Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 41, No. 8, p. 863–870 (August 2004)
TECHNICAL REPORT
Application of Proton-conducting Ceramics and Polymer Permeable Membranes for Gaseous Tritium Recovery Yamato ASAKURA1; , Takahiko SUGIYAMA1 , Takao KAWANO1 , Tatsuhiko UDA1 , Masahiro TANAKA2 , Naruhito TSUJI2 , Koji KATAHIRA3 and Hiroyasu IWAHARA4 1
National Institute for Fusion Science, Oroshi-cho, Toki-shi, Gifu 509-5292 Nippon Kucho Service Co. Ltd., Terugaoka, Meitou-ku, Nagoya 465-0042 3 TYK Co. Ltd., Ohbata-cho, Tajimi-shi, Gifu 507-8607 4 Professor Emeritus, Nagoya University, Shikenya, Moriyama-ku, Nagoya 463-0034 2
(Received December 12, 2003 and accepted in revised form May 12, 2004) In order to carry out deuterium plasma experiments on the Large Helical Device (LHD), the National Institute for Fusion Science (NIFS) is planning to install a system for the recovery of tritium from exhaust gas and effluent liquid. As well as adopting proven conventional tritium recovery systems, NIFS is planning to apply the latest technologies such as proton-conducting ceramics and membrane-type dehumidifiers in an overall strategy to ensure minimal risk in the tritium recovery process. Application of these new technologies to the tritium recovery system for the LHD deuterium plasma experiment is evaluated quantitatively using recent experimental data. KEYWORDS: tritium, tritium separation, tritium monitoring, proton-conducting ceramic, hydrogen pump, membrane filter, membrane dehumidifier, tritium recovery system, Large Helical Device
I. Introduction Deuterium plasma experiments are currently being planned for the Large Helical Device (LHD) of the National Institute for Fusion Science (NIFS), following the present hydrogen plasma experiments. Under the deuterium experiment conditions, it is estimated that 430 MBq of tritium will be generated in each discharge shot, involving the injection of a neutral deuterium (D) beam into D plasma, leading to a maximum of 370 GBq (10 Ci) of tritium generated each year.1) The generated tritium must be recovered before emission, with the annual environmental emission specified at less than 3.7 GBq (0.1 Ci). The Pd–Ag membrane permeator is one candidate for tritium recovery in the LHD plasma exhaust gases treatment system. Intensive study has focused on processes using Pd–Ag membrane permeators for application in plasma exhaust processing and recycling, especially for use in the International Thermonuclear Experimental Reactor (ITER).2–6) However, application of these permeators to low tritium-concentrations and to water vapor processing seems difficult. Instead of a Pd membrane, a blanket tritium recovery system with an electrochemical hydrogen pump using a proton-conducting membrane has been proposed.7) In this study, an electrochemical hydrogen pump using a proton-conducting membrane is evaluated in the treatment of plasma exhaust gases from the LHD experiments. Among atmosphere detritiation systems, methods which collect gaseous tritium as tritiated water using the combined processes of catalytic oxidation and adsorption have been widely applied in Japan and overseas.8,9) To develop more compact and cost-effective systems, alternatives to the ad-
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sorption process have also been studied such as application of gas separation membranes.10–14) In this study, a dehumidifier using a permeable polymer membrane instead of molecular-sieve adsorbers is evaluated in the treatment of exhaust gases emitted during inspection and maintenance work on the LHD. Application of these two technologies will be effective in reducing both the size of the treatment equipment and the quantity of radioactive waste after treatment in LHD deuterium experiments.
