23T2
Fusion reactor technology, focused on tritium safety handling
核融合炉工学最前線−−

トリチウム関連を中心として−−

Takumi Hayashi
林巧
Directorates of Fusion Energy Research, Japan Atomic Energy Agency
801-1, Mukouyama, Naka city, Ibaraki Pref., 311-0193, Japan（独）日本原子力研究開発機構•核融合研究開発部門 〒311-0193 茨城県那珂市向山801-1
The recent progress of fusion technology is summarized focused on tritium safety handling. Typicaldesign concept of fusion fuel cycle for DEMO reactor is presented with the tritium distribution in thereactor site, comparing with that for ITER. Effective accountancy of tritium, widely distributed within
the reactor site, will be a key issue for safety and economical point of view. Multiple tritium confinement
concept and effective detritiation system design are also key issues regarding all life-cycle operation offusion reactor at normal & abnormal conditions, in order to receive the public acceptance.

1. Introduction
In the fusion reactor, effective and carefulhandling of DT fuel is a key issue from theeconomical and safety point of view.
This
presentation summarizes the progress of fusion
technology focused on the tritium safety handling,such as 1) overall design concept of fusion fuelcycle, 2) tritium distribution & its accountancy, and3) tritium confinement & detritiation.
2. Design concept of fusion fuel cycle
Figure 1 shows the typical configuration offusion fuel cycle for DEMO in comparison withthat for ITER, which can handle a few kg of tritium.
Tritium cycle mainly consists of main fuelprocessing cycle loop and blanket tritium recovery
loop. The purpose of main fuel processing cycle
loop is to recover/purify DT fuel from plasma
exhaust and to reuse them as a fuel again, because
the one-through burning ratio of DT fuel will be
only about 5% even in DEMO. In the fusion
reactor with 3GW thermal output, about 450 g oftritium is burned every day. Therefore, the main
fuel loop should process at least more than 9 kg oftritium every day. Because more than 450 g oftritium should be generated and recovered withinthe blanket tritium recovery loop every day.
The fuel processing loop mainly consists of fuelclean up (to purify hydrogen, to process impurity),
isotope separation, and fuel storage & delivery
sub-systems.
To purify hydrogen gases fromplasma exhaust, palladium silver membrane diffuser
will be used. If it is required, the purifiedhydrogen gases will be separated to each isotope
gases
bycryogenic
distillation
columns.
Hydrogen isotopes, existed as impurities like wateror various hydrocarbons, are converted to hydrogengases at first by oxidation & reduction, water gas
shift, cracking, or isotope exchange reactions.
Then, the hydrogen gases are recovered by theabove membrane diffuser. Recovered/purified DT
fuel is accounted and delivered again after
adjustment of the isotope composition by metal
hydride fuel storage beds, such as uranium and/orzirconium cobalt etc.
The blanket tritium recovery loop should
optimize depending on the breeding blanket system.
Though there are many design concepts forbreeding blanket system, in case of water-cooled
ceramic breeder blanket, the bred tritium is purgedout by helium gas. Therefore, it is important how
to recover effectively the bred tritium from thepurge gas under very low partial pressure of tritium(~1Pa).
Hydrogen pump technology will beapplied for this recovery using proton conductor.
Because the bred tritium can transfer to coolingwater, R&D of effective barrier to protect thetransfer is very important, as well as the design
optimization of water detritiation system (CECE :Fig. 2 Multiple barrier concept for tritium
confinement and its safety related investigation items.

Combined Electrolysis & Catalytic Exchange) torecover tritium from tritiated cooling water.
3. Tritium distribution and accountancy
As also shown in the Fig. 1, tritium will exist in
various locations in the site. Site inventory oftritium will be determined by the concept of fuelstorage. Normal thermal power plant will storethe fuel for one month’s operation, so, applying thesame concept, about 14 kg of tritium is required tostore in the fusion reactor site. In this figure,various measuring points for tritium accountancy
are indicated separating for regulatory purpose andfor operation purpose. How to control / treat theaccounting uncertainty (accuracy, precision) will befurther discussion point.
4. Tritium confinement and detritiation
In the fusion research facilities, tritium has beenhandled safely under the multiple barrier concept,
as shown in Fig. 2. This concept is applied toITER and also will be applied to DEMO. Primarybarrier has a function of physical containment,
however, secondary or tertiary barriers has eachconfinement functions with using various active
components, such as isolation valves, dumpers,monitors, HVAC, and detritiation systems, etc.
Figure 3 shows conceptual flow diagram for
detritiation systems in case of ITER Tokamak
complex building. MSDS is typical detritiation
system for normal operation, which uses oxidation
by low (423K) and high (773K) temperaturecatalytic reactor, and water adsorption by molecularsieves dryer. SCDS is emergency detritiation
system, which uses oxidation by room temperaturehydrophobic catalyst and wet scrubber column. In
order to keep enough redundancy, multiple modules
are arranged for tritium release accident, regarding
blackout, fire, and maintenance etc.

Fig.3 Conceptual flow diagram for the detritiation systems
in case of ITER Tokamak complex building.