design of the constrained pebble-beds in the high fluence irradiation. In

addition, the compatibility between the ceramic breeder materials and the containment is studied. It has been shown

that platinum has the best compatibility at 800 8C for both the lithium meta-titanate and lithium ortho-silicate

ceramics.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Blanket materials; Breeding materials; Pebble beds

1. Introduction

As a part of the European programme for the

development of the Helium Cooled Pebble Bed

blanket concept a high fluence irradiation, HICU,

in the High Flux Reactor is under development [1].

The HICU project concerns the investigation of

the impact of neutron spectrum and the influence

of constraint conditions on the thermomechanical

behaviour of ceramic breeder pebble beds. The size

of the irradiation specimens is limited due to the

Cd-shielding that is used to tailor the neutron

spectrum. The preliminary designs of the pebble

stacks imply that they might be too small in order

to apply the presently available tools for thermo-

mechanical analysis of pebble beds. In particular,

the ratio of pebble size to stack diameter is so large

that the pebble bed can be no longer considered as

a continuum. This prevents a straightforward

analysis of stresses during irradiation and the

* Corresponding author. Tel.: �/31-224-56-4246; fax: �/31-

224-56-8883.

E-mail address: hegeman@nrg-nl.com (J.B.J. Hegeman).

Fusion Engineering and Design 69 (2003) 425�/429

www.elsevier.com/locate/fusengdes

0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0920-3796(03)00086-3

determination of pebble swelling and the effects onheat transfer. Therefore, thermomechanical pre-

testing of the specimens is required. In addition,

the compatibility between the ceramic breeder

materials and the containment is studied.

2. Materials

The breeder materials used for the compatibility

tests as well as for the uniaxial compression tests

(UCTs) of the pebbles stacks are lithium ortho-

silicate produced by the melt-spraying process and

lithium meta-titanate produced by the extrusion�/

spheronization�/sintering process. The Li4SiO4 has

a pebble size distribution from 0.25 to 0.63 mm

and a pebble density of 2.35 g/cm3, which is 98% ofthe theoretical density. The size distribution of

Li2TiO3-pebbles is 0.9�/1.2 mm with a density of

3.11 g/cm3.

3. Compatibility

The pre-design thermal analyses of pebble

stacks under neutron irradiation for DEMOrelevant conditions showed that during irradiation

the cladding will have temperatures over 550 8C,

even up to 800 8C. At the highest temperatures,

the interaction between the cladding and the

breeder material may lead to the formation of a

reaction layer on the cladding or may cause

degradation of the cladding. Moreover, those

interactions may influence the thermomechanicalproperties, swelling and heat transfer i.e. key

properties to be studied during the high fluence

irradiation. Furthermore, since the central tem-

perature needs to be measured, the thermocouple

cladding has to be compatible with the breeder

material at 800 8C. Therefore, the compatibility of

various structural materials with ceramic breeder

materials has been studied.Solid breeder pebbles have been crushed and

subsequently poured in tubes of different cladding

materials. The specimens were heated up to

800 8C and were purged with helium, 0.1%

hydrogen for times varying from 200 to 4000 h.

The tests have been performed with crushed

pebbles ortho-silicate and meta-titanate breeder

materials in contact with Eurofer-97, AISI 321

(pre-oxidised), Inconel 718 (with Ni coating), 15�/

15 Ti steel and platinum foil. After the heat

treatment the tubes were impregnated, cut and

polished. SEM and EDS analyses were used to

study interaction, i.e. the reaction layers on the

cladding and on the breeder material.

A summary of the compatibility tests is given in

Table 1. In Figs. 1 and 2, SEM micrographs of the

cross-section of the compatibility tests are shown.

In Fig. 1, a clear interaction between the ortho-

silicate and the AISI 312 stainless steel is demon-

strated. Various oxide reaction layers are observed

on the stainless steel, like iron-chromiumoxide and

chromiumoxide. Also, on the breeder ceramic,

reaction layers have been observed. For the AISI

321 cladding with a platinum protection foil no

interaction is observed after 4000 h at 800 8C (Fig.

2). However, EDS analyses revealed that there is

little chromium diffusion from the steel and silicon

diffusion (may be from the steel or from the ortho-

silicate) into the Pt foil but there is no chromium

diffusion into the ortho-silicate breeder after

4000 h.

Table 1

Results of the compatibility tests at 800 8C purged with He�/

0.1% Ne for ortho-silicate and meta-titanate pebbles

Materials Time (h) MTi pebbles OSi pebbles

AISI 321 500 �//�/ �//�/

1000 �//�/ �/

Inconel 718 500 �//�/ �//�/

1000 �/ �/

Eurofer97 500 �//�/ �/

Inconel 718 Ni-coated 500 �//�/ �/

15�/15 Ti steel 200 �/ �/

500 �/ �/

Pre-oxidised AISI 321 200 �//�/ �/

500 �/ �/

Platinum 200 �/ �/

1000 �/ �/

2000 �/ �/

4000 �/ �/

‘�/’ are serious interactions between cladding and pebbles,

‘�//�/’ some reactions and ‘�/’ means negligible interactions.

J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425�/429426

From Table 1, it can be concluded that ortho-

silicate is more susceptible to interaction with the

cladding material than meta-titanate. Both breeder

materials can sufficiently be protected from inter-

action with the structural material at 800 8C by aplatinum foil of approx. 100 mm.

