Structural materials in the reactor pressure vessels are exposed to a harsh environment, resulting in a number of material degradation processes. Irradiation generates a number of point defects in the atomic structure of a material. In addition, plastic slip localization occurs on the grain level size where highly-deformed narrow bands of material appear already at the moderate strain levels. These bands are called channels or clear bands, because they are almost empty of irradiation defects, whereas the surrounding matrix is still full of them. Clear bands are very thin with a thickness of a few tens of nm. It is thought that these clear bands contribute significantly to yield-stress increase and loss of work-hardening and ductility under irradiation. Additionally, high stress concentrations are generated at points where clear bands impinge on the grain boundaries, resulting in grain boundary damage and increasing the possibility of intergranular cracking. Continuum-based structural-mechanics models are not able to predict the initiation or the evolution of grain-level plastic slip localization. New approaches like strain-gradient crystal plasticity are being developed to tackle these issues. In the present work a numerical approach is presented where the application of strain-gradient crystal plasticity is extended to aggregates containing up to tens of grains. Plastic slip localization is demonstrated within the corresponding finite-element model.