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Routine operation of the gas chromatographic isotope separation system of the Tritium Laboratory Karlsruhe
Fusion Engineering and Design 39 – 40 (1998) 987 – 993
Routine operation of the gas chromatographic isotope separation system of the Tritium Laboratory Karlsruhe G. Neffe a,*, U. Besserer a, J. Dehne a, E. Hutter a, H. Kissel a, R.D. Penzhorn a, J. Wendel a, H. Brunnader b b
a Forschungszentrum Karlsruhe, Tritium Laboratorium Karlsruhe, Postfach 3640, D-76021, Karlsruhe, Germany Canadian Fusion Fuels Technology Project, 2700 Lakeshore Road W., Mississauga, Ontario, LSJ 1K377, Canada
Abstract The Gas chromatographic Isotope Separation System (ISS) of the Tritium Laboratory Karlsruhe (TLK) is in operation with tritium since 1995. The first test runs with up to 3.7× 1012 Bq of HT – DT mixtures revealed the need for modifications and improvements in the areas of process conduction and tritium detection. Upon completion and verification of the improvements by appropriate performance tests, the ISS was taken into routine operation. Until early 1997, approximately 2000 l of hydrogen isotopes with an activity of 2.2 × 1014 Bq tritium have been processed. The H2 and HD fractions released to the Central Tritium Retention System of the TLK contained less than 1010 Bq m − 3 of tritium. Protium was isolated by several consecutive isotopic scrambling/isotopic separation steps and released to the Central Tritium Retention System of the TLK. Deuterium/tritium fractions containing less than 0.1% of protium were sent back to the infrastructure of the TLK. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction In general, nearly all stoichiometric deuterium/ tritium mixtures are used in the experiments performed at the Tritium Laboratory Karlsruhe (TLK). This mixture is progressively diluted with deuterium and/or protium during the course of the experiments and must therefore be processed upon completion of the runs in order to recover a deuterium/tritium mixture of at least the original composition and to release all other hydrogen isotopes. For this process, a gas chromatographic Isotope Separation System (ISS) manufactured by CFFTP is available at the TLK [1,2]. The facility * Corresponding author.
is installed in a 34 m3 glove box kept at a pressure of approximately 10 mbar (Fig. 1) containing very dry air to avoid condensation of moisture on the LN2 servicing lines, which are practically not isolated. The separation takes place with two consecutive liquid nitrogen cooled adsorption columns (C1 and C2) held at 105 and 108 K, respectively. The carrier gas is helium; it is maintained at a pressure of 3 bar and recirculated at 40 slpm in closed loop operation (Fig. 2). The separated isotopic hydrogen species are recognized by thermal conductivity detectors (TC) and, when tritiated, by small volume (effective volume 4.6 cm3) ionization chambers (IC). The separated hydrogen isotope fractions are collected individually on one of five storage beds, each containing approxi-
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(97)00163-4
988
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
mately 5 kg of depleted, powderized uranium. Immobilized hydrogen isotopes are released from the storage beds by heating the metal hydride up to 400°C. The thermally liberated gas is collected in one of two tanks and then subjected to gas analysis. Depending upon the composition of the released fractions, they are either discharged to the central tritium retention system (tritium-lean fractions), delivered to the experiments (tritiumrich fractions) or further processed in the ISS.
2. Improvements of the ISS From the experience gained during the first ten campaigns with trace amounts of tritium, a number of required improvements were found (Fig. 2), i.e.: Installation of a small volume ionization chamber at the reception tank to provide a tritium analysis of the gas to be injected, and replacement of the originally supplied proportional counters for the recognition of tritiated peaks after columns C1 and C2 by small volume IC’s (effective volume 4.6 cm3). A small volume IC was also installed at the tail-end of the ISS to control the hydrogen isotope retention by the uranium storage beds. The performance of the ionization chambers was found to be highly satisfactory, e.g. their low background levels allowed a tritium detection in the range of 1011 – 1014 Bq m − 3 (gas exiting the LN2 cooled columns has a very low dew point). An advantage of the IC’s is that they can be placed directly into the gas stream, thus requiring no gas bypass, as is the case with the proportional counters installed by the manufacturer. In addition, ionization chambers have a much wider dynamic range than proportional counters and require no externally supplied counting gas. Installation of a bypass to column C2 to permit direct feed from column C1 to the uranium beds. With this modification, tritium-lean fractions (H2 and HD) could be trapped immediately after C1 in the selected uranium bed and the total separation time be reduced from 34 to 22 h in the runs aimed at a separation and release of tritium-depleted hydrogen isotope fractions.
