Plasma and Fusion Research: Regular Articles

Volume 11, 2405055 (2016)

Observations of the Gas Stream from the Large Helical Device
for the Design of an Exhaust Detritiation System∗)
Masahiro TANAKA, Naoyuki SUZUKI, Hiromi KATO, Tomoki KONDO
and the LHD Upgrade TeamNational Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan(Received 30 November 2015 / Accepted 9 March 2016)
A small amount of tritium is produced by deuterium–deuterium reactions in the core plasma during deuterium plasma experiments conducted in the Large Helical Device (LHD). From the viewpoints of tritium safety
handling and public acceptance, any waste gas streams arising from the vacuum vessel of the LHD and Neutral
Beam Injection (NBI) system will be treated by an exhaust detritiation system (EDS). The exhaust gas stream
produced during the 18th cycle of LHD plasma experiments was observed to design the EDS. The rough pumping
gas streams from the LHD and NBIs produced a high flow rate of 300 Nm3 /h. The vacuum exhaust gas stream
from the LHD had a high hydrogen concentration more than 7% and a flow rate of less than 5 Nm3 /h during
plasma experiments. The regeneration gas stream of the cryopump systems had a flow rate of more than 5 Nm3 /h.
Based on these observations, the proposed EDS utilizes two types of tritium recovery system. One comprises twotypes of catalyst and dual absorbent beds. Other comprises an oxidation catalyst and polymer membrane system.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: tritium recovery, vacuum exhaust gas, exhaust gas flow rate, large fusion test device, exhaust detritiation system, tritium safety handling
DOI: 10.1585/pfr.11.2405055

1. Introduction
The development of nuclear fusion devices has remarkably progressed in recent years. Due to advances in
performance when using plasma, deuterium and tritium
are used as the operating gases in the fusion test device.
To comply with radiation protection and safety assessments, exhausted tritium from the fusion facility will berecovered by an exhaust detritiation system (EDS) to minimize tritium release into the environment. A similar EDS
has been utilized in the JET Active Gas Handling System(AGHS) [1–4]. The EDS comprises a two-stage catalyticrecombiner for the oxidation of tritiated compounds suchas tritiated hydrogen and tritiated hydrocarbons and threemolecular sieve driers in parallel for the recovery of water
vapor [1, 2]. The role of the EDS is to provide air flow into
the torus vacuum vessel and AGHS openings during maintenance and to detritiate the exhaust gases in the case of an
accident [4].
In the Large Helical Device (LHD), a small amountof tritium is produced by deuterium–deuterium reactionsin the core plasma. A portion of this tritium is exhausted
from the vacuum vessel and the rest is retained in the firstwall and the diverter tiles. Although the exhausted tritium
is a comparatively small amount, any waste gas streams
arising from the vacuum vessel and the auxiliary equipment must be treated by the EDS to comply with safety
author’s e-mail: tanaka.masahiro@nifs.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

requirements. To design the EDS for a large fusion test device, the behavior of the exhaust gas stream has to be understood quantitatively under the various operation modes.
This entails measuring variables such as the flow rate, fluctuations in the flow, and gas compositions. The exhaust gas
streams originate from (1) the plasma discharge operation,(2) regeneration by cryopumps and ice pellet injection systems, (3) rough pumping of the vacuum vessels of both thefusion test device and the Neutral Beam Injection (NBI)
system, (4) a purge gas used in the vacuum vessel during
maintenance.
In this study, the exhaust gas stream from LHD [5],which is the largest helical type fusion test device, was observed under various operating conditions, such as plasmadischarges, regeneration utilizing the cryopump system,and the rough pumping of the vacuum vessels. The design of the proposed EDS has enough capacity to treat allthese gas streams.

