The present study investigates the effects of neutron shielding by concrete slabs in the context of nuclear fusion (14 MeV neutrons). MCNP and Geant4 numerical simulations have been performed to characterize neutron transport through concrete slabs with thicknesses ranging from 1 to 160 cm. Using a monoenergetic, normally incident 14 MeV neutron source, we examine neutron-flux attenuation and spectral evolution after propagation through concrete. The results show that multiple elastic and inelastic scattering processes dominate the transport, leading to a strong spectral redistribution and a significant build-up of scattered neutrons. For intermediate thicknesses (50–100 cm), the transmitted flux is largely composed of scattered and secondary neutrons, with a pronounced epithermal slowing-down component approximately following a 1/E behavior, a thermal peak resulting from hydrogen moderation, and a residual high-energy tail corresponding to partially degraded primary neutrons. At larger thicknesses (≥100 cm), absorption and capture of thermalized neutrons become dominant, resulting in a sharp decrease in the transmitted flux. The impact of these spectral modifications on the soft-error rate (SER) of microelectronic devices is evaluated by folding the transmitted neutron spectra with energy-dependent single-event upset (SEU) cross-sections representative of advanced semiconductor technologies. The results indicate that, although total neutron flux decreases significantly with increasing concrete thickness, the spectral redistribution and the persistence of intermediate-energy neutrons can maintain a non-negligible contribution to SER in certain configurations. These findings highlight the importance of using full transport simulations, including spectral effects, when assessing radiation-induced reliability risks in fusion facilities.