Installation of a complete sampling port for gas analysis at the ISS reception tank and at the intermediate storage tank. While the above enumerated alterations required several weeks of operation laydown, they contributed significantly in the improvement of the overall operation performance of the ISS.
3. Operation experience Since the first run in 1995 with tritium 24 separation campaigns have been completed totaling approximately 700 h of operation. On average, each uranium storage bed has gone through approximately 7 thermal cycles, each cycle having a duration of ca. 4 h. The longest operation times have been attained by the recirculation compressor and the vacuum pumps, i.e. approximately 1000 h. Up to the present, a total of 1074 l of hydrogen isotope mixtures have been received by the ISS. Of these, approximately 520 l of H2, HD
Fig. 1. The isotope separation system of the Tritium Laboratory Karlsruhe.
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
989
Fig. 2. Schematic flowsheet of the isotope separation system.
and D2 were separated and released to stack via the Central Tritium Retention System of the TLK. In view that more than one passage through the columns is necessary for an effective hydrogen isotope separation, the actual throughput through the columns is considerably higher, i.e. 2000 l. The separation of a 120 l hydrogen isotope sample into its isotopic constituents requires approximately 34 h of operation, during which the LN2 consumption for the initial and the steady state cooling of the separation columns totals approximately 1750 l. From this, it is clear that the separation costs are comparatively high. When experiments of the HITEX type [3] are carried out, that produce large waste flows, the demands on the facility and on the operation staff are large. The most frequent failures that were encountered, which in part can be attributed to the very long operation times, were: Strong signs of wear of the inlet and outlet valves of the compressor. Exceedingly high abrasion of the carbon rotors of pumps belonging to the large ionization chambers that monitor the secondary off-gases. Abrasion is probably enhanced by the extremely low dew point of the recirculated gases.
Three Bellows rupture and the damage from the valves from the LN2 supply to the ISS (Fig. 3). The valves operate at 77 K at pressures of up to 12 bar and are actuated very frequently. By design, these valves should withstand pressures of up to 150 bar at cryogenic temperatures. Both failures are possibly accelerated by the oversized valve actuation springs. Self loosening and the opening of joints belonging to the LN2 supply lines after thermocycling at pressures of up to 12 bar.
Fig. 3. Broken bellow of a cryogenic valve.
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
990
Table 1 Volume and composition of the samples to be separated Run No.
Vol (l)
H2 (%)
HD (%)
HT (Bq)
D2 (%)
DT (Bq)
T2 (Bq)
13 16 18 19 20
100 120 120 120 56
35 1 — — —
29 16 1 0.7 1.2
2.1 62 1.3 0.8 1.2
31 82 98 99 98
1.6 250 170 77 230
10.0 7.5 4.1 0.7 2.2
All radioactivities are in Bq/1011.
Inertia increase for the thermal release of hydrogen isotopes from the uranium storage beds. This may be caused by the increasing pulverization of the uranium, and/or the plugging of the outlet filters of the beds. More detailed information will be available when the beds are opened and inspected prior to their final disposal. On the basis of these experiences, the routine maintenance frequency for certain components was increased.
4. The isotope separation process
4.1. Gas transfers It is a rule for transfers at the TLK that the tritium receptor pumps the gas from the tritium donator. By using this procedure, over-pressures in the double walled transfer lines are avoided. All hydrogen isotope transfers from the experiments to the ISS necessarily proceed via the Tritium Transfer System. The latter not only distributes the tritium within the laboratory, is also used for tritium accountancy. The inner diameter of the transfer line from the CAPRICE facility to the ISS has only 10 mm. Inspite of this, it was possible to transfer hydrogen isotopes, via a 60 m long transfer line, from a CAPRICE 100 l tank containing gas at 900 mbar in approximately 1 h. The final pressure in the CAPRICE tank at which the transfer was terminated was usually less than 1 mbar. The transferred gas was controlled volumetrically using previously calibrated volumes and the measured pressures.