2. Monitoring the Gas Stream andHydrogen Concentration
The deuterium plasma experiments in the LHD will
be conducted during the 19th cycle of LHD plasma experiments. In preparation for these experiments, some of theexhaust gas lines were rearranged and combined for the observation of the gas stream before the 18th cycle of LHD
experiments.
To observe the gas stream from the LHD, a gasstream monitoring system was installed in the exhaust

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c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 11, 2405055 (2016)

Fig. 1 Gas stream monitoring system in the gas lines from LHDand NBI.
Fig. 2 Variation in the roughing pump flow rate of the LHD vacuum vessel.

gas line as shown in Fig. 1. The exhaust gas line comprises the vacuum exhaust gas line from the LHD’s vacuum pumping system, and the vacuum vessel ventilationline from the roughing pump and other pumping systems. Two thermal mass flow meters (Bronkhorst HighTech BV, F-106AZ [0 - 20 Nm3 /h] and Oval Co., ModelMFT-B [0 - 600 Nm3 /h]), three hydrogen sensors (NewCosmos Electric Co. LTD, PE-2 type), and a hygrometer(VAISALA, Model HMT334) were installed in the exhaust
gas line. The average dew point in the exhaust gas wasless than −20◦ C during plasma operation. Three types ofhydrogen sensors were used: the hot-wire semiconductor
type for high sensitivity monitoring (from 0% to 10%), thethermal conductivity type for wider range (0% to 100%),and the combustion type (0% to 4%). The latter type wereinstalled to discriminate between hydrogen and helium, because the first two react to both hydrogen and helium.
Thermal mass flow meters were installed in the vacuum exhaust gas line and the vacuum vessel ventilation
line. The hydrogen sensors and dew point indicator connected in series collected sample gas from the vacuum exhaust gas line.

3. Monitoring Results
The observation of the gas stream was performed during the preparation phase and during the actual plasma experiments of the 18th cycle of the LHD’s operation.

3.1Fig. 3 Variation of gas stream during the 18th cycle of LHD
plasma experiments: (a) vacuum exhaust gas flow rate,(b) hydrogen concentration.

Flow of rough pumping from the vacuum vessel of the LHD

After maintenance work in the LHD, the vacuum vessel was evacuated prior to conducting plasma experiments.
The exhaust gas composition was room air without hydrogen gas. The volume of the vacuum vessel was approximately 250 m3 . Figure 2 shows the variation in the roughing pump flow rate of the LHD vacuum vessel. As shown,the maximum flow rate reached 309 Nm3 /h after the evacuation was started. The flow rate reduced for approximately
2 hours as the other large vacuum components making up
the NBI vacuum vessel were evacuated for the NBI conditioning operation. The maximum roughing pump flowrate of the NBI vacuum vessel was observed to be similar
to that of the LHD vacuum vessel. Therefore, when thevacuum vessel was pumped from atmospheric pressure,the flow rate through the EDS that requires processing isapproximately 300 Nm3 /h. This was also the same as thetorus purge flow rate during the LHD vacuum vessel maintenance period.

3.2 Vacuum exhaust gas flow and hydrogenconcentration during plasma operation
In the 18th cycle of LHD plasma operation, hydrogen
was used as the main operating gas. Helium was also usedfor discharge cleaning. Figure 3 shows the variation of vacuum exhaust gas flow rate and hydrogen concentration during these experiments. The flow rate depended on exper-2405055-2

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imental conditions and the operation mode. The vacuum
exhaust gas flow rate was 2 - 3 Nm3 /h under normal operation. However, the flow rate reduced to less than 1 Nm3 /hwhen some NBI systems were shut down. The main exhaust gas flow thus might comprise exhaust gas from theNBI system.
In the 18th cycle of LHD plasma experiments, hydrogen concentration temporarily increased to 7% as transitory phenomenon when the cryopump for the LHD vacuum
vessel was regenerated. To avoid a hydrogen explosion in
the EDS, the hydrogen concentration has to be diluted below 4%, which is the explosion limit of hydrogen gas inair. Thus, the EDS has to maintain suﬃcient throughput
for the dilution of hydrogen.
During the hydrogen ice pellet injection experiments,hydrogen concentration was observed to be steady at approximately 2% during operation, although plasma discharge was conducted every 3 min. Hydrogen concentration and vacuum exhaust flow rate were varied and depended on each plasma experiment’s condition and pump
operation.

