Plastics, Elastomers, and Oils

APPENDIX A DOE-HDBK-1017/1-93 Properties of Metals 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-1 illustrates several polymer Figure A-1 Modifications to Polymer Chains Due to Irradiation 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. Cross- linking 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. MS-02 Page A-6 Rev. 0-A