Combined Environments versus Consecutive Exposures for Insulation Life Studies

The service life of an insulating material which will be used in a nuclear radiation environment cannot be predicted by the usual thermal-aging methods; neither can it be predicted from experiments in which thermal aging follows a pre-exposure to radiation at room temperature. To have any reliable significance, the experiment must be conducted in a combined environment of both thermal and nuclear radiation. At the Naval Research Laboratory an apparatus has been designed and used to achieve this exposure condition. Results show that by combining radiation with heat the normal thermal life of several materials is increased by as much as 800 percent, and for one of these materials the increase was over 3500 percent in one combination of radiation and heat. This is probably due to a balancing of the chain-scission and cross-linking mechanisms which occur in polymer reactions. Increased life is not universal for all materials, for in some of these experiments less than normal thermal life was observed.