Properties of Metals
The surface of the graphite will be contaminated with chemically-bound tritium, and
decontamination may be possible by baking the graphite at 500C in the presence of a hydrogen
exchange medium, such as H2, H2O, or NH3. Except for possible surface erosion, graphite will
probably not be degraded mechanically even over a period of several years, as bulk diffusion
and solubility are extremely low.
Various data suggest that tritium gas in the presence of its chemically-activating beta irradiation
energy could reduce silica bonding to -Si-T and -Si-OT species. At temperatures above 300C,
deuterium appears to reduce silica network, and dissolved deuterium in a gamma irradiation field
has the same effect. The migration of tritium into glass structures could, therefore, cause
embrittlement and possibly fracture under stress over several months or years. Evidence also
suggests that activated hydration of glassy silica structures under T2O exposure is possible.
Embrittlement (unexpected fracture) of a Pyrex syringe stored for two to three years after being
used to transfer T2O was experienced at one DOE nuclear facility.
Permeability of silica glasses is one to two orders of magnitude greater than that for stainless
steel over the temperature range 0 to 200C. Tritium-handling systems constructed largely of
glass have nevertheless been widely used, although this material is not in favor today except for
tritium lamp containment. The exchange of tritium with naturally occurring hydroxyl groups in
various glasses and on their surfaces is a source of protium contamination to tritium, perhaps
1% HT into 1 atm tritium within a 1-L glass container after 1 year. Decontaminating a
highly-exposed glass of its bound tritium would require a significant number of water washes
of 300C hydrogen permeation flushes. This effort is likely to be costly and is often not
warranted by the value of the part undergoing decontamination.
Because tritium's solubility, ability to diffuse, and permeability are so much lower for ceramics
than for glasses, ceramics undergo little or no bulk disruption from tritium. However, some
mechanical degradation of regions near the surface is possible. The depth of the area affected
is a function of ability to diffuse and time. Oxygen release from Al2O3 (sapphire) windows in
the presence of liquid T2O has recently been noted, although compatibility with tritium gas has
been described as excellent. The exchange of surface and near-surface protium is likely,
although mutual contamination of tritium and the ceramic should be less than that for glasses.
Tritium-contaminated ceramics can probably be decontaminated by warm water or steam flushes
or by etching in an acidic solution.