analysis of performance data from ceramic breeder irradiation experiments

Fusion Engineering and Design 39–40 (1998) 751–758

Analysis of performance data from ceramic breederirradiation experiments

J.G. van der Laan a,*, R. Conrad b

a Netherlands Energy Research Foundation ECN, P.O. Box 1, 1755 ZG Petten, The Netherlandsb Joint Research Centre JAM, Petten, The Netherlands

Abstract

In the framework of the European blanket development programme a number of ceramic breeder materials havebeen considered for the DEMO blanket concepts under development in the period 1992–1994. Candidate materialshave been irradiated in the HFR Petten in a series of experiments codenamed EXOTIC. The objective of the latestseries, EXOTIC-7, was to reach target and peak lithium burn-ups, representative for so-called DEMO end-of-lifeconditions. Further objectives were to investigate the burn-up effects on the mechanical stability and on the tritiumrelease characteristics of the candidate ceramic breeder materials. In pile performance and results of the postirradiation examinations are discussed for metazirconates and orthosilicates, materials now considered as candidatesfor both ITER and DEMO blankets. © 1998 Published by Elsevier Science S.A. All rights reserved.

1. Introduction

1.1. Background

In the framework of the European blanket de-velopment programme a number of ceramicbreeder materials have been considered as candi-dates. For these materials, both shaped as pelletsand as pebbles, the irradiation behaviour hasbeen investigated in a number of neutron irradia-tion experiments performed with on-line tritiumrelease measurements in the high flux reactor(HFR) in Petten, known as EXOTIC [1–3]. Eachexperiment consisted of four upper and four

lower capsules containing the sample materials.In fact EXOTIC-1 to -7 cover 56 individual ex-periments. The main irradiation parameters weretemperature, time (burn-up), purge gas composi-tion and power.

Analysis showed that tritium residence timesare a good measure for comparing the tritiumrelease characteristics of various ceramic breedingmaterials. In EXOTIC-1 to -6 they were deter-mined by analysing the tritium release curves ob-tained after temperature transients performedduring irradiation at different temperature levelswith He+0.1% H2 as the reference purge gas.Analysis of the tritium release rates during tem-perature transients so far was mostly based on adiffusion model DIFFIT and fit results were inmost cases in good agreement with the experi-* Corresponding author.

0920-3796/98/$19.00 © 1998 Published by Elsevier Science S.A. All rights reserved.

PII S0920-3796(98)00331-7

J.G. 6an der Laan, R. Conrad / Fusion Engineering and Design 39–40 (1998) 751–758752

Fig. 1. Tritium residence times from inventories and derived from transient data by DIFFIT for Li2ZrO3 pellets (capsules 17.1, 17.2,18.1, 21.1 and 24.1) and pebbles (capsule 24.2) irradiated in EXOTIC-5 and -6.

mental values [1]. Tritium residence times deter-mined from post irradiation inventory measure-ments were generally found in reasonableagreement with the values obtained from in-situtransients. The tritium residence time results ofthe first six EXOTIC experiments show a depen-dence on the temperature, the purge gas composi-tion and on the microstructure, in particular ongrain size, open porosity and density [1]. Resultsfor Li2ZrO3 and Li4SiO4 obtained from EXOTIC-5 and 6 are given in Figs. 1 and 2. Note that thequoted temperatures are volume averaged temper-atures accounting for sample size, power distribu-tion and they are based on the actually measuredtemperatures. It was observed for Li2ZrO3-pelletsthat residence times tend to be lower for negativetransients as compared to positive ones. Also thevalues derived from post-irradiation inventorymeasurements are typically lower than the tran-sient data. In EXOTIC-1 to -6 no significantburn-up effect on the tritium residence time hasbeen observed up to a lithium-burn-up of about3%, except for the Li8ZrO6 of EXOTIC-6 whichshowed a tendency to lower residence times byincreasing burn-up.

1.2. Scope of work

In the period 1989–1994 the DEMO-Blanketconcepts under development were breeder-out-of-tube (BOT) and breeder-in-tube (BIT) [4–7]. Theobjective of EXOTIC-7, defined in 1993, was toreach high lithium burn-up values, more or lessrepresentative for so-called DEMO blanket end-of-life conditions and to investigate the effects oflithium burn-up on the mechanical stability andon the tritium release characteristics of candidateceramic breeder materials [2].

