> **Flaking of Co-Deposited Hydrogenated Carbon Layers on the TFTR
> Limiter.**
>
> C. H. Skinner, C. A. Gentile,
>
> Princeton Plasma Physics Laboratory;
>
> Princeton University,
>
> Princeton, New Jersey, USA.
>
> M.M. Menon, R.E. Barry
>
> Oak Ridge National Laboratory;
>
> Oak Ridge, Tennessee, USA.
>
> **Abstract:**
>
> Flaking of co-deposited layers on the inner limiter tiles was recently
> observed in the Tokamak Fusion Test Reactor (TFTR). This phenomenon
> was unexpected and has occurred since the termination of plasma
> operations on 4 April 1997. Flaking affects approximately 15 % of the
> observable tiles and appears on isotropic graphite but not on carbon
> fiber composite (CFC) tiles. Photographic images of the flakes and
> precise measurements of the limiter geometry are reported. The
> mobilizability of tritium retained in co-deposited layers is an
> important factor in safety analyses of future DT reactors. A program
> to analyze the flakes and tiles is underway.

**1. Introduction**

Carbon atoms, sputtered from plasma facing surfaces by a hydrogen
plasma, are likely to be co-deposited along with the hydrogen atoms on
the surrounding surfaces. Co-deposited layers tens of microns thick have
accumulated in tokamaks on regions of graphite tiles shadowed from
direct ion flux and on stainless steel vacuum vessel surfaces. This
phenomenon affect the tokamak operations \[[^1],[^2],[^3]\] and
dominates the retention of tritium in a fusion device with carbon plasma
facing components. Carbon materials are favored as plasma facing
materials as they have excellent thermal properties and carbon
impurities in the plasma lead to only small increases in radiated power,
however, codeposited layers can grow and accumulate hydrogen isotopes
continuously. The chemistry of hydrocarbon films was recently
reviewed\[[^4]\]. Analysis of in-vessel components exposed during the
deuterium phase of TFTR showed the main mechanism for retention was
codeposition\[[^5],[^6]\]. During tritium plasma operations on TFTR,
approximately 51% of the tritium supplied to the plasma was retained in
the vessel\[[^7],[^8],[^9]\]. In the DTE1 campaign in JET, the fraction
of tritium retained was unexpectedly high (40%) \[[^10]\] and dust and
flakes accumulated in the divertor region\[[^11]\]. The long term
tritium retention (after dedicated removal campaigns) in both machines
was approximately 16%. The tritium experience in large tokamaks was the
subject of a recent workshop\[[^12]\]. Tritium retention in co-deposited
layers has emerged as a potentially serious constraint in the operation
of the International Thermonuclear Experimental Reactor\[[^13]\]. ITER
is currently considering reduced cost designs with both an upper and
lower divertor. The potential for flakes falling from the upper divertor
and initiating a disruption is uncertain.

In TFTR, the co-deposited layers of amorphous hydrogenated carbon,
a:C-H, form a hard 'crusty' film some tens of microns thick on areas on
the plasma facing surface and the sides of the limiter tiles. Until the
recent observations, the films were tenaciously attached to the tiles.
Dust produced by plasma wall interactions in tokamaks has been collected
during maintenance periods on four tokamaks and analyzed\[[^14]\]. A
technical basis for predicting and diagnosing the amount of flakes and
dust in future DT reactors is needed to address the associated
radiological hazards associated with mobilizable tritium or activation
products. The chemical toxicity and reactivity of flakes and dust are
also concerns. Studies of metal tritide dust \[[^15]\] suggest tritiated
dust may be significantly more hazardous than HTO because of a longer
biological residence time and biological studies of tritiated graphite
tokamak dust are needed to establish appropriate occupational limits.

Flaking has been observed of co-deposited layers in JET\[11\] and has
been interpreted as evidence for impurity drift\[[^16]\]. Prior to the
tritium experiments on JET, co-deposited layers on graphite that were
more than 100 microns thick swelled due to water absorption and became
detached. In 1987, flaking was observed of carbon deposits on a
molybdenum liner in JT-60\[[^17]\]. Thick carbon flakes have recently
been observed in TEXTOR \[[^18]\]. Flaking has also been observed in
film growth in laboratory experiments due to differential thermal
expansion following high flux bombardment. This paper describes the
first observations of flaking of co-deposited layers in TFTR occurring
after termination of plasma operations. Further studies are planned to
characterize the flakes and understand the factors leading to their
detachment.

