Because they generally have high surface areas, graphite samples adsorb large amounts of hydrogen gas (4 x 10¹⁸ molecules/g for a graphite pellet used in gas-cooled reactors). Methane, protium, and (possibly) water are generated from beta irradiation of the graphite surface.

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, H₂0, 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.

Glasses

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 T₂0 exposure is possible. Embrittlement (unexpected fracture) of a Pyrex syringe stored for two to three years after being used to transfer T₂0 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.

Ceramics

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 A1₂0₃ (sapphire) windows in the presence of liquid T₂0 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.

Organics are easily permeated by tritium (gas or water) and are therefore subject to disruption of their bulk chemistries. There are few or no mechanisms for rapidly delocalizing beta energy, and substantial mobility of organic chains occur within polymer structures (particularly amorphous regions). Once formed, reactive organic intermediates can thus react with each other.

These effects are important when considering the design of tritium systems. Damage to components, such as gaskets, valve tips, and O-rings, must be carefully considered. Component failure during service can cause a major release of tritium. Because elastomer seals often become embrittled, maintenance on nearby sections of piping may cause seals to develop leaks as the result of mechanical movement in the seal area.

Figure A-I illustrates several polymer chain modifications that take place following activation by beta radiation to ionic or excited species. Cross-linking and degradation are the most important processes to the mechanical properties of the polymer. These both compete in a material, but those polymers that are most sterically hindered appear to preferentially degrade. Steric hindrance prevents neighboring chains from linking and also imparts structural strains that are relieved upon chain scissioning. Crosslinking is noted mechanically by an increase in tensile strength and a decrease in elongation, whereas degradation is evidenced by a decrease in tensile strength, an increase in elongation, and softening of the polymer to a gummy consistency.

Several factors effect polymer stability. First, energy- delocalizing aromatic structural groups increase polymer stability by distributing energies of excited states. In addition, halogen atoms within polymers generate free radicals and thus promote radiation damage.

Figure A-1 Modifications to Polymer Chains

Due to Irradiation

Substituents on aromatic groups that extend the delocalized bonding network are further stabilizers. Finally, saturated aliphatics are more radiation resistant than those that are unsaturated; isolated double bonds are readily excited to ions or radicals.

Organic compounds, in order of decreasing radiation resistance, are aromatics, aliphatics, alcohols, amines, esters, ketones, and acids. Extension to beta radiation is probably reasonable. In tritium gas, however, substantial differences in irradiation or polymer surface as compared to bulk can occur. This results from the greater density of tritium (and the much greater range of the beta in the tritium gas) outside the polymer compared to inside the polymer bulk.

Some direct experience of polymers with tritium has been obtained. Teflon, Viton, or Kel-F exposure in tritium produces the acid TF, noted as SiF₄, gas in a glass system. Because of this acid production, tritium + moisture + Teflon in a stainless steel system at pressures of approximately 1300 atm caused catastrophic stress corrosion cracking of 0.76-mm thick stainless steel tube walls in 16 hours. Substituting deuterium for tritium or removing Teflon or moisture caused no failure. Radiation damage to Teflon is more severe than to all other thermoplastics. Teflon is therefore not recommended in the presence of concentrated tritium streams.

Surface and bulk effects have been noted in numerous polymer/tritium studies. In one study, hardening of neoprene occurred throughout the bulk, while hardening of natural rubber primarily occurred at the surface (crack propagation). Total incorporation of tritium into a polyethylene powder was found not to be a function of the amount of powder, but of the exposed surface area. Radiation-induced fluorescence from the surface of high-density polyethylene exposed to tritium was shown to be orders of magnitude greater than that from the bulk.

Polyimides (good in the presence of gamma radiation) appear good in tritium handling and are recommended. Vespel stem tips for valves, when used with sufficient sealing force, continue to seal for several years in tritium (STP). When used with less sealing force, however, leaks have been noted across valve tips, possibly because of surface hardening. Polyimide gaskets under constant sealing load are probably adequate for years.

Saturated hydrocarbon mineral oils (for example, Duo-Seal) require frequent changes in tritium service because of vapor pressure increases (offgassing) and liquid viscosity increases. Silicone oils are rapidly polymerized or solidified. Polyphenyl ether oils last for years in similar service, but are expensive and may absorb significant amounts of tritium.

Fluorinated pump oils are not recommended for tritium service and certainly not for tritiated water vapor service. Tritium fluoride evolution and corrosion may result.