**In-Situ Tritium Measurements Of The Tokamak Fusion Test Reactor Bumper
Limiter Tiles Post D-T Operations**

C.A.Gentile, C. H. Skinner, K. M. Young, M. Nishi ^1^, S. Langish, S.
O\'hira ^1^, A. von Halle, D. Mueller, L. Ciebiera, D.Shaltis, T.
Yamanishi ^1^, T. Hayashi ^1^ W. M. Shu ^1^, A. Carpe,\
K. Isobe ^1^, E. Kearns, Y. Iwai ^1^, C. Bunting, Y. Kawamura ^1^

Princeton Plasma Physics Laboratory, Princeton University, Princeton ,
New Jersey, 08543 USA

^1^Japan Atomic Energy Research Institute,\
Tritium Engineering Laboratory, Tokai, Ibaraki 319-1195, Japan

Abstract - The Princeton Plasma Physics Laboratory (PPPL) Engineering
and Research Staff in collaboration with members of the Japan Atomic
Energy Research Institute (JAERI), Tritium Engineering Laboratory have
commenced in-situ tritium measurements of the TFTR bumper limiter. The
Tokamak Fusion Test Reactor (TFTR) operated with tritium from 1993 to
1997. During this time \~ 53,000 Ci of tritium was injected into the
TFTR vacuum vessel. After the cessation of TFTR plasma operations in
April 1997 an aggressive tritium cleanup campaign lasting \~ 3 months
was initiated. The TFTR vacuum vessel was subjected to a regimen of glow
discharge cleaning (GDC) and dry nitrogen and \"moist air\" purges.
Currently \~ 7,500 Ci of tritium remains in the vacuum vessel largely
contained in the limiter tiles. The TFTR limiter is composed of 1,920
carbon tiles with an average weight of \~ 600 grams each. The location
and distribution of tritium on the TFTR carbon tiles are of considerable
interest. Future magnetically confined fusion devices employing carbon
as a limiter material may be considerably constrained due to potentially
large tritium inventories being tenaciously held on the surface of the
tiles.

In-situ tritium measurements were conducted in TFTR bay L during August
and November 1998. During the bay L measurement campaign open wall ion
chambers and ultra thin thermoluminscent dosimeters (TLD) affixed to a
boom and end effector were deployed into the vacuum vessel. The
detectors were designed to make contact with the surface of the bumper
limiter tile and to provide either real time (ion chamber) or passive
(TLD) indication of the surface tritium concentration. The open wall ion
chambers were positioned onto the surface of the tile in a manner which
employed the surface of the tile as one of the walls of the chamber. The
ion chambers, which are (electrically) gamma insensitive, were landed at
four positions per tile. The geometry for landing the TLD's provided
measurement at 24 positions per tile. The instrumentation was positioned
on the tiles (96 tiles in each bay) by technicians who manipulated the
boom and end effector from outside the vacuum vessel port. In addition
to obtaining bay L tile measurements, 3 bumper limiter tiles were
collected from the vacuum vessel for further analysis. During the bay L
measurement campaign it was observed that several of the tiles in the
lower third of the bumper limiter exhibited considerable surface
flaking.

**I. Introduction**

The Tokamak Fusion Test Reactor which is located at The Princeton Plasma
Physics Laboratory in Princeton, New Jersey operated for the time period
of 1983 through 1997. During the final four years of TFTR operations,
1993 to 1997 approximately 53, 000 Ci of tritium was injected into the
TFTR vacuum vessel mainly through neutral beam injection \[1,2\]. When
TFTR finished operation in preparation for final shut down an aggressive
three month campaign commenced for the purpose of recovering tritium
inventory which was deposited on the internal surfaces of the \> 80,000
liter volume vacuum vessel \[3\]. The TFTR vacuum vessel tritium cleanup
campaign included; baking out the vacuum vessel to 150 ºC, glow
discharge cleaning with deuterium as the working gas (and later with a
mixture of 10 % oxygen in helium), purging the vacuum vessel with dry
nitrogen at pressures from 20 to 600 torr, and "moist air" purging with
40 % relative humidity air \[2,4\]. During the three month tritium
cleanup campaign approximately 40 kCi of tritium was recovered from the
vacuum vessel. Following the vacuum vessel tritium cleanup campaign
several vacuum vessel purges and vacuum vessel vents were performed in
support of the in vessel surface tritium measurements. Two years after
the TFTR vacuum vessel cleanup campaign approximately 7,500 Ci of
tritium still remains tenaciously held on the 1,920 carbon tiles and
stainless steel interior of the vacuum vessel. It was known from earlier
measurements of TFTR and other fusion devices during deuterium
operations that carbon based limiter tiles have an affinity for
accumulating hydrogen isotopes \[5,6\]. The accumulation of tritium onto
the carbon limiter is facilitated by the formation of a co-deposited
layer of tritium and carbon which becomes deposited at various depths on
the surface of the tile depending on the location in the vacuum vessel.

