In Canada, the spent nuclear fuel disposal method proposed is to permanently isolate the spent fuel in deep underground vaults carved in stable granite rock formations within the Canadian Shield, with the integrity of the isolation to be assured for a minimum period of 500 yr. The present work aims at determining the feasibility of using a consolidated composite material made of an advanced polymer called PEEK (Poly Ether Ether Ketone) and continuous graphite fiber to fabricate a container designed to isolate the spent nuclear fuel from the biosphere for such very long time periods. The research focused on submitting the PEEK-based composite material to a thermal and radioactive environment comparable to, and, in some aspects, more aggressive than, the conditions of exposure in the disposal vault. The changes to the physical, mechanical, and chemical properties of the material following prolonged exposure were then determined. The simulation of the environment was achieved by irradiating numerous test specimens in a mixed radiation field produced by a SLOWPOKE-2 nuclear research reactor at controlled ambient temperatures ranging from ~20 to 75°C. The specimens were characterized via several methods: tensile and flexural testing, differential scanning calorimetry, scanning electron microscopy, and wide-angle X-ray scattering. The results confirmed that the PEEK-based composite material was resistant to exposure to high radiation doses (1 MGy), at temperatures between ~20 and 75°C. The mechanical and other properties were barely affected, with values rarely exceeding 1 of the properties of nonirradiated samples, suggesting that the PEEK-graphite fiber composite material can indeed be considered as a very good candidate for this demanding application.