[en] The main purposes of this thesis are: - to point out the spectral effects on the radiation embrittlement of the pressure-vessel steels (A 508-Class 3). - to discern among the damage models proposed, using as gauge of exposure, the one that gives the best approach of the spectral effects observed between different irradiation locations. A simulated vessel-wall environment (called ''special location'') was constructed in the ''Melusine'' reactor of ''Centre d'Etudes Nucleaires de Grenoble'' that provided for measurements of steel embrittlement increase on Charpy-V notch samples and neutron flux detector activations as well as two other additionnal standard locations. Neutron spectra for these locations were calculated using Discrete Ordinate Transport Codes (ANISN and DOT 3.5.). Six campaigns of measurements of neutron spectrum by multiple foils activations were performed. Good agreement with results obtained from the calculations and the measurements were observed. The measured increases in steel transition temperature from the experimental locations revealed an important spectrum effect between the standard and special locations. If the data are expressed in (phi.t) > 1 MeV, this effect is about 50%. None of the tested damage models (displacement zones, Lindhard model, ...) does represent the observed effects