Development of fusion fuel cycle technology at the Tritium Laboratory Karlsruhe: the experiment CAPRICE

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Fusion Engineering and Design 28 (1995) 348-356

Fusion Engineering and Design

Development of fusion fuel cycle technology at the Tritium Laboratory Karlsruhe: the experiment CAPRICE M. Glugla, R.-D. Penzhorn Kernforschungszentrum Karlsruhe, Institut fi~r Radioehemie, Postfach 3640, D 76021 Karlsruhe, FRG

Abstract

A facility carrying the acronym CAPRICE has been erected at the Tritium Laboratory Karlsruhe to demonstrate fuel clean-up technology for ITER with tritium. The clean-up process for the recovery of molecular and chemically bonded tritium and deuterium from all reactor exhaust streams is based on the combination of hydrogen isotope permeation through palladium-silver with catalytic process steps involving the thermal decomposition of hydrocarbons and the reduction of water vapour by carbon monoxide. On the basis of the CAPRICE process and from recent results on mathematical modelling of a permeator-catalyst combination, a simple continuous clean-up concept for ITER with low tritium inventories is proposed.

1. Introduction

The main tasks of the fuel clean-up system within the fusion fuel cycle are the recycling of unburned fuel, the recovery of chemically bonded tritium and the removal and processing of molecular hydrogen isotopes and impurities from the torus conditioning system, the pellet injection and the blanket tritium recovery unit. In addition, the fuel clean-up system has to convert tritiated water, collected from exhaust gases after air inbreak into the torus, to molecular hydrogen. For delivery to the isotope separation system the hydrogen isotopes need to be of high purity (less than 1 ppm v total impurities). At the Nuclear Research Center Karlsruhe (KfK) an experimental technical facility has been constructed carrying the acronym CAPRICE (CAtalytic PuRifiCation Experiment). The process concept, which constitutes the basis of the CAPRICE facility, was initially formulated in 1985. Since then, laboratory as well as technical

experiments have accompanied the development of this fuel clean-up concept [1]. Important input came from two campaigns using relevant concentrations of tritium performed in collaboration with scientists from TSTA at the Los Alamos National Laboratory [2,3]. At the end of 1991 it was decided to build CAPRICE at a scale of about 1/8th NET II/ITER-CDA, with N N C in Knutsford, U K as an industrial partner. Over the years the exhaust flow rate in the burn and dwell phase of the next tokamak passed through a considerable evolution from NET I via NET II/ITER-CDA up to ITER-EDA. With respect to the current design option, which contemplates recycling of 90% of the unburned fuel directly into the plasma chamber, CAPRICE is nearly 1:1 scale, as shown in Table 1. The main design criteria of CAPRICE include processing of all reactor exhaust streams (burn and dwell, glow discharge, bake out and conversion of water collected after air inbreak), only marginal conversion of hydrogen isotopes into water and low overall tritium

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M. Glugla, R.-D. Penzhorn / Fusion Engineering and Design 28 (1995) 348-356

349

Table 1 Throughputs for burn and dwell (tool h ~)

DT H2 He Impurities

NET I 1987

NET II/ITER-CDA 1990

ITER-EDA 1993-1994

ITER-EDA 90% recycle, 1994

CAPRICE (target limits), 1991

20.0 -1.00 0.50

70.5 0.75 2.25 1.50

120 0.30 3.60 0.60

12.0 0.03 3.60 0.60

9.60 0.40 1.20 0.50

inventory. The plant was required to be simple, to have high throughput, high availability and be of reasonable cost. However, the plant design should give good scaleup capacity and flexibility for future experiments. It is emphasized that CAPRICE is an experimental facility to investigate the performance of components and catalytic process steps and to demonstrate the overall concept with tritium. Advanced components such as a combined catalyst-permeator or other units developed elsewhere may be integrated into the loop at a later stage. Space has been allocated for an increase in the number of glove boxes and for the installation of additional peripheral equipment. Particularly with respect to the number of components, control loops and instrumentation, the catalytic clean-up facility for ITER is expected to be much simpler than CAPRICE.