4.2. Isotope separation Irrespective of the composition of the gas mixture to be separated, the ISS is presently always operated at the same prefixed constant temperatures, carrier gas throughput, and carrier gas pressure. Table 1 summarizes typical gas inventories in the ISS reception tank and typical compositions of mixtures from five separation campaigns. In runs 13 and 16, only a separation through the first column was carried out, due to the aim of these runs being to recover and release only H2 and HD. Under these conditions, the separation was completed in approximately 22 h of total operation. Over the past months, most gases processed by the ISS were exhausted from experiments performed with the facility CAPRICE. The more recent experiments included PERMCAT [4] and HITEX runs [3]. While both processes are based on catalytic isotopic exchange, the PERMCAT unit works under counter-current conditions and therefore produces more than two orders of magnitude less waste gas than the HITEX process. Table 2 gives a comparison of the nominal hydrogen isotope waste gas production rates for all Table 2 Comparison of the burden to the ISS by the impurity processing facility CAPRICE and the final polishing detritiation steps HITEX and PERMCAT at their respective nominal flow rates Process
Generation rate of tritium contaminated hydrogen (l h−1)
CAPRICE PERMCAT HITEX
12 2–4 1300
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
991
experiments at the TLK, deuterium/tritium mixtures of composition D/T 5 1 are needed, it is concluded that the ISS of the TLK adequately fulfils the requirements. Due to the fact that, for a complete separation, a given gas mixture needs to be processed more than once, the effective throughput of the ISS is below the original specification of 3 mol day − 1, i.e. approximately 1.5 mol day − 1.
4.3. Storage and release of hydrogen isotopes 6ia uranium getter beds Fig. 4. Gas chromatogram after column 2.
three processes. The waste gas from all three processes contains tritium at trace levels. Under the present status of the gas chromatographic ISS, a complete isotopic separation of all fractions is not possible in a single run, due to the base line separations being insufficient. However, it is possible at all times to separate, with very little tritium contamination, the non-tritiated fractions (H2 and HD) that elute first from the columns. For this purpose, the first eluate is collected on an uranium storage bed until the ionization chamber that monitors the effluent of C2 shows an appearance signal (HT peak, Fig. 4). As soon as this is the case, the uranium storage bed is valved-out and the effluent gas stream is directed towards another uranium bed. By several consecutive ‘one column’ separations, it is possible to recover and release the unwanted gases, i.e. H2, HD, and to a large extent a large D2 fraction. The tritiated gases, i.e. HT, DT and T2, are retained inside the ISS for further processing. Gettered hydrogen isotopes will undergo isotopic scrambling upon their release from the uranium bed, whereby the small remaining HT fraction in a large D2 fraction is converted into HD and DT. After several separation/isotopic scrambling steps, it is possible to reduce the protium content in the gas mixture to very low levels, with minimum tritium losses. Fig. 5 illustrates the procedure for a mixture of an assumed starting composition of 40% D, 40% H, and 20% T. Once protium has been removed, a large fraction of deuterium can be separated and released. In view that, for most
For the interim storage and handling of hydrogen isotopes, five vacuum insulated uranium beds are employed. The insulation vacuum is maintained at low levels with an appropriate vacuum pump set. The uranium storage beds of the ISS were originally foreseen as storage for the separated H2/HD, HT, D2, DT, and T2 fractions. This procedure strategy had to be abandoned due to sharp separations not being achieved, at least under the present operation conditions. Only one uranium bed, i.e. that used for the immobilisation of H2 and HD, was consistently kept uncontaminated. A rough estimation of the amount of gas collected during the course of a separation on a single uranium bed was obtained from the thermal conductivity detector signal and from the ionization chamber signal placed in the effluent stream of the columns. Additional information was provided by the rise in temperature register-
Fig. 5. Protium removal by sample equilibration and reprocessing with the ISS (calculated).
