Fusion Engineering and Design 58–59 (2001) 683–687

Tritium release from catalytic breeder materials

Kenzo Munakata a,*, Yoshihiro Yokoyama a, Atsushi Baba a,Takahiro Kawagoe a, Toshiharu Takeishi a, Masabumi Nishikawa a,

Ralf Dieter Penzhorn b, Hirotake Moriyma c, Keizo Kawamoto c,Yasuomi Morimoto d, Kenji Okuno d

a Department of Ad�anced Energy Engineering Science, Interdisciplinary Graduate School of Engineering Science,Kyushu Uni�ersity, Kasuga 816-8580, Japan

b Tritium Laboratory Karlsruhe, Research Center Karlsruhe, 76021 Karlsruhe, Germanyc Research Reactor Institute, Kyoto Uni�ersity, Osaka 590-0494, Japan

d Faculty of Science, Radiochemistry Research Laboratory, Shizuoka Uni�ersity, Shizuoka 422-8529, Japan

Abstract

In most current designs of D–T fusion reactor breeding blankets employing Li-based ceramic as breeder materials,the use of a helium sweep gas containing 0.1% of hydrogen is contemplated to extract tritium via isotopic exchangereactions. However, at lower temperatures, the release process of tritium from the breeders is known to be ratherslow. For this reason, there is still a need to develop techniques that promote the release of bred tritium. In order toimprove recovery of tritium from blankets over a wide range of temperature, platinum and palladium were depositedon the solid breeder materials such as Li4SiO4 and Li2TiO3 by the incipient wet impregnation method. Out of piletritium release experiments were conducted using the ceramic breeders irradiated in a research reactor. Theexperimental results of this work reveal the benefit of the addition of catalytic additive metals, which is very effectiveto increase the tritium release rate from ceramic breeder materials especially at comparatively lower temperatures.© 2001 Elsevier Science B.V. All rights reserved.

Keywords: D–T fusion reactor; Blanket; Solid breeder; Catalyst; Noble metal; Isotope exchange

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1. Introduction

In most current designs of D-T fusion reactorblankets with ceramic breeder materials, tritiumbred in breeders such as Li2O, Li4SiO4, LiAlO2,Li2ZrO3 and Li2TiO3 is extracted using heliumsweep gases containing 0.1% of hydrogen. Hydro-

gen is added to the sweep gases to improve theextraction of tritium via isotopic exchange reac-tions. However, these exchange reactions takingplace predominantly on the grain surface of theceramic breeders are rather slow at low tempera-tures. Thus, the development of methods to pro-mote the isotope exchange reactions on the grainsurfaces are of substantial interest, which couldaccelerate the recovery of bred tritium particularlyat moderate temperatures of design inherent.

* Corresponding author. Present address: Hakozaki 6-10-1,Higashi-ku, Fukuoka 812, Japan. Tel./fax: +81-92-642-3784.

E-mail address: kenzo@nucl.kyushu-u.ac.jp (K. Munakata).

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0920 -3796 (01 )00530 -0

K. Munakata et al. / Fusion Engineering and Design 58–59 (2001) 683–687684

In previous studies, the isotope exchange reac-tions and water adsorption at the surface of sev-eral ceramic blanket materials were investigated[1–3]. The results indicate that the isotope ex-change reaction at the breeder gas/solid interfaceproceeds fast only at relatively elevated tempera-tures, thus a considerable decrease in the reactionrate would take place as temperature is decreased.Taking into consideration that there is a broadtemperature distribution within a blanket module[2], it is anticipated that the tritium bred in re-gions of lower temperature is poorly recovered,which could result in an increased overall steadystate tritium inventory in the blanket module.

