24D10
Effect of Outer Tritium Supply to Tritium Balanceof a D-T Fusion Power Reactor
核融合炉のトリチウムバランスに与える外部供給源の効果
Masabumi Nishikawa, Hideki Yamasaki, Hideaki Matsuura* and Yasuyuki Nakao*
西川正史, 山崎英樹，松浦秀明，中尾安幸
Graduate School of Engineering Science, Kyushu University, Fukuoka 812-8581, Japan
九州大学総合理工学研究院，〒812-8581 福岡市東区箱崎6-10-1*Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan*九州大学工学研究院，〒819-0395 福岡市西区本岡744(TBR)BS attainable, which is obtained from the neutron usage and the recovery efficiency of bred tritium inthe blanket system, must be larger than the required tritium breeding ratio, (TBR)BS required, whichrepresents the whole tritium consumption in a D-T fusion reactor. Recent plasma-wall interaction studies
have revealed that a considerable amount of tritium is trapped when a re-deposition layer forms. The effectof tritium trapping on the tritium balance is discussed in this study. Effect of tritium addition from the outer
tritium supply to assist the maintenance of tritium balance is also discussed in this paper.

1. Introduction
Recent plasma-wall interaction studies haverevealed that a considerable amount of tritium
injected into the plasma vacuum vessel is trappedwhen the re-deposition layer forms and that thetime constant of trapping phenomena is rather longas reported by Roth et al.[1] The tritium balance ina D-T fusion reactor has been discussed by thepresent author by comparing the amount of tritium
consumed in the vacuum vessel with the amount oftritium supplied from the blanket system taking theeffect of tritium trapped to re-deposition layer into
account.[2-5] The author proposed the way torecover tritium from co-deposits periodically toease the tritium balance.[3] It is recognized that theinitial tritium inventory of the first reactor must be
supplied from an outer tritium supply. Then, use ofthis tritium source is a possible way to compensate
the lack of tritium in a fusion reactor consideringthat the target value of the Net TBR is only 1.05 indesign of DEMO reactors in JAEA.[6, 7]
2. Theoretical consideration of tritium balance
The way to evaluate the required value for thetritium breeding ratio, (TBR)BS required, iscompared with the way to obtain the attainablevalue of the tritium breeding ratio in the blanket
system, (TBR)BS attainable, in Figure. 1.
The value of (TBR)BS attainable is decided fromthe usage of neutron in the blanket system and therecovery efficiency of the bred tritium.[4](TBR)BS attainable= (TBR)0 βVV βMD βBT .
(1).

Initial inventory& Doubling time

(4)(TBR)R,nonat operation

Outer tritium
supply(3)

Tritium balance
invacuum vessel(TBR)R,Net
require d(1)(TBR)BSattainable(TBR)BS
require d

Recovery ofbred tritium

D-Tneutron(2)
Net TBR

Neutron usageat blanket

Local
TBR

Fig. 1 Tritium flow in a DT fusion reactor.

Neutron usageat first wallFig. 1. (TBR)BS attainable and (TBR)BS reqired.

Fig.1 ways to two (TB R)sThe reactor base tritium breeding ratio in Fig. 1,(TBR)R,Net required, is decided when the amount oftritium to be prepared for a next reactor and thetime for preparation are decided.[4](TBR)R,Net required = ntotal /(TWT f ’Decay). (2)Here, ntotal, TWT and f ’Decay are equivalentburning days corresponding to total inventory in afusion reactor, tritium doubling time and correction
factor for the beta decay in storage with constantaccumulation rate, respectively.
Then, (TBR)BS required is evaluated from thetritium balance as follows.
(TBR)BS required= (TBR)R,Net required + (δT)overall/(η)overall+ (θP)overall/(η)overall + (QT)Decay/Tburn–{(δT)overall/(η)overall}f ’Decay (βtrap)VV .
(3)
Some tritium will be trapped to the re-deposition
layers in the vacuum vessel and the overall trapping
factor is represented by (δT)overall.

contribution to preparation for the next reactor ismuch larger than the contribution to trapping toco-deposits in the condition of this figure.
Effect of the capacity of outer tritium supply iscompared in Figure 3 for the case when thetrapping factor is 3x10-4. It is known from thisfigure that addition of tritium from the outer tritium
supply is helpful to maintain the tritium balance ofa D-T fusion reactor at the beginning stage of thefusion era. Even in the case when the attainable
overall burning efficiency of tritium turn out to be
0.5%, the tritium balance of a 3GWth power reactor
with (TBR)BS of 1.05, (θP)overall of 1x10-4, TWT of 3years and (δ)overall of 3x10-4 can be maintained if 6.3
kg/y of tritium which corresponds to the breeding
ratio of 0.043 is obtained from an outer tritium
supply as known from Fig. 6.

