Behaviour of tritium accumulated in the surface layer of beryllium tiles

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Behaviour of tritium accumulated in the surface
layer of beryllium tiles
E. Kolodinska1, G. Kizane1, J.P.Coad2, A.
Vitin1, V. Tilika1, I. Duenkova1 1
Laboratory of Solid State Radiation Chemistry,
Institute of Chemical Physics University of
Latvia, Kronvalda blvd. 4, Latvia,
elina.kolodinska_at_lu.lv 2 Culham Science Centre,
EURATOM UKAEA Fusion Association, Abingdon, UK
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Outline

• Samples
• Methods
• Tritium distribution in depth of beryllium
  surface and deposition layer
• Chemical forms of tritium and chemical
  composition of deposition layer
• Changes of beryllium structure after exposure in
  plasma chamber
• Tritium release under different conditions
• Summary

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Samples

• 2 Upper belt limiter beryllium tiles (A and B)
  exposed in the Joint European Torus (JET) during
  D D and D T experiments in 1989 1994
• Tile A tritium activity 10 60 kBqcm-2
• Tile B tritium activity 2.4 4.8 kBqcm-2
• Toroidal limiter beryllium tile un-exposed

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Methods

• Lyo-method (tritium chemical forms and
  distribution)
• The method is based on beryllium dissolution
  process and tritium reactions with chemical
  scavengers (tritium detection gas flow
  detector, liquid scintilation detector).

• Beo 2H Be2 2Ho
• Ho Ho H2 (g) K1.1010mol-1 s-1
• Ho To HT (g) K1.1010mol-1 s-1
• To To T2(do not occur, because To ,lt
  10-6 M)
• T2 (s) T2 (g)
• T (s) T (liq)
• A sum(AT2 ATo)g ATliq
• 6Ho Cr2O7-2 4H2SO4 SO42- Cr2 (SO4) 3
  7H2O K2.6.1010 mol-1 s-1
• Asum (AT2 (1-x) ATo) g ( AT x ATo)liq

• Scanning Electron Microscopy SEM (surface
  structure, grain sizes)
• (Hitachi S-4800 and JSM 6490 )
• in addition of
• Energy Dispersive X-ray detection EDX (chemical
  impurities) (EDAX Sapphire Si(Li) Detecting Unit
  with Ultra Thin window technology, for superior
  light element analysis down to Beryllium )

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Tritium monitor behind safety wall
Methods

• Thermoannealing in the Radiation Thermomagnetic
  Rig under different conditions (tritium release)
• Temperature (773 K, constant rate 5Kmin-1 )
• Temperature and radiation (accelerated electrons
  (E5MeV) radiation of 14MGyh-1)
• Temperature and magnetic field (1.7 2.35 T)
• Simultaneous action of all three factors
  temperature, radiation, magnetic field

Thermomagnetic rig on the basis of electron
accelerator LINAC-4
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Distribution of tritiumin the surface of
beryllium tiles
Tritium in upper belt limiter Be tile was found
in the surface layer up to 150mm with maximum
concentration at 10-40 mm from the surface
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Thickness of deposited layer on the surface of
upper limiter beryllium tiles
The thickness of deposited layer on upper belt
limiter beryllium tile has been found to be in
range from 10 35 mm (average thickness
20 mm )
Cross-section of beryllium tile Structure of
beryllium deposited layer boundary
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Scheme of distribution of tritium against
thickness of deposited layer

• Intensity of red colour corresponds to
  the amount of tritium (darker regions correspond
  to higher tritium concentration)

Highest concentration of accumulated tritium was
found at the boundary between beryllium and
deposited layer.
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Chemical forms of tritium in different parts of
the surface of beryllium tiles (B tile)
Operating surface Lateral surface Lateral surface
between teeth (castellation)
Melted surface
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Chemical composition of surface of beryllium
tile (non-melted and melted parts)
Non-melted
Melted
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Chemical composition of surface and tritium
chemical forms
In the melted parts of beryllium surface tritium
was found mostly as T. In non-melted areas
there were found all three chemical forms of
tritium T2 (47-73), To (16-25), T
(11-33). In the melted parts up to 66 wt
oxygen was found, that could form a chemical bond
with tritium OT-. Non-melted parts mostly
contains beryllium, but beryllium tritide is not
stable at the temperatures of plasma chamber wall
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Changes of beryllium structure
Surface of un-exposed beryllium tile
Distribution of grain sizes has been estimated
both on exposed and un-exposed beryllium
tiles. Average grain size for un-exposed
beryllium is 3.5 mm, but for exposed 6.5 mm.
The increase of grain size of beryllium material
has been observed after exposure in plasma
chamber by factor 2
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Tritium release

• Tritium diffusion (release) efficiency
  depends on grain size, impurities and structure
  defects
• During the exposure in plasma chamber the
  structure of beryllium changes, growth of grains
  occurs.

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Tritium release by thermoannealing
The effect of different factors on tritium
release depends on properties of sample. Tiles A
and B have different initial distribution of
chemical forms of tritium ( in the B tile there
is more T form than in the A tile) and structure
(grain size, impurities, dislocations, etc.).
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• The main diffusing particle of tritium in Be is
  To.
• (25 )T ..T (singlet) T2 (recomb.)
• Diff. To To To ....To
• Uncorrelatated (75 )T ..T
  (triplet) To To (further diff.)
• The MF can affect singlet-triplet transformation
  and also change the ratio of To / T2
• The dissociation of the T2 is going under
  irradiation
• T2 (T2) To To (E dissoc.
  4.6 eV, G (To) 17.6 at./100 eV)
• Correlated pair
• A pair To To is created in singlet state with
  life time t gt10-9 s always, MF transforms it
  into triplet state and increases To amount.

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Summary

• Most tritium accumulated in the beryllium tiles
  was found on the boundary of deposited layer and
  beryllium
• Distribution of tritium chemical forms depends on
  chemical composition of surface layer, and it is
  different in areas that have been melted by the
  plasma
• Changes of beryllium structure after exposure in
  plasma chamber have been observed, grain size has
  grown by factor 2.
• The facilitating effect of tritium release under
  simultaneous action of temperature, radiation and
  magnetic field observed previously depends on the
  properties of particular sample.