Tritium Retention in Graphite and Carbon Composites

1
Tritium Retention in Graphite and Carbon
Composites
Rion Causey Sandia National
Laboratories Livermore, CA 94550

Sandia National Laboratories
2
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

3
Carbon

• Tritium Retention in Carbon
• Saturated layer
• Absorption and diffusion along porosity
• Intergranular diffusion and trapping
• Codeposition

Sandia National Laboratories
4
Absorption and Diffusion Along Graphite Porosity

Tritium Profile in POCO Graphite

• 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.

Sandia National Laboratories
5
Codeposition is Expected to be a Major Source of
In-Vessel Tritium Inventory

• 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 (DT)/C on the louvers)
• The thickness of the codeposited layer increases
  monotonically with discharge time.

Codeposited film on TFTR bumper limiter (B.
Mills, SNL)
6
Other Useful Codeposition Information

• As the thermal decomposition data to the right
  shows, codeposition can not occur on hot (gt1000
  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 (DT)/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.

Thermal Stability of the Codeposited Layer
Sandia National Laboratories
7
Removal of the Codeposited Layer

• 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.

Stability of the Codeposited Layer in Air
Sandia National Laboratories
8
Novel laser detritiation technique shows promise

• 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 PFCs
• no HTO to process

Laser spot
TFTR tile with codeposit
Pyrometer

• 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 websitehttp//www.p
  ppl.gov/ PPPL-3603, PPPL-3630, PPPL-3604,
  PPPL-3662

Tritium release
Charles Skinner, PPPL.
9
Intergranular Diffusion with Trapping in Graphite

• 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-gt 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.

Sandia National Laboratories
10
Intergranular Diffusion with Trapping

• 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.

Tritium Trapping in N3M Graphite
Causey et al., Fusion Technol. 19 (1991) 1585
Sandia National Laboratories
11
Radiation Induced Trapping in Graphite

• 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.

Causey et al., Fus. Technol. 19 (1991) 1587
Sandia National Laboratories
12
Intergranular Diffusion with Trapping in Graphite

• 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.

Irradiated at Room Temperature
Sandia National Laboratories
13
Inventory Predicted for Neutron Irradiated
Graphites or Composites (DIFFUSE Code)
Inventory After 3 Years of Continuous Exposure to
Tritium Gas (10 µm grain size)
Temperature (K) Graphite with Graphite
with 100 appm traps 1000 appm traps
4 gm/m3 17 gm/m3 23 gm/m3 22 gm/m3 21 gm/m3 20
gm/m3 19 gm/m3
14 gm/m3 56 gm/m3 164 gm/m3 225 gm/m3 220
gm/m3 209 gm/m3 191 gm/m3
1000 1100 1200 1300 1400 1500 1600
It may not be possible to have 1000 appm
trapping at these elevated temperatures
Sandia National Laboratories
14
Conclusion
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)
Sandia National Laboratories