Studies of irradiation on elastomeric insulation by gamma and neutron flux and thermal radiation degradation

Polymeric insulating materials such as are used in electric cables when used in a radioactive environment will in normal operation be subjected to a background irradiation throughout its operating life. The irradiation will consist of gamma rays and neutrons. In an accident situation, the insulation would have to withstand high levels of irradiation. In the present work, irradiation was carried out in a reactor on power. The plaques 200 mm square and varying in thickness from 2.2 to 3.3 mm, depending on material, were folded into aluminium cans and fed into a reactor channel, the same channel and position in the channel being used so that samples received the same radiation. Samples were subjected to two levels of irradiation, the higher being 2×10⁷ rads gamma and 5×10¹⁶ neutrons/cm². This was followed by high pressure steam and temperature cycles. Changes in the dielectric properties, capacitance and loss tangent, were measured in the frequency domain using a modern bridge with a frequency range of from 30 Hz to 300 kHz. The results of changes in loss tangent for the ethylene methyl acrylate copolymer are given. Changes in mechanical properties were followed by tensile tests and stress relaxation at a constant value of strain which varied between 60 and 75%, of strain at break, depending on the materials. The materials were examined by both TGA (thermo-gravimetric analysis) and DTA (differential thermal analysis)