II. Specifications of Targeted Treatment System Configurations of the exhaust gas/liquid treatment systems are shown in Figs. 1(a) and (b). Figure 1(a) shows the system designed for recovering tritium during the plasma experiment and Fig. 1(b) details the system during inspection and maintenance of the LHD vacuum vessel. The design conditions of these two systems are summarized in Table 1. Tritium concentrations at the different units of these systems are shown in Table 2. In the deuterium experiment, exhaust gas from the vacuum pump is stored in the temporary storage tank. Details of the tritium chemical forms and composition of the stored gas are shown in Sec. III-2(1)(a). The stored gases (He gas including H, D and T) are introduced into the vacuum pumping-gas treatment unit and after tritium removal the gas is released from the stack. Treatment flow rate was assumed to be 0.02 N m3 /h and the tritium concentration of the processed gas was calculated to be 5,000 Bq/cm3 , the mean value for treating the annual tritium emission (10 Ci/yr) over 4,000 h operation. Tritium-recovery efficiency was fixed at 99%. At this efficiency and after dilution by ventilation with non-radioactive gas flow, the tritium concentration at the stack outlet would be reduced to a value less than
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During LHD Operation
Stack
Large Helical Device(LHD) Building
Ventilation system
LHD plasma vacuum vessel
Tritium gas monitor
5
T
(Inlet duct) (Outlet duct)
Pumping–gas storage tank
Vacuum pumping system Tritium safety storage unit
Vacuum pumping–gas treatment unit
Tritium gas monitor (Conventional)
(Hydrogen absorbing alloy bed,etc)
Development item
Flow of gas (a)
During LHD Maintenance
Stack
Large Helical Device(LHD)Building
Ventilation system
(from inlet duct)
Air dehumidifier
5
T
LHD plasma vacuum vessel
Tritium gas monitor
(Inlet duct) (Outlet duct)
Ventilation equipment
Vacuum-vessel purge–gas treatment unit
Recovery tritium enrichment unit Recovery water tank
Tritium gas monitor
Deliver vessels to collecting agency Enriched-water storage vessel
Flow of water Flow of gas
Tritium gas monitor
Tritium monitor (Liquid scintillation counter)
Drainage to sewerage
Development item Waste water tank
(b)
Fig. 1 (a) Exhaust gas and effluent liquid treatment system for LHD during operation (b) Exhaust gas and effluent liquid treatment system for LHD during maintenance
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Table 1 Conditions for gaseous tritium recovery system of LHD Equipment
Vacuum pumping-gas treatment unit
Vacuum-vessel purge-gas treatment unit
Maximum treatment flow rate (N m3 /h) Operation time (h/yr) Tritium generation amount (GBq/yr) Tritium concentration in treatment gas (Bq/cm3 )
0.02 4,000 370 5,000
1,000 4,000 370 0.1
Table 2 Tritium concentrations for different units for application of the gaseous tritium recovery system Vacuum pumping-gas treatment unit
Equipment Tritium-recovery efficiency (%)
Vacuum-vessel purge-gas treatment unit
99
99
3
Tritium conc. of the gas (Bq/cm ) Inlet of treatment unit Outlet of treatment unit Outlet of stacka) Regulation limit concentration (Bq/cm3 ) Chemical form of water vapor Chemical form of hydrogen gas a)
5,000 50 8105
0.1 0.001 8105 5103 70
Total exhaust gas flow rate from the stack is 12,500 N m3 /h
5104 Bq/cm3 , which is one-tenth of the regulation limit for release as water vapor. Considering that the regulation limit for release as hydrogen gas is 70 Bq/cm3 , recovery of tritium in the form of hydrogen gas is preferable to that of water vapor. During inspection and maintenance work of the inside of the LHD vacuum vessel, the vessel is ventilated with dry air, and the remaining tritium is purged along with the air. The tritium-purging gas (air including H, D and T) is flowed to the vacuum-vessel purge-gas treatment unit and after tritium removal the air is released from the stack. For this study, the treatment flow rate was assumed to be a maximum of 1,000 N m3 /h and tritium concentration in the processed gas calculated to be 0.1 Bq/cm3 , the mean value for treating the generated tritium during continuous operation. Tritium recovery efficiency was again fixed at 99%. At this efficiency and dilution by ventilation with non-radioactive gas flow, the tritium concentration at the stack outlet would be less than 5104 Bq/cm3 . The tritium is recovered in the form of water, and the tritiated water accumulated in a tank. It is expected that by using dry air instead of natural, humid air the generation rate of tritiated water will be reduced to 30 t/yr. The tritiated water accumulated in the tank will be reduced in volume and thus enriched in tritium before handling by a collecting agency. As well as considering these two tritium recovery units, the in-line monitor, which can detect tritium released from the stack with a sensitivity of less than 5104 Bq/cm3 , is planned to be developed based on an electrochemical hydrogen pump.