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G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
ing in the storage bed, which is of course related to the amount of hydrogen isotopes trapped by the uranium and to their reaction rate with the getter. The final mass and activity balances are carried out by volumetric and gas analysis after regeneration of the beds. Each of the uranium storage beds of the ISS are loaded with 5 kg of uranium. Thus, their theoretical maximum loading capacity is calculated to be 720 l hydrogen per bed. Experimentally, it was observed, however, that already a loading of 30% of the maximum capacity brings about a significant pressure drop of the helium carrier gas, which is attributed to the considerable volume expansion of uranium upon hydration. As a consequence, the normal carrier gas throughput of 40 slpm was reduced down to half that value during a hydrogen trapping step in accordance with the loading history of the bed. For this reason, the storage capacity of the uranium beds was maintained at less than 240 l. For the recovery of gettered gas, the uranium beds are heated to approximately 400°C and the released gases are collected in a tank. For the complete release of approximately 240 l hydrogen, approximately 4 h are needed. It was observed that the release performance of some of the beds deteriorated with usage time. The cause of this phenomenon is not well understood. If the performance of beds continue to worsen, the whole bed will have to be replaced. A replacement of the getter material is believed to be very difficult, due to all crucial zones of the beds being welded.
Hayesep preparation column held at 70°C for the separation of hydrocarbons and carbon oxides; (2) an Alox column held at LN2 temperature for the separation of hydrogen isotopes; and (3) a molecular sieve column held at 160°C for the separation of N2, O2 and CO. Two mini ionization chambers (effective volume : 3 cm3) are used to detect tritiated species. The detection limit of these IC’s is estimated to be 6000 Bq. Until recently, an Omegatron type mass spectrometer complements the analytical instrumentation of the TLK.
5. Analytics
References
The ISS has no analytical instrumentation of its own. Grab sampling was therefore chosen to perform gas analysis prior to injection and of the various separated fractions. Most gas analysis were carried out by radio gas chromatography (GC) employing a custom made tritium compatible Sichromat 2.8 gas chromatograph from Siemens [5]. The GC is equipped with a: (1)
6. Outlook To improve data collection, data reduction and storage, the Siemens S5 process control system of the ISS will be upgraded with a Win CC data visualization software. Other areas for improvements of the ISS include an increase in the separation capacity and of the quality of the separations. The present rather long separation times and the urgent need for isotope separations to satisfy the current TLK research programme for ITER have impeded the performance of systematic parametric studies so far. As the situation at the TLK relaxes, a considerable amount of extra time will be invested to improve the process. In the long term, tests with column materials other than aluminum oxide and the incorporation of analytics (mass spectrometry or laser Raman spectroscopy) into the glove box are also planned.
[1] G. Neffe, J. Dehne, E. Hutter, H. Kissel, H. Brunnader, Separation of Tritiated Hydrogen Species with the TLK Isotope Separation System, 19th Symp. Fusion Technology, 16 – 20 September, Lisbon, Portugal, 1996. [2] G. Neffe, E. Hutter, H. Brunnader, Construction and commissioning of the hydrogen isotope separation system at the Forschungszentrum Karlsruhe, Proc. 5th Topical Meeting on Tritium Technology in Fission, Fusion and Isotopic Applications in Belgirate, Italy, Fusion Technol. 28 (1995) 1365.
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993 [3] M. Glugla, R.D. Penzhorn, P. Herrmann, H.J. Ache, Advanced catalytic plasma exhaust clean-up process for ITER-EDA, Proc. 18th Symp. on Fusion Technol., 22 – 26 August, Karlsruhe, Germany, 1994, pp. 1135. [4] J.M. Miller, L. Rodrigo, J. Senohrabek, Experimental Demonstration of the HITEX process for fusion fuel clean-up, Proc. 5th Topical Meeting on Tritium Technol-
.
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ogy in Fission, Fusion and Isotopic Applications, Belgirate, Italy, Fusion Technol., 28 (1995). [5] J. Wendel, H. Wertenbach, M. Glugla, R.-D. Penzhorn, B. Spelta, I. Ricapito, G. Baratti H. Dworschak, Quantitative analysis of hydrogen isotopes with a mordenite column by radio gas chromatography, Proc. 5th Topical Meeting on Tritium Technology in Fission, Fusion and Isotopic Applications Belgirate, Italy, Fusion Technol., 28 (1995) 1090.