**2. The TFTR Inner Limiter**

TFTR began operations in 1982\[[^19]\] and a toroidal limiter was
installed in the mid 80\'s on the inner wall of the vacuum vessel to
handle the increasing neutral beam power. The total area of the limiter
is 22 m2 of which approximately 7 m2 contacts the plasma. The limiter
extends over ±60° from the midplane and is divided into 20 sectors
(labeled A-T), each composed of 24 rows of tiles, four tiles wide. The
midplane tiles are 125 mm wide, 81 mm high. Each sector is curved and
the center extends out about 5 mm from a true toroidal surface. The
limiter experienced erosion, codeposition and neutron flux from the DT
plasmas. After exposure to many plasma discharges each limiter sector
developed a poloidally asymmetric 'footprint' of net co-deposition,
surrounded by net erosion areas\[6\]. The pattern is related to the
angle of the incident flux on the individual tiles and the slight
toroidal asymmetry of the limiter\[[^20]\]. The rise in bulk limiter
temperature remained below 50 °C during a discharge\[[^21]\], however
infra-red camera measurements indicated that the surface temperature in
localized hot spots could increase up to 1000 °C or more depending on
the discharge conditions and auxiliary heating power\[[^22]\].

Tile material was initially 100% Union Carbide AXF-5Q isotropic
graphite. In 1993 some damage was noted on the top and bottom rows of
tiles\[[^23]\]. In areas of heavy damage, the tiles were replaced with
Fiber Materials Inc. 4D coarse weave carbon fiber composite (CFC) tiles
and Hercules 3-D fine weave CFC tiles. CFC tiles were installed on the
bottom row of the limiter, the less damaged top row was replaced with
redesigned isotropic graphite tiles and the limiter was realigned to
reduce hot spots. Approximately 45% of tiles are now CFC. A video
inspection of the inside of the vessel on 23 October 1996 showed no
evidence for flaking. Samples of dust removed at that time from the
bottom of vertical diagnostic viewing pipes did contain a small amount
of millimeter-scale flakes\[[^24]\]. An inspection in April 1999 of 12
tiles removed from the vessel prior to DT operations and stored in
plastic bags showed a hard codeposited layer without flaking. However
the limited sample size makes it difficult to be certain that exposure
to DT plasmas and/or the subsequent conditions in the vessel are a
necessary condition for flaking.

A novel feature of the final 3 months of TFTR operations were 4 days
devoted to experiments involving the introduction of 3-6 g of lithium
into the plasma by a new laser assisted delivery device
"DOLLOP"\[[^25],[^26]\]. After the termination of plasma operations, a
sequence of glow discharges, boronization and air ventilations was
undertaken to remove tritium from the vessel. This procedure was similar
to that used after two prior maintenance periods with the exception that
air ventilation was performed at an elevated temperature of 150 C as
well as the room temperature conditions used previously\[[^27]\]. The
co-deposited layers firmly adhered to the substrate up to the end of
plasma operations April 1997 and no flaking was observed on the first
tile removal on 2 October 1997.

**3. Observations of Flaking**

Changes in limiter surface were first observed on 17 August 1998 during
work on tritium technology issues as part of a PPPL/JAERI collaboration.
A large 32 inch by 36 inch port was opened to provide access for in-situ
measurements of the distribution of surface tritium on one sector of the
limiter tiles (Bay L) and to remove tiles for ex-vessel
analysis\[[^28]\]. To date five limiter tiles from the bumper limiter
have been baked in a specialized oven to release and measure the
co-deposited tritium. During the tile removal operation, the technicians
reported that the tiles showed signs of scarring, which were initially
thought to be related to removal of some tiles in the previous year. On
the next opening in 4 November 1998, photographic images were obtained
and then it became clear that the codeposited layers in the observable
bays (I, K, L, M) were beginning to flake off (Figs. 1-3). The
co-deposited region was visible as a broad diagonal band across each
limiter sector. The flaking appeared to be concentrated on the lower
third of the limiter and some unflaked areas showed signs of blistering
and corrugation. The vessel was again opened at Bay K on 26 January 1999
and at that time the flaking was observed also on the upper part of the
limiter. White deposits were also visible on some areas of the surface
and sides of the tiles (Fig. 1) and were also observed on the graphite
poloidal RF limiter (not shown) at the bottom of the torus.

Flake samples were removed from three tiles to assay the tritium
content. On baking 0.24 g of flakes at 500° C for 1 hour in air, 0.72 Ci
of tritium was released. This amount is similar to tile bake out
measurements of tiles with undisturbed co-deposited layers and confirms
that the flakes are detached co-deposited layers that contain most of
the retained tritium. These measurements are consistent with in-vessel
tritium inventory estimates based on the difference between the input
and exhaust of tritium and are an important confirmation of the
difference inventory methodology used for tritium inventory control.