The overall permeation factor (θP)overall represents
the permeation phenomena in the fusion reactor .
The overall burning efficiency of tritium in theplasma vacuum vessel (η)overall) is defined by Eq.
(4) from the burning rate of tritium Tburn and theflow rate into the plasma vacuum vessel.[4](η)overall = Tburn /(QT)VVin)= ηPL1 ηTR1 ηBUP1 (QT)VVin ,(4)where ηPL1, ηTR1 and ηBUP1 are efficiency inplsmadization of tritium in the vacuum vessel,efficiency in transportation of plasmadized tritium
to the burning core and tritium burn-up in the core,respectively.4
3. Discussion on tritium balance
Change of Δ(TBR) (=(TBR)BS required –1) withchange of the overall burning efficiency of tritiumis shown in Figure 2. The overall trapping factor of3x10-4 is evaluated for the condition of all tungstenfirst wall and 3x10-3 for the condition of Be wallwith tungsten divertor by the author4 using thereport by Roth et al for ITER operation.[3] Recent
discussion among material scientists implies thatthe trapping factor may become below one-tenth of
the ITER condition at the wall temperature of apower reactor.[8,9] Then, the trapping factors usedin Fig 2 ,3x10-5, may corresponds to the trapping
factor at the condition of all tungsten first wall in a
power reactor though the effect of neutron
irradiation is not considered. It is known from thisfigure that the tritium balance in a reactor cannot be
maintained even when the trapping factor is so low
as 3x10-5 if the (TBR)BS attainable is 1.05-1.07.
Contribution of tritium breeding ratio requiredfor (1) preparation of the initial inventory of thenext reactor, (2) that for beta decay of the tritium
inventory, (3) that for the loss due to permeation
through the wall of vacuum vessel, and (4, 5) thatfor compensation of tritium trapped to co-deposits
are also compared in this figure. It is shown that

0.14

Total (90% recovery)

0.06

Total(no recovery)

TWT : 3 yrs

1%(η)ove rall
= 4%2%(TBR)BS attainable = 1.05

0.04-3100.019 (2.8)

0.08

0.5%

0.023 (3.3)

TBR

0.100.030 (4.3)

0.070 (10.3 kg/y)

0.12

Tburn=0.4kg/day (1GWe)
ntotal=57days
Permeation (θT)overall=1x10 -4
Trap (δT)overall=3x10 -4

0.043 (6.3)

0.25%-210()overall

FigureCountermeasure
Tritium
Supply.
Fig. 3. Tritium
required3---Outerfrom outer
tritium
supply.
-4-4(Trapping factor
3x10-5 , Permeationfactor(Trappingfactor:3x10) 1x10 )10Fig.7 Δ TB R(90)57 days,0.0003OTS’

4. Conclusion
It is effective to have an outer tritium supply withthe capacity of annual production rate of 5-10 kg/y.
Then, it becomes possible to use the first wallmaterial of which trapping factor is 3x10-4 to keep
the tritium balance even when the overall burning
efficiency of tritium in plasma is 0.5%

0.10

Total (no recovery)
0.08

TBR

0.06

(η)ove rall =0.5%(1)(TBR)BS attainable=1.05

0.04

(3)

0.02

(4)
(2)
(5)

0.00
-310(η)ove rall =2%Total (90%recovery)(1)Preparation (57days, 3yr)(2)Decay (ntotal=57days)(3)Permeation (θT)overall=1x10 -4(4)Trap (δT)overall=3x10 -5(5)Trap (90% recovery)-210()overall

10EffectFig.
of (1)(2)) isbalance.
so large. (Trapping factor:3x10 -5)
2.(andTritium-1

Fig.3 Δ TB R(P0.0001)57days,0.00003-1

References[1] J. ROTH et al., J. Nucler. Mater., 390-391 (2009) 1.
[2] M. NISHIKAWA, Fusion Sci & Tech. 57 (2010) 120.
[3] M. NISHIKAWA, Fusion Sci & Tech., 59 (2011) 350.
[4] M. NISHIKAWA, Fusion Sci. & Tech., Accepted.
(Reported in 9th Int. Tritium Conf., Nara, 2010)[5] M. NISHIKAWA and T. TANABE, Fusion Eng. Des.,85 (2010) 987.
[6] M. ENOEDA et al., Nuclear Fusion, 43, (2003) 1837.
[7] K. TOBITA et al., Nucl. Fusion, 49 (2009) 075029.
[8] G. De TEMMERMAN and R. P. DOERNER, J. Nucl.
Mater., 389 (2009) 479.
[9] V. Kh. ALIMOV et al., J. Nucl. Mater., (2011), doi:
10.1016 /j.jnucmat.2011.01.088.