Routine operation of the gas chromatographic isotope separation system of the Tritium Laboratory Karlsruhe G. Neffe a,*, U. Besserer a, J. Dehne a, E. Hutter a, H. Kissel a, R.D. Penzhorn a, J. Wendel a, H. Brunnader b b
a Forschungszentrum Karlsruhe, Tritium Laboratorium Karlsruhe, Postfach 3640, D-76021, Karlsruhe, Germany Canadian Fusion Fuels Technology Project, 2700 Lakeshore Road W., Mississauga, Ontario, LSJ 1K377, Canada
Abstract The Gas chromatographic Isotope Separation System (ISS) of the Tritium Laboratory Karlsruhe (TLK) is in operation with tritium since 1995. The first test runs with up to 3.7× 1012 Bq of HT – DT mixtures revealed the need for modifications and improvements in the areas of process conduction and tritium detection. Upon completion and verification of the improvements by appropriate performance tests, the ISS was taken into routine operation. Until early 1997, approximately 2000 l of hydrogen isotopes with an activity of 2.2 × 1014 Bq tritium have been processed. The H2 and HD fractions released to the Central Tritium Retention System of the TLK contained less than 1010 Bq m − 3 of tritium. Protium was isolated by several consecutive isotopic scrambling/isotopic separation steps and released to the Central Tritium Retention System of the TLK. Deuterium/tritium fractions containing less than 0.1% of protium were sent back to the infrastructure of the TLK. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction In general, nearly all stoichiometric deuterium/ tritium mixtures are used in the experiments performed at the Tritium Laboratory Karlsruhe (TLK). This mixture is progressively diluted with deuterium and/or protium during the course of the experiments and must therefore be processed upon completion of the runs in order to recover a deuterium/tritium mixture of at least the original composition and to release all other hydrogen isotopes. For this process, a gas chromatographic Isotope Separation System (ISS) manufactured by CFFTP is available at the TLK [1,2]. The facility * Corresponding author.
is installed in a 34 m3 glove box kept at a pressure of approximately 10 mbar (Fig. 1) containing very dry air to avoid condensation of moisture on the LN2 servicing lines, which are practically not isolated. The separation takes place with two consecutive liquid nitrogen cooled adsorption columns (C1 and C2) held at 105 and 108 K, respectively. The carrier gas is helium; it is maintained at a pressure of 3 bar and recirculated at 40 slpm in closed loop operation (Fig. 2). The separated isotopic hydrogen species are recognized by thermal conductivity detectors (TC) and, when tritiated, by small volume (effective volume 4.6 cm3) ionization chambers (IC). The separated hydrogen isotope fractions are collected individually on one of five storage beds, each containing approxi-
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(97)00163-4
988
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
mately 5 kg of depleted, powderized uranium. Immobilized hydrogen isotopes are released from the storage beds by heating the metal hydride up to 400°C. The thermally liberated gas is collected in one of two tanks and then subjected to gas analysis. Depending upon the composition of the released fractions, they are either discharged to the central tritium retention system (tritium-lean fractions), delivered to the experiments (tritiumrich fractions) or further processed in the ISS.
2. Improvements of the ISS From the experience gained during the first ten campaigns with trace amounts of tritium, a number of required improvements were found (Fig. 2), i.e.: Installation of a small volume ionization chamber at the reception tank to provide a tritium analysis of the gas to be injected, and replacement of the originally supplied proportional counters for the recognition of tritiated peaks after columns C1 and C2 by small volume IC’s (effective volume 4.6 cm3). A small volume IC was also installed at the tail-end of the ISS to control the hydrogen isotope retention by the uranium storage beds. The performance of the ionization chambers was found to be highly satisfactory, e.g. their low background levels allowed a tritium detection in the range of 1011 – 1014 Bq m − 3 (gas exiting the LN2 cooled columns has a very low dew point). An advantage of the IC’s is that they can be placed directly into the gas stream, thus requiring no gas bypass, as is the case with the proportional counters installed by the manufacturer. In addition, ionization chambers have a much wider dynamic range than proportional counters and require no externally supplied counting gas. Installation of a bypass to column C2 to permit direct feed from column C1 to the uranium beds. With this modification, tritium-lean fractions (H2 and HD) could be trapped immediately after C1 in the selected uranium bed and the total separation time be reduced from 34 to 22 h in the runs aimed at a separation and release of tritium-depleted hydrogen isotope fractions.
