Using a detailed high-resolution finite element model for a full fusion tokamak device, we assess effects of radiation exposure of the tokamak structure to energetic neutrons produced by the deuterium-tritium plasma. The spatially-varying neutron exposure, evaluated using a bespoke finite element method-compatible Monte Carlo neutron transport approach, generates location-dependent populations of defects, producing spatially-varying elastic stress and strain fields. Exposure to neutrons also gives rise to material-dependent swelling, hardening, embrittlement, and radiation-induced magnetization. Application of critical stress or fracture failure criteria, relating local operating conditions for materials to the structural integrity of the device as a whole, provides a means for assessing the lifetime of an operating fusion device. The study illustrates a multiscale computational approach to modelling nuclear devices, where macroscopic parameters, evaluated using a full device simulation, self-consistently define boundary conditions for the phenomena occurring in materials on microscopic and mesoscopic scales.