2. The facility CAPRICE

The design concept of the CAPRICE facility is based on laboratory experiments including studies on the kinetics of the catalytic decomposition of saturated and unsaturated hydrocarbons, rate measurements of the isotope exchange reactions between methane and deuterium, permeation of hydrogen isotopes through commercial palladium-silver permeators, self-radiolysis of polytritiated compounds relevant to the fusion fuel cycle, tritium inventory analysis in catalysts and solid waste, and pumping characteristics of scroll and bellows pumps. Model calculations and technical size experiments have been carried out to support the engineering. All major components of the facility have been tested with relevant gases or their mixtures. The process chemistry was verified in runs with tritium that included all process steps [3]. The aims of the technical experiments with tritium were mainly the following: to provide, if required for ITER, a database for scale-up;

to determine the minimum achievable tritium concentration in the waste gas; to estimate the transient inventories in all major components; to study isotope effects and hydrogen stripping in the permeator; to examine the chemical phenomena induced by tritium decay (/~-radiation, recoil effects, HeT + reactions); to obtain effective time constants of homogeneous and heterogeneous chemical reactions; to optimize the process with respect to tritium inventory and to simplify it further. A basic flow diagram of the CAPRICE facility is shown in Fig. 1. Zone (1) comprises the torus mock-up, zone (2) corresponds to the separation of unburned fuel by a permeator, zone (3) includes the impurity processing lo0p, and zone (4) contains the components used to process the tritiated water arising for example from an air inbreak. At the beginning of a typical experiment deuterium and tritium are mixed in the 20 1 primary loop torus mock-up buffer vessel. Deuterated methane is then added and tritiated in the isotope exchange reactor containing a few grams of a nickel catalyst operated at 300 °C. Next, so called simulant gas made up of helium, carbon oxides and water is added to yield a process gas of composition as specified for the different sources within the ITER fuel cycle. This gas mixture is passed through the primary loop permeator (zone (2)) with the help of the primary circulator Metal Bellows 601 pump. The permeated hydrogen isotopes are removed employing a Normetex scroll pump of 150 m 3 h-1 throughput, backed by a Metal Bellows 601 pump. A spill-back line from the outlet to the inlet of the scroll pump is used to vary the actual pressure at the secondary side of the permeator independently from the permeate flow rate. The feed pressure of the permeator is kept constant at about 0.1 MPa using a remote controlled needle valve at the outlet and a pressure sensor at the inlet of the primary loop permeator.

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M. Glugla, R.-D. Penzhorn / Fusion Engineering and Design 28 (1995) 348-356 The helium and impurity stream bleeding from the permeator begins to fill the secondary loop buffer vessel (zone (3)), until a predetermined pressure is attained. Once this pressure is reached a valve opens and the impurity processing loop is filled up to a certain pressure using the secondary loop Metal Bellows 601 circulator, while the bleed of the main permeator continues to flow into the secondary loop buffer vessel. The gas in the impurity loop is processed by first passing it through a nickel catalyst, where hydrocarbons are decomposed into elements, and then through a water gas shift catalyst, where carbon monoxide reacts with water to form hydrogen and carbon dioxide. The carbon accumulating on the nickel cayalyst is partially gasified by reaction with carbon dioxide to give carbon monoxide (see also Section 3). Hydrogen liberated from the decomposition of hydrocarbons and from the conversion of water vapour is continuously removed from the impurity processing loop with the help of another permeator. To achieve a high recovery of deuterium and tritium, this permeator is evacuated with a tritium compatible Seiko 600 high throughput turbo molecular pump, capable of sustaining inlet pressures between 500 Pa and a vacuum of 10 - s Pa. The turbo pump is backed by a 15 m 3 h -1 Normetex scroll pump followed by a Metal Bellows 601 pump. The hydrogen isotope streams from both permeators are united and recycled into the torus mock-up vessel. The decontaminated gas is sent to waste with the plant vacuum pump, a 15 m3h 1 Normetex scroll pump backed by a Metal Bellows 601 pump. For ultimate decontamination a cold trap operated at - 8 0 °C is available if necessary. In case of an air inbreak into the plasma chamber of ITER (total volume of torus and ducts is 5400 m 3) the main concern is tritiated water arising from the interaction between tritium in the first wall of the reactor and atmospheric moisture. To address this problem the processing of low tritiated water has been considered. Within CAPRICE tritiated water is gasified employing a vaporizer and using carbon monoxide as carrier gas. The resulting gas mixture is passed through a water gas shift catalyst bed, where the water vapour reacts with carbon monoxide to yield carbon dioxide and molecular hydrogen, the latter being removed by the primary loop permeator. To achieve high conversion the process gas may be passed through a further catalyst reactor containing the same catalyst and through the secondary loop permeator. The experimental programme of CAPRICE presently in progress is focused on the following: parametric integral tests with protium and deuterium; evaluation and o~timization of process control modi; determination of overall decontamination factors.