![](media/image1.emf){width="3.1256944444444446in"
height="2.147222222222222in"}
![](media/image2.emf){width="3.045138888888889in"
height="2.227777777777778in"}

  ----------------------------------- -----------------------------------
  FIG. 1 Lower part of the Bay K and  FIG. 2 Close up of tiles on Bay J 6
  Bay J limiter sectors 17 February   & 7th row up, 3 tiles from the
  1999. Flaking can be observed on    right edge of Fig. 1. Note the lack
  the left of the diagnostic          of flakes on the CFC tiles on the
  penetrations on Bay K and to the    right.
  right on Bay J.                     

  ----------------------------------- -----------------------------------

![](media/image3.emf){width="3.109027777777778in"
height="2.196527777777778in"}
![](media/image4.emf){width="3.236111111111111in"
height="2.0416666666666665in"}

  ----------------------------------- -----------------------------------
  FIG. 3. Close up of flakes on upper FIG. 4. Laser range finder
  Bay K tile D16. As in Fig. 2 the    lineouts. The y axis is the range
  flaking tile is Union Carbide       of the tile surface from the laser
  AXF-5Q isotropic graphite and the   head. The curves represent the
  non flaking tile on the left:       topography along two horizontal
  CFC/Fiber Materials Inc. 4D coarse  lines shown in Fig. 3. The vertical
  weave. Horizontal lines refer to    excursions correspond to the gap
  lineouts in Fig.4.                  between the tiles and the screw
                                      holes.

  ----------------------------------- -----------------------------------

**4. Measurements of Tile Alignment**

During the vessel opening on 27 January 1999 the tile geometry in
selected areas was precisely measured by a coherent laser range
finder\[[^29]\]. This system was developed to remotely verify
sub-millimeter alignment of plasma-facing components in next generation
fusion reactors and this was the first use of the system on a tritiated
radiological environment. The range is determined by frequency
modulating the laser and measuring the beat frequency between the laser
and reflected light. The aim of the measurements was to determine if
there was a correlation of the location of the flaking with the degree
of tile alignment. Leading edges might cause heavy erosion leading to a
thicker than average co-deposited layer that might be more likely to
flake off. The range accuracy was approximately 200 microns and limited
by residual vibrations. Fig. 4 shows a comparison of the topography of
the tiles seen in Fig. 3, in flaking region and non-flaking regions. The
tile alignment is seen to be similarly good in both cases and occurrence
of flaking does not appear to be correlated with the degree of tile
alignment in the local area.

**5. Potential Causes of Flaking**

The flakes occur on about 15% of the tiles while approximately half of
the limiter shows co-deposition. One factor correlated to the occurrence
of flaking is the tile material - flaking appears to be limited to
isotropic graphite tiles. Possibly the surface texture of CFC tiles
helps maintain attachment of the co-deposited layers. The timing of the
flaking (occurring a year after the conclusion of plasma operations)
remains unexplained. Several mechanisms have been proposed for the
flaking:

> \(i\) A slow process of water absorption in thick codeposited layers.
>
> \(iii\) Lithium catalyzed water absorption. An unusually large amount
> of lithium was introduced into the vessel during last 3 month of
> operations. Boron is also a potential factor but boronization has been
> used since 1990\[[^30]\] and the amount introduced in the last few
> years was lower than average.
>
> \(iii\) Beta catalyzed water absorption. In this scenario, beta
> particles from the radioactive decay of tritium break chemical bonds
> along their path in the carbon and these sites then take up OH. The
> layers swell and detach. If verified, this mechanism clearly has high
> significance for the next generation of DT machines.

The vessel was held at approximately 725 torr of air with 40% relative
humidity since the end of plasma operations (with the exception of
occasional air recirculation to remove tritium when the returned air was
dried to 10% humidity). It is possible that some factor other than water
absorption or a combination of factors is decisive. Elemental analysis
of the flake material is planned. Controlled experiments to promote
flaking in different environments may be needed to conclusively
demonstrate the critical factors.

**Acknowledgments**

We wish to acknowledge informative discussions with D. Loesser, the
photography of\
E. Starkmann, and the dedicated work of C. Bunting, A. Carpe, L.
Ciebiera, K. Isobe, Y. Iwai, E. Kearns, S. Langish, D. Mueller, D.
Shaltis, W. Shu, T. Yamanishi, Y. Kawamura. The encouragement and
support of J. Hosea, K. Young and S. O'hira is gratefully acknowledged.
Support is provided by the Annex IV to the JAERI/DOE Implementing
Arrangement on Cooperation in Fusion Research and Development, U.S. DOE
Contract Nos. DE-AC02-76CH03073 and DE-AC05-96OR22464 .