Surface measurements of the distribution of tritium on the limiter tiles
were needed to collect empirical data for developing a model for the
accounting of tritium both in TFTR and in future fusion devices for
tritium material control and accounting purposes. Accurate tritium
accountability in tokamaks and other tritium burning fusion devices is
critical in maintaining the in vessel tritium inventory to quantities
which comply with site specific criteria and associated safety analysis.
It was these reasons that motivated the need to perform surface tritium
measurements of the TFTR limiter.

# II. Measurements Techniques

The in-situ surface measurements of the bay L TFTR limiter tiles were
collected to measure the tritium activity on the surface of the bumper
limiter tiles. Technicians worked from outside the TFTR vacuum port with
a remote handling boom and were able to "land" tritium surface detection
equipment on the surface of limiter tiles mounted on the inner wall at
the small major radius. Three measurement techniques were used to
collect the tritium surface measurements. The first was the deployment
of an open wall ion chamber which, when landed on the surface of the
tile, completed the ion chamber geometry as the surface of the tile
became the dynamic wall of the ion chamber. The ion chamber was
effectively placed at four different locations on each tile. The second
surface measurement technique used ultra thin (50 micron)
thermoluminscent dosimeters which, when placed directly on the surface
of the limiter tile, gave a strong (passive) signal, the result of
tritium beta decay emanating from the surface of the tile. In addition
the TLD's were positioned in a matrix holder such that 24 discrete
locations could be measured for each tile in the limiter (Fig. 1).

![](media/image1.emf){width="3.0083333333333333in"
height="1.1388888888888888in"}

> a\) b)

Figure 1.

> a\) Ion chamber tile measurement locations
>
> b\) TLD tile measurement locations

The third surface measurement technique chosen was a pin diode employed
as a solid state charged particle detector. The use of pin diodes to
measure tritium surface contamination had been employed for similar
purposes prior to the TFTR limiter measurements with success \[7\]. In
the case of the TLD and pin diode tritium detection methods the active
component of the detector needed to be physically placed on the surface
of the tile to ensure that the relatively low energy tritium beta
particles (E Beta max 18.6 keV) would be efficiently detected in an air
atmosphere.

For the first set of bay L limiter tile measurements it was found that
the employment of the open wall ion chamber resulted in the best
technique for collecting data employing the remote boom and end effector
(Fig. 2). Upon the opening of the TFTR vacuum vessel it was found that
the interior of the vessel was coated with carbon dust. Radiological
smear surveys of the dust indicated that the material was highly
contaminated with tritium and ranged from 10 million dpm / 100 cm^2^ to
100 million dpm / 100 cm^2^. In addition the tritiated dust exhibited a
static charge. Instruments placed inside the vacuum vessel with the
remote position boom would become plated with dust and required
extremely careful handling to ensure that the spread of tritium
contamination did not occur outside of the TFTR vacuum vessel.
Instrumentation remotely positioned into the vacuum vessel would within
minutes become covered with the highly tritiated dust. The contamination
of the in vessel measuring devices became further amplified when
instrumentation contact was made with the limiter wall. Of the three
techniques, the open wall ion chamber attached to the remote positioning
boom was the least susceptible to the tritium-contaminated dust when it
was placed on the surface of the carbon tiles, and that reproducible
measurements were possible. TLD's placed on the surface of the carbon
tiles became highly tritium contaminated and required many hours to
decontaminate prior to analysis. The length of time required to remove
tritiated dust from the TLD's masked the signal resulting from placing
the TLD on the surface of the carbon tile for 10 minutes. The pin diode
detector experienced similar tritium contamination issues and in
addition is extremely sensitive to light. Positioning the detectors
required a great deal of lighting for technicians as to ensure that the
detector head was landed at the correct tile measurement coordinates.
The open wall ion chamber exhibited only minor increases in background
signals resulting from the plating out of tritiated dust on the face of
the detector (background signals were subtracted during the
measurement). In addition the detector was electrically configured to
mask out the \~ 45 mR / hr internal vacuum vessel gamma field which is
the result of the activation of the steel, on which the tiles are
mounted, during the 1993 to 1997 D-T campaign. As a result of its
reliability in producing good quality data during this first entry into
the vacuum vessel for performing the bay L in situ measurements the
first complete set of bay L tritium surface data were collected with the
open wall ion chamber method.