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III. Evaluating Application of New Technology 1. Application Plan of New Technology From the viewpoint of minimizing the risk of tritium handling, the following basic development concepts are considered: ‹ Treatment of tritium in the chemical form of hydrogen instead of water › Reduction of radioactive waste contaminated with tritium. (1) Vacuum Pumping-Gas Treatment Unit The conventional method of oxidation of tritium and removal of the tritiated moisture using adsorbents such as molecular sieves could be applied to the LHD system. However, for the present unit, it is expected that small amounts of tritiated water will be generated but with a comparatively high concentration compared to the vacuum-vessel purge-gas treatment unit. Considering development concept ‹, the new system targets the recovery of tritium in the form of isotopically enriched hydrogen gas.15) In order to pump out the hydrogen gas selectively, proton-conducting ceramics7,16–20) have been investigated. A proton-conducting ceramic is not suitable for generating perfectly dry hydrogen gas as in the case of a Pd-alloy diffusion membrane. However, it has the ability to decompose hydrogen compounds such as water vapor and methane into hydrogen molecules and can also extract small amounts of hydrogen molecules selectively by application of a direct voltage between electrodes of the proton-conducting ceramic. That is, when a direct current is applied to an electrochemical cell with proton-conducting solid electrolyte and hydrogen gas is supplied to the anode, the hydrogen ionizes to form protons. These protons are transported through the electrolyte to the porous cathode, where they
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Stack
LHD plasma vacuum vessel
Tritium gas monitor
Tritium gas monitor
Pumping−gas storage tank
Hydrogen gas pumping apparatus (Proton conducting ceramic)
Hydrogen isotope separation apparatus
(Pressure Swing Adsorption)
Flow of gas
Hydrogen absorbing bed Fig. 2 Design of the vacuum pumping-gas treatment unit
are discharged to form hydrogen gas.18) The configuration and process flow of the treatment system for the proton-conducting ceramic device are shown in Fig. 2. In this system, vacuum pumping gas accumulated in the temporary storage tank is delivered to the proton-conducting ceramic device at a constant flow rate of 0.02 N m3 /h, and the hydrogen gas containing tritium is selectively extracted from the processing gas. The tritium included in the hydrogen gas will then be enriched by Pressure Swing Adsorption (PSA) equipment operated at liquid nitrogen temperature. Tritium-enriched hydrogen gas can then be stored in a stable form as a metallic hydride. (2) Vacuum-Vessel Purge-Gas Treatment Unit For this unit, it is difficult to apply the same system as for the vacuum pumping-gas treatment unit because the gas treatment capacity is about 1,000 times larger. With respect to development concept ›, waste generation could be reduced through the use of a polymer membrane dehumidifier15,21) instead of an absorbent column. When using molecular sieves, a dew point of less than 60 C is readily achieved at room temperature if the amount of absorbed water is 10% or less. However, if a dew point of less than 60 C could be obtained using a polymer membrane dehumidifier, the equipment could be reduced in size and a more stable dehumidifying performance could be expected. Molecular-sieve adsorption equipment requires regeneration by alternately switching two columns. The configuration and process flow of the system applying the polymer membrane dehumidifier are shown in Fig. 3. The air used to purge the tritium from the vacuum vessel is delivered to the catalytic oxidation equipment. The tritiated water vapor is recovered by the polymer membrane dehumidifier. To reduce the volume of tritiated water, dry air is used to purge the vacuum vessel. The polymer membrane dehumidifier has additional application in producing this dry air. (3) High Sensitivity In-line Tritium Monitor The tritium concentration in exhaust gas can be monitored