**References**

[^1]: \[?\] Wilson, K.L., and Hsu, W.L., J. Nucl. Mater. 145-147 (1987)
    121.

[^2]: \[?\] Winter, J., J. Nucl. Mater. 145-147 (1987) 131.

[^3]: \[?\] Causey, R.A., J. Nucl. Mater. 162-164 (1989) 151.

[^4]: \[?\] Jacob, W., Thin Solid Films 326 (1998) 1-42

[^5]: \[?\] Wampler, W.R. et al., J. Vac. Sci. Technol. A6 (1988) 2111.

[^6]: \[?\] Mills B.E., Buchanauer, D.A., Pontau, A., and Ulrickson, M.,
    J. Nucl. Mater., 162-164 (1989) 343.

[^7]: \[?\] Skinner C.H. et al. J. Vac. Sci. Technol. A14 (1996) 3267.

[^8]: \[?\] Skinner C.H. et al. J. Nucl. Mater. 241-243 (1997) 214.

[^9]: \[?\] Skinner, C.H. et al. "Tritium Experience in the Tokamak
    Fusion Test Reactor" in Proceedings of the 20th Symposium on Fusion
    Technology, Marseille, Sept. 7-11, 1998 v.1, p153-156, Association
    EURATOM-CEA, Cadarache, France, September 1998.

[^10]: \[?\] Andrew P., J. Nucl. Mater. 266-269 (1999) 153.

[^11]: \[?\] Peacock A.T. et al., J. Nucl. Mater. 266-269 (1999) 423.

[^12]: \[?\] Skinner C.H. et al. Nuclear Fusion 39, (1999) 271.

[^13]: \[?\] Federici G., et al. J. Nucl. Mater.266-269 (1999) 14.

[^14]: \[?\] McCarthy K.A. et al., Fusion Technology, 34 (1998) 728.

[^15]: \[?\] Cheng Y. S. et al., Health Physics 76 (1999) 120.

[^16]: \[?\] Coad J.P. et al., \"Evidence for Impurity Drift in the
    Scrape-off Layer of JET\", Proceedings of the 26th EPS Conference on
    Controlled Fusion and Plasma Physics, Maastricht, The Netherlands,
    June 14th-18th, 1999 to be published in Europhysics Conference
    Abstracts (European Physical Society).

[^17]: \[?\] Masaki K. (personal communication)

[^18]: \[?\] J. von Seggern et al., "Properties of thick layers
    deposited on plasma facing wall components of TEXTOR-94" 8th Int.
    Workshop on Carbon materials, Sept. 3-4th (1998) Julich Germany to
    be published in Physica Scripta.

[^19]: \[?\] Hawryluk R.J., et al., Phys. of Plasmas 5(1998) 1557.

[^20]: \[?\] McGrath R.T., and Brooks J.N., J. Nucl. Mater. 162-164
    (1989) 350 .

[^21]: \[?\] Janos A., et al., J. Nucl. Mater. 196-198 (1992) 602-606.

[^22]: \[?\] Ramsey A.T., et al., Nucl. Fusion 31 (1991) 1811 .

[^23]: \[?\] McSmith M. D., Loesser G.D. and Owens D.K., Fusion Tech. 26
    (1994) 498.

[^24]: \[?\] McCarthy K. A. et al., Fus. Tech. 34 (1999) 728.

[^25]: \[?\] Mansfield D.K., et al. Phys. Plasmas 3 (1996) 1892.

[^26]: \[?\] Mansfield D.K. personal communication.

[^27]: \[?\] Nagy A. et al, " Tritium Recovery from the TFTR vessel" in
    Proceedings of the 17th IEEE/NPSS Symposium on Fusion
    Engineering,San Diego, October 6-10, 1997, IEEE, Piscataway, NJ, USA
    (1998) 317.

[^28]: \[?\] Gentile C.A., et al. "In situ Tritium Measurements of The
    Tokamak Fusion Test Reactor Bumper Limiter Tiles Post D-T
    Operations" in Proceedings of the 5th Symposium on Fusion Nuclear
    Technology, ISFNT-5, 19-24 September 1999, Rome, ITALY.

[^29]: \[?\] Spampinato P.T., et al., Fus. Tech. 34 (1998) 1151

[^30]: \[?\] Mueller D. et al., J. Vac. Sci. Technol. A9 (1991) 2713.