Tritium Retention in Graphite and Carbon Composites

Sandia National Laboratories

Rion CauseySandia National Laboratories

Livermore, CA 94550

Carbon

•Graphite occurs naturally.

•Nuclear graphites are made by Acheson process•Crush, mill, size•Add coal tar pitch and anneal at 1200 K•Graphitize at 2900 K to 3300 K•Product has density of 1.8 to 1.9 g/cm3 (very porous)•Grain size is ~ 10 µm•Grain composed of microcrystallites (~5 nm)

•Carbon composites are very similar to graphite•High strength-to-weight ratio•Can be tailored to have directional properties

Carbon

Sandia National Laboratories

Tritium Retention in Carbon

• Saturated layer

• Absorption and diffusion along porosity

• Intergranular diffusion and trapping

• Codeposition

Absorption and Diffusion Along Graphite Porosity

Sandia National Laboratories

Examination of tiles removed from JET and TFTR have shown tritium profiles extending completely through the tiles. It is the diffusion along porosity that produces these profiles. Removal may be difficult by heating alone. Heating may just drive the tritium into the grains where much higher temperatures will be required. Heating to moderate temperatures (300 to 500 C) in the presence of atomic hydrogen or oxygen should effectively remove this tritium. The inventory associated with the process is small.

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573 K773 K

Distance into Sample (µm)

POCO AXF-5Q Graphite100 eV 1.5 Hr. Exposure

Causey et al.

Tritium Profile in POCO Graphite

Codeposition is Expected to be a Major Source of In-Vessel Tritium Inventory

Plasma

Erosion

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T

Graphite or Composite

Redeposition onnear-by surface

•Carbon erosion from high flux areas results in redeposition of carbon along with tritium.

•Tritium concentrations of 0.4 T/C are expected in a typical DT device.(JET had 1.0 (D+T)/C on the louvers)

•The thickness of the codeposited layer increases monotonically with discharge time.

Codeposited film on TFTR bumper limiter (B. Mills, SNL)

Other Useful Codeposition Information

Sandia National Laboratories

•As the thermal decomposition data to the right shows, codeposition can not occur on hot (>1000 K) surfaces.

•Most codeposited layers are really coimplanted layers (energetic hydrogen neutrals strike the redeposited carbon layer).

•The codeposited layer found on the JET louvers had a (D+T)/C ratio of approximately 1.0.

•ITER-FEAT is predicted to have 1 to 2 grams of tritium codeposit with carbon per pulse. (Federici et al. J. Nucl.Mater. 290-293 (2001) 260)

•Glow discharge (and He/O glow discharge) cleaning is too slow to be effective in removing the codeposited layer.

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Winter et al.a-C:H Film

Causey et al.TFTR Codeposited Film

Thermal Stability of the Codeposited Layer

Removal of the Codeposited Layer

Sandia National Laboratories

•One proposed technique for the removal of the codeposited layer is to heat the entire vessel to a temperature of about 550 K in the presence of air (see figure to the right).

•Concerns with the heating in air technique include damage to the vacuum vessel as well as the reconditioning of the vessel after the layer removal.

•Similar techniques include the use of UV and ozone at lower temperatures. The same concerns listed above apply to these techniques.

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Maruyama et al.

Causey et al.

Stability of the Codeposited Layer in Air

Novel laser detritiation technique shows promise

Pyrometer

TFTR tile with codeposit

Scanning Nd laser heats surface to ≈1500 C and thermally desorbs tritium

Up to 87% of tritium has been removed from TFTR and JET carbon tile samples

Advantages for tokamak application: fiber optic coupling to in-vessel scanner fast - potential overnight cleanup in a

next-step machine. no oxygen to decondition PFC’s no HTO to process

Heating by scanning laser mimics heat loads in

slow transient off-normal events in tokamaks.

Opens new technique for studying high heat flux

interactions, and “brittle destruction”.

Preprints available on PPPL website

“http://www.pppl.gov/”

PPPL-3603, PPPL-3630, PPPL-3604, PPPL-3662

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KC15-21C

Tritium release

TileSampleScanning

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Nd filter

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Charles Skinner, PPPL.

Intergranular Diffusion with Trapping in Graphite

Sandia National Laboratories

•At temperatures of 1000 K and above, tritium begins to diffuse into the graphite grains where it it trapped at high energy trap sites (4.3 eV).

•Each of the ~10 µm grains to the right-> are composed on smaller microcrystallites (5 nm). We think that the hydrogen diffusion occurs along the edges of the smaller crystallites. We also think that the high energy traps only occur along the prism plane (not on the chemically inert basal plane). Graphites and composites with large crystals (low surface to volume ratio) or large basal plane to prism plane ratios exhibit lower trapping densities.

Intergranular Diffusion with Trapping

Sandia National Laboratories

•Unirradiated graphites typically have a tritium trap density of 10 to 20 appm. Neutron irradiation can increase this trap density to values above 1000 appm.

•It is almost certain that irradiation at higher temperatures will limit the production rate of these traps. The higher temperatures simply allow some recovery of the radiation damage.

•Experimental determination of the tritium trap production as a function of temperature is needed.

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Tritium Trapping in N3M Graphite

Causey et al., Fusion Technol. 19 (1991) 1585

Radiation Induced Trapping in Graphite

Sandia National Laboratories

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N3M Ion SimulationH-451 Ion SimulationN3M Neutron IrradiatedH-451 Neutron Irradiated

Damage Fluence (dpa)

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Causey et al., Fus. Technol. 19 (1991) 1587

•The trap density appears to saturate at rather low dose levels

•The saturation damage fluence of 0.1 dpa is equivalent to that expected for the old ITER Physics Phase.

Intergranular Diffusion with Trapping in Graphite

Sandia National Laboratories

•Experiments on two pitch based fiber composites demonstrated resistance to tritium trap generation.

•These composites were known to have very limited fractions of prism planes.

•Unfortunately, these fiber composites are extremely expensive.

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N3M GraphiteFMI 222MKC-1PH

Radiation Damage (dpa)

- Causey et al. (1991)

Tritium Trapping Densities in N3M Graphite and Two Pitch Based Carbon Composites

Irradiated at Room Temperature

Inventory Predicted for Neutron Irradiated Graphites or Composites (DIFFUSE Code)

Sandia National Laboratories

Temperature (K) Graphite with Graphite with100 appm traps 1000 appm traps*

Inventory After 3 Years of Continuous Exposure to Tritium Gas (10 µm grain size)

1000110012001300140015001600

4 gm/m3

17 gm/m3

23 gm/m3

22 gm/m3

21 gm/m3

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19 gm/m3

14 gm/m3

56 gm/m3

164 gm/m3

225 gm/m3

220 gm/m3

209 gm/m3

191 gm/m3

* It may not be possible to have 1000 appm trapping at these elevated temperatures

Conclusion

Sandia National Laboratories

Carbon used in fusion reactors can retain large quantities of tritium through either codeposition or trapping at 4.3 eV traps

Codeposition is a low temperature process

Trapping at 4.3 eV traps is a high temperature process

Experiments are needed on tritium trapping in graphites and composites irradiated at higher temperatures

Limited experiments on silicon carbide suggest this material to present less trapping at elevated temperatures (appears to be very radiation resistant)