In order to accelerate the recovery of tritiumfrom ceramic breeder materials over a broadrange of temperatures, the effect of catalyticallyactive metal additives, such as platinum and palla-dium, on the heterogeneous isotope exchange re-actions at the solid breeder/sweep gas interfacewas examined in our previous work [4]. The re-sults indicate that the isotope exchange reactionson the surface of Li4SiO4 are greatly enhancedwith such catalytic additives. Evidently, out ofpile annealing tests with an irradiated Pt/Li4SiO4

breeder revealed that the release of tritium fromthe breeder is accelerated particularly lower tem-peratures [5,6]. In this work, the authors investi-gated the effect of the deposition of Pd on theLi4SiO4 breeder on the release of tritium from thebreeder. Furthermore, the effect of the depositionof Pd on the Li2TiO3 on the tritium release wasalso studied.

2. Experimental

Platinum or palladium was deposited onLi4SiO4 pebbles (0.51–0.94 mm (average 0.68mm) in diameter, �98% TD) and Li2TiO3 peb-bles (16–24 mesh, �90% TD) by the incipientwet impregnation method generally used for thefabrication of catalysts. A solution of platinumtetra ammonium nitrate Pt(NH3)4(NO3)2 or palla-dium tetra ammonium nitrate Pd(NH3)4(NO3)2

was used to deposit the catalyst metals in thebreeder materials. Details of the fabrication pro-cedure are cited in our previous reports [4–6]. The

breeders fabricated in this way and tested in theout of pile annealing experiments were 1.5% Pt/Li4SiO4, 2.0% Pt/Li4SiO4, and 4.1% Pd/Li2TiO3.

The breeder materials were irradiated in theKyoto University Research Reactor. The breederpebbles were first dried under a dry He stream for12 h by raising the temperature stepwise up to400 °C. Then, the breeder pebbles encapsulated inthe quartz tubes were irradiated in the thermalreactor with a neutron flux of 2.75×1013/cm2/s.The irradiation time of Li4SiO4, Pt/Li4SiO4, andPt/Li4SiO4 was 10 min, and the irradiation time ofLi2TiO3 and Pd/ Li2TiO3 was 5 min. After irradia-tion, the quartz capsules were mechanically bro-ken, and the breeder materials were removed andplaced in a reactor tube made of quartz for exper-iments. All these procedures were carried out in aglove box, and thus the breeder materials werenever exposed to the atmosphere before and dur-ing experiments.

The temperature of the reactor (containingbreeder materials of 0.2 g) was controlled with aconventional ribbon heater and an infrared imagefurnace. A 0.1% H2/Nitrogen gas was used as thepurge gas, and the gas flow rate was controlledwith conventional mass flow controllers. Thegases employed were purified with a trap contain-ing 5A molecular sieve to remove residual watervapor. The concentrations of tritium in the inletand outlet streams of the reactor were traced withan ionization chamber. Details of the experimentswere cited in our pervious literature [5,6].

3. Results and discussion

Fig. 1(a) shows the change in tritium concentra-tion in the outlet stream of the reactor as a resultof an out of pile annealing tests with the virginLi4SiO4 pebbles irradiated in the thermal reactor.The flow rate of the sweep gas (0.1% H2/N2) was200 ml/min. The reactor temperature was raisedstepwise from ambient temperature to300, 500, 700 and finally 900 °C with the infraredimage furnace. At each step, the temperature washeld until the tritium level became almost same asthe background level. Finally, the sweep gas waschanged into a (0.1% H2/N2+0.5% H2O)/N2 gas

K. Munakata et al. / Fusion Engineering and Design 58–59 (2001) 683–687 685

at 900 °C. The total amount of tritium releasedand the fraction of tritium released at each tem-perature are listed in Table 1. As seen in this Fig.1a, although the tritium level become the almostbackground level at the final stage of each tem-perature, tritium was released when temperaturewas raised in step. Ninety-four percent of the totaltritium was released by 700 °C, but a small

amount of tritium was released when temperaturewas raised to 900 °C and the sweep gas wasswitched to a N2 gas containing 0.05% of H2 and0.5% of H2O. In total, 139 �Ci of tritium wasreleased. The largest amount of tritium was re-leased at 500 and 700 °C. At 300 °C, only 7% ofthe total amount of tritium was released.