3.3

Flow during regeneration of the NBI
cryopump

The largest exhaust gas flow rate is during the regeneration process of the cryopump for the NBI system, because nitrogen gas is used to purge the shaft of the roughing pump. Figure 4 shows the variation of exhaust gas flowrate by the regeneration of each NBI cryopump. High flowrate and variation caused by the shaft seal gas of roughing pumps and regenerated gas was observed. When all
NBI cryopump systems were regenerated simultaneously,
the total exhaust gas volume was estimated to be 42.5 Nm3
for a few hours. Thus, the exhaust gas stream from theregeneration of NBI cryopumps is the most critical issue
to the EDS. The regeneration gas of cryopumps has to be
temporarily stored in a large tank to mitigate the gas flow.

Fig. 4 Variation of exhaust gas flow rate by the regeneration ofNBI cryopumps: “TMP” is turbo molecular pump.

4. Conceptual Design of the EDS forLHD
The EDS design policy for the LHD was determinedon the basis of the observations of the exhaust gas stream.
According to the safety management plan for the LHD [6],the detritiation factor is designed to be more than 20, corresponding to a tritium recovery rate of 95%. The exhaust gas flow rate and the hydrogen (tritium) concentration are broadly divided into the following two specifications: (1) high hydrogen (tritium) concentration and low
flow rate during plasma operation and (2) low hydrogen(tritium) concentration and high flow rate during the vacuum vessel maintenance period and during roughing pump
operation. The former demands high reliability because
the tritium concentration in the exhaust gas would be highunder plasma operation. Thus, conventional tritium removal systems comprising an oxidation catalyst and absorbent such as molecular sieves are applied. A compact
system is required due to limited space. A high throughput is required because the maximum flow rate would be300 Nm3 /h. Thus, a polymer membrane that can recoverwater vapor continuously [7, 8] would be applied insteadof an absorbent. The conceptual diagram of the EDS isshown in Fig. 5. The proposed EDS comprises a molecular sieve (MS) system, a polymer membrane (PM) system,
and a water storage and delivery system.

4.1 MS system in the EDS
The MS system comprises three components. The firstcomponent comprises temporary gas storage tanks for theNBIs. The regeneration gas from the cryopump with highflow rate is treated by the MS system because the tritiumconcentration in the exhaust gas will be high. In the conceptual design, the regeneration gas from the cryopump isstored by the large buﬀer tanks in the first component system. Subsequently, the gas processing takes some time tointroduce gas into in the main process line. The total volume of the buﬀer tanks would be designed to store 90 m3 ofgas at 0.65 MPa, because the total regeneration gas volume
from the NBIs was estimated to be 42.5 Nm3 .
The second component is an oxidation catalyst system
for the combustion of hydrogen and hydrocarbons. Thethird component is the recovery system for water vapor,using two absorbents. These main component systems areapplied in conventional tritium recovery systems. However, because the exhaust gas from the vacuum vessel does
not contain oxygen and a high hydrogen concentration in
the exhaust gas was observed, (Fig. 3), dry air is introduced
upstream of the catalyst reactor to dilute the hydrogenand add oxygen for hydrogen combustion. The designed
process flow rate after dilution would be 20 Nm3 /h. Because the vacuum exhaust gas flow rate was approximately
2 Nm3 /h under normal operation, the hydrogen concentration would be diluted to less than 1%. Moreover, hydrocarbons are observed in the vacuum exhaust gas of large fusion test devices such as JT-60U and JET [9,10]. Thus, two

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Fig. 5 Conceptual diagram of EDS: (a) vacuum exhaust gas processing system [MS] for high hydrogen concentration and low flow rateand (b) maintenance purge gas processing system [PM] for low hydrogen concentration and high flow rate.

