S-P4-3
Isotope effects in hydrogen recycling
燃料リサイクリングにおける水素同位体効果
Tetsuo Tanabe
田辺哲朗
Department of Advanced Energy Engineering Science, Interdisciplinary Graduate School of Engineering Science,
Kyushu University、6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
九州大学総合理工研究院 〒812-8581 福岡市東区箱崎6-10-1
Burning efficiency in a fusion fuel cycle is, unfortunately, very poor; i.e. only a few % or less of input
tritium burns and the majority must be recovered to recycle. This requires huge fuel through-put. Inaddition, large in-vessel fuel retention rate would result in the huge in-vessel tritium inventory, which ishard to remove and recover. All these are directly related to tritium safety. In this presentation, aftersummarizing problems on fuel retention and fuel cycles, research proposal which can be done in thedeuterium operation in LHD is made.

1. Introduction
Large through-put owing to poor burning
efficiency and fuel retention in a reactor vessel
concerns us on tritium safety [1]. Before the DT
phase of ITER will start, reliable estimations ofthem must be completed.
At the moment,
however, no large tokamaks are available to study
realistic fuel cycles in a reactor. In this respect,
deuterium operation in LHD will give niceopportunity to study fuel recycling and isotopic
effect on them.
Here, after summarizing
problems on fuel retention and fuel cycles,
research proposal which can be done in the
deuterium operation in LHD will be made.
2. Fuel retention in a reactor vessel
In vessel-tritium inventory and/or tritium
accountability in a reactor is the most serious safetyconcern. In the present tokamaks, there is always
significant imbalance between input and output offuels (mostly measured by deuterium), i.e. 10-20%of the input fuels is continuously retained andimmobilized [2]. Although such in-vessel retention
is very likely caused by incorporation of fuels in
redeposited materials at plasma shadowed area,
estimation of tritium inventory even in ITER arescattered more than three orders of magnitudedepending on retention models.
Difficulty of quantitative analysis of tritium in
the in-vessel components adds additional problems.
It is ironical that the accuracy in detecting low
levels of tritium (below 109 Bq) which utilizes
β-electron is better than that in the very high levels
which are determined by mass and/or pressure
measurements and calorimetry with the accuracy of
only 10-2 to 10-3. In addition, one cannot measuretritium existing in solid deeper than a few μm. The