The results of out of pile annealing tests for thePt/Li4SiO4 breeder are shown in Fig. 1b. As men-tioned before, the temperature was held until thetritium level became almost same as the back-ground level. In total, 117 �Ci of tritium wasreleased (see Table 1). Even at room temperature,an increase in tritium concentration in the outletstream of the reactor was observed, which wasnot observed in the case of the virgin Li4SiO4

breeder. The largest amount of tritium (67% ofthe total amount of tritium) was released at500 °C, and 99.8% of the total tritium was re-leased by 700 °C. No tritium release was observedwhen the sweep gas was changed to the N2 gascontaining 0.05% of H2 and 0.5% of H2O at900 °C. These results are similar to the resultsobtained in our previous study [5,6], indicatingthat the addition of Pt to the Li4SiO4 breeder is aneffective way to enhance the tritium release rate atlow temperatures.

The results of out of pile annealing tests for thePd/Li4SiO4 breeder are shown in Fig. 1c. Onehundred and seventeen microcurie of tritium wastotally released (see Table 1). As observed in thecase of the Pt/Li4SiO4 breeder, an increase intritium concentration in the outlet stream of thereactor was also observed. The largest amount oftritium (58% of the total amount of tritium) wasreleased at 300 °C, and 92.1% of the total tritiumwas released by 500 °C, which is the largestamount of tritium released by 500 °C amongtritium released from the Li4SiO4 based breedersexperimented in this work. This result indicatesthat the addition of Pd to the Li4SiO4 breeder isalso an effective way to enhance the tritium re-lease rate at low temperatures. The tritium releaseperformance of the Pd/Li4SiO4 breeder appears tobe better than that of the Pt/Li4SiO4 breeder, butit also should be noted that not small amounts oftritium were released at 700 and 900 °C when thereactor temperature was raised.

Fig. 1. Out of pile annealing test for: (a) Li4SiO4; (b) 1.5%Pt/Li4SiO4; and (c) 2.0% Pd/Li4SiO4 with 0.1% H2/N2 sweepgas (Amount of breeder: 0.2 g, Flow rate: 200 ml/min, Irradi-ation time: 10 min, Neutron flux: 2.75×1013 cm2/s).

K. Munakata et al. / Fusion Engineering and Design 58–59 (2001) 683–687686

Table 1Amount of tritium released from: (a) Li4SiO4; (b) 1.5% Pt/Li4SiO4; and (c) 2.0% Pd/Li4SiO4 at each temperature

Sweep gasTemperature (°C) Li4SiO4 (%) Pt/Li4SiO4 (%) Pd/Li4SiO4 (%)

–25–30 1.60.1% H2/N2 2.87.0 10.0300 58.10.1% H2/N2

44.4 66.80.1% H2/N2 31.15000.1% H2/N2700 42.6 21.4 2.70.1% H2/N2900 4.8 0.2 5.2

1.2 –(0.05% H2+0.5% H2O)/N2 –900

139 �CiTotal 117 �Ci 117 �Ci

Fig. 2a shows the change in tritium concentra-tion in the outlet stream of the reactor as a resultof an out of pile annealing tests with the Li2TiO3

pebbles. The flow rate of the sweep gas (0.1%H2/N2) was 100 ml/min. The reactor temperaturewas raised at the rate of 3.1 °C/min from ambienttemperature to 400 °C with a conventional rib-bon heater. Then, the reactor temperature wasraised stepwise from 400 to 900 °C with the in-frared image furnace; the ribbon heater was re-placed with the infrared image furnace in severalminutes. The sweep gas was changed into the N2

gas containing 0.05% of H2 and 0.5% of H2O at800 °C. The peak concentration of tritium in theoutlet stream of the reactor was observed at300 °C and 85.4% of the total tritium was re-leased by 400 °C (see Table 2). Although theoutlet tritium level became the background levelat 400 °C, tritium was still released when temper-ature was raised stepwise and the sweep gas waschanged.