351

Tritium operation is foreseen to begin by the end of 1994.

3. Process chemistry In experimental fuel clean-up investigations methane is usually selected as the only hydrocarbon impurity because it is the most abundant and most resistant to chemical treatment. However, as known from plasma graphite interaction experiments and from measurements performed at JET [4], in addition to carbon monoxide and carbon dioxide, a large number of hydrocarbons are formed. According to Yamada [5], the main hydrocarbon produced by the interaction of hydrogen ions with graphite is methane, followed by acetylene, ethylene and some other higher hydrocarbons. Thus the chemistry of higher hydrocarbons was also investigated. If beryllium is used as a first wall material, the product distribution will change and the total amount of impurities will be considerably less. For the processing of hydrocarbons, catalytic reactions were selected not only because they occur at comparatively low temperatures, but also because the chemical reactions take place only on the catalyst surface. Bulk processes, such as those occurring in getters or adsorbers, are avoided in view of the large tritium hold up (inventories) involved. The only bulk process in CAPRICE is permeation; the mass of palladium-silver in a permeator is, however, very small. From basic chemistry it is known that high conversion yields can be achieved when products are removed from the reaction vessel. Therefore catalytic reactions were combined with permeation through a palladiumsilver membrane to remove hydrogen isotopes selectively from the reacting gas mixture and thereby shift chemical equilibria towards dehydrogenation. 3. I. Decomposition o f hydrocarbons The following reactions basically describe the chemistry of the CAPRICE experiment: CH 4 + Ni ~ Ni(C) + 2H2

(1)

Ni(C) + CO 2 .--, 2CO

(2)

CO + H20 --* CO2 + H2

(3)

CO + 3H2 ~ CH4 + H20

(4)

Hydrocarbons are ultimately decomposed into carbon and molecular hydrogen (reaction (1)). Carbon reacts with carbon dioxide according to the Boudouard

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M. Glugla, R.-D. Penzhorn / Fusion Engineering and Design 28 (1995) 348-356

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Fig. 2. Catalytic decomposition of polytritiated methanes (timescale not applicable to CAPRICE).

reaction to yield carbon monoxide (reaction (2)). This reaction helps to keep the carbon deposit on the catalyst at tolerable levels. With carbon monoxide water is reduced to hydrogen (water gas shift, reaction (3)). Reaction (4) is a side reaction, which need not be considered, when molecular hydrogen continuously permeates out of the impurity processing loop. After the gas has been cycled through the impurity processing loop a few times, quantitative conversion of hydrocarbons and water is achieved. Fig. 2 shows the results from a typical loop experiment using a mixture of HT and methane tritiated to 50% [3]. Once the catalyst is heated to 500 °C methane is decomposed to thermodynamic equilibrium (roughly up to 50%). The total tritium activity remains practically constant (the small decrease observed is due to adsorption of residual water by the catalyst). When the heated permeator is integrated into the loop all gaseous tritiated hydrogen permeates out and the remaining methane decomposes further until a quantitative conversion is reached. The tritium activity drops simultaneously with the permeation of the hydrogen isotopes.

The remaining activity shown in Fig. 2 is due to a memory effect of the ionization chamber. The true tritium concentration of the gas phase was measured with a non-contaminated ionization chamber to be about 20-50 Ci m -3. Fig. 3 shows the catalytic decomposition of ethylene as an example of olefinic hydrocarbon decomposition. At low temperature ethylene is converted into ethane and methane. When the temperature is raised to 500 °C, ethane is converted quantitatively into methane and hydrogen. As soon as the heated permeator is integrated into the loop hydrogen is removed by permeation and the remaining methane is also decomposed. 3.2. Conversion of water vapour into hydrogen

The water gas shift reaction occurs at 200 °C at high rate on the surface of a zinc-stabilized copper chromite catalyst [6]. Yoshida showed that at 200 °C a ratio of carbon monoxide to water vapour of at least 3 is necessary to attain a very high degree of conversion. This is substantiated by calculations carried out by McKay et al. [7] for the isotopes protium, deuterium and tritium. At KfK it was demonstrated that the palladium-silver membrane is resistant to CO. In fact, the hydrogen permeation rate of a technical permeator exposed to 60 kPa carbon monoxide remained unchanged over at least 500 h at 400 °C.