![](media/image2.emf){width="3.017361111111111in"
height="1.2458333333333333in"}

Figure 2. Remote boom and end-effector with open walled ion chamber

In addition to the tritiated carbon dust it was also observed that many
of the tiles located in the bottom third of the TFTR limiter were
flaking \[8\]. This phenomena had never been previously observed on the
TFTR limiter tiles. The flakes are extremely fragile and have a tendency
to fall off the face of the tiles with any mechanical agitation. The
thin flakes (\~ 10 - 50 microns thick) range in size from several mm to
\~ 1.5 cm in length and are ultra fragile (Fig. 3).

![](media/image3.emf){width="2.7395833333333335in"
height="3.0034722222222223in"}

Figure 3. TFTR carbon limiter tile flaking

**III. Results**

The bay L limiter wall is composed of 96 carbon tiles configured in four
columns with 24 tiles per column. Typical tile dimensions are \~8 cm
wide, \~ 12 cm long, \~ 3.5 cm deep with an average weight of
approximately 600 grams per tile. During these measurements the open
wall ion chamber was positioned at four locations per tile and data was
collected from each of the measured locations. Surface measurements
employing the open wall ion chamber ranged from 25µCi / cm^2^ to 400µCi
/ cm^2^ with the majority of the measurements ranging between 110µCi /
cm^2^ to 220µCi / cm^2^. The bay L limiter is missing 11 tiles in column
C, the result of an intervention during the summer of 1997 into the
vacuum vessel, employing remote tools to collect bay L limiter tiles for
external analysis. A graphical representation of the bay L tritium
surface measurements is shown in figure 4.

![](media/image4.emf){width="4.0in" height="2.9770833333333333in"}

Figure 4. Bay L tritium surface measurements

External vessel tritium measurements for five of the bay L tiles and one
flake were made employing a bake-out oven at temperatures of 350 ºC and
500 ºC . The bake-out of the 5 bay L tiles supported the data collected
during the in-situ bay L surface measurements which is that the
distribution of tritium entrapped on the carbon limiter tiles in bay L
is not homogenously distributed. Tritium recovered from the individual
tiles ranged from 0.065 Ci to 1.590 Ci. (Fig. 5.). The flake which
weighed 0.24 grams released 0.74 Ci of tritium during bake-out. It
appears that a significant portion of the tritium contained in the
carbon tile is entrapped in the surface flakes. Since the first
observation of tile flaking (November 1998) in the bottom third of the
bay L limiter it has since been observed in other bays during later
vacuum vessel openings. Currently the flaking is more pronounced and can
be observed in the middle and top sections of the TFTR limiter tiles.

