Plastics, Elastomers, and Oils

Properties of Metals DOE-HDBK-1017/1-93 APPENDIX A 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. Rev. 0-A Page A-7 MS-02