Visual Tritium Imaging Of In-Vessel Surfaces

C. A. Gentile, S. J. Zweben, C. H. Skinner, K. M. Young, S. W. Langish,
M. F. Nishi^a^, W. M. Shu^a^, J. Parker, K. Isobe^a^

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

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

**Abstract** **:**

A imaging detector has been developed for the purpose of providing a
non-destructive, real time method of determining tritium concentrations
on the surface of internal TFTR vacuum vessel components. The detector
employs a green phosphor screen (P31, zinc sulfide: copper) with a wave
length peak of 530 nm, a charge-coupled device (CCD) camera linked to a
computer, and a detection chamber for inserting components recovered
from the vacuum vessel. This detector is capable of determining tritium
concentrations on the surfaces. The detector provides a method of
imaging tritium deposition on the surfaces in a fairly rapid fashion.

Keywords: tritium retention, tritium co-deposition, tritium and tritide

1.  **Introduction**

There is a need within the fusion energy community to better understand
the physics associated with tritium deposition on internal vessel
components. Future fusion energy machines will require relatively large
quantities of tritium to operate. Therefore it is in the best interest
of future fusion devices to accurately characterize tritium surface hold
up in the vacuum vessel and, to maintain deposited and co-deposited
tritium to a minimum.

The employment of tritium as a component of TFTR fusion fuel commenced
during 1993 and continued on through 1997. During this time the TFTR
tritium systems processed \~ 1 Mega Curie of tritium, of which \~ 50,
000 Ci of tritium was supplied to the vacuum vessel through Neutral Beam
Injection or by direct vessel gas injection \[1\]. At the completion of
the TFTR D-T operational phase in April of 1997 the vacuum vessel was
subject to a regimen of tritium cleanup operations which included vessel
bake-out, deuterium Glow Discharge Cleaning, Helium / Oxygen Glow
Discharge Cleaning, Pulse Discharge Cleaning, and moist air purges. At
the end of this process it was estimated that \~ 1 gram of tritium still
remained tenaciously bound to the surfaces inside the TFTR vacuum vessel
\[2\].

Commencing in 1998, a collaboration was developed between PPPL and
members of the JAERI Tritium Engineering Staff to investigate the
co-deposition of tritium on the internal surfaces of the TFTR vacuum
vessel \[3\]. In support of the PPPL / JAERI collaboration various
tritium-assaying techniques were deployed to accurately determine the
deposition characteristics associated with the tritium remaining on the
surfaces of vacuum vessel components in the co-deposited layer, both on
carbon and stainless steel surfaces \[4\].

Several methods for determining tritium activity on surfaces have been
developed. These include the deployment of open wall ion chambers,
TLD's, smear surveys, and pin diodes \[5,6 \].

The major advantages of the tritium imaging system described here
compared to non-imaging techniques are that (a) the tritium imaging
system used is non-destructive, as the surface of the item being imaged
is not disturbed, (b) this imagining method provides a real time
assaying technique, and (c) it is fast and relatively inexpensive
\[7,8,9\].

The major components of the detector are a P31 zinc sulfide: copper
(green) phosphor plate, which emits a wavelength of 530 nm \[10\] and a
long-time-integrating CCD camera. For ex-vessel detection the tritiated
surface to be assayed is enclosed in a polycarbonate chamber with the
phosphor plate maintained at a pre-determined distance above (never
touching) the surface to be imaged. The polycarbonate chamber is placed
in a light tight observation enclosure where an internally cooled CCD
camera is positioned to capture photons emitted from the surface of the
P31 phosphor plate, the result of tritium beta particles interacting
with the phosphor coated onto the plate.

![](media/image1.wmf){width="1.7236111111111112in" height="2.1375in"}

Figure 1. Polycarbonate Imaging Chamber

The polycarbonate imaging chamber incorporates an internal adjustable
platform for positioning components of various physical dimensions to a
pre determined distance under the fixed phosphor coated plate. Typically
a distance of 1 mm between the surface to be measured and the phosphor
plate is used. The chamber is capable of being pumped down to
subatmospheric presuure or backfilled with lighter gases.

**2. Configuration**

Two configurations have been developed employing the tritium imaging
system. The first is used for imaging tritiated surface components which
have been removed from the TFTR vacuum vessel. In this ex-vessel
configuration tritiated components are placed in a polycarbonate imaging
chamber. The chamber can be evacuated and backfilled with various gases
(typically helium), or pumped down to sub-atmospheric pressure to
provide an environment where more beta particles from the surface of the
tritiated material interact with the phosphor imaging plate. Tritium
beta particles decay with an average energy of 5.6 KeV and have a range
in air of \~ 4 mm - 6 mm. The range of the tritium beta particles at
subatmospheric pressure or in lighter gases (He) can be increased
causing more beta particles to strike the phosphor plate.

For instances where in-situ imaging is desired the P31 phosphor plate is
fixed to the end of a coherent fiber optic bundle with an appropriate
lens. In this configuration the P31 phosphor plate is positioned 1 mm
above an area (surface) of interest. In this configuration a soft collar
(light seal) is jacketed around the phosphor coated plate and lens. When
the area to be imaged is determined, the foam collar is positioned in a
fashion, which reduces the background from visible light that may be
present in the environment. The coherent fiber optic bundle transmits
photons (generated by the interaction of tritium beta particles on the
phosphor plate) to the CCD camera where the image is recorded.

Employing either of these configurations provides versatility in various
measuring environments. Another advantage of surface tritium detection
employing imaging is that mixed (gamma) radiation environments do not
effect the imaging head (phosphor plate). In addition the system in
(both) the in-vitro configuration and in-situ configuration provides
reliable measurements at sub atmospheric and atmospheric pressures with
a variety of background gases.

