Fusion Engineering and Design 58–59 (2001) 411–415

Analytic of tritium-containing gaseous species at the TritiumLaboratory Karlsruhe

R. Lasser *, C. Caldwell-Nichols, L. Dorr, M. Glugla, S. Grunhagen,K. Gunther, R.-D. Penzhorn

HVT-TL, Forschungszentrum Karlsruhe, Tritium Labor, Postfach 3640, D-76021 Karlsruhe, Germany

Abstract

At the Tritium Laboratory Karlsruhe (TLK) laser Raman spectroscopy, gas chromatography, mass spectroscopy,calorimetry and ionisation chambers are used to determine the composition of tritium gas mixtures. For the first timea laser Raman experiment was assembled with an actively controlled resonator which yields a 50 times higher Ramansignal and with all components (laser, optics, Raman cell and spectrometer) installed inside a glove box. Three gaschromatographs, each with up to six detectors, can determine the gases and their tritiated fractions expected in fusiondevices down to the sub-ppm range. Tritium in solids, liquids and gases is determined by means of three calorimeterswith a dynamic ranges of up to five orders of magnitude and a lower detection limit of 1 GBq. Since any of thesetechniques has its shortcomings the best analytical approach is to analyse a sample by more than one method. © 2001Elsevier Science B.V. All rights reserved.

Keywords: Laser Raman spectroscopy; Tritium; Fusion devices; Calorimetry; Gas Chromatography; Omegatron

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1. Introduction

Major tasks of the Tritium Laboratory Karl-sruhe (TLK) [1] are the development and demon-stration of effective methods for the processing ofexhaust gases of fusion devices [2]. This includestesting of new storage getter materials and ofcommercial components under tritium relevantconditions, development of safe handling of largetritium amounts in general including new tech-

niques for the detritiation of tritiated componentssuch as the tiles [3] from the DTE phase at JETand treatment of tritium contaminated waste forlong term storage.

Tritium gas mixtures change their compositionnot only because of the tritium decay, but alsoespecially due to radiation-induced reactions. Tri-tium reacts with gases adsorbed on the surfaces ofits containment and is capable of removing dis-solved material such as carbon in steel. In anytritium handling facility independent of whetherengaged in basic research, for commercial use, orin applications for fusion reactors, a knowledge ofthe composition of the gases to be handled is offundamental importance.

* Corresponding author. Tel.: +49-7247-82-3136; fax: +49-7247-82-2868.

E-mail address: rainer.laesser@hvt.fzk.de (R. Lasser).

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

PII: S0920 -3796 (01 )00473 -2

R. Lasser et al. / Fusion Engineering and Design 58–59 (2001) 411–415412

Hence the development of new analytical meth-ods, the enhancement of existing techniques aswell as their technical demonstration under tri-tium conditions are important topics at the TLK.This paper briefly presents the analytical toolsavailable at the TLK for determination of gascompositions.

2. Laser Raman spectroscopy

The main components of a laser Raman experi-ment (LARA) are the powerful laser, the optics,the Raman cell and the spectrometer. In tritiumlaboratories until now mainly the Raman cell wasplaced inside a glove box with most of the othercomponents outside. The laser beam was fed tothe cell and to the spectrometer either by specialoptical components [4] or fibre optics [5]. At TLKa new type of LARA is installed with all compo-nents inside a glove box. The small size (0.46×0.14×0.11 m3) of the 5 W Nd:YVO4 laser and itslow heat production (�50 W) make this possible.In addition, a special actively stabilised externalresonator locked to the laser frequency was in-stalled for the first time in a LARA, achieving anenhancement factor in laser Raman sensitivitybetter than 50 with the Raman cell installed insidethe external resonator. These improvements inlaser Raman spectroscopy permit the detection ofH2 at the partial pressure of 1 Pa within a 1 minmeasurement time [6].

3. Gas chromatography

Gas chromatographic separation of the six hy-drogen molecular species is achieved at low tem-peratures (77–150 K) [7]. At the TLK threedifferent gas chromatographs measure the hydro-gen species and the other gases expected in fusiondevices. Fig. 1 shows a schematic flow diagram ofone of the gas chromatograph. The sample isinjected via the automatic valve VA1 into thecapillaries A and C and compressed to 1500 mbarby opening VA2. The contents in the sampleloops are injected via Valco-A and Valco-C intoSystems 1 and 2, respectively. Valco-B transfers

Fig. 1. Schematic flow diagram of a gas chromatograph usedat the TLK.

the first eluding gases helium and hydrogen fromcolumn-A to column-B and is then rotated intothe position shown in Fig. 1 which sends all othergases leaving column-A to the helium ionisationdetector (HeD-A) and the ionisation chamber(IC-A) for analysis. The separation of the hydro-gen species occurs in column-B at 77 K and theanalysis by the thermal conductivity detector(TCD-B) and the ionisation chamber (IC-B). Asmall flow of H2 is added in front of TCD-B toavoid the anomaly in the thermal conductivity ofH2–He mixtures [8]. System 2 uses N2 as carriergas and allows the detection of He-4 and He-3 asa single peak. Chromatograms of the six detectors

Fig. 2. HeD-A and IC-A chromatrograms of gas mixture listedin Table 1.

R. Lasser et al. / Fusion Engineering and Design 58–59 (2001) 411–415 413

Fig. 3. TCD-B and IC-B chromatrograms of gas mixture listedin Table 1.

