This medium allocation proposal supports a focused computational materials modelling project at KTH. The project is embedded in the nuclear materials research environment in Sweden, with active contributions to NuMaP, SUNRISE work packages and the forthcoming NEXUS initiative. It addresses key degradation mechanisms in structural materials for Generation-IV reactor systems, especially under irradiation and in contact with liquid lead or lead-bismuth coolants. We actively pursue multiscale modelling of structural materials to establish predictive links between atomic-scale mechanisms, microstructural evolution and macroscopic materials performance, with close coupling to experimental activities for validation and interpretation. Our research group consisting of approximately 20 senior researchers, 10 postdocs and 25 PhD students, actively carries out the following connected computational modelling activities: 1) density-functional-theory calculations of defect energetics, migration barriers, phase stability, phonon properties and electronic transport properties; 2) kinetic modelling of point-defect migration, accumulation and irradiation-induced microstructural evolution; 3) first-principles molecular dynamics simulations of liquid-metal/solid interfaces to describe local chemical interactions, wetting and early corrosion processes; 4) molecular dynamics simulations of radiation damage, defect clustering, liquid-metal embrittlement and corrosion-related mechanisms; and 5) development of machine-learning interatomic potentials, which is required to extend accurate liquid/solid interface and irradiation-damage simulations to larger length scales and longer time scales. The requested NAISS resources are essential for generating reliable DFT and AIMD reference data, sampling relevant alloy compositions, defect structures, temperatures and interface configurations, and delivering predictive models for advanced nuclear structural materials.