A carbon-carbon composite brazed with OFHC copper divertor structure has been irradiated by neutrons at 673K to 10 dpa. After the irradiation, microstructures and transmutation in the substrate and in the joint interface region were investigated by TEM. Void formation, which results in a degradation of brazed material, was prominent in interfaces (between C/C and a filler metal or between copper phase and silver-copper phase) and in OFHC copper substrate.