M. Glugla, R.-D. Penzhorn / Fusion Engineering and Design 28 (1995) 348 356 The water gas shift reaction using carbon monoxide as the only carrier gas for the vaporization of water was demonstrated with a pilot facility. Up to 2 1 day 1 of liquid water were converted into hydrogen at a carbon monoxide flow rate of 13 mol h 1 employing 1 kg zincstabilized copper chromite catalyst bed operated at 200 °C. The water concentration in the bleed flow was less than 1% and could be reduced further down to ppm levels when hydrogen was removed with a permeator and the gas passed through a second catalyst bed to attain again the water gas shift equilibrium. Of concern is a partial conversion of CO into methane, particularly at high concentrations of tritium. Corresponding research activities are presently under way at the Institute of Radiochemistry. The design throughput of CAPRICE for the conversion of liquid heavy water containing up to 1% tritium is 8.9 mol h 1. After permeative removal of the hydrogen isotopes from the first catalytic conversion step, the remaining product stream consists of carbon monoxide and carbon dioxide with trace amounts of hydrogen isotopes and residual water. To increase further the degree of conversion, this gas mixture is passed through the water gas shift catalyst and the permeator of the impurity processing loop. 3.3. Description o f main components 3.3.1. Permeators A permeator is shown schematically in Fig. 4. The feed gas into the permeator (grey zone) is composed of hydrogen isotopes, helium and impurities. It is first preheated and then heated up to the operation temperature of 350 °C. Since only hydrogen isotopes will permeate through the thin palladium-silver tubes, very high purity levels (greater than 99.9999%) [8] are achieveable. The non-permeated gas, mainly consisting of helium and impurities, is removed from the permeation tubes via capillary tubes inserted into the palladiumsilver fingers. To improve the permeation rate a large cross-section is maintained throughout the secondary side of the permeator by arranging the 54 palladiumsilver tubes in the form of a wreath. To assure proper characterization of the permeator, its operation is controlled by several temperature, pressure, moisture and tritium activity sensors. Additionally, the feed, bleed and permeate gas flow rates are measured. One of the pressure sensors and three of the temperature sensors are hard wired to the TLK central safety system. All other sensors deliver warnings, alarms and may cause a safety trip via the local computer.

353

3.3.2. Catalyst vessels Recuperative coaxial tube heat exchangers have been installed in all catalyst vessels to optimize the energy balance. The feed gas is preheated to the operation temperature of the catalyst bed with a separate heater and control. Pressure sensors and tritium activity sensors are installed in such a way that inlet and outlet pressures and activities can be followed. An impactor at the bottom of the vessel eliminates the transport of particles out of the catalyst bed. In addition, each catalyst vessel is provided with a filter at the outlet. To cope with hydrogen isotopes permeated through structural materials, all catalyst reactors are equipped with an outer containment. The interspace between the inner and outer containments is evacuated to provide thermal insulation and monitored by individual pressure sensors. Permeated hydrogen isotopes can be purged with helium or removed by evacuation into a calibrated test buffer vessel to quantify the permeation. 3.3.3. Vaporizer A newly developed vaporizer has been installed in CAPRICE (see Fig. 5). The ideal tube diameter is as small as feasible to give a long gas path through the water and thus ensure a high degree of saturation. The vaporizer must, however, operate in the bubble regime rather than in slug flow to obtain efficient vaporization and to avoid liquid discharge via the outlet. Laboratory tests with a glass vaporizer supported the choice of a vaporizer diameter of about 48 mm to avoid gas-liquid vertical lift. Owing to the incorporation of gas into the liquid its volume increases significantly. To avoid spreading of liquid into the rest of the facility the vaporizer was provided with a funnel-shaped disengagement zone having a larger upper cross-section.

4. Glove boxes

Four glove boxes, each 1.5 m in length, constitute the secondary containment of the experiment. The large 150 m 3 h 1 scroll pump is housed in an additional, specially designed secondary containment. The total volume of both containments is 16.6 m 3. A nitrogen glove box atmosphere is used to avoid the formation of tritiated water after an inleakage into the primary system. However, an oxygen content of 0.5%- 1% is maintained in the glove box to assure oxidation of tritium by the noble metal catalyst used in the tritium retention systems. Two pressure and two oxygen control systems monitor the box atmospheres. In accordance with the volume of each of the containments a 60 m 3 h - I and a

354

M. Glugla, R.-D. Penzhorn / Fusion Engineering and Design 28 (1995) 348-356

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120 m 3 h-1 tritium retention system have been installed. Four ionization chambers are used to measure the actual tritium content in the glove boxes as well as the efficiency of the tritium retention systems. The boxes interface with the mobile tritium transfer system and with the mobile tritium retention system of the TLK.

