[en] The effects of combined neutron and gamma radiation on the viscoelastic behaviour of two industrial semi-crystalline PEEK grades (VICTREX 150P and 450P) were investigated. Tensile test samples were processed on an ENGEL55 injection moulder, then irradiated in a reactor pool environment, against the reactor vessel wall of the SLOWPOKE-2 facility at the Royal Military College of Canada (RMC), for exposures resulting in dosages ranging from 10 kGy to 320 kGy. First, the morphology of the irradiated resins was characterized by differential scanning calorimetry (DSC), X-ray diffraction and density measurements. Secondly, the viscoelastic behaviour of the material was studied at three thermodynamic states, in order to define the nature of the structural damage suffered by PEEK, by comparing its degrees of molecular motion with the dose received. Therefore, standard mechanical testing was performed at room temperature, stress relaxation near the glass transition temperature (T_g), at 140 degrees C, and melt viscosity above the melting point (T_m), at 350 degrees C. Results confirmed that PEEK is highly resistant to radiation, but also suggested that PEEK degrades faster under a combined neutron and gamma flux, in a reactor pool environment, than observed in previous work under electron beam or gamma alone. The density increased slightly over the range of irradiation (150P: 1300 to 1303 kg/m³ ; 450P: 1296 to 1299 kg/m³), while the crystallinity (150P: 27 %, 450P: 24 %), and the thermal properties, like T_g and T_m, remained unaffected. Therefore, the increases in density were attributed to transformations occurring mostly in the amorphous phase, which is also consistent with previous work. The viscoelastic behaviour analysis provided the following damage assessment for PEEK: the amorphous phase is subject to both crosslinking and chain scission; the effects of chain scission, on the viscoelastic behaviour of PEEK, are predominant for doses high than 100 kGy; tie-molecules, at the crystalline and amorphous phase interface, are more sensitive and are prone to chain scission under radiation; and, crosslinking occurs predominantly near chain-ends. Overall, this research confirmed PEEK as a good candidate material for application's in nuclear reactor pool environments and for the disposal of radioactive waste. (author)