The operation of many nuclear Pressurized Water Reactors (PWR) is being extended beyond their design lifetime threshold. In the perspective of a possible further lifetime extension, satisfying safety requirements is a priority. The characterization of the structural integrity of the reactor pressure vessel (RPV) is an important issue as it is a guiding parameter that influences the reactor lifetime. The embrittlement of the RPV material is primarily induced by the bombardment of fast neutrons (with energies greater than 1 MeV). Consequently, the fast neutron fluence is one of the quantities used by safety authorities to characterize the structural integrity of the RPV. As reactors are aging, the global bias and uncertainty of fast neutron fluence calculations are analyzed precisely in order enhance nuclear safety assessments. Fast neutron fluence estimates are based on fast neutron flux assessments that can be carried out using a two-step approach. The first step is to compute the full core fission neutron source term and the second step consists in the transport of neutrons from the core to the RPV, using the fission neutron distribution determined in the previous step. Such a two-step methodology implies some approximations on the accuracy of the modeling of fission spectra. Therefore, the present study discusses the sensitivity of neutron flux at the RPV to fission spectrum modeling. The analysis was performed by comparing the fast neutron flux predictions of decoupled schemes that consider fission spectra at a fuel assembly or a reactor core scale with those of a reference Monte-Carlo fundamental mode calculation.