types of platinum and palladium catalyst with diﬀerent operating temperatures are designed for the oxidation of hydrogen and hydrocarbons. The platinum catalyst has highactivity for hydrogen oxidation and the palladium catalyst
for hydrocarbon oxidation [11, 12]. According to the experimental results given in ref. 11, the commercially available catalysts based on Al2 O3 would be operated at 200◦ C
for hydrogen oxidation and at 450◦ C for hydrocarbon oxidation.
The third component is the recovery system for tritiated water vapor and uses molecular sieves as absorbents.
The dew point after the molecular sieve bed reaches lessthan −60◦ C, corresponding to a water vapor concentrationof 20 ppmv at atmospheric pressure. The dual absorbent
beds packs are designed for the recovery of tritiated watervapor. The dual bed system is used with one bed on adsorption and the other on regeneration or standby. Regen-eration requires taking the absorbent bed oﬀ-line, passing
gas at 350◦ C through it, and then condensing the tritiatedwater with a cooling unit [13].
Tritium in the main process line would be at such alow concentration that it might not be possible to monitor
it in real time. Moreover, because the ionization chamber
for tritium monitoring is aﬀected by the gas composition, it
would be diﬃcult to perform calibrations of the ionizationchamber for tritium monitoring in an MS system. Thus,
instead of the ionization chamber, hydrogen sensors andhygrometers will be installed in the EDS to evaluate thehydrogen isotope recovery rate. The tritium concentrationin the main process line will be measured by another monitoring system such as a tritium sampler system with waterbubbler.

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PM system in the EDS

dioisotope Association at regular intervals.

The PM system also comprises three components asshown in Fig. 5 (b). The main process gas is room air.
The dew point in room air is controlled to within a few
degrees, corresponding to a water vapor concentration ofseveral thousand ppmv. The designed process flow rate is
300 Nm3 /h to treat the torus purge flow during the LHD
vacuum vessel maintenance period and the roughing pumpflow from the vacuum vessels. The first component of thePM system is the oxidation catalyst system. The platinum
catalyst, which is the same catalyst used for hydrogen oxidation in the MS system, is applied to oxidize hydrogen at200◦ C.
The second component is the recovery system for water vapor using the commercially available hollow fiber
polyimide membrane module. The feed pressure for thismodule is designed to be 0.6 MPa. Because the specification of the process flow rate of the largest polyimide membrane module unit is approximately 15 Nm3 /h, at a dew
point of −50◦ C, the required number of membrane module units is 20. The polyimide membrane module has tobe operated at a constant flow rate. Therefore, this system treats the torus purge gas flow from the vacuum vessel
during the maintenance period. The recovery rate of water
vapor by the polyimide membrane module is measured by
the hygrometers.
The third component is the recovery system for theroughing pump flow using an absorbent. The process gas
flow from the roughing pumps of the LHD and the NBIs,
which changes with time as shown in Fig. 2, bypasses thepolyimide membrane module and then passes through theabsorbent recovery system.
Tritium concentration at the inlet to the PM system
will be monitored by a proportional counter. When thetritium concentration is extremely low, the process gas isbypassed to the main process line.

4.3

Water storage and delivery system in theEDS

The tritiated water recovered by the MS and PM
systems is transported from local storage tanks to a large
temporary wastewater storage tank by the water deliverysystem. The tritiated water in this storage tank will betransferred to 25 L bottles and delivered to the Japan Ra-

5. Summary
The exhaust gas from the LHD and the NBI during
the 18th cycle of LHD plasma experiments was observed
for the design of an EDS. The roughing pump gas streams
from the LHD and the NBIs were observed to have a highflow rate of 300 Nm3 /h. This flow rate is similar to that ofthe purge gas during torus maintenance. The vacuum exhaust gas stream from the LHD has a high hydrogen concentration greater than 7% and a low flow rate less than
5 Nm3 /h during plasma experiments. The regeneration gas
stream from the cryopump systems has a high flow rate.
The total regeneration gas volume from the NBIs cryopumps was estimated to be 42.5 Nm3 .
Based on the observed exhaust gas stream, an EDS
was designed. This design uses two systems for tritium recovery. The first is the MS system comprising two typesof catalyst and dual absorbent beds. The second is a PM
system comprising an oxidation catalyst system and two
recovery systems, one for water vapor and the other forroughing pump flow. The detailed design and the construction of the EDS are intended to be completed before the19th cycle of LHD plasma experiments.

Acknowledgments
This study was supported by the NIFS budget(NIFS14ULAA023 and ULGG801).
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