Rising global energy demand and its adverse environmental impact,especially greenhouse gas emissions,have led to renewed interest in nuclear energy.The increased use of nuclear power poses challenges for reactor materials and immobilizing high-level nuclear waste to meet environmental,non-proliferation,and security concerns.Reactor materials,such as SiC and ferritic steels,and ceramic nuclear waste forms,such as pyrochlore compounds,will suffer radiation damage either from high-energy neutrons or from the alpha-decay of actinides and become structurally unstable.Specifically,recent developments of multiscale simulations of radiation effects and materials performance in a fission or fusion environment,including ab initio calculations,self-consistent accelerated molecular dynamics,advanced kinetic Monte Carlo(KMC)methods,and phase field modeling,will be discussed.I will focus on one of the applications of multiscale modeling to study He behavior and materials performance in a fusion environment.These simulations provide important knowledge related to the binding properties and the relative stability of small He clusters,nucleation of He bubbles,the emission of self-interstitial atoms and dislocation loops from He clusters,as well as their interactions with microstructural features.Further,I will discuss how the atomic-level results can be used to inform mesoscale methods to study the growth of helium bubbles and their effects on the mechanical properties of RAF/M steels.