Installation of a complete sampling port for gas analysis at the ISS reception tank and at the intermediate storage tank. While the above enumerated alterations required several weeks of operation laydown, they contributed significantly in the improvement of the overall operation performance of the ISS.
3. Operation experience Since the first run in 1995 with tritium 24 separation campaigns have been completed totaling approximately 700 h of operation. On average, each uranium storage bed has gone through approximately 7 thermal cycles, each cycle having a duration of ca. 4 h. The longest operation times have been attained by the recirculation compressor and the vacuum pumps, i.e. approximately 1000 h. Up to the present, a total of 1074 l of hydrogen isotope mixtures have been received by the ISS. Of these, approximately 520 l of H2, HD
Fig. 1. The isotope separation system of the Tritium Laboratory Karlsruhe.
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
989
Fig. 2. Schematic flowsheet of the isotope separation system.
and D2 were separated and released to stack via the Central Tritium Retention System of the TLK. In view that more than one passage through the columns is necessary for an effective hydrogen isotope separation, the actual throughput through the columns is considerably higher, i.e. 2000 l. The separation of a 120 l hydrogen isotope sample into its isotopic constituents requires approximately 34 h of operation, during which the LN2 consumption for the initial and the steady state cooling of the separation columns totals approximately 1750 l. From this, it is clear that the separation costs are comparatively high. When experiments of the HITEX type [3] are carried out, that produce large waste flows, the demands on the facility and on the operation staff are large. The most frequent failures that were encountered, which in part can be attributed to the very long operation times, were: Strong signs of wear of the inlet and outlet valves of the compressor. Exceedingly high abrasion of the carbon rotors of pumps belonging to the large ionization chambers that monitor the secondary off-gases. Abrasion is probably enhanced by the extremely low dew point of the recirculated gases.
Three Bellows rupture and the damage from the valves from the LN2 supply to the ISS (Fig. 3). The valves operate at 77 K at pressures of up to 12 bar and are actuated very frequently. By design, these valves should withstand pressures of up to 150 bar at cryogenic temperatures. Both failures are possibly accelerated by the oversized valve actuation springs. Self loosening and the opening of joints belonging to the LN2 supply lines after thermocycling at pressures of up to 12 bar.
Fig. 3. Broken bellow of a cryogenic valve.
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
990
Table 1 Volume and composition of the samples to be separated Run No.
Vol (l)
H2 (%)
HD (%)
HT (Bq)
D2 (%)
DT (Bq)
T2 (Bq)
13 16 18 19 20
100 120 120 120 56
35 1 — — —
29 16 1 0.7 1.2
2.1 62 1.3 0.8 1.2
31 82 98 99 98
1.6 250 170 77 230
10.0 7.5 4.1 0.7 2.2
All radioactivities are in Bq/1011.
Inertia increase for the thermal release of hydrogen isotopes from the uranium storage beds. This may be caused by the increasing pulverization of the uranium, and/or the plugging of the outlet filters of the beds. More detailed information will be available when the beds are opened and inspected prior to their final disposal. On the basis of these experiences, the routine maintenance frequency for certain components was increased.
4. The isotope separation process
4.1. Gas transfers It is a rule for transfers at the TLK that the tritium receptor pumps the gas from the tritium donator. By using this procedure, over-pressures in the double walled transfer lines are avoided. All hydrogen isotope transfers from the experiments to the ISS necessarily proceed via the Tritium Transfer System. The latter not only distributes the tritium within the laboratory, is also used for tritium accountancy. The inner diameter of the transfer line from the CAPRICE facility to the ISS has only 10 mm. Inspite of this, it was possible to transfer hydrogen isotopes, via a 60 m long transfer line, from a CAPRICE 100 l tank containing gas at 900 mbar in approximately 1 h. The final pressure in the CAPRICE tank at which the transfer was terminated was usually less than 1 mbar. The transferred gas was controlled volumetrically using previously calibrated volumes and the measured pressures.