Table 1Typical composition of a residual gas mixture after absorptionof Q2 in a cold getter bed

19 ppm HT, 12 ppm DT, 121 ppm T2, 16.3 ppm CQ4, 3.0ppm CO2, 0.52 ppm C2Q4, 1.36 ppm C2Q6, balance: He-3.Approximately 90% of Q in the hydrocarbons are tritiated.

tected although He-4 is used as carrier gas [9].This is possible because the thermal conductivityof He-3 is different from He-4. The TCD-Cchromatogram of Fig. 4 shows clearly a strongHe-3 peak and a very weak Q2 peak. The threeunseparated tritiated hydrogen species createagain a far stronger signal in IC-C than the sumof all hydrogen molecules in the TCD-C. Detailson the columns and detectors used, on the lowerand upper detection limits, will be given else-where [10].

4. Mass spectroscopy

The Omegatron mass spectrometer used in theTLK for tritium analysis functions in the fol-lowing way: an electron beam confined by thefield of an external magnet ionises molecules.The ions are continuously accelerated by an or-thogonal oscillating electrical field, but areforced by the magnetic field B (approximately0.4 T) on curved paths. When the condition forthe Larmor precession (�=eB/m) is fulfilled,the ions follow a spiral path and strike a collec-tor after many orbits and the resulting current ismeasured by an electrometer. Omegatrons areespecially suitable for the analysis of hydrogenbecause their resolution is proportional to theinverse of the mass (m/�m=2500/m). A furtheradvantage of the Omegatron is its small sizewhich allows its mounting on a simple CF-35flange. Disadvantages for tritium analysis arethe need of tritium compatible pumps, the useof special gas inlet systems and the high ionisa-tion energies. A detailed description of the per-formance of an Omegatron in the quantitativeanalysis of tritium gas mixtures is given in aseparate paper [11].

are given in Figs. 2–4 for a residual gas mixture(see Table 1) observed in stainless steel pipesafter the absorption of the tritium mixture in acold ZrCo-getter bed. Fig. 2 shows the integralcontributions of CQ4 (Q stands for H, D andT), CO2 and higher hydrocarbons observed byHeD-A and the tritiated fractions by the IC-A.The three peaks of IC-B in Fig. 3 belong to thetritiated hydrogen species HT, DT and T2, nosignal is seen by the TCD due to its far lowersensitivity. The large negative peak of the TCD-B chromatogram is caused by He-3. He-3 is de-

Fig. 4. TCD-C and IC-C chromatrograms of gas mixture listedin Table 1.

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5. Calorimetry

Normally very different techniques are neces-sary to determine tritium inventories in gases,liquids and solids. The main advantage of tri-tium calorimetry is that the thermal power pro-duced by the decay is independent of thechemical nature of the tritium in the sample.Tritium (1 g) always generates a decay power of0.324 W �0.3%.

Three calorimeters (one isothermal calorimeter[12] and two inertial guidance calorimeters [13])are used in the TLK for determination of tri-tium in various samples. In the isothermalcalorimeter the sample volume is surrounded bythree cylinders which are kept at progressivelylower, but constant temperatures. The constanttemperature profile is achieved by heating thecylinders and extracting the heat of the sampleby a Peltier cooler. If a sample is placed intothe calorimeter the electrical power to the innercylinder needed to keep the system at constanttemperatures is reduced by the heat produced inthe sample. The reduction in electrical power istaken as the measure of the thermal heat gener-ated. The measurement chamber of the inertialguidance calorimeter is connected by specialheat flow sensors (thermopiles) to a thick alu-minium base plate. Peltier heat pumps are posi-tioned between the base and the supportstructure which is controlled to a temperature of300�5×10−4 K by a Pt 1000 sensor. A largemetal block, the inertial mass, sits on ther-mopiles mounted on the base plate mentionedabove and the whole equipment is installed inan evacuated volume. Any heat transport occursvia the metal structure through the installed sen-sors as radiation losses are negligible. The sensi-tivity of the calorimeter is determined by thetemperature stability of the base plate. Any tem-perature change of the base monitored by thethermopiles next to the inertial mass causes acorrective cooling/heating request to the Peltierelements next to the support structure. In thisway a temperature stability of the base of betterthan 1 nK/s is achieved. The calibration factorof the calorimeter was determined in the rangefrom 1 �W to 10 W and found to be constantover five orders of magnitude. Further proper-

ties of the isothermal and inertial guidancecalorimeters are compared in the Table 2.

The TLK calorimeters are used to assay pur-chased tritium for accountancy, to measure tri-tium trapped in graphite or CFC tiles from theJET DTE campaign, to analyse tritium in liquidsolution with high concentrations where the useof liquid scintillation would have required largedilution with the unavoidable errors and tocharacterise the amount of tritium in compo-nents which have to be disposed.

6. Summary

The main tools available at the TLK for anal-ysis of tritium gas mixtures— laser Raman spec-trometer, gas chromatographs, massspectrometer and calorimeters—have been de-scribed briefly. The best analytical approach forthe characterisation of any tritium gas sample isthe use of more than one of the discussed meth-ods, because each has its disadvantages: Ramaninsensitive gas species such as the noble gasesare not observed by laser Raman spectroscopy;gas species of the same type as the carrier gasare not detected by gas chromatography; the in-terpretation of mass spectra can become cum-bersome due to the overlapping contributionsfrom gas species, cracking products and possiblyeven trimers; and calorimetry can suffer fromheat contributions caused by other reactionsthan the tritium decay.

Table 2Comparison of isothermal and inertial guidance calorimeter

Isothermal Inertial guidancecalorimetercalorimeter

Sample volume 1.2 l 0.5 lStability and �0.5 �W�1 mW

resolution1 mW–3 WRange 1 �W–10 W

109 Bq (0.03 Ci)1012 Bq (3 Ci)Detection limit fortritium

�15 ppmAccuracy at 1 W �1000 ppm4–5 hMeasurement time 4–5 h

for 10 mW

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