Glasses

Properties of Metals DOE-HDBK-1017/1-93 APPENDIX A 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. Rev. 0-A Page A-5 MS-02