Behavio u r of tritium accumulated in the surface layer of beryllium tiles

9th International Workshop on Hydrogen Isotopes in Fusion Reactor materials, Salamanca, Spain, June 2 -3, 2008

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Behaviour of tritium accumulated in the surface layer of beryllium tiles

E. Kolodinska1, G. Ķizāne1, J.P.Coad2, A. Vītiņš1, V. Tīlika1, I. Dušenkova1

1 Laboratory of Solid State Radiation Chemistry, Institute of Chemical Physics University of Latvia, Kronvalda blvd. 4, Latvia, [email protected] 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 kBq·cm-2

Tile B tritium activity 2.4 – 4.8 kBq·cm-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).

•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 )

• Beo + 2H+ Be2+ + 2Ho

• Ho + Ho H2 (g) K~1.1010mol-1 s-1

• Ho + To HT (g) K~1.1010mol-1 s-1

• To + To T2(do not occur, because [To] ,< 10-6 M)

• T2 (s) T2 (g)

• T+ (s) T+ (liq)

• A sum=(AT2 +ATo)g+ AT+liq

• 6Ho + Cr2O7-2 + 4H2SO4 SO4

2- + Cr2 (SO4) 3 + 7H2O K=2.6.1010 mol-1 s-1

• Asum =(AT2 +(1-x) ATo) g + ( AT+ + x ATo)liq


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•Thermoannealing in the Radiation Thermomagnetic Rig under different conditions (tritium release)

•Temperature (773 K, constant rate 5K·min-1 )•Temperature and radiation (accelerated electrons (E=5MeV) radiation of 14MGy·h-1)•Temperature and magnetic field (1.7 – 2.35 T)•Simultaneous action of all three factors: temperature, radiation, magnetic field

Methods

Thermomagnetic rig on the basis of electron accelerator LINAC-4

Tritium monitor behind safety wall


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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 150m with maximum concentration at 10-40 m 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 m (average thickness ~ 20 m )

Cross-section of beryllium tile

Structure of beryllium – deposited layer boundary


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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.

Scheme of distribution of tritium against thickness of 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)

Element,line

Wt%, non- melted

part

Wt%,Melted

part

E(line),keV

Be K 60 – 100 12 - 70 0.109

B K - 11 - 52 0.183

C K 0-15 0 - 37 0.277

O K 0 - 20 8 - 66 0.525

Cr K 0 – 0,05 0 - 1 5.411

Fe K, 0 – 0,05 0 - 26 6.398

Ni K - 0 - 1 7.471

W M 0 - 1 - 1.774

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

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 m, but for exposed – 6.5 m. The increase of grain size of beryllium material has been observed after exposure in plasma chamber by factor ~2

Surface of un-exposed beryllium tile


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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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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.

Behavio u r of tritium accumulated in the surface layer of beryllium tiles

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