![](media/image5.emf){width="2.7993055555555557in"
height="5.397916666666666in"}

Figure 5. Curies removed from bay L tiles by bakeout

# IV. Future Activities

Future plans include people entering the TFTR vacuum vessel through the
bay K port, employing positive pressure bubble suits for radiological
protection against the tritiated carbon dust. Bay K is in pristine
condition since there have been no remote tile removal or in situ
measurement activities which might have disturbed the deposition of
tritium on limiter tiles in this region of the TFTR vacuum vessel. The
goal of the bay K entry will be to perform additional in situ tritium
measurements with an enhanced pin diode detector head configuration to
mitigate the effects of light and tritium dust contamination, a new
slightly off set (\< 0.5 mm) TLD matrix holder to reduce the tritated
dust plating to the TLD's, and open wall ion chambers. Measurements will
be made of the limiter tiles and other surfaces (stainless steel) inside
the vacuum vessel. In addition 41 bay K limiter tiles for external
vessel analysis will be collected along with activation coupons which
were fixed in TFTR prior to the use of tritium in the device. An attempt
will be made to collect flakes (intact) and to recover other
unidentified deposits (Fig. 6.) plated out on the lower section of the
bay K limiter for external analysis. It is also planned to perform a
comprehensive photographic survey of the inside of the vacuum vessel
prior to the decommissioning of TFTR.

![](media/image6.emf){width="3.0208333333333335in"
height="2.404166666666667in"}

Figure 6. Unidentified deposits

V. Conclusion

Data obtained by the in-situ bay L tritium surface measurement and
subsequent bakeout of the tiles indicate that the deposition of tritium
in the TFTR vacuum vessel is not homogenously distributed. The formation
of flakes on the surface of the limiter tiles have been analyzed and
contain a relatively high concentration of tritium. Further in-situ and
external vessel measurements and analysis are required to better
understand the dynamics of the distribution of tritium in a magnetically
confined tokamak configuration. Instrumentation employed for in-situ
surface tritium measurements in carbon lined (D-T fusion) vacuum vessels
need to be compatible with the highly tritium contaminated dust
environment.

**VI. Acknowledgments**

The authors would like to acknowledge the efforts of the PPPPL Health
Physics Staff for safely and efficiently controlling radiological
conditions associated with the in situ tritium measurements of the TFTR
bay L limiter tiles. Funding for the in-situ measurements was provided
by JAERI through the Annex IV to the JAERI / DOE Implementing
Arrangement on Cooperation in Fusion Research and Development, and by
USDOE Contract numbers DE-AC02-76CH03073 and DE-AC05-96OR22464.

**VII. References**

\[1\] Gentile, C.A., Kalish, M., et al., "The Operation of The TFTR
Tritium System" 17th IEEE/NPSS Symposium Fusion Engineering. Vol. 1, No.
97CB36131.

\[2\] Mueller, D., Blanchard, W., et al., "Tritium Retention and Removal
on TFTR". 17th IEEE/NPSS Symposium Fusion Engineering. Vol. 1, No.
97CB36131.

\[3\] Nagy, A., Amerescu, E., et al., "Tritium Recovery from the TFTR
Vessel" 17th IEEE/NPSS Symposium Fusion Engineering. Vol 1, No.
97CB36131.

\[4\] Hosea, J., et al "The Tritium Fuel Cycle on TFTR and Related
Issues" Presented at IEA Workshop, Tritium Experience in Large Tokamaks;
Application to ITER. Princeton Plasma Physics Laboratory, Princeton,
N.J. March 16 - 18, 1998 Nuclear Fusion, Vol. 39 page 271 (1999).

\[5\] Wampler, W.R., et al., "Deposition of carbon, deuterium, and
metals on the wall and limiters of the Tokamak Fusion Test Reactor".
Journal Vacuum Science Technology. A6, 2111-2115. (1998).

\[6\] Wampler, W.R., Doyle, B.L., Pontau, A.E., "Hydrogen and deuterium
in graphite limiters from TFTR, PDX and PLT". Journal of Nuclear
Materials. 145 - 147, 353 - 356. (1987).

\[7\] Wampler , W.R., Doyle, B.L., "Low-energy beta spectroscopy using
pin diodes to monitor tritium surface contamination". Nuclear
Instruments and Methods in Physics Research. A 349 473 - 480. (1994).

\[8\] Skinner, C.H., Gentile, C.A., Menon, M.M., Barry, R.E. "Flaking of
co-deposited hydrogenated carbon layers on the TFTR limiter". Nuclear
Fusion, Vol. 39, No. 9. (1999).