Two 6 kW heat exchanger units are used to keep the maximum glove box temperature below 40 °C. The glove box atmosphere flow rate through the external cooling duct of the main glove box is about 1000 m 3 h - l . The heat exchangers are cooled with water at 13 °C provided from an external chiller. The chiller in turn is cooled with water at 20 °C supplied by the TLK infrastructure.

M. Glugla, R.-D. Penzhorn I Fusion Engineering and Design 28 (1995) 348-356

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M. Glugla, R.-D. Penzhorn / Fusion Engineering and Design 28 (1995) 348-356

5. Commissioning of CAPRICE Commissioning of the facility CAPRICE has taken place at the manufacturers site, that is N N C in Knutsford (UK), as well as in the TLK. After installation and setting the software into operation, control of all safety systems and control and instrumentation systems was carried out. The process units were then tested first at room temperature and afterwards at the corresponding operation temperatures. Each of the process cycles, i.e. burn and dwell and pump down, were tested separately. A demonstration run with the integral facility, including the secondary containment, was performed using protium-deuterium and typical ITER impurities.

6. Inventories and wastes An estimation of the tritium inventory in each of the principal vessels of CAPRICE is given in Fig. 1. The total tritium inventory in the process relevant components is rather low. For operation with DT it will only be about 2.8 g. This was mainly achieved by the use of catalytic reaction steps and minimization of the number and size of the buffer vessels. From an extrapolation of laboratory measurements less than 100 mg of tritium will be given to waste with the catalysts (total of 8.5 kg). The amount of wasted tritium can be further reduced by extended in situ !sotopic swamping.

nickel catalyst, while a countercurrent flow of protium is employed to produce a decontaminated waste stream via isotope exchange and permeation. The pure hydrogen stream and the impurity stream are at all times separated by a palladium-silver membrane. With the methane cracker, the permcat and the permeator the process becomes continuous. The permcat is presently being tested experimentally and modelled mathematically. Once the concept is proven it will be tested in the CAPRICE facility on a 1:1 scale with respect to ITER-EDA.

8. Conclusions The primary system of the CAPRICE facility has been completed within the contractual schedule and costs. The commissioning tests without tritium, but with all gaseous impurities, indicate that the facility CAPRICE basically fulfills the functional specifications. The beginning of the experimental phase of CAPRICE is anticipated for spring 1994.

Acknowledgement Many valuable contributions from D.K. Murdoch are gratefully acknowledged. This work has been performed in the framework of the Nuclear Fusion Project of the Kernforschungszentrum Karlsruhe and is supported by the European Communities within the European Fusion Technology Program.

7. Advanced catalytic clean-up process for ITER-EDA While the impurity separation using a palladium-silver permeator is a continuous process, the catalytic impurity processing is still of batch type. In order to overcome this disadvantage an improved concept is under development [9]. It combines hydrocarbon cracking and the water gas shift reaction with isotopic swamping. According to this concept a catalyst operated at about 480 °C at the inlet and center of the bed and at about 400 °C at the outlet of the bed is employed for hydrocarbon cracking and the water gas shift reaction. This bed reduces the impurity partial pressures and increases the partial pressures of molecular hydrogen isotopes. Downstream of the catalyst bed a combined catalyst-permeator ('permcat') is placed for isotopic swamping. Within the permcat, held at 400 °C, the tritiated impurity stream is passed over a

References [1] M. Glugla, R.-D. Penzhorn, R. Rodriguez and D. Herbrechter, Report 322/88-8//FU-D, NET-No 88-173, June 1990. [2] M. Glugla, R.-D. Penzhorn, J.L. Anderson and J.R. Bartlit, Fusion Technol., 14 (1988) 683. [3] M. Glugla, R.-D. Penzhorn, R.S. Willms and J.L. Anderson, unpublished work. [4] G. Saibene, personal communication. [5] R. Yamada, J. Nucl. Mater., 145-147 (1987) 359. [6] H. Yoshida, H. Takeshita, H. Ohno, T. Kurasawa and H. Watanabe, Nucl. Technol. Fusion, 5 (1984) 178. [7] A.M. McKay, C.H. Cheh and R.W. Glass, Report CFFTP-G-83017, 1983. [8] R. Goto, Chem. Econ. Eng. Rev. (October 1970) 44. [9] M. Glugla, R.D. Penzhorn, to be presented at 18th SOFT, Karlsruhe, 1994.