4.2. Isotope separation Irrespective of the composition of the gas mixture to be separated, the ISS is presently always operated at the same prefixed constant temperatures, carrier gas throughput, and carrier gas pressure. Table 1 summarizes typical gas inventories in the ISS reception tank and typical compositions of mixtures from five separation campaigns. In runs 13 and 16, only a separation through the first column was carried out, due to the aim of these runs being to recover and release only H2 and HD. Under these conditions, the separation was completed in approximately 22 h of total operation. Over the past months, most gases processed by the ISS were exhausted from experiments performed with the facility CAPRICE. The more recent experiments included PERMCAT [4] and HITEX runs [3]. While both processes are based on catalytic isotopic exchange, the PERMCAT unit works under counter-current conditions and therefore produces more than two orders of magnitude less waste gas than the HITEX process. Table 2 gives a comparison of the nominal hydrogen isotope waste gas production rates for all Table 2 Comparison of the burden to the ISS by the impurity processing facility CAPRICE and the final polishing detritiation steps HITEX and PERMCAT at their respective nominal flow rates Process
Generation rate of tritium contaminated hydrogen (l h−1)
CAPRICE PERMCAT HITEX
12 2–4 1300
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
991
experiments at the TLK, deuterium/tritium mixtures of composition D/T 5 1 are needed, it is concluded that the ISS of the TLK adequately fulfils the requirements. Due to the fact that, for a complete separation, a given gas mixture needs to be processed more than once, the effective throughput of the ISS is below the original specification of 3 mol day − 1, i.e. approximately 1.5 mol day − 1.
4.3. Storage and release of hydrogen isotopes 6ia uranium getter beds Fig. 4. Gas chromatogram after column 2.
three processes. The waste gas from all three processes contains tritium at trace levels. Under the present status of the gas chromatographic ISS, a complete isotopic separation of all fractions is not possible in a single run, due to the base line separations being insufficient. However, it is possible at all times to separate, with very little tritium contamination, the non-tritiated fractions (H2 and HD) that elute first from the columns. For this purpose, the first eluate is collected on an uranium storage bed until the ionization chamber that monitors the effluent of C2 shows an appearance signal (HT peak, Fig. 4). As soon as this is the case, the uranium storage bed is valved-out and the effluent gas stream is directed towards another uranium bed. By several consecutive ‘one column’ separations, it is possible to recover and release the unwanted gases, i.e. H2, HD, and to a large extent a large D2 fraction. The tritiated gases, i.e. HT, DT and T2, are retained inside the ISS for further processing. Gettered hydrogen isotopes will undergo isotopic scrambling upon their release from the uranium bed, whereby the small remaining HT fraction in a large D2 fraction is converted into HD and DT. After several separation/isotopic scrambling steps, it is possible to reduce the protium content in the gas mixture to very low levels, with minimum tritium losses. Fig. 5 illustrates the procedure for a mixture of an assumed starting composition of 40% D, 40% H, and 20% T. Once protium has been removed, a large fraction of deuterium can be separated and released. In view that, for most
For the interim storage and handling of hydrogen isotopes, five vacuum insulated uranium beds are employed. The insulation vacuum is maintained at low levels with an appropriate vacuum pump set. The uranium storage beds of the ISS were originally foreseen as storage for the separated H2/HD, HT, D2, DT, and T2 fractions. This procedure strategy had to be abandoned due to sharp separations not being achieved, at least under the present operation conditions. Only one uranium bed, i.e. that used for the immobilisation of H2 and HD, was consistently kept uncontaminated. A rough estimation of the amount of gas collected during the course of a separation on a single uranium bed was obtained from the thermal conductivity detector signal and from the ionization chamber signal placed in the effluent stream of the columns. Additional information was provided by the rise in temperature register-
Fig. 5. Protium removal by sample equilibration and reprocessing with the ISS (calculated).