4. Thermomechanical behaviour

UCTs have been performed with small pebble

stacks with a large height to diameter ratio H/D. It

had been shown previously that this ratio has a

significant influence on the thermomechanical

behaviour of ceramic pebble beds [2,3]. When the

H/D ratio is large, friction forces between the walland the pebbles become important. Besides, the

small ratio of stack diameter to pebble size affects

the packing factor and packing density. Although

the pebble stack is not representative for the

blanket breeder bed, the thermomechanical beha-

viour needs to be studied to analyse the stresses,

swelling and heat transfer during high fluence

neutron irradiation.Nimonic steel tubes with a platinum foil pre-

venting direct contact between the breeder materi-

als have been designed with inner diameters of 4, 8

and 18 mm for the UCTs. The design allows

compression from both sides of the cylinder in

order to reduce friction effects with the wall, which

will result in a non-homogeneous stress distribu-

tion in the pebble bed. For the UCTs an IN-STRON tensile test-machine is used. After filling,

the specimens were tapped and vibrated to densify

the pebble beds. Subsequently, the bed height,

filling weight and the packing factor were mea-

sured at room temperature. Filling procedures

have been repeated 10 times to obtain accurate

results for the filling factors.

Then, the specimens were isothermally heatedup to the test-temperature and, after stabilising in

order to allow the granular material to expand

freely, the compression test was performed with a

loading/unloading speed of 0.8 MPa/min and a

holding time of 5 min at 5 MPa. The specimens of

¥ 8�/12 mm at 800 8C were compressed 3 times

using the same (un)loading speed and holding

time. The strain was calculated using the displace-ment of the actuator and the measured initial

length after vibrating at room temperature. The

test matrix, listed in Table 2 includes the values of

the packing factors. It can be observed that for

stack diameters smaller than approx. 10�/ the

average pebble diameter, the packing factor de-

creases with decreasing stack diameter. For the

present stack diameters, this effect is more ex-pressed for meta-titanate compared to ortho-

silicate due to the larger pebble size.

The results of the UCTs are plotted in Figs. 3

and 4. At ambient temperature, the beds are much

stiffer due to wall friction effects compared to

standard UCTs performed with a H/D of 0.16,

compare [2,3]. At 800 8C, the pebble beds show a

Fig. 1. SEM micrographs of Pre-oxidised AISI 321 (4 h) 500 h

in contact with OSi at 800 8C.

Fig. 2. SEM micrograph of the reaction between 321SS steel

with Pt foil*/4000 h in contact with OSi at 800 8C.

J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425�/429 427

significant amount of creep. Thermal creep for the

type of meta-titanate used in the present experi-

ments is larger than for ortho-silicate, which is

consistent with previous experiments [4]. The

quantitative comparison of the present data with

the previous creep results is difficult due to the

small creep period in the present experiments. The

creep strain is larger for the smaller beds. This

indicates an additional influence of the ratio of

pebble diameter to stack diameter. During the first

unloading, still some creep can be noticed from the

slight increase in strain in the graphs, which is not

the case during unloading at room temperature.

No significant fragmentation of pebbles was ob-

served after loading up to 5 MPa during the UCT.

5. Conclusions

A dedicated irradiation experiment, HICU, is

being designed to assess the impact of neutron

spectrum and neutron fluence on individual cera-

mic breeder pebbles and*/if possible*/the influ-

ence of constraint conditions on the thermo-

mechanical behaviour of the pebbles in the irradia-

tion capsules considered as pebble beds. From the

pre-irradiation tests of representative stack geome-

tries the following results were obtained:

. Pt has been found as the most suitable material

that is compatible with breeder ceramics at

800 8C in blanket typical purge gas condition

Table 2

Test matrix of the UCT of breeder ceramics for various H/D ratios

Material Stack dimension (mm�/mm) H/D Bed density (g/cm3) Packing factor (%) Test temperature (8C)

Li4SiO4 ¥4.14�/15.8 3.8 1.45 61 803

¥4.18�/40.9 9.8 1.48 62 795

¥8.10�/40.4 5.0 1.53 64 794

¥8.10�/40.3 5.0 1.53 64 22

¥12.13�/39.7 3.3 1.50 64 791

Li2TiO3 ¥4.18�/16.3 3.9 1.68 54 815

¥4.18�/40.7 9.7 1.74 55 790

¥8.12�/39.4 4.8 1.90 61 795

¥8.12�/39.6 4.9 1.90 61 22

¥12.16�/40.1 3.3 1.93 62 789

Fig. 3. UCT for various stack heights and diameters of ortho-

silicate at 800 8C and room temperature.

Fig. 4. UCT for various stack heights and diameters of meta-

titanate at 800 8C and room temperature.

J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425�/429428

(He�/0.1% H2). Pt-foil enhances the number ofstructural materials that can be used for the

design.

. Pebble stack size and constrains are being

selected carefully to be reasonably representa-

tive for a pebble bed based breeding blanket!

. After the performance of UCTs, no significant

fragmentation was observed after loading up to

5 MPa for OSi and MTi.. UCTs with irradiation capsule relevant dimen-

sions have shown distinct differences to stan-

dard UCTs. This is due to wall friction effects

that influence strongly the present results (in-

fluence of stack height to diameter ratio) and

fact that the pebble diameter is no longer

always negligibly small compared to the stack

diameter (most expressed for the meta-titanatepebbles and the smallest stack diameter). There-

fore, considering the pebble bed assemblies in

the irradiation capsules as pebble beds and

using the corresponding thermomechanical

modelling tools might be justified for ortho-

silicate and stack diameters larger than 8 mmbut not for other conditions.

References

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