In this field of work, the focus lies on shielding staff and the public from the ionizing radiation directly emitted by radioactive waste. Spent fuel assemblies, in particular, pose high shielding requirements due to the combination of simultaneously occurring neutron and gamma radiation and their spatial distribution within the fuel assembly. For effective protection, GNS has been developing multiple thick-walled CASTOR® types casks with a complex radial and axial structure. For the computational demonstration of effective shielding, WTI uses 3D Monte Carlo methods, such as MCNP®, to analyse the expected dose rate field around the containers before the actual loading. These methods are also required for analyzing radiation transport processes inside and outside buildings to accurately assess radiation transport through intake and exhaust openings as well as scattering processes in the air and soil. The computationally predicted compliance with dose rate limits is confirmed for each container loading at the time of container storage through an incoming measurement. Additionally, the calculation methods used at WTI are validated by various detailed measurement campaigns on loaded containers and benchmark experiments.