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G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993
ing in the storage bed, which is of course related to the amount of hydrogen isotopes trapped by the uranium and to their reaction rate with the getter. The final mass and activity balances are carried out by volumetric and gas analysis after regeneration of the beds. Each of the uranium storage beds of the ISS are loaded with 5 kg of uranium. Thus, their theoretical maximum loading capacity is calculated to be 720 l hydrogen per bed. Experimentally, it was observed, however, that already a loading of 30% of the maximum capacity brings about a significant pressure drop of the helium carrier gas, which is attributed to the considerable volume expansion of uranium upon hydration. As a consequence, the normal carrier gas throughput of 40 slpm was reduced down to half that value during a hydrogen trapping step in accordance with the loading history of the bed. For this reason, the storage capacity of the uranium beds was maintained at less than 240 l. For the recovery of gettered gas, the uranium beds are heated to approximately 400°C and the released gases are collected in a tank. For the complete release of approximately 240 l hydrogen, approximately 4 h are needed. It was observed that the release performance of some of the beds deteriorated with usage time. The cause of this phenomenon is not well understood. If the performance of beds continue to worsen, the whole bed will have to be replaced. A replacement of the getter material is believed to be very difficult, due to all crucial zones of the beds being welded.
Hayesep preparation column held at 70°C for the separation of hydrocarbons and carbon oxides; (2) an Alox column held at LN2 temperature for the separation of hydrogen isotopes; and (3) a molecular sieve column held at 160°C for the separation of N2, O2 and CO. Two mini ionization chambers (effective volume : 3 cm3) are used to detect tritiated species. The detection limit of these IC’s is estimated to be 6000 Bq. Until recently, an Omegatron type mass spectrometer complements the analytical instrumentation of the TLK.
5. Analytics
References
The ISS has no analytical instrumentation of its own. Grab sampling was therefore chosen to perform gas analysis prior to injection and of the various separated fractions. Most gas analysis were carried out by radio gas chromatography (GC) employing a custom made tritium compatible Sichromat 2.8 gas chromatograph from Siemens [5]. The GC is equipped with a: (1)
6. Outlook To improve data collection, data reduction and storage, the Siemens S5 process control system of the ISS will be upgraded with a Win CC data visualization software. Other areas for improvements of the ISS include an increase in the separation capacity and of the quality of the separations. The present rather long separation times and the urgent need for isotope separations to satisfy the current TLK research programme for ITER have impeded the performance of systematic parametric studies so far. As the situation at the TLK relaxes, a considerable amount of extra time will be invested to improve the process. In the long term, tests with column materials other than aluminum oxide and the incorporation of analytics (mass spectrometry or laser Raman spectroscopy) into the glove box are also planned.
[1] G. Neffe, J. Dehne, E. Hutter, H. Kissel, H. Brunnader, Separation of Tritiated Hydrogen Species with the TLK Isotope Separation System, 19th Symp. Fusion Technology, 16 – 20 September, Lisbon, Portugal, 1996. [2] G. Neffe, E. Hutter, H. Brunnader, Construction and commissioning of the hydrogen isotope separation system at the Forschungszentrum Karlsruhe, Proc. 5th Topical Meeting on Tritium Technology in Fission, Fusion and Isotopic Applications in Belgirate, Italy, Fusion Technol. 28 (1995) 1365.
G. Neffe et al. / Fusion Engineering and Design 39–40 (1998) 987–993 [3] M. Glugla, R.D. Penzhorn, P. Herrmann, H.J. Ache, Advanced catalytic plasma exhaust clean-up process for ITER-EDA, Proc. 18th Symp. on Fusion Technol., 22 – 26 August, Karlsruhe, Germany, 1994, pp. 1135. [4] J.M. Miller, L. Rodrigo, J. Senohrabek, Experimental Demonstration of the HITEX process for fusion fuel clean-up, Proc. 5th Topical Meeting on Tritium Technol-
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ogy in Fission, Fusion and Isotopic Applications, Belgirate, Italy, Fusion Technol., 28 (1995). [5] J. Wendel, H. Wertenbach, M. Glugla, R.-D. Penzhorn, B. Spelta, I. Ricapito, G. Baratti H. Dworschak, Quantitative analysis of hydrogen isotopes with a mordenite column by radio gas chromatography, Proc. 5th Topical Meeting on Tritium Technology in Fission, Fusion and Isotopic Applications Belgirate, Italy, Fusion Technol., 28 (1995) 1090.