![](media/image2.wmf){width="3.341666666666667in" height="2.8875in"}

Figure 2. Ex-Vessel Tritium Imaging System Configuration

In this configuration the surface to be imaged is contained within a
polycarbonate imaging chamber. Internal chamber conditions are varied to
optomize the effect of the tritium beta particles interacting with the
phosphor imaging plate. The phosphor plate is fixed at a pre-determined
position above the surface to be imaged, the same distance used during
calibration, typically 1 mm above the surface of the tritiated material.
The polycarbonate imaging chamber is placed in a light tight observation
enclosure where light generated from the phosphor plate are collected
onto a CCD camera and downloaded to a PC.

![](media/image3.wmf){width="3.1902777777777778in"
height="2.9930555555555554in"}

Figure 3. In-Situ Tritium Imaging System Configuration

In this imagining configuration a coherent fiber optic bundle is
employed for imaging tritiated surfaces in-situ. In this case a surface
(bumper limiter tile in the vacuum vessel) can be scanned By hovering
the head of the detector at a fixed distance above the surface. A light
shield is employed to mitigate the effects of background light.

**3. System Calibration**

The system is calibrated by imaging known tritium standards of various
concentrations and sizes under the same atmospheric pressure conditions
and gas compositions as the tritium samples to be evaluated.
Commercially available electroplated tritium standards are employed
which can be fabricated in a variety of sizes, which can simulate many
of the surface geometries of materials to be imaged. Typically it is
best to use tritium standards that closely approximate the level of
tritium on the surface of the item to be imaged. In the case of the TFTR
D-T tiles tritium imagining calibration standards with a tritium
activity in the hundreds of uCi / cm 2 range were employed to calibrate
the system.

During calibration, the visible light intensity is recorded by a Roper
Scientific Sensys model KAF1400- G2 CCD camera with wavelength
sensitivity of 400 - 1100 nm. Light from the interaction of the phosphor
plate with a typical tritium calibration standard is integrated for a
period of 100 seconds with the camera shutter open. The image of the
phosphor plate intensity is recorded by the CCD camera and stored in the
calibration file of the PC.

**4. Imagining**

The response of a P31 phosphor plate was calibrated using an
electroplated tritium standard in atmospheric pressure at a distance of
1 mm. The same phosphor plate was then fixed in position within the
polycarbonate chamber 1 mm above TFTR D-T tile I-C-13 at atmospheric
pressure. The polycarbonate chamber was placed in a light tight
enclosure where photons generated by the interaction of tritium beta
particles from the surface of the tile interacting with the phosphor
coated onto the plate were collected by a CCD camera and compared to
calibration data stored in the PC. The tile was then measured employing
an open wall ion chamber which was placed on the surface of the tile and
gave measurements within 20 % on tile surface areas employing the
imagining technique.

One major limiting factor affecting surface tritium detectors (open wall
ion chambers, pin diodes, TLD\'s) is that the devices come into physical
contact with the tritiated surfaces to be measured, are susceptible to
cross contamination, and in some instances can change the conditions of
fragile surfaces being measured. Tritium imaging employing the
techniques described in this paper does not physically interact with the
surface being measured and is therefore non-destructive and non invasive
in determining surface tritium activity.

![](media/image4.wmf){width="4.954166666666667in"
height="3.9868055555555557in"}

Figure 4. Surface Tritium Activity for TFTR D-T Tile #I-C-13

In-situ measurements of in-vessel components are planned for the summer
of 2000 during the next planned TFTR vacuum vessel entry. To date
tritium standards have been imaged employing the in-situ configuration
with good success. An engineered off-set is being designed to ensure
that the phosphor imagining plate and lens is maintained at the
(calibrated) fixed distance above the surface to be imaged.

**5. Conclusion**

The use of a tritium imaging system provides a non-destructive
examination (no part of the surface to be measured is physically
impacted by the detector) technique for measuring tritium surface
contamination under a wide range of environmental conditions. The system
is relatively inexpensive, and provides quantitative data fairly quick.
The system can be employed for either ex-vessel or in-situ tritium
imaging measurements. Such a detector provides a tool which can be
employed in vacuum vessels to scan wall surfaces for tritium deposition,
and determine tritium "hide-out" locations where tritium build up due to
co-depositing may be increased due to wall geometry or plasma operating
conditions.

**6. Acknowledgements**

The authors would like to acknowledge the contributions of Lloyd
Ciebiera, William Walker, and Denis Shaltis for their craftsmanship in
the fabrication of the tritium imaging components. Dr. Thomas Venhaus
from Sandia National Laboratory who consulted and participated with the
PPPL Tritium Staff in collecting tritium surface measurements from TFTR
D-T tiles.

Funding for the development of the Visual Tritium Imaging System 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-AC05-960R22464.

**7. References**

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\[3\] C.H. Skinner, C.A.Gentile, K.M. Young, \" Observations of Flaking
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\[4\] C.H. Skinner, C.A. Gentile, et al. These PSI Proceedings.

\[5\] C.A. Gentile, C.H. Skinner, K.M. Young, et al. \"In-situ tritium
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\[6\] W.R. Wampler, B.L. Doyle, \" Low-energy beta spectroscopy Using
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\[7\] I. Youle, A.A. Haasz, \"Profiling with Tritium Imagining\",
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\[8\] C.A. Gentile, S.J. Zweben, C.H. Skinner, et al. "A Visual
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\[9\] S.J. Zweben, C.A. Gentile, D. Mueller, et al, "In-vessel tritium
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\[10\] Phosphor Resource Manual for Industrial and Military Cathode Ray
Tubes, Section 3 Standard Phosphors.