The tested breeder materials were:pellets: Li2ZrO3 (2), Li8ZrO6 and LiAlO2;pebbles: Li2ZrO3 and Li4SiO4, two Li4SiO4+Bemixtures.The HFR irradiation of EXOTIC-7 started in

February 1994 and was completed in February1995. The temperatures ranged between 280 and780°C. Total lithium burn-ups in a range between6 and 18% have been achieved within 261 fullpower days [2]. Although the thermal loading ofspecimens was about twice projected DEMO in-service conditions, the pellet-stacks and pebble-beds have remained essentially intact [2]. Firstresults of lithium burn-up measurements, neutron

J.G. 6an der Laan, R. Conrad / Fusion Engineering and Design 39–40 (1998) 751–758 753

Fig. 2. Tritium residence times for Li4SiO4 pebbles irradiated in EXOTIC-5 and -6.

metrology and post-irradiation annealing tests forLi4SiO4 and Li2ZrO3 were presented elsewherealong with experimental results for the residualtritium inventory [3].

Within the European Blanket R&D pro-gramme, started in 1996, the orthosilicate andmetazirconate materials are most important.Therefore results of the in-pile behaviour of se-lected capsules from EXOTIC-7 are presentedalong with the associated PIE results.

2. EXOTIC-7 in-pile behaviour

2.1. In-pile beha6iour of Li2ZrO3

Metazirconates were irradiated as pellets (sup-plied by CEA) in capsules 25.1 and 27.1 and aspebbles (supplied by AECL) in 25.2 (Table 1).The annular pellets had 2 and 4 mm inner andouter radius respectively, the stack length was 40mm. The central temperatures ranged from 450 to640°C and 525 to 700°C. The radial temperaturegradient was about 90°C. The pebble-bed had anouter radius of 3.45 and a 40 mm stack length.Due to the lower thermal conductivity of the

pebble-bed the temperature gradient was about180°C and central temperatures ranged from 475to 660°C. Several tens up to 50 in-situ tempera-ture transients were performed. Fig. 3 shows thetritium release signal for capsule 25.1 during vari-ous transients in the last reactor cycle. The tritiumrelease behaviour of the metazirconates was gen-erally quite good with a relatively stable steadystate and fast adjustments to transients.

2.2. In-pile beha6iour of Li4SiO4

Orthosilicate pebbles were irradiated in capsules28.1 and in mixed-beds with beryllium in 26.2 and28.2, see Table 1. Capsules 28.1 and 26.2 consistedof 3 sections of 20 mm stack length in order toachieve different temperature levels for the post-ir-radiation analyses. Therefore the tritium produc-tion is the sum for the connected sections andtritium release signals during transients are a com-position. Temperature ranges were 410–500, 435–525 and 450–540°C for the three sections ofcapsule 26.2 and 485–580, 560–665, 665–760°Cfor 28.1, respectively. The temperature gradientswere in the range of 10–20°C for capsule 26.2 and80–120°C for 28.1, respectively.


Table 1Ceramic breeding materials (pellets/pebbles) tested in 56 capsules of EXOTIC-1 to -7

E-4 E-5 E-6Material Suppliers E-1 E-7E-2 E-3

Li2SiO3 2NRL, SCK 4 3 4Li2O NRL, ANL, JAERI 4 2a 1

122LiAlO2 NRL, ENEA, CEA 4 11 1Li6Zr2O7 NRL, ECN 11 1Li8ZrO6 NRL, ECN 1 1

23Li2ZrO3 2NRL, SCK, CEA, ENEA 321 1Li2ZrO3 Pebbles: FZK, AECL

111Li4SiO4 Pebbles: FZK2Li4SiO4 Pebbles: FZK

a Including irradiation experiments performed as a contribution to the BEATRIX-I programme.

Capsule 28.2 differed significantly from the oth-ers. It consisted of a mixed bed of Li4SiO4 peb-bles, sized 80 mm length and 11.35 mm diameter,with small and large beryllium pebbles (FZK) andits additional objective was the in-situ investiga-tion of beryllium swelling. The bed packing waswith the highest possible density with a contain-ment tube made from MANET steel, with wallthickness 1.75 mm. The central temperaturesranged from 420 to 480°C. The radial temperaturegradients were initially about 40°C, but graduallydecreased to 25°C at the end of irradiation. Apartfrom the power decreasing with burn-up thiscould be attributed to either swelling of the beryl-lium or thermal ratcheting of the pebble-bed or acombination of these effects. The related postirradiation results for this capsule are not yetavailable.

