22D10
Modeling of fuel retention in JT-60U
JT-60U炉内水素蓄積モデルの構築
Masafumi Yoshida1, Tetsuo Tanabe1, Takao Hayashi2, Tomohide Nakano2,Masakatsu Fukumoto2, Junnichi Yagyu2, Yasuhiko Miyo2, Kei Masaki2 and Kiyoshi Itami2
吉田雅史1, 田辺哲朗1，林 孝夫2，仲野友英2,福本正勝2, 柳生純一2, 三代康彦2, 正木 圭2, 伊丹 潔2
1

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
九州大学大学院 総合理工学府 〒812-8581 福岡県福岡市東区箱崎6-10-1
2

Japan Atomic Energy Agency
801-1 Mukoyama, Naka-shi, Ibaraki-ken 311-0193, Japan
日本原子力研究開発機構 那珂核融合研究所 〒311-0193 茨城県那珂市向山801-1
A model for fuel retention build-up in vacuum vessel of JT-60U was derived by using accumulated data for
analyzes of erosion/re-deposition and fuel retention in carbon tiles used for the JT-60U. In the model, four
different hydrogen retention mechanisms for re-deposited layers, highly and slightly eroded surfaces andbulk were applied depending on the location of the plasma facing tiles.

1. IntroductionMore accurate fuel retention model is urgently
required for ITER and next step fusion devices
from viewpoints of tritium safety and economy[1]. After extensive studies on time dependenceof erosion/re-deposition and fuel retention incarbon tiles used for JT-60U [2], we havesummarized fuel retention mechanisms in thecarbon tiles and derived a model for fuel retention
build-up in JT-60U.
2. Analysis
Analyzed carbon tiles were retrieved fromdivertor region, baffle plate and first wall in JT-60U.
The carbon tiles were exposed to DD plasma heatedby D-NBI with different experimental campaign of1992 to 2004. At end of each experimental
campaign, HH discharge heated by H-NBI wasperformed to remove tritium (T) produced by DDreactions. Tile temperature changed from baking
temperature of 573 K to 700 K - 1400 K during
discharges. Erosion/re-deposition of the plasma
facing surface of the tiles was observed by SEM,and H+D retentions (fuel retention) in not only
plasma facing surface but also bulk of the tiles weremeasured by TDS.
Retention mechanisms were divided into fourdifferent regions (i.e., re-deposited layers, highlyand slightly eroded surfaces and bulk).
3. Retention mechanisms in JT-60U
3.1 Retentions in re-deposited layers
In the re-deposited layers, fuel retention is causedby re-deposition on the tiles and increases linearlywith increasing thickness of the layers. Growth rateof the layers depends on location of the tiles, and itchanges from 1 to 4 × 1020 C·s-1. One can notesthat hydrogen concentration (= (H+D)/C) in thelayers is saturated. (H+D)/C in the re-depositedlayer varies from 0.13 to 0.08 depending on tile
temperature.
3.2 Retentions in eroded surfaces
The fuel retention in the eroded regions rapidly
increased to ~2 × 1022 H+D atoms·m-2 in shortdischarge time. Afterwards, it increased very slowly
to 3~4 × 1022 H+D atoms·m-2 different from that for
the re-deposited layers. Since the plasma facing
surface of the eroded surfaces was exposed to very
high particle flux, hydrogen concentration of theirsurface must be immediately saturated to producehydrogen saturated layers in earlier discharge time.
At the same time, the hydrogen saturated layers
were eroded by subsequently impinging particles.
Accordingly, the fuel retention rate for the eroded
tile was determined by a balance between growth
and recession of the thickness of the saturatedlayers. Since the retention rates of the outer
divertor and the first wall, which are eroded, bothare quite different due to the different heat load ortemperature rise. Therefore highly eroded surface(outer divertor) and slightly eroded surface (firstwall) should be separately taken into accounts.
3.3 Retention in bulk
Retention in bulk is caused by penetration of4. Model for fuel retention build-up in JT-60U
Fuel retention rates for four different regions aresummarized in table I. Using these results, fuelretention build-up in the vacuum vessel of JT-60U
is derived.
Figure 1 shows the total fuel retention build-up inJT-60U with separation of four different regimes.
For a earlier discharge time, retention in the bulk
caused by penetration of gases through open poresof carbon tiles could occupy a significant part of thetotal fuel retention. But it would saturate soon andhardly increase to become minor. With increasing
the discharge time, retentions in the slightly eroded
surface exceeds bulk retention but they also saturate
a little later. Since the retention rates in the highly
eroded surface are very small, the contribution stays
very small, even though it continues to grow.
For prolonged operation, retention in there-deposited layers becomes dominant in the total
fuel retention.
General tendency of increase of the total fuelretention in JT-60U is very similar to that for
ITER’s estimation by Roth et al. [1]. Highertemperature operation of JT-60U results inappreciably smaller retention compared with theRoth’s estimation for ITER. This is a clear
indication that higher tile temperature operation
would significantly reduce the T retention inmachines using carbon as PFM.
Table I. Retention rates for plasma facing surfaces
Retention rate
152710Total retention
Fuel retention [atoms]

neutral particles and/or gases through open pores ofcarbon tiles. The penetrating particles are retained
only at the surfaces of graphite grains facing to theopen pores and the retention rapidly saturates
because of extremely small diffusion coefficient ofhydrogen into graphite grains. Therefore, thehydrogen isotopes (H,D) were uniformly distributed
in mm scale in the bulk of the tiles and the retention
in the bulk saturates with early discharge timewith a saturation concentration of 2~10 × 10-4.

2510Bulk

231021

10-110110310510710

Time [sec]Fig.1. Fuel retention build-up in JT-60U according to thepresent modeling with four different mechanisms
for retentions in re-deposited layers, two different
eroded surfaces and bulk. Note that T inventory
limit of ITER (700 g T) is 1.4 × 1026 atoms.

5. Summary and conclusion
A model for fuel retention build-up in JT-60U
was derived in which four different fuel retention
mechanisms for re-deposited layers, highly andslightly eroded surfaces and bulk were applied
depending on the location of the plasma facing tiles.
According to the model, although the tendency of
increase of fuel retention is similar to the previous
estimation in ITER, the fuel retention rates wereappreciably smaller because of higher temperature
operation in JT-60U. This suggests that the higher
temperature operation would significantly reduce
the T retention in machines using carbon as PFM.
Acknowledgments
This work was supported by Grand-in-Aid for
Scientific Research, Priority area 467, ‘‘Tritium
Science and Technology for Fusion’’ (No.
19055008) and JSPS Research Fellow for Young
Scientists (201000005).

Temp. (K)

Re-deposited layer
1.3× 10 [/s]800-10000.13
Highly eroded(1022×time) (9 m2) [/m2/s]
1400
0.16
Slightly eroded (1018×time)(200 m2) [/m2/s]
700-4
Bulk
2~10 × 10 [/s]700-1400

References[1] J. Roth, et al: J. Nucl. Mater. 390-391 (2009) 1.
[2] T. Tanabe: Fus. Eng. Des. 81 (2006) 139.
[3] T. Tanabe, et al.: Phys. Scr. T138 (2009) 014006.
[4] M. Yoshida, et al.: J.Nucl.Mater. 390-391 (2009) 635.
[5] M. Yoshida, et al.: Proc. 13th Int. Workshop on
PFMC, Rosenheim, Germany, (2011).