Properties of MetalsDOE-HDBK-1017/1-93APPENDIX ASubstituents on aromatic groups that extend the delocalized bonding network are furtherstabilizers. Finally, saturated aliphatics are more radiation resistant than those that areunsaturated; 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 comparedto bulk can occur. This results from the greater density of tritium (and the much greater rangeof 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-Fexposure in tritium produces the acid TF, noted as SiF4, gas in a glass system. Because of thisacid production, tritium + moisture + Teflon in a stainless steel system at pressures ofapproximately 1300 atm caused catastrophic stress corrosion cracking of 0.76-mm thick stainlesssteel tube walls in 16 hours. Substituting deuterium for tritium or removing Teflon or moisturecaused 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 primarilyoccurred at the surface (crack propagation). Total incorporation of tritium into a polyethylenepowder was found not to be a function of the amount of powder, but of the exposed surfacearea. Radiation-induced fluorescence from the surface of high-density polyethylene exposed totritium 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 arerecommended. Vespel stem tips for valves, when used with sufficient sealing force, continueto seal for several years in tritium (STP). When used with less sealing force, however, leakshave been noted across valve tips, possibly because of surface hardening. Polyimide gasketsunder constant sealing load are probably adequate for years.Saturated hydrocarbon mineral oils (for example, Duo-Seal) require frequent changes in tritiumservice because of vapor pressure increases (offgassing) and liquid viscosity increases. Siliconeoils 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 tritiatedwater vapor service. Tritium fluoride evolution and corrosion may result.Rev. 0-A Page A-7MS-02
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