The tritium release behaviour of the capsuleswith orthosilicate showed a considerably higherscatter as compared to the other capsules, buttemperatures were also relatively low. Fig. 4shows the tritium release signal for capsule 28.1during various transients in the last reactor cycle.The steady state signals for the orthosilicate cap-sules were rather unstable and the recovery fromtransients was slow and often incomplete. Fastfluctuations of tritium release rate were observedat lower temperatures, in particular when thecentral temperature fell below 450°C for 28.2 andbelow 550°C for 28.1. The magnitude of the re-lease rate fluctuations decreased with increasingtemperature. As this effect cannot be attributed to

flow fluctuations, which has been checked for allcapsules, it is possibly inherent to the tritiumrelease dynamics of a pebble-bed.

3. Tritium production and lithium burn-up

The instrumentation of EXOTIC-7 included 40neutron activation monitor sets and 26 gamma-scan-wires. Nearly all of them have been mea-sured, in order to enable a proper neutronicanalysis [8]. The derived range of fast (54Mn)neutron fluence was 1–2×1025 n m−2 (peak).The derived range of thermal (60Co) neutronfluence was 1–1.6×1025 n m−2 (peak). The de-pression of the thermal flux at capsule centers wasa factor 2–4 [8]. Lithium burn-up values weremeasured by mass spectrometry and ranged be-tween 6 and 18% [3]. Nuclear analysis has beenperformed to get a consistent analysis of neutronfluence and fluence rate, tritium production rateand lithium burn-up. The analysis was performedwith the nuclear code collection WIMS, treatingthe specimen configuration in 2-dimensions. Theanalysis uses data from HFR-TEDDI, which de-scribes the actual irradiation history of the wholereactor core including loaded experiments. Thecalculations account for the effect of actual burn-up on tritium production rates in the course of theirradiation and from the associated changes in theneutron spectrum. Tritium production rates varyfrom 5 to 25 mCi g hr−1 for the various capsules.Average nuclear power densities ranged from 20to 110 W cc−1.


Fig. 3. Example of tritium release and temperature histories for capsule 25.1 (Li2ZrO3 pellets) during last HFR-cycle.

Examples are given for capsule 25.1 (Li2ZrO3

annular pellets) and 28.1 (Li4SiO4 pebble-bed) inFigs. 5 and 6, respectively. The figures show theradial profiles of tritium production rates for the1st, 6th and 11th (last) HFR cycle. The ratio ofinner/outer production rates is about a factor 3for 25.1 and about a factor 5 for 28.1. In bothcases it is seen that the tritium production rate atthe inner side of the specimens increases withirradiation time, i.e. with lithium burn-up. Fig. 7shows the derived profile of total lithium burn-upin capsule 28.1 accumulating with the number ofHFR irradiation cycles.

The neutron flux levels averaged over the sam-ple positions were in reasonable to excellentagreement with the experimental values from themetrology [8]. Considering the steep radial gradi-ents of burn-up in the specimens, the averagevalues for lithium burn-up are found in reason-able to good agreement with the available massspectrometry results. The agreement is better forthe pelletcontaining capsules as well as for capsule

28.2 and less for the monolithic pebblebeds of25.2 (Li2ZrO3) and 28.1 (Li4SiO4).

Temperatures and profiles have been calculatedwith a stationary heat flow code. Figs. 5 and 6also show typical temperature profiles for thecapsules 25.1 and 28.1.

4. Residence times from inventory and transientanalyses

The second objective of EXOTIC-7 was to de-termine burn-up effects on tritium release. Only alimited number of in situ transients have beenanalyzed with the diffusion model DIFFIT. Re-sults obtained for the metazirconate pellets sug-gested that the residence times had decreased athigher burn-up levels, at least for the lowest tem-peratures applied [2]. However no correlation wasmade sofar with an evaluation of differential tri-tium inventories, e.g. like that which has beendone for EXOTIC-6 [9,10]. Values for the differ-


Fig. 4. Example of tritium release and temperature histories for capsule 28.1 (Li4SiO4-pebbles) during last HFR-cycle.

ential tritium residence times or inventories weredetermined for selected capsules and transients byintegrating the area under the transient curve.Since the temperature steps were modest, the dif-ferential values were used to derive total invento-

ries for the various temperatures. Such analysisconfirms that in general the tritium release be-haviour of the eight capsules did not changeduring the irradiation. A more elaborate analysis

Fig. 6. Calculated radial profile of tritium production rate(n,a-power) at start, middle and end of irradiation and typicaltemperature fields in the three sections of capsule 28.1(Li4SiO4).