easy isotopic exchange reactions of tritium withhydrogen in water and hydrocarbons result in easy
contamination of the surroundings and cansignificantly affect the analysis.
Large isotopic effects between D and T would
significantly influence their retention rates in PFM,
confinement times fuelling efficiencies in plasmaand exhausting ratio. Until now no systematicstudies on intentionally mixed hydrogen (H and D)
plasma have been made.
3. Tritium issues in burning plasma
Burning efficiency in a fusion fuel cycle is,unfortunately, very poor; i.e. only a few % or lessof input tritium burns and the majority must be
recovered to recycle. In addition, large in-vessel
fuel retention rate would result in the huge in-vessel
tritium inventory, which is hard to remove andrecover. Since exhausted fuel from the vessel
includes all hydrogen isotopes (H, D, and T), He
and other impurities like water and hydrocarbons,the hydrogen isotopes must be refined andisotopicaly separated with each other to be recycled
as fuels.
Figure 1 [3, 4] summarizes the fuel flow in areactor. In the figure, the burning efficiency andthe in-vessel retention rate are assumed to be 1%
and 10% respectively. For a total throughput ofD/T, as 50/50, only 1/1 fuel is burned to produce 1
neutron, while 5/5 of D/T is retained in the vacuum
vessel (VV).
Accordingly, D/T of 44/44 isexhausted.
In present plasma devices, lighthydrogen (H) always remains in the deuterium
fuelling operation. The origin is partly residual H2O
in the VV. Since any materials retains some
impurity gases in their bulk, their use in vacuum
systems always causes out-gassing.
Anotherfuelling independently. It is, however,quite
difficulttomeasuretheconcentration of D and T separately.
Plasma opacity could disturb opticalmeasurements like Thomson scattering.
Separate detection of neutrons withenergies of 14 MeV (produced by DTreactions) and of 4.7 MeV (DD reactions)is one of the most promising ways, but
the signal does not necessarily give
spatial distribution of D/T ratio in theplasmas. Different confinement times of
D and T would result in theirinhomogeneous distribution particularlyin their radial distribution.
Furthermore, retention rates of D and T
inthe plasma facing wall is notFig. 1. Fuel balance in a reactor assuming
necessarily the same, thus their releaseburning efficiency = 1% or neutron yield = 1 [4]rates from the wall. This means that if
source is back-flow from vacuum pumps. Whateversomelocalthermal load like an ELM hits the wall,
the sources are, contamination of D and T by H isthermalreleaseof D and T are different in theirunavoidable and additional effort to remove H in anamounts
andthis
disturbs the D/T ratio in theisotope separation process is required.
plasma.
Owingtothehuge wall inventory, a small
For enhancement of radiative cooling of thechangeintheD/T
ratio
of the wall retention could
plasma and also for disruption mitigation, impurity
cause significant change the D/T ratio of theseeding such as Ne and Ar would be employed,burning plasma. Thus we are going to facewhich could significantly dilute the fuels. In
difficulty in controlling the DT ratio in the plasma
addition, various chemical forms of hydrocarbons
to attain and keep efficient burning.
will be formed from carbon plasma facing materials,
if they are used. Thus D and T fueled into the VV
are exhausted as a heavily contaminated gas with H,
hydrocarbons and inert gasses (He, Ne and Ar).
Since only D and T must be recycled, the exhaustgas must be recovered, refined, isotopicaly
separated and refueled.
It should be noted that the behavior of D and T
are significantly different from each other, not onlyin the plasma but also in materials [4], owing totheirlarge mass difference; for instance,
confinement times, fuelling efficiencies, escaping
fluxes from the plasma, retention rates in materials,
and evacuation rates from the VV will be different
for D and T. [3,4] Simple kinetic theory indicatesthat the square root of mass ratio of D/T can becorrelated with those properties, but more or less nodata are available for D and T until now.
In Fig.1, the same fueling rate is assumed for Tand D, but the fueling rate is necessarily the sameand must be optimized to attain the highest burning
efficiency. Therefore, it will not be easy tomaintain the appropriate DT ratio in the plasma forcontinuing the most efficient burning or keeping
DT burning efficiency the highest.
The
concentration of D and T in the burning plasma
must be separately measured to achieve feedback

4. Proposal in relate experiments in LHD
Since in the deuterium operation in LHD, all
exhaust gas will be once stored in surge tanks fortritium safety, quantitative analysis of exhausting H
and D becomes possible first time. This makes
possible to detailed study on above mentioned
problems, such as, fueling efficiency, recycling,fuel retention, and pumping efficiency including
isotope effects, required to establish safe andeconomical DT burning.
References[1] T. Tanabe: Tritium management in a fusion reactor safety, handling and economical issues -, Proc.2nd
ITER International Summer School: Confinement:
Ed. S. -I. Itoh, S. Inagaki, M. Shindo, M.Yagi, (Amer.
Inst. Phys., 2009) pp.112-126H. Ohta and S.
Hamaguchi: J. Vac. Sci. Technol. A 19 (2001) 2373.
[2] J. Roth, E. Tsitrone, T. Loarer, et al.: Plasma Phys.
Control. Fusion, 50 (2008) 103001(20p)[3] T. Tanabe, Tritium issues in plasma wall interactions,Plasma Interactions in Controlled Fusion Devices,
Proc. 3rd ITER International Summer School: Ed. S.
Benkadda (Amer. Inst. Phys., 2010) pp.106-121.
[4] T. Tanabe, J. Nucl. Mater. 417(2011) 545.