with sufficient accuracy to meet regulations using a liquid scintillation method. However, a real-time monitor has not been commercially available for concentrations under 5104 Bq/cm3 , which is 1/10 of the regulation value for tritiated water vapor in the exhaust gas. Using the hydrogen pump described in Chap. I, it is possible to lower the effective detection limit by greater than an order of magnitude, by concentrating the hydrogen-isotope gas (including tritium) and by removing the radon gas which is mixed in at the monitoring stage. The configuration and process flow of the monitoring system applying the proton-conducting ceramic are shown in Fig. 4. The monitoring gas is introduced to the proton-conducting ceramic device and only the hydrogen-isotope gas is extracted to the circulating carrier gas. Extraction of hydrogen continues until the tritium concentration in the carrier gas is higher than the detection limit of the radiation detector. A proportional counter is used as a radiation detector for this high sensitivity monitoring system. 2. Evaluation of Potential Application (1) Application of Proton-Conducting Ceramic to Vacuum Pumping-Gas Treatment Unit (a) Gas Amount and Composition The estimated amount and composition of the gas to be treated are shown in Table 3. The processed gas is a mixture of the exhaust gas from helium glow discharge operations during cleaning of the vacuum vessel wall and the exhaust gas during plasma experiments, in which hydrogen is the main isotope. The maximum quantity of exhaust gas from the deuterium plasma experiments is estimated to be 80 N m3 /yr. Helium is the main component of the processed gas stored in the temporary storage tank, and is present in the exhaust gas from both deuterium plasma experiments and helium discharge cleaning operations. The hydrogen component composition is estimated to be 8.3% hydrogen, 6.3% water vapor and 0.2% methane. That is, total hydrogen content in the processed gas is 15% by molecular ratio or JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
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Application of Proton-conducting Ceramics and Polymer Permeable Membranes for Gaseous Tritium Recovery
(From inlet duct)
Stack
Membrane dehumidifier
Compressor
Compressor
LHD Plasma vacuum vessel
Membrane dehumidifier
Tritium gas monitor
Drain separator Flow of water Flow of gas
Catalytic oxidation apparatus
Temporary storage tank
Fig. 3 Design of the vacuum-vessel purge-gas treatment unit
Carrier gas
Circular pump Exhaust
Monitoring gas
Tritium enriched gas
Hydrogen gas pumping apparatus
Proportional counter
(Proton conducting ceramic)
Exhaust
Flow of gas Fig. 4 Design of the in-line gas phase tritium monitoring unit
Table 3 Composition of the vacuum pumping gas stored in the temporary storage tank Gas component Deuterium discharge Cleaning discharge (He-glow discharge) Total
Q2 (%)
Q2 O (%)
CQ4 (%)
He (%)
59 1
15 5
1 0.1
0 89
0.2
77.8
8.3
6.3
12 N m3 by gas volume. (b) Required Performance for Proton-conducting Ceramic When evaluating performance of the proton-conducting ceramic, both treatment capacity and stability of the device should be considered. In this treatment, operation was assumed to be continuous for half a year (4,000 h/yr). Thus, the required treatment flow rate to treat the annual quantity of exhaust gas of 80 N m3 over these 4,000 h is about VOL. 41, NO. 8, AUGUST 2004
CO2 etc. (%)
Gas volume (N m3 )
25 4.9
10 70
7.4
80
350 ml/min. Considering the hydrogen component composition in Table 3, the required hydrogen-permeation capacities are calculated as 29 ml/min for hydrogen gas and 22 ml/min for water vapor. The total annual volume of hydrogen gas included in the exhaust gas is thus estimated at about 12 N m3 . This volume corresponds to a total proton current value of about 7 A using the proton-conducting ceramic device for extracting the hydrogen.
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Y. ASAKURA et al. Table 4 Performance of electrochemical hydrogen pumps using proton-conducting ceramic tubes Vacuum exhaust-gas treatment
Experimental condition
Hydrogen-pump performance
Electrolyte Electrode Heating temperature Anode gas Cathode gas Hydrogen extraction rate Hydrogen extraction density Reference a)
c)
Water-vapor electrolysis performance
SrZr0:9 Yb0:1 O3 Plated platinum 700 C 1% H2 + 1.2% H2 O + Ar 1.2% H2 O + Ar 1.2% H2 O + Ar 1.2% H2 O + Ar 0.85 ml/min at 0.8 V 0.61 ml/min at 2 V (2.1 ml/min at 2 Vb) ) 2 b) 45 ml/min/cm 13 ml/min/cm2 23) 23)
Sensitive tritium monitor Water-vapor electrolysis performance CaZr0:9 In0:1 O3 Pasted platinum 800 C 1.2% H2 O + 20% O2 + Ar Dry Ara) 0.67 ml/min at 3.5 V 14 ml/min/cm2 27)
H2 O