Fig. 5. Calculated radial profile of tritium production rate(n,a-power) at start, middle and end of irradiation and typicaltemperatures in capsule 25.1 (Li2ZrO3).


Fig. 7. Radial profile of 6Li burn-up in capsule 28.1 (Li4SiO4) accumulating per reactor cycle.

of the transient data and differential inventories isrequired to determine the origin of the data trendsfrom the previous DIFFIT-based results. Becauseof the significant radial gradients in both powerdensity and temperature a presentation of resi-dence times versus volume average temperature isfound inappropriate. Means are being developedto account for the higher tritium production ratesand lower mobility at the specimen outer rims andto apply such analysis for the more complexcapsules with three sections as the monolithicLi4SiO4 pebble-bed (28.1).

5. Conclusions

(1) Analysis of results compiled for metazir-conate (pellets/pebbles) and orthosilicates (peb-bles) from EXOTIC-5 and -6 shows that tritiumresidence times are a good measure for comparing

the tritium release characteristics of various ce-ramic breeding materials.

(2) The EXOTIC-7 experiment, which wasended in February 1995 has been successful inachieving total Li-burn-ups of 6–18% during 11HFR cycles (261 FPD). The thermal loadingswere two to three times DEMO values and al-though fracture of pellets and pebbles has beenobserved, the pellet-stacks and pebble-beds werefound essentially intact.

(3) The in-pile tritium release behaviour of thesix EXOTIC-7 capsules containing metazirconateor orthosilicate did not show significant changesduring the irradiation. Fast fluctuations in therelease signal were observed for all orthosilicatesat lower temperatures.

(4) Results from nuclear analysis were obtainedfor burn-up and neutronics of the EXOTIC-7experiments. Profiles for power density and burn-up show strong gradients over the sample radius,


ranging from a factor of 3 to 5. Calculated neu-tron to fluences and lithium burn-ups agreed rea-sonable to excellent with metrology values andmass-spectrometry results.

(5) Post-irradiation inventories for the metazir-conates and orthosilicates agree reasonably withthe in-pile results and seem to be in similar rangesas compared to EXOTIC-5 and -6. Detailed anal-ysis is required to account for the higher tritiumproduction rates and lower mobility at the speci-men outer rims and to apply such analysis for themore complex capsules (28.1 and 26.2).

Acknowledgements

The work at ECN has been performed in theframework of the European Blanket DevelopmentProgramme, with financial support from the Eu-ropean Commission and in cooperation with N.Roux (CEA). H. Werle (FZK), S. Casadio(ENEA) and R.A. Verrall (AECL), who haveprovided most of the material specimens. Theauthors acknowledge C. Sciolla, W.E. Freudenre-ich and R. May for nuclear and thermal analyses,M. Stijkel, H. Buurveld and J. Baard for post-ir-

radiation analyses and H. Kwast, who was projectleader for the EXOTIC experiments until mid1995, for the data compilation of EXOTIC-1 to -6[1].

References

[1] H. Kwast, M. Stijkel, R. Muis, R. Conrad, EXOTIC:Development of Ceramic Tritium Breeding Materials forFusion Reactor Blankets, ECN Report ECN-C-95-123,December, 1995.

[2] J.G. van der Laan, et al., J. Nucl. Mater. 233 (1996)1446–1451.

[3] J.G. van der Laan, et al., Proc. 19th SOFT Lisbon, 1996.[4] M. Dalle Donne, Proc. 17th SOFT, Rome, 1992, p.

1326–30[5] M. Dalle Donne, European DEMO BOT Solid Breeder

Blanket, KfK Report 5249, November, 1994.[6] M. Eid (Ed.), Helium-cooled Ceramic-Breeder-In-Tube

Blanket Line, CEA Report DMT 94/5766 SERMA 1682,December, 1994.

[7] F. Alessandrini, et al., Lithium Aluminate CeramicBreeder Development for BIT DEMO Relevant Blankets,ENEA Report INN/NUMA/MATAV (95)2.

[8] J.H. Baard, Neutron Metrology in the HFR, ExperimentR212-25/26/27/28 (EXOTIC-7), Report ECN-I-95-045,November 1995.

[9] H. Kwast, et al., Proc. 17th SOFT, Rome, 1992, 1409–13.[10] H. Werle, private communication, April, 1993.

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analysis of performance data from ceramic breeder irradiation experiments

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