The results of out of pile annealing tests for thePd/Li2TiO3 breeder are shown in Fig. 2(b). Theexperiment was carried out in the same way as theprevious run. In this case, in total 41.4 �Ci oftritium was released (see Table 2). As seen in thecase of the Pt/Li4SiO4 and Pd/Li4SiO4 breeders, asmall amount of tritium was released even atroom temperature, which was not observed in thecase of the virgin Li2TiO3 breeder. The peakconcentration of tritium in the outlet stream ofthe reactor was observed at 200 °C, which islower than that in the case of Li2TiO3 by 100 °C.Moreover, the peak tritium level was approxi-mately two times as large as that in the case of the

virgin Li2TiO3 breeder. The outlet tritium levelbecame the background level at 400 °C and notritium release was observed even when tempera-ture was raised and the sweep gas was changed,which suggests that all the tritium bred was re-leased by 400 °C. These results suggest that mostof tritium was released at considerably lower tem-

Fig. 2. Out of pile annealing test for: (a) Li2TiO3; and (b) 4.1%Pd/ Li2TiO3 with 0.1% H2/N2 sweep gas (Amount of breeder:0.2 g, Flow rate: 100 ml/min, Irradiation time: 5 min, Neutronflux: 2.75×1013 cm2/s).

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Table 2Amount of tritium released from: (a) Li2TiO3; and (b) 4.1% sPd/ Li2TiO3 at each temperature

Temperature (°C) Li2TiO3 (%)Sweep gas Pd/Li2TiO3 (%)

0.127 4.10.1% H2/N2

27–400 0.1% H2/N2 85.3 95.8600 0.1% H2/N2 4.9 –

3.40.1% H2/N2 0.1800(0.05% H2+0.5% H2O)/N2800 4.0 –

900 2.3(0.05% H2+0.5% H2O)/N2 –

52.6 �CiTotal 41.4 �Ci

peratures compared with the virgin Li2TiO3

breeder and that the addition of Pd to the Li2TiO3

breeder is very effective for the enhancement oftritium release rate at low temperatures.

As reported in our previous report [4], theisotope exchange reactions over the Pd/Li4SiO4

and Pt/Li4SiO4 breeders proceeds much fasterthan that over Li4SiO4, and thus the enhancementof tritium release from the Pt/Li4SiO4, Pd/Li4SiO4

and Pd/Li2TiO3 breeder could be attributed to theenhanced exchange reaction on the surface of thebreeder material helped with the catalytically ac-tive metal.

4. Conclusions

Catalytic breeder materials were produced byimpregnating Li4SiO4 and Li2TiO3 breeders witheither Pt or Pd. Out of pile annealing experimentswere conducted using irradiated ceramic breeders.The results of the experiments indicate that tri-tium bred in the catalytic breeder material isreleased at lower temperatures compared with thebreeder material with no catalyst. Therefore, itwas experimentally confirmed that the deposition

of catalyst metals such as Pt and Pd on theLi4SiO4 and Li2TiO3 breeders is effective for theenhancement of tritium releases rate particularlyat lower temperatures.

References

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[2] K. Munakata, M. Nishikawa, K. Yoneda, Effect of waterin an Li2O bed on tritium inventory, Fus. Technol. 15(1989) 1451–1457.

[3] A. Baba, M. Nishikawa, T. Eguchi, Isotope exchangereaction on Li2ZrO3, J. Nucl. Mater. 250 (1997) 29–35.

[4] K. Munakata, A. Baba, M. Nishikawa, R.D. Penzhorn,Development of an improved ceramic tritium breeder—ce-ramic breeder with catalytic function, J. Nucl. Sci. Tech-nol. 35 (1998) 511–514.

[5] K. Munakata, A. Baba, T. Kawagoe, T. Takeishi, Y.Yokoyama, M. Nishikawa, R.D. Penzhorn, H. Moriyama,K. Kawamoto, K. Okuno, Tritium release from improvedceramic breeder with catalytic function, J. Nucl. Sci. Tech-nol. 36 (1999) 962–964.

[6] K. Munakata, A. Baba, T. Kawagoe, T. Takeishi, Y.Yokoyama, M. Nishikawa, R.D. Penzhorn, H. Moriyama,K. Kawamoto, K. Okuno, Enhancement of tritium releasefrom ceramic breeders with impregnated catalytic addi-tives, to be published in Fus. Eng. Des.